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Full text of "Sewerage and sewage treatment"

FIG. 1. Construction of Peck's Run Sewer, Baltimore, Maryland. 

Frontispiece. 



FEW 

3 



SEWERAGE 

AND 

SEWAGE TREATMENT 



BY 

HAROLD E. BABBITT, M.S. 

Assistant Professor, Municipal and Sanitary Engineering, 

University of Illinois; Associate Member 

American Society of Civil Engineers 




NEW YORK 

JOHN WILEY & SONS, INC. 

LONDON: CHAPMAN & HALL, LIMITED 
1922 



Copyright, 1922, by 
HAROLD E. BABBITT, M.S. 



PRESS OF 

BRAUNWOHTH ft CO. 

BOOK MANUFACTURERS 

BROOKLYN. N. V. 



PREFACE 



THIS book is a development of class-room and lecture notes 
prepared by the author for use in his classes at the University 
of Illinois. He has found such notes necessary, since among 
the many books dealing with sewerage and sewage treatment 
he has found none suitable as a text-book designed to cover the 
entire subject. The need for a single book of the character 
described has been expressed by engineers in practice, and by 
students and teachers for use in the class-room. This book 
has been prepared to meet both these needs. It is hoped that 
the searching questions propounded by students in using the 
original notes, and the suggestions and criticisms of engineers 
and teachers who have read the manuscript, have resulted in a 
text which can be readily understood. 

The ground covered includes an exposition of the principles 
and methods for the designing, construction and maintenance of 
sewerage works, and also of the treatment of sewage. In covering 
so wide a field the author has deemed it necessary to include some 
chapters which might equally well appear in works on other 
branches of engineering, such as the chapter on Pumps and 
Pumping Stations. Special stress has been laid on che funda- 
mentals of the subject rather than the details of practice, although 
illustrations have been drawn freely from practical work. The 
quotation of expert opinions which may be in controversy, or the 
citation of examples of different methods of accomplishing the 
same thing, has been avoided when possible in order to simplify 
explanations and to avoid confusing the beginner. 

The work is to some extent a compilation of notes and quota- 
tions which have been collected by the author during years 
of study and teaching the subject. Credit has been given 
wherever due, and at the same time references have pointed out 
the original sources whenever possible. These references, which 

v 



vi PREFACE 

have been supplemented by brief bibliographies at the end of 
certain chapters, will be useful to the student and engineer inter- 
ested in further study. Occasionally the original reference has 
been lost or the phraseology of a quotation has been so altered 
by class-room use, as to make it impossible to trace the original 
source, so that in some few instances full credit may be lacking. 

The author is indebted to many of his friends for their criti- 
cisms and suggestions in the preparation of the manuscript; 
but he desires particularly to acknowledge the assistance of 
Professor A. N. Talbot, Professor of Municipal and Sanitary 
Engineering at the University of Illinois, and of Professor M. L. 
Enger, Professor of Mechanics and Hydraulics at the University 
of Illinois, in the entire work; also that of Mr. T. D. Pitts, Prin- 
cipal Assistant Engineer of the Baltimore Sewerage Commis- 
sion during the construction of the Baltimore sewers, for his 
suggestions on the first half of the book; and to Mr. Paul Hansen, 
consulting engineer, of Chicago, and to Mr. Langdon Pearse, 
Sanitary Engineer of the Sanitary District of Chicago, for 
their help on the section covering the treatment of sewage; and 
to Professor Edward Bartow, Professor of Chemistry at the 
University of Iowa, for his review of the chapter on Activated 
Sludge; in general his thanks are due to all others who have 
furnished suggestions, illustrations, or quotations, acknowledg- 
ments of which have been included in the text. 

H. E. B. 
URBANA, ILLINOIS, 1922. 



TABLE OF CONTENTS 



CHAPTER I 

INTRODUCTION 

PAGES 

1. Sewerage and the Sanitary Engineer. 2. Historical. 3. Methods 
of Collection. 4. Methods of Disposal. 5. Methods of Treat- 
ment. 6. Definitions. . 1-8 



CHAPTER II 
WORK PRKLIMINARY TO DESIGN 

7. Division of Work. 8. Preliminary. 9. Estimate of cost. METH- 
ODS OF FINANCING. 10. Bond Issues. 11. Special Assessment. 
12. General Taxation. 13. Private Capital. PRELIMINARY 
WORK. 14. Preparing for Design. 15. Underground Surveys. 
16. Borings 9-23 

CHAPTER III 
QUANTITY OP SEWAGE 

17. Dry Weather Flow. 18. Methods for Predicting Population. 
19. Extent of Prediction. 20. Sources of Information on 
Population. 21. Density of Population. 22. Changes in Area. 
23. Relation between Population and Sewage Flow. 24. Char- 
acter of District. 25. Fluctuations in Rate of Sewage Flow. 
26. Effect of Ground Water. 27. Resume 1 of Method for 
Determination of Quantity of Dry-weather Sewage. QUANTITY 
OF STORM WATER. 28. The Rational Method. 29. Rate of 
Rainfall. 30. Time of Concentration. 31. Character of Sur- 
face. 32. Empirical Formulas. 33. Extent and Intensity of 

Storms 24-50 

vii 



viii CONTENTS 

CHAPTER IV 

HYDRAULICS OF SEWERS 

PAGES 

34. Principles. 35. Formulas. 36. Solution of Formulas. 37. Use 
of Diagrams. 38. Flow in Circular Pipes Partly Full. 39. Sec- 
tions Other than Circular. 40. Non-Uniform Flow 51-77 

CHAPTER V 
DESIGN OF SEWERAGE SYSTEMS 

41. The Plan. 42. Preliminary Map. 43. Layout of the Separate 
System. 44. Location and Numbering of Manholes. 45. 
Drainage Areas. 46. Quantity of Sewage. 47. Surface Profile. 
48. Slope and Diameter of Sewers. 49. The Sewer Profile. 
DESIGN OF A STORM-WATER SEWER SYSTEM. 50. Planning the 
System. 51. Location of Street Inlets. 52. Drainage Areas. 
53. Computation of Flood Flow by McMath Formula. 54. 
Computation of Flood Flow by Rational Method 78-98 

CHAPTER VI 

APPURTENANCES 

55. General. 56. Manholes. 57. Lampholes. 58. Street Inlets. 
59. Catch-basins. 60. Grease Traps. 61. Flush-tanks. 62. 
Siphons. 63. Regulators. 64. Junctions. 65. Outlets. 66. 
Foundations. 67. Underdrains 99-126 



CHAPTER VII 
PUMPS AND PUMPING STATIONS 

68. Need. 69. Reliability. 70. Equipment. 71. The Building. 
72. Capacity of Pumps. 73 Capacity of Receiving Well. 74. 
Types of Pumping Machinery. 75. Sizes and Descriptions of 
Pumps. 76. Definitions of Duties and Efficiency. 77. Details 
of Centrifugal Pumps. 78. Centrifugal Pump Characteristics. 
79. Setting of Centrifugal Pumps. 80. Steam Pumps and 
Pumping Engines. 81. Steam Turbines. 82. Steam Boilers. 
83. Air Ejectors. 84. Electric Motors. 85. Internal Com- 



CONTENTS ix 

PAGES 

bustion Engines. 86. Selection of Pumping Machinery. 87. 
Costs of Pumping Machinery. 88. Cost Comparisons of Dif- 
ferent Designs. 89. Number and Capacity of Pumping Units. 127-163 



CHAPTER VIII 
MATERIALS FOR SEWERS 

90. Materials. 91. Vitrified Clay Pipe. 92. Cement and Concrete 
Pipe. 93. Proportioning of Concrete. 94. Waterproofing of 
Concrete. 95. Mixing and Placing Concrete. 96. Sewer Brick. 
97. Vitrified Clay Sewer Block. 98. Cast Iron, Steel, and Wood. 164-193 

CHAPTER IX 
DESIGN OF THE SEWER RING 

99. Stresses in Buried Pipe. 100. Design of Steel Pipe. 101. 
Design of Wood Stave Pipe. 102. External Loads on Buried 
Pipe. 103. Stresses in Circular Ring. 104. Analysis of Sewer 
Arches. 105. Reinforced Concrete Sewer Design 194-210 

CHAPTER X 

CONTRACTS AND SPECIFICATIONS 

106. Importance of the Subject. 107. Scope of the Subject. 108. 
Types of Contracts. 109. The Agreement. 110. The Advertise- 
ment. 111. Information and Instructions for Bidders. 112. 
Proposal. 113. General Specifications. 114. Technical Specifi- 
cations. 115. Special Specifications. 116. The Contract. 
117. The Bond. . . 211-232 



CHAPTER XI 
CONSTRUCTION 

118. Elements. WORK OF THE ENGINEER. 119. Duties. 120. 
Inspection. 121. Interpretation of Contract. 122. Unex- 
pected Situations. 123. Cost Data and Estimates. 124. 
Progress Reports. 125. Records. EXCAVATION. 126. Speci- 
fications. 127. Hand Excavation. 128. Machine Excavation. 



X CONTENTS 

PAGES 

129. Types of Machines. 130. Continuous Bucket Exca- 
vators. 131. Cableway and Trestle Excavators. 132. Tower 
Cableways. 133. Steam Shovels. 134. Drag Line and Bucket 
Excavators. 135. Excavation in Quicksand. 136. Pumping 
and Drainage. 137. Trench Pump. 138. Diaphragm Pump. 
139. Jet Pump. 140. Steam Vacuum Pumps. 141. Centrif- 
ugal and Reciprocating Pumps. 142. Well Points. 143. Rock 
Excavation. 144. Power Drilling. 145. Steam or Air for Power. 
146. Depth of Drill Hole. 147. Diameter of Drill Hole. 

148. Spacing of Drill Holes. SHEETING AND BRACING. 

149. Purposes and Types. 150. Stay Bracing. 151. Skeleton 
Sheeting. 152. Poling Boards. 153. Box Sheeting. 154. 
Vertical Sheeting. 155. Pulling Wood Sheeting. 156. Earth 
Pressures. 157. Design of Sheeting and Bracing. 158. Steel 
Sheet Piling. LINE AND GRADE. 159. Locating the Trench. 
160. Final Line and Grade. 161. Transferring Grade and 
Line to the Pipe. 162. Line and Grade in Tunnel. TUN- 
NELLING. 163. Depth. 164. Shafts. 165. Timbering. 166. 
Shields. '167. Tunnel Machines. 168. Rock Tunnels. 169. 
Ventilation. 170. Compressed Air. EXPLOSIVES AND BLASTING. 
171. Requirements. 172. Types of Explosives. 173. Per- 
missible Explosives. 174. Strength. 175. Fuses and Deto- 
nators. 176. Care in Handling. 177. Priming. Loading, and 
Firing. 178. Quantity of Explosive. PIPE SEWERS. 179. The 
Trench Bottom. 180. Laying Pipe. 181. Joints. 182. Labor 
and Progress. BRICK AND BLOCK SEWERS. 183. The Invert. 
184. The Arch. 185. Block Sewers. 186. Organization. 
187. Rate of Progress. CONCRETE SEWERS. 188. Construction 
in Open Cut. 189. Construction in Tunnels. 190. Materials 
for Forms. 191. Design of Forms. 192. Wooden Forms. 193. 
Steel-lined Wooden Forms. 194. Steel Forms. 195. Rein- 
forcement. 196. Cost of Concrete Sewers. BACKFILLING. 197. 
Method 233-331 

CHAPTER XII 

MAINTENANCE OF SEWERS 

198. Work Involved. 199. Causes of Troubles. 200. Inspection. 
201. Repairs. 202. Cleaning of Sewers. 203. Flushing Sewers. 
204. Cleaning Catch-basins. 205. Protection of Sewers. 206. 
Explosions in Sewers. 207. Valuation of Sewers 332-351 

CHAPTER XIII 
COMPOSITION AND PROPERTIES OF SEWAGE 

208. Physical Characteristics. 209. Chemical Composition. 210. 
Significance of Chemical Constituents. 211. Sewage Bacteria. 



CONTENTS xi 

PAGES 

212. Organic Life in Sewage. 213. Decomposition of Sewage. 
214. The Nitrogen Cycle. 215. Plankton and Macroscopic 
Organisms. 216. Variations in the Quality of Sewage. 217. 
Sewage Disposal. 218. Methods of Sewage Treatment 352-371 

CHAPTER XIV 
DISPOSAL BY DILUTION 

219. Definition. 220. Conditions Required for Success. 221. Self- 
purification of Running Streams. 222. Self-purification of 
Lakes. 223. Dilution in Salt Water. 224. Quantity of Diluting 
Water Needed. 225. Governmental Control. 226. Prelimi- 
nary Treatment. 227, Preliminary Investigations 372-382 

CHAPTER XV 

SCREENING AND SEDIMENTATION 

228. Purpose. 229. Types of Screens. 230. Sizes of Openings. 
231. Design of Fixed and Movable Screens. PLAIN SEDIMEN- 
TATION. 232. Theory of Sedimentation. 233. Types of Sedi- 
mentation Basins. 234. Limiting Velocities. 235. Quantity 
and Character of Grit. 236. Dimensions of Grit Chambers. 
237. Existing Grit Chambers. 238. Number of Grit Chambers. 
239. Quantity and Characteristics of Sludge from Plain Sedi- 
mentation. 240. Dimensions of Sedimentation Basins. CHEM- 
ICAL PRECIPITATION. 241. The Process. 242. Chemicals. 
243. Preparation and Addition of Chemicals. 244. Results 383-409 

CHAPTER XVI 

SKITICIZATION 

245. The Process. 246. The Septic Tank. 247. Results of Septic 
Action. 248. Design of Septic Tanks. 249. Imhoff Tanks. 
250. Design of Imhoff Tanks. 251. Imhoff Tank Results. 
252. Status of Imhoff Tanks. 253. Operation of Imhoff Tanks. 
254. Other Tanks 410-430 

CHAPTER XVII 

FILTRATION AND IRRIGATION 

255. Theory. 256. The Contact Bed. 257. The Trickling Filter. 
258. Intermittent Sand Filter. 259. Cost of Filtration. IRRI- 
GATION. 260. The Process. 261. Status. 262. Preparation 
and Operation. 263. Sanitary Aspects. 264. The Crop 431-464 



xii CONTENTS 

CHAPTER XVIII 

ACTIVATED SLUDGE 

PAGES 

265. The Process. 266. Composition. 267. Advantages and 
Disadvantages. 268. Historical. 269. Aeration Tank. 270. 
Sedimentation Tank. 271. Reaeration Tank. 272. Air Dis- 
tribution. 273. Obtaining Activated Sludge. 274. Cost 465-479 

CHAPTER XIX 

ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION 

275. The Miles Acid Process. ELECTROLYTIC TREATMENT. 276. 

The Process. DISINFECTION. 277. Disinfection of Sewage .... 482-493 

CHAPTER XX 

SLUDGE 

278. Methods of Disposal. 279. Lagooning. 280. Dilution. 281. < 

Burial. 282. Drying 495-505 

CHAPTER XXI 
AUTOMATIC DOSING DEVICES 

283. Types."* 284. Operation. 285. Three Alternating Siphons. 
286. Four or More Alternating Siphons. 287. Timed Siphons. 
288. Multiple Alternating and Timed Siphons 506-512 



SEWERAGE AND SEWAGE TREATMENT 



CHAPTER I 
INTRODUCTION 

1. Sewerage and the Sanitary Engineer. Present day concep- 
tions of sanitation are based on the scientific discoveries which 
have resulted so much in the increased comfort and safety of 
human life during the past century, in the increase of our material 
possessions, and the extent of our knowledge. The danger to 
health in the accumulation of filth, the spreading of disease by 
various agents, the germ theory of disease, and other important 
principles of sanitation can be counted among the more recent 
scientific discoveries and pronouncements. Experience has shown, 
and continues to show, that the increase of population may be 
inhibited by accumulations of human waste in populous districts. 
The removal of these wastes is therefore essential to the existence 
of our modern cities. 

The greatest need of a modern city is its water supply. With- 
out it city life would be impossible. The next most important 
need is the removal of waste matters, particularly wastes con- 
taining human excreta or the germs of disease. To exist without 
street lights, pavements, street cars, telephones, and the many 
other attributes of modem city life might be possible, although 
uncomfortable. To exist in a large city without either water or 
sewerage would be impossible. The service rendered by the sani- 
tary engineer to the large municipality is indispensable. In 
addition to the service necessary to the maintenance of life in 
large cities, the sanitary engineer serves the smaller city, the 
rural community, the isolated institution, and the private estate 
with sanitary conveniences which make possible comfortable 



2 INTRODUCTION 

existence in them, and which are frequently considered as of 
paramount necessity. Training for service in municipal sanita- 
tion is training for a service which has a more direct beneficial 
effect on humanity than any other engineering work, or any 
other profession. W. P. Gerhard states: 

A Sanitary Engineer is an engineer who carries out those 
works of civil engineering which have for their object; 

(a) The promotion of the public and individual health; 

(6) The remedying of insanitary conditions; 

(c) The prevention of epidemic diseases. 

A well-educated sanitary engineer should have a 
thorough knowledge of general civil engineering, of archi- 
tecture, and of sanitary science. The practice of the sani- 
tary engineer embraces water supply, sewerage, and 
sewage and garbage disposal for cities and for single build- 
ings; the prevention of river pollution, the improvement 
of polluted water supplies; street paving and street clean- 
ing, municipal sanitation, city improvement plans, the 
laying-out of cities, the preparation of sanitary surveys, 
the regulation of noxious trades, disinfection, cremation, 
and the sanitation of buildings. 

The need of the work of the sanitary engineer in the provision 
of sewers and drains is thrust upon us in our daily experience by 
the clogging of sewers, the flooding of streets by heavy rains, 
filthy conditions in unsewered districts, increased values of prop- 
erty and improved conditions of living in sewered districts, and 
in many other ways. The increasing demand for sewerage and 
the amount of money expended on sewer construction is indicated 
by the information given in Table I. 

2. Historical. An ordinance passed by the Roman Senate in 
the name of the Emperor about A.D. 80, states: 

I desire that nobody shall conduct away any excess 
water without having received my permission or that of my 
representatives ; for it is necessary that a part of the supply 
flowing from the delivery tanks shall be utilized not only for 
cleaning our city, but also for flushing the sewers. 1 

Neither the sewers mentioned nor the distributing pipes of 
the public water supply were connected to individual residences. 
The contributions to the sewers came from the ground and the 
street surface. The streets were the receptacles of liquid and 

1 Frontinus and the Water Supply of Rome, p. 81, by Clemens Herschel. 



HISTORICAL 



3 



solid wastes and were often little more than open sewers. A 
promenade after dark in an ancient, medieval, or early modern 
city was accompanied not only by the underfoot dangers of an 
uneven pavement or an encounter with a footpad, but with the 
overhead danger from the emptying of slops into the streets from 
the upper windows. Sewers were used for the collection of sur- 
face water; the discharge of fecal matter into them was pro- 
hibited. The problem of the collection of sewage remained 
unsolved until the Nineteenth Century. 

TABLE 1 
POPULATION TRIBUTARY TO SEWERAOE SYSTEMS 





1905* 


1915f 


1920 1 


Population discharging raw sewage into 
the sea or tidal estuaries 


6500,000 


8,500000 




Population discharging raw sewage into 
inland streams or lakes. .... 


20,400,000 


26,400,000 




Population connected to systems where 
sewage is treated in some way 


1,100,000 


6900000 




Population connected with sewerage sys- 
tems 


28,000,000 


41,800,000 


46,300 000 











* Estimated by G. W. Fuller. Trans. Am. Society of Civil Engineers, Vol. 44, 1906, 
p. 148. The total population connected with sewerage systems was assumed to be the 
total imputation in the United States in cities over 4000 in population. 

t Estimated by Metcalf and Eddy, American Sewerage Practice, Vol. Ill, p. 240. 

t Computed from report of the United States Census, 1920, on the same basis as 
Fuller's estimate for 1905. 

The development of the London sewers was commenced 
early in the Nineteenth Century. The sewerage system of Ham- 
burg, Germany, was laid out in 1842 by Lindley, an English 
engineer who with other English engineers performed similar 
work in other German cities because of their earlier experience 
in English communities. Berlin's present system dates from 1860. 
The construction of storm water drains in Paris dates from 1663. l 
They were intended only as street drains but are now included in 
the comprehensive system of the city. The first comprehensive 
sewerage system in the United States was designed by E. S. 
Chesbrough for the City of Chicago in 1855. Previous to this 
1 Cosgrove, History of Sanitation. 



4 INTRODUCTION 

time sewers had been installed in an indifferent manner and with- 
out definite plan. The installation of a comprehensive sewerage 
system in Baltimore in 1915 marks the completion of installation 
of sewerage systems in all large American cities. 

In the early days of sewerage design it was considered unsafe 
to discharge domestic wastes into the sewers as the concentration of 
so much sewage was expected to create great nuisances and 
dangers to health. That the fear that the concentration of large 
quantities of sewage would create a nuisance was not ill founded 
is proven by the conditions on the Thames at London in 1858-59. 
Dr. Budd states: l 

For the first time in the history of man, the sewage 
of nearly three millions of people had been brought to 
seethe and ferment under a burning sun in one vast open 
cloaca lying in their midst. 

The result we all know. Stench so foul we may well 
believe had never before ascended to pollute this lower 
air. Never before at least had a stink risen to the height 
of an historic event. . . For months together the topic 
almost monopolized the public prints . . . 'India is in 
revolt and the Thames stinks' were the two great facts 
coupled together by a distinguished foreign writer, to mark 
the climax of a national humiliation. 2 

The problem of sewage disposal followed the more or less 
successful solutions of the problem of sewage collection. In 
England the British Royal Commission on Sewage Disposal was 
appointed in 1857 and issued its first report in 1865. The first 
studies in the United States were started in 1887 by the establish- 
ment of an experiment station at Lawrence, Massachusetts, where 
valuable work has been done. The station is under the State 
Board of Health, which issued its first report containing the 
results of the work at the station, in 1890. 

Various methods of sewage treatment preparatory to disposal 
have been devised from time to time. Some have fallen into 
disuse, such as the A. B. C. (alum, blood and clay) process, and 
others have taken a permanent place, such as the septic tank. 
The unsolved problems of sewage collection, and the number of 

1 Sedgwick: Sanitary Science and Public Health. 

* No detrimental effect on the public health was noted as a result of this 
condition however. It has never been conclusively proven that such nuisances 
are detrimental to the public health. 



METHODS OF COLLECTION 5 

persons still unserved by sewerage and sewage disposal opens a 
wide field to the study and construction of sewerage works. 

3. Methods of Collection. The method of collection which 
involves the removal of night soil from a privy vault, the pail 
system which involves the collection of buckets of human excreta 
from closets and homes, indoor chemical closets, and other make- 
shift methods of collection are of extreme importance where no 
sewers exist, but they are not properly considered as sewerage 
systems or sewerage works. These methods of collection are 
generally confined to rural districts and to outlying parts of 
urban communities. They require constant attention for their 
proper conduct and little skill for their installation, the principal 
requirements being to make the receptacles fly-proof. 

The pneumatic system was introduced by Liernur, a Dutch 
engineer. 1 It is used in parts of a few cities in Europe, but it is 
not capable of use on a large scale. It consists of a system of 
air-tight pipes, connecting water closets, kitchen sinks, etc., with 
a central pumping station at which an air-tight tank is provided 
from which the air is partly exhausted. As little water as possible 
is allowed to mix with the fecal matter and other wastes in order 
not to overtax the system. Solid and liquid wastes are drawn 
to the central station when the waste valve on the plumbing 
fixture is opened. 

The collection of sewage in a system of pipes through which it 
is conducted by the buoyant effect and scouring velocity of water 
is known as the water carriage system. This is the only method 
of sewage collection in general use in urban communities. In 
this system solid and liquid wastes are so highly diluted with 
water as either to float or to be suspended therein. The mixture 
resulting from this high dilution follows the laws of hydraulics as 
applied to pure water, or water containing suspended matter. 
It will flow freely through properly designed conduits and will 
concentrate the sewage wastes at the point of ultimate disposal. 

4. Methods of Disposal. Sewage is disposed of by dilution in 
water, by treatment on land, or occasionally by discharging it 
into channels that contain no diluting water. Some forin-of treat- 
ment to prepare sewage for ultimate disposal is frequently neces- 
sary and will undoubtedly be required in a comparatively short 
time for all sewage discharged into watercourses. The solid 

1 Moore and Silcock, Sanitary Engineering. P- 67, 1909. 



6 INTRODUCTION 

matters removed by treatment may be buried, burned, dumped 
into water, or used as a fertilizer. 

If the volume of diluting water, or the area and character of 
land used for disposal are not as they should be, a nuisance will 
be created. The aim of all methods of sewage treatment has so 
far been to produce an effluent which could be disposed of without 
nuisance and in certain exceptional cases to protect public water 
supplies from pollution. Financial returns have been sought 
only as a secondary consideration. A few sewage farms and irri- 
gation projects might be considered as exceptions to this as the 
value of the water in the sewage as an irrigant has been the primary 
incentive to the promotion of the farm. 

It is to be remembered that since the aim of all sewage treat- 
ment is to produce an effluent that can be disposed of without 
causing a nuisance, the simplest process by which this result can 
be attained under the conditions presented is the process to be 
adopted. No attempt is made to purify sewage completely, or 
on a practical scale to make drinking water. 

5. Methods of Treatment. Screening and sedimentation 
are the primary methods for the treatment of sewage. By these 
methods a portion of the floating and settleable solids are removed, 
preventing the formation of unsightly scum and putrefying sludge 
banks. Chemicals are sometimes added to the sewage to form a 
heavy flocculent precipitate which hastens sedimentation of the 
solid matters in the sewage. The process in these methods is 
mechanical and the solid matters removed from the sewage must 
be disposed of by other methods than dilution with the sewage 
effluent. More complete methods of treatment are dependent on 
biologic action. Under these methods of treatment complete 
stabilization of the effluent is approached, and in the most com- 
plete treatment an effluent is produced which is clear, sparkling, 
non-odorous, non-putrescible, and sterile. Sterilization of sewage, 
usually with chlorine or some of its compounds, has been used, not 
to reduce the amount of diluting water necessary, but to reduce 
the number of pathogenic germs and to minimize the danger of 
the transmission of disease. 

6. Definitions. Sewage and sewerage are not synonymous 
terms although frequently confused. Sewage is the spent water 
supply of a community containing the waste from domestic, 
industrial or commercial use, and such surface and ground water 



DEFINITIONS 7 

as may enter the sewer. 1 Sewerage is the name of the system of 
conduits and appurtenances designed to carry off the sewage. 
It is also used to indicate anything pertaining to sewers. 

A difference is made between sanitary sewage, storm sew- 
age, and industrial wastes. Sanitary sewage, sometimes called 
domestic sewage, is the liquid wastes discharged from residences 
or institutions, and contains water closet, laundry and kitchen 
wastes. Storm sewage is the surface run-off which reaches the 
sewers during and immediately after a storm. Industrial wastes 
are the liquid waste products discharged from industrial 
plants. 

A sewer is a conduit used for conveying sewage. 

The names of the conduits through which sewage may flow 
are: 

Soil Stack. A vertical pipe in a building through which waste 
water containing fecal matter or urine is allowed to flow. 

Waste Pipe. A vertical pipe in a building through which 
waste water containing no fecal matter is allowed to flow. 

House Drain. The approximately horizontal portion of a 
house drainage system which conveys the drainage from the soil 
stack or waste pipe to the point of discharge from the build- 
ing. 

House Sewer. The pipe which leads from the outside wall of 
the building to the sewer in the street. 

Lateral Sewer. The smallest branch in a sewerage system, 
exclusive of the house sewers. 

Sub-main or Branch Sewer. A sewer from which the sewage 
from two or more laterals is discharged. 2 

Main or Trunk Sewer. A sewer into which the sewage from 
two or more sub-main or branch sewers is discharged. 3 

Intercepting Sewer. A sewer generally laid transversely to a 
sewerage system to intercept some portion or all of the sewage 
collected by the system. 

Relief Sewer. A sewer intended to carry a portion of the flow 
from a district already provided with sewers of insufficient capacity 
and thus preventing overtaxing the latter. 4 

1 Similar to the definition proposed by the Am. Public Health Assn. 
1 Definition recommended by Am. Public Health Assn. 
1 Ibid. 
4 Ibid. 



8 INTRODUCTION 

Outfall Sewer. That portion of a main or trunk sewer below 
all branches. 

Flushing Sewer. A conduit through which water is conveyed 
for flushing portions of a sewerage system. 

Force Main. A conduit through which sewage is pumped 
under pressure. 



CHAPTER II 
WORK PRELIMINARY TO DESIGN 

7. Division of Work. Engineering work on sewerage can be 
divided into four parts, namely: preliminary, design, construc- 
tion and maintenance. An engineer may be engaged during 
any one or all of these periods on the same sewerage system, and 
should therefore be acquainted with his duties during each period. 

8. Preliminary. The demand for sewerage normally follows 
the installation or extension of the public water supply. It may 
be caused by: a lack of drainage on some otherwise desirable 
tract of real estate; from a public realization of unpleasant or 
unhealthful conditions in a built-up district; or through the 
realization by the municipal administration of the necessity for 
caring for the future. In whatever way the demand may be 
created the engineer should take an active part in the promotion 
of the work. 

The engineer's duties during the preliminary period are: to 
make a study of the possible methods by which the demand for 
sewerage can be satisfied ; to present the results of this study in 
the form of a report to the committee or organization responsible 
for the promotion of the work; and so to familiarize himself with 
the conditions affecting the installation of the proposed plans 
as to be able to answer all inquiries concerning them. This work 
will require the general qualities of character, judgment, efficiency 
and the understanding of men in addressing interested persons 
individually and collectively on the features of the proposed 
plans, and the exercise of engineering technique in the survey 
and the drawing of the plans. The engineer should assure him- 
self that all legal requirements in the drawing of petitions, adver- 
tising, permits, etc., have been complied with. This requires 
some knowledge of national, state, and local laws. Although 
none the less essential their description is not within the scope of 
this book. 

9 



10 WORK PRELIMINARY TO DESIGN 

The engineer's preliminary report should contain a section 
devoted to the feasibility of one or more plans which may be 
explained in more or less detail with a statement of the cost and 
advantages of each. A conclusion should be reached as to the 
most desirable plan and a recommendation made that this plan be 
installed. Other sections of the report may be devoted to a history 
of the growing demand, a description of the conditions necessitat- 
ing sewerage, possible methods of financing, and such other sub- 
jects as may be pertinent. The making of the preliminary plan 
and the design of sewerage works are described in subsequent 
chapters. 

9. Estimate of Cost. In making an estimate of cost the 
information should be presented in a readable and easily compre- 
hended manner. It is necessary that the items be clearly defined 
and that all items be included. The method of determining the 
costs of doubtful .items such as depreciation, interest charges, 
labor, etc., and the probability of the fluctuation of the costs of 
certain items should be explained. 

The engineer's estimate may be divided somewhat as follows : 

Labor. 

Material. 

Overhead. This may include construction plant, 
office expense, supervision, bond, interest on borrowed 
capital, insurance, transportation, etc. The amount of 
the item is seldom less than 15 per cent and is usually 
over 20 per cent of the contract price. 

Contingencies. This allowance is usually 10 to 15 per 
cent of the contract price. 

Profit. This should be from 5 to 10 per cent of the 
sum of the four preceding items. 

The contract price is the sum of these items. To this may be 
added : 

Engineering. 2 to 5 per cent of the contract price. 
Extra Work. Zero to 15 per cent of the contract price; 
dependent on the character of the work, the completeness 
of the preliminary information, the completeness of the 
plans, etc. 

Legal expense. 

Purchase of land, rights of way, etc., etc. 

The cost of the sewer may be stated as so much per linear 
foot for different sizes of pipe, including all appurtenances 



ESTIMATE OF COST 11 

such as manholes, catch-basins, etc., or the items may be sep- 
arated in great detail somewhat as follows: 

Earth excavation, per cu. yd. 

Rock excavation, per cu. yd. 

Backfill, per cu. yd. 

Brick manholes, 3 feet by 4 feet, per foot of depth. 

Vitrified sewer pipe with cement joints, in place, 

inches in diameter, to 6 feet deep 

6 to 8 feet deep 
8 to 10 feet deep 

Repaving, macadam per sq. yd. 
asphalt per sq. yd. 

Flush tanks, gal. capacity, per tank. 

Service pipes to flush tanks, per linear foot., etc., etc. 

These methods represent the two extremes of presenting cost 
estimates. Each method, or modification thereof, may have its 
use, dependent on circumstances. 

Reliable cost data are difficult to obtain. Lists of prices of 
materials and labor are published in certain engineering and trade 
periodicals. The Handbook of Cost Data by H. P. Gillette 
contains lists of the amount of material and labor used on certain 
specific jobs and types of construction. The price of labor and 
materials on the local market can be obtained from the local 
Chamber of Commerce, contractors and other employers of labor, 
and dealers in the desired commodities. Contract prices for 
sewerage work published in the construction news sections of 
engineering periodicals may be a guide to the judgment of the 
probable cost of proposed work, but are generally dangerous to 
rely upon as full details are lacking in the description of the work. 
A wide experience in the collection and use of cost data is the 
desirable qualification for making estimates of cost. It is pos- 
sessed by few and is not an infallible aid to the judgment. 

Having completed the design and summary of the bills of 
material and labor necessary for each structure or portion of the 
sewerage system, the product of the unit cost and the amount 
of each item plus an allowance for overhead will equal the cost 
of the item. The total cost will be the sum of the costs of each 
item. The items should be so grouped that the cost of the differ- 
ent portions of the system are separated in order that the effect 
on the total cost resulting from different combinations of items 
or the omission of any one item may be readily computed. 



12 



WORK PRELIMINARY TO DESIGN 



A method for estimating the approximate cost of sewers, 
devised by W. G. Kirchoffer 1 depends upon the use of the diagram 
shown in Fig. 2. The factors for local conditions are shown in 
Table 2. For example, let it be required to find the cost of a 
15-inch vitrified pipe sewer at a depth of 9 feet, if the unit costs 



468 



Diameter of Vitrified Pipe in Inches. 
10 12 14 16 18 20 22 24 26 28 30 32 34 36 




30 



20 30 40 50 60 70 &0 90 ^ 



2 3456 

'Cost of Sey/er in Dollars per Lineal Foot. 



7 8 9 10 II n 14 



FIG. 2. Diagram for Estimating the Cost of Sewers. 
Eng. News, Vol. 76, p. 781. 

of labor and material and the conditions are the same as shown 
in Table 3. 

Solution 

First: To find the factor depending on local condi- 
tions, enter the diagram at the 10-inch diameter and 
continue down until the intersection with the depth of 
trench at 8.2 feet is found. Now go diagonally parallel 
to lines running from left to right upwards to the inter- 

!Eng. News, Vol. 76, 1916, p. 781. See also Eng. News-Record, Vol. 85, 
1920, pp. 22, 1175. 



ESTIMATE OF COST 



13 



section with the vertical line through a cost of 45 cents 
per foot. The diagonal line running from left to right 
downwards through this intersection corresponds to a 
factor of about 11. 

TABLE * 

FACTORS FOR COSTS OF SEWERS TO BE USED WITH FIGUBE 2 



Character of Material 



Factor 



Character of Material 



Factor 



Clay, gravel and boulders, 
Medford 

Mostly sand, deep trenches 
sheeted. Wages medium. 
Richland Center 

Sandy clay. Wages medium. 
Labor conditions good at 
Kiel 

Sand. Sandy clay, some 
water. Labor conditions 
good. Pipe prices medium 
at Manston 

Gravelly clay, -n^h laid 
concrete at Burlington . . 

Sandy clay, some water, sheet- 
ing at La Farge 

Sand with water 

Gravel and boulders. High 
wages 

Clay soil. Good digging. . . . 

Sandy clay. Some water 



22-26 
21-22 
15-20 

14-20 
13-22 

17-23 
20 

26 
17 
23 



Clay 2 miles inland. Laborers 
boarded at sanitarium, 
Wales 

Clay, gravel and boulders at 
Plymouth 

Sand, clay and good digging 
at Lake Mills 

Red clay. Machine work at 
North Milwaukee 

Good digging. Wages me- 
dium at West Salem 

Sandy soil, bracing only re- 
quired. No water. Wages 
and pipe medium 

Red sticky clay 

Good digging in any soil 
Work scarce 

Red clay. No bracing 

Work inland from railroad. 
Boarding laborers and 
other expenses 



35 



20-27 



16-19 



20-24 



17-19 



14 
24 

15 
20 



35 



Second: To find the cost of 15-inch pipe at a depth of 
9.0 feet, enter the diagram at a diameter of 15 inches 
and continue down until the intersection with a depth of 
trench at 9 feet is found. Now go diagonally parallel to 
lines running from left to right upwards to the intersection 
with the diagonal line running from left to right downwards 
corresponding to the factor of 11 found above. The 
vertical line passing through this point shows the cost to 
be 67 cents per foot. 



14 



WORK PRELIMINARY TO DESIGN 



TABLE 3 

COST OF SEWER CONSTRUCTION AT ATLANTIC, IOWA 
(From Gillette's Handbook of Cost Data) 

Material: Clay, not difficult to spade and requiring little or no bracing and 
practically no pumping. All hand work except backfill which was done by 
team and scraper. Depth of trench averaged 8.2 feet; width 30 inches. 
Diameter of pipe 10 inches. 



Item 


Wage, 
Cents 
per 
Hour 


Cost, 
Cents 
per 
Foot. 


Item 


Wage, 
Cents 
per 
Hour 


Cost, 
Cents 
per 
Foot. 


Pipe 




0.20 
.003 

.001 
.006 
.014 
.014 
.027 

.130 
.002 
.002 


Trenching. Bracing 
men 


17 
17 

30 

17 
10 
30 


.002 
.010 

.008 

.005 
.006 
.022 


Hauling team and 
driver 


30 
17 


Backfilling. Shovel . 
Backfilling. Team 
and scraper 


Hauling. Man help- 
ing . . 


Cement and sand. . . 


Backfilling. Man 
and scraper 


Pipe layers 


22 
17 
17 

17 
17 
17 


Pipe layer's helper. . 
Trenching. Top men 
Trenching. Bottom 
men 


Water boy 


Foreman 


Total 




.450 


Trenching. Scaffold 
men . 









METHODS OF FINANCING 

The construction of sewerage works may be paid for by the 
issue of municipal bonds, by special assessment, by funds available 
from the general taxes, or by private enterprise. 

10. Bond Issues. A municipal bond is a promise by the 
municipality to pay the face value of the bond to the holder at a 
certain specified time, with interest at a stipulated rate during 
the interim. The security on the bond is the taxable property 
in the municipality. The legal restrictions thrown around muni- 
cipal bond issues, the value of the taxable property in the munici- 
pality, all of which may be used as security for municipal bonds, 
and the fact that a municipality can be sued in case of default, 
make municipal bonds desirable and provide a good market for 



SPECIAL ASSESSMENT 15 

their sale. The funds available from a municipal bond issue are 
limited by the amount that the legal limit is in excess of the out- 
standing issues. The legal limit varies in different states from 
about 5 to 15 per cent of the assessed value of the property in 
the municipality. In some cases the amount available from 
municipal bonds has been increased by forming a municipality 
within a municipality such as a sanitary district, a park district, 
a drainage district, etc., which comprises a large portion or all 
of an existing municipal corporation. This case is well illustrated 
in some parts of the City of Chicago where the municipal taxing 
powers are shared by the City government, the Sanitary District, 
and Park Commissioners. The right to create a new municipal 
corporation must be granted by the state legislature. Knowledge 
of fixed bonds, serial bonds, life of bonds, sinking funds, etc. is an 
important part of an engineer's education. 1 

Bond issues must usually be presented to the voters for approval 
at an election. If approved, and other legal procedure has been 
followed, the bonds may be bought by some of the many bonding 
houses, or by private individuals, and the money is immediately 
available for construction. The bonds are redeemed by general 
taxation spread over the period of the issue. 

11. Special Assessment. A special assessment is levied against 
property benefited directly by the structure being paid for. 
Special assessments are used for the payment for the construction 
of lateral sewers which are a direct benefit to separate districts 
but are without general benefit to the city. In case the construc- 
tion of an outfall sewer or the erection of a treatment plant, 
which may be of some general benefit, is necessary to care for a 
separate district, a part of the expense may be borne by funds 
available from general taxation. The legal procedure for the 
raising of funds by special assessment and the purpose to which 
the funds so raised may be applied are stipulated in great detail 
in different states and their directions must be followed implicitly. 
Illinois procedure, which is similar to that in some other states, 
is as follows: a meeting of the interested property owners is called 
by a committee or board of the municipal government, as the 
result of a petition by interested persons or through the inde- 
pendent action of the Board. At this preliminary meeting or 

1 For a more extensive treatment of the subject see Principles and Methods 
of Municipal Administration by W. B. Munro, 1916. 



16 WORK PRELIMINARY TO DESIGN 

public hearing arguments for and against the proposed improve- 
ment are heard. The engineer is present at this meeting to 
answer questions and to advise concerning the engineering 
features of the plan. If approval is given by the Board the plan 
and specifications are prepared complete in every detail and 
incorporated in an ordinance which is presented to the legislative 
branch of the city government for passage. If the project is 
adopted it is taken to the county court. An assessment roll is 
prepared by a commissioner appointed by the court. This roll 
shows the amount to be assessed against each piece of property 
benefited. A hearing is then held in the county court at which 
the owner of any assessed property may voice objections to the 
continuation of the project. The project may be thrown out of 
court for many different reasons, such as the misspelling of a street 
name, an error in an elevation, an error in the description of a 
pavement, but most important of all is definite proof that the 
benefit is not equal to the assessment. The many minor irregu- 
larities which may nullify the procedure in a special assessment 
differ in different states and in different courts in the same state, 
but in general no court can approve an assessment greater than 
the benefits given. After the project has passed through the 
county court and the assessment roll has been approved, bonds 
may be issued for the payment of the contractor. Special assess- 
ment bonds are liens against the property assessed and have not 
the same security as a general municipal bond. For this reason 
a city which has reached its legal limit of municipal bond issues 
can still pay for work by special assessment. 

The funds available from special assessments are limited only 
by the benefit to the property assessed. The amount of the 
benefit is difficult to fix and may lead to much controversy. It 
should not exceed the amount demanded for similar work in other 
localities, unless unusual and well-understood reasons can be 
given. 

12. General Taxation. In paying for public improvements 
by general taxation the money is taken from the general municipal 
funds which have been apportioned for that purpose by the 
legislative department of the municipal government. This 
method of raising funds for sewerage construction is seldom used 
unless the political situation is unfavorable to the success of a 
bond issue or special assessment and the need for the improvement 



PREPARING FOR DESIGN 17 

is great. It is usually difficult to appropriate sufficient funds for 
new construction as the general tax is apportioned to support 
only the operating expenses of the city, and statutory provisions 
limit the amount of tax which can be levied. 

13. Private Capital. Private capital has been used for financ- 
ing sewerage works in some cases because of the aversion of the 
public in some cities to the payment of a tax for the negative 
service performed by a sewer. Sewers are buried, unseen, and 
frequently forgotten, but knowledge of their necessity has spread 
and the number of privately owned sewerage works is diminishing 
because of the better service which can be provided by the munici- 
pality. 

Franchises are granted to private companies for the construc- 
tion of sewers only after the city has exhausted other methods for 
the raising of capital. The return on the private capital invested 
is received from a rental paid by the city, or paid directly by the 
users of the system, an initial payment usually being demanded 
for connection to the system. To be successful the enterprise 
must be popular and must fill a great need. This method of 
financing sewerage works is seldom employed as favorable con- 
ditions are not common. 

PRELIMINARY WORK 

14. Preparing for Design. Methods for the design of sewerage 
systems are given in Chapter V. Before the design is made 
certain information is essential. A survey must be made from 
which the preliminary map can be prepared as described in Art. 
42. Other necessary information which is the basis of subsequent 
estimates of the quantity of sewage to be cared for must be obtained 
by a study of rates- of water consumption and the density and 
growth of population, the measurement of the discharge from 
existing sewers, and the ' compilation of rainfall and run-off data. 
If no rainfall data are available estimates must be made from 
the nearest available data. Observations of rainfall or run-off 
for periods of less than 10 to 20 years are likely to be misleading. 
Methods for gathering and using this information are explained 
in subsequent chapters. 

Underground surveys are desirable along the lines of the 
proposed sewers to learn of obstructions, difficult excavation 



18 WORK PRELIMINARY TO DESIGN 

and other conditions which may be met. All such data are seldom 
gathered except for sewerage systems involving the expenditure 
of a large amount of money. For construction in small towns 
or small extensions to an existing system the funds are usually 
insufficient for extensive preliminary investigation. The saving 
in this respect is paid unknowingly to the contractor as com- 
pensation for the risk in bidding without complete information. 

15. Underground Surveys. These may be more or less exten- 
sive dependent on the character of the district in which construc- 
tion is to take place. In built-up districts the survey should be 
more thorough than in sparsely settled districts where only the 
character of the excavated material is of interest and no obstruc- 
tions are to be met. 

Underground surveys furnish to the engineer and to prospect- 
ive bidders on contract work information on which the design 
and estimate of cost and the contractor's bid may be based and 
without which no intelligent work can be done. By removing 
much of the uncertainty of the conditions to be met in the con- 
struction of the sewer, the design can be made more economical 
and the contractor's bfd should be markedly lower, sufficiently 
so to repay more than the expense of the survey. The information 
to be obtained consists of the location of the ground-water level, 
and the location and sizes of water, gas, and sewer pipes, tele- 
phone and electric conduits, street-car tracks, steam pipes, and all 
other structures which may in any way interfere with subsurface 
construction. These structures should be located by reference 
to some permanent point on the surface. The elevation of the 
top of the pipes, except sewers, rather than the depth of cover 
should be recorded, as the depth of cover is subject to change. 
The elevation of sewers should be given to the invert rather than 
to the top of the pipe. 

A portion of the map of the subsurface conditions at Wash- 
ington, D. C., is shown in Fig. 3. Many of the dimensions and 
notations are not shown to avoid confusion on this small repro- 
duction. 1 Colors are generally used instead of different forms of 
cross hatching to show the different classes of pipe and structures. 
In addition to a record of the underground structures the char- 
acter of the ground and the pavement should be recorded. A 
comprehensive underground survey is seldom available nor does 
1 Eng. Record, Vol. 74, 1916, p. 263. 



UNDERGROUND SURVEYS 



19 




l-s 



I-V 

* 4 



E 3 
S-g.3 



o 

Q I 



A. 

I 



u a * 

5 8.5 



s 



* 2 



2 
ff 

o - 



a ssi! 
2 e-f H 



Inf 



1*4,1* 

& ^ H s 1 

rl! 



^- 03 

if 



-p 

o 

1 

ro 



8- s ^ 
" 6g 

> fc; M 



IMP 

-g 

Sal 

^^ 

ii 

a| 

S S r 

Js o 

i la 

ll=.- 

iijf 



20 



WORK PRELIMINARY TO DESIGN 



time usually permit its being made preliminary to the design of a 
sewerage system. The character of the material through which 
the sewer is to pass should be determined in all cases. 

Underground pipes and structures are located by excavations, 
which may be quite extensive in some cases. Their position is 
fixed by measurements referred to manholes and other under- 
ground structures which are somewhat permanent 
in position. A city engineer should grasp every 
opportunity to record underground structures when 
excavations are made in the streets. The character 
of the material through which the sewer is to pass is 
determined by borings. 

16. Borings. Methods used for the investigation 
of subsurface conditions preliminary to sewer con- 
struction are: punch drilling, boring with earth 
auger, jet boring, wash boring, percussion drilling, 
abrasive drilling, and hydraulic drilling. The last 
three methods named are used only for unusually 
deep borings or in rock. 

Punch drills are of two sorts. The simplest punch 
drill consists of an iron rod f of an inch to 1 inch in 
diameter, in sections about 4 feet long. One section 
is sharpened at one end and threaded at the other 
so that the next section can be screwed into it with- 
ou t increasing the diameter of the rod, as shown in 
Fig. 4. The drill is driven by a sledge striking upon 
a piece of wood held at the top of the drill to pre- 
vent injury to the threads. The drill should be turned as it is 
driven to prevent sticking. It is pulled out by a hook and lever as 
shown in Fig. 5. It is useful in soft ground for soundings up to 
8 to 12 feet in depth. Another form of punch drill described 
by A. C. Veatch 1 consists of a cylinder of steel or iron, one 
to two feet long split along one side and slightly spread. The 
lower portion is very slightly expanded and tempered into a 
cutting edge. In use it is attached to a rope or wooden poles 
and lifted and dropped in the hole by means of a rope given a few 
turns about a windlass or drum. By this process the material 
is forced up into the bit, slightly springs it, and so is held. When 
the bit is filled it is raised to the surface and emptied. Much 
1 Professional paper No. 46, United States Geological Survey, 1906, p. 97. 



FIG. 4. 
Punch Drill. 



BORINGS 



21 




; 






FIG. 5. Lever for Pulling Punch Drill. 



r 



deeper holes can be 

made with this than 

with the sharpened 

solid rod. 

Types of earth 

augers about l- 

inches in diameter 

are shown in Fig. 6. 

They are screwed 

on to the end of 

a section of the 

pipe or rod and as the hole is deepened successive lengths of pipe 

or rod are added. The device is operated by two men. It is 

pulled by straight lifting or with the assistance of a link and 

lever similar to that shown 
in Fig. 5. The device is 
suitable for soft earth or 
sand free from stones, and 
can be used for holes 1 5 
to 25 feet in depth. For 
deeper holes a block and 
tackle should be used for 
lifting the auger from the 
hole. It is not suitable 
for holes deeper than about 
35 feet. 

In the jetting method 
water is led into the hole 
through a f-inch or 1-inch 
pipe, and forced down- 
ward through the drill bit 
or nozzle against the bot- 
tom of the hole. The 
complete equipment is 
shown in Fig. 7. 1 It is 
not always necessary to 
case the hole as shown in 
the figure as the muddy 

PIG. 6. Earth Augers. water and the vibration 

; United States Geological Survey, Water Supply paper No. 257, 1911. 







22 



WORK PRELIMINARY TO DESIGN 



Drive Weight 
Wooden Platform . 
Clamped to 
Casing 



Wooden Buffer 
--Drive Head 
Force Pump 



-Drive Weight Pope 
fo Hois-ring Drum or Spool 



Drill Rod Pope 
fo Hoisting Drum 



-- RopeSupporting Drill Rods 



,- Rubber Hote to Font 

Pump 



Clamp or Wrench 
for Turn ing 
Drill Pipe- 



/ Pipe Wrench for 
' Turning Casing . 




FIG. 7. Jetting Outfit. 

U. S. Geological Survey, Water Supply Paper, No. 257 

1. Simple Jetting Outfit. 2. Jetting Process. 3. Common Jetting Drill. 

4a and 46. Expansion Bit or Paddy. 5. Drive Shoe. 



BORINGS 23 

of the pipe puddle the sides so that they will stand alone. The 
jet pipe may be churned in the hole by a rope passing over a block 
and a revolving drum. In suitable soft materials such as clay, 
sand, or gravel, holes can be bored to a depth of 100 feet and 
samples collected of the material removed. An objection to the 
method is the difficulty of obtaining sufficient water. 

Methods of drilling in rock up to depths of 20 feet are described 
in Chapter XI under Rock Drilling. For deeper holes percussion, 
abrasive, or hydraulic methods as used for deep well drilling must 
be employed. 



CHAPTER III 
QUANTITY OF SEWAGE 

17. Dry-weather Flow. Estimates of the quantity of dry- 
weather sewage flow to be expected are ordinarily based on the 
population, the character of the district, the rate of water con- 
sumption, and the probable ground-water flow. Future condi- 
tions are estimated and provided for, as the sewers should have 
sufficient capacity to care for the sewage delivered to them during 
their period of usefulness. 

18. Methods for Predicting Population. Methods for the 
prediction of future population are given in the following para- 
graphs. 

The method of graphical extension. This is the quickest and 
most simple of all. In this method a curve is plotted on rect- 
angular coordinates to any convenient scale, with population as 
ordinates and years as abscissas. The curve is extended into 
the future by judgment of its general tendency. An example is 
given of the determination of the population of Urbana, Illinois, 
in 1950. Table 4 contains the population statistics which have 
been plotted on line A in Fig. 8 and extended to 1950. The 
probable population in 1950 is shown by this line to be about 
21,000. 

The method of geometrical progression. In this method the 
rate of increase during the past few years or decades is assumed 
to be constant and this rate is applied to the present population 
to forecast the population in the future. For example the rate 
of increase of population in Urbana for the past 7 decades has 
varied widely, but indications are that for the next few decades 
it will be about 20 per cent. Applying this rate from 1920 to 
1950 the population in 1950 is shown to be about 17,800. It is 
evident that this method may lead to serious error as insufficient 
information is given in the table to make possible the selection of 
the proper rate of increase. 

24 



METHODS FOR PREDICTING POPULATION 



25 



TABLE 4 
POPULATION STUDIES 





Urbana, Illinois 


Population of 






Abso- 


Per 
















Year 




















Ann 




Popu- 
lation 


lute 
Increase 
for Each 


ent 
Increase 
forEach 


Decatur 


Dan- 
ville 


Cham- 
paign 


Kanka- 
kee 


Peoria 


Bloom- 
ington 


Arbor, 
Michi- 






Decade 


Decade 














gan 


1850 


210 








736 






5,095 


1,594 




1860 


2,038 


1828 


85.6 


3,839 


1,632 


1,727 


2,984 


14,045 


7,075 


5,097 


1870 


2,277 


239 


10.5 


7,161 


4,751 


4,625 


5,189 


22,849 


14,590 


7,368 


1880 


2,942 


665 


22.6 


9,547 


7,733 


5,103 


5,651 


29,259 


17,180 


8,061 


1890 


3,511 


569. 


16.2 


16,841 


11,491 


5,839 


9,025 


41,024 


20,484 


9,431 


I'.mn 


5,728 


2217 


38.7 


20,754 


16,354 


9,098 


13,595 


56,100 


23,286 


14,509 


1910 


8,245 


2517 


30.5 


31,140 


27,871 


12,421 


13,986 


66,950 


25,786 


14.817 


1920 


10,230 


1985 


19.4 


43,818 


33,750 


15,873 


16,721 


76,121 


28,638 


19,516 



Decades in Life of City of Urbana 
1870 1680 1890 1900 1910 1920 1930 1940 1950 




50,000 



40,000 



30,000_ 



20,000 r 



10,000 



60 50 40 30 20 10 10 20 30 40 50 60 
Years Before and After which each City had a Population of 10,230 



FIG. 8. Diagram Showing Methods for Estimating Future Population. 



26 QUANTITY OF SEWAGE 

The method of utilizing a decreasing rate of increase. This 
method attempts to correct the error in the assumption of a con- 
stant rate of increase. After a certain period of growth, as the 
age of a city increases its rate of increase diminishes. In applying 
this knowledge to a prediction of the future population of a city 
the population curve is plotted, as in the graphical method and a 
straight line representing a constant rate or increase is drawn 
tangent to the curve at its end. The curve is then extended at a 
flatter rate in accordance with the rate of change of a similar 
nearby larger city. This method has not been applied to any of 
the cities included in Table 4, as none has reached that limiting 
period where the rate of increase has begun to diminish. 

The method of utilizing an arithmetical rate of increase. This 
method allows for the error of the geometrical progression which 
tends to give too large results for old and slow-growing cities. 
This method generally gives results that are too low. The abso- 
lute increase in the population during the past decade or other 
period is assumed to continue throughout the period of prediction. 
Applying this method to the same case, the increase in the popula- 
tion during the past decade was 2,000. Adding three times this 
amount to the population in 1920, the population of Urbana in 
1950 will be about 16,000. 

The method involving the graphical comparison with other 
cities with similar characteristics. In this method population 
curves of a number of cities larger than Urbana but having 
similar characteristics, are plotted with years as abscissas and 
population as ordinates, with the present population of Urbana 
as the origin of coordinates. The population curve for Urbana 
is first plotted. It will lie entirely in the third quadrant as shown 
by the heavy full line in Fig. 8. The population curves of some 
larger cities are then plotted in such a manner that each curve 
passes through the Origin at the time their population was the 
same as that of the present population of Urbana. These curves 
lie in the first and third quadrants. The population curve of the 
city in question is then extended to conform with the curves of 
older cities in the most probable manner as dictated by judgment. 
Such a series of plots has been made in Fig. 8. The results indi- 
cate that the population of Urbana in 1950 will be about 25,500. 

The last method described will give the most probable result 
as it is the most rational. For quick approximations the geo- 



EXTENT OF PREDICTION 27 

metrical progression is used. The arithmetical progression is 
useful only as an approximate estimate for old cities. 

19. Extent of Prediction. The period for which a sewerage 
system should be designed is such that each generation bears its 
share of the cost of the system. It is unfair to the present genera- 
tion to build and pay for an extensive system that will not be 
utilized for 25 years. It is likewise unfair to the next generation to 
construct a system sufficient to comply with present needs only, 
and to postpone the payment for it by a long term bond issue. 
An ideal solution would be to plan a system which would satisfy 
present and future needs and to construct only those portions 
which would be useful during the period of the bond issue. 
Unfortunately this solution is not practical, because, 1st, it is less 
expensive to construct portions of the system such as the outfall, 
the treatment plant, etc., to care for conditions in advance of 
present needs, and 2nd, the life of practically all portions of a 
sewerage system is greater than the legal or customary time limit 
on bond issues. 

A compromise between the practical and the ideal is reached 
by the design of a complete system to fulfill all probable demands, 
and the construction of such portions as are needed now in accord- 
ance with this plan. The payment should be made by bond 
issues with as long life as is financially or legally practical, but 
which should not exceed the life of the improvement. 

The prediction of the population should therefore be made 
such that a comprehensive system can be designed with intelli- 
gence. Practice has seldom called for predictions more than 50 
years in the future. 

20. Sources of Information on Population. The United 
States decennial census furnishes the most complete information 
on population. Unfortunately it becomes somewhat old towards 
the end of a decade. More recent information can be obtained 
from local sources. Practically every community takes an annual 
school census the accuracy of which is fairly reliable. The gen- 
eral tendencies of the population to change can be learned by a 
study of the post office records showing the amount of mail matter 
handled at various periods. Local chambers of commerce and 
newspapers attempt to keep records of population, but they are 
often inaccurate. Another source of information is the gross 
receipts of public service companies, such as street railways, water, 



28 



QUANTITY OF SEWAGE 



gas, electricity, telephone, etc. The population can be assumed 
to have increased almost directly as their receipts, with proper 
allowance for change in rates, character of management, and other 
factors. 

21. Density of Population. So far the study of population 
has been confined to the entire city. It is frequently necessary 
to predict the population of a district or small section of a city. 
A direct census may be taken, or more frequently its population 
is determined by estimating its density based on a comparison 
with similar districts of known density, and multiplying this 




FIG. 9. Density, Area, and Population, Cincinnati, Ohio. 1850 to 1950. 

density by the area of the district. In determining the density, 
statistics of the population of the entire city will be helpful but 
are insufficient for such a problem. A special census of the area 
involved would be conclusive but is generally considered too expen- 
sive. A count of the number of buildings in the district can be 
made quickly, and the density determined by approximating the 
number of persons per building. Statistics of the population of 
various districts together with a description of the character of 
the district are given in Table 5. 

The density of population in Cincinnati from 1850 to 1913 with 
predictions to 1950 is given in Fig. 9. 1 This shows the densities 
for the entire city and is illustrative of the manner in which future 
1 From Eng. Cont., Vol. 41, 1914, p. 698. 



DENSITY OF POPULATION 



29 



TABLE 5 
DENSITIES OF POPULATION 



City 



Character of District 



Area, 
Acres 



Density 
per Acre 



Philadelphia Thomas Run. Residential. Most'y pairs 
of two and three-story houses. 1204 acres 
settled 1,840 59 

Pine Street. Residential. Mostly solid 
four to six-story houses. 156 acres 
settled 160 97 

Shunk Street. Residential. Mostly pairs 
of two and three-story houses. 539 acres 
settled 539 119 

Lombard Street. Tenements and hotels, 
145 acres settled 147 113 

York Street. Residential and manufactur- 
ing. 354 acres settled 358 94 

New York City Residential. Three-story dwellings with 
18-foot frontage, and four-story flats with 

20-foot frontage 100 

Residential. Five-story flats 520-670 

Residential. Six-story flats 800-1000 

Residential. Six-story apartments. High 
class 300 

Chicago 1st Ward. Retail and commercial. The 

"Loop" 1,440 20.5 

2d Ward. Commercial and low-class resi- 
dential solidly built up 800 53 . 5 

3d Ward. Low-class residential 960 48. 1 

5th Ward. Industrial. Some low-class 

residences. Not solidly built up 2,240 25 . 51 

6th Ward. Residential. Four and five- 
story apartments. A few detached resi- 
dences 1,600 47 . 

7th Ward. Same as Ward 6. Not solidly 

built up. Contains a large park 4,160 21 .7 

8th Ward. Industrial. Sparsely settled .. 13,624 4.8 
9th Ward. Industrial and low-class resi- 
dential. Solidly built up 640 70.0 

10th Ward. Same as Ward 9 640 80.8 

13th Ward. Low-class residential. Solidly 
built with three and four-story flats 6.100 36. 7 



30 



QUANTITY OF SEWAGE 



TABLE 5 Continued 
DENSITIES OF POPULATION 



Citv 



Character of District 



Area, 
Acres 



Density 
per Acre 



Chicago 16th Ward. Middle-class residential. 

Some industries. Well built up 800 81 . 5 

19th Ward. Industrial and commercial. 

Some-low class residences 640 90. 7 

20th Ward. Low-class residential. Some 

industries. Entirely built up 800 77 . 1 

21st Ward. Industrial. Entirely built up 960 49.9 
23d Ward. Industrial and residential. . . . 800 55.4 
24th Ward. Residential apartment houses 

and middle-class residences 1,120 46.8 

25th Ward. Residential. High-class 
apartments. Wealthy homes. Contains 

a large park 4,160 24.0 

26th Ward. Residential. Middle-class 
homes and apartments. Fairly well built 

up 4,640 16.1 

27th Ward. Residential. Sparsely settled . 20,480 5.5 
29th Ward. Low -class residential. Two- 
story frame houses. " Back of the Yards" 6,400 12 . 8 

30th Ward. The Stock Yards 1,280 40 . 1 

32d Ward. Scattered residences 8,480 8.3 

33d Ward. Scattered residences 12,944 5 . 5 

35th Ward. Scattered residences 4,960 12.0 

General average The most crowded conditions with five- 
story and higher, contiguous buildings in 

poor class districts 750-1000 

Five and six-story contiguous flat buildings 500- 750 

Six-story high-class apartments 300- 500 

Three and four-story dwellings, business 
blocks and industrial establishments. 

Closely built up 100- 300 

Separate residences, 50 to 75-foot fronts, 
commercial districts, moderately well 

built up 50-100 

Sparsely settled districts and scattered 
frame dwellings for individual families 0- 50 



CHANGES IN AREA 31 

conditions were predicted for the design of an intercepting sewer. 
The data given in Table 5 are of value in estimating the densities 
of population in various districts. The Committee on City Plan 
of the Board of Estimate and Apportionment of New York City 
obtained some valuable information on this point, especially in 
Manhattan. Three-story dwellings with 18-foot frontage, or four- 
story flats with 20-foot frontage, presumably contiguous, were 
found to hold 100 persons to the acre. Five-story flats held 520 
to 670 persons per acre. Six-story flats held 800 to 1,000 persons 
per acre, and high class six-story apartments held less than 300 
per acre. 

22. Changes in Area. In order to determine the probable 
extent of a proposed sewerage system it is important to estimate 
the changes in the area of a city as well as the changes in the 
population. With the same population and an increased area 
the quantity of sewage will be increased because of the larger 
amount of ground water which will enter the sewers. Predictions 
of the area of a city are less accurate than predictions of popula- 
tion because the factors affecting changes cannot be so easily 
predicted. An area curve plotted against time would be helpful 
in guiding the judgment, but its extension into the future based 
on past occurrences would be futile. A knowledge of the city, 
its political tendencies, possibilities of extension, and other factors 
must be weighed and judged. The engineer, if he is ignorant of 
the city for which he is making provision, is dependent upon the 
testimony of real estate men, business men and others acquainted 
with the local situation. 

23. Relation between Population and Sewage Flow. The 
amount of sewage discharged into a sewerage system is generally 
equal to the amount of water supplied to a community, exclusive 
of ground water. The entire public water supply does not reach 
the sewers, but the losses due to leakage, lawn sprinkling, manu- 
facturing processes, etc., are made up by additions from private 
water supplies, surface drainage, etc. The estimated quantity 
of water used but which did not reach the sewers in Cincinnati 
is shown in Table 6. The amount shown represents 38 per cent 
of the total consumption. Unless direct observations have been 
made on existing sewers or other factors are known which will 
affect the relation between water supply and sewage, the average 
sewage flow exclusive of ground water, should be taken as the 



32 QUANTITY OF SEWAGE 

average rate of water consumption. Experience has shown that 
water consumption increases after the installation of sewers. 

TABLE 6 

ESTIMATED QUANTITY OF WATER USED BUT NOT DISCHARGED INTO THE 
SEWERS IN CINCINNATI 

Expressed in gallons per capita per day, and based on a total consumption 
of 125 to 150 gallons per capita per day. 



Steam railroads 


6 to 7 


Manufacturing and me- 




Street sprinklers 


6 to 7 


chanical 


6 to 7 


Consumers not sewered . . . 


9 to 10 


Lawn sprinklers 


3 to3 






Leakage 


18 to 21 











The public water supply is generally installed before the sewer- 
age system. By collecting statistics on the rate of supply of 
water a fair prediction can be made of the quantity of sewage 
which must be cared for. The rate of water supply varies widely 
in different cities. It is controlled by many factors such as meters, 
cost and availability of water, quality of water, climate, popula- 
tion, etc. In American cities a rough average of consumption is 
100 gallons per capita per day. Other factors being equal the 
rate of consumption after meters have been installed will be 
about one-half the rate before the meters were installed. Low 
cost, good quantity and good quality will increase the rate of 
consumption, and the rate will increase slowly with increasing 
population. Statistics of rates of water consumption are given 
in Table 7. 

24. Character of District. The various sections of a city are 
classified as commercial, industrial, or residential. The residential 
districts can be subdivided into sparsely populated, moderately 
populated, crowded, wealthy, poor, etc. Commercial districts 
may be either retail stores, office buildings, or wholesale houses. 
Industrial districts may be either large factories, foundries, etc., 
or they may be made up of small industries housed in loft build- 
ings. 

In cities of less than 30,000 population the refinement of such 
subdivisions is generally unnecessary in the study of sewage flow, 
all districts being considered the same. The data given in Tables 
8 and 9 indicate the difference to be found in different districts of 



CHARACTER OF DISTRICT 



33 



large cities. The Milwaukee data are presented in a form avail- 
able for estimates on different bases. These data are shown in 
Table 10. 

TABLE 7 
RATES OF WATER CONSUMPTION 

From Journals of American and New England Water Works Associations 









Con- 








Con- 




Popu- 




sump- 




Popu- 




sump- 




lation 


Per 


tion, 




lation 


Per 


tion, 


City 


in 


Cent 


Gal. 


City 


in 


Cent 


Gal. 




Thou- 


Metered 


per 




Thou- 


Metered 


per 




sands 




Capita 




sands 




Capita 








per Day 








per Day 


Tacoma Wash . . . 


100 


11.6 


460 


Jefferson City, Mo 


13.5 


34.4 


100 


Buffalo N Y 


450 


4.9 


310 


Muncie Ind 


30 


23.8 


95 


Cheyenne, Wyo .... 


13 




270 




24 


4.5 


90 


Erie, Pa 


72 


3.0 


198 


Council Bluffs, la . . 


32 


75.5 


80 


Philadelphia, Pa 


1611 


4.6 


180 


San Diego, Cal .... 


85 


100 


80 


St. Catherines, Ont. 


17 


3.2 


160 


Monroe "Wis . . . 


3 


100 


80 


Port Arthur, Ont.. . 


18 


14.7 


145 


Yazoo City, Miss . . 


7 


84.1 


75 


Ogdensburg, N. Y . . 


18 


0.2 


140 


Oak Park, Illinois. . 


26 


100 


70 


Los Angeles, Cal. . . . 


516 


77.9 


140 


Portsmouth, Va. . . . 


75 


8.1 


65 


Wilmington, Del. . . 


92 


43.7 


125 


New Orleans, La. . . 


360 


99.7 


60 


Lancaster, Pa 


60 


34.6 


120 


Rockford, 111 


53 


93.0 


55 


Richmond, Va 


120 


75.2 


115 


Fort Dodge, la. ... 


20 


96.0 


50 


St. Louis, Mo 


730 


6.7 


110 


Manchester, Vt. . . . 


1.5 


69.0 


45 


Springfield, Mass. . . 


100 


94.4 


110 


Woonsocket, R. I. . 


47.5 


95.6 


35 


Keokuk, la 


14 


64.5 


105 



























Attempts have been made to express the rate of sewage flow 
in different units other than in gallons per capita per day. A unit 
in terms of gallons per square foot of floor area tributary has been 
suggested for commercial and industrial districts. It has not 
been generally adopted. The rates of flow in New York City as 
reported in this unit by W. S. McGrane are given in Table 1 1 . 

The most successful way to predict the flow from commercial 
or industrial districts is to study the character of the district's 
activities and to base the prediction on the quantity of water 
demanded by the commerce and industry of the district affected. 

25. Fluctuations in Rate of Sewage Flow. The rate of flow 
of sewage from any district varies with the season of the year, 
the day of the week, and the hour of the day. The maximum 
and minimum rates of sewage flow are the controlling factors in 
the design of sewers. The sewers must be of sufficient capacity 



34 



QUANTITY OF SEWAGE 



to carry the maximum load which may be put upon them, and 
they must be on such a grade that deposits will not occur during 
periods of minimum flow. The maximum and minimum rates of 
flow are usually expressed as percentages of the average rate of 
flow. 

TABLE 8 
SEWAGE FLOW FROM DIFFERENT CLASSES OF DISTRICTS 

Arranged from data by Kenneth Allen in Municipal Engineer's Journal, 
Feb., 1918. 



District 



Gallons 

per 

Capita 
per Day 



Gallons 

per 

Acre 
per Day 



Buffalo, N. Y. From Report of International Joint Com- 
mission on the Pollution of Boundary Waters: 

Industrial: Metal and automobile plants. Maximum 13,000 

Industrial: Meat packing, chemical and soap '. . . 16,000 

Commercial: Hotels, stores and office buildings 60,000 

Domestic: Average 80 

Domestic : Apartment houses 147 

Domestic: First-class dwellings 129 

Domestic: Middle-class dwellings 81 

Domestic: Lowest-class dwellings 35.5 

Cincinnati, Ohio. 1913 Report on Sewerage Plan: 

Industrial, in addition to residential and ground water 9,000 

Commercial, in addition to residential and ground water .... 40,000 
Domestic 135 

Detroit, Mich.: 

Domestic 228 

Industrial, in addition to residential and ground water 12,000 

Commercial, in addition to residential and ground water .... 50,000 

Milwaukee, Wis. 1915 Report of Sewerage Commission: 

Industrial, maximum 81 16,600 

Industrial, average 31 8,300 

Commercial, maximum 60,500 

Commercial, average 37,400 

Wholesale commercial, maximum 20,000 

Wholesale commercial, average 9,650 



CHARACTER OF DISTRICT 



35 



TABLE 9 

OBSERVED WATER CONSUMPTION IN DIFFERENT CLASSES OF DISTRICTS IN 

NEW YORK CITY 

From data by Kenneth Allen in Municipal Engineers Journal, for 1918 















Daily 




Daily 




Daily 




Cons. 




Cons. 




Cons. 


Hbtels 


Gals, per 
1000 
Sq. Ft. 


Tenements 


Gals, per 
1000 
Sq. Ft. 


Office and Loft 
Buildings 


Gals, per 
1000 
Sq. Ft. 




Floor 




Floor 




Floor 




Area 




Area 




Area 


Building 


* 

M 


| 


Location 


* 

M 

- 


M 


Building 


* 

M 

a 


| 


Hotel Biltmor.' 
Hotel McAlpin. 


470 

7 VI 


MM 

094 


78th-79th St. and 
IV way 


256 


192 


Mi-draw Bldg. . . . 
N Y Telephone 


309 


200 


Hotel Plaza 


630 


578 


410 E 65th St. . . . 


350 


295 


Bldg 




194 


Hotel Waldorf 






30th St. and Madi- 






Met. Life Bldg. . 




256 




618 


482 


son Ave 


306 


Iss 


42d St Bldg 




271 




732 


492 


27 Lewis St 


307 


250 






118 


Hotel Vanderbilt . . 


604 


545 


258 Delancey St . . 


207 


226 


Equitable Bldg .... 


366 


268 




634 


526 


Average 


207 


230 




338 


219 









* Max. represents only the average maximum, not the greatest maximum. 

TABLE 10 

SEWAGE FLOW FROM DIFFERENT CLASSES OF DISTRICTS BASED ON 1915 
REPORT OF MILWAUKEE SEWERAGE COMMISSION 






Ratio of maximum to average rate for department store district .... 

Ratio of maximum to average rate for hotel district 

Ratio of maximum to average rate for office building district 

Ratio of maximum to average rate for wholesale commercial district. 



1.755 
1.65 
1.51 
2.1 



Average and maximum gallons per thousand square feet of 


Avg. 


Max 


For department store district 


232 


407 


For office building district 


541 


891 


For wholesale commercial district .... 


164 


344 


For all districts except wholesale commercial. . 


381 


618 


Average and maximum gallons per day : 
For all districts except wholesale commercial 


17,700 


29,800 


For wholesale commercial district . . 


9.650 


20.000 



36 



QUANTITY OF SEWAGE 



TABLE 11 

RATES OF CONSUMPTION PREDICTED FOU DIFFERENT DISTRICTS IN NEW YORK 

CITY 





o 




-d ' 


13 - 




c 


4> 


o 


s> 


b 










M 









a 


a 






jj g 


C 

o 


2ji 


a> 


m 




S ^ 




00 


J 






o 






C 














.S3 


5 


.S*' 


S 


o 
O 


.a 1. 


H 


Q". 


^ 

Q . 


Q 


District 


o! k. C 

-2-2 


o 


a c- 


A . 

OCB 


c 
1 


l-l 


Xft 

Q 


sl 


SL2 


s| 




"^ Q) 01 





o^ 


O 


^ 


<^ 


. ( ; 


*^" 


^ 


w 




| 


B 

3 


|8 


"2 


o 

c 


3J^ 


"8. 


sJ 

~ 


ll 


ll 




3 t. fi 


*z\ 


(M ^ 


> 


u 


OS 


tj ^ 


"E a g 


3 g 


= g 




-g D.U 


H 




1 


1 


c^ <^ 2. 


is 


"I < 


1^4 


i^-< 




fc 


^ 




















S 


s 


Hotel and midtown 


24,800 


15 


634 


526 


500 


.20 


.29 


.34 


1.04 


.146 


Midtown and financial. 


24,800 


15 


338 


219 


300 


. 12 


.18 


.23 


.078 


.110 


East and West of midtown. . . . 


24,800 


10 


297 


230 


300 


.074 


.12 


.15 


.057 


.097 


Apartment, 59th to 155th Sts. . 


20,400 


7 




230 


300 


.043 


.06 


.09 






Manhattan north of 155th St. . 


20,400 


5 




230 


300 


.031 


.05 


.08 







Midtown district consists of department stores, large railroad terminals, industrial and 
loft buildings, and sky-scraper office buildings. 

It is difficult to set any definite figure for the percentage which 
the maximum rate of flow is of the average. Fluctuations above 
and below the average are greater the smaller the tributary 
population. This relation can be expressed empirically as 



= - 
pn* 

in which M represents the per cent which the maximum flow is 
of the average, and P represents the tributary population in 
thousands. The expression should not be used for populations 
below 1,000 nor above 1,000,000. Having determined the expected 
average flow of sewage by a study of the population, water con- 
sumption, etc., the maximum quantity of sewage is determined 
by multiplying the average flow by the per cent which the maxi- 
mum is of the average. In this connection W. G. Harmon 1 offers 
the relation 



,_> 

4+Vp' 

which was used in the design of the Ten Mile Creek intercepting 

sewer at Toledo, Ohio. For rough estimates and for comparative 

purposes the ratio of the average to the minimum flow can be 

1 Eng. News-Record, Vol. 80, page 1233, 1918. 



FLUCTUATIONS IN RATE OF SEWAGE FLOW 



37 



taken the same as the ratio of the maximum to the average flow, 

unless direct gaugings or other information show it to be otherwise. 

The fluctuations of flow in commercial and industrial districts 

are so different from those in residential districts that the formulas 



zoo 




23456789 10 II 
A.M. 



23456789 
P.M. 



10 II 



1. Toledo 

2. Toledo 

3. Toledo 

4. Toledo 

5. Toledo 



FIG. 10. Daily and Hourly Variations of Sewage Flow. 



().; Manufacturing average. 

O. ; Manufacturing, Monday. 

<).; Manufacturing, Sunday. 

O.; Residential, average. 

O.; Residential, Monday. 



6. Toledo, O.; Residential, Sunday. 

7. Cincinnati, O., Industrial, average. 

8. Cincinnati, O.; Residential, average. 

9. Cincinnati, O. ; Commercial, average. 
10. Average of 7 cities. 



given should not be used in the design of sewers other than those 
draining residential areas. It is reasonable to suppose that 
fluctuations in rates of flow from industrial districts are dependent 



38 QUANTITY OF SEWAGE 

upon the character of the tributary industries. A study of these 
industries will give valuable light on the maximum and minimum 
rates at which sewage will be delivered to the sewers. 

Hourly, daily, and seasonal fluctuations in rates of sewage 
flow are of interest in the design of pumping stations to give 
knowledge of the rates at which the pumps must operate at 
various periods. The fluctuations in rates of sewage flow during 
various hours and days in different cities and districts are shown 
in Fig. 10. Fluctuations in rate of flow of sewage lag behind 
fluctuations in rate of water consumption, the time being depend- 
ent on the distance through which the wave of change must 
travel in the sewer. 

26. Effect of Ground Water. Sewers are seldom laid with 
water-tight joints. Since they usually lie below the ground 
water level it is inevitable that a certain amount of ground water 
will enter. Various units have been suggested for the expression 
of the inflow of ground water in an attempt to include all of the 
many factors. Some of these units are : gallons per acre drained 
by the sewer per day, gallons per mile of pipe per day, gallons per 
inch diameter per mile of pipe per day, etc. Since the ground 
water enters pipe sewers at the joints, the longer the joints the 
greater the probability of the entrance of ground water. The 
last unit is therefore the most logical but the accuracy of the 
result is scarcely worthy of such refinement and the unit usually 
adopted is gallons per mile of pipe per day. 

No definite figure can be given for the amount of ground 
water to be expected in sewers since the character of the soil and 
the ground water pressure must be considered. Relatively 
normal infiltration may be found from 5,000 to 80,000 gallons per 
mile of pipe per day. The minimum is seldom reached in wet 
ground and the maximum is frequently exceeded. Table 12 
shows the amount of ground water measured in various sewers 
as given by Brooks. 1 

27. Resume of Method for Determination of Quantity of Dry- 
weather Sewage. The steps in the determination of the quantity 
of sewage are: determine the period in the future for which the 
sewers are to be designed; estimate the population and tributary 
area at the end of this period ; estimate the rate of water consump- 

1 Infiltration of Ground Water into Sewers. Transactions of the American 
Society of Civil Engineers, Vol. 76, 1913, p. 1909. 



EFFECT OF GROUND WATER 



39 






I* 

S <y 

02 -a 

8 w 
2 I 

* S 

g o 

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^>. "** 

^* "3 

M 

s 4 

H o 8 

M h 3) 

O 

2 * i 

Hog 
B H 

tf -2 

I -i 

|J 

i ^ 

H 



DATA ON 



w 



o 



QMOQQOQ<P7t!QeQQ<eeS ;? 

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ob^riiJic^c^Tfli-irtth-i-iS 

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00 "5 dCO ^OiOOO^f CO >~ ' >~l 



od 

o; -I -3 

p .5 oo e3 p i>-ri> t-. os 8 

IN O OO i-iT}ilNI>. 48 

flj 4-l 

i : 'So- 

s ** 

i '. : ;o -^ 

6 

ii 

o d 

O----00--IO-CC-*--- -O. 

w ^ 

Q, 

2S. I 

1 * 

ft. 
_ - * 

os ^>!;j > !;;;;j>''-^ > ~>! > -i. > ^.i:- ( ;.c3c S> 

CO rf 
CO . (N . 

p - M oo -oo 2^^^ 'co^^^-s g 

1 i 
1 

* 'rv^Ko tc S * 
"laM pq 

1 

^ :::::::::::::::: | 

JS 

-s 1 ^, 

OQ 

= 






40 QUANTITY OF SEWAGE 

tion and assume the sewage flow to equal the water consumption ; 
determine the maximum and minimum rates of sewage flow; 
and finally, estimate the maximum rate of ground water seepage 
and add it to the maximum rate of sewage flow to give the total 
quantity of sewage to be carried by the proposed sewers. 

QUANTITY OF STORM WATER 

28. The Rational Method. The water which falls during a 
storm must be removed rapidly in order to prevent the flooding 
of streets and basements, and other damages. The quantity of 
water to be cared for is dependent upon : the rate of rainfall, the 
character and slope of the surface, and the area to be drained. 
All methods for the determination of storm water run-off, whether 
rational or empirical, depend upon these factors. 

The so-called Rational Method can be expressed algebraically, 
as, 

Q = AIR, 

in which Q = rate of run-off in cubic feet per second; 

A =area to be drained expressed in acres; 

7 = percentage imperviousness of the area; 

R = maximum average rate of rainfall over the entire 
drainage area, expressed in inches per hour, which 
may occur during the time of concentration. 

The area to be drained is determined by a survey. A discussion 
of R and 7 follows in the next two sections. An example of the 
use of the Rational Method is given on page 95. 

29. Rate of Rainfall. Rainfall observations have been made 
over a long period of time by United States Weather Bureau 
observers and others. Continuous records are available in a few 
places in this country showing rainfall observations covering 
more than a century. Such records have been the bases for a 
number of empirical formulas for expressing the probable maximum 
rate of rainfall in inches per hour, having given the duration of 
the storm. Table 13 is a collection of these formulas with a 
statement as to the conditions under which each formula is appli- 
cable. The formula most suitable to the problem in hand should 
be selected for its solution. 1 

1 A comprehensive discussion of rainfall formulas will be found in Vol. 54 
of the Transactions Am. Society of Civil Engineers, 1905. 



RATE OF RAINFALL 



41 



TABLE 13 
RAINFALL FORMULAS 



Name of 
Originator 


Conditions for which Formula is 
Suitable 


Formula 


E. S. Dorr 




150 
'~*+30 
360 


A. N. Talbot. ... 
A. N. Talbot 


Maximum storms in Eastern United 
States 


<+30 
105 


Ordinary storms in Eastern United 
States 


Emil Kuichling. . . 
L. J. Le Conte.. . . 

Sherman 


t+15 

120 
i= , etc. 
<+20 

<-7/f* 

t = 25.12/*- 687 
= 18/K 
i = 12/t- 6 
105 


Heavy rainfall near New York City. . 

For San Francisco. See T. A. S. C. E. 
v. 54,p. 198 


Maximum for Boston, Mass 


Sherman 


Extraordinary for Boston, Mass 


Webster . 


Ordinary for Philadelphia, Pa 


Hendrick 


Ordinary storms for Baltimore. Eng. 
& Cont., Aug. 9. 1911 


J. de Bruyn-Kops. 
C D Hill 


t + 10 
163 

'~t+27 
120 


Ordinary storms for Savannah, Ga .... 
For Chicago 111 


Metcalf and Eddy 
W. W. Homer. . . . 

R. A Brackenbuj 

Metcalf and Eddy 
Metcalf and Eddy 
Kenneth Allen . . . 


Louisville, Ky. Am. Sew. Prac., Vol I. 
St. Louis, Mo. Eng. News, Sept. 29, 
1910 


t+15 
>-U/fH 

t = 56/f<+5)' 86 
i 2392 10154 


For Spokane, Wash. Eng. Record, Aug. 
10, 1912 


-<+2.15 l(U5 
i = 19/'^ 

*4 

"<-M 

400 * 


New Orleans 


For Denver, Colo 


Central Park, N. Y 51-Year Record. 
Eng. News-Record, April 7, 1921, 
p. 588.. 


2/+40 





* Formula devised by H. E. Babbitt from Allen's 25-year curve. 

30. Time of Concentration. By the time of concentration is 

meant the longest time without unreasonable delay that will be 

required for a drop of water l to flow from the upper limit of a 

drainage area to the outlet. Assuming a rainfall to start sud- 

1 See Note under TaWe 14. 



42 QUANTITY OF SEWAGE 

denly and to continue at a constant rate and to be evenly dis- 
tributed over a drainage area of 100 per cent imperviousness and 
even slope towards one point, the rate of run-off would increase 
constantly until the drop of water from the upper limit of the area 
reached the outlet, after which the rate of run-off would remain 
constant. In nature the rate of rainfall is not constant. The 
shorter the duration of a storm the greater the intensity of rain- 
fall. Therefore the maximum run-off during a storm will occur 
at the moment when the upper limit of the area has commenced 
to contribute. From that time on the rate of run-off will decrease. 
The time of concentration can be measured fairly well by 
observing the moment of the commencement of a rainfall, and the 
time of maximum run-off from an area on which the rain is falling. 
A prediction of the time of concentration is more or less guess 
work. As the result of measurements some engineers assume the 
time of concentration on a city block built up with impervious 
roofs and walks, and on a moderate slope, is about 5 to 10 minutes. 
This is used as a basis for the judgment of the time of concentra- 
tion on other areas. For relatively large drainage areas such a 
method cannot be used. The procedure is to measure the length 
of flow through the drainage channels of the area, to assume the 
velocity of the flood crest through these channels and thus to 
determine the time of concentration. Table 14 shows the flood 
crest velocities in various streams of the Ohio River Basin under 
flood conditions. The velocity over the surface of the ground 
may be approximated by the use of the formula 1 

V = 2,0007 VS, 

in which V = the velocity of flow over the surface of the ground 

in feet per minute; 

7 = the percentage imperviousness of the ground; 
$ = the slope of the ground. 

For areas up to 100 acres where natural drainage channels are not 
existent this formula will give more satisfactory results than guesses 
based on the time of concentration of certain known areas. 

Having determined the time of concentration, the rate of rain- 
fall R to be used in the Rational Method is found by substitution 
in some one of the rainfall formulas given in Table 13. 
1 Sewerage by A. P. Folwell. 



TIME OF CONCENTRATION 



43 



CO 

i-H 

o> 



1* 

a 
& 

W02 

> t. 

5-2 
<a 




-i 



/ 

"* 



-5. a 




H-i 

03 te i 



. 
fl 



C5 < 

C 

a 1.1 cj- 

-S P-0.9 



Sta 






O Tj< rt* <N rt< F-H O Oi <N CO Tt< 00 iO CO -H CO rt< I CO O - 

___._- _ ,_H . 1 O5 T}< 1-1 I-H C^'-'' 1 I I 



O IM COCO t^-i-HOCKN O5COCO CO <N 



C^GOrt<OiO'-iO5t^OOcOt~-r^(Na><N 
O5 O O CO I-H O iO CO ^H I-H O5 <N (N CO CO CO <M * * iO N CO 



CO CO * Tj< CO t GO O5 



OiGOi^r-t^cocOcOiO-^cO'-i' 



8(M(M 
CCi-H 



CO iO CO CO 00 CC O -^ O H t^ CO id Tt< >O 



QeO 
O> OO 





3* 

BJS 

i: 

S-o 

,3 
I- 

I! 

58 

It 

4). a 
-* 
**' 

.-H 

w ^ 
O 

IS 

H 



the 
Th 



and are n 
entration. 



wav 
f co 



rest of the flo 
ing the tim 



ties 
in 



NOTE. The velocities shown are the v 
of the crest of the flood wave should be u 
because of the storage in the river basin. 



44 QUANTITY OF SEWAGE 

31. Character of Surface. The proportion of total rainfall 
which will reach the sewers depends on the relative porosity, or 
imperviousness, and the slope of the surface. Absolutely impervi- 
ous surfaces such as asphalt pavements or roofs of buildings will 
give nearly 100 per cent run-off regardless of the slope, after the 
surfaces have become thoroughly wet. For unpaved streets, 
lawns, and gardens the steeper the slope the greater the per cent 
of run-off. When the ground is already water soaked or is frozen 
the per cent of run-off is high, and in the event of a warm rain on 
snow covered or frozen ground, the run-off may be greater than the 
rainfall. The run-off during the flood of March, 191 3, at Columbus, 
Ohio, was over 100 per cent of the rainfall. Table 15 l shows the 
relative imperviousness of various types of surfaces when dry 
and on low slopes. The estimates for relative impervious- 
ness used in the design of the Cincinnati intercepter are given in 
Table 16. 

TABLE 15 

VALUES OF RELATIVE IMPERVIOUSNESS 

Roof surfaces assumed to be watertight . 70-0 . 95 

Asphalt pavements in good order 85- . 90 

Stone, brick, and wood-block pavements with tightly cemented 

joints 75- . 85 

The same with open or uncemented joints 50- .70 

Inferior block pavements with open joints 40- . 50 

Macadamized roadways 25- . 60 

Gravel roadways and walks 15- . 30 

Unpaved surfaces, railroad yards, and vacant lots 10- . 30 

Parks, gardens, lawns, and meadows, depending on surface slope 

and character of subsoil 05- . 25 

Wooded areas or forest land, depending on surface slope and char- 
acter of subsoil 01- . 20 

Most densely populated or built up portion of a city 70- . 90 

C. E. Gregory 2 states that I, in the expression Q = AIR is a 
function of the time of concentration or the duration of the storm. 
If t represents the time of concentration and T represents the 
duration of the storm, then when T is less than t 

7 = 0.175^, 

1 From an article by E. Kuichling in Transactions American Society of 
Civil Engineers, Vol. 65, 1909, p. 399. 

2 Trans. Am. Society Civil Engineers, Vol. 58, 1907, p. 483. 



CHARACTER OF SURFACE 



45 





02 



If. 

5 
5 
g 

O 




46 QUANTITY OF SEWAGE 

but when T is greater than t, 



L 

Gregory condenses Kuichling's rules with regard to the per cent 
run-off, as follows: 

1. The per cent of rainfall discharged from any given 
drainage area is nearly constant for heavy rains lasting 
equal periods of time. 

2. This per cent varies directly with the area of imper- 
vious surface. 

3. This per cent increases rapidly and directly or uni- 
formly with the duration of the maximum intensity of the 
rainfall until a period is reached which is equal to the time 
required for the concentration of the drainage waters from 
the entire area at the point of observation, but if the rain- 
fall continues at the same intensity for a longer period this 
per cent will continue to increase at a much smaller rate. 

4. This per cent becomes larger when a moderate rain 
has immediately preceded a heavy shower on a partially 
permeable territory. 

Gregory's formulas have not been generally accepted and are 
not widely used in practice. Marston stated: 1 

All that engineers are at present, warranted in doing is 
to make some deduction from 100 per cent run-off . . . the 
deduction . . . being at present left to the engineer in 
view of his general knowledge and his familiarity with local 
conditions. 

Burger states 2 in the same connection : 

In its application there will usually be as many results 
(differing widely from each other) as the number of men 
using it. 

In spite' of these objections the Rational Method is in more favor 
with engineers than any other method. 

32. Empirical Formulas. The difficulty of determining run- 
off with accuracy has led to the production by engineers of many 
empirical formulas for their own use. Some of these formulas 
have attracted wide attention and have been used extensively, 

1 Trans. American Society of Civil Engineers, Vol. 58, 1907, p. 498. 

2 Ibid. 



EMPIRICAL FORMULAS 47 

in some cases under conditions to which they are not applicable. 
In general these formulas are expressions for the run-off in terms 
of the area drained, the relative imperviousness, the slope of the 
land, and' the rate of rainfall. 

The Burkli-Ziegler formula, devised by a Swiss engineer for 
Swiss conditions and introduced into the United States by Rudolph 
Hering, was one of the earliest of the empirical formulas to attract 
attention in this country. It has been used extensively in the 
form 



in which Q = the run-off in cubic feet per second; 

i = the maximum rate of rainfall in inches per hour over 
the entire area. This is determined only by ex- 
perience in the particular locality, and is usually 
taken at from 1 to 3 inches per hour; 
S = the slope of the ground surface in feet per thousand, 
A = the area in acres; 

C an expression for the character of the ground sur- 
face, or relative imperviousness. In this form of 
the expression C is recommended as 0.7. 

The McMath formula was developed for St. Louis conditions 
and was first published in Transactions of the American Society 
of Civil Engineers, Vol. 16, 1887, p. 183. Using the same notation 
as above, the formula is, 



McMath recommended the use of C equal to 0.75, i as 2.75 inches 
per hour, and S equal to 15. The formula has been extended 
for use with all values of C, i, S, and A ordinarily met in sewerage 
practice. Fig. 11 is presented as an aid to the rapid solution of 
the formula. 

Other formulas have been devised which are more applicable 
to drainage areas of more than 1,000 acres. 1 Such areas are met 
in the design of sewers to enclose existing stream channels drain- 
ing large areas. Kuichling's formulas, published in 1901 in the 

1 The principles governing the run-off from large areas are explained in 
Elements of Hydrology, by A. F. Meyer, 1917. 



48 



QUANTITY OF SEWAGE 



T 



3 
-4 

-S 

-6 

8 

10 



: 20 

-30 

-40 
-0 
-60 



-200 

^300 

AW 
500 
^600 

1(800 
1000 



2000 

-j3000 
-4000 

-SOOO 
j-6000 

^8000 
: .IO,000 



Values of c 
0.1 0.2 03 04 OS 0.7 1.0 




FIG. 11. Diagram for the Solution of McMath's Formula, 

<*** 



EMPIRICAL FORMULAS 



49 



report of the New York State Barge Canal, were devised for areas 
greater than 100 square miles. The following modification of 
these formulas for ordinary storms on smaller areas was published 
for the first time in American Sewerage Practice, Volume I, by 
Metcalf and Eddy : 

25,000 




30 40 50 60 70 

Quantity in Cubic Feet per Second. 

FIG. 12. Comparison of Empirical Run-off Formulas. 

It is to be noted that the only factor taken into consideration is 
the area of the watershed. It is obvious that other factors such 
as the rate of rainfall, slope, imperviousness, etc., will have a 
marked effect on the run-off. 

There are other run-off formulas devised for particular con- 
ditions, some of which are of as general applicability as those 
quoted. Two formulas which are frequently quoted are: Fan- 
ning's, Q = 200M s/i and Talbot's Q= 500M*, in which M is the area 
of the watershed in square miles. A comprehensive treatment 
of the subject is given in American Sewerage Practice, Vol. I, 
by Metcalf and Eddy. 

A comparison of the results obtained by the application of a 
few formulas to the same conditions is shown graphically in Fig. 
12. It is to be noted that the divergence between the smallest 



50 QUANTITY OF SEWAGE 

and largest results is over 100 per cent. As these formulas are 
not all applicable to the same conditions, the differences shown are 
due partially to an extension of some of them beyond the limits 
for which they were prepared. 

33. Extent and Intensity of Storms. In the design of storm 
sewers it is necessary to decide how heavy a storm must be pro- 
vided for. The very heaviest storms occur infrequently. To 
build a sewer capable of caring for all storms would involve a 
prohibitive expense over the investment necessary to care for the 
ordinary heavy storms encountered annually or once in a decade. 
This extra investment would lie idle for a long period entailing a 
considerable interest charge for which no return is easily seen. 
The alternative is to construct only for such heavy storms as are 
of ordinary occurrence and to allow the sewers to overflow on 
exceptional occasions. The result will be a more frequent use of 
the sewerage system to its capacity, a saving in the cost of the 
system, and an occasional flooding of the district in excessive 
storms. The amount of damage caused by inundations must be 
balanced against the extra cost of a sewerage system to avoid the 
damage. A municipality which does not provide adequate 
storm drainage is liable, under certain circumstances, for damages 
occasioned by this neglect. It is not liable if no drainage exists, 
nor is it liable if the storm is of such unusual character as to be 
classed legally as an act of God. 

Kuichling's studies of the probabilities of the occurrence of 
heavy storms are published in Transactions of the American 
Society of Civil Engineers, Vol. 54, 1905, p. 192. Information 
on the extent of rain storms is given by Francis in Vol. 7, 1878, 
p. 224, of the same publication. Kuichling expresses the intensity 
of storms which will occur, 

. 105 
once in 10 years as i = 



120 
once in 15 years as t = 



in which i is the intensity of rainfall in inches per hour and t is 
the duration of the storm in minutes. 






CHAPTER IV 
THE HYDRAULICS OF SEWERS 

34. Principles. The hydraulics of sewers deals with the 
application of the laws of hydraulics to the flow of water through 
conduits and open channels. In so far as its hydraulic proper- 
ties are concerned the characteristics of sewage are so similar to 
those of water that the same physical laws are applicable to both. 
In general it is assumed that the energy lost due to friction between 
the liquid and the sides of the channel varies as some function of 
the velocity, usually the square, and that the total energy passing 
any section of the stream differs from the energy passing any 
other section only by the loss of energy due to friction. 

The general expression for the flow of sewage would then be, 

h=(f)V n , 

in which h is the head or energy lost between any two sections, 
and V is the average velocity of flow between these sections. 
It is to be noted in this general expression that the quantity and 
rate of flow past all sections is assumed to be constant. This 
condition is known as steady flow. Problems are encountered 
in sewerage design which involve conditions of unsteady flow, 
and methods of solution of them have been developed based on 
modifications of this general expression. The average velocity 
of flow is computed by dividing the rate (quantity) of flow past 
any section by the cross-sectional area of the stream at that 
section. This does not represent the true velocity at any par- 
ticular point in the stream, as the velocity near the center is faster 
than that near the sides of the channel. The distribution of 
velocities in a closed circular channel is somewhat in the form of 
a paraboloid superimposed on a cylinder. 

The laws of flow are expressed as formulas the constants of 
which have been determined by experiment. It has been found 
that these constants depend on the character of the material 

51 



52 THE HYDRAULICS OF SEWERS 

forming the channel and the hydraulic radius. The hydraulic 
iadius is defined as the ratio of the cross-sectional area of the 
stream to the length of the wetted perimeter, or line of contact 
between the liquid and the channel, exclusive of the horizontal 
line between the air and the liquid. 

35. Formulas. The loss of head due to friction caused by 
flow through circular pipes flowing full as expressed by Darcy is, 

% f lV2 
h=f d2g> 

in which h is the head lost due to friction in the distance I, V is 
the velocity of flow, g is the acceleration due to gravity, and / is a 
factor dependent on d and the material of which the pipe is made. 
A formula for / expressed by Darcy as the result of experiments 
on cast iron pipe is, 



in which d is the diameter in feet. In using the formula with 
this factor the units used must be feet and seconds. 

Another form of the same expression is known as the Chezy 
formula. It is an algebraic transformation of the Darcy formula, 
but in the form shown here, by the use of the hydraulic radius, 
it is made applicable to any shape of conduit either full or partly 
full. The Chezy formula is, 



in which R is the hydraulic radius, S the slope ratio of the hydraulic 
gradient, and C a factor similar to / in the Darcy formula. 

Kutter' s formula was derived by the Swiss engineers, Gan- 
guillet and Kutter, as the result of a series of experimental observa- 
tions. It was introduced into the United States by Rudolph 
Bering and its derivation is given in Hering and Trautwine's 
translation of " The Flow of Water in Open Channels by Gan- 
guillet and Kutter." In English units it is, 



V = 



, 7 , .0028 
67H s 



VRS, 



FORMULAS 53 

in which n is a factor expressing the character of the surface of 
the conduit and the other notation is as in the Chezy formula. 
V is the velocity in feet per second, S is the slope ratio, and R the 
hydraulic radius in feet. The values of n to be used in all cases 
are not agreed upon, but in general the values shown below are 
used in practice, 

VALUES OF n IN KUTTER'S FORMULA 

n CHARACTER OF THE MATERIALS 

0.009 Well-planed timber. 

0.010 Neat cement or very smooth pipe. 

0.012 Unplaned timber. Best concrete. 

0.013 Smooth masonry or brickwork, or concrete 

sewers under ordinary conditions. 

0.015 Vitrified pipe or ordinary brickwork. 

0.017 Rubble masonry or rough brickwork. 

0.020 1 

QOK / Smooth earth 



0.030 
0.050 



> Rough channels overgrown with grass. 



Kutter's formula is of general application to all classes of material 
and to all shapes of conduits. It is the most generally used for- 
mula in sewerage design. 

The cumbersomeness of Kutter's formula is caused somewhat 
by the attempt to allow for the effect of the low slopes of the 
Mississippi River experiments on the coefficients. The correct- 
ness of these experiments has not been well established and the 

0028 

slopes are so flat that the omission of the term - - will have 

o 

no appreciable effect on the value of V ordinarily used in sewer 
design. The difference between the value of V determined by 
the omission of this term and the value of V found by including 
it is less than 1 per cent for all slopes greater than 1 in 1,000 
for 8 inch pipe (72 = 0.167 feet). As the diameter of the pipe or 
the hydraulic radius of the channel increases up to a diameter of 
13.02 feet (72 = 3.28 feet), the difference becomes less and at this 
value of R there is no difference whether the slope is included or 
not. For larger pipes the difference increases slowly. For a 
16 foot pipe (R = 4 feet) on a slope of 1 in 1,000 the difference is 
less than 0.2 per cent, and on a slope of 1 in 10,000 the difference 
is approximately 1 per cent. Flatter slopes than these are 



54 THE HYDRAULICS OF SEWERS 

seldom used in sewer design, except for very large sewers where 
careful determinations of the hydraulic slope are necessary. It 
is therefore safe in sewer design to use Kutter's formula in the 

0028 
modified form shown below in which the term '- - has been 

o 

omitted. 



Bazin's formula is 



in which a and /3 are constants for different classes of material. 
For cast-iron pipe a is 0.00007726 and ft is 0.00000647. This 
formula is seldom used in sewerage design. 

Exponential formulas have been developed as the result of 
experiments which have demonstrated that V does not vary as 
the one half power of R and S but that the relation should be 
expressed as, 

V=CR P S 9 , 

in which p and q are constants and C is a factor dependent on 
the character of the material. The various formulas coming 
under this classification have been given the names of the experi- 
menters proposing them. Examples of these formulas are: 
Flamant's, in English units, for new cast iron pipe, which is, 

7=232 J R' 715 >S- 572 , 

and Lampe's for the same material which is, 
F=203.3# 694 S 555 . 

These formulas are useful only for the material to which they 
apply, but they can be used for conduits of any shape. A. V. 
Saph and E. W. Schoder have shown l that the general formula 
for all materials lies between the limits, 

V= (93 to 50 to ' 5563 to " 



1 Transactions of the American Society of Civil Engineers, Vol. 51, 1903, 
p. 11. 



SOLUTION OF FORMULAS 55 

Hazen and Williams' formula is in the form, 



in which C is a factor dependent on the character of the material 
of the conduit. The values of C as given by Hazen and Williams 
are, 

C CHARACTER OP MATERIAL 

95 Steel pipe under future conditions. (Riveted 

steel.) 
100 Cast iron under ordinary future conditions and 

brick sewers in good condition. 
110 New riveted steel, and cement pipe. 
120 Smooth wood or masonry conduits under ordinary 

conditions. 
130 Masonry conduits after some time and for very 

smooth pipes such as glass, brass, lead, etc., 

when old, and for new cast-iron pipe under 

ordinary conditions. 

This formula is of as general application as Kutter's formula and 
is easier of solution, but being more recently in the field and 
because of the ease of the solution of Kutter's formula by dia- 
grams it is not in such general use. Exponential formulas are 
used more in waterworks than in sewerage practice. 
Manning's formula is in the form, 



n 

in which n is the same as for Kutter's formula. Charts for the 
solution of Manning's formula are given in Eng. News-Record, 
Vol. 85, 1920, p. 837. 

36. Solution of Formulas. The solution of even the simplest 
of these formulas, such as Flamant's, is laborious because of the 
exponents involved. Darcy's and Kutter's formulas are even 
more cumbersome because of the character of the coefficient. 
The labor involved in the solution of these formulas has resulted 
in the development of a number of diagrams and other short cuts. 
Since each formula involves three or more variables it cannot be 
represented by a single straight line on rectangular coordinate 
paper. The simplest form of diagram for the solution of throe 
or more variables is the nomograph, an example of which is shown 



56 



THE HYDRAULICS OF SEWERS 



72 
-48 J 

O 

-36JE 

B 

24 o 



0.00003 



0.0001 



-0.001 



-0.01 



O.I 



1.5- 



2 - 



3 - 



5 - 



7- 



in Fig. 13 for the 'solution of Flamant's formula. A straight-edge 

placed on any two points of 
the scales of two different ver- 
tical lines will cross the other 
line at a point on the scale cor- 
responding to its correct value 
in the formula. Such a diagram 
is in common use for the 
solution of problems for the 
flow of water in cast-iron 
pipe. 

Fig. 14 has been prepared 
to simplify the solution of 
Hazen and Williams' formu- 
la. The scales of slope for 
'OH different classes of material 

FIG. 13.-Diagram for the Solution of are shown On Vertical Unes 
Flamant's Formula for the Flow of to tn e left of the slope line. 
Water in Cast-iron Pipe. For use these scales must be 

projected horizontally on the 

slope line. The scales for other factors are shown on independent 

reference lines. 

For example let it be required to find the loss of head in 
a 12 inch pipe carrying 1 cubic foot per second when the 
coefficient of roughness is 100. A straight-edge placed 
at 1.0 cubic feet per second on the quantity scale, and 12 
inches on the diameter scale crosses the slope line at 0.00092 
opposite the slope scale for c= 100. It crosses the velocity 
line at 1.31 feet per second. 

Kutter's formula is the most commonly used for sewer design 
and has been generally accepted as a standard in spite of its 
cumbersomeness. Fig. 15 is a graphical solution of Kutter's 
formula for small pipes, and Fig. 16 for larger pipes. The dia- 
grams are drawn on the nomographic principle and give solutions 
for a wide range of materials, but they are specially prepared for 
the solution of problems in which w=.015. In their preparation 
the effect of the slope on the coefficient has been neglected. Fig. 
17 is drawn on ordinary rectangular coordinate paper and can be 
used only for the solution of problems in which n = .015. Both 
diagrams are given for practice in the use of the different types. 



SOLUTION OF FORMULAS 



67 



Values of C 



-2 



-0.9 



0.7 S. 

0.6?. 



rO.4 



-0.3' 



-0.2 



Mil 



48' 
42' 

36- 
33' 
30' 
27' 

24' 

22' 
|? 

20" 

18- 
15' 

ir 

io- 

9- 
8'h 



I 

4 
o 




c ~ 



(f) 



08- 

0.9 

1.0- 



1.5- 



2- 



Q- 3- 



.E 3.5 - 



4.5- 

5- 

5.5 

6- 

65 
7 

7.5 



Fia. 14. Diagram for the Solution of Hazen and Williams' Formula. 



58 



TIIF: rrvDHAi'Lifs m si:\vi.i;s 



e 



9 
8 
7 

1 

5 

4 
3 

M 


2 - 

i- 

01 

4- 
0) 


Values of n 
5c> 5! c^^ cj 




N 


(ft 


7C" 


- 


. ? S 








*x* 




S 

S 

K . 
- 
s 

N. 




JO 

33' 

30' 
28" 
26" 
24 
IT 


N \J. 

P 

X x ^^ 







, ^^ 


* v ^ 


^ s 


X ^^ 




*) A." 








Ift" 




"* v 


. s 




" < h 




s 




^ 


u 
C 

I5"n 

I2" u 

fl'" 

J 
-R* 




\ 






N 


' 




^V k,. 














r 






\ 




,i 






- 






V /^ 




k, 










s 




> 


' 




<o 

Q 

1.0 , 

0.9 o 
<j 

0.8$ 
0.7 g_ 

0.6 2 

05* 

3 
u 

0.4 = 
| 

< 
Q2 

0, 


X 


N 


1 1 










s 




s 




N. 


s ^ 




NV 


s 




N^ 






\ 










. 

r6 ' 
-4- 














\ 








N 






\ 


















1 






X 






^ 





























\ 












\ ^ 






S ^ 














s 






\ 






^ 







_o . 

tn : 



-)000> 



000? 

0003 

0004 
0005 
0006 
0007 
0008 

001 



002 

003 
004 
005 
006 
.007 
.008 
.01 



-.2 



FIG. 15. Diagram for the Solution of Kutter's Formula. 

For values of n between 0.010 and 0.020. Specially arraiiKod for n = 0.015. 
Q from G.I to 10 second-feet. 



1.5- 



2.0- 



35- 
4.0^ 

5-0 : 
5.5: 

6.0: 
6.5 ^ 
7.Q\ 

Values of 



SOLUTION OF FORMULAS 



59 



Values of n 



KLVlS^ 






qnn ^.^^^i 


J k > 






v " ^ 


19* 


or\f\ ^ >* 5 


' s s 


!' 


oUU x ^ ^ 


> X ^ s, 


17* 


N^ S, / 


^ ^ S 




700 S 5,* 


* X 


Ih' 




? S s S 


1C* 


"*Si s^ y 


i . S 




iOO r5^- 




14* 




s x 




"V x 




1 3* 


cnn ^ _ ^ / 


s ^ 








\r 


"S. i. j 


^ 




"N 




ir 


y*y* S,, 


V . 




[00 s ^^ 


^ ^ v 


1 A' 


*s. N 


> ^ 




22 


X ^ 




*- ^ s 




9' 


s^ . v 


, s 




300 x ^j 










W 




X ^ s 






^ s 


90V 


^s^ %^ 


"-V N 


OA" 


*\ w J 


' r x 


o't 


^v * v - 


P s 


78' -i 


s^ ^ v a 


v s 




200 f!s *5 


p ** ^ 


17' ~ 


V "V 


S, N 




. C ^ * 


|> ^ ^ 




V 


x, ^i 


OD , 


o> S ^ 








^ s 


hir 


<U ^ 


^*xT ^ 




EX 


T X 


_ .. 




S 


G 




x ^ 




O X. s 


' ^ 




v 


X 


-48' H 




^ ^ 




100 ^ 


a ^ 




^ 




-4^" 


90^ *5i: 


X 




^ s 


V 


i 


jO ^ ^ " 






o 


>. 1 


36 


7fl 03 ^ 


^ 




<n "*\~ 


^ 




V "^ - 










Tfim 




! 


oU 


tt, t V j 


. 




cn+- *. 


X 


-27' 


50 2 K 


x^^ 






x. 






S^ " Vs - 


^ * 


o ^ 


^ . 


L*f 


40 5 


s 






J \ 


Ol> 


3 s - 


s 


tl 


N^ 








C 




7ft >. 


9 v 


. Ifl* 




s 


io 


C 








Z 










3 
a 

20 
10 












UUUI 
0002 


. 






4 


0003 


i 
4-- 






-^ 


0004 


I 






^ 


0005 


. 






d 


0006 
0007 
0008 


5- 


o-* 
o 




5 


001 


; 


+~ 




I 




~ 


"(0 


tn 


: 




! 


c- 


o 







6~ 


UJ 





J 


002 


: 


c 


4~ 


~ 







L 










o 


CO 

D 


J 


003 


7- 





o~ 


B 






o 


UJ 


- 


004 





c 




c 


o - 


005 


8- 


o 




Jr 




^ 

<0 - 

tt - 


006 
007 
008 




LO 

L 
Q> 
Q- 


V 


o_- 


01 




0> 





: 




10- 


0> 

u_ 










c 






.02 


II- 





1 


J 


.03 


12- 


5 

o 

Z 






.04 


13- 


> 


1 




.05 


14- 








.06 











.07 


(5 








.08 

1 


16- 




> 




.1 


17- 










18- 








.2 


19- 








.3 










.4 








.5 








.6 








I 

1.0 





FIG. 16. Diagram for the Solution of Kutter's Formula. 

For values of n between 0.010 and 0.020. Specially arranged for n- 0.015. Values of 
Q from 10 to 1,000 second-feet. 



60 



THE HYDRAULICS OF SEWERS 



puooag_Jad 



core? 
oeo'o 

080'0 
OZO'O 



000 O O 
000 O O 




ZOOO'O 



s 



USE OF DIAGRAMS 



61 




FIG. 18. Conversion Factors for 
Kutter's Formula. 



In Figs. 15 and 16 the diameter scales are varied for different 
values of the roughness coefficient n. The velocity scale is shown 
only for a value of n=.015. 
The velocity for other values 
of n can be determined by the 
method given in the following 
paragraphs. 

37. Use of Diagrams. 
There are five factors in 
Kutter's formula: n, Q, V, d 
(or R), and S. If any three of 
these are given the other two 
can be determined, except when 
the three given are Q, V, and d. 
These three are related in the 
form Q = AV, which is inde- 
pendent of slope or the char- 
acter of the material. There 
are only nine different com- 
binations possible with these 
five factors, which will be met 

in the solution of Kutter's formula. The solution of the 
problems by means of the diagrams is simple when the data 
given include n=.015. For other given values of n the solu- 
tion is more complicated. Results of the solution of types of 
each of the nine problems are given in Table 17 and the 
explanatory text below. 

// n is given and is equal to .015, the solution is simple. 

For example in Table 17 case 1, example 1; to be solved 
on Fig. 15. Place a straight-edge at 1.0 on the Q line and 
at 6 inches on the diameter line for n= .015. The slope 
and the velocity will be found at the intersection of the 
straight-edge with these respective scales. 

All problems in which n is given as .015 and the solution for which 
falls within the limits of Fig. 15 or 16 should be solved by placing 
a straight-edge on the two known scales and reading the two 
unknown results at the intersection of the straight-edge and the 
remaining scales. 

For example in case 1, example 2 find the intersection 
of the horizontal line representing Q= 100 with the sloping 



62 



THE HYDRAULICS OF SEWERS 



diameter line representing d = 48 inches. The vertical 
slope line passing through this point represents S= .0065 
and the sloping velocity line passing through this point 
represents 8.5 feet per second. 

In general problems in which n=.015, can be solved on Fig. 17 
by finding the intersection of the two lines representing the given 
data, and reading the values of the remaining variables represented 
by the other two lines passing through this point. 

TABLE 17 

SOLUTIONS OF PROBLEMS BY KUTTRR'S FORMULA 



Case 


Ex- 
ample 


Given 


Found 


n 


Q 


V 


d 


S 


n 


Q 


V 


d 


S 


1 
1 
1 
1 
2 
2 
3 
3 
4 
4 
5 
6 
7 
7 
8 
9 


1 
2 
3 

4 
1 
2 
1 
2 
1 
2 
1 
1 
1 
2 
1 
1 


0.015 
.015 
.020 
.020 
.015 
.010 
.015 
.018 
.015 
.011 
.015 
.018 


1.0 
100.0 
1.0 
100.0 
5.0 
5.0 


2.5 


6 








5 




0.0575 
.0065 
.13 
.0125 








8.5 






6 








5.0 






48 








8 5 








0.0003 
.0003 
.002 
.0008 






1.2 
1.7 
2 25 


28 
23.5 














18 
18 




4 










2.0 


1 1 






2.0 
2.0 


2.5 
2.5 
5.0 
5.0 

2.5 
4.2 








12 
12 


.00475 
.0022 
.0038 












36 

18 
36 

66 






35 




3.0 
50.0 
6.0 


.001 
.002 
.005 
.003 
.00059 


0.019 
.012 
.018 
.011 


185.0 


1 7 


80 






7.0 










21 




100.0 

















// n is given and is not equal to .015 the solution is not so simple. 
In Fig. 15 and 16 the diagram is so drawn that the position of 
the diameter scales for all values of n is fixed on the vertical 
" diameter " line. The scales of diameter change for each value 
of n. These scales of diameter are shown for each value of n 
from .010 to .020 on vertical lines to the left of the " diameter " 
line. For use, the proper diameter scale for any given value of n 
must be projected horizontally upon the vertical " diameter " 
line. The velocity can be determined on Fig. 15 and 16, only 






USE OF DIAGRAMS 63 

when the diameter has been determined and then only when the 
diameter scale for n equal .015 is used, since the only scale shown 
for velocity is for n= .015. 

For example, in Case 1, Example 3 there are given 
n .020, Q, and d. Find the intersection of the vertical 
line for n = . 020 with the sloping diameter line for d = 6 
inches. Project the intersection horizontally to the right 
to the vertical "diameter" line. Place a straight-edge at 
this point and at Q = 1 . on the quantity scale. The 
required value of S is read at the intersection of the straight- 
edge and the slope scale and is equal to 0.13. The inter- 
section of the straight-edge in this position with the velocity 
scale is not the required value of the velocity since the 
velocity scale is made out for n= .015 and not .020. It is 
necessary to change the position of the straight-edge so that 
it may lie on Q equal 1 . and on d equal 6 inches for n 
equal . 015. The value of V is shown in this position as 5 
feet per second. 

The reverse process for Fig. 15 and 16 is illustrated 
by Case 4, Example 2 in which n= .011 and Q and V are also 
given. When Q and V are given the value of d is fixed 
independent of all other factors. Therefore the value of d 
can be read from the scale with n= .015 and is found to be 
12 inches. Now find the value of d= 12 inches on the scale 
for n= .011 and project on to the "diameter" line. Place 
the straight-edge at this point and at Q = 2. The required 
slope is read as . 0022. 

Fig. 17 is prepared for the solution of problems in which 
n=.015 only. For problems in which n has some other value it 
is necessary to transform the data to equivalent conditions in 
which n=.015. This is done by means of the conversion factors 
shown in Fig. 18. The given slope or velocity is multiplied by 
the proper factor to convert from or to the value of n= .015. 

For example in Case 1, Example 4 there are given 
n = . 020, Q, and d. With Q and d given the value of V can 
be read from Fig. 17 without conversion. The correspond- 
ing value of S for n= .015 is .0065. It is now necessary to 
use the transformation diagram Fig. 18. The hydraulic 
radius of the given pipe is one foot. On Fig. 18 at the inter- 
section of the slope line for R = 1 . foot and n = . 020 the 
value of the factor is read as 1 . 92. Since the given n is 
for rougher material than that represented bv n= .015 the 
required slope must be greater than for n= .015 to give the 



64 THE HYDRAULICS OF SEWERS 

same velocity. It is therefore necessary to multiply 
. 0065 X 1 . 92 and the required slope is . 0125. 

In Case 6, Example 1 there are given n .018, d, and S, 
The remaining factors are to be solved by Fig. 17. Solve 
first as though n= .015 in order to find an approximate 
value of d or R. In this case it is evident that d is greater 
than 57 inches. The value of R is therefore about 1.25. 
Referring to Fig. 18 the conversion factor for the slope for 
n= .018 is about 1.52. Since the given slope for n= .018 
is .001, for an equal velocity and for n=.015 the slope 
should be less. Therefore in reading Fig. 17 it is necessary 

001 
to use a slope of j-^= .00066. The diameter is found to 

1 . i 

be about 80 inches. Since this is nearer to the correct 
diameter the value of the conversion factor must be cor- 
rected for this approximation. The hydraulic radius for an 
80 inch pipe is 1 . 67 feet, and the conversion factor from 
Fig. 18 is about 1.48. The slope for n=.015 should be 

therefore ^TQ .000675 and from Fig. 17 the required 

diameter and quantity are read as 80 inches and 185 second 
feet, respectively. 

If n is not given but must be solved for, the solution on Fig. 
15 and 16 is relatively simple. The desired value of n is read at 
the intersection of the sloping diameter line representing the 
known diameter and the horizontal projection of the intersection 
of the straight-edge with the vertical " diameter " line. 

For example in Case 7, Example 1 there are given Q, 
d, and S. Lay the straight-edge on the given values of 
Q = 3 and S = . 002. At the point where the straight-edge 
crosses the vertical "diameter" line project a horizontal 
line to the sloping diameter line for d=18 inches. The 
vertical line passing through this point represents a value of 
n= .019. In order to find the value of V lay the straight- 
edge on Q = 3 and d= 18 inches for n= .015. The value of 
V is read as 1.7. 

A slightly different condition is illustrated in the solu- 
tion of Case 8, Example 1 in which Q, V and S are given. 
Determine first the value of d as though n= .015. Then 
proceed to determine n as in the preceding examples. 

The solution for an unknown value of n on Fig. 17 is not so 
simple. It must be determined by working backwards from the 
conversion factor. 



FLOW IN CIRCULAR PIPES PARTLY FULL 65 

For example in Case 7, Example 2 there are given Q, 
d, and S. The value of V is read directly as though n = .015 
as 7 feet per second. The value of S read for n= .015 is 
is .0075. But the given slope is .005. Since the given 
slope is flatter than that for n= .015 the conversion factor 

005 
is less than unity and is therefore -^^. = . 67. With this 



value of the conversion factor and the value of R given as 
0.75 the value of n is read from Fig. 18 as slightly greater 
than .012. 

38. Flow in Circular Pipes Partly Full. The preceding 
examples have involved the flow in circular pipes completely 
filled. The same methods of solution can be used for pipes 
flowing partly full except that the hydraulic radius of the wetted 
section is used instead of the diameter of the pipe. Diagrams 
are used to save labor in finding the hydraulic radius and the 
other hydraulic elements of conduits flowing partly full. 

The hydraulic elements of a conduit for any depth of flow are : 
(a) The hydraulic radius, (6) the area, (c) the velocity of flow, 
and (d) the quantity or rate of discharge. The velocity and 
quantity when partly full as expressed in terms of the velocity 
and quantity when full as calculated by Kutter's formula will 
vary slightly with different diameters, slopes and coefficients of 
roughness. The other elements are constant for all conditions 
for the same type of cross-section. The hydraulic elements for 
all depths of a circular section for two different diameters and 
slopes are shown in Fig. 19. The differences between the 
velocity and quantity under the different conditions are shown 
to be slight, and in practice allowance is seldom made for this 
discrepancy. 

In the solution of a problem involving part full flow in a cir- 
cular conduit the method followed is to solve the problem as 
though it were for full flow conditions and then to convert to 
partial flow conditions by means of Fig. 19, or to convert from 
partial flow conditions to full flow conditions and solve as in the 
preceding section. 

For example let it be required to determine the quantity 
of flow in a 12-inch diameter pipe with n= .015 when on a 
slope of . 005 and the depth of flow is 3 inches. First find 
the quantity for full flow. From Fig. 15 this is 2.0 cubic 
feet per second. The depth of flow of 3 inches is one-fourth 



66 



THE HYDRAULICS OF SEWERS 

or 0.25 of the full depth of 12 inches. From Fig. 19, run- 
ning horizontally on the 0.25 depth line to meet the quantity 
curve, the proportionate quantity at this depth is found to 
be on the 0.13 vertical line, and the quantity of flow is 
therefore 2 X0.13 = 0.26 cubic feet per second. 




O.I 0.2 0.3 0.4 0.5 0.6 0.1 0.8 0.9 1.0 1.1 U 
Hydraulic Elements in Terms of Hydraulic Elements for Full Section. 

FIG. 19. Hydraulic Elements of Circular Sections. 

<i = 12'0" s = .0004 n = .015 

d= I'O" s = .01 n = .013 

Another problem, involving the reversal of this process is 
illustrated by the following example : 

Let it be required to determine the diameter and full 
capacity of a vitrified pipe sewer on a grade of 0.002 if the 
velocity of flow is 3.0 feet per second when the sewer is 
discharging at 30 per cent of its full capacity, the depth of 
flow being 12 inches. From Fig. 19 the depth of flow when 
the sewer is carrying 30 per cent of its full capacity is . 38 
of its full depth. Since the partial depth is 12 inches 

12 
the full diameter is =31.6 inches. The velocity of 

U . oo 

flow at 38 per cent depth is 86 per cent of the full velocity. 
Since the velocity given is 3.0 feet per second, the full 

3 
velocity is -^ = 3.5 feet per second. With a full ve- 

.oO 

locity of 3.5 feet per second and a diameter of 31.6 inches 
from Fig. 16 the full capacity of the sewer is 18 cubic feet 
. per second. 



SECTIONS OTHER THAN CIRCULAR 67 

39. Sections Other than Circular. The ordinary shape used 
for small sewers is circular. The difficulty of constructing large 
sewers in a circular shape, special conditions of construction such 
as small head room, soft foundations, etc., or widely fluctuating 
conditions of flow have led to the development of other shapes. 
For conduits flowing full at all times a circular section will carry 
more water with the same loss of head than any other section 
under the same conditions. In any section the smaller the flow 
the slower the velocity, an undesirable condition. The ideal 
section for fluctuating flows would be one that would give the 
same velocity of flow for all quantities. Such a section is yet to 
be developed. Sections have been developed that will give rela- 
tively higher velocities for small quantities of flow than are given 
by a circular section. The best known of these sections is the 
egg shape, the proportions and hydraulic elements of which are 
shown in Fig. 20. Other shapes that have the same property, 
but which were not developed for the same purpose are the rect- 
angular, the U-shape, and the section with a cunette. The egg- 
shaped section has been more widely used than any other special 
section. It is, however, more difficult and expensive to build 
under certain conditions, and has a smaller capacity when full 
than a circular sewer of the same area of cross-section. Various 
sections are illustrated in Fig. 22 and 23. 

The U-shaped section is suitable where the cover is small, or 
close under obstructions where a flat top is desirable and the 
fluctuations of flow are so great as to make advantageous a special 
shape to increase the velocity of low flows. The proportions of a 
U-shaped section are shown in Fig. 23 (6). Other sections used 
for the same purpose are the semicircular and special forms of 
the rectangular section. 

The proportions and the hydraulic elements of the square- 
shaped section are shown in Fig. 21. This is useful under low 
heads where a flat roof is required to carry heavy loads, and the 
fluctuations of flow are not large. 

Sections with cunettes have not been standardized. A cunette 
is a small channel in the bottom of a sewer to concentrate the low 
flows, as shown in Fig. 22 (7). A cunette can be used in any 
shape of sewer. 

Sections developed mainly because of the greater ease of con- 
struction under certain conditions are the basket handle, the gothic, 



68 



THE HYDRAULICS OF SEWERS 




O.I 



O.t 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 I.I 1.2 
Hycfraulic Elements in Terms of Hydraulic Elements for Full Section 

FIG. 20. Hydraulic Elements of an Egg-shaped Section. 
d = 6'y s = .00065 n = .015 




0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 I.I I.? 
Hydraulic Elements in Terms of Hydraulic Elements fbrFull Section. 

FIG. 21. Hydraulic Elements of a Square Section. 



s = .0004 



n = .015 



SECTIONS OTHER THAN CIRCULAR 



69 



the catenary, and the horse shoe. Some of these shapes are shown 
in Fig. 22 and 23. They are suitable for large sewers on soft 
foundations, where it is desirable to build the sewer in three 
portions, such as, invert, side walls, and arch. They are also 
suitable for construction in tunnels where the shape of the sewer 
conforms to the shape of the timbering, or in open cut work where 
the shape of the forms are easier to support. 

Problems of flow in all sections can be solved by determining 
the hydraulic radius involved, and substituting directly in the 
desired formula, or by the use of one of the diagrams after con- 
verting to the equivalent circular diameter. The determina- 
tion of the hydraulic radius of these special sections is laborious, 
and hence other less difficult methods are followed. Problems 
are more commonly solved by converting the given data into an 
equivalent circular sewer, solving for the elements of this cir- 
cular sewer and then reconverting into the original terms, or by 
working in the other direction. The hydraulic elements of vari- 
ous sections when full are given in Table 18. 

TABLE 18 
HYDRAULIC ELEMENTS OF SEWER SECTIONS. 









Vert. 






Area 




Dia. D 






in 


Hy- 


in Terms 








draulic 








Terms 




of 








I vu 1 nis in 






Section 


Vertical 


terms of 


Dia. d 


Source 




Diameter 


Verticul 


of Equiv- 






Squared 


DIA D 


alent 






D* 


j /iii . / ' . 


Circular 










Section 




Circular 


0.7854 


0.250 


1.000 




Ear.. 


0.5150 


. 1931 


1.295 


Eng. Record, Vol. 72: 608 


Ovoid 


0.5650 


.2070 


1.208 


Eng. Record, Vol. 72: 608 


Semi-elliptical 


0.8176 


.2487 


1.041 


Eng. News, Vol. 71 : 552 


Catenary 


0.6625 


.2237 


1.1175 


Eng. Record, Vol. 72: 608 


Horseshoe 


0.8472 


.2536 


0.985 


Eng. Record, Vol. 72: 608 


Basket handle. . 


0.8313 


.2553 


0.979 


Eng. Record, Vol. 72 608 


Rectangular 


1.3125 


2865 


.7968 


Hydraulic Dgins. and This. 










Garrett 


Square (3 sides wet) . 


1.0000 


.333 


.7500 


Eng. Record, Vol. 72: 608 


Square (4 sides wet) . 


1.0000 


.250 


1.0000 


Eng Record, Vol. 72: 608 



70 



THE HYDRAULICS OF SEWERS 





1. Standard Egg-shaped Section, North 2. Rectangular Section, Omaha, Nebraska, 
Shore Intercepter, Chicago, Illinois. Eng. Contracting, Vol. 46, p. 49. 




3. Trench in firm 4. Trench in 

ground. Rock. 

NOTE Underdrains and Wedges to be 
used only when Ordered by the 
Engineer. 



7. Brick and Concrete Sewer showing 
cunette. 



-IS',4 Rings fD 
of Brick 




Concrete 



\ '-brave! or 
Broken Stone/ 

5. Soft Founda- 
tion. 




JU 



6. Wet 
ground. 



t-3*K---i ar\ i 
13-10- J 

8. Brick and Concrete Sewer, Evanston, 
111., Eng. Contracting, Vol. 46, p. 227. 

FlG. 22. 



SECTIONS OTHER THAN CIRCULAR 



71 



PayLinefor 

Excavation 
and Concrete. 



.. Pay Line forxcoration , - 

i^jt i ( . Pay line -for 
js-*".^-^ Concnte 




1. Tunnel Sections. 2. Open Cut Sections. 

Type A. Type B. Type C. Type D. 

Where Rock is Where Rock is Where Rock is Where Rock 16' 6" Sewer. Where Rock 
more than 16' more than 7' between drops below 25' Fill is above 

above Spring- an less than Springing Line Springing Line Springing 

ing Line. 16' above and 7' above on either Side. Line 

Springing Line Springing Line 
on both Sides, on both Sides. 
Mill Creek Sewer, St. Louis, Eng. Record, Vol. 70, pp. 434, 435. 



Limits of 
Excavation 




Soft Hard 

Ground. Ground. 

3. Circular Concrete Section in Soft and Hard 
Ground, Eng. Record, Vol. 59, p. 570. 



4. Semi-Elliptical Section, Louisville, Ky., 
Eng. News, Vol. 62, p. 416. 




-36-3 



v*T> M ir' 

29 -o ->rk: 



In Rock. 



In Soft Ground. 
5. Reinforced Concrete Sewer, Harlem Creek, 
St. Louis, Eng. News, Vol. 60, p. 131. 

FIG. 23. 




-*- 

-5 Vit. Bricks 



6. U-Shaped Section, San Francisco, 
Eng. News, Vol. 73, p. 310. 



72 THE HYDRAULICS OF SEWERS 

Equivalent sections are sections of the same capacity for the 
same slope and coefficient of roughness. They have not neces- 
sarily the same dimensions, shape, nor area. The diameter of 
the equivalent circular section in terms of the diameter of each 
special section shown is given in Table 18. The inside height of 
a sewer is spoken of as its diameter. 

For example let it be required to determine the rate of 
flow in a 54-inch egg-shaped sewer on a slope of 0.001 when 
n=.015. First convert to the equivalent circle. From 

Table 18 the diameter of the equivalent circle is , nnp , times 

1.295 

the diameter of the egg-shaped sewer, which becomes in 
this case 43 inches. From Fig. 16 the capacity of a circular 
sewer of this diameter with S = 0.001 and n= .015 is 28 
cubic feet per second, which by definition is the flow in the 
egg-shaped sewer. 

As an example of the reverse process let it be required 
to find the velocity of flow in an egg-shaped sewer flowing 
full and equivalent to a 48-inch circular sewer. Both sewers 
are on a slope of . 005 and have a roughness coefficient of 
n= .015. It is first necessary to find the quantity of flow 
in the circular sewer, which by definition is the quantity of 
flow in the equivalent egg-shaped sewer. The velocity of 
flow in the egg-shaped sewer is found by dividing this 
quantity by the area of the egg-shaped section. As read 
from the diagram the quanity of flow is 90 cubic feet per 
second. From Table 18 the area of the egg-shaped sewer is 
. 5 ID 2 where D is the diameter of the egg-shaped sewer, and 
D = 1 . 295d where d is the diameter of the equivalent cir- 
cular sewer. Therefore the area equals (0 . 5 1 ) X ( 1 . 295 X 4) 2 

90 

= 13.5 square feet and the velocity of flow is 10 _ =6.7 

lo.o 

feet per second. This is slightly less than the velocity 
in the circular section. 

Some lines for egg-shaped sewers have been shown on Fig. 17 
by which solutions can be made directly. For other shapes, and 
for sizes of egg-shaped sewers not found on Fig. 17 the preceding 
method or the original formula must be used for solution. Prob- 
lems in partial flow in special sections are solved similarly to 
partial flow in circular sections, by converting first to the condi- 
tions of full flow or by working in the opposite direction. 

40. Non-uniform Flow. In the preceding articles it is assumed 
that .the mean velocity and the rate of flow past all sections are 



NON-UNIFORM FLOW 73 

constant. This condition is known as steady, uniform flow. In 
this article it will be assumed that conditions of steady non- 
uniform flow exist, that is, the rate of flow past all sections is 
constant, but the velocity of flow past these sections is different 
for different sections. Under such conditions the surface of the 
stream is not parallel to the invert of the channel. If the velocity 
of flow is increasing down stream the surface curve is known as 
the drop-down curve. If the velocity of flow is decreasing down 
stream the surface curve is known as the backwater curve. The 
hydraulic jump represents a condition of non-uniform flow in 
which the velocity of flow decreases down stream in such a manner 
that the surface of the stream stands normal to the invert of the 
channel at the point where the change in velocity occurs. Above 
and below this point conditions of uniform flow may exist. 

Conditions of non-uniform flow exist at the outlet of all sewers, 
except under the unusual conditions where the depth of flow hi 
the sewer under conditions of steady, uniform flow with the given 
rate of discharge would raise the surface of water in the sewer, at 
the point of discharge, to the same elevation as the surface of the 
body of water into which discharge is taking place. By an appli- 
cation of the principles of non-uniform flow to the design of out- 
fall sewers, smaller sewers, steeper grades, greater depth of cover, 
and other advantages can be obtained. 

The backwater curve is caused by an obstruction in the sewer, 
by a flattening of the slope of the invert, or by allowing the sewer 
to discharge into a body of water whose surface elevation would 
be above the surface of the water in the sewer, at the point of 
discharge, under conditions of steady, uniform flow with the given 
rate of discharge. 

The drop-down curve is caused by a sudden steepening of the 
slope of the invert; by allowing a free discharge; or by allowing a 
discharge into a body of water whose surface elevation would be 
below the surface of the water in the sewer, at the point of dis- 
charge, under conditions of steady, uniform flow with the given 
rate of discharge. The last described condition is common at 
the outlet of many sewers, hence the common occurrence of the 
drop-down curve. 

The hydraulic jump is a phenomenon which is seldom consid- 
ered in sewer design. If not guarded against it may cause trouble 
at overflow weirs and at other control devices, in grit chambers, 



74 THE HYDRAULICS OF SEWERS 

and at unexpected places. The causes of the hydraulic jump 
are sufficiently well understood to permit designs that will avoid 
its occurrence, but if it is allowed to occur the exact place of the 
occurrence of the jump and its height are difficult, if not impos- 
sible, to determine under the present state of knowledge con- 
cerning them. The hydraulic jump will occur when a high veloc- 
ity of flow is interrupted by an obstruction in the channel, by a 
change in grade of the invert, or the approach of the velocity to 
the " critical " velocity. The " critical " velocity is equal to 
V0S, where h is the depth of flow and g is the acceleration due to 
gravity. The velocity in the channel above the jump must be 
greater than Vghi, where hi is the depth of flow in the channel 
above the jump. The velocity in the channel below the jump 
must be greater than Vgrfo, where /i2 is the depth of flow below 
the jump. The jump will not take place unless the slope of the 

invert of the channel is greater than -^, in which C is the coeffi- 



cient in the Chezy formula. With this information it is possible 
to avoid the jump by slowing down the velocity by the installation 
of drop manholes, flight sewers, or by other expedients. 

The shape of the drop-down curve can be expressed, in some 
cases, by mathematical formulas of more or less simplicity, 
dependent on the shape of the conduit. The formula for a circu- 
lar conduit is complicated. Due to the assumptions which must be 
made in the deduction of these formulas, the results obtained by 
their use are of no greater value than those obtained by approxi- 
mate methods. A method for the determination of the drop- 
down curve is given by C. D. Hill. 1 In this method it is necessary 
that the rate of flow past all sections shall be the same.; that the 
depth of submergence at the outlet shall be known; and that the 
depth of flow at some unknown distance up the stream shall be 
assumed. The shape and material of construction of the sewer 
and the slope of the invert should also be known. The problem is 
then to determine the distance between cross-sections, one where 
the depth of flow is known, and the other where the depth of flow 
has been assumed. This distance can be expressed as follows: 

T _(d 2 -d 1 )-(H l -H 2 )_d'-H' 
S-Si S' ' 

1 Municipal and County Engineering, Vol. 58, 1920, p. 164. 



NON-UNIFORM FLOW 



75 



in which L = the distance between cross-sections; 

d\ = the depth of flow at the lower section; 

c?2 = the depth of flow at the upper section; 
HI = the velocity head at the lower section; 
#2 = the velocity head at the upper section; 

*S = the hydraulic slope of the stream surface; 

<Si = the slope of the invert of the sewer. 

In order to solve such problems with a satisfactory degree of 
accuracy the difference between d\ and cfe should be taken suffi- 
ciently small to divide the entire length of the sewer to be investi- 
gated into a large number of sections. The solution of the prob- 
lem requires the determination of the wetted area, the hydraulic 
radius, and other hydraulic elements at many sections. The 
labor involved can be simplified by the use of diagrams, such as 
Fig. 19, or by specially prepared diagrams such as those accom- 
panying the original article by C. D. Hill. The solution of the 
problem can be simplified by tabulating the computations as 
follows: 

DROP-DOWN CURVE COMPUTATION SHEET 

Uniform discharge. Varying depth 



Q- A. V=<>= *- L-*^-' 
A 61 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


11 


12 


1.3 


Depth 


R 


H 


Hi 


di-f/i 


V 


S 


Si 


L 


Elevation 


D 


d 


</i 


Sewer 


W. L. 

















































































At the head of the computation sheet should be recorded the 
diameter of the sewer in feet, the assumed volume of flow, the 
area of the full cross-section of the sewer, the velocity of the 
assumed volume flowing through the full bore of the sewer, and 
the gradient or slope of the invert. In the 1st column enter the 



76 THE HYDRAULICS OF SEWERS 

assumed depth in decimal parts of the diameter for each cross- 
section; in the 2nd column enter the same depth in feet; in the 
3rd column enter the difference in feet between the successive 
cross-sections; in the 4th column enter the hydraulic radius 
corresponding to the depth at each cross-section ; in the 8th column 
enter the velocity, equal to the volume divided by the wetted 
area, for each cross-section; in the 5th column enter the cor- 
responding velocity head; in the 6th column enter the difference 
between the velocity heads at successive cross-sections; in the 
7th column enter the difference between the quantities in the 
third and in the sixth columns; in the 9th column enter the hydrau- 
lic slope corresponding to the velocity and hydraulic radius of 
each cross-section; in the 10th column enter the difference between 
the hydraulic slope and the slope or gradient of the sewer; in the 
llth column enter the computed distance between successive 
cross-sections; in the 12th column enter the elevation of the 
bottom of the sewer at each cross-section; and in the 13th column 
enter the corresponding elevation of the surface of the water. 

The table should be filled in until the distance to the required 
section is determined, or if the distance is known, it should be 
filled in until the depth of flow with the assumed rate of discharge 
has been checked. 

If only the depth of flow at some section is known and it is 
required to know the maximum rate of flow with a free discharge, 
or a discharge with a submergence at the outlet less than the 
depth of flow with the maximum rate of discharge, it is necessary 
to make a preliminary estimate of the maximum rate of flow in 
order to fill in the quantity Q at the head of the table. The 
procedure should be as follows; 

1st. Assume a depth of flow at the outlet. 

2nd. Compute the area (A) and the hydraulic radius (R) 
at the known section and at the outlet. 

3rd. Determine the area and the hydraulic radius half way 
between these two sections as the mean of the areas 
and the hydraulic radii of the two sections. 

4th. Determine the rate of flow through the sewer from the 
condition that the difference in head at the two 
sections is the head lost due to friction caused by 
the average velocity of flow between the sections 

(IV 2 \ ( 

equals ^5 ) plus the gain in velocity head ( equals 



NON-UNIFORM FLOW 77 

Vi 2 



\ 

j , which when combined and transposed 



result in the expression : 



/ 2Rgh 

Q - AAlA2 \2A l 2 A 2 2 gl+(A 1 -A 2 )(A 2 C 2 R) 

in which Q = rate of flow; 

A =the area determined in the 3rd step; 

AI = the area at the upper cross-section; 

_A 2 = the area at the lower cross-section; 

C =the coefficient in the Chezy formula; 

g = the acceleration due to gravity; 

h =the difference in elevation of the surface of the 

stream at the two cross-sections; 
I =the distance between the cross-sections; 
R = the hydraulic radius determined in the third step. 
5th. Continue this process by assuming different depths at 
the outlet until the maximum rate of discharge has 
been found by trial. 

With this rate of discharge and depth of flow at the outlet, the 
depth of flow at the known section can be checked. If appreci- 
ably in error a correction should be made by the assumption of 
a diiferent depth of flow at the outlet. The approximate character 
of the method is scarcely worthy of the refinement in the results 
which will be obtained by checking back for the depth of flow at 
the known section. It will be sufficiently accurate to assume 
the rate of flow obtained by trial from the preceding expression, 
as the maximum rate of discharge from the sewer. 



CHAPTER V 
DESIGN OF SEWERAGE SYSTEMS 

41. The Plan. Good practice demands that a compre- 
hensive plan for a sewerage system be provided for the needs of a 
community for the entire extent of its probable future growth, 
and that sewers be constructed as needed in accordance with 
this plan. 

Sewerage systems may be laid out on any one of three systems: 
separate, storm, or combined. A separate system of sewers is 
one in which only sanitary sewage or industrial wastes or both 
are allowed to flow. Storm sewers carry only surface drainage, 
exclusive of sanitary sewage. Combined sewers carry both 
sanitary and storm sewage. The use of a combined or a separate 
system of sewerage is a question of expediency. Portions of the 
same system may be either separate, combined, or storm sewers. 

Some conditions favorable to the adoption of the separate 
system are where: 

a. The sanitary sewage must be concentrated at one 
outlet, such as at a treatment plant, and other outlets 
are available for the storm drainage. 

6. The topography is flat necessitating deep excavation, 
and steeper grades for the larger combined sewers. 

c. The sanitary sewers must be placed materially deeper 
than the necessary depth for the storm water drains. 

d. The sewers are to be laid in rock, necessitating more 
difficult excavation for the larger combined sewers. 

e. An existing sewerage system can be used to convey 
the dry weather flow, but is not large enough for the storm 
sewage. 

/. The city finances are such that the greater cost of the 
combined system cannot be met and sanitary drainage is 
imperative. 

g. The district to be sewered is an old residential section 
where property values are not increasing and the assess- 
ment must be kept down. 

78 



PRELIMINARY MAP 



79 



Some additional points given in a report by Alvord and Burdick 
to the city of Billings, Montana, are: 

The separate system of sewerage should be used, where: 

1st. Storm water does not require extensive under- 
ground removal, or where it can be concentrated in a few 
shallow underground channels: 

2nd. Drainage areas are short and steep facilitating 
rapid flow of water over street surfaces to the natural water 
courses. 

3rd. The sanitary sewage must be pumped. 

4th. Sewers are being built in advance of the city's 
development to encourage its growth. 

5th. The existing sewer is laid at grades unsuitable for 
sanitary sewage, it can be used as a storm sewer. 

A combined system must be relatively larger than a 
separate storm sewer as the latter may overflow on ex- 
ceptional occasions, but the former never. 

A combined system of sewerage should be used where: 
1st. It is evident that storm and sanitary sewerage 
must be provided soon. 

2nd. Both sanitary and storm sewage must be pumped. 
3rd. The district is densely built up. 

42. Preliminary Map. The first step in the design of a sewer- 
age system is the preparation of a map of the district to be served 
within the limits of its probable growth. The map should be on 
a scale of at least 200 feet to the inch in the built up sections or 
other areas where it is anticipated that sewers may be built, and 
where much detail is to be shown a scale as large as 40 feet to the 
inch may have to be used. The adoption of so large a scale will 
usually necessitate the division of the city or sewer district into 
sections. A key map should be drawn to such a scale that the 
various sections represented by separate drawings can all be 
shown upon it. In preparing the enlarged portions of the map 
it is not necessary to include these portions of the city in which 
it is improbable that sewers will be constructed, such as parks and 
cemeteries. 

The contour interval should depend on the character of the 
district and the slope of the land. In those sections drawn to a 
scale of 200 feet to the inch for slopes over 5 per cent, the contour 
interval need not be closer than 10 feet. For slopes between 1 and 
5 per cent the contour interval should be 5 feet. For flatter 



80 DESIGN OF SEWERAGE SYSTEMS 

slopes the interval should not exceed 2 feet, and a one foot interval 
is sometimes desirable. In general the horizontal distances 
between contours should not exceed 400 feet and they should be 
close enough to show important features of the natural drainage. 
Elevations should also be given at street intersections, and at 
abrupt changes in grade. For portions of the map on a smaller 
scale the contours need be sufficiently close to show only the 
drainage lines and the general slope of the land. 

The following may be shown on the preliminary map: the 
elevation of lots and cellars; the character of the built up districts, 
whether cheap frame residences, flat-roof buildings, manufacturing 
plants, etc.; property lines; width of streets between property 
lines and between curb lines; the width and character of the 
sidewalks and pavements; street car and railroad tracks; exist- 
ing underground structures such as sewers, water pipes, telephone 
conduits, etc.; the location of important structures which may 
have a bearing on the design of the sewers such as bridges, rail- 
road tunnels, deep cuts, culverts, etc.; and the location of possible 
sewer outlets and the sites for sewage disposal plants. 

Fig. 24 shows a preliminary map for a section of a city, on 
which the necessary information has been entered. The map is 
made from survey notes. All streets are paved with brick. The 
alleys are unpaved. The entire section is built up with high-class 
detached residences averaging one to each lot. The lots vary from 
1 to 3 feet above the elevation of the street. 

43. Layout of the Separate System. Upon completion of the 
preliminary map a tentative plan of the system is laid out. The 
lines of the sewer pipe are drawn in pencil, usually along the center 
line of the street or alley in such a manner that a sewer will be 
provided within 50 feet or less of every lot. The location of the 
sewers should be such as to give the most desirable combination 
of low cost, short house connections, proper depth for cellar drain- 
age, and avoidance of paved streets. Some dispute arises among 
engineers as to the advisability of placing pipes in alleys, although 
there is less opposition to so placing sewers than any other utility 
conduit. The principal advantage in placing sewers in alleys is 
to avoid disturbing the pavement of the street, but if both street 
and alley are paved it is usually more economical to place the 
sewer in the street as the house connections will be shorter. On 
boulevards and other wide streets such as Meridian Avenue in 



LOCATION AND NUMBERING OF MANHOLES 81 

Fig. 24, the sewers are placed in the parking on each side of the 
street, rather than to disturb the pavement and lay long house 
connections to the center of the street. 

All pipes should be made to slope, where possible, in the direc- 
tion of the natural slope of the ground. The preliminary layout 
of the system is shown in Fig. 24. The lowest point in the portion 
of the system shown is in the alley between Alabama and Tennessee 
Streets. The flow in all pipes is towards this point, and only 
one pipe drains away from any junction, except that more than 
one pipe may drain from a terminal manhole on a summit. 

44. Location and Numbering of Manholes. Manholes are 
next located on the pipes of this tentative layout. Good practice 
calls for the location of a manhole at every change in direction, 
grade, elevation, or size of pipe, except in sewers 60 inches in diam- 
eter or larger. The manholes should not be more than 300 to 500 
feet apart, and preferably as close as 200 to 300 feet. In sewers 
too small for a man to enter the distance is fixed by the length of 
sewer rods which can be worked successfully. In the larger 
sewers the distances are sometimes made greater but inadvisedly 
so, since quick means of escape should be provided for workmen 
from a sudden rise of water in the sewer, or the effect of an asphyx- 
iating gas. In the preliminary layout the manholes are located 
at pipe intersections, changes in direction, and not over 300 to 
500 feet apart on long straight runs at convenient points such as 
opposite street intersections where other sewers may enter. 

No standard system of manhole numbering has been adopted. 
A system which avoids confusion and is subject to unlimited 
extension is to number the manholes consecutively upwards from 
the outlet, beginning a new series of numbers prefixed by some 
index number or letter for each branch or lateral. This system 
has been followed with the manholes on Fig. 24. 

45. Drainage Areas. The quantity of dry weather sewage is 
determined by the population rather than the topography. Lot 
lines and street intersections or other artificial lines marking the 
boundaries between districts are therefore taken as watershed 
lines for sanitary sewers. The quantity of sewage to be carried 
and the available slope are the determining factors in fixing the 
diameter of the sewer. Since there may be no change in diameter 
or slope between manholes the quantity of sewage delivered by a 
sewer into any manhole will determine the diameter of the sewer 



82 



DESIGN OF SEWERAGE SYSTEMS 




LAYOUT OF THE SEPARATE SYSTEM 







I 

i 



3 

oo 



o 





84 DESIGN OF SEWERAGE SYSTEMS 

between it and the next manhole above. In order to determine 
the additional amount contributed between manholes a line is 
drawn around the drainage area tributary to each manhole. 
This line generally follows property lines and the center lines of 
streets or alleys, its position being such that it includes all the 
area draining into one manhole, and excludes all areas draining 
elsewhere. An entire lot is usually assumed to lie within the 
drainage area into which the building on the lot drains. In 
laying out these areas it is best to commence at the upper end of a 
lateral and work down to a junction. Then start again at the 
upper end of another lateral entering this junction, and continue 
thus until the map has been covered. 

The areas are given the same numbers as the manholes into 
which they drain. The dividing lines for the drainage areas on 
Fig. 24 are shown as dot and dash lines, and the areas enclosed are 
appropriately numbered. If more than one sewer drains into 
the same manhole the area should be subdivided so that each 
subdivision encloses only the area contributing through one 
sewer. Such a condition is shown at manhole C2. The areas 
are designated by subletters or symbols corresponding to the 
symbol used for the sewer into which they drain. For example, 
the two areas contributing to manhole C2 are lettered C2 K and 
C2z>. The sewer from manhole C3 to C2 receives no addition, it 
being assumed that all the lots adjacent to it drain into the sewer 
on the alley. There is therefore no area C2. Likewise there is 
no area Al c . 

46. Quantity of Sewage. The remaining work in the compu- 
tation of the quantity of sewage is best kept in order by a tabula- 
tion. Table 19 shows the computations for the sewers discharging 
from the east into manhole No. 142. The computation should 
begin at the upper end of a lateral, continue to a junction, and then 
start again at the upper end of another lateral entering this junc- 
tion. Each line in the table should be filled in completely from 
left to right before proceeding with the computations on the next 
line. In the illustrative solution in Table 19, computations for 
quantity have not been made between manholes where it was 
apparent that there would be an insufficient additional quantity 
to necessitate a change in the size of the pipe. 

In making these computations the assumptions of quantity 
and other factors given below indicate the sort of assumptions 



QUANTITY OF SEWAGE 85 

which must be made, based on such studies as are given in Chapter 
III. The density of population was taken as 20 persons per acre, 
the assumption being based on the census and the character of 
the district. The average sanitary sewage flow was taken as 
100 gallons per capita per day. The per cent which the maximum 

500 
dry weather flow is of the average was taken as M = p^-, in which 

P is the population in thousands. The per cent is not to exceed 
500 nor to be less than 150. The rate of infiltration of ground 
water was assumed as 50,000 gallons per mile of pipe per day. 

In the first line of Table 19, the entries in columns (1) to (6) 
are self-explanatory. There are no entries in columns (7) to (10), 
as no additional sewage is contributed between manholes 3.5 and 
3.4. In column (11), 2250 persons are recorded as the number 
tributary to manhole No. 3.5 in the district to the north and west. 
These people contribute an average of 100 gallons per person per 
day, or a total of 0.346 second foot. This quantity is entered in 
column (13). The figure in column (14) is obtained from the 

500 
expression M = p^-. Column (15) is .01 of the product of columns 

(13) and (14). Column (16) is the product of the length of pipe 
between manholes 3.5 and 3.4, and the ground water unit reduced 
to cubic feet per second. Column (17) is the sum of column (16), 
and all of the ground water tributary to manhole 3.5, which i? 
not recorded in the table. Column (18) is the pum of columns 
(15) and (17). 

No new principle is represented in the second and third lines. 

In the fourth line the first 10 columns need no further explana- 
tion. The (llth) column is the sum of the (10th) column, and the 
(llth) column in the third line. It represents the total number 
of persons tributary to manhole 3.4 on lateral No. 8. Column 
(13) in the fourth line is the sum of column (13) in the third line 
and the (12th) column in the fourth line, and the (15th) column 
in the fourth line is the product of the 2 preceding columns in the 
fourth line. Note that in no case is the figure in column (15) 
the sum of any previous figures in column (15). With this intro- 
duction the student should be able to check the remaining figures 
in the table, and should compute the quantity of sewage entering 
manhole No. 142 from the west, making reasonable assumptions 
for the tributary quantities from beyond the limits of the map. 






86 



DESIGN OF SEWERAGE SYSTEMS 



TABLE 

COMPUTATIONS FOR QUANTITY OF SEWAGE 



On Street 


From Street 


To Street 


From 
Man- 
hole 


To 
Man- 
hole 


Length 
Feet 


Mark 
of 
Added 
Areas 


Nebraska St 


Map margin 


Alley S. Grant St. . . 


3.5 


3.4 


338 




Alley S of Grant St . 


Railroad 


E. of Missouri St 


8 3 


8 2 


328 


8 2 


Alley S.of Grant St. 
Alley S.of Grant St. 


E. of Missouri St. . . 
E. of Kansas St . . . . 


E. of Kansas St. ... 
Nebraska St 


8.2 
8 1 


8.1 
3 4 


355 
340 


8.1 
3 4s 


Nebraska St 


Alley S. of Grant St. 


Alley S. of Meridian. 


3.4 


3.3 


380 








Nebraska St 


7 2 


3 3 


800 


7.1 
3 3? 


Nebraska St 


Alley S. Meridian. . . 


Alley S. of Smith Av. 


3.3 


3.2 


304 




Alley S.of Smith Ave. 
Nebraska St 


Railroad 
Alley S. of Smith Ave. 


Nebraska St 
S. of Cordovez St. . . 


6.2 
3. '2 


3.2 
3.1 


609 
300 


6.1 
3.2, 


S of Cordovez St 






4 1 


3 1 


410 


3 14 


S of Cordovez St 






5 1 


3 1 


380 


3 U 


Nebraska St 


S of Cordovez St. 




3 1 


148 


172 




Long St 


Map margin 


Nebraska St 


149 


148 


380 


148 


Long St 


Nebraska St 


N Carolina St 


148 


147 


492 




Long St 


N Carolina St . . 




147 


146 


430 




Long St 
Long St 


Georgia St 
Harris St 


Harris St 
Tennessee St 


146 
145 


145 
143 


419 
725 


146 
2.1 

143-145 


Column No. (1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 



* Industrial waste. 

TABLE 

COMPUTATIONS FOR SLOPE AND DIAMETER OF 



On Street 


From Street 


To Street 


From 
Man- 
hole 


To 

Man- 
hole 


Length, 
Feet 


Nebraska St 




Alley S. of Grant St. . 


3 5 


3 4 


338 


Alley S. of Grant St . 


Railroad 


East of Missouri St . . 


8 3 


8.2 


328 


Alley S. of Grant St . 
Alley S of Grant St 


East of Missouri St . 


East of Kansas St. . . . 
Nebraska St . . 


8.2 
8 1 


8.1 
3 4 


355 
340 


Nebraska St. . . . . . . 


Alley S. of Grant St. 


Alley S. of Meridian. . 


3.4 

7 2 


3.3 

7 1 


380 
400 


Alley S of Meridian 




Nebraska St ... 


7.1 


3.3 


400 


Nebraska St . . . 


Alley S. of Meridian. 


Alley S. of Smith Ave. 


3.3 


3.2 


304 


AlleyS of Smith Ave 






6 2 


6.1 


305 


AlleyS of Smith Ave 




Nebraska St 


6.1 


3.2 


304 


Nebraska St 


Alley S. of Smith Ave. 


Alley S. of Cordovez. . 
Nebraska St 


3.2 
5.1 


3.1 

3.1 


300 
380 


Alley S of Cordovez 




Nebraska St 


4.1 


3.1 


410 


Nebraska St 




Long St' 


3 1 


148 


172 


Long St 




Nebraska St 


149 


148 


380 


Long St. . . 
Long St 


Nebraska St 
North Carolina St . . 


North Carolina St . . . 
Georgia St 


148 

147 


147 
146 


492 
430 


Long St 


Georgia St 


Harris St 


146 


145 


419 


Alley S of Janis St 






2.2 


2.1 


350 


Harris St. 


Alley N of Janis St. 


Long St 


2.1 


145 


135 


Long St 
Long St 
Tarbell Ave 


Harris St 
Kentucky St 


Kentucky St 
Tennessee St 
Long St 


145 
144 
1.1 


144 
143 
143 


258 

282 
417 


Long St 




Alley W. of Tenn. St.. 


143 


142 


185 


Column No. (1) 


(2) 


(3) 


(4) 


(5) 


(6) 



QUANTITY OF SEWAGE 



87 



19 

FOR A SEPARATE SEWERAGE SYSTEM 













Cumu- 


Per 




Incre- 








Area. 
Acres 


Popu- 
lation 
per 
Acre 


Num- 
ber 
of 
Per- 
sons 


Total 
Per- 
sona 
Tribu- 
tary 


v * 
Sani- 
tary 
Flow, 
C.F.S. 


lative 
Avg. 
Sani- 
tary 
Flow, 


cent 
Max. 
Sani- 
tary 
is of 


Total 
Max. 
Sani- 
tary, 
C.F.S. 


ment 
of 
Ground 
Water, 
Cv <a 


Cumu- 
lative 
Ground 
Water, 
C.F.S. 


Total 
Flow, 
C.F.S. 


Lino, 
Num- 
ber 












C.F.S. 


Average 




.r .0. 














2250 


O.(MMM) 


0.346 


425 


1.47 


0.005 


0.0187 


1.66 


1 


2.7 


'26 


'54 


54 


.0084 


.0084 


500 


0.041 


.0048 


.0048 


0.046 


2 


3.41 


20 


68 


122 


.0106 


.0190 


500 


0.095 


.0052 


.010 


0.105 


3 


2.68 


20 


54 


176 


.0084 


.0274 


500 


0.137 


.0050 


.015 


0.152 


4 








2426 


.0000 


.373 


423 


1.58 


.0056 


.208 


1.79 


5 


7.14 


20 


142 


142 


.0221 


.0221 


500 


0.111 


.0117 


.0117 


0.123 


'6 








2568 


.0000 


.395 


414 


1.63 


.0045 


.224 


1.85 


7 


3.82 


20 


76 


76 


.0119 


.0119 


500 


0.060 


.0089 


.0089 


0.069 


8 








2644 


.0000 


.407 


414 


1.68 


.0044 


.237 


1.92 


9 


s.'io 


20 


"62 


62 


.0096 


.0096 


500 


0.048 


.006 


.006 


0.054 


10 


2.69 


20 


54 


54 


.0084 


.(K)S : 1 


500 


0.042 


.0056 


.0056 


0.048 


11 








2760 


.0000 


.425 


409 


1.74 


.0025 


.251 


1.99 


12 


iiss 


'20 


"si 


31 


.0048 


.0048 


500 


0.024 


.0056 


.0056 


0.030 


13 








2791 


.0000 


.430 


409 


1.76 


.0072 


.264 


2.02 


14 








2791 


1 000* 


.430 


400 


1.76 


.0064 


1.27 


3.03 


15 


6!si 


20 


"ie 


2807 


.0025 


.433 


407 


1.76 


.0061 


1.28 


3.04 


16 


6.6 


20 


132 


2936 


.0205 


.454 


403 


1 83 


.024 


1.30 


3.13 


17 


(8) 


(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


(17) 


(18) 





Treated as ground water. 



20 

PIPES FOR A SEPARATE SEWERAGE SYSTEM 



El. of Surface 


Total 
Flow, 
C.F.S. 


Slope 


Dia. 
of 
Pipe, 
Inches 


Velocity 
when 
Full, 
Ft. per 
Second 


Capacity 
when 
Full. 
Second 
Feet 


El. of Invert 


Line 
Number 


Upper 
Man- 
hole 


Lower 
Man- 
hole 


Upper 
Manhole 


Lower 
Manhole 


105.8 
113.5 
112.0 
107.7 
102.4 
111.8 
107.0 
100.7 
109.3 
105.3 
99.3 
100.8 
104.6 
101.1 
103.8 
98.7 
103.8 
99.1 
105.2 
98.1 
96.9 
94.4 
98.7 
92.6 

(7) 


102.4 
112.0 
107.7 
102.4 
100.7 
107.0 
100.7 
99.3 
105.3 
99.3 
101.1 
101.1 
101.1 
98.7 
98.7 
103.8 
99.1 
96.9 
98.1 
96.9 
94.4 
92.6 
'.-' ti 

IB. a 

(8) 


1.66 
0.046 
0.105 
0.152 
1.79 


0.0108 
.00575 
.0110 
.0156 
.oo:<s.-, 
.0120 
.0157 
.0042 
.0131 
.0197 
.00213 
.00574 
.00854 
.00213 
.0134 
.00213 
.0016 
.0016 
.0203 
.0088 
.00353 
00085 
.0146 
.0016 

(10) 


10 
8 
8 
8 
12 
8 
8 
12 
8 
8 
15 
8 
8 
15 
8 
15 
18 
18 
8 
8 
18 
18 
8 
18 

(11) 


3.25 
2.00 
2.78 
3.27 
2.28 
2.90 
3.28 
2.36 
3.00 
3.70 
2.00 
2.00 
2.46 
2.00 
3.04 
2.00 
2.00 
2.00 
3.78 
2.53 
2.98 
4.00 
3.18 
2.00 

(12) 


1.78 
0.71 
0.98 
1.18 
1.79 
1.03 
1.18 
1.85 
1.08 
1.32 
2.45 
0.71 
0.87 
2.45 
1.08 
2.45 
3.50 
3.50 
1.35 
0.89 
5.20 
7.00 
1.14 
3.50 

(13) 


97.80 
105.50 
103.61 
99.69 
94.07 
103 . 80 
98.99 
92.37 
101.30 
97.29 
90.84 

'.)!>., SO 

96.60 
90.04 
95.80 
BB.Sfl 

NX. H'.l 

87.99 
97.20 
90.09 
87.31 
86.39 
90.70 
83.77 

(14) 


94.40 
103.62 
99.70 
94.40 
92.61 
99.00 
92.70 
91.09 
97.30 
91.30 
90.20 
90.62 
93.10 
89.87 
90.70 
88.94 
88.00 
87.32 
90. 10 

SS.(H) 

86.40 
84.60 
84.60 
83.47 

(15) 


1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 


0.123 
1.85 


0.069 
1.92 


0.054 
1.99 
0.030 
2.02 
3.03 
3.04 










3.13 
(9) 



88 DESIGN OF SEWERAGE SYSTEMS 

47. Surface Profile. A profile of the surface of the .ground 
along the proposed lines of the sewers should be drawn after the 
completion of the computations for quantity. An example of a 
profile is shown in Fig. 26 for the line between manholes No. 3.5 
and No. 147. The vertical scale should be at least 10 times the 
horizontal. A horizontal scale of 1 inch to 200 feet can be used 
where not much detail is to be shown, but a scale of one 1 to 
100 feet is more common and more satisfactory and even one inch 
to 10 feet has been used. The information to be given and the 
method of showing it are illustrated on Fig. 26. The profile 
should show the character of the material to be passed through 
and the location of underground obstacles which may be encoun- 
tered. The method of obtaining this information is taken up in 
Chapter II. The collection of the information should be com- 
pleted as far as possible previous to design, and borings and other 
investigations made as soon as the tentative routes for the sewers 
have been selected. 

48. Slope and Diameter of Sewers. After the quantity of 
sewage to be carried has been determined, and the profile of the 
ground surface has been drawn, it is possible to determine the 
slope and diameter of the sewer. A table such as No. 20 is made 
up somewhat similar to No. 19, or which may be an extension of 
Table 19 since the first 6 columns in both tables are the same. 
The elevation of the surface at the upper and lower manholes is 
read from the profile. 

The depth of the sewer below the ground surface is first 
determined. Sewers should be sufficiently deep to dram cellars 
of ordinary depth. In residential districts cellars are seldom more 
than 5 feet below the ground surface. To this depth must be 
added the drop necessary for the grade of the house seweri Six- 
inch pipe laid on a minimum grade of' 1.67 per cent is a common 
size and slope restriction for house drains or sewers. An addi- 
tional 12 inches should be allowed for the bends in the pipe and 
the depth of the pipe under the cellar floor. Where the eleva- 
tion of the street and lots is about the same, and the street is not 
over 80 feet in width between property lines, a minimum depth 
of 8 feet to the invert of sewers, 24 inches or less in diameter is 
satisfactory. This is on the assumption that the axes of the 
house drain and the sewer intersect. For larger pipes the depth 
should be increased so that when the street sewer is flowing full, 



SURFACE PROFILE 



89 




90 DESIGN OF SEWERAGE SYSTEMS 

sewage will not back up into the cellars or for any great distance 
into the tributary pipes. 

The grade or slope at which a sewer shall lie may be fixed by : 
the slope of the ground surface; the minimum permissible self- 
cleansing velocity; a combination of diameter, velocity, and 
quantity; or the maximum permissible velocity of flow. Sewers 
are laid either parallel to the ground surface where the slope is 
sufficient or where possible without coming too near the surface 
they are laid on a flatter grade to avoid unnecessary excavation. 
The minimum permissible slope is fixed by the minimum permis- 
sible velocity. 

The velocity of flow in a sewer should be sufficient to prevent 
the sedimentation of sludge and light mineral matter. Such a 
velocity is in the neighborhood of 1 foot per second. Since 
sewers seldom flow full this velocity should be available under 
ordinary conditions of dry weather flow. The minimum velocity 
when full should therefore be about 2 feet per second. Under 
this condition, the velocity of 1 foot per second is not reached 
until the sewer is less than 18 per cent full. The velocity in small 
sewers should be made somewhat faster than in large sewers 
since the velocity of flow for small depths in small pipes is less 
than for the same proportionate depth in large pipes. The 
maximum permissible velocity of flow is fixed at about 10 feet 
per second in order to avoid excessive erosion of the invert. If the 
sewer is carefully laid this limit may be exceeded in sanitary sewers. 

The method for determining the grade and diameter of sewers 
is best explained through an illustrative problem which is worked 
out in Table 20 for the profile shown on Fig. 26. The figures 
are inserted in the table from left to right in each line, one line 
being completed before the next one is commenced. The head- 
ings in the first 6 columns are self-explanatory. The elevations 
of the surface at the upper and lower manholes are read from the 
profile. The total flow is read from column (18) in Table 19. 
The slope of the ground surface is then computed, and with the 
quantity, slope, and coefficient of roughness, the diameter of the 
pipe and the velocity of flow are read from Fig. 15. 

The following conditions may arise: 

(1) The diameter required is less than 8 inches. Use a 
diameter of 8 inches as experience has shown that the use of 
smaller diameters is unsatisfactory. 



SLOPE AND DIAMETER OF SEWERS 91 

(2) The velocity of flow when the sewer is full is less than 
2 feet per second. Increase the slope until the velocity 
when full is 2 feet per second. 

(3) The diameter of the pipe required is not one of the 
commercial sizes shown in Fig. 15. Use the next largest 
commercial size. 

(4) The slope of the ground surface is steeper than 
necessary to maintain the required minimum velocity 
and the upper end of the sewer is deeper than the required 
minimum depth. Place the sewer on the minimum per- 
missible grade, or upon such a grade that its lower end 
will be at the minimum permissible depth. 

(5) The slope of the ground surface is so steep as to 
make the velocity of flow greater than the maximum rate 
permissible. Reduce the grade by deepening the sewer at 
the upper manhole and using a drop manhole at this point. 

It is not permissible to use a pipe larger than that called for 
by the above conditions. This is attempted sometimes in order 
to reduce the grade and thereby save excavation, under the rule 
of a minimum velocity of 2 feet per second when full. It is 
better to use the smaller pipe on the flat grade as the quantity of 
sewage is insufficient to fill the larger sewer and the minimum 
permissible velocity is more quickly reached. 

Having determined the slope, the diameter, and the capacity 
of the pipe to be used, these values are entered in the table. 
The elevations of the invert of the pipe at the upper and lower 
manholes are next computed and entered in the table. This 
method is followed until all of the diameters, slopes, and eleva- 
tions have been determined. 

The slopes are computed from center to center of manholes, 
but an extra allowance of 0.01 of a foot is allowed by some design- 
ers for the increased loss in head in passing through the manhole. 
When it becomes necessary to increase the diameter of the sewer 
the top of the outgoing sewer is placed at the same elevation or 
below the top of the lowest incoming sewer. No extra allow- 
ance is made to compensate for loss in head in the manhole in this 
case. This case is illustrated in columns (14) and (15) in lines 
(16) and (17) of Table 20. All of the conditions listed above are 
illustrated in Table 20, except the condition for a velocity greater 
than 10 feet per second. 

The first condition is met at the head of practically every 
lateral, and is illustrated in the second line. 



92 DESIGN OF SEWERAGE SYSTEMS 

The second condition is also illustrated in the second line. 
The slope of the ground surface is 0.0046, which gives a velocity 
of only 1.8 feet per second in an 8-inch pipe. The slope is there- 
fore increased to 0.00575, on which the full velocity is 2 feet per 
second. 

The third condition is met in the first line. The diameter 
called for to carry 1.66 cubic feet per second on a slope of 0.0108 
is slightly less than 10 inches. A 10-inch pipe is therefore used 
and its full capacity and velocity are recorded. 

The fourth condition is illustrated in the fourteenth line. 
The cut at manhole No. 3.1 is 11.1 feet. The slope of the ground 
is 0.014, much steeper than is necessary to maintain the minimum 
velocity in a 15-inch pipe. The pipe is therefore placed on the 
minimum permissible slope, and excavation is saved. The student 
should check the figures in Table 20 and be sure that they are 
understood before an attempt is made to make a design inde- 
pendently. 

49. The Sewer Profile. The profile is next completed as 
shown in Fig. 26, the pipe line being drawn in as the computa- 
tions are made. The cut is recorded to the nearest -roth of a foot 
at each manhole, or change in grade. It should not be given else- 
where as it invites controversy with the contractor. The cut is 
the difference of the elevation of the invert of the lowest pipe in 
the trench at the point in question, and the surface of the ground. 

The stationing should be shown to the nearest roth of a foot. 
It should commence at 0+00 at the outlet and increase up the 
sewer. The station of any point on the sewer may show the dis- 
tance from it to the outlet, or a new system of stationing may be 
commenced at important junctions or at each junction. 

Elevations of the surface of the ground should be shown to 
the nearest iVth of a foot, and the invert elevation to the nearest 
Tiro-th of a foot. 

Only the main line sewer is shown in profile in Fig. 26. The 
profiles of the laterals computed in Table 20, have not been shown. 
The approximate location of all house inlets are shown on the 
profile and located exactly, and are made a matter of record 
during construction. 



PLANNING THE SYSTEM 93 

DESIGN OF A STORM WATER SEWER SYSTEM 

50. Planning the System. Storm sewer systems are seldom 
as extensive as separate or combined sewer systems, since storm 
sewage can be discharged into the nearest suitable point in a 
flowing stream or other drainage channel, whereas dry weather 
or combined sewage must be conducted to some point where its 
discharge will be inoffensive. The need of a comprehensive 
general plan of a storm sewer system is quite as great, however, 
as for a separate system. The haphazard construction of sewers 
at the points most needed for the moment results in the duplica- 
tion of forgotten drains, expense in increasing the capacity of 
inadequate sewers, and difficult construction due to underground 
structures thoughtlessly located. A comprehensive plan permits 
the construction of sewers where they are needed as they are 
required, and enables all probable future needs to be cared for at a 
minimum of expense. 

The same preliminary survey, map, and underground informa- 
tion are necessary for the design of a storm sewer system as for a 
separate sewer system. The map shown on Fig. 25 has been used 
for the design of a storm-water sewer system. 

The steps in the design of a storm- water sewer system are: 
1st. Note the most advantageous points to locate the inlets 
and lay out the system to drain these inlets. 2nd. Determine 
the required capacity of the sewers by a study of the run-off from 
the different drainage areas. 3rd. Draw the profile and compute 
the diameter and slope of the pipes required. 

51. Location of Street Inlets. The location of storm sewers 
is determined mainly by the desirable location of the street inlets. 
The inlets must therefore be located before the system can be 
planned. In general the inlets should be located so that no water 
will flow across a street or sidewalk, in order to reach the sewer. 
This requires that inlets be placed on the high corners at street 
intersections, in depressions between street intersections, and at 
sufficiently frequent intervals that the gutters may not be over- 
loaded. City blocks are seldom so long as to necessitate the loca- 
tion of inlets between crossings solely on account of inadequate 
gutter capacity. The capacity of a gutter can be computed 
approximately by the application of Kutter's formula. Inlet 
capacities are discussed in Chapter VI. When the area drained 



94 DESIGN OF SEWERAGE SYSTEMS 

is sufficiently large to tax the capacity of the gutter or inlet, an 
inlet should be installed regardless of the location of the street 
intersections. 

The street inlets are located on the map as shown in Fig. 25. 
The sewer lines are then located so as to make the length of pipe 
to pass near to all inlets a minimum. Storm sewers are seldom 
placed near the center of a street because of the frequent crowded 
condition on this line. 

52. Drainage Areas. The outline of a drainage area is drawn 
so that all water falling within the area outlined will enter the 
same inlet, and water falling on any point beyond the outline will 
enter some other inlet. This requires that the outline follow 
true drainage lines rather than the artificial land divisions used 
in locating the drainage lines in the design of sanitary sewers. 
The drainage lines are determined by pavement slopes, location 
of downspouts, paved or unpaved yards, grading of lawns and 
the many other features of the natural drainage which are altered 
by the building up of a city. The location of the drainage lines 
is fixed as the result of a study of local conditions. 

The watershed or drainage lines are shown on Fig. 25 by means 
of dot and dash lines. A drainage line passes down the middle of 
each street because the crown of the street throws the water to either 
side and directs it to different inlets. A watershed line is drawn 
about 50 feet west of such streets as Kentucky St., Florida St., etc., 
because the downspouts from the houses on those streets discharge 
or will discharge into the street on which they face. The location 
of any watershed line within 20 feet more or less is, in most 
cases, a matter of judgment rather than exactness. Each area 
is given an identifying number or mark which is useful only in 
design. It usually corresponds to the inlet number. 

53. Computation of Flood Flow by McMath Formula. 
McMath's Formula is used as an example of the method pursued 
when an empirical formula is adopted for the computation of 
run-off, and because of its frequent use in practice. Other formu- 
las may be mbre satisfactory under favorable conditions. 

Computations should be kept in order by a tabulation such as 
is shown in Table 21, in which the quantity of storm flow discharged 
from the sewer at the foot of Tennessee St., on Fig. 25, has been 
computed by means of the McMath Formula, using the constants 
suggested for St. Louis conditions, 2 = 2.75, and c=0.75. The 



FLOOD FLOW BY RATIONAL METHOD 95 

solutions of the formula have been made by means of Fig. 11. 
The column headings in the Table are explanatory of the figures 
as recorded. The computation should begin at the upper end 
of a lateral, proceed to the first junction and then return to the 
head of another lateral tributary to this junction. They should 
be continued in the same manner until all tributary areas have 
been covered. Special computations will be necessary for the 
determination of the maximum quantity of storm water entering 
each inlet to avoid the flooding of an inlet or gutter. These 
computations have not been shown as they are so easily made by 
the application of McMath's Formula to each area concerned. 

The determination of the average slope ratio is a matter of 
judgment, based on the average natural slope of the surface of 
the ground and an estimate of the probable future conditions. 

54. Computation of Flood Flow by Rational Method. The 
rational method for the computation of storm water run-off is 
described in Chapter III. An example of its application to storm 
sewer design is given here for the district shown in Fig. 25. 1 
The computations are shown in Table 21. As in the preceding 
designs the table has been filled in from left to right and line by 
line. Computations have started at the upper end of laterals 
tributary to each junction. The column headed / represents the 
imperviousness factor in the expression Q = A IR. It is based on 
judgment guided by the constants given in Chapter III concern- 
ing imperviousness. The column headed " Equivalent 100 per 
cent 7 acres " is the product of the two preceding columns. It 
reduces all areas to the same terms so that they can be added for 
entry in the column headed " Total 100 per cent 7 acres." It 
may be necessary to record the values for this column on several 
lines where the imperviousnesses of the tributary areas are differ- 
ent. This condition is illustrated in the last line of the table, 
for the length of sewer nearest the outlet. In the preceding lines 
the imperviousness recorded represents an average for all the 
tributary areas. 

The time of concentration in minutes is assumed by judgment 
for the first area. For all subsequent areas it is the sum of the 
time of concentration for the area or areas tributary to the inlet 
next above and the time of flow in the sewer from the inlet next 

1 For diagrams for the Solution of the Rational Method, see Eng. News- 
Record, Vol. 83, 1919, p. 868 and Vol. 85, 1920, p. 151. 



96 



DESIGN OF SEWERAGE SYSTEMS 



TABLE 
COMPUTATIONS FOE THE QUANTITY OF STORM SEWAGE 



On Street 


From Street 


To Street 


Identifying 
Number of Areas 


By McMath's Formula 


B 




2 


c 


oi 

fa 
U 








Drained 


1*8 


^"2 


do 


e 










5 'a 


< 
* 

f 


Slope of 


to 

O 

a 

tf 


State 


N. Carolina 


S. Carolina. . 


91 and 92 


2.35 


2.35 


0.005 


5.5 


State 


S. Carolina 


Georgia 


88, 89 and 90 


3.0 


5.35 


.005 


10.8 


State 


Georgia 


Florida 


85, 86 and 87 


3.0 


8.35 


.007 


16.5 


State 


Florida 


Kentucky. . . 


81, 83 and 84 


3.0 


11.35 


.009 


22.0 


State 
State 


Kentucky 
Texas 


Tennessee. . . 
Louisiana. . . . 


79, 80 and 82 
76 and others 
73, 74 and 75 
70, 71 and 72 
68, 69, 77 and 78 
65, 66 and 67 
64 and 64a 


3.0 
3.8 

3.7 
3.0 
4.3 
2.8 
0.7 


14.35 
3.8 
7.5 
10.5 
29.15 
2.8 
29.85 


.010 
.005 
.007 
.006 
.15 
.018 
.15 


28.0 
8.3 
15.0 
19.0 
52 
8.4 
55 


State 


Louisiana 
Alabama . . 


Alabama. . . . 
Tennessee. . . 
Talon 
Tennessee. . . 
Burnside .... 


State 


Tennessee. . 
Talon 


State 
Albemarle 
Talon 


Tennessee . . 


Burnside. . . 


N. Carolina 


S. Carolina. . 


57, 58 and 59 


2.84 


2.84 


.008 


7.2 


Burnside . . . 


S. Carolina 


Georgia 


54, 55 and 56 


3.88 


6.72 


.010 


14.9 


Burnside. . . 


Georgia 


Florida 


50, 52 and 53 


3.88 


10.60 


.012 


22 


Burnside. . . 


Florida 


Kentucky. . . 


47, 48 and 51 


3.88 


14.48 


.013 


30 


Burnside. . . 


Kentucky 


Tennessee. . . 


44, 45 and 46 


3.88 


18.36 


.013 


36 


Tennessee. . 


Burnside 


Elm 


42 and 43 


2.84 


51.05 


.015 


82 


Elm 


Above Chetwood . 


Chetwood . . . 


Included in next line below 






Elm 
Elm 


Chetwood 
Albemarle 
Elm 


Albemarle. . . 
Tennessee. . . 
Varennes. . . . 


31, 32 and 33 
27, 28, 29 and 30 
25, 26 and 41 


2.75 
5.75 
2.62 


2.75 

8.50 
62.17 


.007 
.016 
.017 


7.0 
20 
100 


Tennessee. . 


Varennes. . . 


S. Carolina 


Georgia 


17, 18 and 19 


3.17 


3.17 


.010 


8.3 


Varennes. . . 


Georgia 


Florida 


14, 15 and 16 


3.17 


6.34 


.011 


14.5 


Varennes. . . 


Florida 


Kentucky. . . 


11, 12 and 13 


3.17 


9.51 


.013 


21 


Varennes. . . 


Kentucky 


Tennessee. . . 


8, 9 and 10 


3.17 


12.68 


.013 


26 


Tennessee. . 


Varennes 


Boulevard . . . 


6 and 7 


2.32 


77.17 


.017 


120 


Tennessee . . 


Boulevard 


Outlet 


1, 2, 3, 4 and 5 


4.72 


81.89 


.017 


122 



above to the inlet in question. For example, in line 2 the time 
8.1 minutes is the sum of 7.0 minutes time of concentration to 
the inlet at the corner of State and North Carolina St., and the 
time of flow of 1.1 minute in the sewer on State St. from North 
Carolina St. to South Carolina St. Where two sewers are con- 
verging as at the corner of Varennes Road and Tennessee St. the 
longest time is taken. For example, the tune of concentration 



FLOOD FLOW BY RATIONAL METHOD 



97 



21 

AT THE FOOT OF TENNESSEE STREET ON FIGURE 25 



By Rational Method 


Line Number 


1 

3 

of 

V 

j 


I 


Equivalent 
100 Per Cent 
/ Acres 




u 4) 
l\ 

8^ 

Til 

J 


Time of Con- 
centration, 
Minutes 


R 





S 


V 


Sewer Length, 
Feet 


Time in Sewer 


2.35 


0.50 


1.17 


1.17 


7.0 


4.8 


5.6 


0.011 


4.6 


300 


1.1 


1 


3.00 


.50 


1.50 


2.67 


8.1 


4.6 


12.2 


.010 


5.5 


300 


0.9 


2 


3.00 


.50 


1.50 


4.17 


9.0 


4.4 


18.3 


.012 


5.8 


300 


0.9 


3 


3.00 


.50 


1.50 


5.67 


9.9 


4.2 


23.9 


.009 


6.0 


300 


0.8 


4 


3.00 


.50 


1.50 


7.17 


10.7 


4.1 


29.3 


.009 


6.2 


300 


0.8 


5 


3.80 


.35 


1.33 


1.33 


10.0 


4.2 


5.6 


.009 


3.2 


370 


1.9 


6 


3.70 


.40 


1.48 


2.81 


11.9 


3.9 


11.0 


.011 


5.2 


300 


1.0 


7 


3.00 


.45 


1.35 


4.16 


12.9 


3.8 


15.8 


.002 


3.2 


300 


1.6 


8 


4.30 


.50 


2.15 


13.48 


14.5 


3.6 


48.5 


.019 


9.8 


450 


0.8 


9 


2.80 


.40 


1.12 


1.12 


8.0 


4.6 


5.2 


.004 


3.0 


210 


1.2 


10 


0.70 


.20 


0.14 


14.74 


15.3 


3.5 


51.5 


.006 


5.0 


120 


0.4 


11 


2.84 


.55 


1.56 


1.56 


10.0 


4.2 


6.5 


.008 


4.5 


300 


1.1 


12 


3.88 


.55 


2.13 


3.69 


11.1 


4.0 


14.8 


.007 


4.7 


300 


1.1 


13 


3.88 


.55 


2.13 


5.82 


12.2 


3.9 


22.7 


.011 


5.8 


300 


0.9 


14 


3.88 


.55 


2.13 


7.95 


13.1 


3.7 


29.4 


.016 


7.5 


300 


0.7 


15 


3.88 


.55 


2.13 


10.08 


13.8 


3.7 


37.3 


.019 


9.2 


300 


0.5 


16 


2.84 


.45 


2.28 


26.10 


15.7 


3.4 


88.8 


.015 


10.2 


280 


0.5 


17 
























18 


2.75 


.40 


1.10 


1.10 


8.0 


4.6 


5.1 


.020 


5.3 


480 


1.5 


19 


5.75 


.45 


2.59 


3.69 


9.5 


4.3 


15.8 


.012 


6.1 


410 


1.1 


20 


2.62 


.50 


1.31 


30.00 


16.2 


3.4 


102 


.012 


10.2 


180 


0.3 


21 


3.17 


.55 


1.74 


1.74 


9.0 


4.4 


7.7 


.012 


5.2 


270 


0.9 


22 


3.17 


.55 


1.74 


3.48 


9.9 


4.2 


14.6 


.010 


5.7 


300 


0.9 


23 


3.17 


.55 


1.74 


5.22 


10.8 


4.1 


21.4 


.017 


7.7 


300 


0.6 


24 


3.17 


.55 


1.74 


6.96 


11.4 


4.0 


27.8 


.015 


7.8 


300 


0.6 


25 


2.32 
0.18 
1.38 
2.80 
0.36 


.55 
.80 
.50 
.55 
.75 


1.28 
0.14 
0.69 
1.54 
0.27 


32.84 
Area 

Arc;l 

Areas 
35.48 


16.5 
No. 1 
No. 2 
No. 3 
16.9 


3.3 


108 


.012 


10.2 


230 


0.4 


26 
27 
28 
29 
30 














and 4 
3.3 












117 


Areas 


No. 1-5 inclua 

1 


ive 



down Varennes Road to Tennessee St. is shown in line 25 as 
11.4+0.6= 12.0 minutes. The time to the same point down Ten- 
nessee St. is shown in line 21 as 16.2+0.3 = 16.5 minutes. This 
time is therefore used in line 26. 

R, the rate of rainfall in inches per hour is determined by 
Talbot's formula. 

Q, is in cubic feet per second and is the product of the 8th 



98 



DESIGN OF SEWERAGE SYSTEMS 



and 10th columns. Since the 8th column is the sum of the prod- 
ucts of the 5th and the 6th columns for the lines representing 
tributary areas, then the llth column is the product of A, I, and R. 

S, is the slope on which it is assumed that the sewer will be 
laid. It is usually assumed as parallel to . the ground surface 
unless the velocity for this slope becomes less than 2 feet per sec- 
ond. In such a case the slope is taken as one which will cause 
this velocity. 

V, the velocity in feet per second, is computed from diagrams 
for the solution of Kutter's formula. The length in feet is scaled 
from the map as the distance between inlets or groups of inlets, 
and the time is the length in feet divided by the velocity in feet 
per minute. 

Having computed the quantity of flow to be carried in the 
sewer, the design is completed by. drawing the profile and com- 
puting the diameters and slopes by the same method as used in 
the design of separate sewers. 






CHAPTER VI 
APPURTENANCES 

55. General. The appurtenances to a sewerage system are 
those devices which, in addition to the pipes and conduits, are 
essential to or are of assistance in the operation of the system. 
Under this heading are included such structures and devices 
as: manholes, lampholes, flush-tanks, catch-basins, street inlets, 
regulators, siphons, junctions, outlets, grease traps, foundations 
and underdrains. 

56. Manholes. A manhole is an opening constructed in a 
sewer, of sufficient size to permit a man to gain access to the sewer. 
Manholes are the most common appurtenances to sewerage sys- 
tems and are used to permit inspection and the removal of obstruc- 
tions from the pipes. The details of the Baltimore standard 
manholes are shown in Fig. 27 and a manhole on a large sewer in 
Omaha is shown in Fig. 28. The features of these designs which 
should be noted are the size of the opening and working space, 
and the strength of the structure. Manhole openings are seldom 
made less than 20 inches in diameter and openings 24 inches in 
diameter are preferable. A man can pass through any opening 
that he can get his hips through provided he can bend his knees 
and twist his shoulders immediately on passing the hole. For this 
reason the manhole should widen out rapidly immediately below 
the opening, as shown in Fig. 27 and 38. 

The walls of the manhole may be built either of brick or of 
concrete. Brick is more commonly used, as the forms necessary 
for concrete make the work more expensive unless they can be 
used a number of times. The walls of the manhole should be at 
least 8 inches thick. Greater thicknesses are used in treacherous 
soils and for deep manholes, or to exclude moisture. A rough 
expression for the thickness of the walls of a brick manhole more 

d 
than 12 feet deep in ordinary firm material is ^ = ^+2, in which t 

99 



100 



APPURTENANCES 



is the thickness in inches and d is the depth in feet. The thick- 
ness of brick walls may be changed every 5 to 10 feet or so. Con- 
crete walls may be built thinner than brick walls. 

The bottoms of brick manholes are frequently made of con- 
crete as shown in Fig. 27. The floor slopes towards the center 
and is constructed so that the sewage flows in a half round or 
U-shaped channel of greater capacity than the tributary sewers. 
The sides of the channel should be high enough to prevent the 
overflow of sewage onto the sloping floor, which should have a 




Straight Through 
Manhole. 



Manhole. 



Junction Manhole. 
FIG. 27. Baltimore Standard Manhole Details. 

pitch of about one vertical to 10. or 12 horizontal. In manholes 
where two or more sewers join at approximately the same level 
the channels in the bottom should join with smooth easy curves. 
Where the inlet and outlet pipes are not of the same diameter 
the tops of the pipes should ordinarily be placed at the same 
elevation to prevent back flow in the smaller pipes when the 
larger pipes are flowing full. 

The dimensions of the manhole should not be less than 3 feet 
wide by 4 feet long for a height of at least 4 feet, when built in 
the form of an ellipse, or 4 feet in diameter when built circular. 
No standard method for the reduction of the diameter of the man- 
hole near the top is observed, the rate being more or less dependent 



MANHOLES 



101 



on the depth of the manhole. The use of sloping sides above the 
frost line is desirable as such a form is more resistant to heaving 
by frost action. 

For sewers up to 48 inches in diameter the manhole is usually 
centered over the intersection of the pipes and has a special founda- 
tion. For larger sewers the manhole walls spring from the walls 
of the sewer as shown in Fig, 28. 

In the case of a decided drop in the elevation of a sewer, or of 
a tributary sewer appreciably higher than an outlet in any man- 
hole, the sewage is allowed to drop vertically at the manhole, 




Cross Section. Longitudinal Section. 

Manhole at Omaha 



Well Hole at St. Paul. 

From Eng Record. Vol. 66. p.S7<V 



FIG. 28. Details of a Manhole and a Well Hole. 

hence the name drop manhole. The Baltimore standard drop 
manhole is shown in Fig. 27. A well hole is an unusually deep 
drop manhole in which the force of the vertical drop of sewage 
is broken by a series of baffle plates, or by a sump at the bottom 
of the well hole. Fig. 28 shows a well hole at St. Paul, Minn. 
The use of drop manholes can be avoided in large sewers by the 
construction of a flight of steps or flight sewer as shown in Fig. 29, 
which allows the use of a steep grade and serves to break the 
velocity of the sewage. 

The specifications of the Sanitary District of Chicago, cover- 
ing the construction of manhole covers and frames are : 

All castings shall be of tough, close grained, gray iron, 
free from blow holes, shrinkage and cold shuts, and sound, 
smooth, clean and free from blisters and all defects. 



102 APPURTENANCES 

All castings shall be made accurately to dimensions to 
be furnished and shall be planed where marked or where 
otherwise necessary to secure perfectly flat and true sur- 
faces. Allowance shall be made in the patterns so that the 
specified thickness shall not be reduced. 

All castings shall be thoroughly cleaned and painted 
before rusting begins and before leaving the shop with 
two coats of high grade asphaltum or any other varnish 
that the Engineer may direct. After the castings have 
been placed in a satisfactory manner, all foreign adhering 
substances shall be removed and the castings given one 
additional coat of asphaltum. No castings shall be 
accepted the weight of which shall be less than that due to 
its dimensions by more than 5 per cent. 




<- 8-1"-- 4 



*" *'-*&% 

- 4 ?8 -6- :* 



FIG. 29. Flight Sewer at Baltimore. 

. Eng. Record, Vol. 59, p. 161. 

The weights of frames and covers in use vary from 200 to 600 
pounds, the, weight of the frame being about 5 times that of the 
cover. The lightest weights are used where no traffic other than 
an occasional pedestrian will pass over the manhole. Frames 
and covers weighing about 400 pounds are commonly used on 
residential streets, whereas 600 pound frames and covers are 
desirable in streets on which the traffic is heavy. The frames 
should be so designed that the pavement will rest firmly against 
it and wear at the same rate as the surrounding street surface. 
Experience has shown that vertical sides should be used for the 
outside of the frame to approach this condition, and that the frame 
should not be less than 8 inches high. The cover should be rough- 
ened in some desirable pattern as shown in Fig. 30. Smooth 
covers become dangerously slippery. Where the ventilation of 



MANHOLES 



103 



^Corrugations 
for Type C 
Cover 



the sewers is not satisfactory the manhole covers are sometimes 
perforated. This is undesirable from other points of view as the 
rising odors and vapors are 
obnoxious at the surface and 
the entering dirt and water are 
detrimental to the operation 
of the sewer. The stealing 
and destruction of manhole 
covers and the unauthorized 
entering of sewers has occa- 
sionally required the locking 
of the covers to the frame when 
in place. The locks commonly 
used consist of a tumbler which 
falls into place when the man- 
hole is closed, and which can be 
opened only by a special wrench 




4>K 



J^andCCovs&r Cover Cfobeused 
, onallSidcwalh and 

+ Crown e/senrhtre as dinchd. 
" '-^ jframe and Coyer oiphalr. 
~~ -ed.Corrugo- 




- J'-2" 

FIG. 30. Baltimore Standard Manhole 



Frame and Cover. 



or hook. Adjustable frames 
are sometimes used where the 
street grade is settling, or may 
be raised in order that the 

elevation of the top of the cover may be made to conform to that 
of the street surface, without reconstructing the top of the man- 
hole. One type of adjustable cover is shown in Fig. 31. Man- 




FIG. 31. Adjustable Manhole Frame and Cover. 

hole covers should be so marked that the sanitary sewer can be 
distinguished from the storm water sewer, and both from the 
telephone conduit, etc. 

Iron steps are set into the walls of the manhole about 15 
inches apart vertically to allow entrance and exit to and from the 
manhole. Galvanized iron is preferable to unprotected metal as 
the action of rust is particularly rapid in the moist air of the sewer. 



104 



APPURTENANCES 



One type of these manhole steps is shown in Fig. 27. The steps 
should have a firm grip in the wall as a loose step is a source of 
danger. 

57. Lampholes. A lamphole is an opening from the surface 
of the ground into a sewer, large enough to permit the lowering 
of a lantern into the sewer. Lampholes are used in the place of 

manholes to permit the inspec- 
tion or the flushing of sewers, 
and to avoid the expense of a 
manhole. They are located 
from 300 to 400 feet from 
the nearest manhole in such 
a manner that a lamp lowered 
in the lamp hole can be seen 
from the two nearest man- 
holes. 

Lampholes should be con- 
structed of 8- to 12-inch tile 
or cast-iron pipe. The lower 
section should be a cast iron 
T on a firm foundation, but 
if constructed of tile it should 
be reinforced with concrete 
to take up the weight of the 
shaft. The details of the 
Baltimore standard lamphole 
are shown in Fig. 32. 
Lampholes are not com- 
monly used on sewerage sys- 
tems on account of their 
lack of real usefulness and 
the troubles encountered by 
breaking of the pipe below the shaft. 

58. Street Inlets. A street inlet is an opening in the gutter 
through which storm water gains access to the sewer. The types 
used in different cities vary widely. Details of an inlet in success- 
ful use are shown in Fig. 33. The figure shows also a common 
form of connection to the sewer. A water-seal trap is sometimes 
used to prevent the escape of odors from the sewer. Care must 
be taken in design that such traps do not freeze in winter nor dry 




FIG. 32. Baltimore Standard Lamphole. 



STREET INLETS 



105 





<- - 


.... 


~2'-i 


" * ~^i 


~>r 










j 




f^ 


n] 








L* 


h 








17" 


~~P 




i 




l"C^ 


i 




p 
V 




fe= 


.JJ 

n 


s 

c 






!"*> 


II 


*o 






i* 


1 1 

u 








jV 


~T| 
J 1 








tr 


-^.J 




j 






M^^MM 















out in summer, although it is not always possible to prevent 
these contingencies. 

The important features to be observed in the design of a street 
inlet are: height and length of opening, character of grating, and 
location. The general location of inlets is discussed in Chapter V. 
The clear height of opening commonly used is from 5 to 6 inches, 
with a clear length of 24 to 30 
inches or longer. Inlets of this 
size have given satisfaction on 
paved streets with moderate slopes, 
where the drainage area is not 
greater than 10,000 to 12,000 
square feet of pavement. W. W. 
Horner states: 1 

The St.Louis type of inlet 
" old " style was a vertical 
opening in the curb 8 inches 
high and 4 feet in length 
with a horizontal bar mak- 
ing the net opening about 
5 inches. It has a broad 
sill extending under the 
sidewalk. The " new " style 
inlet is 4 feet long with a 
clear opening of 6 inches 
and no bar. The sill is done 
away with and the opening 
drops down directly from 
the curb. Tests were made 
of the capacity of this inlet 
on pavements on different 
slopes with sumps of depths 
varying from to 6 inches 
in front of the inlet, extend- 
ing out 3 feet from the gutter, 

and returning to the elevation of the gutter at a slope of 3 
inches to the foot. The results of these tests are shown in 
Table 22. The capacity of the inlet is expressed as the 
amount of water entering just before some water begins to 
lap past. If a large amount of water is allowed to flow past 
much more water will enter the inlet thus furnishing a 
factor of safety for large storms. It was noted that by 
beginning the rise in the pavement about opposite the 

1 Municipal and County Engineering, October, 1909. 




k~ 26"- 

FIG. 33. Details of an Untrapped 
Street Inlet, without Catch-Basin. 



106 



APPURTENANCES 



middle of the inlet the capacity of the inlet was increased 
from 20 to 50 per cent. 

TABLE 22 

CAPACITIES OF ST. Louis STREET INLETS 
From tests by W. W. Horner. Cubic feet per second 



Slope in Per Ct. 


0.42 


1.5 


2.85 


4.5 


Depth of Sump, 


0.0 


2 


4 
1 27 


6 


0. 


2 


4 


6 



0.03 
03 


2 
0.25 
?8 


4 
0.78 
87 


6 
1.49 
1 62 



02 


2 
15 


4 
45 


6 

i n 


Capacity, old 


Capacity, new 


It 1 


5 


1 5 


> ", 


08 


4 


1 1 


? 1 





































Gratings with horizontal bars will admit more water than 
gratings with vertical bars, but they will also admit more rubbish 
such as sticks, papers, leaves, etc., which serve to clog the sewers. 
Vertical barred gratings and gratings in the bottom of the gutter 
clog more quickly than other types. In the selection of the type 
of grating to be used a decision must be made as to whether it is 
more desirable to clean the sewer or catch-basin, or to flood the 
street as a result of clogged inlets. Where catch-basins are used 
or the sewers are large, horizontal bars are more satisfactory. 
The openings between bars should be small enough to prevent 
the entrance of a horse's hoof or objects of sufficient size to clog 
the sewer. Four inches in the clear for vertical openings and 6 
inches for horizontal openings are reasonable sizes. 

The location of the inlets at the intersection of the two curb 
lines at a corner results in a lower first cost but on heavily traveled 
streets this may result in a higher maintenance cost than for other 
locations because of the concentration of traffic at street corners, 
hammering the inlet 'casting out of shape or position. Vehicles 
making short turns will tend to climb the curb and will intensify 
the wear upon the inlet. These objections can be overcome by 
the use of two inlets at each corner, set back far enough from the 
curb intersection to avoid interference with the cross-walks. 
This also makes it possible to raise the cross-walks without the 
use of gutters under them. 

The size of the pipe from the inlet to the catch-basin or sewer 
should be large enough to care for all of the water which may enter 



CATCH-BASINS 



107 



the inlet. As the capacity of the inlet is seldom known with accu- 
racy and the capacity of the pipe is difficult of determination, it 
has become customary to use a 10-inch or a 12-inch connecting 
pipe for each ordinary independent inlet. 

59. Catch-basins. Catch-basins are used to interrupt the 
velocity of sewage before entering the sewer, causing the deposi- 
tion of suspended grit and sludge and the detention of floating 
rubbish which might enter and clog the sewer. A separate catch- 




FIG. 34. CATCH-BASIN. 

Outlets are not always trapped. 

basin may be used for each inlet, or to save expense, the pipes from 
several inlets may discharge into one catch-basin. 

The types in successful use are extremely varied, but the dis- 
tinguishing feature of all is an outlet located above the floor of 
the basin. A common form of catch-basin is shown in Fig. 34. 
It is constructed similar to a manhole with a diameter of about 
4 or 4i feet and a depth of retained water from 3 to 4 feet. Catch- 
basins of this size will care for the sewage from the inlets at the 
four corners of a street intersection, each draining a city block. 



108 



APPURTENANCES 



In unusual situations it may be necessary to install a larger basin, 
but too large a catch-basin is less desirable than one which is too 
small, as the former stinks and the latter is useless. Traps are 
sometimes used to prevent the escape of odors from the sewer 
into the street, but odors are often created in the catch-basins 
themselves. Some engineers arrange the trap so that it can be 
opened for observation down the sewer as in Fig. 34, thus com- 
bining the advantages of a manhole with the catch-basin. 

The use of catch-basins is objectionable because: they furnish 
a breeding place for mosquitoes and other flying insects; the 
septic action in them produces offensive odors; if on a combined 
sewer they permit the escape of offensive odors in dry weather 
when the water seal in the trap has evaporated; and the freezing 
of the water seal in the trap prevents the entrance of water to 
the sewer. The sole advantage lies in the prevention of the 
clogging of the sewers, but this may be sufficient to overbalance 
all of the disadvantages. In general catch-basins should be 
provided on paved streets which are cleaned by flushing the 
material into the sewers, or where the drainage is from an unim- 
proved or macadamized street, sandy country, or into sewers in 
which the velocity of flow is less than 2 feet per second. 

60. Grease Traps. The presence of grease in sewers results 
in the formation of incrustations which are difficult to remove 

and which cause a material loss in 
the capacity of the sewer. The 
presence of oil and gasoline has re- 
sulted in violent and destructive ex- 
plosions as is described in Chapter 
XII. A type of grease trap used on 
the drains' from hotels, restaurants, 
or other large grease producing indus- 

trieS . is sh Wn in Fi S' 35 ' . The tra P 
i g similar to a catch-basin except 

that it is too small for a man to 
enter, and the outlet is necessarily trapped in order to pre- 
vent the escape of grease. The details of a gasoline and oil 
separator approved by the New York City Fire Department are 
shown in Fig. 36. * 

1 "Cleaning and Flushing Sewers. " Journal of the Association of Engineer- 
ing Societies, Vol. 33, 1904, p. 212. 




FIG. 35.-Diagrammatic Sec- 
tion through a Grease Trap. 



FLUSH-TANKS 



109 



2"Kr/i/Ar4 

u 




FIG. 36. Gasoline and Oil Separator. 



61. Flush-Tanks. These are devices to hold water used in 
flushing sewers. They are. required only on sanitary and com- 
bined sewers. Their use tends to prevent the clogging of sewers 
laid on flat grades and permits 
flatter grades in construction 
than could otherwise be adopt- 
ed. Flush-tanks may be oper- 
ated either by hand or auto- 
matically. Automatic operation 
is more common than hand 
operation. The hand-operated 
tanks are similar to manholes 
so arranged that the inlet and 
outlet sewers can be plugged 
while the manhole or tank is 
being filled with water either 
from a hose or a special service 
connection. When sufficient 
water has been accumulated 
the outlet is opened and 

the sewer is flushed by the rush of water. A sluice gate, flap 
valve, or a specially fitted board is sufficient to fit over the mouth 
of the inlet and outlet during the filling of the tank. Such an 
arrangement has the advantage of being cheap, simple, and 
satisfactory, though somewhat crude. In some ca,ses water is 
run into the tank at the same rate that it is discharged through 
the open outlet, maintaining a depth of 4 or 5 feet in the tank 
until the water passing the manhole below runs clean. The 
volume of water required by this method is large. Flushing 
water under a relatively high head is sometimes obtained by the 
use of tank wagons which are quickly emptied into the sewer 
through a canvas pipe dropped down a manhole. In all such 
cases if not well constructed the manhole is subject to caving due 
to the rush of water around the outlet. Precautions should be 
taken to minimize this danger by limiting the depth of water 
which may be accumulated. This can be done by constructing 
an overflow at a height of 4 or 5 feet above the bottom of the man- 
hole, discharging into the sewer through an outside drain. 

Automatic flush-tanks are constructed similar to a manhole, 
but special care should be taken to make them water-tight. The 



110 



APPURTENANCES 



Regula- 




apparatus for providing the automatic discharge may operate 
either with or without moving parts,- the latter being preferable 
as they require less attention and are not so liable to get out of 
order. An automatic flush-tank of the Miller type is shown in 
Fig. 37. It is a patented device manufactured by the Pacific 

Flush Tank Company. The 
small pipe at the left is a 
service connection to the water 
main. Water is allowed to flow 
continuously into the tank at 
such a rate as to fill it in the 
required interval between dis- 
charges. The tanks are dis- 
charged as nearly once a day 
as it is practicable to regulate 
them. The rate of flow into 
the tank is determined by trial 
and varies to some extent with 
the water pressure. The regu- 
lator shown in the figure is 
desirable as the continuous flow 
through the ordinary cock soon wears it away. Some waters 
will cause deposits to form in the small passages of the cocks or 
regulators, thus cutting off the flow. 

The tank operates as follows: when the water rising in the 
tank reaches the bottom of the bell, air is trapped in the bell and 
prevented from escaping through the main trap by the water at A. 
As the water continues to rise in the tank the air in the bell is 
compressed, the water level at A is driven down and water trickles 
from the siphon at C. The height of the water in the tank above 
the level of the water in the bell is equal at all times to the height 
of C above the lowering position of A. When A reaches the 
position of B a small amount of air is released through the short 
leg of the trap and a corresponding volume of water enters the 
bell. The head of water above the bell then becomes greater 
than the head of water in the short leg of the trap, which results 
in the discharge of all of the air in the long leg of the trap and 
the rapid discharge of the water in the tank through the siphon. 
The discharge is continued until the siphonic action is broken by 
the admission of air when the water level in the tank is lowered 



FIG. 37. Automatic Flush-Tank. 

Pacific Flush Tank Co. 



FLUSH-TANKS 



111 



to the bottom of the bell. The size of the siphons is fixed by the 
diameter of the leg of the siphon. Table 23 shows the capacity 
and size of sewers for which the different sizes of siphons are 
recommended by the manufacturers. 1 

TABLE 23 

SIZES OF SIPHONS TO BE USED WITH AUTOMATIC FLUSH-TANKS 



Diameter 
of 


Diameter 
of Tank 


Total 
Discharge 


Average 
Rate 


Diameter 
of 


Height of 
the 


Siphon 


at the 
Discharge 


for One 

Flush 


of 
Discharge 


Sewer 


Discharge 
Line above 


in 
Inches 


Line 
in Feet 


in 
Gallons 


in 
Sec.-ft. 


in 
Inches 


the Edge of 
the Bell 


4 


3 


60 


0.35 


4 to 6 


1 ft. 2 in. 


5 


3 


100 


0.73 


6 to 8 


1 ft. 11 in. 


6 


4 


240 


1.06 


8 to 10 


2 ft. 6 in. 


8 


4 


280 


2.12 


12 to 15 


2 ft. 11 in. 



When flush-tanks are placed at the upper end of laterals 
provision should be made for inspecting and cleaning the sewer 
by the construction of a separate manhole, or by combining the 
features of a manhole and a flush-tank in the same structure. 
Such a combination is shown in Fig. 38 from a design by Alex- 
ander Potter. 

Except under unusual conditions flush-tanks are used only on 
separate sewers. They should be placed at the upper end of 
laterals in which the velocity of flow when full is less than 2 to 
4 feet per second. The capacity of the tank or the volume of the 
dose is dependent on the diameter and slope of the sewer. The 
most effective flush is obtained by a volume of water traveling 
at a high velocity and completely filling the sewer. A large 
volume allowed to run slowly through the sewer will have but 
little if any flushing action. Data on the quantity of flushing 
water needed are given in Table 24. 2 As the result of a series of 
experiments conducted by Prof. H. N. Ogden on the flushing of 

1 Notes on the Design and Principles of Sewage Siphons, Eng. News-Record, 
Vol.85, 1920, p. 1041. 

2 From A. E. Phillips, Trans. Am. Society of Municipal Improvements, 
1898, p. 70. 



112 



APPURTENANCES 




Sectional 
Plan. 



FIG. 38. Automatic Flush-Tank and Manhole. 

Miller-Potter Design. Pacific Flush Tank Co. 

TABLE 24 
GALLONS OF WATER NEEDED FOR FLUSHING SEWERS 



CI1 


Diameter of Sewer in Inches 


blope 


8 


10 


12 


0.005 


80 


90 


100 


.0075 


55 


65 


80 


.01 


45 


55 


70 


.02 


20 


30 


35 


.03 


15 


20 


24 



SIPHONS 113 

sewers, 1 the conclusion was reached that the effect of a flush of 
about 300 gallons in an 8-inch sewer on a grade less than 1 per 
cent would not be effective beyond 800 to 1,000 feet, but that on 
steeper grades much smaller quantities of water would produce 
equally good results. 

Engineers do not agree upon the advisability of the use of 
automatic flush-tanks, some believing that they are a needless 
expense that can be avoided by hand flushing, and others feeling 
that a flush-tank should be placed at the upper end of every lateral. 
These diverse opinions are the result of different experiences in 
different cities. 

62. Siphons. There are two forms of siphons used in sewerage 
practice, a true siphon and an inverted siphon. A true siphon 
is a bent tube through which liquid will flow at a pressure less 
than atmospheric, first upwards and then downwards, entering 
and leaving at atmospheric pressure. An inverted siphon is a bent 
tube through which liquid will flow at a pressure greater than 
atmospheric first downwards and then upwards, entering and 
leaving at atmospheric pressure. 

In sewerage practice the word siphon refers to an inverted 
siphon unless otherwise qualified. Siphons, both true and 
inverted, are used in sewerage systems to pass above or below 
obstacles. True siphons are seldom used as they must be kept 
constantly filled with liquid. 2 Accumulated gas must be removed 
in order to prevent the breaking of the siphon which results in the 
cessation of flow. By the breaking of a true siphon is meant the 
stoppage of siphonic action due to the accumulation of air or gas 
at the peak of the siphon. Since the rate of flow of sewage fluc- 
tuates widely it is extremely difficult to control the flow so that a 
true siphon may be completely filled with liquid at all times. 

In the design of inverted siphons care must be taken to pre- 
vent sedimentation, and to permit inspection and cleaning. 
Sedimentation is prevented by maintaining a velocity greater 
than a fixed minimum, usually taken at about 2 feet per second. 
This minimum is attained by providing a number of channels. 
The smallest channel is designed to convey the least expected 
flow at the minimum velocity. Each of the other channels is 
made as small as possible, within the limits of economy and sim- 

1 Trans. Am. Society of Civil Engineers, Vol. 15, 1886. 

1 True Siphon at East Providence, Eng. News-Record, Vol. 85, 1920, p. 862. 



114 



APPURTENANCES 



plicity, in order that the minimum velocity shall be exceeded 
quickly after flow has commenced in them. The last channel or 
channels to be filled are made somewhat larger, because the 
sewage conveyed in them contains less settleable matter than is 
contained in the more concentrated dry weather flow. The type 
of siphon used in New York to pass under the subway is shown in 
Fig. 39. Note should be taken of the clean-out manhole provided 
on the 14-inch pipe. The other pipes are large enough for a man 
to enter and clean. 



Old 

4-10x3-6" 
Sewer 
ci.---| 



4-6'C/rcu/arffei'nf.Concrefe 



'/4 "C. /.(Dry WeathejPips 
Sectional Plan 



'-Gleanout Chamber 
i Cle an out Manhole 







WC.I. 
Section A-A. 



2,4'-6"(Storm) Pipes-' 

Longitudi'nal Section. 



Section B-B. 



FIG. 39. Sewer Siphon under New York Subway. 

Eng. News Vol. 76, p. 443. 

The computations involved in the design of a siphon are 
illustrated in the following example, in which it is desired to con- 
struct a siphon to pass under the railway cut shown in Fig. 40. 
The first step is to determine the limiting diameter and slope of 
the smallest pipe in the siphon. One-sixth of the capacity of the 
6-foot approach sewer or 19 cubic feet per second will be assumed 
as the minimum flow. The diameter of the pipe necessary to 
carry 19 cubic feet per second at a velocity of 2 feet per second is 
42 inches. The hydraulic gradient should have a slope of 0.0005 
if the material used has a roughness coefficient of .015. This is 
the minimum permissible slope of the siphon. The selection of a 
steeper slope will necessitate the laying of the sewer at a greater 
depth, and will permit the use of smaller pipes for the siphon. 



SIPHONS 



115 



The selection of the exact slope must then be based on judgment 
with the minimum limitation above placed. The slope will be 
arbitrarily selected as 0.001, the same as that of the approach 
sewer. The diameter of the dry weather pipe will therefore be 
36 inches, with a capacity of 18 second-feet, which is approximately 
the assumed dry-weather flow. The velocity of flow will be 
2.5 feet per second. The length of flow along the siphon is 150 feet. 
The next step should be the determination of the elevation at 
the lower end of the 36-inch pipe. This is done by multiplying 



40 1 ^j, 20-0"- 

- El. 100.0 



- 60 "6torm Sewer 
,42"Storm Sewer 




36 Dry 
' Weather 
Sewer 




Vertical Cross Section 



60 " Sewer 



42. Sewer 



36 " Sewer 



Plan under Retaining Wall 
FIG. 40. Diagram for the Design of an Inverted Siphon. 

the assumed grade by the equivalent length of straight pipe, and 
subtracting the product from the elevation at the upper end. 
The length of straight pipe which will give the same loss of head 
as the siphon is called the equivalent pipe. It is determined as 
follows: 

First, determine the head loss at entrance. This will vary 
between nothing and one velocity head, dependent on the arrange- 
ment at the entrance. 1 The length of straight pipe which will 

1 "The Effect of Mouthpieces on The Flow of Water Through a Sub- 
merged Short Pipe," by F. B. Seely. Bulletin No. 96, 1917, of the Eng'g. 
Experiment Station of the University of Illinois. 



116 APPURTENANCES 

give this same loss can be computed from the expression 1=-^, 

using for S the assumed slope of the hydraulic gradient. 

Second, determine the head loss due to the bends. This is 
determined from the expression 

h-^ V - 
~d2g 

in which /i = the head loss in the bend; 
Z = the length of the bend; 
d=the diameter of the pipe; 
v = the average velocity of flow ; 
</ = the acceleration due to gravity; 
/=a factor dependent on the radius (R) of the bend 
and d. 

The relation between /, R, and d, for 90 bends is shown as 

follows: 1 

R/d 24 16 10 6 4 2.4 

/ 0.036 0.037 0.047 0.060 0.062 0.072 

After the head loss has been determined, the equivalent length of 
straight pipe is determined as above. 

Third. The equivalent length of pipe will be the sum of the 
actual length of pipe and the equivalent lengths as found above. 

In the problem in hand the head lost at the entrance will be 
assumed as one-third of a velocity head, or 0.0324 foot. With 
the assumed slope of 0.001 this is equivalent to 32 feet of pipe. 
The radius of the bend is about 20 feet and the length for a 45 
central angle is about 16 feet. The head loss for this angle will 
probably be a little more than one-half that for a 90 angle. The 

V 2 

expression will therefore be taken as about 0.2 ^ and for two 

^9 

bends is equivalent to about 40 feet of pipe. The equivalent 
length of pipe is therefore 150+32+40 = 222 feet. The elevation 
at the lower end should therefore be: the elevation at the upper 
end, 92.07 - 222 X. 001 = 91.85. 

The diameters of the remaining pipes in the siphon are 

determined so that the sewage in the approach sewer is backed 

up as little as is consistent with good judgment before each pipe 

comes into action. This is done satisfactorily by a method of 

1 Trans. Am. Society of Civil Engineers, Vol. 49, 1902. 



REGULATORS 117 

cut and try. Let it be assumed that the siphon will be composed 
of three pipes: the dry-weather pipe taking 18 second-feet, the 
second pipe taking 28 second-feet, and the third pipe taking the 
remaining 70 second-feet. The diameters of the two larger pipes 
on the assumed slope of 0.001 will therefore be 42 inches and 60 
inches respectively. Other combinations might be used which 
would be equally satisfactory. There are many methods by which 
the sewage can be diverted into the different channels of the 
siphon. For example, the openings into the different pipes may 
be placed at the same elevation, and the sewage allowed to enter 
them in turn through automatically or hand-controlled gates, or 
in another method of control the openings may be placed at such 
elevations that when the capacity of one pipe has been exceeded 
the sewage will flow into the next largest pipe as shown in Fig. 40. 
The outlets from the different pipes are ordinarily placed at the 
same elevation, thus leaving each pipe standing full of sewage. 
Stop planks should be provided at the outlet in order that the 
pipes may be pumped out for cleaning. The objection to this 
arrangement is that the larger pipes may operate at a velocity 
less than 2 feet per second, and they will be standing full of 
sewage which might become septic. However, as they will take 
nothing but the storm flow near the top of the sewer no difficulty 
should be encountered from sedimentation in them, and all are 
large enough for a man to enter for inspection or cleaning. 

63. Regulators. Regulators are commonly used to divert the 
direction of flow of sewage in order to prevent the overcharging 
of a sewer or to regulate the flow to a treatment plant. Sewer 
regulators are of two types, those with moving parts and those 
without moving parts. An example of the moving part type is 
shown in Fig. 41. In this type as the sewage rises the float closes 
the gate to the inlet sewer, thus preventing the entrance of sewage 
under head from the larger sewer. There are many variations in 
the details of float-controlled regulators, but the principle of opera- 
tion is similar in all. These regulators can be adjusted to fix the 
maximum rate of flow to a relief channel or sewage treatment 
plant, or during times of storm to cut off the outlet to the dry 
weather channel. Another form of the moving part type is shown 
in Fig. 42. 1 It has been used extensively by the Milwaukee 

1 Described by W. L. Stevenson before the Boston Society of Civil Engi- 
neers in 1916 . 



118 



APPURTENANCES 



-Storm Stwtr 



Sewerage Commission. In its operation the dry-weather flow is 
diverted by the dam into the intercepter. It passes under the 
movable gate on its way to the treatment plant. As the flow 

increases the dam is 
overtopped and flood 
waters are discharged 
down the storm chan- 
nel. The movable gate 
is hung on a pivot 
placed below center. 
As the water rises in 
the intercepter, the 
.-Copper Float pressure against the 
upper portion of the 
gate becomes greater 
than that against the 
lower portion, and 
the gate closes. An 

opening is left at the bottom to allow an amount of sewage equal 
to the dry-weather flow to escape beneath the gate to prevent 
clogging or sedimentation in the intercepter channel. 

Objections to all moving part regulators are their need of 
attention and liability to become clogged. 




FIG. 41. Coffin Sewer Regulator. 



Direction 
of Flow 




Regulator 

Gate-, 




- StormSewer- 
Longitudinal Section 



- Dry Weather- -Storm Flow- 

Transverse Sections through Regulator. 



FIG. 42. Moving Part Regulator without Float. 

The overflow weir and the leaping weir have no moving parts 
and are used for the regulation of the flow in sewers. A leaping 
weir is formed by a gap in the invert of a sewer through which 
the dry-weather flow will fall and over which a portion or all of 
the storm flow will leap. One form of leaping weir is shown in 
Fig. 43. An overflow weir is formed by an opening in the side of 
a sewer high enough to permit the discharge of excess flow into a 
relief channel. A weir at San Francisco is shown in Fig. 44. A 
series of tests were run on leaping weirs and overflow weirs in the 
hydraulic laboratory of the University of Illinois. The type of 



REGULATORS 



119 



leaping weir tested was formed by the smooth spigot end of a stand- 
ard vitrified sewer pipe. The overflow weirs were formed by a 

it-tiri'dr 



.-Brick 



Cast Iron 
Orating 

Concrete 
Iron 
Casting 



Section of Inlet. 
Iron Casting to here 

/ JM Detail n 




Plan of Inlet. 
FIG. 43. Leaping Weir at Danville, Illinois. 




""Long. Steel Ban &'--. Section throu'g 

Reinforcement under Manhole. 
Sectional Plan. 

e'..i 

Reinforct- 





Section A-A. Section B-B. Section C~C. 



FIG. 44. Overflow Weir at San Francisco. 

Eng. News, Vol. 73, p. 307. 

steel knife edge in the side of the pipe parallel to its axis as shown 
in Fig. 45. Tests were made in 18-inch and 24-inch pipes on various 
slopes from 0.018 to 0.005, for both leaping weirs and overflow 



120 



APPURTENANCES 



weirs. The overflow weirs were varied in length from 16 inches 
to 42 inches and were placed at various heights from 25 per cent 
to 50 per cent of the diameter above the invert of the sewer. As 
the result of the observations the following formulas were 
developed. For the leaping weir the expressions for the coordi- 
nates of the curve of the surfaces of the falling stream, are : 

For the outside surface z = 
For the inside surface x = 




FIG. 45. Overflow Weir in Action. 

Shadow of steel knife edge which forms the lip of the weir can be seen through the 
falling sewage. 

in which x and y are the coordinates. The origin is in the upper 
surface of the stream vertically above the end of the invert of the 
pipe. The ordinate y is measured vertically downwards. V is 
the velocity of approach in feet per second. These expressions 
are applicable to any diameter of sewer up to 10 or 15 feet. They 
should not be used for depths of flow greater than about 14 inches; 
nor for slopes of more than 25 per 1,000; nor for velocities less 
than 1 foot per second nor more than 10 feet per second. The 
expression for the ordinate of the inside curve is not good for less 



JUNCTIONS 121 

than 6 inches nor more than 5 feet. The expression for the ordi- 
nate of the outside curve is limited to values between the origin 
and 5 feet below it. 

The expression for the length of an overflow weir of the type 
shown in Fig. 45, necessary to discharge a given quantity, is in the 
form, 



in which I =the length of the weir in feet; 

V =the velocity of approach in feet per second; 
d =the diameter of the pipe in feet; 
hi =the head of water on the upper end of the weir; 
h,2 = the head of water on the lower end of the weir. 

In the design of an overflow weir by this formula the height of the 
weir above the invert of the sewer and the flow over the weir 
should be determined arbitrarily. The height should be sub- 
tracted from the computed depth of water above the weir to 
determine the value of h\. The difference between the flow over 
the weir and the flow above the weir will represent the rate of 
flow in the sewer below the weir. The value of h? can then be 
computed as the difference in the depth of flow below the weir 
and the height of the weir above the invert. The value of V is 
computed from Kutter's formula. The length of the weir is 
determined by substituting these values in the formula. 

64. Junctions. At the junction of two or more sewers the 
elevation of the inverts should be such that the normal flow lines 
are at the same elevation in all sewers. The sewers should 
approach the junction on a steep grade to prevent sewage backing 
up in one when the other is flowing full. The velocity of flow at 
the junction should not be decreased and turbulence should be 
avoided in order to prevent sedimentation and loss of head. 
The junction should be effected on smooth easy curves with radii 
at least five times the diameter of the sewer where possible. 
Curves with short radii cause backing up of sewage thus reducing 
the capacity of the sewers. 

The terms bellmouth or trumpet arch are sometimes applied 
to the junction of sewers large enough to be entered by a man 
In small sewers the Y branches and special junctions are manu- 
factured so that the center lines of the pipes intersect, and the 



122 APPURTENANCES 

junctions of mains and laterals are made in manholes. In the 
construction of a bellmouth the arch is carried over all the sewers. 
A manhole should be constructed at these junctions as clogging 
frequently occurs there, due to swirling and back eddies which 
cannot be avoided. 

65. Outlets. The outlets to a sewerage system discharging 
into a swiftly running stream must be protected against wash 
and floating debris. In a stream or other body of water subject 
to wide variations in elevation the backing up of the sewage during 
high water should be avoided. Where tidal flats or low ground 
about the outlet may be alternately submerged and uncovered 
the discharge should always be into swiftly running water. In 
quiescent bodies of water such as lakes and harbors, and in rivers 
where the dilution is low, and in many other cases, the sewer 
outlet should be submerged. 

Outlets are protected against wash and the impact of debris 
by the construction of deep foundations and heavy protecting 
walls. Although the construction of an outlet in a slow current 
or a back eddy would avoid danger from wash and debris, the 
discharge of the sewage into the most rapid current possible aids 
in the prevention of a local nuisance. A row of batter piles on 
the upstream or exposed side of the sewer is desirable, or it may 
be necessary to construct a break-water to prevent the wash of 
the current from dislodging the pipe. These break-waters are 

low dams of wood or broken 
stone, more or less loosely 
thrown together. The back- 
ing up of water into the 
sewer can be prevented by 
constructing the sewer above 
FIG. 46. Tide Gate. the outlet on a steep grade. 

Where this is not possible 

the use of tide gates will be helpful. A tide gate, one form of 
which is shown in Fig. 46, is a special form of check valve placed 
on the end of the sewer. 

Sewer outlets are sometimes constructed on long trestles in 
order to reach deep or running water. Such a trestle is shown 
in Fig. 47. In Boston the outlet sewers are submerged under the 
harbor and discharge through outlets well out in the tidal currents. 
The sewage is discharged under pressure and the pumps are 




OUTLETS 



123 



operated at some of the stations only at such times as the tidal 
currents will carry the sewage away from the harbor. It is not 
always necessary in a combined sewerage system to carry the 

if. 4-4 L 0- ^ 

Expanded ....32-6'- 

Metal-. 
Portland 

Concrete -\ 

I2"xl2" 

FenderWalt-W^, 




Half Cross Section. 



Half Elevation. 



6 Facing Port!. Concrete, A'g-'/g Granite Powdtr-^ --Porfl. Concrete, 1-24 




Longitudinal Section. 
FIG. 47. Sewer Outlet on a Trestle. 

Eng. News, Vol. 49, p. 9. 

storm flow to a distant submerged outlet. A double outlet can 
be constructed as shown in Fig. 48 in which the dry-weather flow 
is carried to the channel in a submerged sewer and the storm 



124 



APPURTENANCES 



flow is discharged on the bank. 1 Cast-iron pipe should be used 
for submerged outlets as the sewer is subject to disturbance by 
the currents, anchors, ice, and other causes. 

66. Foundations. Sewers constructed in firm dry soil require 

no special foundation to dis- 
tribute the weight over the sup- 
porting medium. In soft ma- 
terials the lower half of the 
sewer ring may be spread 
as shown in Fig. 22, and in 
rock the pressures on sewer 
pipes are evenly distributed 




Dry Weather Outlet 



by a 



In 



cushion of sand. 

FIG. 48.-Dry Weather and Storm wet g rOUnd Such . as 

Sewer Outlet at Minneapolis, Min- sand > mud , swamp land, etc., 

nesota. a foundation must be con- 

En g . Record, Vol. es, p. 383. structed if the water cannot be 

drained off. 

The permissible intensities of pressure on foundations in 
various classes of material allowed by the building codes in differ- 
ent cities are given in Table 25. These figures are based on the 
assumption that the material is restrained laterally, which is 
generally the condition in sewer construction. In the softer 
materials it becomes necessary to spread the foundations not 
only to reduce the intensity of pressure, but also to care for the 
thrust of the sewer arch. The arch thrust may be found by one 
of the methods of arch analysis, and the haunches spread to care 
for this, or the sewer invert may be transversally reinforced to 
assist in caring for this action. Some sewer sections in hard and 
soft material are shown in Fig. 22 and 23. 

On soft foundations such as swamps or for outfalls on the muck 
bottom of rivers the sewer may be carried on a platform. For 
small sewers 2-inch planks, 2 to 4 feet longer than the diameter 
of the pipe are laid across the trench, and the sewer rests directly 
upon them. For large sewers imposing a heavy concentrated 
load, a pile foundation should be constructed. The foundation 
may consist of piles alone, pile bents, or a wooden platform sup- 
ported on pile bents. The load which can be carried by a pile is 

1 Multiple Outlet for Calumet Intercepting Sewer, by S. T. Smetters, Eng. 
News : Record, Vol. 83, 1919, p. 728. 



FOUNDATIONS 

TABLE 25 
ALLOWABLE BEARING VALUE ON SOILS IN VARIOUS CITIES 

From Proc. Am. Soc. Civil Engre., Vol. 46, 1920, p. 906 



125 



Quicksand and alluvial soil 


J to 1 ton per sq. ft. for Providence, R. I., } ton per 
sq. ft. for 6 cities 


Soft clay 


1 ton per sq. ft. for 27 cities, 1 ton per sq. ft. for New 
Orleans, 2 to 3 tons for Providence, R. I. 


Moderately dry clay and fine sand, 
clean and dry 


2 tons for 7 cities, 1J to 2J for Chicago, 2} tons for 
Louisville, 2 to 4 tons for Providence, 3 tons for 
Grand Rapids and Los Angeles 


Clay and sand in alternate layers 


2 tons for 19 cities, 1} to 2J for Chicago, 3 to 5 tons 
for Providence 


Firm and dry loam or clay, or hard 
dry clay or fine sand 


3 tons for 24 cities, 2] tons for 2 cities, 2 to 3 tons for 
Atlanta, 3J tons for Philadelphia, 4 tons for 6 cities 


Compact coarse sand, stiff gravel or 
natural earth 


4 tons for 25 cities, 3} tons for Buffalo, 3 to 4 tons for 
Atlanta, 4 to 5 tons for Cincinnati, 5 tons for 
Denver, 4 to 6 tons for 3 cities, 6 tons for Rochester, 

N. Y. 


Coarse gravel, stratified stone and 
clay, or rock inferior to best brick 
masonry 


6 tons for 3 cities, 5 tons for 2 cities, 8 tons for 1 city 


Gravel and sand well cemented 


8 tons for 5 cities, 6 tons for 2 cities, 8 to 10 tons for 
1 city 


Good hard pan or hard shale 


10 tons for 4 cities, 6 tons for 2 cities, 8 tons for 1 city 


Good hard pan or hard shale unex- 
POSM! to air, frost or water 


8 tons for 1 city, 10 to 15 tons for 1 city, 12 to 18 
tons for 1 city 


Very hard native bed rock 


20 tons for 5 cities, 15 tons for 1 city, 10 tons for 
1 city, 25 to 50 tons for 1 city 


Rock under caisson 


24 tons for Baltimore, 25 tons for Cleveland 



determined with accuracy only by driving a test pile and placing 
a load on it. Where piles do not penetrate to a firm stratum the 
load they will support can be determined by any one of the various 
formulas, either theoretical or empirical, which have been devised. 
Probably the best known of these formulas are the so-called 
Engineering News formulas one of which is: 

2Wh . 
P = TT for a pile driven by a drop hammer, 



126 APPURTENANCES 

in which P = the safe load on the pile in pounds. The factor 

of safety is 6; 

TF = the weight of the hammer in pounds; 
7i = the fall of the hammer in feet; 

$ = the penetration of the pile in inches at the last 
driving blow. The blow is assumed to be driven 
on sound wood without rebound of the hammer. 

Reference should be made to engineering handbooks for other 
forms of pile formulas. The accuracy of all of these formulas is 
not of a high degree. 

The piles are driven at about 2 to 4 feet centers, to a depth of 
from 8 to 20 feet, unless hard bottom is struck at a lesser depth. 
The butt diameter of the piles used for the smallest sewers is 
about 6 to 8 inches. If bents are used, 2 or 3 piles are driven in a 
row across the line of the sewer and are capped with a timber. 
For brick, block, pipe, and some concrete sewers, a wooden plat- 
form must be constructed between the pile bents for the support 
of the sewer. 

67, Underdrains. The construction of special foundations 
can sometimes be avoided by laying drains under the sewers, 
thus removing the water held in the soil. The laying of the under- 
drains facilitates the construction of the sewer and reduces the 
amount of ground water entering the sewer. The underdrains 
usually consist of 6- or 8-inch vitrified tile laid with open joints 
from 1 to 2 feet below the bottom of the sewer as shown in Fig. 1. 
If the sewers are large, parallel lines of drains may be laid beneath 
them. An observation hole should be constructed from the bottom 
of the manhole to each underdrain. This hole usually consists 
of a 6- or 8-inch pipe, embedded in concrete, connected to the 
drain and open at the top. It is too small to permit effective 
cleaning of the underdrains, which are usually neglected after 
construction, and which as a result clog and cease to function. 
Since the principle period of usefulness of the drains is during 
construction, their stoppage after the work is completed is not 
serious. The hollow tile used in vitrified block sewers serve as 
underdrains after construction, but are of little or no assistance 
to the draining of the trench during construction, 



CHAPTER VII 
PUMPS AND PUMPING STATIONS 

68. Need. In the design of a sewerage system it is occasion- 
ally necessary to concentrate the sewage of a low-lying district at 
some convenient point from which it must be lifted by pumps. 
In the construction of sewers in flat topography the grade 
required to cause proper velocity of sewage flow necessitates deep 
excavation. It is sometimes less expensive to raise the sewage 
by pumping than to continue the construction of the sewers with 
deep excavation. 

In the operation of a sewage-treatment plant a certain amount 
of head is necessary. If the sewage is delivered to the plant at a 
depth too great to make possible the utilization of gravity for the 
required head, pumps must be installed to lift the sewage. In 
the construction of large office buildings, business blocks, etc., 
the sub-basements are frequently constructed below the sewer 
level. The sewage and other drainage from the low portion of 
the building must therefore be removed by pumping. Because 
pumps are often an essential part of a sewerage system, their 
details should be understood by the engineer who must write the 
specifications under which they are purchased and installed. 

69. Reliability. If the only outlet from a sewerage system is 
through a pumping station, the inability of the pumps to handle 
all of the sewage delivered to them may so back up the sewage as 
to flood streets and basements, endangering lives and health and 
destroying property. Such an occurrence should be guarded 
against by providing sufficient pumping capacity and machinery 
of the greatest reliability. 

70. Equipment. The equipment of a sewage pumping station, 
in addition to pumping machinery, may include a grit chamber, a 
screen, and a receiving well. The grit chamber and screen are 
necessary to protect the pumps from wear and clogging. Grit 
chambers are not necessary in sewage devoid of gritty matter, 

127 



128 



PUMPS AND PUMPING STATIONS 



such as the average domestic sewage, unless reciprocating pumps 
are used. Sufficient gritty matter is found in average domestic 
sewage to have an undesirable effect on reciprocating pumps. 
Receiving wells are used in small pumping stations where the 
capacity of the pumps is greater than the average rate of sewage 
flow. The pumps are then operated intermittently, the pumps 
standing idle during the time that the receiving well is filling. 

Except for a few types of pumps of which the valve openings 
are unsuitable, any machine capable of pumping water is capable 
of pumping sewage which has been properly screened. The 
principles of sewage pumps are then similar to principles of water 
pumps. The conditions under which these principles are applied 
differ but slightly in the character of the liquid, and a smaller 
range of discharge pressures. Pumps with large passages, dis- 
charging under low heads are more commonly found among 
sewage pumps. 

71. The Building. The pumping station should, if possible, 
be of pleasing design and should be surrounded by attractive 
grounds. The Calumet Sewage Pumping Station in Chicago is 
shown in Fig. 49. Its architecture is pleasing particularly in 




FIG. 49. Calumet Sewage Pumping Station, Chicago, Illinois. 

contrast with its location and immediate surroundings. Such 
structures tend to remove the popular prejudice from sewerage 
and to arouse interest in sewerage questions. Service to the 



CAPACITY OF PUMPS 129 

public is of value. It can be rendered more easily by arousing 
public interest and co-operation by the erection of attractive 
structures, than by feeding popular prejudice by the construction 
of miserable eyesores. 

72. Capacity of Pumps. The capacity of the pumping equip- 
ment should be sufficient to care for the maximum quantity of 
sewage delivered to it, with the largest pumping unit shut down, 
and the provision of such additional capacity as, in the opinion 
of the designer, will provide the necessary factor of safety. 

Pumps can usually be operated under more or less overload. 
Power pumps and centrifugal pumps driven by constant speed 
electric motors have no overload capacity. A power pump or a 
centrifugal pump may be overloaded up to the maximum horse- 
power of any variable speed motor or steam engine driving it, 
provided the pump has been designed to permit it. Direct-acting 
steam pumps which are designed for proper piston speed and 
valve action at normal loads, can carry a 50 per cent overload for 
short periods, although the strain on the pump is great. They 
will carry a 20 to 25 per cent overload for about eight hours with less 
vibration and strain. The use of pumps capable of working at 
an appreciable overload is somewhat of an additional factor of 
safety, but the overload factor should not be taken into considera- 
tion in determining the capacity of the pumping equipment. 

The load on a pumping station consists of the quantity of 
sewage to be pumped and the height it must be lifted. Variations 
in the quantity are discussed in Chapter III. The head against 
which the pumps must operate fluctuates with the level in the 
tributary sewer or pump well, and in the discharge conduit. 
For a free discharge or discharge into a short force main the greater 
the rate of sewage flow the smaller the lift, as the depth of flow 
in the tributary sewer increases more rapidly than that in the 
discharge conduit. If the discharge is into a large body of water 
or under other conditions where the discharge head is approxi- 
mately constant, the fluctuations in total head should not exceed 
the diameter of the tributary sewer. Such fluctuations are of 
minor importance in the operation of direct-acting steam pumps, 
but may be of great importance in the operation of centrifugal 
pumps, as is brought out in Art. 78. 

73. Capacity of Receiving Well. The use of receiving wells 
is restricted to small installations which require, in addition to 



130 PUMPS AND PUMPING STATIONS 

the standby unit, only one pump, the capacity of which is equal 
to the maximum rate of sewage flow. When the receiving well 
has been pumped dry the pump stops, allowing the well to fill 
again. Although the use of a large receiving well, or an equaliz- 
ing reservoir, for a large pumping station would permit the opera- 
tion of the pumps under more economical conditions, the storage 
of sewage for the length of time required would not be feasible. 
The sewage would probably become septic, creating odors and 
corroding the pumps. The extra cost of the reservoir might not 
compensate for the saving in the capacity and operation of the 
pumps. 

The capacity of the receiving well should be so designed that 
the pump when operating will be working at its maximum capacity, 
and the period of rest during conditions of average rate of flow 
should be in the neighborhood of 15 to 20 minutes. For example, 
assume an average rate of flow of 2 cubic feet per second, with a 
maximum rate of double this amount. The pump should have a 
capacity of 4 cubic feet per second, and if the receiving well is to 
be filled in 15 minutes by the average rate of sewage flow its capac- 
ity should be 15X5X60X7.5 or 14,000 gallons. Under these 
circumstances the pump will operate 15 minutes and rest 15 
minutes, during average conditions of flow. 

74. Types of Pumping Machinery. The two principal types 
of pumping machines for lifting sewage are centrifugal pumps and 
reciprocating pumps. A centrifugal pump is, in general, any 
pump which raises a liquid by the centrifugal force created by a 
wheel, called the impeller, revolving in a tight casing, as shown in 
Fig. 50. A reciprocating pump is one in which there is a periodic 
reversal of motion of the parts of the pump. 

Centrifugal pumps are sometimes classified as volute pumps 
and turbine pumps. A volute pump is a centrifugal pump with a 
spiral casing into which the water is discharged from the impeller 
with the same velocity at all points around the circumference, as 
shown in Fig. 51. A turbine pump is a centrifugal pump in which 
the water is discharged from the impeller through guide passages 
into a collecting chamber, in such a manner as to prevent loss of 
energy in changing from kinetic head to pressure head. A tur- 
bine pump is shown in section in Fig. 51. Centrifugal pumps are 
sometimes classified as single stage and multi-stage. A centrif- 
ugal, pump from which the water is discharged at the pressure 



TYPES OF PUMPING MACHINERY 



131 



created by a single impeller is called a single-stage pump. If the 
water in the pump is discharged from one impeller into the suction 
of another impeller the pump is known as a multi-stage pump. 




Woodruff Keys 



FIG. 50. Section through de Laval Single-Stage, Double-Suction Centrifugal 

Pump. 



375 Lubricating ring. 554 

380 Oil hole cap. 555 

383 Oil drain tube. 555-1 

404 Shaft sleeve lock nut. 556 

440 Driving coupling. 560 

441 Driven coupling. 563 
443 Coupling check nut. 567R 

450 Coupling bolt. 

451 Coupling bolt nut. 567L 

452 Coupling rubber. 

453 Coupling rubber steel tube. 583 
600 Pump case. 567 ' 

550 Bearing bracket cap. 583 / 

551 Bearing. 600 

552 Shaft. 692 

553 Shaft sleeve, right hand thread 815 
PW Impeller. 815-1 



Shaft sleeve, left hand thread. 

Shaft stop collar, inner. 

Shaft stop collar, outer. 

Guide ring. 

Packing gland. 

Bearing. 

Impeller protecting ring, right hand 

thread. 
Impeller protecting ring, left hand 

thread. 
Pump case protecting ring. 

Labyrinth packing. 

Pump case cover. 
Impeller key. 
Bearing bracket, outer. 
Bearing bracket, inner. 



The number of impellers operating at different pressures deter- 
mines the number of stages of the pump. A three-stage pump is 
shown in Fig. 52. 



132 



PUMPS AND PUMPING STATIONS 



Reciprocating pumps are generally driven by steam and are 
either direct-acting, or of the crank-and-fly-wheel type. Power 
pumps are reciprocating machines which may be driven by any 
form of motor, to which they are connected by belt, chain or shaft. 
A Deming triplex power pump is shown in Fig. 53. Power 



- Impeller 



Impeller Eye 





-'Diffusion 
Vanes 



Volute Pump. TurbmePump, Circular Case. 

FIG. 51. Types of Centrifugal Pumps. 

pumps can be used only where the character of the sewage will 
not clog the valves nor corrode the pump. A direct-acting steam 
pump is one in which the steam and water cylinders are in the 
same straight line and the steam is used at full boiler pressure 
throughout the full length of the stroke. The crank-and-fly- 




FIG. 52. Section of a Multi-Stage Centrifugal Pump. 

Courtesy DeLaval Steam Turbine Co. 

wheel type of pumping engine permits the use of steam expan- 
sively during a part of the stroke, the energy stored in the fly- 
wheel carrying the machine through the remainder of the stroke. 
Reciprocating pumps are sometimes classified as plunger pumps 
and piston pumps. In the action of a plunger pump the water is 
expelled from the water cylinder, by the action of a plunger 



133 




53. Triplex Power Pump. 

Courtesy, The Deming Co. 



which only partly fills the water cylinder, as shown in Figs. 54 
and 55. In a piston pump the water is expelled from the water 
cylinder by the action of a piston which completely fills the water 
cylinder, as shown in Fig. 
63, which illustrates a direct- 
acting piston pump. 

Plungers are better than 
pistons for pumping sewage 
as the wear between the pis- 
tons and the inside face of 
the cylinder soon reduces the 
efficiency of the pump. Out- 
side packed plungers are 
better than the inside packed 
type because the packing can 
be taken up without stopping 
the pump and the leakage 

from the pump is visible at all times. Outside packed pumps 
are more expensive in first cost, but are easier to maintain and 
have a longer life than piston pumps. 

In selecting a pump to perform certain work the size of the 
water cylinder and the speed of the travel of the piston should be 
_____________________________ investigated to insure proper 

capacity. The average linear 
travel of the piston for slow 
speed pumps is estimated at 
about 100 feet per minute, 
dependent on the length of 
stroke and the valve area. 
For short strokes and small 
valve areas the speed may 
be as low as 40 feet per min- 
ute, and for long stroke fire 
pumps with large valves 
the piston can be operated 
at a speed of 200 feet per 
minute. 1 Vertical, triple-expansion, crank-and-fly- wheel, outside- 
packed plunger pumps with flap valves can be operated at speeds 
of 200 feet per minute when lifting sewage, and when equipped 
1 " Direct Acting Steam Pumps," by F. R. Nickel, 1915. 




FIG. 54. Water End of Inside Center- 
Packed Plunger Pump. 



134 



PUMPS AND PUMPING STATIONS 



with mechanically operated valves and lifting water they can be 
run at speeds of 400 to 500 feet per minute. The speed of travel 
multiplied by the volume of piston or plunger displacement, with 
proper allowance for slippage, will give the capacity of the pump. 
The slippage allowance may be from 3 to 8 per cent for the best 
pumps, and for pumps in poor conditions it may be a high as 30 to 
40 per cent, 




Channel Way 
f * Pump 



Main Suction Pipe 



FlG. 55 Water End of Outside Center-Packed Plunger Pump. 

Courtesy Allis-Chalmers Co. 



There are two types of ejector pumps used for lifting sewage. 
One of these depends on the vacuum created by the velocity of a 
stream of water or steam passing through a small nozzle. The 
operation of this pump is described in Art. 139 and it is illustrated 
in Fig. 97. The other type of ejector pump is known as the com- 
pressed-air ejector. It is operated by means of compressed air 
which is turned into a receptacle containing sewage. The details 
of this type are explained in Art. 83 and are illustrated in Fig. 68. 



SIZES AND DESCRIPTION OF PUMPS 135 

75. Sizes and Description of Pumps. The size of a centrif- 
ugal pump is expressed as the diameter of the discharge pipe in 
inches. It has nothing to do with the head for which the pump 
is suited. On the assumption of a velocity of flow of 10 feet per 
second through the discharge pipe the capacity of the pump can be 
approximated. 

The size of a reciprocating pump involves the expression of the 
diameters of the steam cylinders, the water cylinder, and the length 
of the stroke in inches, in the order named, beginning with the 
steam cylinder with the highest pressure. A complete descrip- 
tion of a steam pumping engine might be; compound, duplex, 
horizontal, condensing, crank-and-fly-wheel, outside-center- 
packed, 12"X24"X18"X24" pump. The word compound 
means that there are a high-pressure and a low-pressure steam 
cylinder; the word duplex means that there are two of each of 
these cylinders; the word horizontal means that the axes of these 
cylinders are in a horizontal plane; the word condensing means 
that the steam is discharged from the low-pressure cylinders into a 
condenser; the name crank-and-fly-wheel is self-explanatory; 
the name outside-center-packed means that the water cylinder is 
divided into two portions between which the plunger is exposed 
to the atmosphere, and that the packing rings are on the outside 
of the two portions of the cylinder as shown in Fig. 55 ; the figures 
shown mean that the high-pressure steam cylinder is 12 inches in 
diameter, the low-pressure 24 inches in diameter, the water cylin- 
der is 18 inches in diameter, and the stroke of the pump is 24 
inches. 

76. Definitions of Duty and Efficiency. The duty of a pump 
is the number of foot pounds of work done by the pump per 
million B.T.U., per thousand pounds of steam, or per hundred 
pounds of coal, consumed in performing the work. These units 
are only approximately equal as 100 pounds of coal or 1,000 pounds 
of steam do not always contain the same number of B.T.U. and 
may only approximately equal 1,000,000 B.T.U. 

Since 1,000,000 B.T.U. are equal to 778,000,000 foot-pounds 
of work, a pump with a duty of 778,000,000 will have an effi- 
ciency of 100 per cent. The efficiency of a pump is therefore its 
duty based on B.T.U. divided by 778,000,000. The efficiencies 
or duties of various types of pumps are given in Table 26. 1 
1 From Heat Engines, by Allen and Bursley. 



136 PUMPS AND PUMPING STATIONS 

TABLE 26 
APPROXIMATE DUTIES OF STEAM PUMPS 

Small duplex, non-condensing 10,000,000 

Large duplex, non -condensing 25,000,000 

Small simple, flywheel, condensing 50,000,000 

Large simple, flywheel, condensing 65,000,000 

Small compound, flywheel, condensing 65,000,000 

Large compound, flywheel, condensing 120,000,000 

Small triple, flywheel, condensing 150,000,000 

Large triple, flywheel, condensing 165,000,000 

77. Details of Centrifugal Pumps. A section of a centrifugal 
pump with the names of the parts marked thereon is shown in 
Fig. 50. Among the important parts which require the attention 
of the purchaser are: the impeller (PW), the impeller packing 
rings (567 R & L), the bearings (551, 563), the thrust bearings 
(555-1), the shaft (552), and the shaft coupling (440). 

The impeller should be of bronze, gun metal, or other alloy, 
because there is no rusting or roughening of the surface, and the 
efficiency does not fall with age. Normal fresh sewage is not 
corrosive, but septic sewage and sludge are usually so corrosive 
that iron parts cannot be used with success in contact with them. 
The impeller should be machined and polished to reduce the fric- 
tion with the liquid. Impellers are made either closed or open, 
i.e., either with or without plates on the sides connecting the 
blades to avoid the friction of the liquid against the side of the 
casing. The closed type of impeller is shown in Fig. 50. Closed 
impellers are slightly more expensive, but generally give better 
service and higher efficiencies than the open type. Single impeller 
pumps should have an inlet on each side of the impeller to aid in 
balancing the machine, unless the plane of the impeller is to be 
horizontal when operating. Multi-impeller pumps usually have 
single inlet openings for each impeller. Vibration in the pump is 
sometimes caused by an unbalanced impeller. The moving parts 
may be balanced at one speed and unbalanced at another. To 
determine if the moving parts are balanced the pump should be 
run free at different speeds and the amount of vibration observed. 
If the impeller is removed from the pump its balance when at 
rest can be studied by resting it on horizontal knife edges. If 
there is a tendency to rotate in any direction from any position 
the impeller is not perfectly balanced. 



DETAILS OF CENTRIFUGAL PUMPS 



137 



Packing rings are used to prevent the escape of water from 
the discharge chamber back into the suction chamber. These 
rings should be made of the same material as the impeller. 
Labyrinth type rings, as shown in Fig. 50, are sometimes used as 
the long tortuous passages are efficient in preventing leakage. 

The bearings must be carefully made because of the high speed 
of the pump. They are usually made of cast iron with babbitt 
lining. They should be placed on the ends of the shaft on the 
outside of the pump casing, as shown in Fig. 50, and should be 
split horizontally so as to be easily renewed. Exterior bearings 
are oil lubricated by means of ring or chain oilers with deep oil 
wells. Where interior bearings are necessary, because of the length 
of the shaft, they should be made of hard brass and should be 
water lubricated. 

Thrust bearings or thrust balancing devices are used to take 
up the end thrust which occurs in even the best designed pumps. 
To overcome this pumps are designed with double suction, 
opposed impellers, or two pumps with their impellers opposed 
may be placed on the same shaft. Due to inequalities in wear, 
workmanship or other conditions, end thrust will occur and must 
be cared for. Various types of thrust bearings are in successful 
use, such as: the piston, ball, roller or marine types. The marine 
type thrust bearing is shown in Fig. 56. The piston type of 






FIG. 56. Marine Type Thrust Bearing. 
Courtesy, DeLaval Steam Turbine Co. 

hydraulic balancing device is shown in Fig. 57. In the figure A 
represents the impeller, and B a piston fixed to the shaft and 
revolving with it. There is a passage for water through the open- 
ings (1), (2), and (3) leading from the impeller chamber to the 
atmosphere or to the suction of the pump. If the impeller tends 
to move to the right opening (1) is closed resulting in pressure on 




138 PUMPS AND PUMPING STATIONS 

the right of the impeller forcing it to the left. If the impeller 
moves to the left (1) is opened thus transmitting pressure to the 
piston B forcing the impeller to the right. The flange C is not 
essential, but is advantageous in pumps handling gritty matter. 
As the channel (2) wears larger the pressure against the piston 
decreases allowing it to move to the left. This partially closes 
(3) building up the pressure again. 

Flexible shaft couplings should be used if the shaft of the 
driving motor and the pump are in the same line, as direct align- 
ment is difficult to obtain or to 
maintain. Where connected to steam 
turbines, reduction gearing and rigid 

couplings are usually used on sewage 

& pumps to obtain slow speed and per- 

'l 2'' H mit large passages. Flexible coup- 

FIG. 57. Piston Type of Thrust lingsare of various types, oneof which 
Balancing Device. is shown in Fig. 50. A rigid coup- 

ling would be formed by bolting 

the flanges firmly together. Shaft couplings are usually not 
necessary where the pump is driven by belt connection to the 
engine or motor, or where the pump and pulley rest on only two 
bearings. 

The stuffing box shown in Fig. 50 is packed loosely with two 
layers of hemp between which is a lantern gland, in order to permit 
a small amount of leakage. A drip box is placed below this gland 
to catch the leakage and return it to the pump. The leakage is 
permitted as it aids in lubrication and the tightening of the gland 
will cause binding of the shaft. The gland on the suction side of 
the pump should be connected by a small pipe to the discharge 
chamber in order to keep a constant supply of water for lubrica- 
tion and to prevent the entrance of air to the suction end of the 
pump. 

78. Centrifugal Pump Characteristics. The capacity of a 
centrifugal pump is fixed by the size and type of its impeller and 
by the speed of revolution. Roughly, the capacity of a pump, 
for maximum efficiency, varies directly as the speed of revolution, 
the discharge pressure varies as the square of the speed, and the 
power varies as the cube of the speed. These relations are found 
not to hold exactly in tests because of internal hydraulic friction 
in. the pump. 



CENTRIFUGAL PUMP CHARACTERISTICS 



139 



The characteristic curves for a centrifugal pump, or the so- 
called pump characteristics, are represented graphically by the 
relation between quantity and efficiency, quantity and power 
necessary to drive, and quantity and head, all at the same speed. 
The quantities are plotted as abscissas in every case. The curve 
whose ordinates are head and whose abscissas are quantities is 
known as " the characteristic." The curve showing the relation 
between quantities and speeds is sometimes included among the 
characteristics. Characteristics of pumps with different styles 
of impellers are shown in Fig. 58. Fig. 59 shows the character- 
istics of a pump run at different speeds, the efficiencies at these 




Type 1. 



10 20 30 40 50 60 70 80 
Capacity in Gallons per Minute 

FIG. 58. Characteristics of Centrifugal Pumps with Different Styles of 
Impellers at Constant Speed. 

speeds when pumping at different rates, and the maximum effi- 
ciency at different speeds. It is to be noted that the informa- 
tion given in this figure is more extensive than that in Fig. 58. 
The operating conditions under any head, rate of discharge, and 
speed are given. The curves of constant speed are parallel, and 
their distances apart vary as the square of the speed. The 
line of maximum efficiency is approximately a parabola. 

A study of the characteristics of any particular pump should be 
made with a view to its selection for the load and conditions under 
which it is to be used. Among the important things to be con- 
sidered in the selection of a centrifugal pump for the expected 
conditions of load are: the capacity required, the maximum and 
minimum total head to be pumped against, the maximum varia- 
tions in suction and discharge heads, and the nature of the drive. 
For example, the pump, whose characteristics are shown in Fig. 



140 



PUMPS AND PUMPING STATIONS 



59, should be operated at about 800 revolutions per minute. 
Under total heads between 40 and 50 feet, the discharge for the 




100 200 300 400 500 .600 700 800 900 1000 1100 1200 1300 
Capacity in Gallons perJMfnute 

FIG. 59. Efficiency and Characteristic Curves of a Centrifugal Pump at 

Different Speeds. 

best efficiency will vary between 600 and 670 gallons per 

minute. 

The efficiencies of centrifugal pumps increase with their 
capacities as is shown approximately 
in Fig. 60. 

79. Setting of Centrifugal Pumps. 
In setting a centrifugal pump, care 
should be taken to provide a firm 
foundation to hold the shafts of the 
pump and the electric motor or the 
reduction gearing in good alignment, 
or to prevent the pump from being 
displaced by the pull of a belt. It is 
desirable that the foundation be level. 
Centrifugal pumps should be set sub- 
merged for small pumping stations 

automatically controlled. Sludge pumps must be set submerged 

as otherwise they will not prime successfully. Provision should be 





Jeo 

UJ 

50 
40 














s 


^ 


** 




A 


* 


















I 










10 20 30 40 50 



Size of Pumps in Inches 

FIG. 60. Efficiencies of Cen- 
trifugal Pumps. 



SETTING OF CENTRIFUGAL PUMPS 



141 



made by which the pump can be lifted from the sewage, or sludge, 
for inspection and repair. In many cases the pump can be made 
self-priming by setting it in a dry. water-tight vault below the low 
level of sewage flow. Where possible it is desirable not to set the 
pump submerged as it will receive better care when easily acces- 
sible. 

The suction pipe should be free from vertical bends where air 
might collect and should be straight for at least 18 to 24 inches 
from the pump casing. An elbow on the suction pipe, attached 
directly to the casing of the pump gives a lower efficiency than a 
suction pipe with a short straight run. Centrifugal pumps will 
operate with as high a suction lift as reciprocating pumps, but at 
the start they must be primed and some provision must be made 
for priming them. The suction pipe should be equipped with 
foot valves to hold the priming, or some method may be provided 
for exhausting the air from the suction pipe. The foot valves 
should be so installed as to form no appreciable obstruction to the 
flow of water. They should have an area of opening at least 50 
per cent greater than the cross-section of the suction pipe. A 
strainer on the suction pipe is un- 
desirable as it becomes clogged 
and is usually in an inaccessible 
position for cleaning. A screen 
should be placed at the en- 
trance to the suction well to 
prevent the entrance of objects 
that are likely to clog the pump. 
A gate-valve and a check-valve 
should be provided on the dis- 
charge pipe, the former to assist 
in controlling the rate of dis- 
charge and the latter to prevent 
back flow into the pump when 
it is not operating. 

Centrifugal pumps are well 
adapted to service in either large 
or small units. An installation 
in a manhole at Park Point, 

Duluth, is shown in Fig. 61. This station is controlled by an 
automatic electric device which is operated by a float in the suc- 




24"M.H. 
Cover, 



10 

Overflo 



Float- 



61. Centrifugal Pump in Man- 

hole at Duluth, Minn. 
Eng. Contracting, vol. 43. 1915, p. 310. 



142 



PUMPS AND PUMPING STATIONS 



tion pit. Such automatic control is an added advantage of the 
use of electrically driven centrifugal pumps. The Calumet 
Pumping Station in Chicago, shown in Fig. 49, has a capacity of 
approximately 1,000 cubic feet per second. The simplicity of the 
layout of this station is shown in Fig. 62. 



Suction Basin Gages 

Dry Weaf her Discharge Basin Gag* 
~''-6"-\~&. -24'- 

- ifn ~ 



'Storm Water 
Discharge 
Basin Oages \,/ 

-34-3- >k 




FIG. 62. Interior Arrangement of the Calumet Sewage Pumping Station, 

Chicago. 

Eng. News-Record, Vol. 85, 1920, p. 872. 



80. Steam Pumps and Pumping Engines. The direct-acting 
steam pump, one type of which is shown in Fig. 63, is adapted to 
pumping sewage the character of which will not corrode or clog 
the valves. In this form of pump it is necessary to utilize the 
steam at full pressure throughout the entire length of the stroke, 
which results in high steam consumption. A fly-wheel permits 
the use of steam expansively during a part of the stroke, thus 
increasing the economy of operation. Other devices used for the 
same purpose are known as compensators. They are not in 
general use. 

Steam engines are classified in many different ways, for 
example ; according to the type of valve gear, as, plain slide valve, 
Corliss, Lentz, etc.; or according to the number of steam expan- 



STEAM PUMPS AND PUMPING ENGINES 



143 



sions, as, simple, compound, triple expansion, etc.; or according 
to the efficiency of the uoachine as low duty or high duty; or as 




FIG. 63. Section of Duplex Piston Steam Pump. 

Courtesy, The John H. McGowan Co. 



STEAM END 

2 Steam cylinder and housing combined. 
8 Steam piston head. 
Steam piston follower. 

10 Steam piston inside ring. 

11 Steam piston outside ring (2). 

12 Steam cylinder head. 
14 Steam chest. 

16 Steam chest cover. 

17 Steam slide valve. 

18 Steam valve rod. 

20 Steam valve rod, pin and nut. 

22 Steam valve rod, collar and set screw. 

23 Steam valve rod, stuffing box. 

24 Steam valve rod, stuffing box, nut and 

gland. 
38 Piston rod. 

47 Piston rod stuffing box. 

48 Piston rod, stuffing box, nut and gland. 
40 Valve gear stand. 

51 Long valve crank and shaft. 

52 Short valve crank and shaft. 



PUMP END 
115 Pump body. 
127 Brass liner. 

129 Water piston head. 

130 Water piston follower. 
137 Cylinder head. 

139 Valve plate. 

140 Cap 

152 Suction flange. 

161 Discharge flange. 

162 Valve seat, suction or discharge. 

163 Valve, suction or discharge. 

164 Suction valve spring. 

167 Discharge valve spring. 

168 Valve plate, suction or discharge. 

169 Valve stem, suction or discharge. 

STEAM END 

55 Crank pin. 

56 Valve rod link. 

61 Long rocker arm. 

62 Short rocker arm. 

63 Rocker arm wiper. 
69 Cross head. 



condensing or non-condensing, etc. Throttling engines or auto- 
matic engines refer to the method of control of the steam by the 
governor. In throttling engines the governor controls the amount 



144 



PUMPS AND PUMPING STATIONS 



of opening of the throttle valve, in automatic engines the governor 
controls the position of the cut-off. 

The simple slide-valve, low-duty, non-condensing, throttling 
engine, is the lowest in first cost and the most expensive in the 
consumption of fuel. The triple-expansion Corliss, or the non- 
releasing Corliss, high-duty pumping engine is the most expensive 
in first cost but consumes less steam for the power delivered than 
any other form of reciprocating engine. For pumps of very small 
capacity the cost of fuel is not so important an item as the first 
cost of the machine. For this reason and because of the lower 



9x6 ' Non Condensing, Jhrottlincf 
High Speed , Single Valve 



125 K. W. Non Condensing, Automatic, 
High Speed. Single Valve 



den sing, Rotary 4 Valve, Mediumope 



125 K,W. Compound Condensing, 
Automatic, Highspeed, SingleValve-- 



-450H.P. De Laval Turbine 



400 K.W. Compound 

Condensing, Medium Speed, Popped Valve 




40 60 80 

Per Cent of Rated Load 



120 



20 
10 ( 


V 


1 1 
s^ ^Condensing 


,Me4 

H=J 
ines 


ium S 


Deed, 


Rotary 


'41/al 


/e 










\ 


i 1 


,.Ct 


mpour 


id Con 


densin 


j, Medi 


jmSpe 


ed, Pof. 


pettVa 


Ive 


























) 200 400 600 600 1000 1200 140 
Horse Power 



FIG. 64. Diagram Showing Rates of Steam Consumption for Different Size 
Units under Different Loads. 



cost of attendance low-duty pumps are more frequently found in 
small pumping stations. 

The steam consumption per indicated horse-power, better 
known as the water rate of the engine, for various types of engines 
at full and at part load is shown in Fig. 64. The steam consump- 
tion of other types at full load is shown in Table 27. The indi- 
cated horse-power (I.H.P.) of a steam engine is the product of 
the mean effective pressure (M.E.P.), the area of the steam 



STEAM PUMPS AND PUMPING ENGINES 



145 



Type of Engine 


Power, 
K.W. 


Per Cent of Full Load 


Boiler 
Press, 
Lbs. 


25 


50 


75 


100 


125 


Single cylinder, high speed, non-condensing 


25 
250 


33 

42 


27 
37.5 


M.I 

35 


27.0 
34.0 


27.5 
34.0 


100 to 
150 


Automatic, flat four valve, high speed 


150 
250 




32 
33 


30 
31 


26.5 
28 


29.0 
30.0 


100 to 
125 






Tandem compound condensing, high speed 


125 




23 

25 


19 
20 


17 
19.5 


18 
21 


100 to 
150 








Cross compound, condensing, high speed 




30 


26 


24 


23 


23.5 


125 




Cross compound, non-condensing, highspeed 




39 


31 


27 


26 


27.5 


125 




Single cylinder Corliss, condensing 


120 
500 


23.7 
26.3 


20.4 
22.8 


19 
21.3 


18.5 
20.8 


19.0 
21.3 


100 
125 


Compound Corliss, condensing 




16.5 
22.2 


14 
19 


12.5 

17.0 


12.1 
16.5 


12.5 
17.0 


100 
150 






Single cylinder, rotary four valve, non-con- 
densing 


75 
400 


26.2 
35.0 


22.3 
27.2 


21.3 
26.4 


21.6 
26.0 


22.8 
26.8 


100 
180 


Rotary four valve, tandem compound non- 
condensing 


125 
600 


32.0 
40.0 


22.0 
28.3 


20 
23.2 


18.25 
22.5 


18.5 
22.7 


100 
150 


Cross compound, non-condensing rotary 
four valve 


125 
600 


25 
39.4 


21 

28 


19.1 
22.3 


18.5 
20.6 


19.0 

20.7 


100 
150 


Single cylinder, poppett valve, non-con- 
densing 


120 
600 


22.7 
28.5 


20.5 
26.0 


19.7 
25.0 


19.1 
24.3 


20.1 
25.5 


100 
150 


Single cylinder, poppett valve, condensing 


120 

600 


18.5 
24.6 


16.7 
22.3 


16.1 
21.4 


15.6 
20.8 


16.4 
21.9 


100 
150 


Compound condensing, poppett valve 


200 

1200 


14.2 
18.4 


13.0 
16.9 


12.5 
16.3 


12.2 
15.9 


12.9 
16.8 


100 
150 


Uniflow 


125 
600 


14.6 
15.0 


13.7 
14.3 


13.4 
13.7 


13.4 

13.5 


13.3 
14.0 


150 


Steam turbines, condensing, Allia-Chalmers 


300 
2000 




24 
31.9 


17 
26.3 


160 
23.8 


16.5 
23.0 


125 
175 


Steam turbines, condensing, Westinghouse 


300 

2000 




13.7 
18.2 


12.8 
16.9 


12.2 
16.2 


12.6 
16.8 


125 
175 


Steam turbines, high pressure, non-con., 12* 
to 36" wheel, 1000 to 3600 R.P.M. 


4 to 8 
stages 








28.5 
116.5 


















Ditto. Condensing, 26-inch 










17.3 
112.0 















146 PUMPS AND PUMPING STATIONS 

pistons, the length of the stroke, and the number of strokes per 
unit of time. A common form of this expression is, 

PLAN 

33,000 ' 

in which P=the M.E.P. in pounds per square inch; 
L = the length of the stroke in inches; 
A = the sum of the areas of the pistons in square inches; 
N = the number of revolutions per minute. 

The I.H.P. multiplied by the mechanical efficiency of the machine 
will give the brake or water horse-power, that is, the horse-power 
delivered by the machine. The product of the M.E.P., the sum 
of the areas of the steam pistons and the mechanical efficiency of 
the machine, should equal the product of the total head of water 
pumped against expressed in pounds per square inch and the sum 
of the areas of the water pistons or plungers. The M.E.P. is 
determined from indicator cards taken from the steam cylinders 
during operation. These cards show the steam pressure on the 
head and crank ends of each cylinder at all points during the stroke. 
81. Steam Turbines. Among the advantages in the use of 
steam turbines as compared with reciprocating steam engines for 
driving centrifugal pumps are their simplicity of operation, the 
small floor space needed, their freedom from vibration requiring a 
relatively light foundation, and their ability to operate success- 
fully and economically either condensing or non-condensing 
under varying steam pressure. They can be operated with steam 
at atmospheric or low pressure, thus taking the exhaust from 
other engines. The greatest economy of operation for the tur- 
bine alone will be obtained by operating with high pressure, super- 
heated steam and with a vacuum of 28 inches. In large units 
the economy of operation of steam turbines is equal to that of the 
best type of reciprocating engines. In order to develop the high- 
est economy turbines are operated at speeds from about 3,600 to 
10,000 r.p.m. or greater, the smaller turbines operating at the 
higher speeds. As these speeds are usually too great for the 
operation of centrifugal pumps for lifting sewage, reduction gears 
must be introduced between the turbine and the pump. Although 
the best form of spiral-cut reduction gears may obtain efficiencies 
of 95, to 98 per cent, or even higher, their use, particularly in small 



STEAM PUMPS AND PUMPING ENGINES 



147 



units, is an undesirable feature of the steam turbine for driving 
pumps. 

The steam consumption of DeLaval turbines of different 
powers, and the steam consumption of a 450 horse-power DeLaval 
turbine at different loads are shown in Fig. 64. Some steam con- 
sumptions of other turbines are recorded in Table 27. It is to be 
noted that the steam consumption of the 450 horse-power turbine 
at part loads is not markedly greater than that at full loads. 
This is an advantage of steam turbines as compared with recipro- 
cating engines. The steam consumption of any turbine is depend- 
ent on the conditions of operation and is lower the higher the 
vacuum into which the exhaust takes place. 

There are two types of turbines in general use, the single stage 
or impulse machines, and the compound or reaction type. The 
DeLaval is a well-known make 
of the single stage or impulse type. 
The principle of its operation is 
indicated in Fig. 65, which is the 
trade mark of the DeLaval Steam 
Turbine Co. The energy of the 
steam is transmitted to the wheel 
due to the high velocity of the 
steam impinging against the 
vanes. In the compound or re- 
action type of machine the steam 
expands from one stage to the 
next imparting its energy to the 
wheel by virtue of its expansion 
in the passages of the turbine. 
For this'i reason the single-stage 

or impulse type is operated at higher speeds than the compound 
or reaction machines. 

82. Steam Boilers. Among the important points to be con- 
sidered in the selection of a steam boiler for a sewage pumping 
station are: the necessary power; the quality of the feed water; 
the available floor space; the steam pressure to be carried; and 
the quality and character of the fuel. Tubular boilers of the 
type shown in Fig. 66, are lower in first cost than other types of 
boilers. They are not ordinarily built in units larger than 250 to 
300 horse-power and where more power is desired a number of 




FIG. 65. The DeLaval Trade 
Mark, Illustrating the Principle 
of the DeLaval Steam Turbine. 

Courtesy, DeLaval Steam Turbine Co. 



148 



PUMPS AND PUMPING STATIONS 




FIG, 66. Horizontal Fire-tube Boiler. 



units must be used. They are objectionable because of the 

relatively large floor space 
required, and because of their 
relatively poor economy of 
operation. The efficiencies 
of water-tube boilers of dif- 
ferent types are given in 
Table 28. Large power units 
of the water-tube type, as 
shown in Fig. 67, although 
more expensive in first cost, 
require less floor space. Al- 
most any desired steam 
pressure can be obtained 
from either type but water- 
tube boilers are more com- 
monly used for high pres- 
sures. The grate or stoker 

can be arranged to burn almost any kind of fuel under either 
water-tube or fire-tube boilers. The use of poor quality of water 
in water-tube boilers is un- 
desirable as the tubes are 
more likely to become clogged 
than the larger passages of 
the fire-tube boilers. If nec- 
essary, a feed-water puri- 
fication plant should be 
installed, as it is usually 
cheaper to take the inpurities 
out of the water than to take 
the scale out of the boiler. 

Not less than two boiler 
units should be used in any 
power station, regardless of 
the demands for power, and 
if the feed water is bad, three p IG 67. Babcock and Wilcox Water- 
or even four units should tube Boiler, 

be provided, as two units 
may be down at any time. 




An appreciable factor of safety is 



provided by the ability of a boiler to be operated at 30 to 50 per 



STEAM BOILERS 



149 



cent overload, if sufficient draft is available, but with resulting 
reduction in the economy of operation. The number of units 
provided should be such that the maximum load on the pumping 
station can be carried with at least one in every 6 units or less, 
out of service for repairs or other cause. 

TABLE 28 

EFFICIENCIES OF STEAM BOILERS 
From Marks' Mechanical Engineer's Handbook 











Per 




Evap. 


Com- 










Cent 


B.T.U. 


from and 


bined 








Sq. Ft. 


of 


per 


at 212 


Effi- 


Type 




Furnace 


Grate 


Rated 


Lb. 


per 


ciency 








Area 


Capac- 


Dry 


Lb. 


of Boiler 










ity 


Coal 


Dry 


and 










D'vTd 




Coal 


Furnace 


Babcock <fc Wilcox 


300 




84 


118 7 


11 912 


8 81 


71 8 


Babcock & Wilcox 


640 


Hand-fired 


118 


121 5 


14,602 


10 83 


72 




1128 


B. & W. chain grate . 


187 


198.3 


12,130 


9.51 


76 1 


Rust 


335 


Hand-fired 


68 


210 5 


13,202 


9 42 


68 9 




400 


Green chain grate. . . 


83.5 


123.8 


11,608 


8.79 


73 5 


Maximum efficient 


y recor 


led 










83 













The steam delivered by a boiler is the basis of the measurement 
of its capacity or power. A boiler horse-power is the delivery of 
33,320 B.T.U. per hour. It is approximately equal to the raising 
of 30 pounds of water per hour from a temperature of 100 
Fahrenheit, to steam at a pressure of 70 pounds per square inch, 
or to 34 pounds of water per hour changed to steam from and at 
212 Fahrenheit, at atmospheric pressure. The horse-power of a 
boiler is sometimes approximated by the area of its grate or heat- 
ing surface. Such a method of measuring has a low degree of 
accuracy on account of the variations in the quality of the fuel, 
and the rate of combustion. For example, the rate of combustion 
under a locomotive boiler is high and there is less than ^th of a 
square foot of grate area and about 4.5 square feet of heating 
surface per boiler horse-power. The Scotch Marine type of boiler 
used on steam ships, has slightly more grate area and slightly less 
heating surface than the locomotive type of boiler, because the 
rate of combustion is lower. Stationary water-tube boilers may 
have 2 to 3 times as much grate area and heating surface per 



150 



PUMPS AND PUMPING STATIONS 



horse-power as is found in locomotive boilers. If a poor type of 
fuel is to be used the area of the grate should be increased about 
inversely as the heat content of the fuel. The approximate heat 
content of various types of fuels is shown in Table 29. 

TABLE 29 
APPROXIMATE HEAT VALUE OF FUELS 



Fuel 


B.T.U. 
per 
Pound 


Pounds of Water 
Evaporated from 
and at 212 F. 
All heat utilized 


Anthracite 


13,500 


14 


Semi-bituminous, Pennsylvania 


15,000 


15 5 


Semi-bituminous, best, West Virginia 


15,000 


15 8 


Bituminous, best, Pennsylvania 


14,450 


15 


Bituminous, poor, Illinois 


10,500 


10 9 


Lignite, best, Utah 


11,000 


11 4 


Lignite, poor, Oregon 


8,500 


8 8 


Wood, best oak 


9,300 


9 6 


Wood, poor ash 


8,500 


8 8 









83. Air Ejectors. The Ansonia compressed-air sewage ejector 
is shown in Fig. 68. In its operation, sewage enters the reservoir 
through the inlet pipe at the right, the air displaced being expelled 
slowly through the air valve marked B. The rising sewage lifts 
the float which actuates the balanced piston valve in the pipe 
above the reservoir when the reservoir fills. The lifting of the 
valve admits compressed air to the reservoir. The air pressure 
closes valve A and the inlet valve at the right, and ejects the 
sewage through the discharge pipe at the left. As the float drops 
with the descending sewage it shuts off the air supply and opens 
the air exhaust through the small pipe at the top center. Sewage 
is prevented from flowing back into the reservoir by the check 
valve in the discharge pipe. Other ejectors operating on a similar 
principle are the Ellis, the Pacific, the Priestmann and the Shone. 

84. Electric Motors. The most common form of alternating 
current electric motor used for driving sewage pumps where con- 
tinuous operation and steady loads are met is the squirrel-cage 
polyphase induction motor. These motors operate at a nearly 



ELECTRIC MOTORS 



151 



constant speed which should be selected to develop the maximum 
efficiency of the pump and motor set. While Fig. 59 shows the 
best efficiency under varying heads to be obtained with variable 
speed, the advantages of cost, attention, and availability make 
the use of a constant speed motor common. 1 This type of motor 
is undesirable where stopping and starting are frequent because 
it has a relatively small starting torque and it requires a large 




FIG. 68. Ansonia Compressed-Air Sewage Ejector. 

starting current. Such motors can be constructed in small sizes 
for high starting torques by increasing the resistance of the rotor, 
but at the expense of the efficiency of operation. 

Alternating current motors are more generally used than direct- 
current motors because of the greater economy of transmission of 
alternating current, but where direct current is available constant 
speed shunt wound motors should be adopted. 

1 " The Economy Resulting from the Use of Variable Speed Induction 
Motors for Driving Centrifugal Pumps " by M. L. Enger and W. J. Putnam. 
Journal Am. Water Works Ass'n., 1920, Vol. 7, p. 536. 



152 PUMPS AND PUMPING STATIONS 

In the selection of a motor to drive a centrifugal pump it is 
important that the motor have not only the requisite power, but 
that its speed will develop the maximum efficiency from the pump 
and motor combined. If the pump and motor operate on the 
same shaft the speed of the two machines must be the same. If 
the two are belt connected, the size of the pulleys may be selected 
so as to give the required speed. If the motor is to be connected 
to a power pump an adequate automatic pressure relief valve 
should be provided on the discharge pipe from the pump, to pre- 
vent the overloading of the motor or bursting of the pump in case 
of a sudden stoppage in the pipe. The motor must be selected to 
suit the conditions of voltage, cycle, and phase on the line. Trans- 
formers are available to step the voltage up or down to practically 
any value. Rotary converters are used to change direct to alter- 
nating current or vice versa. 

85. Internal Combustion Engines. Internal combustion 
engines are used for driving pumps. Units are available in size 
from fractions of 1 horse-power to 2,000 horse-power or more, 
although the use of the larger sizes is exceptional. These engines 
are not commonly used for sewage pumping but when used they 
are ordinarily belt connected to a centrifugal pump, or to an 
electric generator which in turn drives electric motors which 
operate centrifugal pumps. This type of engine is more com- 
monly adapted to small loads, although not entirely confined to 
this field, as they serve admirably as emergency units to supple- 
ment an electrically equipped pumping station. The fuel effi- 
ciency of internal combustion engines is higher than for steam 
engines as is indicated in Table 30, but the fuel is more expensive. 

The four-cycle gas engine shown in Fig. 69 is the type most 
commonly used. Its horse-power is the product of: the mean 
effective pressure, the length of the stroke, the area of the piston, 
and the number of explosions per second divided by 550. The 
M.E.P. is dependent on the character of the fuel used and the 
compression of the gas before ignition. Producer gas will furnish 
mean effective pressures between 60 and 70 pounds per square 
inch, natural gas and gasoline, 85 to 90 pounds per square inch, 
and alcohol from 95 to 110 pounds per square inch. 

The Diesel Engine is the most efficient of internal combustion 
engines. The original aim of the inventor, Dr. Rudolph Diesel, 
was to avoid the explosive effect of the ordinary internal com- 



INTERNAL COMBUSTION ENGINES 



TABLE 30 
COMPARATIVE FUEL COSTS FOR PRIME MOVERS 






Type of Engine 


Quantity of Fuel 
per H.P. Hour 


Cost of Fuel 
in Cents per 
Horse-power 
Hour 


Reciprocating steam engines, simple, non- 
condensing, 25 to 200 H.P 


21 to 8 Ib. coal 


4.2to 1.6 


Triple condensing, 2000 to 10,000 H.P. . 


2. 3 to 1.91b. coal 


0.46 to 0.37 


Steam turbines, high pressure, non-con- 
densing, 
200 to 500 K.W 


6. 5 to 4. 2 Ib. coal 


1.3 to 0.86 


500 to 3000 K.W 


2. 6 to 1.9 Ib. coal 


0.52 to 0.37 


Condensing 5000 to 20,000 K.W. . . 


1 8 to 1 43 Ib coal 


36 to 0.28 








Gas engines 
Natural gas, 50 to 200 H.P 


19 to 11 cu. ft. 




Producer gas, 50 to 200 H P 


2 to 1 5 cu ft 




Illuminating gas, 10 to 75 H P . 


26 to 19 cu. ft 


2.1to 1.5 


Gasoline, 10 to 75 H P 


1 5 to 8 pints 


5.6to 3.0 








Oil engines, 100 to 500 H.P 


1 . 1 to 75 Ib oil 











NOTE. Coal assumed at $4.00 per ton, illuminating gas at 80 cents per thousand 
cubic feet, and gasoline at 30 cents per gallon. 




FIG. 69. Bessemer Oil Engine. 
Twin Cylinder, Valve Side. 



154 PUMPS AND PUMPING STATIONS 

bustion engine by injecting a fuel into air so highly compressed 
that its- heat would ignite the fuel, causing slow combustion of 
the fuel thus utilizing its energy to a greater extent. The fuel 
and air were to be so proportioned as to require no cooling. 
Although the ideal condition has not been attained, the heat 
efficiency of Diesel engines is high. They will consume from 
0.3 to 0.5 of a pound of oil (containing 18,000 B.T.U. per pound) 
per brake horse-power hour, giving an effective heat efficiency of 
25 to 30 per cent. Although not now in extensive use in the 
United States it is probable that this engine will be more generally 
adopted for conditions suitable for internal combustion engines. 

86. Selection of Pumping Machinery. Centrifugal pumps 
are particularly adapted to the lifting of sewage because of their 
large passages, and their lack of valves. The low lifts, nearly 
constant head, and the possibility of equalizing the load by 
means of reservoirs are particularly suited to efficient operation 
of centrifugal pumps. They require less floor space than recipro- 
cating pumps of the same capacity, and because of their freedom 
from vibration they do not demand so heavy a foundation. The 
discharge from the pump is continuous thus relieving the piping 
from vibration. In case of emergency the discharge valve can 
be shut off without shutting down the pump, an important point 
in " fool proof " operation. 

Volute pumps are better adapted to pumping sewage as their 
passages are more free and they are better suited to the low lifts 
met. Gritty and solid matter will cause wear on the diffusion 
vanes of turbine pumps in spite of the most careful design. 
Although turbine pumps can possibly be built with higher effi- 
ciency than volute pumps, their efficiency at part load falls rapidly 
and the fluctuations of sewage flow are sufficient to affect the 
economy of operation. Turbine pumps are more expensive and 
heavier than volute pumps on account of the increased size neces- 
sitated by the diffusion vanes. 

Multi-stage pumps are used for high lifts and are seldom if 
ever required in sewage pumping. As ordinarily manufactured, 
each stage is good for an additional 40 to 100 pounds pressure, 
but wide variations in the limiting pressures between stages are 
to be found. 

Reciprocating plunger pumps are sometimes used for sewage 
pumping where the character of the sewage is such that the 



SELECTION OF PUMPING MACHINERY 155 

valves will not be clogged nor parts of the pump corroded. These 
pumps are seldom used in small installations or for low lifts. 
They are not adapted to automatic or long distance control as 
are electrically driven centrifugal pumps. The use of recipro- 
cating pumps for sewage pumping is practically restricted to very 
large pumping stations with capacities in the neighborhood of 
50,000,000 gallons per day or more. Steam-driven pumps are 
the most common of the reciprocating type, but power pumps are 
sometimes used in special cases for small installations and may be 
driven by either a steam or gas engine or an electric motor. 

Compressed air ejectors, as described in Art. 83 are used for 
lifting sewage and other drainage from the basement of buildings 
below the sewer level. 

Centrifugal pumps electrically driven are, as a rule, the most 
satisfactory for sewage pumping. Electric drive lends itself to 
control by automatic devices, which are particularly convenient 
in small pumping stations. The control can be arranged so that 
the pump is operated only at full load and high efficiency, and 
when not operating no power is being consumed, as is not the 
case with a steam pump where steam pressure must be maintained 
at all times. The electric driven pump is thrown into operation 
by a float controlled switch which is closed when the reservoir 
fills, and opens when the pump has emptied the reservoir. The 
choice between steam and electric power for large pumping stations 
is a matter of relative reliability and economy. 

The selection of the proper type of pump, whether recipro- 
cating or otherwise, requires some experience in the consideration 
of the factors involved. Fig. 70 is of some assistance. In dis- 
cussing this figure, Chester states: 

" Fig. 70 attempts to represent graphically, the writer's 
ideas under general conditions, of the machines that should 
be selected for certain capacities for both principal engine 
and alternate and the station duty they may be expected 
to produce, but you must realize that this intends the 
principal engine doing at least 90 per cent of the work and 
that the head, the cost of coal, the load factor, the cost 
of real estate . . . the boiler pressure, and the space avail- 
able, and finally .'. . the funds available, are factors which 
may shift both the horizontal and curved lines. In the 
field of low service pumps of 10,000,000 capacity or over, 
the centrifugal pump reigns supreme, and for constant 



156 PUMPS AND PUMPING STATIONS 

low heads of 20,000,000 capacity or over the turbine driven 
centrifugal usurps the field." 

A reciprocating pump of any type would have to be specially 
built for pumping sewage not carefully screened or otherwise 
treated, as the valves, ordinarily used in such pumps for lifting 
water, would clog. The vertical triple-expansion pumping 
engine with special valves and for large installations, and the 
centrifugal pump for large or small installations are the only suit- 



+Direct-Actinq Triple Expansion./. 

'////A.-/ 't///*?-, I, 1 / *//// sv /,///. 




o 10 



1 2 3 456 7 8 9 10 11 ft 13 R 15 16 17 18 19 20 
Capacity in MiHions of Gallons per Day. 

FIG. 70. Expectancy Curves for Pumping Engines Working against a Pres- 
sure of 100 Pounds per Square Inch. 

J. N. Chester, Journal Am. Water Works Ass'n, Vol. 3, 1916, p. 493. 

able types for pumping sewage. With steam turbine or electric 
drive the centrifugal has the field to itself. 

87. Costs of Pumping Machinery. The cost of pumping 
machinery can not be stated accurately as the many factors 
involved vary with the fluctuations in the prices of raw materials, 
transportation, labor, etc. The actual purchase price of machinery 
can be found accurately only from the seller. The costs given in 
this chapter are useful principally for comparative purposes and 
for exercise in the making of estimates. The costs of complete 
pumping stations are shown in Table 3 1. 1 These figures repre- 
sent costs in 1911. 

1 C. A. Hague in Trans. Am. Society of Civil Engineers, Vol. 74, 1911, 
p. 20. 



COST COMPARISONS OF DIFFERENT DESIGNS 



157 



TABLE 31 

COSTS OF COMPLETE PUMPING STATIONS 

These costs include the best type of triple expansion engines, high-pressure 
boilers, brick or inexpensive stone building with slate roof, chimney and intake. 
Cost of land is not included. 



,b 


I* 


L 


y 


AS 

O U 


00 

- c 


L 


s| 


,b 


- s 
.-2 


L 


fei 

BJ 


ft* 


u * 


E* 


00 0) 




b * 


^ 


00 a) 


b 




* 


0) 


oS oT^ 

3 w 


bl 

iii 


t. Dolla 
orse-po 


<aO 

21 


o! ^^^ 
"S 3 C 1 


ll 


t, Dolla 
orse-po 


jgC3 


S v*~ 


la"? 

5. c & 

sll 


41 
Si 

^ o 


y 




S^o, 


H 


o"* 




~- ' 


B 


o*' 


. See 


SMM 


; ~ 




g 


X 








5 


s 


6 





Q 


B 





c3 


30 


12 


562 


6,750 


70 


28 


277 


7,750 


110 


44 


200 


8,750 


40 


16 


438 


7,000 


80 


32 


250 


8,000 


120 


48 


187 


9,000 


50 


20 


362 


7,250 


90 


36 


229 


8,250 


130 


52 


192 


10,000 


60 


24 


312 


7,500 


100 


40 


213 


8,500 











88. Cost Comparisons of Different Designs. In the design of 
a pumping station and its equipment the relative costs of different 
designs should be compared, and the least expensive design 
selected, due consideration being given to serviceability, reliability, 
and other factors without definite financial value. In comparing 
the costs of different types of machinery, all items in connection 
with the pumping station should be considered. For example, 
the cost of an electrically driven centrifugal pump and equipment 
may be less than the total cost of a steam driven reciprocating 
pump and equipment because of the saving in the cost of boilers, 
boiler house, etc., but a comparison of the capitalized cost of the 
two might show in favor of the reciprocating steam pump because 
of the lower cost of operation. 

The total cost of a plant, or any portion thereof, may be 
considered as made up of three parts: (1) The first cost, (2) opera- 
tion and maintenance and, (3) renewal. The total cost *S can be 
expressed as 



hi which 



C = the first cost; 

= the annual expenditure for operation and mainte- 

nance; 

R = the amount set aside to cover renewal ; 
r = the rate of interest. 



158 PUMPS AND PUMPING STATIONS 

S is called the capitalized cost of a plant. The annual payment 
necessary to perpetuate a plant is 



The value of R is useful when expressed in terms of the life of the 
plant or machine and the current rate of interest. It is sometimes 
called the depreciation factor or capitalized depreciation. If it 
is borne in mind that R is the amount to be set aside at compound 
interest for the life of the plant, at the end of which time the 
accrued interest should be sufficient to renew the plant, it is evi- 
dent that 



in which n is the period of usefulness, or life of the plant, expressed 
in years, no allowance being made for scrap value. 

A comparison of the annual expense of three different plants is 
shown in Table 32. It is evident from this comparison that the 
machinery with the least first cost is not always the least expen- 
sive when all items are considered. 

A sinking fund is a sum of money to which additions are made 
annually for the purpose of renewing a plant at the expiration of 
its period of usefulness. The annual payment into the sinking 
fund is equivalent to the term Rr in the expression for annual 
cost, or in terms of C, r, and n, the annual payment is 

Cr 
(l+r) n -r 

It is the same as the capitalized depreciation multiplied by the 

T 

rate of interest. The expression ,.._, \ n _i i g sometimes called 

the rate of depreciation. 

The present worth of a machine is the difference between its 
first cost and the present value of the sinking fund. If m repre- 
sents the present age of a plant in years, then the present worth is 



Where straight-line depreciation is spoken of it is assumed that 
the worth of a machine depreciates an equal part of its first cost 



COST COMPARISONS OF DIFFERENT DESIGNS 



159 



< 
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to r- C* C 
i-i IN 00 t^ C 


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160 



PUMPS AND PUMPING STATIONS 



each year. For example, if the life of a plant is assumed to be 
20 years, straight-line depreciation will assume that the plant 
loses ^ of its original value annually. The present worth of a 
plant under this assumption would be the product of its first cost 
and the ratio between its remaining life and its total life. This 
method of estimating depreciation and worth is frequently used, 
particularly for short-lived plants and for simplicity in book- 
keeping, but it is less logical than the method given above. 

89. Number and Capacity of Pumping Units. In order to 
select the number and capacity of pumping units for the best 
economy, a comparison of the costs of different combinations of 
units should be made and the most economical combination 
determined by trial. The principles outlined in the preceding 
articles should be observed in making these comparisons. In a 
steam pumping station, when the number of units operating is 
less than the average daily maximum for the period, steam must 
nevertheless be kept on a sufficient number of boilers to operate 
the maximum number of pumps. This, and corresponding 
standby losses must not be overlooked, as they may show that a 
smaller number of larger units is ultimately more economical. 

TABLE 33 

SUMMARY OF FLUCTUATIONS OF SEWAGE FLOW AT A PROPOSED 
PUMPING STATION 





o 








a 








d 








c 








c 








c 






SC 








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oi 






>> O 


09 






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0) 


V 


CS fa b 

Qgg 
-oO> 


3 ir, 

o o 


40 
B 


b 


5ji 




H oo GJ 


B 

V 


E 


fe-S 


a_o"3 




1 


QJ ^J G 


c o"S 


h 


O 

a 


S- 


3 o'S 




1 


o c3 O 




_e 


1 




=3_g 


.2 


A 


1 ^ ^ O 


.* j r; 


a 


i 


S^a 


ill 


* 


E 
o 


i^.s 




jg 


. o 


l^.s 


SoS 


4. 


o 


* 


E 




a 


Z 


E 


3 


B 


55 


E 


3 


n 


1 


293 


6.0 


450 


45 


51 


13.4 


173 


12 


29 


15.0 


ill 


8 


163 


8.6 


354 


41 


50 


13.5 


169 


8 


24 


15.6 


95 


15 


119 


10.0 


300 


30 


45 


13.8 


158 


5 


20 


16.0 


79 


18 


106 


10.6 


284 


28 


44 


13.9 


154 


3 


16 


16.5 


65 


23 


88 


11.2 


249 


23 


40 


14.2 


143 


2 


14 


16.8 


58 


31 


69 


12.2 


211 


21 


38 


14.4 


137 


1 


6.5 


18.0 


29 


32 


65 


12.4 


204 


18 


35 


14.6 


129 











Total horse-power days for one year, 102,000. 
Average load in horse-power, 280. 

For example, the sewage flow expected at a proposed pumping 
station is shown in Table 33. The steps involved in the selection 



NUMBER AND CAPACITY OF PUMPING UNITS 



161 



3 



a 

$ 



H 

2 





do 



o 



1 


.K.ii) I UO p.ij j 1 1:_ ) 

psoq JBJOJ, 


8SS3iiS2S3iSiS^3 89 


*Computed on the assumption that the pumps may be operated at 50 per cent overload for short periods, the rated capacity being equal to the 
oads given in Table 33. fFor description of type see note under Table 35. 


JBa.\ Ul 


- o<, Q cco- M -,oaoco^ ox5<N- 




200 Horse-power 
Typeot 


ooo'oi iwn 

UII.M is spunoj 


t-o "5 ;;;'.;;* 




2S 2isll^lii^S :::!:: | ' 


Ul pBO'J 


...... 


SSl22gilS2SS2 ;::::: : 


>H d H 
.M! HiiMis spunoj 










JO JU3O J3J 


1 -.o<Nfflwg-i<Nw-Hoogog :::::: ' 


' ..... 


I 

>. 

KH 
8 


spunoj 
OOO'OI 8iufl 
uiB*g spunoj 






Ul pBO'J 




'O C 1 ) O Oi C5 O O ' ' 


""OH d'H 
aad uiBdjg spunoj 


C^OKNC*W WW N - - 


JO JU3Q J3J 


sis- ;;;;;;;; ^ ; ; ; 


60 Horse-power 
Type It 


spunoj 
OOO'OI SIIUQ 
iHBaig spunoj 


XXW O IO...CO 


" N<NCO : : : : : :^ : : : 3 


jaMOd-as-ioji 
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*oci ^.<N *.::: 


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j.ni un:.( js spunoj 




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spunoj 
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>OOOOOO>O U5 . .10 10 


USO WONQ -H O5 




"nH d H 
jad in i; >is spunoj 


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jo ?uao joj 


-o^ow^- co ::::::: : 




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g-t- 
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spunoj 

OOO'Ot B !a 
uiBajg spunoj 




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ja.vvod-.jB.iou 
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M.I IIIK.HS spunoj 


in*in -tit) .10 -us-* _;" 


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222 "* : : : :2 : : : :2 : :*S - 



162 



PUMPS AND PUMPING STATIONS 



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^3 







NUMBER AND CAPACITY OF PUMPING UNITS 163 

of the number and capacity of pumping units to care for these 
quantities are as follows: (1) Determine the rated capacity of 
the equipment to be provided. In this case the capacity will be 
taken as 450 horse-power, which is the maximum load to be placed 
on the pumps. (2) Select any number of units of such different 
types and capacities as are available for comparison, and arrange 
them in different combinations so that each unit will operate as 
nearly as possible at its rated capacity. The work involved in 
such a study for 5 units is shown in Table 34. The weight of 
steam consumed per indicated horse-power hour corresponding 
to the per cent of the rated capacity at which the unit is operating 
is read from Fig. 64 or other data. (3) Repeat this step for other 
numbers and types of units. (4) Prepare a table showing the 
annual costs of combinations of different numbers and types of 
units as shown for this example in Table 35. The figures in Table 
35 show that the least expensive of the combinations of the units 
studied is one 200 horse-power unit, and one 250 horse-power 
unit, with a 250 horse-power unit in reserve. It is to be noted 
that a reserve unit has been provided in each combination, the 
capacity of which is equal to that of the largest unit of the com- 
bination. 



CHAPTER VIII 
MATERIALS FOR SEWERS 

90. Materials. The materials most commonly used for the 
manufacture of sewer pipe are vitrified clay and concrete. Cast 
iron, steel, and wood are also used, but only under special condi- 
tions. For pipes built in the trench, concrete, concrete blocks, 
brick, and vitrified clay blocks are used. Concrete is being used 
to-day more than bricks or blocks because it is cheaper. A decade 
or more ago all large sewers were built of bricks. Vitrified clay 
and concrete are used for manufactured pipe 42 inches and less in 
diameter. Concrete is used almost exclusively for larger sizes of 
pipe, particularly for pipe constructed in place, although a brick 
invert lining is advisable when high velocities of flow are expected. 

The character of the external load, the velocity of flow and the 
quality of sewage are important factors in determining the material 
to be used in the construction of sewers. Reinforced concrete 
should be used for large sewers near the surface subjected to 
heavy moving loads. A high velocity of flow with erosive sus- 
pended matter demand a brick wearing surface on the invert. 
Many engineers consider concrete less suitable than vitrified clay 
or brick for conveying septic sewage or acid industrial wastes, as 
concrete deteriorates more rapidly under such conditions. Con- 
crete should be used on soft yielding foundations, whereas a hard 
compact earth, which can be cut to the form of the sewer, is suit- 
able to the use of brick or concrete. 

Cast-iron pipe with lead joints is used for sewers flowing under 
pressure, or where movements of the soil are to be expected. If 
the sewage is not flowing under pressure, cement joints are some- 
times used in the cast-iron pipe. Movements of the soil are to 
be expected on side hills, under railroad tracks, etc. Steel pipe 
is used on long outfalls or under other conditions where external 
loads are light and the cost is less than for other materials. 
Because of the thin plates used and the liability to corrosion steel 
is not frequently used, It should never be deeply buried nor 

164 



VITRIFIED CLAY PIPE 



165 



externally loaded because of its weakness in resisting such forces. 
Like wood pipe, its lightness is favorable to use on bridges, but the 
greater heat conductivity of steel than wood necessitates protection 
against freezing in exposed positions. Wood is preferable only where 
the economy of its use is pronounced and the pipe is running full 
at all times. It is desirable that the wood pipe should be always 
submerged as the life of alternately wet and dry wood is short. 

Corrugated galvanized iron and unglazed tile have been used for 
sewers, but usually only in emergencies or as a make-shift. Corru- 
gated iron is not suitable on account of its roughness and liability 
to corrosion, and unglazed tile because of its lack of strength. 

91. Vitrified Clay Pipe. In general the physical and chemical 
qualities of clays before burning are not sufficient to cause their 
condemnation or approval by 
the engineer, as their behavior in 
the furnace is quite individual 
and depends greatly on the man- 
ner in which they are fired. The 
engineer is interested in the re- 
sult and writes his specifications 
accordingly. 

In the manufacture of clay 
pipe,the clay as excavated is taken 
to a mill and ground while dry, to 
as fine a condition as possible. It 
is then sent to storage bins from 
which it is taken for wet grind- 
ing and tempering. In this proc- 
ess the clay is mixed with water 
to the proper degree of plasti- 
ity. A variation of 1 to 1$ per 
cent in the moisture content will 
mean failure. Too wet a mix- 
ture will not have sufficient 
strength to maintain its shape 

in the kiln. Too dry a mixture 
.... ... . FIG. 71. Diagrammatic Section 

will show laminations as it is through Clay-pipe Press, 

pressed through the discs. 

A press used in the manufacture of clay pipe is shown in 
cross-section in Fig. 71. With the piston heads in the steam and 




166 



MATERIALS FOR SEWERS 



mud cylinders at their extreme upward positions, the mud cylinder 
is filled with clay of the proper consistency. Steam is then turned 
into the steam cylinder under pressure and the clay is squeezed 
into the space between the inner and outer shells of the die and 
mandrel to form the hub of the pipe. The pressure on the clay 
may be from 250 to 600 pounds per square inch. When clay 
appears at the holes, marked hh at the bottom of the mud cylinder, 




FIG. 72. Clay-pipe Press. 

Courtesy, Blackmer and Post Manufacturing Co. 

the bottom plate and the center portion of the die are removed 
and the remainder of straight portion of the pipe is formed by 
squeezing the clay between the mandrel and the outer wall of the 
die. A completely formed pipe can be seen issuing from the press 
in Fig. 72. Any sized pipe that is desired can be formed from the 
same press by changing the size of the dies and mandrel. 

Curved pipes are made in two ways by bending directly as 
they issue from the press, or by shaping by hand in plaster of 
paris molds. Junctions are made by cutting the branch pipe to 
the shape of the outside of the main pipe, fastening the branch 



VITRIFIED CLAY PIPE 167 

in place with soft clay and then cutting out the wall of the main 
pipe the size of the branch. Special fittings are usually made by 
hand in plaster molds. 

After being pressed into shape the pipes are taken to a steam- 
heated drying-room where a constant temperature is maintained 
in order to prevent cracking of the pipes. They remain in the 
drying room from 3 to 10 days until diy, when they are taken to 
the kilns. If taken to the kilns when moist blisters will be pro- 
duced. 

The dried pipes are piled carefully in the kiln so that heat and 
weight may be as evenly distributed as possible, and the fire is 
then started in the kiln. The process of burning can be roughly 
divided into five stages: 

1st. Water smoking, which lasts about 72 hours during which 
the temperature is raised gradually to 350 degrees Fahrenheit. 

2nd. Heating, during which the temperature is raised to 800 
degrees Fahrenheit in 24 hours. 

3rd. Oxidation, during which the temperature is raised to 
1,400 degrees Fahrenheit in 84 hours. 

4th. Vitrification, in which the temperature is raised to 2,100 
degrees Fahrenheit in 48 hours, and finally, 

5th. Glazing, during which the temperature is unchanged but 
salt (NaCl) is thrown in and allowed to burn. 

Oxidation must be complete before vitrification is started as 
otherwise blisters will be raised due to imprisoned carbon dioxide. 
The important points in vitrification are to make the required 
temperature within a reasonable time and to maintain a uniform 
distribution of heat throughout the kiln. When vitrification is 
complete as shown by a glassy fracture of a broken sample taken 
from the kiln, glazing is accomplished by throwing a shovelful of 
salt on the hottest part of the fire. About five to six applications 
of salt from two to three hours apart may be needed. The kiln 
is then allowed to cool and the manufacture of the pipe is com- 
plete. The completeness of vitrification is indicated by the 
amount of water that the finished pipe will absorb. Completely 
vitrified pipe will absorb no moisture. Soft-burned pipe may 
absorb as much as 15 per cent moisture. 

Vitrified clay blocks are made of the same material and in the 
same manner as vitrified clay pipe. 

The following data on vitrified pipe have been abstracted from 



168 MATERIALS FOR SEWERS 

the specifications for vitrified pipe adopted by the American 
Society for Testing Materials. 

Pipes shall be subject to rejection on account of the following: 

(a) Variation in any dimension exceeding the per- 
missible variations given in Table 36. 

(6) Fracture or cracks passing through the shell or 
hub, except that a single crack at either end of a pipe not 
exceeding 2 inches in length or a single fracture in the hub 
not exceeding 3 inches in width nor 2 inches in length will 
not be deemed cause for rejection unless these defects 
exist in more than 5 per cent of -the entire shipment or 
delivery. 

(c) Blisters or where the glazing is broken or which 
exceed 3 inches in diameter, or which project more than 
| inch above the surface. 

(d) Laminations which indicate extended voids in the 
pipe material. 

(e) Fire cracks or hair cracks sufficient to impair the 
strength, durability or serviceability of the pipe. 

(/) Variations of more than inch per linear foot in 
alignment of a pipe intended to be straight. 

(0) Glaze which does not fully cover and protect all 
parts of the shell and ends except those exempted in Sect. 
31. Also glaze which is not equal to best salt glaze. 

(h) Failure to give a clear ringing sound when placed 
on end and dry tapped with a light hammer. 

(1) Insecure attachment of branches or spurs. 

Workmanship and Finish 

(29) Pipes shall be substantially free from fractures, 
large or deep cracks and blisters, laminations and surface 
roughness. 

(31) The glaze shall consist of a continuous layer of 
bright or semi-bright glass substantially free from coarse 
blisters and pimples. . . . Not more than 10 per cent 
of the inner' surface of any pipe barrel shall be bare of 
glaze except the hub, where it may be entirely absent. 
Glazing will not be required on the outer surface of the 
barrel at the spigot end for a distance from the end equal 
to | the specified depth of the socket for the corresponding 
size of pipe. Where glazing is required there shall be 
absence of any well defined network of crazing lines or 
hair cracks. 

(32) The ends of the pipe shall be square with their 
longitudinal axis. 

(33) Special shapes shall have a plain spigot end and 



VITRIFIED CLAY PIPE 



169 



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3 



I 



S OF CLAY S 
ons of the Am 



PROPER 
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170 



MATERIALS FOR SEWERS 



a hub end corresponding in all respects with the dimensions 
specified for pipes of the corresponding internal diameter. 

(a) Slants shall have their spigot ends cut at "an angle 
of approximately 45 degrees with the longitudinal axis. 



Decreaser 1-8 Curve Increaser 




Double T Branch Running Trap with- 
out Hand Hole 



Breeches 




Straight Pipe 

Running Trap with Double Y Branch 
. Side Hand Hoie 

FIG. 73. Standard Clay Pipe Specials. 

Courtesy, Blaekmer and Post Manufacturing Co. 

(6) Curves shall be at angles of 90, 45, 22fc and 11J 
degrees as required. They shall conform substantially to 
the curvature specified. 

(c) ... All branches shall terminate in sockets. 

In Fig. 73 are shown the various forms of vitrified pipe and 
specials which are ordinarily available on the market. 



VITRIFIED CLAY PIPE 



171 



The life of vitrified clay sewers and some observations on the 
results of the inspection of the sewers in Manhattan are discussed 
in Chapter XII. The strength of vitrified sewer pipes is shown 
in Table 37. 



TABLE 37 
STRENGTH OF SEWER PIPE 

Strength in pounds per linear foot to carry loads from ditch filling material 
such as ordinary sand and thoroughly wet clay, with the under side of the 
pipe bedded 60 to 90 by ordinary good methods. From Proc. Am. Society 
for Testing Materials, Vol. 20, 1920, page 604. 





Breadth of the Ditch a Little Below the Top of the Pipe 


Height 












of Fill 


1 Foot 


2 Feet 


3 Feet 


4 Feet 


5 Feet 


Above 












Top of 
Pipe, 


Ditch Filling Material 


Feet 
























sand 


clay 


sand 


clay 


sand 


clay 


sand 


clay 


sand 


clay 


2 


265 


280 


615 


635 


970 


990 


1330 


1,350 


1,690 


1,710 


4 


400 


450 


1055 


1125 


1745 


1825 


2455 


2,535 


3,165 


3,250 


6 


470 


545 


1370 


1500 


2370 


2525 


3405 


3,575 


4,460 


4,740 


8 


505 


605 


1600 


1790 


2875 


3115 


4215 


4,495 


5,595 


5,890 


10 


525 


640 


1765 


2015 


3275 


3610 


4900 


5,295 


6,590 


7,020 


12 


535 


660 


1880 


2185 


3600 


4030 


5485 


6,000 


7,460 


8,035 


14 


540 


675 


1965 


2320 


3855 


4380 


5975 


6,620 


8,225 


8,950 


16 


545 


680 


2025 


2425 


4065 


4675 


6395 


7,165 


8,890 


9,775 


18 


545 


685 


2070 


2505 


4230 


4920 


6750 


7,630 


9,480 


10,520 


20 


545 


690 


2100 


2565 


4365 


5130 


7050 


8,060 


9,995 


11,190 


22 


545 


690 


2125 


2610 


4470 


5305 


7305 


8,425 


10,445 


11,795 


24 


545 


690 


2140 


2645 


4560 


5445 


7525 


8,750 


10,840 


12,340 


26 


545 


690 


2150 


2675 


4630 


5575 


7705 


9,035 


11,185 


12,830 


28 


545 


690 


2160 


2695 


4685 


5680 


7860 


9,280 


11,490 


13,270 


30 


545 


690 


2165 


2715 


4725 


5765 


7990 


9,500 


11,755 


13,670 


Very great 


545 


690 


2180 


2770 


4910 


6230 


8725 


11,075 


13,635 


17,305 



92. Cement and Concrete Pipe. Although there is no general 
recognition of a difference between cement and concrete pipe, 
there is a tendency to term manufactured pipe of small diameter 
cement pipe, and large pipes or pipes constructed in place, con- 
crete pipe. Cement, unlike clay, is used in the manufacture of 



172 MATERIALS FOR SEWERS 

pipe in the field or by more or less unskilled operators in " one 
man " plants. Great care should be used in the selection of 
cement, aggregate, and reinforcement for precast cement pipe 
since the shocks to which it is subjected in transit are more liable 
to rupture it than the heavier but steadier loads imposed on it in 
the trench. 

The United States Government, various scientific and engi- 
neering societies, and other interested organizations have col- 
laborated in the preparation of specifications for cement and 
cement tests. These specifications can be found in Trans. Am. 
Soc. Civil Engineers, Vol. 82, 1918, p. 166, and in other publica- 
tions. 

The following abstracts have been taken from the proposed 
tentative specifications for Concrete Aggregates, of the Am. 
Society for Testing Materials, issued June 21, 1921: 

1. Fine aggregate shall consist of sand, stone screen- 
ings, or other inert materials with similar characteristics, 
or a combination thereof, having clean, hard, strong, 
durable uncoated grains, free from injurious amounts of 
dust, lumps, soft or flaky particles, shale, alkali, organic 
matter, loam or other deleterious substances. 

2. Fine aggregates shall preferably be graded from 
fine to coarse, with the coarser particles predominating, 
within the following limits: 

Passing No. 4 sieve 100 per cent 

Passing No. 50 sieve, not more than 50 per cent 

Weight removed by elutrition test, not more 

than 3 per cent 

Sieves shall conform to the U. S. Bureau of Standards 
specifications for sieves. 

3. The fine aggregate shall be tested in combination 
with the coarse aggregate and the cement with which it 
is to be used and in the proportions, including water, in 
which they are to be used on the work, in accordance with 
the requirements specified in Section 6. ... 

7. Coarse aggregate shall consist of crushed stone, 
gravel or other approved inert materials with similar 
characteristics, or a combination thereof, having clean, 
hard, strong, durable, uncoated pieces free from injurious 
amounts of soft, friable, thin, elongated or laminated pieces, 
alkali, organic or other deleterious matter. 

The following Table indicates desirable gradings, in per- 
centages, for coarse aggregate for certain maximum sizes. 






CEMENT AND CONCRETE PIPE 
OF COARSE AGGREGATES 



173 



Maximum 
Size of 
Aggregate 
Inches 


Circular Openings, Inches 


Passing Screen Hav- 
ing Circular Open- 
ings J Inch in diam- 
eter, not more than 


3 


2i 


2 


1* 


U 


1 


1 


i 


3 
2* 
2 
U 

H 

l 

a 


100 




40-75 










15 per cent 
15 per cent 
15 per cent 
15 per cent 
15 per cent 
15 per cent 
15 per cent 


100 




40-75 












100 




40-75 












100 




40-75 










100 




35-70 
40-75 










100 














100 

















The manufaeture of small size cement pipe requires relatively 
more skill than equipment. As a result great care must be 
observed in the inspection of cement pipe and in the enforcement 
of specifications. For large size concrete pipe and reinforced 
concrete pipe the difficulty of holding the pipe together during 
transportation and lowering into the trench aid in insuring a good 
product. 

Cement pipe is made by ramming a mixture of cement, sand, 
and water into a cylindrical mold and allowing it to stand until set. 
The mold is then removed and the pipe stands for a further period 
of time to become cured. The selection and proportion of 
materials, the amount of water, the method of ramming, the 
period of setting, the length of time of curing, and the control of 
moisture and temperature during this period are of great impor- 
tance in the resulting product. E. S. Hanson l states that the 
most conservative engineers recommend a mixture of one sack of 
cement to 1\ cubic feet of aggregate measured as loosely thrown 
into the measuring box. In making up the aggregate, clean gravel 
or broken stone up to \ inch in size is used. The American Con- 
crete Institute recommends that 100 per cent pass a ^-inch screen, 
70 per cent a j-inch screen, 50 per cent a No. 10, 40 per cent a 
No. 20, 30 per cent a No. 30, and 20 per cent a No. 40. The 
materials should be carefully graded by experiment and not 
guessed at, as the behavior of all aggregates is not the same. 
Too coarse an aggregate is difficult to handle in manufacturing. 
1 Proceedings Illinois Society of Engineers, 1916, page 81. 



174 



MATERIALS FOR SEWERS 




It causes loss of pipe when the jacket or mold is removed and 
results in rough pipe, stone pockets, and pin holes through which 
water spurts when pressure tests are applied. Too fine an aggre- 
gate causes loss of strength and with ordinary mixtures tends to 
produce a pipe which will show seepage under internal pressure 
tests. The amount of water in the mixture will vary, from 15 to 
20 per cent. The mixture should appear dry but should ball in 
the hand under some pressure. 

The mixture can be rammed into the molds by hand or machine. 
A machine-made pipe is preferable as it produces a more even and 
stronger product. There are two types of machines for this 
purpose. One type consists of a number of tamping feet which 
deliver about 200 blows to the minute with a pressure of about 
800 pounds per square inch of area exposed. In the other type a 
revolving core is drawn through the pipe, packing and polishing 
the concrete as it is pulled through, with special provision for 

packing the bell of the pipe. 
The tamping machines can 
make 1,500 feet of small size 
pipe to 300 feet of 24-inch 
pipe in a day. Machines of 
the second type can make 750 
feet of 8-inch to 200 feet of 
30-inch pipe in 30-inch lengths 
in 9 hours. The inside and 
outside forms for a 24-inch 
pipe are shown in Fig. 74 as 
used with the tamping ma- 
chines. The forms are swab- 
bed with oil before being filled 

m in order to facilitate their 

Elevation. removal. In making a Y- 

branch or other special, a 
hole is cut in the pipe or 

Bottom view. Bottom view. mold the size of the joining 

FIG. 74. Details of 24-Inch Concrete pipe which is then set in place 
Pipe Form. an d the joint wiped smooth 

with cement. 

After the removal of the mold the pipe may be cured by the 
water or the steam process. Hanson states: 




Inside 
Form 




\ 


Top View. 
.- -3-4- -| 




TTT1 


1 


< 2 ^-_> 




Outside 





Form. 




=.-_-_^=.-=.-= S 




H -f 11 . i 

II fl! li II) ! !. 




CEMENT AND CONCRETE PIPE 



175 



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176 MATERIALS FOR SEWERS 

By the former the pipe are simply set on the floor of 
the plant and as soon as they are sufficiently strong so 
that they can be sprinkled with water without falling 
down; sprinkling is commenced and continued at such 
intervals for 6 or 7 days that the pipe will be moist at all 
times. This is a slower process than steam curing. It is 
also less uniform and less subject to control than where 
the product is cured by steam. 

In the steam process the pipe is exposed to low-pressure steam 
with plenty of moisture in a closed receptacle for 24 hours, or 
until hardened. It has been found by tests that pipes sprinkled 
for 28 days are as strong as steam-cured pipes. 

The dimensions of cement concrete sewer pipe as recommended 
by the Am. Society for Testing Materials are shown in Table 38. 

The following has been abstracted from the description of the 
manufacture of one form of concrete pipe by G. C. Bartram. 1 
All pipe are manufactured in 4-foot lengths near the site at which 
they are to be installed because of their great weight, for example, 
36-inch pipe weighs one ton. The plant for the manufacture of 
the pipe consists of cast-iron bottom and top rings for each size 
to be used on the job, and inside and outside steel casings. 
There are three bases for each steel casing as the pipes stand on 
the bases for 72 hours and the steel casing remains on for only 
24 hours after the concrete has been poured. The pipes are then 
lifted off the bases and stored for aging. The pipes are cast with 
the spigot end up. 

The concrete is ordinarily mixed in the proportions of 1 : 2 : 4. 
The materials are placed in the mixer in the following order: 
first, the stone, then the sand, then the cement, and finally the 
water. Sufficient water is added to make the concrete flow freely. 
In cold weather or for a hurry-up job the molds are covered with 
canvas and are steamed for 2 or 3 hours immediately after the 
concrete is poured. The molds are then removed but the pipe 
should be steamed before use. Otherwise they are allowed to 
stand 72 hours, as explained above. In cold weather the steam is 
used to prevent freezing and not to hasten the completion of the 
pipe. 

One layer or ring of reinforcement is used for sizes from 24 to 
48 inches and two layers or rings for larger pipe. A type of rein- 

1 Municipal Engineers' Journal for April, 1918. 



CKMFAT AND CONCRKTK PIPK 



177 



forcement sometimes used is the American Steel and Wire Com- 
pany's Triangular Mesh, an illustration of which is shown in 
Fig. 75. The wire mesh is cut to fit and is placed in a slot in the 
cast-iron base. The slot is then filled with sand so that the con- 




t'AMesft /i* 7 ~ 8 ~ 

Style *ZZ- 
Section B-B Section A-A. 

FIG. 75. Triangle Mesh Reinforced Concrete Pipe. 

As made by the Am. Concrete Pipe and Pile Co., Chicago. 

w v_ 

/Mortar 




Tongue and Groove 
Joint 



Lock Joint, 
Patented 



Hub and Spigot Joint. 

A Three Types of Joints Commonly Used m Reinforced Concrete Pipe 
Sewer Construction 






Single Line Double Line Elliptical 

Reinforcing Reinforcing Reinforcing. 

B Three Methods of Reinforcing Concrete Pipe 

FIG. 76. Methods of Joining and Reinforcing Concrete Pipe. 

Crete cannot enter, thus leaving a portion of the reinforcement 
exposed. The inside reinforcement extends through and out of 
the spigot of the completed pipe. In the trench the two rein- 
forcements overlap in the key-shaped space left on the inside of 



178 



MATERIALS FOR SEWERS 



the pipe by the design of the bell and spigot. This space is shown 
in Fig. 76 A. When the pipe is placed in the trench the key-shaped 
space is plastered with mortar and a piece is knocked out of the 
bell to receive the grout with which the joint is closed. A spring 
steel band is then put on the outside of the joint and grout poured 
into the hole at the top. The band is removed as soon as the 
joint materials have set. 

The rules for the reinforcement of concrete pipe recommended 
in Volume XV, 1919, of the Transactions of the Concrete Insti- 
tute are as follows: 

No reinforcement is approved for pipe between 30 and 
60 inches in diameter or in rock or hard soils. For pipe 
36 inches in diameter or less the minimum thickness of 
shell shall be 5 inches. For 60-inch pipe the minimum 
thickness shall be 7 inches with intermediate sizes in pro- 
portion. Reinforcement for circular pipe shall consist of 
one or two rings of circular wire fabric or rods of the areas 
shown in Table 39. All sewers near the surface and sub- 
ject to vibration should be reinforced. For sewers 6 feet 
or less in diameter the reinforcement should consist of 
at least \ of 1 per cent of the area of the concrete. It 
should be placed near the inside at the crown and near 

TABLE 39 

REINFORCEMENT FOR CIRCULAR CONCRETE SEWER PIPE 
(See Vol. XV, Proceedings Am. Concrete Institute) 





Mini- 




Cross 




Mini- 




Cross 


Diam- 
eter 


mum 
Thick- 


Number 


Sec- 
tional 


Diam- 
eter 


mum 
Thick- 


Number 


Sec- 
tional 


in 
Inches 


ness 
of Shell 


of Rings 


Area of 
Each 


in 
Inches 


ness 
of Shell 


of Rings 


Area of 
Each 




in 
Inches 




Ring 




in 
Inches 




Ring 


24 


3 


1 


.058 


48 ' 


5 


2 


.107 


27 


3 


1 


.068 


54 


6J 


2 


.123 


30 


3* 


1 


.080 


60 


6 


2 


.146 


33 


4 


1 


.107 


66 


6| 


2 


.168 


36 


4 


1 


.146 


72 


7 


2 


.180 


39 


4 


1 


.146 


84 


8 


2 


.208 


42 


4* 


1 


.153 


96 V 


9 


2 


.245 



PROPORTIONING OF CONCRETE 179 

the outside at the haunches. If large horizontal pressures 
are expected the pipe should be reinforced for these reverse 
stresses, which involves placing the reinforcement near 
the outside at the crown and near the inside at the 
haunches. The minimum thickness of the walls of sewers 
greater than 6 feet in diameter with flat bottom and arch, 
with or without side walls, should be 8 inches. 

Three methods for the reinforcement of concrete sewers are shown 
in Fig. 76 B. 

93. Proportioning of Concrete. In the proportioning of con- 
crete questions of strength, of permeability, and of workability 1 
may need consideration. All of these qualities are affected by 
the amount of cement, the nature and gradation and relative 
proportions of the fine and the coarse aggregate, and the amount 
of mixing water used. 

Other things being equal the strength varies with the amount 
of cement put into the concrete. For the same amount of cement 
and the same consistency of the mixture, the strength increases 
with increased density of concrete (that is, with decreased voids), 
and the effort should be made so to proportion the fine and coarse 
aggregates as to produce the densest concrete (least voids) with 
the aggregates available. For the same consistency, the strength 
then will vary with the ratio of the amount of cement to the 
amount of the voids. 

So far as the mixing water is concerned, the greatest strength 
in the concrete will be attained at a rather dry mix; that which 
produces the least volume of concrete. The addition of more 
water results in a concrete of less strength; 40 per cent more 
water may give a concrete of less than half the normal strength. 
The reduction in strength is then very marked for the wetter 
mixes, and the water content used is a feature of considerable 
importance in the design of concrete mixtures. 

Permeability is affected by the same elements as strength, 
but the size and discontinuity of the pores have a greater influence. 

Workability is an important quality; in some respects it will 
have to be obtained at the expense of strength. Increasing the 
amount of mixing water increases the workability of the mixtures, 
with a resulting decrease in strength which may have to be 
accepted or else overcome by increasing the cement in the mix. 

1 Workability involves ease in placing and smoothness of working. 



180 MATERIALS FOR SEWERS 

An excess of water is often used unnecessarily through ignorance 
of the injurious results. A high- proportion of coarse aggregate, 
up to a certain limit, will give concrete of high strength, but the 
mixture will be harsh-working and not easy to place. Lower 
proportions of coarse aggregate will give greater workability and 
better uniformity of product, the latter being an important 
matter. It is apparent that the degree of workability of the mix- 
ture needed will depend upon the nature of the construction for 
a pavement where the concrete will receive substantial tamping 
or working the water content may be much less than that which 
may need to be used in placing concrete around reinforcement in 
narrow members, or where little tamping or spading can be done. 
The nature of the work will affect the standard of consistency to be 
specified. 

The proportioning of the concrete should then be dependent 
upon the needs of the structure and the manner of placing the 
concrete. The proportions selected should be carefully adhered 
to and especially should care be taken to see that the right quan- 
tity of mixing water is used. 

The materials are commonly measured volumetrically (by 
bulk). Because of the variations which are introduced by volu- 
metric measurement of the materials by the presence of varying 
degrees of moisture, measurements by weight would be more 
accurate, but these would also be affected by differences in the 
specific gravity of the materials. The methods of measuring, 
the allowance for moisture, as well as the proportions of the 
materials, should be specified. 

The methods for proportioning concrete are: 

(1) Arbitrarily selected proportions. 

(2) Proportions based on minimum voids. 

(3) Proportions based on trial mixtures. 

(4) Proportions based on a sieve analysis curve. 

(5) Proportions based on the surface area of the aggre- 
gates. 

(6) Proportions based on the water-cement ratio and 
the fineness modulus. 

(7) Proportions based on mortar-voids and cement- 
voids ratio. 

Arbitrarily selected proportions are in quite general use; 
they are intended to apply to the materials most commonly used 



PROPORTIONING OF CONCRETE 181 

in the vicinity of the work. The most common practice is to 
use twice as great a volume of coarse aggregate as fine aggregate, 
as for instance 1 part cement, 2 parts fine aggregate, and 4 parts 
coarse aggregate. Decreasing the ratio of coarse aggregate to 
fine aggregate may give a more easily worked mix or require 
relatively less water for a given workability, and in some cases it 
will be proper to increase tnis ratio and thus secure an increase of 
strength. Judgment and experience with given materials may 
warrant changes from a stated ratio. The proportions are now 
frequently given as one part cement to a certain number of parts 
of the mixed aggregate, leaving the proportions of the fine to 
coarse to be determined otherwise, since small variations in the 
relation of these will not greatly affect the strength. Proportions 
in common use are: 1 

Mortar for 

Laying brick and stone masonry from 1:0 to 1 : 3 

Filling joints in sewer pipe 1:0 to 1 : 2 

Surfaces, floors, sidewalks, pavements . . 1 : to 1 : 2 

Waterproof linings 1:0 tol:2 

Cement, bricks, and blocks 1 : 2 to 1 : 4 

Concrete for 

Gravity retaining walls, heavy founda- 
tions, structures needing mass more 

than strength from 1:3:6 to 1 : 4 : 8 

Retaining walls, piers, sewers, pave- 
ments, foundations, and work requir- 
ing strength. (Compressive strength 
in 28 days, 1,500 to 2,000 pounds per 

square inch) from 1:2:^ to 1 : 3 : 6 

Floors, beams, pavements, reinforced 
concrete, arch bridges, low-pressure 
tanks. (Compressive strength in 28 
days, 2,000 to 3,000 pounds per square 

inch) from 1 : 1 : 3 to 1 : 2 : 4 

Reinforced concrete columns, conduit 
pipe, impervious concrete. (Com- 
pressive strength in 28 days, 3,000 to 
4,000 pounds per square inch) . . .from 1:1:2 to 1 : 1 : 3 

The usual method of proportioning based on minimum voids 

is to assume that the particles of fine aggregate should fill the 

voids in the coarse aggregate and that the particles of the cement 

will fill the voids in the fine aggregate. About 5 to 10 per cent 

1 Johnson's Materials of Construction, 5th Edition, 1918, p. 432. 



182 



MATERIALS FOR SEWERS 



additional fine aggregate is generally added to push the particles 
of the coarse aggregate apart and thus give a more easily worked 
concrete and one freer from void spaces. This method is inaccu- 
rate, principally because of the effect of the moisture on the volume 
of the voids, and because the effect on the volume by the addi- 
tion of water is unknown. 

Trial mixtures may be made by carefully weighing each of the 
ingredients and then combining them to give a workable concrete. 
Using a given amount of cement, the proportion of ingredients, of 
the same total weight, which will give the least volume and there- 
fore the densest concrete is adopted. When making the compari- 
son the consistency of the mixes must be maintained constant. 

Proportioning may be based on an ideal sieve analysis curve 
of the mixed cement and aggregates. The sieve analysis of the 
aggregates is made by screening a predetermined weight of the 
sample through a series of 5 to 8 sieves graded in size from slightly 
below the size of the largest particle to slightly above the smallest 
particle of the aggregate. The analysis is then expressed in the 
form of a curve. The ideal curve, according to Fuller, 1 is shown 
in Fig. 77. 
100 




0.25 0.50 0.75 1.00 1.25 1.50 

Diameter of Particle in Inches. 

FIG. 77. Gravel Analysis. 

The dotted line indicates the ideal combination of the coarse and fine portions. The heavy 
full line indicates the combination attained. 

1 Trans. Am. Society of Civil Engineers, Vol. 59, 1907, p. 146. 



WATERPROOFING CONCRETE 183 

The method of proportioning concrete by surface areas is 
based on the theory that the strength of a concrete depends on the 
amount of cement used in proportion to the surface area of the 
aggregates. 1 

The proportioning of concrete on the basis of a water-cement 
ratio and a fineness modulus was introduced by Prof. D. A. 
Abrams. 2 It is based on the theory that with fixed conditions 
of aggi'egate, moisture, etc., the ratio of water to cement deter- 
mines the strength of the concrete. 

A method of proportioning concrete by determining experi- 
mentally the voids in mortars made up with a given amount of 
sand and definite proportions of cement, and then calculating 
the voids in the concrete made up by adding a definite amount 
of coarse aggregate to the mixture, has been developed. 3 Ibc 
method is based on the theory that the strength of the concrete 
is a known function of the ratio of the volume of cement to the 
volume of the voids in the concrete. The effect of varying the 
proportion of the ingredients, including an increase in the amount 
of mixing water beyond that required to give the densest mixture, 
may be found by the method, and a comparison may be made of 
results obtainable with different classes of fine and coarse aggre- 
gates. 

Arbitrarily selected proportions, proportions based on voids, 
and proportions based on trial mixtures are usually satisfactory 
for small jobs where the amount of materials involved is not large. 
Where the saving in materials will permit, more accurate methods 
should be used. The methods can be studied more fully by 
reference to the original articles quoted in the footnotes, or to the 
following texts: 

Materials of Construction, Johnson, 5th Edition, 1918. 
Materials of Engineering, H. F. Moore, 2d Edition, 1920. 
Masonry Construction, I. O. Baker, 10th Edition, 1912. 
Concrete Engineer's Handbook, Hool and Johnson, 1918. 
Concrete, Plain and Reinforced, Taylor and Thompson, 1916. 

1 L. N. Edwards, Trans. Am. Society Testing Materials, 1918, and R. B. 
Young, Eng. News-Record, Vol. 82, 1919, p. 33. 

1 Bulletin No. 1, Structural Materials Research Laboratory, Lewis Insti- 
tute, Chicago, Illinois. 

3 Proportioning Concrete by Voids in the Mortar, A. N. Talbot, read 
before Am. Society Testing Materials, June 22, 1921. Abstract in Eng. 
News-Record, Vol. 87, 1921, p. 147. 



184 MATERIALS FOR SEWERS 

94. Waterproofing Concrete. The waterproofing of concrete' 
is most satisfactorily done by making dense mixtures. In practice 
such substances as hydrated lime, clay, alum and soap, and pro- 
prietary compounds such as Ceresit, Medusa, etc., are frequently 
mixed with the concrete under the theory that these very fine 
substances will fill any remaining voids and render the concrete 
impervious. The specifications of the Joint Committee issued on 
June 4, 1921, are much briefer and contain less detailed instruc- 
tion than those issued earlier. 1 The earlier instructions follow. 

Many expedients have been resorted to for making 
concrete impervious to water. Experience shows, however, 
that when mortar or concrete is proportioned to obtain 
the greatest practicable density and is mixed to the proper 
consistency, the resulting mortar or concrete is impervious 
under moderate pressure. 

On the other hand concrete of dry consistency is more 
or less pervious to water, and, though compounds of vari- 
ous kinds have been mixed with the concrete or applied 
as a wash to the surface, in an effort to offset this defect, 
these expedients have generally been disappointing, for 
the reason that many of these compounds have at best 
but temporary value, and in time lose their power of impart- 
ing impermeability to the concrete. 

In the case of subways, long retaining walls, and reser- 
voirs, provided the concrete itself is impervious, cracks 
may be so reduced, by horizontal and vertical reinforcement 
properly proportioned and located, that they will be too 
minute to permit leakage, or will be closed by infiltration 
of silt. 

Asphaltic or coal tar preparations applied either as a 
mastic or as a coating on felt cloth or fabric, are used for 
waterproofing, and should be proof against injury by liquids 
or gases. 

For retaining and similar walls in direct contact with 
the earth, the application of one or two coatings of hot 
coal tar pitch, following a painting with a thin wash of 
coal tar dissolved in benzol, to the thoroughly dried sur- 
face of concrete is an efficient method of preventing the 
penetration of moisture from the earth. 

Tar paper and asphaltic compounds are not often used in 
sewer work as absolute imperviousness is seldom necessary. 

95. Mixing and Placing Concrete. Careful workmanship is 
desirable in the mixing and placing of concrete in sewers since 

1 Trans. Am. Society of Civil Engineers, Vol. 81, 1917, p. 1122. 



MIXING AND PLACING CONCRETE 185 

water-tight construction is desired. Because of the difficulty of 
inspecting concrete in wet, dark and crowded excavations, and 
the careless habits of workmen experienced in concrete Sewer 
construction, the highest class of concrete work cannot be expected. 
The situation is met by designing thick walls as shown in the 
sections illustrated in Fig. 22 and 23. 

In the report of the Joint Committee on Concrete and Rein- 
forced Concrete in Transactions of the American Society of 
Civil Engineers for 1917, on page 1101 the recommendation 
is made concerning the mixing and placing of concrete as 
follows: 1 

The mixing of concrete should be thorough and should 
continue until the mass is uniform in color and is homo- 
geneous. As the maximum density and greatest strength 
of a given mixture depends largely on thorough and com- 
plete mixing, it is essential that this part of the work 
should receive special attention and care. 

Inasmuch as it is difficult to determine by visual 
inspection whether the concrete is uniformly mixed, especi- 
ally where aggregates having the color of cement are used, 
it is essential that the mixing should occupy a definite 
period of time. The minimum time will depend on whether 
the mixing is done by machine or hand. 

(a) Measuring Ingredients: Methods of measurement 
of the various ingredients should be used which will secure 
at all times separate and uniform measurements of cement, 
fine aggregate, coarse aggregate and water. 

(6) Machine Mixing: The mixing should be done hi 
a batch machine mixer of a type which will insure the 
uniform distribution of the materials throughout the mass, 
and should continue for the minimum time of 1^ minutes 
after all the ingredients are assembled in the mixer. For 
mixers of 2 or more cubic yards capacity, the minimum 
time of mixing should be 2 minutes. Since the strength 
of the concrete is dependent on thorough mixing, a longer 
tune than this minimum is preferable. It is desirable 
to have the mixer equipped with an attachment for auto- 
matically locking the discharging device so as to prevent 
the emptying of the mixer until all the materials have been 
mixed together for the minimum time required after they 
are assembled in the mixer. Means should be provided 

1 See also Tentative Specifications for Concrete and Reinforced Concrete 
submitted by the Joint Committee to its Constituent Organizations, June 
4, 1921. 



186 MATERIALS FOR SEWERS 

to prevent aggregates being added after the mixing has 
commenced. The mixer should also be equipped with 
water storage, and an automatic measuring device which 
can be locked if desired. It is also desirable to equip 
the mixer with a device recording the revolutions of the 
drum. The number of revolutions should be so regulated 
as to give at the periphery of the drum a uniform speed. 
About 200 feet per minute seems to be the best speed in 
the present state of the art. 

(c) Hand Mixing: Hand mixing should be done on 
a watertight platform and especial precautions taken 
after the water has been added, to turn all the ingredients 
together at least 6 times, and until the mass is homo- 
geneous in appearance and color. 

(d) Consistency: The materials should be mixed wet 
enough to produce a concrete of such a consistency as will 
flow sluggishly into the forms and about the metal reinforce- 
ment when used, and which at the same time can be con- 
veyed from the mixer to the forms without separation 
of the coarse aggregate from the mortar. The quantity 
of water is of the greatest importance in securing concrete 
of maximum strength and density; too much water is as 
objectionable as too little. 

(e) Retempering : The remixing of concrete and mortar 
that has partly reset should not be permitted. 

Placing Concrete 

(a) Methods: Concrete after the completion of the 
mixing should be conveyed rapidly to the place of final 
deposit; under no circumstances should concrete be used 
that has partly set. 

Concrete should be deposited in such a manner as will 
permit the most thorough compacting such as can be 
obtained by working with a straight shovel or slicing tool 
kept moving up and down until all the ingredients are in 
their proper place. Special care should be exercised to 
prevent the formation of laitance; where laitance has 
formed it should be removed, since it lacks strength and 
prevents a proper bond in the concrete. 

Care should be taken that the forms are substantial 
and thoroughly wetted (except in freezing weather) or 
oiled, and that the space to be occupied by the concrete 
is free from all debris. When the placing of concrete 
is suspended, all necessary grooves for joining future work 
should be made before the concrete has set. 

When work is resumed concrete previously placed 
should be roughened, cleansed of foreign material and 



MIXING AND PLACING CONCRETE 187 

laitance, thoroughly wetted and then slushed with a mortar 
consisting of one part Portland cement and not more than 
2 parts of fine aggregate. 

The surfaces of concrete exposed to premature drying 
should be kept covered and wet for at least 7 days. 

Where concrete is conveyed by spouting, the plant 
should be of such a size and design as to insure a practically 
continuous stream in the spout. The angle of the spout 
with the horizontal should be such as to allow the concrete 
to flow without separation of the ingredients; in general 
an angle of about 27 degrees or 1 vertical to 2 horizontal 
is good practice. The spout should be thoroughly flushed 
with water before and after each run. The delivery from the 
spout should be as close as possible from the point of deposit. 
Where the discharge must be intermittent, a hopper should 
be provided at the bottom. Spouting through a vertical 
pipe is satisfactory when the flow is continuous; when it 
is checked and discontinuous it is highly objectionable unless 
the flow is checked by baffle plates. 

(6) Freezing Weather: Concrete should not be mixed 
or deposited at a freezing temperature, unless special 
precautions are taken to prevent the use of materials 
covered with ice cystals or containing frost, and to prevent 
the concrete from freezing before it has set and sufficiently 
hardened. 

As the coarse aggregate forms the greater portion of 
the concrete, it is particularly important that this material 
be warmed to well above the freezing point. 

The enclosing of a structure and the warming of a space 
inside the enclosure is recommended, but the use of salt 
to lower the freezing point is not recommended. 

(c) Rubble Concrete: Where the concrete is to be 
deposited in massive work, its value may be improved 
and its cost materially reduced by the use of clean stones 
saturated with water, thoroughly embedded in and com- 
pletely surrounded by concrete. 

(d) Under Water: In placing concrete under water, it is 
essential to maintain still water at the place of deposit. 
With careful inspection the use of tremies, properly designed 
and operated, is a satisfactory method of placing concrete 
through water. The concrete should be mixed very wet 
(more so than is ordinarily permissible) so that it will flow 
readily through the tremie and into place with practically 
a level surface. 

The coarse aggregate should be smaller than ordinarily 
used and never more than one inch in diameter. The 
use of gravel facilitates the mixing and assists the flow. 
The mouth of the tremie should be buried in the concrete 



188 MATERIALS FOR SEWERS 

so that it is at all times entirely sealed and the surrounding 
water prevented from forcing itself into the tremie. The 
concrete will then discharge without coming in contact 
with the water. The tremie should be suspended so that 
it can be lowered quickly when it is necessary either to 
choke off or to prevent too rapid flow. The lateral flow 
preferably should not be over 15 feet. 

The flow should be continuous in order to produce a 
monolithic mass and to prevent the formation of laitance 
in the interior. 

In case the flow is interrupted it is important that all 
laitance be removed before proceeding with the work. 

In large structures it may be necessary to divide the 
mass of concrete into several small compartments or units 
to permit the continuous filling of each one. With proper 
care it is possible in this manner to obtain as good results 
under water as in the air. 

A less desirable method is the use of the drop bottom 
bucket. Where this method is used the bottom of the 
bucket should be released when in contact with the surface 
of the place of deposit. 

Concrete sewers should be constructed in longitudinal sections 
in a continuous operation without interruption for the entire 
invert, side walls, or arch. In pouring the concrete it should be 
kept level in the forms and should rise evenly on each side of the 
sewer. All rough places in the concrete should be finished smooth 
by brushing with a grout of neat cement and water and honey- 
combs should be filled with neat cement or a one-to-one mortar. 

96. Sewer Brick. The quality of brick used in sewers is 
seldom specified with the minute care that is taken in the speci- 
fications for concrete, iron, and certain other materials of con- 
struction, as inferior materials in brick are more easily detected. 
The specifications of the Baltimore Sewerage Commission for 
sewer brick are: 

Sewer brick shall be whole, new bricks of the best 
quality, of uniform standard size, with straight and parallel 
edges and square corners; they shall be of compact texture, 
burned hard and entirely through, free from injurious 
cracks and flaws, tough and strong, and shall have a clear 
ring when struck together. The sides, ends and faces of 
all bricks shall be plane surfaces at right angles and parallel 
to each other. Bricks of any one make shall not vary 
more than y^th of an inch in thickness, nor more than 



VITRIFIED SEWER BLOCK 189 

|th of an inch in width or length, from the average of the 
samples submitted for approval. 

The truest bricks shall be used in the face of the masonry 
and the exposed surfaces shall be true and smooth planes. 

All bricks delivered for use shall be culled by the Con- 
tractor when required. No brick thrown out in the culling 
shall be used in any work done under any contract of the 
Sewerage Commission, except that the best of the culls 
may "be used in manholes, above the level of the top of 
the sewer, if permitted by the Engineer. 

The average amount of water absorbed by the bricks, 
after being thoroughly dried and then immersed for 24 
hours, shall not exceed 6 per cent. All bricks shall be 
uniform in quality and percentage of absorption. 

Whenever vitrified bricks are required in the invert 
of the sewer, they shall be smooth, hard, tough, and of 
such durability as will fit them for this use. They shall 
be of standard size, well and uniformly burned, thoroughly 
vitrified throughout, and free from warps, cracks, and 
other defects. The surfaces and edges shall be true and 
straight and the corners sharp and square. They shall 
be in every respect satisfactory to the Engineer, and in all 
respects equal to the sample in the office of the Engineer. 

The remaining paragraphs of the specifications deal with the 
manner in which samples shall be submitted and the necessity for 
conformity between the samples submitted and the bricks used. 

A cotnmon size of brick in use for sewers is 2jX4X8j inches, 
but the variations in size are many. The bricks in use on any 
one job should be as near the same size as possible as the extra 
mortar filling necessary to make up for small brick detracts from 
the strength of the sewer. Small brick are undesirable as the 
cost of laying small and large bricks is the same, but the thickness 
of the finished sewer is less. Sewer brick should not absorb 
more than 10 to 20 per cent moisture by volume, in 24 hours; 
except the special paving brick used to prevent erosion at the 
invert which should absorb less than 5 per cent moisture. 

97. Vitrified Sewer Block. Blocks and bricks are manu- 
factured in a manner similar to the manufacture of vitrified sewer 
pipe described in Art. 91. J. M. Egan describes two types of 
sewer blocks 1 as follows: 

There are on the market two designs of blocks, one 
being a single-ring block and the other a double-ring block. 
1 Journal Illinois Society of Engineers for 1916, p. 75. 



190 MATERIALS FOR SEWERS 

The former has a ship-lap joint on the ends and a tongue- 
and-groove joint on the sides. In the double block the laps 
and joints are made in the construction of the sewer and 
the blocks are placed one on top of the other as in a two- 
ring brick sewer. The blocks are hollow longitudinally 
with web braces. They are made for sewers from 30 inches 
to 108 inches in diameter and weigh from 40 to 120 pounds. 
They are 18 inches to 24 inches long, 9 to 15 inches wide, 
and 5 to 10 inches thick. Short lengths are made for 
convejoience in construction and for use on sharp curves. 
Special blocks are made for connections and junctions. 

A special block is also made for inverts, which has occasionally 
been used with brick sewers to avoid the difficulty of constructing 
with brick at this point. Such blocks are objectionable, as they 
leave a line of weakness along the longitudinal joint so formed. 
They are not used frequently in present-day practice. 

Vitrified blocks are generally cheaper than bricks, but they 
do not make so strong a structure. In some cases it is possible to 
lay vitrified block without the expense of high-priced bricklayers, 
thus saving on the cost of the sewer and obtaining a conduit with 
a smoother interior finish. 

98. Cast Iron, Steel ; and Wood. Cast iron, steel, and wood 
pipe belong more to the field of waterworks than of sewerage, as 
they are not extensively used in the construction of sewers. 
There are, however, some special conditions under which these 
materials may be serviceable. 

The iron used in cast-iron pipe for sewers, and in castings for 
manhole covers, inlet frames, etc., is seldom carefully or definitely 
specified. The standard specifications of the American Water 
Works Association with regard to the quality of iron for water 
pipe are: 

All pipe and special castings shall be made of cast iron 
of good quality and of such character as shall make the 
metal of the castings strong, tough, and of even grain and 
soft enough to satisfactorily admit of drilling and cutting. 
The metal shall be made without the admixture of cinder 
iron or other inferior metal, and shall be remelted in a 
cupola or air furnace. 

The specifications of the Sanitary District of Chicago for the 
quality of iron to be used in manhole covers, etc., are given on 
page 101. 



CAST IRON, STEEL, AND WOOD 191 

Although sewer pipes are not ordinarily subjected to internal 
pressure, cast-iron pipe for sewers should be as heavy or heavier 
than water pipe to resist the corrosive action of the sewage and 
the external stresses that are to be imposed upon it. The sizes 
and details of standard cast-iron pipe used for both water works 
and sewerage can be found in specification of the American and 
New England Water Works Associations. 

The quality of steel used for reinforcing concrete should be 
carefully specified because of the possibility of the substitution of 
inferior material. The specifications for " Billet Steel Concrete 
Reinforcement Bars," of the American Society for Testing 
Materials 1 are the standard for engineering practice, or the fol- 
lowing specifications may be used : 

All reinforcement shall be free from excessive rust, 
scale, paint, or coatings of any character which will tend 
to destroy the bond. The bars shall be rolled from new 
billets. No rerolled material will be accepted. All rein- 
forcement bars shall develop an ultimate tensile strength 
of not less than 70,000 pounds per square inch. The test 
specimen shall bend cold around a pin, whose diameter 
is two times the thickness of the bar, 180 degrees without 
cracking on the outside portion. The reinforcing bars 
shall in all respects fulfill the requirements of the standard 
specifications of the American Society for Testing Materials 
for Billet Steel Concrete Reinforcing Bars serial designa- 
tion A 15-14. 

The steel used in pipe should be a soft, open-hearth steel with 
an ultimate tensile strength of 60,000 pounds per square inch, an 
elastic limit of 30,000 pounds per square inch, an elongation in 
8 inches before fracture between 22 and 25 per cent, and a reduc- 
tion in area before fracture of 50 per cent. The working strength 
of the steel is taken at 16,000 to 20,000 pounds per square inch in 
tension, 10,000 to 12,000 pounds per square inch in shear, and 
20,000 to 24,000 pounds per square inch in bearing. A liberal 
allu\vance should be made for corrosion. The standard specifi- 
cations for Open Hearth Boiler Plate and Rivet Steel of the Ameri- 
can Society for Testing Materials, Aug. 16, 1919, include " flange 
steel," which is suitable for the manufacture of plates, and extra 
soft steel which is suitable for rivets. 

Steel pipe should be coated both inside and out to protect it 
1 See A. S. T. M. Standards for 1918, p. 148. 



192 MATERIALS FOR SEWERS 

against corrosion. The various proprietary coatings are mainly 
coal-tar pitches, or mixtures of coal-tar pitch and asphalt. A 
coal-tar pitch is a distillate of coal tar from which the naphtha has 
been removed and to which about one per cent of heavy linseed oil 
has been added. The coating is applied to the pipe at a tempera- 
ture of about 300 degrees Fahrenheit, by dipping hot pipe in the 
heated coating material. The pipe should be carefully cleaned 
and all rust and scale removed before it is dipped. In some 
cases the steel is pickled before dipping. This consists in rolling 
the cold plates to a short radius to loosen the scale, heating them 
to about 125 degrees, and dipping them in a warm 5 per cent acid 
solution for about 3 minutes, and finally rinsing in a weakly basic 
wash water. 

The woods commonly used for the manufacture of wood pipe 
are spruce, Oregon fir, Douglas fir, and California redwood. 
Wood pipe lines have been constructed of other kinds of lumber 
but only in more or less unusual conditions. The following has 
been abstracted from the specifications for California redwood 
given by J. F. Partridge. 1 

The staves shall be of clear, air-dried, California red- 
wood, seasoned at least one year in the open air, and shall 
be free from knots (except small knots appearing on one 
face only), sap, dry rot, wind shakes, pitch, pitch seams, 
pitch pockets, or other defects which would materially 
impair their strength or durability. The sides of the 
staves shall be milled to conform to the inside and outside 
radii of the pipe; and the edges shall be beveled to true 
radial planes. The staves shall be milled from stock sizes 
of lumber, the net finished thickness of the stave, for the 
various diameters of pipe, shall be as given in Table 40. 
The ends shall be cut square and slotted to receive the 
metallic tongues which form the butt joints. The slots 
shall appear in the same position on each stave, and shall 
be cut to make a tight fit with the tongues in all directions. 
The staves shall have an average length of at least 15 ft. 
6 in. and not more than one per cent shall have a length 
of less than 9 ft. 6 in. Staves shorter than 8 ft. will not 
be accepted. 

The bands shall be spaced on the pipe with a factor of 
safety of at least four, and shall consist of round, mild 
steel rods, connected with malleable iron shoes. Either 

1 Trans. Am. Society Civil Engrs., Vol. 82, 1918, p. 459. 



CAST IRON, STEEL, AND WOOD 



193 



open-hearth or Bessemer steel may be used. . . . The ulti- 
mate strength shall be from 55,000 to 65,000 Ib. per sq. in. 

The original reference should be consulted for complete details 
and for specifications for various kinds of wood and classes of 
pipe. The discussion following the specifications is of value. 

Machine-made wood pipe is superior to stave pipe put together 
in the field. It is seldom manufactured in sizes large enough for 
use in sewers, which results in the almost exclusive use of field 
constructed stave pipe. The steel bands used to hold the staves 
together should be coated similarly to steel plates. All lumber, 
except California redwood should receive a preservative coating 
of creosote 1 or other material. One of the best methods of pre- 
serving the wood is to keep it submerged and to maintain the 
pipe under internal pressure. 

TABLE 40 

DETAILS OF DESIGN FOR CONTINUOUS STAVE WOOD PIPE 

CLASSES A, B, AND C 
(B*y J. F. Partridge, Trans. A. S. C. E., Vol. 82, page 461) 





Stave 


Stock 


Size 


Top Width 


Spacing of 


Diameter, 
Inches 


Thickness, 
Standard, 


Size of 
Lumber, 


of Band, 

Inches 


of Staves, 
Standard, 


Bands for 
100 Feet 




Inches 


Inches 




Inches 


Head 


12 


If 


2X4 


i 


3.56 


6.38 


18 


1& 


2X4 


A 


3.66 


5.76 


24 


1* 


2X4 


A 


3.70 


4.34 


30 


u 


2X6 


* 


5.48 


4.53 


36 


i* 


2X6 


i 


5.62 


3.77 


42 


H 


2X6 


i 


5.51 


3.23 


48 


If 


2X6 


iorf 


5.60 


2.84or4.41 


60 


2i 


3X6 


I 


5.56 


3.54 


72 


3* 


4X6 


for* 


5.69 


2. 95 or 4. 24 


84 


3i 


4X6 


i 


5.65 


3.63 


120 


3f 


4X6 


! 


5.68 


2.54 


144 


3f 


4X6 


iorj 


5.64 


2. 12 or 2. 89 



1 See Trans. Am. Society Civil Eng., Vol. 82, 1918, p. 482. 



CHAPTER IX 
DESIGN OF THE SEWER RING 

99. Stresses in Buried Pipe. The stresses which sewer pipe 
should be designed to resist are: internal bursting pressure, for 
sewers flowing under pressure; stresses due to handling, for 
precast pipe; temperature stresses; and external loads. The 
latter is by far the most important and frequently is the only 
stress considered in design. 

The thickness of a pipe to resist internal stress should be 

PR 
ft' 

in which P=the intensity of internal pressure; 

R = the radius of the inside of the pipe, and 
ft = the unit-strength of the material in tension 

The derivation of this expression is simple. The stresses due 
to handling cannot be computed and are cared for by a thickness 
of material dictated by experience. These thicknesses are given 
for vitrified clay and cement pipe in the specifications in the pre- 
ceding chapter. Temperature stresses are not allowed for in the 
design of the pipe ring, but allowance must be made for them in 
long rigid pipe lines exposed to wide variations in temperature. 
Such a condition seldom exists in sewerage works. 

The external forces are ordinarily the controlling features in 
the design of sewer rings. The simplest problems arise in the 
design of a circular pipe. If the external loading is uniform about 
the circumference of the pipe the internal stresses will all be com- 
pression. Almost all other forms of loading will cause bending 
moments resulting in tension and compression in different parts 
of the pipe. The maximum bending is caused by two concen- 
trated loads diametrically opposed. As such a condition is 
extreme it is not cared for in ordinary design, but a loading between 

194 



DESIGN OF STEEL PIPE 195 

this condition and perfect distribution is assumed, as explained 
in Art. 103. 

100. Design of Steel Pipe. The stresses which may occur in 
steel sewer pipes are commonly caused by the internal or bursting 
pressure of the contained liquid. Occasionally a steel pipe may 
be used as a bridge or as a stressed member of a bridge, but steel 
pipes should not be used to withstand compression normal to the 
axis. In order to avoid such stresses the bursting tensile stresses 
should exceed the external compressive stresses. Such a condi- 
tion in design requires that buried pipes shall never be emptied, a 
condition that cannot always be fulfilled. Precaution should be 
taken, by the installation of proper valves, to prevent the empty- 
ing of the pipe at so rapid a rate that a vacuum is created result- 
ing in the collapse of the pipe. 

Steel pipes are ordinarily made of plates curved to the proper 
diameter, the edges being held together by rivets. The design of 
the pipe consists in the determination of the thickness of the plate 
and the design of the riveted joint. The longitudinal joint and 
the thickness of the plate are first designed. The design of the 
joint consists in determining the diameter and pitch of the rivets 
and the thickness of the plate so that the full strength of the uncut 
metal shall be developed as nearly as possible under bearing, 
tearing, and shearing. This is done by making the efficiency of 
the joint the same under all stresses. The efficiency of the joint 
is the ratio of the strength of the joint under any kind of stress to 
the strength in tension of the unpunched plate. Properties of 
riveted joints are given hi Table 41. 

The diameter of the rivet holes should be computed as ^ of 
an inch larger than the diameter of the rivets. Rivets and plates 
should be designed for the nearest or next largest commercial 
size, and a generous allowance for corrosion should be made in 
determining the thickness of the plate. The distance from the 
edge of the plate to the side of the rivet should not be less than 
lj times the diameter of the rivet. The unit-strengths of the 
metal are given in the preceding chapter. 

The transverse joint must be designed empirically as the 
stresses in it are indeterminate. The common form of joint for 
pipes less than 48 inches in diameter is a single-riveted lap joint, 
and for larger pipes or for pipes exposed to unusual stresses, a 
double-riveted lap joint is used. The same size rivets are used as 



196 



DESIGN OF THE SEWER RING 



in the longitudinal joint. The maximum permissible distance 
between rivets should be used in the transverse ioint. 

TABLE 41 

PROPERTIES OF RIVETED JOINTS 
(Chicago Bridge and Iron Works) 











Effi- 




Type of Joint 


Thickness 
Plate, 
Inch 


Diameter 
of 
Rivet, 
Inch 


Pitch, 
Inches 


ciency 
of 
Joint, 
Per Cent 


Thickness 
Butt 
Plate, 
Inches 


Single riveted lap 


i 


| 


1.88 


49 






i 


f 


2.25 


50 






A 


i 


2.63 


50 




Double riveted lap 


i 

4 


f 


2.50 


70 






A 


3 


3.00 


71 






! 


1 


3.40 


71 




Triple riveted lap 


i 


i 


2.39 


74 






A 


i 


2.96 


74 






3 


i 


3.53 


75 






A 


i 


4.09 


76 




Quadruple riveted lap . 


1 


s 

8 


3.20 


77 






A 


3 

4 


3.90 


78 




Double riveted butt.. . 





1 


3.62 


72 


| 




A 


1 


3.62 


72 


f 




! 


1 


3.62 


72 


1 




U 


1 


3.62 


72 


A 




I 


1 


4.12 


73 


A 




1 


1 


3.82 


71 


1 




i 


1 


3.48 


68 


ft 


Triple rnjeted butt.. . . 


! 


1 


4.94 


80 


5 




f 


1 


5.62 


80 


T6 




1 


1 


5.16 


78 


A 




i 


1 


4.66 


76 


A 










i 




Quadruple riveted butt 


1 


1 


7.13 


84 


3 

4 





! 


1 


6.51 


83 


U 


% 


l 


1 


5.84 


81 


f 



DESIGN OF WOOD STAVE PIPE 197 

Pipes used as compression members of a bridge are stiffened 
by riveting standard rolled steel sections longitudinally on the 
pipe. 

Lock Bar Pipe is a steel pipe with a special form of joint made 
by the East Jersey Pipe Corporation. It is arranged as shown 
in Fig. 78 and has the ad- 
vantage of developing the 
full strength of the plate. 
It is equivalent to a joint 
with 100 per cent efficien- 
cy, which permits the use 
of thinner plates. 

101. Design of Wood 
Stave Pipe. In the de- 
sign of wood stave pipe 1 
the entire bursting pres- Fro. 78. Lock Bar Pipe. 

sure is taken up by steel 

bands wrapped around the outside of wood staves which make 
up the shell of the pipe. The pipe is not designed to resist external 
loads except those which may be overcome by the internal pressure 
in the pipe. The thickness of the staves is fixed by experience. 
The sizes of staves and bands recommended by J. F. Partridge 2 
are given in Table 40. The size of the steel bands can be deter- 
mined from the expression; 




in which S = the total stress in the band; 

R = the radius of the inside of the pipe; 
2 = the thickness of the stave; 

r=the area of bearing per unit length of the band on 
the wood. For circular bands it is assumed as 
the radius of the band; 

C=the crushing strength of wood, usually taken at 
650 pounds per sq. in. 

The preceding expression can be derived easily by the 
application of the laws of mechanics, and from it the 

1 See Trans. Am. Society Civil Engr., Vol. 41, 1899, p. 76, and Vol. 82, 
1918, p. 433, Eng. News, Vol. 74, 1915, p. 400, and Vol. 75, 1916, p. 911. 
* Trans. Am. Soc. Civil Engra., Vol. 82, 1918, p. 433. 



198 



DESIGN OF THE SEWER RING 



expression for the distance between bands follows logically. 
It is, 

S 
p ~PR+kt 

in which S = the strength of the band; 

p = the distance between bands; 
P = the intensity of bursting pressure in the pipe; 
72 = the radius of the inside of the pipe; 
t = the thickness of the staves; 

/c = the swelling strength of wood, usually taken at 
100 pounds per sq. in. 

Transverse joints between staves are closed by inserting 

metal strips between them, or by shaping the edges irregularly 

so that they fit closely together 
with an irregular joint. Trans- 
verse joints between all staves 
at any one point are avoided by 
splitting the joints between staves. 
Longitudinal j oints between 
staves are usually made smooth 

FIG. 79. Shoe for Wood Stave Pipe. an d are closed by steel bands 

which are drawn tight about the 

pipe by inserting the ends in coupling shoes as shown in Fig. 79. 
102. External Loads on Buried Pipe. Prof. Anston Marston 

and H. C. Anderson published 1 the results of a series of experiments 

on the loads on buried pipes which are 

of extreme value in the design of sewer 

pipe. The load on the pipe is given by 

the empirical expression W=CwB 2 , in 

which w is the weight of the backfilling 

material in pounds per cubic foot, B is 

the width of the trench in feet at the 

elevation of the end of a radius making 

an angle of 45 degrees upwards with the 

horizontal diameter of the pipe as illus- 
trated in Fig. 80, and C is a coefficient 

dependent on the character of the backfill and the ratio of the 

1 Bulletin No. 31 of the Engineering Experiment Station of the Iowa 
State College of Agriculture. 





FIG. 80. B in Formula 
W = CwB 2 . 



EXTERNAL LOADS ON BURIED PIPE 



199 



width to the depth of the trench. Values of C are given in 
Table 42. The weights of various classes of backfilling are given 
in Table 43. 

TABLE 42 
APPROXIMATE SAFE WORKING VALUES OP C IN THE EXPRESSION 



From Bulletin No. 31 of the Engineering Experiment Station, Iowa State 
College of Agriculture. 





Approximate Values of C 




Approximate Values of C 


Ratio 
of 
Depth 
to 
Width 


Damp 
Top Soil 
and Dry 
and Wet 
Sand 


Satu- 
rated 
Top 
Soil 


Damp 
Yrilow 
Clay 


Satu- 
rated 
Yellow 
Clay 


Ratio 
of 
Depth 
to 
Width 


Damp 
Top Soil 
and Dry 
and Wet 
Sand 


Satu- 
rated 
Top 
Soil 


Damp 
Yellow 
Clay 


Satu- 
rated 
Yellow 
Clay 


0.5 


0.46 


0.47 


0.47 


0.48 


7.0 


2.73 


2.95 


3.19 


3.55 


1.0 


0.85 


0.86 


0.88 


0.90 


7.5 


2.78 


3.01 


3.27 


3.65 


1.5 


1.18 


1.21 


1.25 


1.27 


8.0 


2.82 


3.06 


3.33 


3.74 


2.0 


1.47 


1.51 


1.56 


1.62 


8.5 


2.85 


3.10 


3.39 


3.82 


2.5 


1.70 


1.77 


1.83 


1.91 


9.0 


2.88 


3.14 


3.44 


3.89 


3.0 


1.90 


1.99 


2.08 


2.19 


9.5 


2.90 


3.18 


3.48 


3.96 


3.5 


2.08 


2.18 


2.28 


2.43 


10.0 


2.92 


3.20 


3.52 


4.01 


4.0 


2.22 


2.35 


2.47 


2.65 


11.0 


2.95 


3.25 


3.58 


4.11 


4.5 


2.34 


2.49 


2.63 


2.85 


12.0 


2.97 


3.28 


3.63 


4.19 


5.0 


2.45 


2.61 


2.78 


3.02 


13.0 


2.99 


3.31 


3.67 


4.25 


5.5 


2.54 


2.72 


2.90 


3.18 


14.0 


3.00 


3.33 


3.70 


4.30 


6.0 


2.61 


2.81 


3.01 


3.32 


15.0 


3.01 


3.34 


3.72 


4.34 


6.5 


2.68 


2.89 


3.11 


3.44 


00 


3.03 


3.38 


3.79 


4.50 



TABLE 43 

APPROXIMATE WEIGHTS OF DITCH FILLING MATERIAL TO BE USED IN THE 
EXPRESSION W = CwB 2 * 



Ditch Filling 



Pounds per Cubic Foot 



Partly compacted top soil (damp) . . 

Saturated top soil 

Partly compacted damp yellow clay . 

Saturated yellow clay 

Dry sand 

Wet sand . . 



90 
110 
100 
130 
100 
120 



* From bulletin No. 31, Engineering Experiment Station, Iowa State College of Aftri- 
culture. 



200 



DESIGN OF THE SEWER RING 



Where surface loads are to be carried on the sewer trench the 
proper proportion of the load to be carried by the sewer is deter- 
mined by the expression L P = CL, in which L p is the equivalent 
backfill load per unit length of the trench, L i \ the surface load 
per unit length of the trench, and C is a coefficient in which allow- 
ance is made for the character of the backfilling, the ratio of depth 
to width o' trench, and the character of the load, whether long or 
short. A long load 's a load extending along the length of the 
trench such as a pile of building material. A short load is one 
extend ng across the trench and for only a short distance along it, 
such as that caused by a street car or road roller crossing the trench. 
Values of C are given in Table 44 for long loads, and in Table 45 
for short loads. Values of long and short loads occasionally met 
in practice are given in Tables 46 and 47 respectively, 

TABLE 44 
RATIO OP LOAD ON PIPE TO LONG LOAD ON TRENCH * 



Ratio 
of 
Depth 
to 
Width 


Sand 
and 
Damp 
Top 
Soil 


Satu- 
rated 
Top 
Soil 


Damp 
Yellow 
Clay 


Satu- 
rated 
Yellow 
Clay 


Ratio 
of 
Depth 
to 
Width 


Sand 
and 
Damp 
Top 
Soil 


Satu- 
rated 
Top 
Soil 


Damp 
Yellow 
Clay 


Satu- 
rated 
Yellow 
Clay 


0.0 


1.00 


1.00 


1.00 


1.00 


3.0 


0.37 


0.41 


0.45 


0.51 


0.5 


0.85 


0.86 


0.88 


0.89 


4.0 


0.27 


0.31 


0.35 


0.41 


1.0 


0.72 


0.75 


0.77 


0.80 


5.0 


0.19 


0.23 


0.27 


0.33 


1.5 


0.61 


0.64 


0.67 


0.72 


6.0 


0.14 


0.17 


0.20 


0.26 


2.0 


0.52 


0.55 


0.59 


0.64 


8.0 


0.07 


0.09 


0.12 


0.17 


2.5 


0.44 


0.48 


0.52 


0.57 


10.0 


0.04 


0.05 


0.07 


0.11 



* From Bulletin No. 31, Engineering Experiment Station, Iowa State College of Agri- 
culture. 

For example, let it be desired to determine the load 
on a 72-inch concrete sewer with a 9-inch shell under the 
following conditions: depth of backfill over the top of 
the pipe, 15 feet; character of backfill, saturated yellow 
clay; superimposed load, pile of building brick 6 feet 
high. The ratio of the depth of backfill to the width of 
the trench is 15-:- 9 or 1.67. The coefficient in the expression 
CwB 2 is 1.39, from Table 42. The weight of saturated 
yellow clay is 130 pounds per cubic foot, from Table 43. 
Therefore the load per foot length of the sewer due to the 
backfill is: 



W = CwB 2 = 1 .39 X 130 X 81 = 14,600 pounds. 



EXTERNAL LOADS ON BURIED PIPE 



201 



TABLE 45 
RATIO OF LOAD ON PIPE TO SHORT LOAD ON TRENCH * 



Ratio 
of 
Height 
to 
Width 
of 


Sand and Damp 
Top Soil 


Saturated 
Top Soil 


Damp Saturated 
Yellow Clay Yellow Clay 


Length of Load Equal to 


Width 


& 
Width 


Width 


A 

Width 


Width 


A 

Width 


Width 


A 

Width 


Trench 


of 


of 


of 


of 


of 


of 


of 


of 




Trench 


Trench 


Trench 


Trench 


Trench 


Trench 


Trench 


Trench 


0.0 


1.00 


1.00 


1.00 


1.00 


1.00 


1.00 


1.00 


1.00 


0.5 


0.77 


0.12 


0.78 


0.13 


0.79 


0.13 


0.81 


0.13 


1.0 
1.5 
2.0 
2.5 
3.0 
4.0 
5.0 
6.0 
8.0 
10.0 


0.59 
0.46 
0.35 
0.27 
0.21 
0.12 
0.07 
0.04 
0.02 
0.01 


0.02 


0.61 
0.48 


0.02 


0.63 
0.51 


0.02 


0.66 
0.54 


0.02 




0.38 




0.40 




0.44 






0.29 




0.32 




0.35 






0.23 




0.25 




29 






0.12 




0.16 




19 






0.09 




0.10 




13 






0.05 




0.06 




08 






0.02 




0.03 




04 







0.01 




0.01 





0.02 





* From Bulletin No. 31, Engineering Experiment Station, Iowa State College of Agri- 
culture. 



TABLE 46 

WEIGHTS OF COMMON BUILDING MATERIAL WHEN PILED FOR STORAGE. 
POUNDS PER CUBIC FOOT 



Brick 120 

Cement 90 

Sand 90 

Broken stone . . . 150 



Lumber 35 

Granite paving 160 

Coal 50 

Pig iron 400 



The pressure of the pile of brick per square foot of trench 
area is, from Table 46, 120X6 = 720 pounds per square 
foot. The value of C from Table 44, is about 0.70. There- 
fore L p is 0.7X9X720=4536 pounds. The equivalent 
depth of backfill weighing 130 pounds per cubic foot is 

4536 

= 3.88 foot. The total equivalent depth of back 



202 



DESIGN OF THE SEWER RING 



fill is therefore 3.88+15 = 18.88 feet. The ratio of depth 

18 88 
to width is ^- =2.98. The coefficient C in the expression 

W = CwB 2 is 2.17. The total load per foot length of 
sewer is therefore TF = 2. 17X130X81 = 22,800 pounds. 

TABLE 47 

WEIGHTS OF SHORT LOADS ON SEWER TRENCHES 
(Adapted from Specifications of the American Bridge Company for Bridges) 



Street railways, heavy 

Street railways, light 

For city streets, heavy traffic 

For city streets, moderate traffic .... 

For city streets, light traffic or coun- 
try roads 



Road rollers . 



A load of 24 tons on 2 axles on 10 foot 

centers. 
A load of 18 tons on 2 axles on 10 foot 

centers. 
A load of 24 tons on 2 axles 10 feet 

apart and 5 foot gage. 
A load of 12 tons on 2 axles 10 feet 

apart and 5 foot gage. 
A load of 6 tons on 2 axles 10 feet 

apart and 5 foot gage. 

Total weight 30,000 pounds. Weight 
on front wheel, 12,000 pounds, and 
on each of two rear wheels, 9,000 
pounds. Width of front wheel, 
4 feet and of each of two rear wheels 
20 inches. Distance between front 
and rear axles 11 feet. Gage of 
rear wheels, 5 feet, c. to c. 



103. Stresses in Circular Ring In Fig. 8 la the loads shown 
indicate the distribution ordinarily assumed in sewer design, the 
forces being uniformly distributed across the diameter. To find 
the bending moment in the pipe caused by this loading, let ah in 
Fig. 816 represent a section of a pipe loaded with equally dis- 
tributed horizontal and vertical forces. Then the vertical com- 
ponent on a strip of differential length ds is wds cos 6 and the 
horizontal component is wds sin and resolving, the resultant 
normal to the surface is wds, in which w is the intensity per unit 
length of the horizontal and vertical forces and 9 is the angle 
which the tangent to ds makes with the horizontal. Thus the load- 
ing of the nature shown in Fig. 81& is equivalent to a loading of 
equally distributed normal forces which give no moment in the ring. 



STRESSES IN CIRCULAR RING 



20:5 



Considering a ring subjected to vertical forces only, the 
moments will be as shown in Fig. 81c and if loaded with horizontal 
forces only, the moments will be as shown in Fig. 81d. Because 
of the symmetry of the figure, moment (1) equals moment (4) 
but is opposite in direction and moment (2) equals moment (3) 
but is opposite in direction. When the horizontal and vertical 
forces are combined on the same ring as in Fig. 816 these moments 
cancel each other as has been proven. Therefore moment (1) 
equals moment (2) and moment (3) equals moment (4). Then 
in Fig. 81e, M a = M b . Now 2Af = for conditions of equilibrium, 

(W \ id\ Wd 

~o~)(l) = ^ an d solving M a = -r^-. This 
& / \4/ lo 

moment occurs at the ends of the horizontal and vertical diameters 
and causes tension on the inside of the pipe at the top and on the 




-b- 




-c- 



-d- 



-e- 



FIG. 81. Distribution of Stresses on Buried Pipe. 



outside at the ends of the horizontal diameter. There will also 
be compression at each end of the horizontal diameter equal to 
one-half of the total load on the pipe. If the material of the 
pipe is homogeneous, the maximum fiber stress / can be found 

My P 
through the expression /=-y--j in which M is the bending 

moment, y is the distance from the neutral axis to the extreme 
fiber of a cross-section of the shell of the pipe of unit length, / is the 
moment of inertia of this cross-section about its neutral axis, P is 
one-half the total load on the pipe, and A is the area of the cross- 
section. For reinforced concrete, the standard formulas should 
be used with this expression for M. The stresses in a circular 
ring subjected to other distributions of loads are shown in Table 
48. An exhaustive study of the stresses in circular rings was 
published by Prof. A. N. Talbot in Bulletin No. 22 of the Engi- 
neering Experiment Station at the University of Illinois, 1908. 



204 



DESIGN OF THE SEWER RING 




Symmetrical 
Vertical Loadings 


Moment 
at Crown 
of Sewer 


Moment 
at End of 
Horizontal 
Diameter 


Com- 
pressive 
Thrust 
at 
Crown 


Com- 
pressive 
Thrust 
at End of 
Horizontal 
Diameter 


Shear 
at 
Crown 


Shear 
at End of 
Horizontal 
Diameter 


Character 


Width 


Concentrated. 
Uniform 



60 
90 
180 


W 
+ . 318ft 
12 

+ . 207ft 
12 

W 
+ . 169 R 
12 

W 
+ . 125ft 


W 
- . 182ft 
12 

W 
- . 168ft 
12 

W 
- . 154ft 
12 

W 
- . 125ft 
12 


0.000 
0.000 
0.000 
0.000 


W 
+ .500 
12 

W 

+ - 50 V 

W 
+ .500- 

W 
+ .500 
12 


W 
0.500 
12 

o.ooo 

12 

W 
0.000 
12 

W 
0.000^- 


0.000 
0.000 
0.000 
0.000 


Uniform 


Uniform 





ft = the radius of the pipe, W = total weight of ditch filling and superimposed load plus 
| of the weight of the pipe itself (usually neglected), expressed in pounds per foot length of 
pipe. Moments are inch-pounds per inch length of pipe. Shears and thrusts are in pounds 
per inch length of pipe. 

104. Analysis of Sewer Arches. The preceding method for 
the determination of the stresses in a sewer ring has referred only 
to a circular pipe uniformly loaded. Other methods must be 
used if the pipe is not circular or the load is not uniformly dis- 
tributed. The simplest method, is the static or so-called vouis- 
soir method. In this method the arch is assumed to be fixed at 
both ends, presumably at the springing line or line of intersection 
between the inside face of the arch and the abutment, and it is so 
designed that the resultant of all the forces acting on any section 
shall lie within the middle third of that section. 

To design an unreinforced sewer arch by the vouissoir method, 
a desired arch is drawn to scale in apparently good proportions 
for the loadings anticipated. The arch is then divided into any 
number of sections of equal or approximately equal length called 
vouissoirs, and the line of action of the resultant load, including 
the weight of the vouissoir is drawn above each vouissoir as shown 
in Fig. 82. The forces are assumed to act as shown in the figure. 
In symmetrically loaded sewer arches there is no vertical reaction 
at the crown. The resultant R is assumed to act at the lower 
middle third of the skewback, which is the inclined joint between 
the arch and the abutment. The upper horizontal force H is 
assumed to ac^. at the upper middle third of the middle or crown 



ANALYSIS OF SEWER ARCHES 



205 



section. The magnitude of H is computed by equating the sum 
of the moments of all forces about the point of application of R 
at the skewback to zero, and solving. The force polygon is then 
drawn as shown in Fig. 83, and the equilibrium polygon is com- 
pleted in Fig. 82 with its rays parallel to the corresponding strings 
drawn from the end of H as origin in Fig. 83. If the equilibrium 
polygon line, called the resistance line, lies wholly within the 
middle third of each vouissoir, the arch is satisfactory to support 
the assumed load without reinforcement. If any portion of the 
resistance line lies outside of the middle third, an attempt should 
be made to find a resistance line which lies wholly within the 
middle third. The true resistance line is that which deviates the 




R v 
FIG. 82. Voussoir Arch Analysis. 




FIG. 83. Force Polygon for 
Voussoir Arch Analysis'. 



least from the neutral axis of the arch. To approximate more 
nearly the true resistance line find two points at which the resist- 
ance line already drawn deviates the most from the neutral axis 
of the arch. Select points M and N on these joints, M being 
nearer the crown than N. Then let T^i and W% be the sum of all 
the loads between the crown and M and N respectively, y repre- 
sent the vertical distance from the crown to N, and y' represent 
the vertical distance between M and N, and x\ and x% represent 
the horizontal distance from W\ and Wz to M and N respectively. 
Then the horizontal thrust H, and a, the distance from the crown 
to the point of application of H, are, 



1 From Vouissoir Arches by Cain. 



206 



DESIGN OF THE SEWER RING 



A resistance line should be drawn with this new horizontal thrust. 
If no resistance line can be found lying wholly within the middle 
third, new sections should be designed until a resistance line can be 
drawn lying wholly within the middle third unless the arch is to 
be reinforced. A number of satisfactory arches should be designed 
and the easiest one to build should be selected. This method is 
limited in its application to sewer arches with rigid side walls and 
it cannot be extended to include the invert. Although an approxi- 
mate method it is accurate within less than 10 per cent of the true 
stresses and is usually quite close. 

The elastic method for the design of arches locates the true 
line of resistance without approximations and is more accurate 
though not so simple to apply as the static or vouissoir method. 
In this method a desired form of arch is drawn as in the static-- 
method and subdivided into vouissoirs so that the distance S 
along the neutral axis between joints is such that the ratio I/S 
shall be the same for all vouissoirs. I is the average of the 
moments of inertia of the surfaces of the two limiting joints about 
the neutral axis. If the thickness of the arch is constant the 
distance between joints will be the same. The method for divid- 
ing the arch into sections such that the ratio I/S shall be a con- 
stant l is as follows: divide the half arch axis into any number of 




FIG. 84. Method for Dividing Arch into Proportion I/S. 

equal parts; measure the radial depth at each point of division; 
lay off the length of the arch axis to scale on a straight line; 
divide this line into the same number of equal parts as the half 
arch, as shown in Fig. 84; at each point erect a perpendicular 
1 Baker's Masonry, 10th Edition, p. 676. 



ANALYSIS OK SK\VKR AHCHKS 



207 



equal in length by scale to the moment of inertia at the correspond- 
ing point on the arch section; draw a smooth curve through the 
tops of these lines; draw a line ab at any slope from the center of 
the original straight line to the curve, and then a line be back to 
the straight line to form an isosceles triangle abc; continue forming 
these triangles in a similar manner thus dividing the original 
straight line in the required ratio. The distance between joints 
is represented by the bases of the triangles. By construction the 
altitude of the triangle represents the average moment of inertia 
between the two limiting joints. The base of each isosceles 
triangle is S, and I/S = $ tan a in which a is the base angle of all 
the isosceles triangles. 




FIG. 85. Elastic Arch Analysis. 






The following steps in the procedure are taken from the second 
edition of the American Civil Engineers Pocket Book, p. 634 : 

In Fig. 85 let the middle points of the joints be marked 
1, 2, 3, etc. and the coordinates x and y from the crown 
be found for each by computation or measurement. For 
a load W placed at one of these points, let z denote the 
distance from it, toward the nearest skewback, to another 
middle point. Let 'Lzx be the sum of the products of all 
the values of z by the corresponding x, and 2zy be the sum 
of all the products of z by the corresponding y; that is, 
each z in the last two summations is multiplied by the x 
or y of the point back of W which corresponds to z. 

For a single load W on the left semi-arch of Fig. 85 
the following formulas are deduced from the elastic theory, 



208 DESIGN OF THE SEWER RING 

n being the number of parts into which the semi-arch is 
divided. 





/TF\n2zi/-2 


yz* m 




V 9 / n Ti/ 2 ( 
\& / fl'^y \ 

%W2z-HZy 


2?/ )2 ' ' W 

f9ft 


Shp^r at, CJrown Vr\~ 


n 
%WZzx 


. . . \i) 

(3) 




2x 2 ' ' 





For symmetrical loading such as W on the left and W on 
the right the horizontal thrust and crown moment due 
to both loads are double those found by the above formulas, 
while the crown shear Fo is zero. For several loads unsym- 
metrically placed the formulas are to be applied to each 
in succession and the results added algebraically, the 
value of FO being taken as negative for the left semi-arch 
and positive for the right semi-arch. 

For any joint whose middle point is at a distance x 
from the crown 

M = M +Hy+Vox- 2Wz, 
F=F -ZTF, 

where 2TF is the sum of all the loads between the joint 
and the crown and 2Wz is the sum of the moments of 
those loads with respect to the middle of the joint. The 
components of the resultant thrust normal and parallel 
to the joints are, 

N = H cosd V sin 8, 



in which 6 is the angle which the plane of the joint makes 
with the vertical. 

The distances from the neutral axis to the resistance 
line are, 

at the crown, eo = -~ t 
li 

at the joint, e= j^- 

The resistance line should be located as in the vouissoir 
method and if not within the middle third a new design should be 
studied. 



REINFORCED CONCRETE SEWER DESIGN 209 

105. Reinforced Concrete Sewer Design. The method to be 
followed in the design of reinforced concrete arches is similar 
except that the moment of inertia should include both the con- 
crete and the steel, that is, 

I = l c +nl t , 

in which 7 is the moment of inertia to be employed, I e is the 
moment of inertia of the concrete, /, is the moment of inertia of 
the steel, and n is the ratio of their moduli of elasticity, generally 
taken as 15. All of the moments of inertia are referred to the 
neutral axis of the beam. The reinforcement called for in pre- 
cast circular pipes is given in Table 39. Sewers cast in place are 
ordinarily designed to avoid reinforcement, except where the 
depth of cover is small and the sewer may be subjected to super- 
imposed loads. 

Concrete sewers are sometimes reinforced longitudinally, with 
expansion joints from 30 to 50 feet apart. This reinforcement is 
to reduce the size of expansion and contraction cracks by dis- 
tributing them over the length of a section. The pipe is divided 
into sections to concentrate motion due to expansion or contrac- 
tion at definite points where it can be cared for. 

The amount of longitudinal reinforcement to be used is a 
matter of judgment. It varies in practice from 0.1 to 0.4 per 
cent of the area of the section. Since the coefficients of expan- 
sion of concrete and of steel are nearly the same, movements of 
the structure are as important as the stresses due to changes in 
temperature. 

Because of the uncertain and difficult conditions under which 
concrete sewers are frequently constructed it is advisable to 
specify the best grade of concrete and not to stress the concrete 
over 450 pounds per square inch hi compression, with no allowable 
stress hi tension. The concrete covering of reinforcing steel 
should be thicker than is ordinarily used for concrete building 
design, because of the possibility of poor concrete allowing the 
sewage to gain access fco the steel, resulting in more rapid deteriora- 
tion than would be caused by exposure to the atmosphere. A 
minimum covering of about 2 inches is advisable, except in very 
thin sections not in contact with the sewage. A minimum thick- 
ness of concrete of about 9 inches is frequently used in design, 
although crown thicknesses of 4$ inches have been used with 



210 DESIGN OF THE SEWER RING 

success. Greater thicknesses should be used near the surface, 
particularly in locations subjected to heavy or moving loads. 

Brick linings are often provided for the invert where moder- 
ately high velocities of about 10 feet per second when flowing full 
are to be expected. For velocities in the neighborhood of 20 feet 
per second the invert should be lined with the best quality vitri- 
fied brick. Although concrete may erode no faster than brick 
under the same conditions, brick linings are more easily replaced 
and at a smaller expense. 



CHAPTER X 
CONTRACTS AND SPECIFICATIONS 

106. Importance of the Subject. Sewers may be constructed 
by day labor or by contract. Under the day labor plan a city 
official or commission is charged with the purchase of material, 
the hiring and firing of employees, and the management of the 
work. Under the contract system a private individual or com- 
pany contracts to supply all the material and labor necessary for 
the completion of the work. 

Under the day labor plan all persons engaged are " working 
for the City." There is not the same sense of individual responsi- 
bility, the same incentive to economize, the same feeling of loyalty 
that is inspired by work under the personality of a contractor. 
Under either the day labor or contract plan unscrupulous politics 
are likely to enter into the relations of the employees of the city 
and the city officials or between the contractor and the city 
officials. Neither the day labor nor the contract plan offer a sure 
cure for unscrupulous political misdealings. Under the contract 
plan the contractor is led to keep his bid as low as possible, realiz- 
ing the competition of other bidders, and during construction he 
will obtain greater efficiency from his labor because of their 
realization of the different conditions under which they are work- 
ing. In some states and cities it is illegal for the municipality to 
do sewer construction except under the contract method. 

The contract method is therefore used in the majority of 
cases, and it is to the interest of the engineer that he be acquainted 
with the essentials of contracts and specifications necessary for 
the proper prosecution of sewer construction. 

107. Scope of Subject. The making of a contract is one of 
the most common episodes of every day life. The contract may 
be an informal verbal agreement to meet at a certain place at a 
certain time, or it may be a formal document hedged about by 
confusing legal phraseology and bearing varieties of penalties and 

211 



212 CONTRACTS AND SPECIFICATIONS 

dire consequences in the event of its breach. The purpose of this 
chapter is to explain only those general features of an engineering 
contract which have particular bearing upon sewerage construc- 
tion. Only the most essential points can be touched in the limited 
space available to this subject, it being presumed that the engi- 
neer is previously grounded in the principles of business law. 1 

108. Types of Contracts. Contracts are known as lump sum, 
cost-plus, unit-price, and by other titles indicating the method of 
payment. 

A lump sum contract is one in which a stated amount is fixed 
upon, before the execution of the contract, to be paid for all the 
work to be done and materials to be furnished under the contract. 
Such an arrangement is not advisable for a sewer contract, as the 
cautious contractor will bid high enough to protect himself in the 
event of a-ny probable emergency. The principal must therefore 
pay whether the emergency or unforeseen difficulty is met or not. 
The advantage of this type of payment is that the principal 
knows exactly the cost of the work to him before construction is 
commenced. 

Cost-plus contracts are those in which the cost of the work to 
the contractor is to be paid by the principal, plus, (a) a fixed sum 
of money, (6) a percentage of the cost of the work, (c) a per- 
centage of the cost of the work but with a fixed limit, (d) a per- 
centage of the difference between the cost of the work and some 
fixed sum, or other variations of this principle. Such contracts 
have the advantage that the principal assumes all the risk in con- 
struction and therefore pays for only those contingencies which 
actually arise. Except for the last named form, they have the 
disadvantage that there is little or no incentive for the contractor 
to keep the cost of the work down. They are most successful 
where the contractor can be selected by the principal, but where 

1 Business Law for Engineers, C. Frank Allen, McGraw-Hill, 1917; En- 
gineering Contracts and Specifications, J. B. Johnson, McGraw-Hill, 1904; 
Contracts in Engineering, J. I. Tucker, McGraw-Hill, 1910; The Law Affect- 
ing Engineers, W. V. Ball, Archibald Constable, 1909; Law and Business 
of Engineering and Contracting. C. E. Fowler, McGraw-Hill, 1909; The 
Economics of Contracting, D. J. Hauer, E. H. Baumgartner, 1915; The 
Elements of Specification Writing, R. S. Kirby, John Wiley & Son, 1913; 
Contracts, Specifications and Engineering Relations, D. W. Mead, McGraw- 
Hill, 1916; Engineering and Architectural Jurisprudence, J. C. Wait, John 
Wiley, 1912. 



THE AGREEMENT 213 

it is necessary to let contracts to the lowest bidder, the " cost- 
plus *' contract is not easily managed. In most states a munici- 
pality cannot make a cost-plus contract. 

A unit-price contract is one in which the amount to be paid is 
fixed in proportion to the amount of work done or materials sup- 
plied. This type of contract is the most suitable for sewer con- 
struction for a municipality where the contract must be let to the 
lowest bidder. The contractor is protected in the event of many 
unforeseen emergencies and the principal is protected against a 
raise in bids to cover such emergencies and against increase in the 
cost of the work in order to increase the profits under a " cost- 
plus " contract. 

It is sometimes desirable for the principal to furnish a portion 
of the materials, the bidders being notified beforehand that this 
material will be furnished. In this manner the quality of material 
is assured, contractors with the necessary skill but small capital 
may be attracted to bid, and uncertainties in the procuring of 
materials is eliminated. 

109. The Agreement. A contract is an agreement between 
two or more interested parties to do a certain thing. A contract 
for the construction of a sewer is an agreement between a muni- 
cipality or individual desiring sewerage facilities and a company or 
individual engaged in the construction of sewers. The latter 
promises to construct a sewer in return for which the former 
promises to pay a certain amount of money. 

The various portions of the agreement which are bound 
together as the complete contract are: I. The Advertisement, 
II. Information and Instructions for Bidders, III. Proposal, 
IV. General Specifications, V. Technical Specifications, VI. 
Special Specifications, VII. Contract, VIII. Bond, and IX. 
Contract Drawings. These should be fastened together in pamph- 
let form and constitute the complete instrument called the con- 
tract. No binding contract and specifications can be drawn upon 
logical deductions alone as legal precedent and tried methods 
must be followed to insure success. To draw up an original con- 
tract requires the combined knowledge of an engineer and a 
lawyer. The engineer of to-day writes his specifications by copy- 
ing copiously from specifications used on work which has been 
completed successfully. In order that selections may be made 
with judgment and discrimination some examples have 



214 CONTRACTS AND SPECIFICATIONS 

been selected from existing published specifications and con- 
tracts. 

110. The Advertisement. This should contain: (1) A head- 
ing indicating the type of work, (2) A statement as to when, 
where and how bids will be received and opened, (3) A brief 
description of the character and amount of work to be done, (4) 
The method of payment, (5) The conditions under which further 
information can be obtained, (6) A statement as to the amount 
of money which must be deposited with the bid, and (7) Any 
other pertinent facts concerning the work. 1 An example of an 
advertisement follows; 

Sewer Construction 
Construction Turkey Creek Sewer 

Kansas City, Missouri. 

Bids for the construction of the Turkey Creek Sewer, two sewage 
pumping stations to be used in connection therewith, and certain 
laterals and extensions of existing sewers thereto, for Kansas City, 
Missouri, will be received up to 2 p. m. August 19, 1919, at the office 
of the Board of Public Works, City Hall, Kansas City, Missouri. 

The main sewer will be about one and one-fifth miles long, and the 
laterals and extensions about three and one-half miles: the main 
sewer will be constructed of reinforced concrete, the laterals and 
extensions will consist of concrete, segment blocks, and clay pipe. 

This work is estimated to cost from $1,500,000 to $1,760.000. 
Payment for the work will be made in four year special tax bills, 
bearing 7 per cent interest, payable one-fourth each year. Time 
600 working days, barring strikes, bad weather, etc. 

Bidders are required to deposit $15,000 in cash or a certified check 
with bid, to insure signing of contract when let. Same to be returned 
on execution of the contract or rejection of bid. 

Complete plans and specifications for the work may be had and 
all information obtained by seeing or writing to A. D. Ludlow, 
Engineer of Sewers, City Hall, Kansas City, Missouri. Twenty-five 
($25.00) Dollars will be required to be deposited for a set of the plans, 
but $20.00 thereof will be refunded upon return of the plans in good 
condition. 

BOARD OF PUBLIC WORKS, 

Kansas City, Missouri, 

by F. E. McCabe, Secretary. 

There are usually legal restrictions which require that the 
advertisement be inserted a certain number of times in specified 
newspapers or other advertising mediums before the opening of 
bids. If the contract is of sufficient size to attract outside con- 

1 See article by E. W. Bush in Eng. News-Record, Vol. 85, 1920, p. 122. 



INFORMATION AND INSTRUCTIONS FOR BIDDERS 215 

tractors, the advertisement should be inserted in engineering and 
contracting journals of wide circulation. Although the adver- 
tisement appears separately from the other portions of the con- 
tract, a copy is usually bound in as the first page of the pamphlet 
containing the contract and specifications and is made an integral 
part thereof. 

111. Information and Instructions for Bidders. This is some- 
what on the order of an introduction to the pamphlet in which the 
specifications, contract, and contract drawings are published. 
As examples of the type of information and instructions given to 
prospective bidders the abstracts below have been taken from the 
" Contract, Specifications, Bond, and Proposal for the North 
Shore Sanitary Intercepting Sewer " by the Sanitary District of 
Chicago. The information 'and instructions to bidders can be 
divided into the following sections: 1st. Examination of Site, 
2nd. Character and Quantity of Work, 3rd. Qualification for 
Bidding, 4th. Instructions for Making out Proposal, 5th. Certified 
Check, and 6th. Rejection of Bids, 

REQUIREMENTS FOR BIDDING AND INSTRUCTIONS TO BIDDERS 

Bidders are required to submit their bids upon the following 
express conditions: 

Bidders must carefully examine the entire sites of the 
work and the adjacent premises, and the various means 
of approach to the sites, and shall make all necessary 
investigations to inform themselves thoroughly as to the 
facilities for delivering and handling materials at the sites 
and to inform themselves thoroughly as to all the difficulties 
that may be involved in the complete execution of all work 
under the attached contract in accordance with the speci- 
fications hereto attached. 

Bidders are also required to examine all maps, plans, 
and data mentioned in the specifications, contract or pro- 
posal as being on file in the office of the Chief Engineer, 
for examination by bidders. No plea of ignorance of 
conditions that exist or that may hereafter exist or of 
conditions or difficulties that may be encountered in the 
execution of the work under this contract, as the result 
of a failure to make the necessary examinations and investi- 
gations, will be accepted as an excuse for any failure or 
omission on the part of the Contractor to fulfill in every 
detail all of the requirements of said contract, specifica- 



216 CONTRACTS AND SPECIFICATIONS 

tions and plans, or will be accepted as a basis for any 
claims whatsoever for extra compensation. Upon applica- 
tion all information in the possession of the Chief Engineer 
will be shown to bidders, but the correctness of such 
information will not be guaranteed by the Sanitary District. 
The following schedule of quantities, although stated 
with as much accuracy as : s possible in advance, is approxi- 
mate only, and is assumed solely for the purpose of com- 
paring bids. 

Then follows an itemized schedule of the quantity of work to be 
done after which comes the following: 

Bidders must determine for themselves the quantities 
of work that will be required, by such means as they may 
prefer, and shall assume all risks as to variations in the 
quantities of the different classes of work actually furnished 
under the contract. Bidders shall not at any time after 
the submission of this proposal, dispute or complain of 
the aforesaid schedules of quantities or assert that there 
was any misunderstanding in regard to the amount or the 
character of the work to be done, and shall not make any 
claims for damages or for loss of profits because of a dif- 
ference between the quantities of the various classes of 
work assumed for comparison of bids and the quantities 
of work actually performed. 

Proposals that contain any omissions, erasures, or 
alterations, conditions or items not called for in the con- 
tract and plans attached hereto, or that contain irregu- 
larities of any kind, will be rejected as informal. 

Bids manifestly unbalanced will not be considered in 
awarding the contract. 1 

No bid will be accepted unless the party making it 
shall furn'sh evidence satisfactory to the Board of Trustees 
of the Sanitary District of Chicago of his experience and 
familiarity with work of the character specified and of 
his financial ability to successfully and properly prosecute 
the proposed work to completion within the specified time. 

Each bid shall be accompanied by a certified check, 
or cash, to the amount of ten (10) per cent of the total 
amount of said bid figured on the quantities given here- 

1 An unbalanced proposal is one in which the bids on some of the items 
are obviously low and on other items are obviously or suspiciously high. 
The purpose of submitting unbalanced bids is to keep secret the true or 
supposed cost of the work to the contractor or to obtain more money by bid- 
ding high on those items which are believed to have been underestimated 
by the Engineer. A low bid is made on other items in order to keep down 
the total amount of the bid. 



PROPOSAL 217 

with, the lowest alternative total being allowed. Said 
amounts deposited with bids, shall be held until all of the 
bids have been canvassed and the contract awarded and 
signed. The return of said check or cash to the bidder to 
whom the contract for said work is awarded will be con- 
ditioned upon his appearing and executing a contract for 
the work so awarded and giving bond satisfactory to said 
Board of Trustees, for the fulfillment of each contract in 
the amount of fifty (50) per cent of the amount of each 
contract. 

The said Board of Trustees reserves the right to re- 
ject any or all bids. 

Accompanying the contract form are plans which, 
together with the specifications, show the work on which 
said tenders are to be made. 

The proposal must not be detached herefrom or from 
the contract by any bidder when submitting a bid. 

112. Proposal. The proposal is a blank printed form on 
which the bidder is required to enter the prices for which he 
proposes to do the work. The proposal blank is necessary in 
order that the bids may be sufficiently uniform for proper com- 
parison. Sewers are often paid for, particularly for small sizes, 
per foot of completed sewer as measured along the center line of 
the pipe parallel to the surface of the ground with the exterior 
length of manholes and other structures deducted. Sometimes, 
under other conditions, a different rate is allowed for each addi- 
tional two feet of depth of sewer, and special structures, such as 
manholes, catch-basins, flush-tanks, etc., are paid for at a unit 
price according to the depth. Water connections to flush-tanks 
are paid for per foot of length of service pipe laid. In especially 
large or difficult work, materials are paid for at a unit price, for 
example, per cubic yard of excavation, per cubic yard of concrete, 
per thousand feet board measure of lumber, etc. 

The following example is taken from the contract for the 
North Shore Intercepting Sewer previously quoted, to indicate 
the type of Proposal used: 



218 CONTRACTS AND SPECIFICATIONS 

PROPOSAL 

FOR THE CONSTRUCTION OF THE NORTH SHORE 
INTERCEPTING SEWER 

To the Honorable, the President and the Board of Trustees 
of the Sanitary District of Chicago: 

Gentlemen: The undersigned hereby certi 

that ha examined the specifications and form 

of contract and the accompanying plans for the construc- 
tion of the North Shore Intercepting sewer, and ha 

also examined the premises at and adjacent to the sites 
of the proposed work, as herein described, and the means 
of approach to the said sites. 

The undersigned ha also examined the fore- 
going " Requirements for Bidding and Instructions to 
Bidders " and propose. ... to do all the work called for in 
said specifications and contract, and shown on said plans, 
and to furnish all materials, tools, labor and all appliances 
and appurtenances necessary to the full completion of 
said work at the rates and prices for said work as follows, 
to-wit: 

(la) For six (6) by nme (9) foot concrete sewer, com- 
plete in place, as specified, the sum of 

Dollars and 

cents ($ ) per linear foot. 

(6a) For manholes, concrete, complete in place, as 
specified the sum of 

Dollars 

and cents ($ ) each 

The following plans showing the work to be performed 
in accordance with the attached specifications, have been 
examined by the undersigned in preparing the foregoing 
proposal, to-wit: 



In accordance with the requirements set forth in the 
attached Information and Instructions for Bidders, there 

is deposited herewith the sum of 

Dollars and 

cents (S ) which 

under the terms therein mentioned entitle . . 



to bid on said work, the said sum to be refunded to. 



upon the faithful performance of all conditions set forth 
in the Information and Instructions for Bidders. 

Name 

Address . . 



GENERAL SPECIFICATIONS 219 

Blanks are provided for each item. No place is left at the 
end for a summary. The proposal ends with an acknowledgment 
that the contract has been examined completely and all prelimi- 
nary directions therein have been complied with. A blank is 
prepared for inserting the amount of the required certified check, 
and finally for the signature of the bidders. 

113. General Specifications. The specifications, both general 
and technical, are occasionally incorporated in the contract form, 
but more frequently they are printed separately and are bound in 
the pamphlet preceding the contract. The general specifications 
relate to the conditions under which all work must be performed 
and are as applicable to the construction of a pumping station as 
to the smallest lateral, unless otherwise specified. It is not pos- 
sible to include a complete set of General Specifications in the 
limited space of this text, but the more important specifications 
will be emphasized by examples taken from specifications in use. 1 

The subjects covered in General Specifications are: 

(1) Definitions of doubtful terms. 

(2) The Engineer to settle disputes. 

(3) Duties of the Engineer. 

(4) Duties of the Contractor. 

(5) Hours and days of work. 

(6) No work to be done in the absence of an inspector. 

(7) Contractor to be represented at all times. 

(8) Time of commencing and completing the work. 

(9) Liquidated damages for delay in completion. 

(10) The City may change the plans. 

(11) The City may increase the amount of the work. 

(12) Inspection and its conduct. 

(13) The Contractor to be acquainted with laws relat- 

ing to the work. 

(14) Contractor responsible for damages to persons or 

property. 

(15) City to be protected against patent claims. 

(16) Abandonment of contract and its remedy. 

(17) Estimates of work done and moneys due. 

(18) Payments for extra work. 

(19) Character of workmen to be employed. 

(20) City may reserve a sum for repairs during stipu- 

lated term after completion. 

1 Taken mainly from specifications of the Sanitary District of Chicago 
and the Baltimore Sewerage Commission, with miscellaneous selections 
from other sources. 



220 CONTRACTS AND SPECIFICATIONS 

(21) City may use money due Contractor to pay claims 

for labor or materials used on the work and not 
paid for by the Contractor. 

(22) The Contractor shall have no claim for damages 

on account of delay or unforeseen difficulties. 

(23) The Contractor may not assign nor sublet the 

contract without the City's consent. 

(24) Cleaning up after completion. 

(25) The Contractor's relations to other contractors. 

(26) The portions composing the contract. 

The following examples cover the subjects named in the pre- 
ceding titles: 

1. Definitions. The word Engineer whenever not 
qualified shall mean the Chief Engineer of the Commission, 
acting either directly or through his properly authorized 
agents, such agents acting severally within the scope of 
the particular duties entrusted to them. 

This article may include words that may be in dispute or ambigu- 
ous such as: Board of Trustees, Elevation, City, Contractor, 
Rock, Earth, etc., etc. 

2. Disputes. To prevent disputes and litigations, the 
Engineer shall in all cases determine the amount, quality, 
and acceptability of the work which is to be paid for under 
the contract; shall decide all questions in relation to said 
work and the performance thereof, and shall in all cases 
decide every question which may arise relative to the ful- 
fillment of the contract on the part of the Contractor. 
His determination, decision and estimate shall be final 
and conclusive, and in case any question shall arise between 
the parties touching the contract, such determination, 
decision, and estimate shall be a condition precedent to 
the right of the Contractor to receive any moneys under 
the contract. 

3. Duties of the Engineer. The Engineer shall make 
all necessary explanations as to the meaning and intentions 
of the specifications and shall give all orders and directions, 
either contemplated therein or thereby, or in every case 
in which a difficulty or unforeseen condition shall arise in 
the performance of the work. Should there be any dis- 
crepancies in or between, or should any misunderstanding 
arise as to the import of anything contained in the plans 
and specifications, the decision of the Engineer shall be 
final and binding. Any errors or omissions in plans and spe- 
cifications may be corrected by the Engineer, when such 
corrections are necessary for the proper fulfillment of their 

, intentions as construed by him. 



GENERAL SPECIFICATIONS 221 

4. Duties of the Contractor. The Contractor shall 
do all the work and furnish all the labor, materials, tools 
and appliances necessary or proper for performing and com- 
pleting the work required by the contract, in the manner 
called for by the specifications, and within the contract 
time. He shall complete the entire work at the prices 
agreed upon and fixed therefor to the satisfaction of the 
Commission and its Chief Engineer and in accordance 
with the specifications, the drawings, and such detailed 
drawings as may be furnished from time to time, together 
with such extra work as may be required for the perform- 
ance of which written orders may be given and received 
as hereinafter provided. 

The Contractor shall place sufficient lights on or near 
the work and keep them burning from twilight to sunrise; 
shall erect suitable railings, fences or other protections 
about all open trenches, and provide all watchmen on the 
work, by day or night, that may be necessary for the public 
safety. The Contractor shall, upon notice from the Engi- 
neer that he has not satisfactorily complied with the fore- 
going requirements, immediately take such methods and 
provide such means and labor to comply therewith as the 
Engineer may direct, but the Contractor shall not be 
relieved of this obligation under the contract by any such 
notice or directions given by the Engineer, or by neglect, 
failure, or refusal on the part of the Engineer to give such 
notice and directions. 

The Contractor shall furnish such stakes and the neces- 
sary labor for driving them as may be required by the 
Engineer. He shall maintain the stakes when set, with 
reasonable diligence, and stakes misplaced due to the careless- 
ness of the Contractor or his workmen shall be reset under 
the direction of the Engineer, at the Contractor's expense. 

5. Night, Sunday, and Holiday Work: 1 No night, 
Sunday, nor holiday work requiring the presence of an 
engineer or inspector will be permitted except in case of 
emergency, and then only to such an extent as is absolutely 
necessary and with the written permission of the Engineer; 
provided that this clause shall not operate in the case of 
a gang organized for regular and continuous night, Sunday, 
or holiday work. 

6. Absence of Engineer or Inspector. Any work done 
without lines, levels, and instructions having been given 
by the Engineer or without the supervision of an assistant 

1 Restrictions are placed on work done outside of ordinary working hours 
in order that the Contractor may not perform work in the absence of an 
engineer or inspector. 



222 CONTRACTS AND SPECIFICATIONS 

or inspector, will not be estimated or paid for except when 
such work is authorized by the Engineer in writing. Work 
so done may be ordered removed and replaced at the 
Contractor's sole cost and expense. 

7. Absence of Contractor. During the absence of the 
Contractor he shall at all times have a duly authorized 
representative on the work. The Contractor shall give 
written notice to the Commission of the name and address 
of said representative and shall state where and how such 
representative can be reached, at any and all hours, whether 
by day or night. 

Whenever the Contractor or his representative is not 
present at any place on the work where it may be necessary 
to give orders or directions, such orders or directions will 
be given by the Engineer and they shall be received and 
promptly obeyed by the superintendent or foreman who 
may have immediate charge of the particular work in rela- 
tion to which the order may be given. 

8. Commencing Work. The Contractor agrees to 

begin the work covered by this contract within 

days of the execution of the contract and to prosecute the 
same with all due diligence and to entirely complete the 
work within days. 

It is understood and agreed that time is of the essence 
of this contract, and that a failure on the part of the Con- 
tractor to complete the work herein specified within the 
time specified will result in great loss and damage to said 
Sanitary District and that on account of the peculiar 
nature of such loss it is difficult, if not impossible, to accu- 
rately ascertain and definitely determine the amount 
thereof. 

9. Liquidated Damages. It is therefore covenanted 
and agreed that in case the said Contractor shall fail or 
neglect to complete the work herein specified on or before 
the date hereinbefore fixed for completion, the said Con- 
tractor shall and will pay the said Sanitary District the 
sum of Dollars for each and every day the Con- 
tractor shall be in default in the time of completion of 
this contract. 

Said sum of Dollars per day is hereby agreed 

upon, fixed and determined by the parties hereto as the 
liquidated damages which said Sanitary District will 
suffer by reason of such defaults, and not by way of a 
penalty. 

10. Changes in Plans. The Board reserves the right 
to change the alignment, grade, form, length, dimensions 
or materials of the sewers or any of their appurtenances, 

> whenever any condition or obstructions are met that 



GENERAL SPECIFICATIONS 223 

render such changes desirable or necessary. In case the 
alterations thus ordered make the work less expensive to 
the Contractor a proper deduction shall be made from 
the contract prices and the Contractor shall have no claim 
on this account for damages or for anticipated profits on 
the work that may be dispensed with. In case such 
alterations make the work more expensive, a proper addi- 
tion shall be made to the contract prices. Any deduction 
or addition as aforesaid shall be determined and fixed by 
the Engineer. 

11. Extensions and Additions. In the event that any 
material alterations or additions are made as herein speci- 
fied which in the opinion of the Engineer will require 
additional time for execution of all the work under this 
contract, then, in that case the time of completion of the 
work shall be extended by such a period or periods of time 
as may be fixed by said Engineer and his decision shall be 
final and binding upon both parties hereto, provided that 
in such case the Contractor, within four (4) days after 
being notified in writing of such alterations and additions, 
shall request in writing an extension of time, but the 
provisions of this paragraph shall not otherwise alter the 
provisions of this contract with reference to liquidated 
damages, and the said Contractor shall not be entitled to 
any damages or compensation from the said Sanitary 
District on account of such additional time required for 
the execution of the work. 

12. Inspection. All materials of whatsoever kind to 
be used in the work shall be subject to the inspection and 
approval of the Engineer and shall be subject to constant 
inspection before acceptance. Any imperfect work that 
may be discovered before its final acceptance shall be cor- 
rected immediately, and any unsatisfactory materials used 
in the work or delivered at the site shall be rejected and 
removed on the requirement of the Engineer. The inspec- 
tion of any work shall not relieve the Contractor of any 
of his obligations to perform proper and satisfactory work 
as herein specified, and all work which, during the progress 
and before the final acceptance, may become damaged 
from any cause, shall be removed and replaced by good 
and satisfactory work without extra charge therefor. The 
Engineer and his assistants shall have at all times free 
access to every part of the work and to all points where 
material to be used in the work is manufactured, procured 
or stored and shall be allowed to examine any material 
furnished for use in the work under this contract. 

All inspection of any and all material furnished for use 
in work to be performed under this contract shall be made 



224 CONTRACTS AND SPECIFICATIONS 

at the site of the work after the delivery of the material, 
provided, that, if requested by the Contractor the Engi- 
neer may at his option perform, or have performed, inspec- 
tion of materials at points other than the site of the work. 
In any such case the Contractor shall pay the Sanitary Dis- 
trict the extra cost of such inspection, including the neces- 
sary expenses of the inspector for the extra time expended 
in performing any such inspection at said other points. 

13. Legal Requirements. The Contractor shall keep 
hinself fully informed of all existing and future national 
and state laws and local ordinances and regulations in 
any manner affecting those engaged or employed in the 
work, or the materials used in the work, or of all such 
orders and degrees of bodies or tribunals having any juris- 
diction or authority over the same, and shall protect and 
indemnify the party of the first part against any claim 
or liability arising from or based on the violation of such 
law, ordinance, regulation, order or decree, whether by 
himself or his employees. 

14. Damages. If any damage shah be done by the 
Contractor or by any person or persons in his employ to 
the owner or occupants of any land or to any real or per- 
sonal property adjoining, or in the vicinity of the work 
herein contracted to be done or to the property of a neigh- 
boring contractor the Engineer shall have the right to esti- 
mate the amount of said damage arid to cause the Sanitary 
District to pay the same to the said owner, occupant, or 
contractor, and the amount so paid shall be deducted 
from the money due said Contractor under this contract. 
Said Contractor covenants and agrees to pay all damages 
for any personal injury sustained by any person growing 
out of any act or doing of himself or his employees that 
is in the nature of a legal liability, and he hereby agrees 
to indemnify and save the Sanitary District harmless 
against all suits or actions of every name and description 
brought against said Sanitary District, for or on account 
of any such injuries, or such damages received or sustained 
by any person or persons; and the said Contractor further 
agrees that so much of the money due to him under this 
contract, as shall be considered necessary by the Board of 
Trustees of said Sanitary District, may be retained by the 
Sanitary District until such suit or claim for damages 
shall have been settled, and evidence to that effect shall 
have been furnished to the satisfaction of said Board of 
Trustees. 

15. Patents. It is further agreed that the Contractor 
shall indemnify, keep and save harmless said Sanitary 
District from all liabilities, judgments, costs, damages and 



GENERAL SPECIFICATIONS 225 

expenses which may in any wise come against said Sanitary 
District, or which may be the result of an infringement 
of any patent by reason of the use of any materials, machin- 
ery, devices, apparatus, or process furnished or used in 
the performance of this contract, or by reason of the use 
of designs furnished by the Contractor and accepted by 
the Sanitary District, and in the event of any claim or 
suit or action at law or in equity of any kind whatsoever 
being made or brought against said Sanitary District, then 
the Sanitary District shall have the right to retain a suffi- 
cient amount of money in the same manner and upon the 
conditions as hereinafter specified. 

16. Abandonment of Contract. If the work to be done 
under the contract shall be abandoned by the Contractor, 
or if at any time the Engineer shall be of the opinion, and 
shall so certify, in writing, to the Commission, that the 
performance of the contract is unnecessarily or unreason- 
ably delayed, or that the Contractor is willfully violating 
any of the conditions of the specifications, or is executing 
the same in bad faith, or not in accordance with the terms 
thereof, or if the work be not fully completed within the 
time named in the contract for its completion, the Com- 
mission may notify the Contractor to discontinue all 
work thereunder, or any part thereof, by a written notice 
served upon the Contractor, as herein provided; and 
thereupon the Contractor shall discontinue the work, or 
such part thereof, and the Commission shall thereupon 
have the power to contract for the completion of said work 
in the manner prescribed by law, or to procure and furnish 
all necessary materials, animals, machinery, tools and 
appliances, and to place such and so many persons as it 
may deem advisable to work at and complete the work 
described in the specifications, or such part thereof, and 
to charge the entire cost and expense thereof to the Con- 
tractor. And for such completion of the work or any part 
thereof, the Commission may for itself or its contractors, 
take possession of and use or cause to be used any or all 
such materials, animals, machinery, tools and implements 
of every description as may be found on the line of the said 
work. The cost and expense so charged shall be deducted 
from, and paid by the City out of such moneys as may be 
due or may become due to the Contractor, under and by 
virtue of the contract. In case such expense shall exceed 
the amount which would have been payable under the con- 
tract, if the same had been completed by the Contractor, 
he shall pay the amount of such excess to the City. When 
any particular part of the work is being carried on by the 
Commission, by contract or otherwise, under the provisions 



226 CONTRACTS AND SPECIFICATIONS 

of this clause of the contract, the Contractor shall continue 
the remainder of the work in conformity with the terms 
of his contract, and in such manner as in no wise to hinder 
or interfere with the persons or workmen employed by the 
Commission by contract or otherwise as above provided, 
to do any part of the work or to complete the same under 
the provisions hereof. 

17. Estimates. The Engineer shall from time to time 
as the work progresses, on or about the last day of each 
month, make in writing an estimate, such as he shall believe 
to be just and fair, of the amount and value of the work 
done and the materials incorporated into the work by the 
Contractor under the specifications, provided however that 
no such estimate shall be required to be made when, in the 
judgment of the Engineer the total value of the work done 
and the materials incorporated into the work since the last 

preceding estimate is less than dollars. Such 

estimates shall not be required to be made by strict measure- 
ments, but they may be approximate only. 

The Contractor shall not be entitled to demand from 
the Commission as a right, a detailed statement of the 
measurements or quantities entering into the several items 
of the monthly estimates, but he will be given such oppor- 
tunities and facilities to verify the estimates as may be 
deemed reasonable by the Commission. 

When in the opinion of the Engineer, the Contractor 
shall have completely performed the contract on his part, 
the Engineer shall make a final estimate, based on actual 
measurements, of the whole amount of the work under 
and according to the terms of the contract, and shall certify 
to the Commission in writing, the amount of the final 
estimate at the completion of the work. After the com- 
pletion of the work the City shall pay to the Contractor 
the amount remaining after deducting from the total 
amount or value of the work, as stated in the final estimate, 
all such sums as have theretofore been paid to the Con- 
tractor under any of the provisions of the contract, except 
such sums as may have been paid for extra work,' and also 
any sum or all sums of money which by the terms thereof 
the City is or may be authorized to reserve or retain; 
provided that nothing therein contained shall affect the 
right of the City, hereby reserved, to reject the whole 
or any portion of the aforesaid work, should the said 
cert'ficate be found or known to be inconsistent with the 
terms of the contract or otherwise improperly given. All 
monthly estimates upon which partial payments have been 
made, being merety estimates, shall be subject to cor- 
, rection in the final estimate, which final estimate may be 



GENERAL SPECIFICATIONS 227 

made without notice thereof to the Contractor, or of the 
measurements upon which it is based. 

18. Extra Work. The Contractor shall do any work 
not herein otherwise provided for, when and as ordered 
in writing by the Engineer or his agents specially authorized 
thereto in writing, and shall when requested by the Engi- 
neer so to do, furnish itemized statements of the cost of 
the work ordered and give the Engineer access to accounts, 
bills, vouchers, etc. relating thereto. If the Contractor 
claims compensation for extra work not ordered as aforesaid, 
or for any damages sustained, he shall within one week 
after the beginning of any such work or the sustaining 
of any such damage, make a written statement of the 
nature of the work performed or the damage sustained, 
to the Engineer, and shall, on or before the fifteenth day 
of the month succeeding that in which any such extra 
work shall have been done or any such damage shall have 
been sustained, file with the Engineer an itemized statement 
of the details and amount of any such work or damage; and 
unless such statement shall be made as so required, his claim 
for compensation shall be forfeited and he shall not be en- 
titled to payment on account of any such work or damage. 

For all such extra work the Contractor shall receive 
the reasonable cost of said work, plus fifteen (15) per cent 
of said cost. 

19. Competent Employees. The Contractor shall 
employ only competent skillful men to do the work; and 
whenever the Engineer shall notify the Contractor, in 
writing, that any man employed on the work is, in his 
opinion unsatisfactory, such man shall be discharged from 
the work and shall not again be employed on it, except 
with the consent of the Engineer. 

20. Money Retained. Upon the completion of the 
work and its acceptance by the City, the City shall reserve 
and retain five (5) per cent of the total value of the work 
done under the contract as shown by the final estimate, 
over and above any and all other reservations which the 
city by the terms thereof is entitled or required to retain 
and shall hold the said five (5) per cent for a period of nine 
(9) months from and after the date of completion and 
acceptance, and the City shall be authorized to apply 
such part of said five (5) per cent so retained to any and 
all costs of repairs and renewals as may become necessary 
during such period of nine (9) months, due to improper 
work done or materials furnished by the Contractor, if 
the Contractor shall fail to make such repairs or renewals 
within twenty-four (24) hours after receiving notice from 
the City so to do. 



228 CONTRACTS AND SPECIFICATIONS 

Upon the expiration of said nine (9) months from and 
after the completion and acceptance of the work, the City 
shall pay to the Contractor the said five (5) per cent hereby 
retained, less such sums as may have been retained here- 
under. 

21. Unpaid Claims against Contractor. The Con- 
tractor shall furnish the City with satisfactory evidence 
that all persons who have done work or furnished materials 
under the contract, and have given written notices to the 
City, before and within ten (10) days after the final com- 
pletion and acceptance of the whole work under the con- 
tract, that any balance for such work or materials is due 
and unpaid, have been fully paid or satisfactorily secured. 
And in case such evidence is not furnished as aforesaid, 
such amount as may be necessary to meet the claims of 
the persons aforesaid shall be fully discharged or such 
notices withdrawn. 

22. Delays and Difficulties. The Contractor shall not 
be entitled to any claims for damages on account of post- 
ponement or delay in the work occasioned by forces beyond 
the control of the City, nor for postponement or delay in 
the work where ten (10) days written notice has been given 
the Contractor of such postponement or delay, nor where 
unforeseen difficulties are encountered in the prosecution 
of the work. In the event of a postponement or delay 
ordered in writing by the City the time of completion of 
the contract shall be extended a number of days equal to 
the number of days that the work has been postponed or 
delayed. 

23. Assignment of Contract. The Contractor shall not 
assign by power of attorney or otherwise, nor sublet the 
work or any part thereof, without the previous written 
consent of the party of the first part, and shall not either 
legally nor equitably assign any of the moneys payable 
under this agreement or his claim thereto unless by and 
with the consent of the party of the first part. 

24. Cleaning Up. On or before the completion of the 
work, the Contractor shall, without charge therefor, tear 
down and remove all buildings and other structures built 
by him, shall remove all rubbish of all kinds from any 
grounds which he has occupied, and shall leave the line 
of the work in a clean and neat condition. 

25. Access to Work and Other Contractors. The Com- 
mission and its engineers, agents and employees may at 
any time and for any purpose enter upon the work and the 
premises used by the Contractor, and the Contractor shall 
provide proper and safe facilities therefor. Other con- 
tractors of the Commission may also when so authorized 



TECHNICAL SPECIFICATIONS 229 

by the Engineer, enter upon the work and the premises 
used by the Contractor for all the purposes which may 
be required by their contracts. Any differences or con- 
flicts which may arise between this Contractor and other 
contractors of the Commission in regard to their work 
shall be adjusted and determined by the Engineer. 

26. The Contract. It is understood and agreed by 
the City and the Contractor that the terms of this contract 
are embodied and included in the Advertisement. Informa- 
tion and Instructions to Bidders, Proposal, Specifications 
of every nature, the Bond and the contract drawings 
hereto attached. 

These few articles have been given as examples of some of the 
essential subjects to be treated in general specifications. It is to 
be understood that these examples do not represent a complete 
set of general specifications and items have been omitted the 
absence of which in a complete contract might be injurious to 
the successful completion of the work. 

114. Technical Specifications. These ordinarily follow the 
general specifications and have to do with the quality of materials, 
the manner of putting them together, and the method of doing 
the work. The subject headings in the Technical Specifications 
on the Baltimore Sewerage Commission are: 

Excavation Cement 

Tunneling Mortar 

Rock Excavation Concrete 

Sheeting Brick 

Sheet Piling Masonry 

Sheeting and Bracing Reinforced Concrete 

Piles Vitrified Pipe 

Blasting Concrete and Brick Sewers 

Pumping and Drainage Vitrified Pipe Sewers and Drains 

Foundations Manholes 

Refilling Iron Castings 

Repaving House Connections 

Underdrains Obstructions 

Buildings Fences 

Inlets and Catch-Basins Flush-Tanks 

Each of these subjects is treated in the appropriate section of 
this book. 

An important part of each section of the technical specifica- 
tions is the clause providing for the method of payment for the 
work specified. This is usually the last clause in the section. 



230 CONTRACTS AND SPECIFICATIONS 

For example, the last clause in the Baltimore Specifications 
relating to Rock Excavation, is: 

"Payment will be made for the number of cubic yards 
of rock measured and allowed as above specified at the 
price of four dollars and fifty cents ($4.50) per cu. yd., meas- 
ured in place. Payment for rock excavation will be made 
in addition to the prices bid for excavation." 

115. Special Specifications. These have to do with problems, 
methods of construction, or materials peculiar to certain contracts 
or certain portions of the work. It frequently occurs that the 
construction of sewerage works will be let out under a number of 
contracts, or bids will be called for on different alternatives to 
which the entire Advertisement, Information and Instructions 
for Bidders, Proposal, and General Specifications are applicable. 
The special specifications will apply only to the contract in ques- 
tion, e.g., in some work done under the direction of the author, 
the sewer on one contract came within twelve inches of the surface 
of a highway. The special specification relating to this piece of 
construction, was: 

" Where crossing under the Chicago Road the pipe sewer 
shall be embedded in concrete as shown on the contract 
drawings. The concrete for this purpose shall be mixed 
in the proportions of one (1) part cement, three (3) parts 
fine aggregate, and six (6) parts coarse aggregate. Pay- 
ment for the concrete so used will be made at the unit 
price stated in the accompanying Proposal." 

In order to avoid confusion the special specifications are either 
incorporated directly in the Contract form, or follow the Technical 
Specifications and are grouped according to the contracts to which 
they apply. 

116. The Contract. The contract is a brief instrument which 
includes a simple statement of the obligations of each party 
involved. The following is an example of a form in successful use: 

CONTRACT 

This agreement made and entered into this 

day of in the year one thousand nine 

hundred and by and between the City of 

by its duly constituted or elected authorities herein acting 

for the City of without personal liability 

to themselves, party of the first part, hereinafter desig- 



THE CONTRACT 231 

nated as the City, and 

party of the second part hereinafter designated as the 
Contractor. 

WITNESSETH, that the parties to these presents each 
in consideration of the undertakings, promises and agree- 
ments on the part of the other herein contained, have 
undertaken, promised and agreed, and do hereby under- 
take, promise and agree, the party of the first part for 

itself, its successors and assigns, and the part of 

the second part for and heirs, executors, 

administrators and assigns as follows, to-wit: 

Art. I. To be bounden by all the articles of the General, 
Technical, and Special Specifications applicable, and by 
the terms of the Advertisement, Information and Instruc- 
tions for Bidders, Proposal and Contract Drawings hereto 
attached, and which are understood and acknowledged 
to be an integral part of this contract. 

Art. II. The work to be completed under this con- 
tract is 

Art. III. The City shall pay and the Contractor shall 
receive as full compensation for everything furnished and 
done by the Contractor under this contract, including all 
work required but not specifically mentioned in the follow- 
ing items, and also for all loss or damage arising from the 
nature of the work aforesaid, or from the action of the 
elements, or from any unforeseen obstruction or difficulty 
encountered in the prosecution of the work and for well 
and faithfully completing the work as herein provided, 
as follows: 

Then follows a copy of the Proposal with the prices bid. The 
contract closes with the final clause: 

In witness whereof the said City of , party 

of the first part have hereunto set their hands and seals, 
and the Contractor has also hereunto set his hand and 
seal and the party of the first part and the Contractor 
have executed this agreement in duplicate, one part to 
remain with the party of the first part and one to be deliv- 
ered to the Contractor this day of 

hi the year one thousand nine hundred and 

City of 

Contractor . 



232 CONTRACTS AND SPECIFICATIONS 

117. The Bond. The bond called for in the Information and 
Instructions for Bidders is bound in the pamphlet following the 
Contract. No uniform practice is followed in the amount of the 
bond required. It varies from 50 to 100 per cent of the contract 
price and may be stated as a lump sum before the contract price 
is known. There is a possibility that the Contractor may fail 
before he has commenced work and the City may be unable to 
procure another contractor to take up the work. The City should 
then be protected by a 100 per cent bond. Such a contingency is 
remote. The Contractor seldom fails until work is well under 
way, and other contractors are usually available, although the 
failure of one contractor tends to increase the bids of other con- 
tractors for the same work. In fixing the amount of the bond 
the judgment of the Engineer is called into play in order that the 
amount may be as low as possible in fairness to the Contractor, 
and high enough to protect the interests to the City. By reducing 
the amount of the bond the expense to the City is also reduced 
as the City ult-'mately must pay its cost. 

Upon the acceptance of the bond and the execution of the 
Contract, the Engineer's duties take him out of the designing 
office and into the construction field. 



CHAPTER XI 
CONSTRUCTION 

118. Elements. The principal elements in construction are: 
labor, materials, tools, and transportation. The lack of or 
inadequateness of any one of these detracts from the effectiveness 
of the others. The engineer should assure himself of the com- 
pleteness of his plans or those of the contractor on each of these 
points. The disposition of labor and the handling of materials 
to obtain the largest amount of good with the least expenditure 
of money and effort are problems which must be solved by the 
engineer or the contractor during construction. 

WORK OF THE ENGINEER 

119. Duties. The duties of the engineer during construction 
consist in giving lines and grades; inspecting materials; inter- 
preting the contract, specifications and drawings; making deci- 
sions when unexpected conditions are encountered; making esti- 
mates of work done; collecting cost data; making progress reports; 
keeping records; and in guarding the interests of the City. 

120. Inspection. In the inspection of workmanship and 
materials, the engineer is assisted by a corps of inspectors and 
assistants who act under his direction. The duties of the inspector 
are to be present at all times that work is in progress and to act 
for the engineer in enforcing the terms of the contract, the 
details of the drawings, and the tests applicable to the workman- 
ship and materials that he is delegated to inspect. He should 
have a copy of the contract, or that portion of it which pertains 
to his work, available at all times. He should examine all 
materials as they are delivered on the job and see that rejected 
materials are removed at once. An ordinary recourse of some 
foremen will be to place rejected material to one side until a brief 
absence of the inspector will present the opportunity for the use 

233 



234 CONSTRUCTION 

of the rejected material. The methods to be followed in the 
inspection of materials and workmanship should be such as to 
discover discrepancies between the specifications and the materials 
delivered or the work done. Other duties of the inspector are: 
to record the location of house connections or to drive a stake 
over them for subsequent location by the engineer; to see that 
plugs are put in the branches left for future house connections; 
to inspect the workmanship in the making of joints in pipe sewers; 
to protect the line and grade stakes from displacement; to check 
the size, depth, and grade of sewers and elevations of special 
structures, etc. 

*Dishonest and unscrupulous workmen have many tricks to 
get by the inspector. These tricks are best learned by experi- 
ence as no academic list can impress them properly on the memory. 
The position of the inspector is not always enviable. He must 
hold the respect of the workmen, of the contractor, and of the 
engineer. To do this he must not be unreasonable or arbitrary 
in his decisions, but when a decision is once made he must be 
firm in following up its enforcement. He must be careful not to 
give directions whose fulfillment he cannot enforce, nor for which 
he cannot give adequate reason to his superiors. His integrity 
must never be questioned. He must not allow himself to become 
under obligations to the contractor by the acceptance of favors 
he cannot return except at the expense of his employer, yet at 
the same time he must not appear priggish by the refusal of all 
favors or social invitations. In brief he must be friendly without 
being intimate, independent without being aloof, and firm without 
being arbitrary. 

The engineer must support his inspectors in their decisions or 
discharge them if he cannot. 

121. Interpretation of Contract. In interpreting the contract, 
specifications and drawings, the engineer is supposedly an 
impartial arbiter between the interests of the city and the con- 
tractor. His decisions, as to the meaning of the contract, must 
be founded on his engineering judgment, and should aim to pro- 
duce the best results without demanding more from the contractor 
than, in his honest opinion, it is the intention of the contract to 
demand. However conscientiously he may attempt to remain 
impartial, and in spite of the honesty of the contractor, his posi- 
tion, as an employee of the city will almost invariably cause him 



UNEXPECTED SITUATIONS 235 

to favor the city in his decisions on close points. The experi- 
enced contractor knows this and fixes his bid accordingly, the 
personality of the engineer sometimes acting as an important 
factor in the amount of the bid. The situation arises through 
the character of the contract, and not through a lack of moral 
integrity on the part of anyone concerned. 

122. Unexpected Situations. When unexpected or uncertain 
conditions are encountered in construction the engineer should 
visit the spot at once and should advise or direct, according to the 
terms of the contract, the procedure to be followed. Such condi- 
tions may be the encountering of other pipes, quicksand, rock, etc. 
Each case is a problem in itself. Water, gas, telephone and elec- 
tric wire conduits can be moved above or below the sewer being 
constructed with comparative ease. Other sewers, if smaller, 
may be permitted to flow temporarily across the line of the sewer 
under construction and finally discharge into the completed sewer, 
or one sewer must be made to pass under the other, either as an 
inverted siphon or by changing the grade of one of the sewers. 
Rock, or other material for which a special rate of payment is 
allowed, must be measured as soon as uncovered in order to avoid 
delaying the work or losing the record of the amount removed. 
When quicksand is met special precautions must be taken to 
safeguard the sewer foundation and to insure that the sewer will 
remain in place until after the backfilling is completed. These 
precautions are described in Art. 135. 

123. Cost Data and Estimates. Cost account keeping and 
the making of monthly or other estimates are closely connected. 
Cost accounts are of value in estimating the amount of work done 
to date, and in making preliminary estimates of the cost of similar 
work. Although the engineer is not always required to keep such 
accounts, they are usually of sufficient value to pay for the labor 
of keeping them. Under some contracts the contractor's accounts 
are open to examination by the engineer. Usually, however, he 
must depend on reports from the inspectors for information con- 
corning the man-hours required on different pieces of work, and 
on his own measurements of materials used and his knowledge of 
their unit costs, in order to make up an estimate of total cost. 

The measurement of a completed structure and a summary of 
the materials used in its construction may act as a check on the 
use of proper materials as called for in the contract. For example, 



236 



CONSTRUCTION 



if it is known that 2,000 bricks are required for the construction 
of a manhole and if only 15,000 have been used in the construction 
of ten manholes, it is probable that some or all of the manholes 
have been skimped. Similar conditions may show in the pro- 
portions of concrete, backfilling in tunnels, sheeting to be left in 
place, etc. 

The statement of a few principles of cost accounting, and the 
illustration of a few blanks in use should be sufficiently suggestive 
to lead a resourceful engineer in the right direction. 1 Costs should 
be divided into four general classifications: labor, materials, 
equipment, and overhead. Labor should be subdivided under 
its several different classifications arranged in accordance with 
rates of pay. The number of laborers under each classification 
and the amount of work done per day should be recorded. Fig. 86 
is an example of a form which may be used for such a purpose. 



FOREMAN'S DAILY PAY ROLL REPORT. 
Loc&iion....l4tk Avenue. ...No, Z....96" Sewer -...Date August 7, 1907 
Itemized Pay Roll. Work Done, Pay Roll Distributed. 


Foreman / days at 
Engineer - / ' 
Labor 1 ' 
S7.4' 
" "*! 

Carts "T ' 
Teams -10 ' 
Hoister 1 ' 
Water boy 1 ' 

Total day's pay 
roll 


i 

3 
1 

1 


00 
60 
00 
75 
60 


4 

a 

3 

47 
17 


00 
50 
00 
05 
70 


General Night Watchman and Water Boy 
Excavation completed to station 18.40 Cost 
Sheeting 18.30 
Foundation pl'nk" 18.00 
Backfilling "' 18.90 
Sheeting Pulled " 17.10 


2 
19 
8 

2 
3 
2 


25 
26 
26 
63 
00 
62 


3 
ff 
g 


00 
00 
00 
76 


3 

1 
2 

83 

\ 


00 
20 
00 
76 

10 


Brick Invert 






Concrete Sides \ " 17.48 
Concrete Roof / '* 


29 


62 


Steel Bars set '" 17.48 
Forms set to station 17.48 
Forms Pipe laid to station 


S 
7 


60 
26 


Manholes built to-day 












Other items Pump 
Teams working Cement 17.48 

Carts working Gravel 17.48 
Total day's pay roll 


1 

3 
183 


63 
SO 

00 
10 


Signed L.W. Foreman. 





FIG. 86. Foreman's Daily Payroll Report. 

From Engineering and Contracting, 1907. 

Materials may be recorded as they are delivered on the job, as 
they are used, or in both cases. Measurements are usually easier 
to make at the time of delivery, but records made at the time 

1 Cost Keeping and Management, by Gillette and Dana. Practical Cost 
Keeping for Contractors, by F. R. Walker. Cost Keeping in Sewer Work, 
by K. O. Guthrie in Eng. Contracting, Vol. 28, p. 238, 1905. Sewer Con- 
struction Records at Scarsdale, Eng. News-Record, Vol. 83, p. Ill, 1919. 



COST DATA AND ESTIMATES 



237 



materials are used are more serviceable. For example, 100 
barrels of cement may be delivered on a job in November, 50 of 
them are used before the job freezes up and the other 50 are held 
over until spring. It would be misleading to charge 100 barrels 
used in November. Fig. 87 is a form in use for an inspector's 



FOREMAN'S DAILY MATERIAL RE 
Location.... 14th Avtnut No. 96* E 


PORT. 

ate, August 7, 1907 




Pull cement bags on hand las 
received to-< 

Total .. 


t night 




84 


lay 




160 








JS44. 


Pull cement used to-day on c< 
' b 

" ; c< 

' IT 

' P 


snc. invert 






rick " 






snc. sides \ 


. 166 




11 roof J 






anholcs 






Dinting up, etc.*. 


1 


167 


Balance on hand to-night 








87 








Empty cement bags on hand 1 
Full bags used to-day 


ast night 




7 






167 


Empty bags sent in to-day 


Total 






164 






140 


Empty bags balance on hand to-night.. 








*4.. 


Materials received. 


From. 


Amounts. 
47 L 16 16-(t.. .. 






1S- 


-4 x 614 
I' roof en 

-I IS ft. 


-ft.. 




E N 


usty 


Stetl Bars. 


Car No. P RR 7284 


ISO 








iso 1 19-ti 


Sheeting and bracing left in 
place 


None 


76 | SO-ft. 







L. W. 


Foreman. 





FIG. 87. Foreman's Daily Material Report. 

From Engineering and Contracting, 1907. 

report on materials. The total cost must be made up in the 
office from these records and a knowledge of unit costs. 

Equipment consists of tools, animals, machinery, and appara- 
tus used in construction. Only equipment that is actually used 
should be charged to the job and a credit should be made at the 
completion of the job for the fair value of the equipment remain- 
ing after the completion of the work. 

Overhead charges include the expense of the office force, 
superintendence, and miscellaneous items such as insurance, rent, 



238 CONSTRUCTION 

transportation, etc., which cannot be charged to any particular 
portion of the work but are equally applicable to all portions. It 
happens frequently that many jobs are handled in the same main 
office. The division of overhead becomes more difficult and is 
frequently arranged on an arbitrary basis, e.g., each job may be 
charged the proportion of overhead that its contract price bears 
to the total contract prices being performed under that office. 
This rule may be modified when it becomes evident that some job 
is taking distinctly more than its share of the overhead. 

Estimates of work done in any period can be made with the 
above data in hand by subtracting the total costs of the work up 
to the beginning of the period from the total costs up to the end 
of the period. Fig. 88 shows a sample blank from the final esti- 
mate sheets used at Scarsdale, N. Y. 

124. Progress Reports. 1 These are kept by the engineer in 
order that he may see that the work is progressing as called for in 
the contract, and any portion which is lagging behind without 
reason may be pushed. Such reports are most useful when the 
information is expressed graphically, as the eye quickly catches 
points where the work is falling behind schedule. 

125. Records. The contract drawings are supposed to show 
exactly where and how construction is to be done. Due to 
unexpected contingencies changes occur, of which a record should 
be made and preserved. These records may be kept in a form 
similar to the contract drawings, or if the changes are not exten- 
sive, they can be recorded on the original contract drawings. 
The location of house and other connections should be recorded 
in a separate note book available for immediate consultation. 
The engineer should keep a diary of the work in which are recorded 
events of ordinary routine as well as those of special interest and 
importance. This diary should be illustrated by photographs 
showing the condition of the streets before and after construction, 
methods of construction, accidents, etc. Such accounts are of 
great value in defending subsequent litigation and their existence 
sometimes prevents litigation. A contractor may wait a year or 
so after the completion of a piece of work until the engineer and 
other city officials have broken their connection with the city. 
Suit is then brought against the city and unless good records are 

1 See Planning and Progress on a Big Construction Job, by Chas Penrose, 
Eng.> News-Record, Vol. 84, 1920, pp. 554 and 627. 



COST DATA AND ESTIMATES 



239 



24." VIT. SEWER 
.6 FT. TO 8 FT. PEEP, INCLUDING A FT. 

1914 LOCATION SCHEDULE FEET PRICE AMOUNT 


Thi 


s siqn indioates that the item has been inclu 


A & 
eftcf in the 


to fa.03$ 406. U* 
monthly estimate 




Y-BRANCHES ON 24" PIPE 
LOCATION SCHEDULE NUnBER PRICE AMOUNT 


Ifrdt 


S &&***. M.H*4-&MH.*S (OtC&i&u) 


A 


* "^ 
6 4.oo . 


2.4-. o* 








r MANHOLES 6FTTo8FT. 

?* 
1914 io LOCATION SCHEDULE 


PRICE AMOUNT 


ZS 


, ^^^^^ 


A 


* * 
svoo , 


so. oa* 


H 








. MANHOLE DEPTHS EXCEEDING 6 FT. 

"*"i 
1914 io LOCATION SCHEDULE FEET PRICE AMOUNT 




?/ W^tiujbrtt.^ tfit&ty&fie. 


K 4-. 


3 ^.00\. , 


2. 10 9 








ROCK 
IN SEWER TRENCH 
191.4 LOCATION SCHEDULE CU.YDS. PRICE AMOUNT 


5* 


*^85^&~& 


c< & 


4-1 ^/f , // 


'4. f3* 








z R "C K 

| g^ IN MANHOLE EXCAVATION 
1914 fc i LOCATION SCHEDULE CU.YDS. WICE AMOUNT 




4* ^>*4*W*yfSFtt 


F : /^ 


* / 

01 Vf . 


JO. 21 



|g CONTINGENT EXTRAS 
5< STANDARD-A-SECTION 
*. CONCRETE FOR SEWERS 
.EN6THS^2 X LOCATION . SCHEDULE CU. YDS KICE AMOUNT 


t7J.3 


,4 


3 ^w^w<*<& 


'A 


J/02 


S.oo 





I? 






3 

Z 

EXTRA ORDER 
NUMBER 


CONTINGENT EXTRAS 

SHEETING ft SHORING 
LOCATION SCHEDULEBQWttFT.Kia AMOUNT 


*,,<, 


/^#&4xt2*^3~44 


P 


//40. 


'35!$ 31. 


ft* 



FIG. 88. Samples of Cost Record Forma. 

From Engineering and Contracting, 1909. 



240 CONSTRUCTION 

available the administration may be forced to buy the claimant 
off or may elect to enter court, only to be beaten. 

EXCAVATION 

126. Specifications. The following abstracts have been taken 
from the specifications on Excavation by the Baltimore Sewerage 
Commission as illustrative of good practice. In conducting the 
work the contractor shall: 

. . . remove all paving, or grub and clear the surface 
over the trench, whenever it may be necessary and shall 
remove all surface materials of whatever nature or kind. 
He shall properly classify the materials removed, separat- 
ing them as required by the Engineer; and shall properly 
store, guard, and preserve such as may be required for 
future use in backfilling, surfacing, repaving or otherwise. 
Ah 1 macadam material removed shall be separated and 
graded into such sizes as the Engineer may direct and 
materials of different sizes shah 1 be kept separate from each 
other and from any and all other materials. 

All the curb, gutter, and flag-stones and all paving 
material which may be removed, together with all rock, 
earth and sand taken from the trenches shah* be stored hi 
such parts of the carriageway or such other suitable place, 
and in such manner as the Engineer may approve. The 
Contractor shah 1 be responsible for the loss of or damage 
to curb, gutter and flag-stones and to paving material 
because of careless removal or wasteful storage, disposal, 
or use of the same. 

. . . When so directed by the* Engineer the bottom of 
the trench shall be excavated to the exact form of the 
lower half of the sewer or of the foundation under the 
sewer. 

The bottom width of the trench for a brick or concrete 
sewer shall be ... not less in any case than the overall 
width of the sewer, as shown on the plans. In case the 
trench is sheeted this minimum width will be measured 
between the interior faces of the sheeting as driven, but in 
no case shah 1 bracing, stringers, or waling strips be left 
within any portion of the masonry of the sewer except by 
permission of the Engineer; and such braces, stringers 
and waling strips shall not, in any case, be allowed to 
remain within the neat lines of the masonry as shown on 
the plans. In case that the distance between faces of the 
- sheeting is less than that called for by the width of the 



SPECIFICATIONS 241 

sewer to be laid in the trench, the Engineer may direct the 
sheeting to be drawn and redriven, or otherwise changed 
and altered; or he may direct that the sewer be reinforced 
in such manner and to such an extent as he may deem 
necessary without compensation to the Contractor, even 
though such narrower trench was not caused by negli- 
gence or other fault on the part of the Contractor. 

Trenches for vitrified pipe shall be at all points at least 
six inches wider in the clear on each side than the greatest 
external width of the sewer, measured over the hubs of the 
pipe . . . Bell holes shall be excavated in the bottoms of 
trenches for vitrified pipe sewers wherever necessary. 

Not more than three hundred feet of trench shall be 
opened at any one time or place in advance of the com- 
pleted building of the sewer, unless by written permission 
of the Engineer and for a distance therein specified. . . . 

The excavation of the trench shall be fully completed 
at least twenty feet in advance of the construction of the 
invert, unless otherwise ordered. 

During the progress of construction the Contractor will 
be required to preserve from obstruction all fire hydrants 
and the carriageway on each side of the line of the work. 

The streets, cross walks, and sidewalks shall be kept 
clean, clear, and free for the passage of carts, wagons, car- 
riages and street or steam railway cars, or pedestrians, 
unless otherwise authorized by special permission in writ- 
ing from the Engineer. In all cases a straight and con- 
tinuous passageway on the sidewalks and over the cross 
walks of not less than three feet in width shall be pre- 
served free from all obstruction. 

Where any cross walk is cut by the trench it shall be 
temporarily replaced by a timber bridge at least three feet 
wide, with side railings, at the Contractor's expense. The 
placing of planks across the trench without proper means 
of connection or fastenings, or pipe or other material, or 
the using of any other makeshift in place of properly con- 
structed bridges, will not be permitted. 

This is equally applicable to certain wagon bridges to be fixed 
upon by the Engineer, on the basis of traffic requirements. 

In streets that are important thoroughfares or in narrow 
streets the material excavated from the first one hundred 
feet of any opening or from such additional length as may 
be required, shall upon the order of the Engineer, be 
removed by the Contractor, as soon as excavated. The 
material subsequently excavated 'shall be used to refill the 
trench where the sewer has been built. 



242 CONSTRUCTION 

The preceding specifications are applicable to open-trench 
excavation. Rigid restrictions are placed about tunneling 
because of the greater difficulty of doing good work, the greater 
danger to life and property and the possibility of later surface 
subsidence if the backfilling is done improperly. A common 
clause in specifications is: 

All excavations for sewers and their appurtenances 
shall be made in open trenches unless written permission 
to excavate in tunnel shall be given by the Engineer. 

127. Hand Excavation. Earth excavation by pick and shovel 
is the simplest and most primitive mode of excavation. Only 
small jobs are handled in this manner in order to save the invest- 
ment necessary in machines or the expense of hiring and moving 
one to the work. The tools used in the hand excavation of trenches 
are: picks, pickaxes, long-handled and short-handled pointed 
shovels, square-edged long- and short-handled shovels, scoop 
shovels, axes, crowbars, rock drills, mauls, sledges, etc. The 
excavating gangs are divided up into units of 20 to 50 men under 
one foreman or straw boss, and among the men may be a few 
higher priced laborers who set the pace for the others. Each 
laborer on excavation should be provided with a shovel, the 
style being dependent on the character of the material being 
excavated and the depth of the trench. In stiff material and 
deep trenches requiring the lifting of the material in the shovel, 
long-handled pointed shovels should be used. In loose sandy 
material loaded directly into buckets short-handled, square 
pointed shovels are satisfactory. Picks are used in cemented 
gravels or where hard obstructions prevent cutting down with 
the edge of the shovel. Very stiff but not hard material can be 
cut out in chunks with a pickaxe and thrown from the trench or 
into a bucket with a scoop shovel. Scoop shovels are also useful 
in wet running quicksand. The number of picks, axes, crowbars, 
and other tools must be proportioned according to the material 
being excavated. Under the worst conditions of excavation in a 
hard cemented gravel it may be necessary to provide each man 
with a pick as well as a shovel, whereas in sand only a shovel is 
necessary. Two or three crowbars, axes, a length of chain, two 
or three screw jacks, etc., are provided per gang in case of an 
unexpected encounter with an obstruction in the trench, such as 
a boulder, a tree stump, a length of pipe, etc. 



HAND EXCAVATION 



243 



In laying out the work the foreman marks the outlines of the 
trench on the ground by means of a scratch made with a pick, 
chalk marks, tape, or other devices. These marks are measured 
from offset or center stakes set by the engineer. Center stakes 
are less conducive to error but are more likely to be disturbed 
before use than are offset stakes, but careless foremen make more 
errors with offset than with center stakes. The inspector should 
assist or be present at the laying out of the trench. After the 
trench has been laid out each laborer should be given a certain 
specific portion of it to dig and this portion is marked out on the 
ground. In this way a check can be kept upon the performance 
of each laborer and the knowledge of this fact tends to a uni- 
formly better performance. The amount of work that can be 
performed by one man with a pick and shovel is as shown in 
Table 49. Some men may exceed these rates, many will not 
attain them. The allotted task must be gaged on the character 
of the ground in order that the tasks may be equal and a spirit of 
competition fostered. The hard worker will set the pace for the 
lazy man. Some contractors have adopted the expedient of dis- 
missing laborers for the day as soon as the allotted task is done. 

TABLE 49 

AMOUNT OF MATERIAL MOVED BY ONE MAN WITH A PICK AND SHOVEL 
(From H. P. Gillette) 



Material 


Cubic Yard 
per Hour 


Material 


Cubic Yard 
per Hour 


Harclpan 


33 


Sand 


1 25 


Common earth 


8to 1 2 


Sandy soil 


8 to 1 2 


Stiff clay 


85 


Clayey earth 


1 3 


Clay.. 


1.00 


Sandy soil (frozen) . . 


75 











The opening of the trench may be facilitated by breaking 
ground with a plow. In hard ground or on paved roads it may 
be necessary to cut through the surface crust with a hammer and 
drill, although in some cases a plow can be used successfully. 
Frozen ground can lx> thawed by building fires along the line of 
the trench, or greater economy may !>' achieved by placing steam 
pipes along the surface with perforations about every 18 inches 



244 CONSTRUCTION 

and either boxing them on the top and sides or burying them in 
the frozen earth with a covering of sand. Another arrangement 
is to blow steam into a line of bottomless boxes in which each box 
is about 8 feet long. Holes are left in the top of the boxes into 
which the pipe is shoved, and after its withdrawal the holes are 
covered. Blasting of frozen earth is sometimes successful but 
cannot be resorted to in built up districts where it is unsafe unless 
properly controlled. Once the frost crust is broken through it 
can be attacked from below and frequently broken down by 
undermining. 

A laborer cannot dig and raise the earth much more than to 
the height of his head, and preferably not quite so high, without 
tiring quickly. After the trench has passed a depth of 4 feet he 
cannot throw the earth clear of the trench. An additional laborer 
is needed then at the surface to throw the earth back. He should 
shovel the earth from a board platform placed at the edge of the 
trench as a protection to the bank. When the trench passes the 
6-foot depth a staging is put in about 4 feet from the top on which 
the lowest laborer piles his materials. It is then passed up to the 
surface by a second laborer on the staging, and a third laborer on 
the surface throws the material back clear of the trench. Stag- 
ings are put in about every 5 or 6 feet for the full depth of the 
trench. 

When the trench has come within half the diameter of the 
pipe of the final grade, if the material is sufficiently firm, the 
remainder of the trench should be cut to conform to the shape of 
the lower half of the outside of the pipe, with proper enlargements 
for each bell. 

128. Machine Excavation. On work of moderately large 
magnitude excavation by machine is cheaper than by pick and 
shovel alone. In comparing the cost of excavation by the two 
methods all items such as sheeting, pipe laying, backfilling, etc., 
should be included, since these items will be affected by the method 
of excavation. The cost of setting up and reshipping the machine 
must be included as this is frequently the item on which the use 
of the machine depends. Because of the cost of setting up and 
shipping, which must be distributed over the total number of 
yards excavated, the cost per cubic yard of excavating by machine 
varies with the number of cubic yards excavated. The point of 
economy hi the use of a machine is reached when the cost by hand 






MACHINE EXCAVATION 245 

and by machine are equal. For all work of greater magnitude, 
excavation by machine will prove cheaper. 1 Items favoring the 
use of machinery which may cause its adoption for small jobs are: 
its greater speed, reliability, ease in handling, economy in sheet- 
ing, economy in labor, and small amount of space needed making 
it useful in crowded streets. Continuous bucket machines, drag 
lines, and occasionally steam shovels are not adapted to conditions 
where rocks, pipes and other underground obstacles are frequently 
met. 

The following problem is an example of the work necessary in 
making a comparison of the relative economy of machine and 
hand excavation: 

It is assumed that a man can excavate 15 feet of trench 
30 inches wide and 8 feet deep in 10 hours. He receives 
55 cents per hour for his work. A machine costing $10,000 
has a life of 6 years. It can be kept busy 150 days in the 
year. When operating it costs $1.25 per hour for the 
operator, fuel and repairs. It will excavate 800 linear feet 
of 30 inch trench to a depth of 8 feet in 10 hours. It is 
assumed that capital is worth 10 per cent on such a venture 
and that the sinking fund will draw 10 per cent. If the 
cost of moving and setting up the machine is $1,800, how 
many cubic yards of excavation must there be to make 
excavation by machine economical. Costs of sheeting, 
pumping, etc., are assumed to be the same for machine or 
hand work. 

Solution. For hand work the man excavated 1.11 
cubic yard per hour at 55 cents. The relative cost of hand 
excavation is then 50 cents per cubic yard. 

The cost of machine work will be divided into: interest 
on first cost; operation and repairs; and sinking fund for 
renewal. The interest on the first cost of $10,000 at 
10 per cent is $1,000 per year. The machine works 1,500 
hours in the year. Therefore the cost per hour is $0.67. 

The sinking fund payment, as found from sinking fund 
tables or the accumulation of $10,000 in. 6 years, is $1,300 
per year or per hour for 1,500 hours is $0.87. 

The cost of operation per hour is given as $1.25. 

The total cost per hour is therefore $2.79. 

The machine excavated 59.3 cubic yards per hour W 
which makes the cost, exclusive of moving, equal to $0.47 ]\ 

1 See also " Ownership and Operation of Trench Excavators by the 
Water Department of Baltimore," by V. B. Seims, presented before Am. 
Water Works Association, June 9, 1921. 



246 CONSTRUCTION 

per cubic yard. In order to equalize the cost of machine 
and hand excavation the cost of moving the machine must 
be divided among a sufficient number of cubic yards so that 
the cost per cubic yard shall be 3 cents. The cost of moving 
is given as $1,800. This amount divided among 60,000 
cubic yards equals 3 cents per cubic yard. Therefore the 
job must provide at least 60,000 cubic yards of excavation 
in order that the use of the machine shall be justifiable 
from the viewpoint of economy alone. 

129. Types of Machines. Machines particularly adapted to 
the excavation of sewer and water pipe trenches are of four types : 
(1) continuous bucket excavators; (2) overhead cable way or track 
excavators; (3) steam shovels; and (4) boom and bucket excava- 
tors. Other types of excavating machinery can be used for 
sewer trenches under special conditions. Machines are ordinarily 
limited to a minimum width of trench of 22 inches. Between 
widths of 22 inches and 36 inches the limit of depth for the first 
class of machines is about 25 feet. For other types of machines 
there is no definite limit, though the economical depth for open 
cut work seldom exceeds 40 feet. 

130. Continuous Bucket Excavators. Continuous bucket 
excavators are of the types shown in Figs. 89 and 90. The 
buckets which do the digging and raising of the earth may be 
supported on a wheel as in Fig. 89 or on an endless chain as in 
Fig. 90. The support of the wheel or endless chain can be raised 
or lowered at the will of the operator so as to keep the trench as 
close to grade as can be done by hand work. In some machines 
the shape of the buckets can be made such as to cut the bottom of 
the trench, in suitable material, to the shape of the sewer invert. 
In operation, the buckets are at the rear of the machine and 
revolve so that at the lowest point in their path they are traveling 
forward. The excavated material is dropped on to a continuous 
belt which throws it on the ground clear of the trench, into dump 
wagons, or on to another continuous belt running parallel with the 
trench to the backfiller, by means of which the excavated material 
is thrown directly into the backfill without rehandling. The 
body of the machine supporting the engine travels on wheels 
ahead of the excavation and is kept in line by means of the pivoted 
front axle. When obstacles are encountered the excavating 
wheel or chain is raised to pass over the obstacle, and allowed to 
dig itself in on the other side. 



CONTINUOUS BUCKET EXCAVATORS 



247 




FIG. 89. Buckeye Wheel Excavator. 

Courtesy, Buckeye Traction Ditcher Co. 




FIG. 90. Buckeye Endless-chain Excavator. 
Courtesy, Buckeye Traction Ditcher Co. 



248 



CONSTRUCTION 



Wheel excavators are not adapted to the excavation of sewer 
trenches over 3 to 4 feet in width and 6 to 8 feet in depth. The 
endless-chain excavators are suitable for depths of 25 feet with 
widths from 22 to 72 inches, and due to the arrangement permit- 
ting buckets to be moved sideways they will cut trenches of differ- 
ent widths with the same size buckets.. This is an advantage 
where there are to be irregularities in the width of the trench 
such as for manholes or changes in size of pipe. With excavating 

machines pipe can be laid 
within 3 feet of the moving 
buckets and the trench back- 
filled immediately, thus mak- 
ing an appreciable saving 
in the amount of sheet- 
ing. In the construction of 
trenches for drain tile at 
Garden Prairie, Illinois, the 
sheeting was built in the 
form of a box or shield, 
fastened to the rear of the 
machine and pulled along 
it as is shown in 




FIG. 91. Movable Sheeting Fastened to 

Traction Ditcher. 
From Eng. News-Record, Vol. 82, 1919, p. 740. 



after 
Fig. 91. 

The performance of this type of excavating machine under 
suitable conditions is large. A remarkable record was made by 
Ryan and Co. in Chicago, 1 with an excavating machine. 1338 
feet of 32-inch trench were excavated to an average depth of 8^ 
feet in 7 hours, or an average of 160 cubic yards per hour. More 
could have been accomplished if it had not been for delays in 
supplies. Another crew at Greeley, Colorado, 2 with a Buckeye 
endless-chain ditcher weighing 17 tons and costing $5200, averaged 
232 cubic yards per day for 300 days, and the cost was 10.7 cents 
per cubic yard. A 15-ton Austin excavator can be expected to 
remove 300 to 500 cubic yards per day. 

The cost of operation of the machines is made up of items 
listed in Table 50. The figures given are merely suggestive. 

In making a comparison of the cost of hand and machine 



1 Eng. and Contracting, Vol. 48, 1917, p. 492. 

2 Earth Excavation by A. B. McDaniel. 



CONTINUOUS BUCKET EXCAVATORS 



249 





Per Day 


Total 


Labor: 
1 Operator at $150 per month 


$6 00 




1 Assistant Operator at $120 per month . . . 


4 00 




4 Laborers at $4 . 00 per day 


16 00 








>(, on 


Fuel: 

20 Gallons of gasoline at 28 cents 


5 60 


5 60 


Miscellaneous: 
Oil, waste, etc 


1 20 




Repairs and maintenance 


10 00 




Interest, 6 per cent on $10,000 for 150 days. . . 


4 00 




Depreciation, 200 working days per year and an 
8 year life 


11 11 


26 31 








Total cost per day : 




s:>7 '.! 









TABLE 51 

COMPARISON OF COST OF HAND EXCAVATION AND MACHINE EXCAVATION 
WITH CONTINUOUS-BUCKET EXCAVATOR 


Hand Work 


Per Day, 

Dollars 


Machine Work 


Per Day, 
Dollars 


Foreman 


4.00 
3.00 
2.50 
80.00 

i 


Engineer 


4.00 
2.50 
5.00 
4.00 
4 00 
3.00 
2.50 
8.00 
4.00 

10.00 


Timberman. 


Fireman. . 


Helper 


Coal 


40 Laborers at $2.00 


Team 


Total 


Foreman 


Pipe layer 


Helper 


2 Teams backfilling 


2 Helpers 


Interest, depreciation and 
repairs 


Total 


95.00 


54.50 







250 



CONSTRUCTION 



excavation the figures given in Table 51 are from " Excavating 
Machinery " by McDaniel, who quotes the cost of machine exca- 
vation from the manufacturers of the Parsons machine issued as 
the result of several years' experience with their excavator. In 
the comparison the hand crew is assumed to dig 315 linear feet 
of trench 28 inches wide by 12 feet deep in a day of 10 hours. 
This assumes that each man will excavate 7 cubic yards per day. 
The machine is assumed to excavate 250 feet of the same trench. 
The comparison indicates that an excavator will work at about 
50 per cent of the cost of hand excavation, if the cost of moving 
the machine is not included. 




FIG. 92. Carson Excavating Machine on Trench Excavation in South Mil- 
waukee. 

Courtesy, Mr. C. F. Henning. 

131. Cableway and Trestle Excavators. Cableway and 
trestle excavators are most suitable for deep trenches and crowded 
conditions. They should not be used for trenches much less than 
8 feet in depth. They differ from the continuous-bucket excava- 
tors in that the actual dislodgment of the material is done by pick 
and shovel, the excavated material being thrown by hand into the 
buckets of the machine. A machine of the Carson type is shown 
in Fig. 92. The machine consists of a series of demountable 
frames held together by cross braces and struts to form a semi- 
rigid structure. An I beam or channel extending the length of 



CABLEWAY AND TRESTLE EXCAVATORS 251 

i 

the machine is hung closely below the top of the struts. The 
lower flange of this beam serves as a track for the carriages which 
carry the buckets. All the carriages are attached to each other 
and to an endless cable leading to a drum on the engine. This 
cable serves to move the buckets along the trench. The buckets 
are attached to another cable which is wound around another 
drum on the engine and serves to lower or raise all t he buckets at 
the same time. In operation there are always at least two buckets 
for each carriage, one in the trench being filled and the other on 
the machine being dumped. There should be a surplus of buckets 
to replace those needing repairs. 

The machines may be from 200 to 350 feet in length, and the 
number of buckets which can be lifted at one time varies from 
one to a dozen or more. On trenches over 5 to 6 feet in width a 
double line of buckets is sometimes used. The entire machine 
rests on rollers and straddles the trench. It is moved along the 
trench by its own power, either by gearing or chains attached to 
the wheels, or by a cable attached to a dead-man ahead. 

The Potter trench machine differs from the Carson in that 
only 2 buckets are used at a time and these are carried on a car 
which travels on a track on top of the trestle. The movement of 
the buckets and the car are controlled by 2 dump men who 
ride on the car and who can raise or lower the buckets inde- 
pendently. 

The organization needed to operate these machines is: a 
lockman who locks and unlocks the buckets on the cable, a 
dumper, as many shovelers as there are buckets on the machine, 
and an engineman who is usually his own fireman. From 50 to 
400 cubic yards of material can be excavated in a day with one of 
these machines, dependent on the character of the material and 
the depth of the trench. H. P. Gillette in his Handbook of Cost 
Data reports that about 190 cubic yards were excavated per day 
with a Potter machine. The machine was 370 feet long. Six 
f-yard buckets were used, 4 in the trench and 2 on the carrier. 
The trench was 10j feet wide and 18 feet deep in wet sand and 
soft blue clay. The organization consisted of an engineman, a 
fireman, 2 dumpmen on the carrier, and from 17 to 21 excavating 
laborers depending on the kind and the amount of the excavation. 
In general the capacity of such machines is limited by the amount 
of material which can be shoveled into them by hand. 



252 CONSTRUCTION 

132. Tower Cableways. These are essentially of the same 
class as the trestle cableway machines. They differ in that the 
carriage supporting the buckets travels on a cable suspended 
between 2 towers instead of on a track supported on a trestle. 
As a rule only one bucket is handled in the machine at a time. 
They are used in sewer work only in exceptional cases as the 
towers must be taken down and re-erected each time that there 
is an advance in the trench greater than the distance between the 
towers. 

133. Steam Shovels. The use of steam shovels for the exca- 
vation of sewer trenches is becoming more prevalent because of 
their growing dependability and durability as compared with other 
machines, their adaptability for small trenches, and the relatively 
large number of widely different uses to which they can be put. 
In excavating a trench the shovel straddles the trench and runs on 
tractors, wheels, or rollers on either side of it. The shovel cuts 
the trench ahead of it. As a result it is difficult to set sheeting 
and bracing close to the end of the trench while the shovel is 
operating. Steam shovels are therefore not suitable for excava- 
tion in unstable material, unless the sheeting is driven ahead of 
the excavation. It is only in the softest ground that ordinary 
wood sheeting can be driven ahead of the excavation. Steel 
sheet piling is more suitable for such use. Fig. 93 l shows a shovel 
at work on a trench in Evanston, Illinois. 

Shovels are equipped with extra long dipper handles to adapt 
them to trench excavation. The dipper handle in the picture is 
longer than the standard for this type of machine. The method 
of supporting the shovel can be seen in the picture under the 
machine and the method of bracing and of finishing the trench 
by hand work are also shown. The excavated material is taken 
out in the shovel and dropped on the bank or into wagons. 

The limiting depth to which trenches can be excavated by 
steam shovels is about 20 to 25 feet, where the trench is too nar- 
row for the shovel to enter. Wider trenches are cut in steps of 
about 15 feet, the shovel working in the trench for additional 
depths. Shovels are now made to cut trenches as narrow as a 
man can enter to lay pipe. The greatest width that can be cut 
from one position of the shovel is from 15 to 40 feet, dependent on 
the size of the shovel. Occasionally , combination of a drag line 
1 Courtesy, Sanitary District of Chicago. 



STEAM SHOVELS 



253 



and a steam shovel can be used, as on the construction of the 
Calumet sewer in Chicago. On this work the first step was cut 
by a steam shovel. It was followed by a drag line resting on the 
step thus prepared, and excavating the remaining distance to 
grade. The depth of 
the trench in this work 
averaged about 25 to 
30 feet. 

Steam shovels are 
rated according to 
their tonnage and the 
capacity of the dipper 
in cubic yards. Both 
are necessary as the 
size of the dipper is 
varied for the same 
weight of machine, 
dependent on the char- 
acter of the material 
being excavated. For 
rock the dipper is 
made smaller than 
for sand. Gillette in 
his Hand Book of 
Cost Data gives the 
coal and water con- 
sumption of steam 
shovels as shown in 
Table 52. The per- 

formance of steam FIG. 93 .-Steam Shovel at Work on Sewer Trench 
shovels is recorded for North Shore Intercepting Sewer, Evanston, 
in Table 53. The Illinois, 
conditions of the 

work have a marked effect on the output of the shovel. A 
shovel in a thorough cut, i.e., in a trench just wide enough 
for the shovel to turn 180 degrees but too narrow to run 
cars or wagons along side of it, will perform less than one- 
half of the work that it can perform in a side cut, i.e., where 
the cars can be run along side the shovel which turns less than 
90 degrees. 




254 



CONSTRUCTION 



TABLE 52 

COAL AND WATER CONSUMPTION BY STEAM SHOVELS 
(From Handbook of Cost Data, by H. P. Gillette) 



Weight in tons 


35 


45 


55 


65 


75 


90 


Dipper, cubic yards 


U 


U 


If 


2 




3 


Coal, tons per 10 hour day . . 




1 






2 




Water, gallons per 10 hour day. . 


1500 


2000 


2500 


3000 


4000 


4500 



TABLE 53 

PERFORMANCE BY STEAM SHOVELS 













Cost in 






Weight 
in 


Dipper 
Cubic 


Depth 
of Cut, 


Width 
of Cut 


10-Hour 
Perform- 


Cents, 
per 


Authority 


Remarks 


Tons 


Yards 


Feet 




ance 


Cubic 
















Yard 






25 


1 


9 


36 in. 


85 


22.6 


R. T. Dana Eng. Rec., 


1 














69:581 




25 


1 


8 


35 in. 


96 


23.5 


do. 


2 


70 


2 


26 


16 ft. 


569 


6.7 


do. 


3 


30 


1 


15-18 


60 in. 


300 




A B McDaniel Excavat- 


4 














ing Machinery 




15 


| 


14 


134 ft. 


400 




Eng. Cont'r, 8-25-09 


5 




8 


36 


f Very 


16 yd. } 






6 








\ wide 


cars / 








55 








296 




H P Gillette's Cost Data 


7 


65 


2i 






280 




do 










Greater 
than 


\ 700 


30. 6 < 


G. C. D. Lenth, Eng. 
News-Record, 85:22 


8 








78 in. 


J 









Remarks: 

1. One runner at $5.00, one fireman at $2.31, two laborers at $1.70 each, supplies at $4.50, 

and interest and depreciation on 200 days per year, $4.00. Total per day, $19.21. 
Material, clay and gravel. 

2. Average of 11 jobs with the same shovel. 

3. Cost per day, one runner at $5.00, one craneman at $3.60, one fireman at $2.00, 

7 roller men at $1.50 each, supplies $9.00 and interest and depreciation on $9000 
at 200 days per year $8.00. Total, 338.10. 

4. Hard clay. 

5. Stiff clay for the basement of a building in Chicago. 

6. Stripping ore. This is a maximum record. The average was about three hundred 

and twenty 16 cubic yard cars per day. 

7. Blasted mica schist. 

8. General average. 



DRAG LINE AND BUCKET EXCAVATORS 255 

134. Drag Line and Bucket Excavators. A drag line exca- 
vator is shown in Fig. 94. The back of the bucket is attached to 
a drum on the engine by means of a cable passing over the wheel 
in the end of the long boom. The front of the bucket is attached 
by another cable directly to another drum on the engine. In 
operation the bucket is raised by its rear end and dropped out to 
the extremity of the boom. It is then dragged over the ground 
towards the machine, digging itself in at the same time. When 
filled the bucket is raised by tightening up on the two cables, 
swung to one side by means of the movable boom, and dumped. 





FIG. 94. Drag Line at Work on Trench for Drain Tile. 

Drag line excavators will perform as much work as steam 
shovels under favorable conditions. They are less expensive in 
first cost and operation, and are equally reliable but they are not 
adapted to the more difficult situations where steam shovels can 
be used to advantage. Drag lines are suitable only for relatively 
wide trenches in material requiring no bracing, and in a locality 
where relatively long stretches of trench can be opened at one 
time. 

The bucket excavator differs from the drag line in that the 
bucket can be lifted vertically only and the types of buckets used 
in the two types of machine are different. The bucket may be 
self filling of the orange-peel or clam-shell type, or a cylindrical 
container which must be filled by hand. A drag line can be 



256 CONSTRUCTION 

easily converted into a boom and bucket excavator. Boom and 
bucket excavators are well adapted to use in deep, closely braced 
trenches and shafts. 

136. Excavation in Quicksand. 1 A sand or other granular 
material in which there is sufficient upward flow of ground water 
to lift it, is known as quicksand. Its most important property, 
from the viewpoint of sewer construction, is its inability to sup- 
port any weight unless the sand is so confined as to prevent flow- 
ing of the sand, or unless the water is removed from the sand. 

Excavation in quicksand is troublesome and expensive and is 
frequently dangerous. The material will flow sluggishly as a 
liquid, it cannot be pumped easily, and its excavation causes the 
sides of the trench to fall in or the bottom to rise. The founda- 
tions of nearby structures may be undermined, causing collapse 
and serious damage. These conditions may arise even after the 
backfilling has been placed unless proper care has been taken. 
The greatest safeguard against such dangers is not only to exer- 
cise care in the backfilling to see that it is compactly tamped and 
placed, but to leave all sheeting in position after the completion of 
the work. 

The ordinary method of combating quicksand and in conduct- 
ing work in wet trenches is to drive water-tight sheeting 2 or 3 feet 
below the bottom of the trench, and to dewater the sand by pump- 
ing. When dry it can be excavated relatively easily. A more 
primitive but equally successful method is to throw straw, brick- 
bats, ashes, or other filling material into the trench in order to 
hold the excavation once made, or this may supplement the 
attempts at pumping, or the wet sand may be bailed out in buckets. 
Successful excavation in quicksand requires experience, resource- 
fulness, and a careful watch for unexpected developments. The 
well points described in Art. 142 are used for dewatering quick- 
sand. 

136. Pumping and Drainage. Ground water is to be expected 
in nearly all sewer construction and provision should be made for 
its care. Where geological conditions are well known or where 
previous excavations have been made and it is known that no 
ground water exists it may be safe to make no provision for 
encountering ground water. Where ground water is to be expected 

1 See article by J. R. Gow, Journal New England Waterworks Ass'n, 
Sept., 1920, also Public Works, Vol. 50, p. 98. 



TRENCH PUMP 



257 



the amount must remain uncertain within certain rather wide 
limits until actually encountered. 

In order to avoid the necessity for pumping, or working in wet 
trenches it is sometimes possible to build the sewer from the low 
end upwards and to drain the trench into the new sewer. The 
wettest trenches are the most difficult to drain in this manner as 
the material is usually soft and the water so laden with sediment 
as to threaten the clogging of the sewer. It is undesirable to run 
water through the pipes until the cement in the joints has set. 
This necessitates damming up the trench for a period which may 
be so long as to flood the trench or delay the progress of the work. 
If it is not possible to drain the trench through the sewer already 
constructed the amount of water to be pumped can be reduced 
by the use of tight sheeting. 

Pumps for dewatering trenches must be proof against injury 
by sand, mud, and other solids in the water. For this purpose 
pumps with wide passages and without valves or 
packed joints are desirable. The types of pumps 
used are: simple flap valve pumps improvised on 
the job, diaphragm pumps, jet pumps, steam 
vacuum pumps, centrifugal pumps, and recipro- 
cating pumps. All are of the simplest of then- 
type and little attention is paid to the economy of 
operation because of the temporary nature of their 
service. 

137. Trench Pump. A simple pump which can 
be improvised on the job is shown in section in 
Fig. 95. Its capacity is about 20 gallons per 
minute but its operation is backaching work. It is 
inexpensive, quickly put together and may be a 
help in an emergency. It is to be noted that the 
passages are large and straight, that there are no FlQ ^ 
packed joints, and that the velocity of flow is so improvised 
small that it is not liable to clogging by picking up Trench Pump, 
small objects. 

138. Diaphragm Pump. The type of pump shown in Fig. 96 
is the most common in use for draining small quantities of water 
from excavations. It is known as the diaphragm pump from the 
large rubber diaphragm on which the operation depends. The 
pump is made of a short cast-iron cylinder, divided by the rubber 




Canvas ' 
or 
Leather 




258 



CONSTRUCTION 



diaphragm or disk to the center of which the handle is connected. 
The valve is shown at the center of the disk. As the diaphragm 
is lifted the valve remains closed, creating a partial vacuum in the 

suction pipe and at the 
same time 'discharging 
the water which passed 
through the valve on the 
previous down stroke. 
When the valve is 
lowered the foot valve 
on the suction pipe 
closes, holding the water 
in place, and the valve 
in the pump opens 
allowing the water to 
flow out on top of the 
^BET^- disk * be discharged 
on the next up stroke. 
Table 54 shows the 
capacities of some dia- 
phragm pumps as rated 
by the manufacturers. The smaller sizes are the more frequently 
used and are equipped with a 3-inch suction hose with strainer and 
foot valve. They are not adapted to suction lifts over 10 to 12 
feet. Where greater lifts are necessary one pump may discharge 
into a tub in which the foot valve of a higher pump is submerged. 

TABLE 54 
CAPACITIES OF DIAPHRAGM PUMPS 




FIG. 96. Diaphragm Pump. 
Courtesy, Edson Manufacturing Co. 



Diameter of 


Diameter of 


Length of 


Capacity per 


Cylinder, 


Suction, 


Stroke in 


Stroke, 


Inches 


Inches 


Inches 


Gallons 


6 


3 


4 


0.49 


81 


4 


6 


1.47 


9* 


2* 




0.75 


12** 


3 




1.25 


12i* 


Power driven by 1 


horse-power engine 


0.58f 



* Diameter of diaphragm, 
t Gallons per minute 



JET PUMP 



259 



139. Jet Pump. The simplicity of the parts of the jet pump 
is shown in Fig. 97. It has a distinct advantage over pumps 

containing valves and moving 

parts in that there are no obstruc- 
tions offered to the passage of 
solids as well as liquids through 
the pump. It is not economical 
in the use of steam, however. It 
operates by means of a steam jet 
entering a pipe at high velocity 
through a nozzle. This action 
causes a vacuum which will lift 
water from 6 to 10 feet. The 
lower the suction lift, however, 
the greater the efficiency of the 
work. The sizes and capacities 
of jet pumps as manufactured by 
the J. H. McGowan Co. are shown 
in Table 55. 




FIG. 97. McGowan Steam Jet 

Pump. 
Courtesy, The John H. McGowan Co. 



TABLE 55 

CAPACITIES OF JET PUMPS 
(J. H. McGowan Co.) 



Size of Pump 






Capacity, 


Approximate 


and 


Discharge Pipe, 


Steam Pipe, 


Gallons 


Horse-power 


Suction Pipe, 






per 


Required 


Inches 


Inches 


Inches 


Minute 




1 


i 


1 


8 


2 


1 


1 


i 


15 


3 


H 


i 


* 


20 


4 


H 


U 


! 


30 


6 


2 


H 


! 


40 


8 


2* 


2 


i 


50 


10 


3 


2 


i 


60 


15 


4 


Si 


U 


85 


25 



140. Steam Vacuum Pumps. This type of pump depends on 
the condensation of steam in a closed chamber to create a vacuum 
which lifts water into the chamber previously occupied by the 



260 



CONSTRUCTION 



steam and from which the water is ejected by the admission of 
more steam. The best known pumps of this type are the Pulsom- 
eter, manufactured by the Pulsometer Steam Pump Co., the 
Emerson, manufactured by the Emerson Pump and Valve Co.; 
and the Nye Pump, manufactured by the Nj^e Steam Pump and 
Machinery Co. 

A section of a Pulsometer is shown in Fig. 98. It consists of 
two bottle-shaped chambers A and B with their necks communi- 




K-2 




K-I 



Sect 
Discharge 



through 
Chamber 



FIG. 98 Pulsometer Steam Vaccum Pump. 

eating at the top and each opening into the outlet chamber 
through a check valve. Steam is admitted at the top and enters 
chamber A or B according to the position of the steam valve C 
as shown. This steam valve is a ball which is free to roll either 
to the right or left and forms a steam-tight joint with whichever 
seat it rests upon. In normal operation chamber A would be 
filled with water as the steam enters the cylinder. At the same 
time a check valve at the top opens to admit a small quantity of 
air which forms a cushion insulating the steam from the water, 
reduces the condensation of the steam, and serves as a cushion 



STEAM VACUUM PUMPS 



261 



for the incoming water on the opposite stroke. The pressure of 
the steam depresses the surface of the water without agitation 
and forces the water through the check valve F into the discharge 
chamber 0. When the water falls to the level of the discharge 
chamber the even surface is broken up and the intimate contact 
of the steam and water condenses the former instantaneously. 
This forms a vacuum in chamber A which, assisted by a slight 
upward pressure in chamber B 
caused by the incoming water, 
immediately pulls the ball C 
over to the other seat and 
directs the steam into chamber 
B. The vacuum in chamber A 
now draws up a new charge of 
water through the suction pipe 
into the chamber. 

A section of the Emerson 
pump is shown in Fig. 99. The 
pump consists of two vertical 
cylinders B and C. Each 
chamber has a suction valve 
L at the bottom, opening up- 
ward from a common chamber 
from which the discharge pipe 
U extends. On the top of 
each chamber is a baffle plate 
G which operates to distribute 
the steam evenly to the two 
chambers and to prevent it 
from agitating the surface of 
the water in the chambers. 
A condenser nozzle F is con- 
nected with the bottom of the opposite chamber by a pipe 
into which a check valve opens upward. As the pressure in 
the chamber alternates water will be injected through F into the 
opposite chamber and condense the steam therein, promptly 
forming a vacuum. An air valve P admits a small quantity of 
air while the chamber is filling with water, the air acting as an 
insulating cushion as in the Pulsometer. Valve 0, just above the 
top connection S is used to regulate the amount of steam that 




FIG. 99. Emerson Steam Vaccum 
Pump. 



262 CONSTRUCTION 

enters the pump. The top connection S has two ports, one leading 
to each chamber. An oscillating valve enclosed in it admits the 
steam through these ports to the two chambers alternately. This 
valve is driven by a small three-cylinder engine, the crank shaft of 
which extends into the top connection in the center of the bearing 
on which the valve oscillates. A positive geared connection is 
made between the valve and the engine and so arranged that the 
engine will run faster than the valve. 

The action of these pumps consists of alternately filling and 
emptying the two chambers. They will continue operation with- 
out attention or lubrication so long as the steam is turned on. In 
view of the simplicity of their operation and make-up, their ability 
to handle liquids heavily charged with solids, and their reasonable 
steam consumption these pumps are widely used for pumping 
water in construction work. They have an added advantage 
that no foundation or setting is required for them as they can be 
hung by a chain from any available support. 

These pumps are manufactured in sizes varying from 25 to 
2500 gallons per minute at a 25-foot head, and with a steam con- 
sumption of about 150 pounds per horse-power hour. They 
reduce about 4 per cent in capacity for each 10 feet of additional 
lift. They will operate satisfactorily between heads of 5 to 150 
feet, with a suction lift not to exceed 15 feet. Lower suction lifts 
are desirable and the best operation is obtained when the pump is 
partly submerged. The steam pressure should be balanced against 
the total head. It varies from 50 to 75 pounds for lifts up to 
50 feet, and increases proportionally for higher lifts. The dryer 
the steam the lower the necessary boiler pressure. 

141. Centrifugal and Reciprocating Pumps. The details of 
these pumps, their adaptability to various conditions, and their 
capacities are given in Chapter VII. The centrifugal is better 
adapted to trench pumping as it is not so affected by water con- 
taining sand and grit, but for clear water, high suction lifts and 
fairly permanent installations, reciprocating pumps can be used 
with satisfaction. 

142. Well Points. In dewatering quicksand a method fre- 
quently attended with success is to drive a number of well, points 
into the sand and connect them all to a single pump. Figure 100 
shows a well point system used on sewer work in Indiana. The 
well points are 3 feet apart and are connected to a 2|-inch header 



ROCK EXCAVATION 263 

which in turn is connected to six Nye pumps, each with a capacity 
of 200 gallons per minute for a lift of 50 feet. The number and 
size of well points and pumps to use will depend on conditions as 
met on the job. On a piece of work in Atlantic City 1 the equip- 
ment consisted of two complete outfits each comprising one hundred 
1^-inch by 36-inch No. 60 well points, one hundred 6-foot lengths 
of rubber hose, about 600 feet of suction main, one hundred 
valved T connections, and a 7 X 8-inch Gould Triplex Pump with 
a capacity of 200 gallons per minute, belted to a 1\ horse-power 
motor. 




FIG. 100. Well Points Pumped by Nye Steam Vacuum Pump. 

143. Rock Excavation. A common definition of rock used in 
specifications is: whenever the word Rode is used as the name of 
an excavated material it shall mean the ledge material removed 
or to be removed properly by channeling, wedging, barring, or 
blasting; boulders having a volume of 9 (this volume may be 
varied) cubic feet or more, and any excavated masonry. No soft 
disintegrated rock which can be removed with a pick, nor loose 
shale, nor previously blasted material, nor material which may 
have fallen into the trench will be measured or allowed as rock. 

Channeling consists in cutting long narrow channels in the 

rock to free the sides of large blocks of stone. The block is then 

loosened by driving in wedges or it is pried loose with bars. It is 

a method used more frequently in quarrying than in trench exca- 

1 Eng. News, Vol. 75, 1916 p. 1050. 



264 



CONSTRUCTION 



vation where it is not necessary to preserve the stone intact. In 
blasting, a hole is drilled in the rock, and is loaded with an explosive 
which when fired shatters the rock and loosens it from its position. 
In drilling rock by hand the drill is manipulated by one man 
who holds it and turns it in the hole with one hand while striking 
it with a hammer weighing about 4 pounds held in the other hand, 
or one man may hold and turn the drill while one or two others 
strike it with heavier hammers. In churn drilling a heavy drill 

is raised and dropped in the hole, 
the force of the blow developing 
from the weight of the falling 
drill. Hand drills are steel bars 
of a length suitable for the depth 
of the hole, with the cutting edge 
widened and sharpened to an 
angle as sharp as can be used 
without breaking. The drill bar 
is usually about |th of an inch 
smaller than the diameter of the 
face of the drill. 

Wedges used are called plugs 
and feathers. They are shown 
in Fig. 101 which shows also the 
method of their use. The feathers 
are wedges with one round and 
one flat face on which the flat 
faces of the plug slide. 

144. Power Drilling. In power drilling the drill is driven by a 
reciprocating machine which either strikes and turns the drill in 
the hole, or lifts and turns it as in churn drilling, or the drill may 
be driven by a rotary machine which is revolved by compressed 
air, steam, or electricity. There are many different types of 
machines suitable for drilling in the different classes of material 
encountered and for utilizing the various forms of power available. 
A jack hammer drill is shown in Fig. 102. In its lightest 
form the drill weighs about 20 pounds and is capable of drilling 
f-inch holes to a depth of 4 feet. Heavier machines are available 
for drilling larger and deeper holes. The same machine can be 
adapted to the use of steam or compressed air. When in use the 
point of the drill is placed against the rock and a pressure on the 




FIG. 101. Plug and Feathers for 
Splitting Rock. 



POWER DRILLING 



265 



handle opens a valve admitting air or steam. The piston is caused 
to reciprocate in the cylinder, striking the head of the drill at 
each stroke. The drill is revolved in the hole by hand or by a 
mechanism in the machine. A hollow drill can be used by means 
of which the operator admits air or steam to the hole, thus blow- 
ing it out and keeping it clean. These machines have the advan- 
tage of small size, portability and simplicity. They can be easily 
and quickly set up and the drills can be changed rapidly. Their 
undesirable features are the vibration transmitted to the operator 
and the dust raised in the trench. 





i St'd Drill 
JhroHle- 



FIG. 102. Jack Hammer Rock Drill. 



FIG. 103. Tripod Drill. 



A type of drill heavier and larger than the jack hammer drill 
is shown in Fig. 103. It requires some form of support such as a 
tripod, or in tunnel work it can be braced against the roof or sides. 
Some data on steam and air drills are given in Table 56. The 
effect of the length of the transmission pipe, temperature of the 
outside air, pressure at the boiler or compressor, etc., will have a 
marked effect on the amount of steam or air to be delivered to the 
drill. Compressed air is affected more than steam by these out- 
side factors, but it has an advantage in that as it loses in pressure 
it increases in volume so that the loss of power is not so marked. 
Gillette states: 



266 CONSTRUCTION 

We may assume that a cubic foot of steam will do 
practically the same work in a drill as a cubic foot of com- 
pressed air at the same pressure, because neither the steam 
nor the air acts expansively to any great extent in a drill 
cylinder, due to the late cut-off. This being so ... one 
pound of steam is equivalent to nearly 30 cubic feet of 
free air ... all at the same pressure of 75 pounds per 
square inch. If a drill consumes at the rate of 100 cubic 
feet of free air per minute ... it would therefore consume 
240 pounds of steam (at 75 pounds pressure) per hour. 
. . . Where not more than three or four drills are to be 
operated, probably no power can equal compressed air 
generated by gasoline. It will require 12 horse-power to 
compress air for each drill, hence 1^ gallons of gasoline will 
be required per hour per drill while actually drilling. 

TABLE 56 

DATA ON ROCK DRILLS 
(From H. P. Gillette) 



Diameter of cylinder in inches 
Length of stroke in inches . . 


2* 
5 


25- 
6 


2! 
6i 


3i 

61 


3i 
6f 


3f 

7i 


Length of drill from end of crank to 
end of piston 


36 


43 


50 


50 


50 


52 


Depth of hole drilled without change 
of bit, inches 


15 


20 


24 


24 


24 


24 


Diameter of supply inlet. Standard 


| 


| 


f 


1 


1 


lj 


Approximate strokes per minute with 


500 


450 


375 


350 


325 


300 


Depth of vertical hole each machine 
will drill easily, feet 


6 


8 


10 


14 


16 


20 


Diameter of holes drilled, inches 
Diameter of octagon steel, inches. . . . 
Best size of boiler to give plenty of 
steam at high pressure, horse-power 
Best size of supply pipe to carry 
steam 100 to 200 feet, inches . . . 


Itoi 


| 


I to 1 
8 
J 


1 to 1J s 

1 to 1J 

8 
J 


s desired 
U toll 

9 

1 


lj to li 
10 
1 


Utoll 
12 
\\ 


Weight of drill unmounted, with 
wrenches and fittings, not boxed, 


128 


190 


265 


315 


385 


390 


Weight of tripod, without weights, 


80 


160 


160 


160 


210 


275 


Weight of holding down weights, 


120 


270 


270 


285 


330 


375 


Cubic feet of free air per minute 
required to run one drill at 100 


92 


104 


126 


146 


154 


160 

















For more than one drill, multiply the value in the above line by the following factors: 
For 2 drills, 1.8; 5by4.1; 10by7.1; 15by9.5; 20 by 11. 7; 30 by 15.8; 40by21.4; 70 by 
33.2. 



POWER DRILLING 267 

Since gasoline air-compressors are self regulating, when the 
drill is not using air very little gasoline is burned by the 
gasoline engine driving the compressor. A gasoline com- 
pressor possesses other very important economic advan- 
tages over a small steam-driven plant. First, there is the 
saving in wages of firemen and second, there is the saving in 
hauling and pumping of water and the hauling of fuel. 
The cost of gasoline is often less than the cost of coal for 
operating a small plant. 

An electric drill 1 operated on the principle of the solenoid 
does away with motor, valves, pipes, vapor, freezing, and other 
difficulties attendant on the use of steam or air. 

The rates of drilling in different classes of rock are shown in 
Table 57. Frequent changes of drills and relocation of tripods 
will materially reduce the performance of a drill, for as much as 
45 minutes may be lost in making a new set up. In this the jack 
hammer drills show their advantage as no time is lost in a set up. 

TABLE 57 
RATES OF ROCK DRILLING 

Rates in Feet per Ten-hour Shift. Vertical Holes 10-20 Feet Deep. 
(From Gillette) 

Hard Adirondack granite 48 

Maine and Massachusetts granite 45-50 

Mica-schist of New York City. Possible 60-70 

Mica-schist of New York City. Average 40-50 

Hard, Hudson River trap rock 40 

Soft red sand stone of Northern New Jersey 90 

Hard limestone near Rochester, N. Y 70 

Limestone of Chicago Drainage Canal 70-80 

Douglass, Indiana, syenite. Difficult set ups 36 

Canadian granite on Grand Trunk R. R 30 

Windmill point, Ontario limestone: 

3f-inch drills 75 

2j-inch drills 00 

2}-inch drills 37 

146. Steam or Air for Power. The choice between steam or 
air is dependent on the conditions of the work. Steam is unde- 
sirable in tunnels on account of the heat produced. In open cut 
1 Mun. Engineering, Vol. 53, p. 6. 



268 



CONSTRUCTION 



work it is at a disadvantage because of the loss of power due to 
radiation from the hose or pipe. The life of the hose is not so 
long as when air is used, escaping steam causes clouds of vapor 
which obscure the work, and serious burns may occur due to hot 
water thrown from the exhaust. It is advantageous since leaks 
may be easily discovered and remedied, it requires less machinery 
than air, and it is sometimes less expensive. With compressed 
air, gasoline or electric motors can be used for operating the com- 
pressors. 

TABLE 58 
ROCK BLASTING 
(From Gillette) 







Depth 


Distance 


Distance 


Character of Material 


Powder Used per Hole 


of 
Hole, 


Back of 
Face, 


Hole to 
Hole, 






Feet 


Feet 


Feet 


Limestone of Chicago 










Drainage Canal 


40 per cent dynamite 


12 


8 


8 


Sandstone s 


200 pounds 


1 20 


18 


14 




black powder 


J 






Granite \ 


2 pounds 


Il2 


4i 


4to 5 




60 per cent dynamite 


J 






Pit Mining, Treadwell, 










Mine, Alaska 




12 


2? 


6 













146. Depth of Drill Hole. The depth of the hole is dependent 
on the character of the work. The deepest holes can be used in 
open cut work where the shattered rock is to be removed by steam 
shovel. The face can be made 10 to 15 feet high. The depth of 
the hole in center cut tunnel facings are from 6 to 10 or even 12 
feet. In the bench the depth is equal to the height of the bench. 
In narrow trenches where the rock is to be removed by derrick or 
thrown into a bucket by hand, the hole should be sufficiently deep 
to shatter the rock to a depth of at least 6 inches below the 
finished sewer. Frequently shooting to this depth at one shot 
cannot be done due to the built up condition of the neighborhood 
or other local factors. The depth of the hole in trench work 
should not much exceed the distance between holes. Deep holes 



SPACING OF DRILL HOLES 269 

are usually desirable as a matter of economy in saving frequent 
set ups, but the holes cannot be made much over 20 feet in depth 
without increasing the friction on the drill to a prohibitive amount. 

147. Diameter of Drill Hole. The diameter of the hole should 
be such as to take the desired size of explosive cartridge. The 
common sizes of dynamite cartridges are from inch to 2 inches in 
diameter. In drilling, the diameter of the hole is reduced about one- 
eighth of an inch at a time as the drill begins to stick. This reduc- 
tion should be allowed for, and experience is the best guide for the 
size of the hole at the start. In general the softer or more faulty 
or seamy the rock, the more frequent the necessary reductions in 
size of bit. 1 For hard homogeneous rock the holes can be drilled 
10 feet or more without changing the size of the drill bit. 

148. Spacing of Drill Holes. The spacing of holes in open 
cut excavation is commonly equal to the depth of the hole. The 
character of the material being excavated has much to do with the 
spacing of the holes. The spacing, diameter and depth of holes 
used on some jobs is shown in Table 58. Gillette states: 

It is obviously impossible to lay down any hard and 
fast rule for drill holes. In stratified rock that is friable, 
and in traps that are full of natural joints 'and seams, it is 
often possible to space the holes a distance apart somewhat 
greater than their depth, and still break the rock to com- 
paratively small sizes upon blasting. In tough granite, 
gneiss, syenite, and in trap where joints are few and far 
between, the holes may have to be spaced 3 to 8 feet apart 
regardless of their depth for with wider spacing the blocks 
thrown down will be too large to handle with ordinary 
appliances. Since in shallow excavations the holes can 
seldom be much further apart than one to one and one-half 
times their depth we see that the cost of drilling per cubic 
yard increases very rapidly the shallower the excavation. 
Furthermore the cost of drilling a foot of hole is much 
increased where frequent shifting of the drill tripod is 
necessary. 

The common practice in placing drill holes is to put 
down holes in pairs, one hole on each side of the proposed 
trench; and if the trench is wide one or more holes are 
drilled between these two side holes 2 but in narrow trench 

1 For types of drill bits see article by T. H. Proske, Mining and Scientific 
Press, March 5, 1910. 

1 These intermediate holes are seldom more than 3 feet apart. 



270 CONSTRUCTION 

work, such as for a 12-inch pipe, one hole in the middle of 
the trench will usually prove sufficient. 

The holes are spaced about 3 feet apart longitudinally. After 
the holes have been completed they should be plugged to keep 
out dirt and water. 

SHEETING AND BRACING 

149. Purposes and Types. Sheeting and bracing are used in 
trenching to prevent caving of the banks and to prevent or retard 
the entrance of ground water. The different methods of placing 
wooden sheeting are called stay bracing, skeleton sheeting, poling 
boards, box sheeting, and vertical sheeting. Steel sheeting is 
usually driven to secure water tightness and if braced the bracing 
is similar to the form used for vertical wooden sheeting. 

150. Stay Bracing. This consists of boards placed vertically 
against the sides of the trench and held in position by cross 
braces which are wedged in place. The purpose of the board 
against the side of the trench is to prevent the cross brace from 
sinking into the earth. The boards should be from 1^X4 inches 
to 2X6 inches and 3 to 4 feet long. The cross braces should not 
be less than 2X4 inches for the narrowest trenches and larger 
sizes should be used for wider trenches. The spacing between the 
cross braces is dependent on the character of the trench and the 
judgment of the foreman. Stay bracing is used as a precautionary 
measure in relatively shallow trenches with sides of stiff clay or 
other cohesive material. It should not be used where a tendency 
towards caving is pronounced. Stay bracing is dangerous in 
trenches where sliding has commenced as it gives a false sense of 
security. The boards and cross braces are placed in position 
after the trench has been excavated. 

151. Skeleton Sheeting. This consists of rangers and braces 
with a piece of vertical sheeting behind each brace. A section of 
skeleton sheeting is shown in Fig. 104 with the names of the differ- 
ent pieces marked on them. This form of sheeting is used in 
uncertain soils which apparently require only slight support, but 
may show a tendency to cave with but little warning. When the 
warning is given vertical sheeting can be quickly driven behind 
the rangers and additional braces placed if necessary. The sizes 
of pieces, spacing and method of placing should be the same as 



POLING BOARDS 



271 



for complete vertical sheeting in order that this may be placed if 
necessary. 

152. Poling Boards. These are planks placed vertically 
against the sides of the trench and held in place by rangers and 
braces. They differ from vertical sheeting in that the poling 
board is about 3 or 4 feet long. It is placed after the trench has 
been excavated; not driven down with the excavation like vertical 
sheeting. An arrangement of poling boards is shown in Fig. 105. 
This type of support is used in material that will stand unsup- 
ported for from 3 to 4 feet in height. Its advantages lie in that no 
driving is necessary, thus saving the trench from jarring; no 




FIG. 104. Skeleton Sheeting. 



FIG. 105. Poling Boards. 

Showing Different Types of Crosa Bracing. 



sheeting is sticking above the sides of the trench to interfere with 
the excavation; and only short planks are necessary. 

The method of placing poling boards is as follows: Excavate 
the trench as far as the cohesion of the bank will permit. Poling 
boards, l inch to 2 inch planks, 6 inches or more in width, are 
then stood on end at the desired intervals along each side of the 
trench for the length of one ranger. The poling boards may be 
held in place by one or two rangers. Two are safer than one but 
may not always be necessary. If one ranger is to be used it is 
placed at the center of the poling board. After the poling boards 
are in position the rangers are laid in the trench and the cross 



272 



CONSTRUCTION 



braces are cut to fit. If wedges are to be used for tightening the 
cross braces, the cross braces are cut about 2 inches short. If 
jacks are to be used the braces are cut short enough to accommo- 
date the jacks when closed, or adjustable trench braces may be used 
as shown in Fig. 106. The use of extension braces saves the labor 
of fitting wooden braces. With everything in readiness in the 
trench, the cross brace is pressed against the ranger which is thus 
held in place. The wedge or jack is then tightened holding the 
poling boards and cross brace in position. 

153. Box Sheeting. Box sheeting is composed of horizontal 
planks held in position against the sides of the trench by vertical 
pieces supported by braces extending across the trench. The 

arrangement of planks and braces 
for box sheeting is shown in Fig. 
106. This type of sheeting is 
used in material not sufficiently 
cohesive to permit the use of 
poling boards, and under such 
conditions that it is inadvisable 
to use vertical sheeting which 
protrudes above the sides of the 
trench while being driven. This 
sheeting is put in position as the 
trench is excavated. No more 
of the excavation than the width 
of three or four planks need be 
unsupported at any one time. In 

Showing Different Types of Cross Bracing, placing the sheeting the trench 

is excavated for a depth of 12 to 

24 inches. Three or four planks are then placed against the sides 
of the trench and are caught in position by a vertical brace which 
is in turn supported by a horizontal cross brace. 

154. Vertical Sheeting. This is the most complete and the 
strongest of the methods for sheeting a trench. It consists of a 
system of rangers and cross braces so arranged as to support a 
solid wall of vertical planks against the sides of the trench. An 
arrangement of complete vertical sheeting is shown in Fig. 107. 
This type can be made nearly water tight by the use of matched 
boards, Wakefield piling, steel piling, etc. Wakefield piling is 
ma.de up of three planks of the same width and usually the same 




FIG. 106. Box Sheeting. 



VERTICAL SHEETING 



273 




thickness. They are nailed together so that the two outside planks 
protrude beyond the inside one on one side, and the inside one 
protrudes beyond the two out- 
side ones on the other side as 
shown in Fig. 108. The pro- 
truding inside plank forms a 
tongue which fits into the groove 
formed by the protruding out- 
side planks of the adjacent pile. 
In placing vertical sheeting 
the trench is excavated as far 
as it is safe below the surface. 
Blocks of the same thickness 
as the sheeting are then placed 
against the bank at the middle 
and at the ends of two rangers 
on opposite sides of the trench. j^ 10?. Vertical Sheeting. 
The ranger rest against blocks, 

and are held away from the sides of the trench by them. Cross 
braces are next tightened into position opposite the blocks to 

hold the tt 10 1 ** in P lace - Alier the 
skeleton sheeting is in place the planks 

forming the vertical sheeting are put in 

FIG. lOS.-Wakefield Sheet P sition ^ a chisel ed S e cut on the 
lower end of the plank, with the flat side 

against the bank. The planks should be 
driven with a maul, the edge of the plank following closely behind 
the excavation. In relatively dry work the driving of the plank 
is facilitated by excavating beneath the edge as 
it is driven. The upper end of the sheeting should 
be protected by a malleable steel or iron cap to 
prevent brooming of the lumber. A cap is shown 
in Fig. 109. A sledge hammer may be used for 
driving when the lumber is protected. If the 
sheeting is to start at the surface and is to be 
Driven by hand, the first length should not exceed 
4 feet unless a platform is erected for the driver. 
Succeeding lengths may be longer, the driver stand- 
ing on planks supported on the cross braces in the trench. Steam 
hammers and pile drivers are sometimes used for driving sheeting. 





FIG. 109. 

Section through 

Malleable Steel 

Driving Cap. 



274 CONSTRUCTION 

The framework of the sheeting should be placed with a cross 
brace for each end of each ranger and a cross brace for the middle 
of each ranger. If the ends of two rangers rest on the same cross 
brace an accident displacing one ranger will be passed on to the 
next and might cause a progressive collapse of a length of trench, 
whereas the movement of an independently supported ranger 
should have no effect on another ranger. The cross braces 
should have horizontal cleats nailed on top of them as shown in 
Fig. 107 to prevent the braces from being knocked out of place by 
falling objects. In driving vertical sheeting a vacant place will 
be left behind each cross brace corresponding to the original block 
placed to hold the ranger away from the bank. This is an unde- 
sirable feature in the use of vertical sheeting. It is ordinarily 
remedied by slipping in planks the width of the slot and wedging 
or nailing them against the convenient cross bracing. In extremely 
wet trenches, after all other pieces of vertical sheeting are in place, 
the original cleat behind the cross brace can be knocked out and a 
piece of sheeting slipped into this opening and driven. Care 
must be taken in this event not to drive the rangers down when 
driving the sheeting. If the bracing begins to drop, it should be 
supported by vertical pieces between the rangers and resting on a 
sill at the bottom of the trench. 

155. Pulling Wood Sheeting. Wood sheeting is pulled after 
the completion of the trench by a device shown in Fig. 110. In 
wet trenches where the removal of the sheeting 
would permit a movement of the banks, result- 
ing in danger to the sewer or other structures, 
the sheeting should be left in place in the trench. 
If sufficient saving can be made the sheeting is 
cut off in the trench immediately above the danger 
line, usually the ground water line. The cutting 

, is done with an axe or by a power driven saw 
FIG. 110. Steel , . , , ,, 
Clamp for Pull- devlsed for the purpose. 

ing Wood Sheet- 156. Earth Pressures. 1 The various theories 

ing. of earth pressure are so conflicting in their 

conclusions as to be confusing. Rankine's 

theory, the most frequently used, assumes that the pressure 

increases with the depth, whereas Meem's theory 2 leads 

1 Earth Pressures, Old Theories and New Test Results, Eng. News-Record, 
VoL 85, 1920, p. 632. 

2 Trans. Am. Society Civil Eng'rs, Vol. 60, 1908. 




EARTH PRESSURES 275 

to an opposite conclusion. The discussion following Meem's 
article is very illuminating. It indicates that no matter how 
good the theory, practical experience together with the use of 
generous sizes and close spacing are the best guides for bracing 
trenches and coffer dams. All are not possessed with the desired 
practical experience and some basis on which to commence work 
is essential. Another factor affecting computations of sizes 
based on theory is the tendency in practice to use the same size 
material for rangers and braces on any one job for all except very 
deep trenches and other special cases. Occasionally where there 
is an independent brace for each end of each ranger, the brace is 
made thinner, but is of the same depth as the ranger. 

The application of Rankine's theory of earth pressure to the 
computation of the sizes of rangers and braces will be shown. 
His formula for the active earth pressure against a retaining 
wall is: 

.cos 6 Vcos 2 6 cos 2 <f> 
P=whcos6 . 

cos 0+vcos 2 6 cos 2 <f> 

in which w = the weight of earth in pounds per cubic foot; 

h = depth in feet at point at which pressure is to be 

determined; 
6 = the angle of surcharge, or the angle which the surface 

makes with the horizontal; 
= the angle of repose of the earth. Usually taken as 

33-41' = l horizontal to 1 vertical; 
P = the intensity of pressure in pounds per square foot on 

a vertical plane in a direction parallel to the surface 

of the ground. 

In studying the pressures for trenches the surface of the ground 
will be assumed as horizontal and the formula reduces to 

1 sin < , 



. 
1+sm < 

167. Design of Sheeting and Bracing. The trench shown in 
Fig. Ill is assumed to be constructed in moist sand weighing 110 
pounds per cubic foot, with an angle of repose of 30 degrees. The 
material used for sheeting and bracing is yellow pine. The steps 



276 



CONSTRUCTION 



I 

I 



, F4"x6"\ 
K B-4 -H 



4"*6" 



6x8<. 



^ E> 

\< -e'-8--\ 



8-0 ^ 

4'iS" ** lo l 



} , p 

k 6-4 H 



ifr 



^ 



: 



taken in the design of the sheeting and bracing for this trench are 
as follows: 

1. Earth Pressure. Substituting the units given in the data, 
in Rankine's formula for earth pressures, 

P = 36.7ft. 

Because the earth has been freshly cut and will not be kept open 
long enough to break up the cohesiveness of the banks it is cus- 
tomary to reduce the assumed 
pressure by dividing by 2, 3, or 
4, according to the natural 
cohesiveness of the material. 
The cohesiveness of sand is not 
great, therefore the pressure 
will be assumed as one-half of 
the amount given by the form- 
ula, or 

p = 18h. 

2. Thickness of Sheeting and 
Spacing of Rangers. It is desir- 
able to use the same thickness 
of sheeting throughout the depth 
of the trench. Computations 
should therefore be commenced 
at the bottom of the trench 
where the pressures are the 
greatest and the thickest 
sheeting will be required. It 
is necessary to determine by 
trial a spacing for the rangers 
and a thickness of sheeting 
so that the sheeting is stressed 
to its full working strength. 

Having determined the thickness of the sheeting at the bottom, 

the remainder of the computations consists in determining the 

spacing of the rangers. 

In the example the lower ranger will be assumed as 3 feet from 

the bottom of the trench and the distance to the next ranger as 

4,feet, 




Diagram 
of Pressures 
on Sheeting. | 



FIG. 111. Diagram for the Design of 
Wood Sheeting. 



DESIGN OF SHEETING AND BRACING 



277 



The intensity of pressure at 22 feet 9 inches is 409.5 

pounds per square foot. 
The intensity of pressure at 26 feet 9 inches is 481.5 

pounds per square fot. 

The distribution of pressures is shown by the diagram on Fig. 111. 
The maximum bending moment is slightly below the point mid- 
way between the rangers and for a 12-inch strip is 10,500 inch 
pounds. 

Assuming 3 inch sheeting the maximum fiber stress is: 

Me 10,400X1.5X12 






12X27 



: = 568 pounds per square inch. 



The working strength of yellow pine as given in Table 59, is 
1200 pounds per square inch. Thinner sheeting should therefore 
be used. 

TABLE 59 
WORKING UNIT STRESSES FOR TIMBER 

The most used value in the Building Codes of Baltimore, Boston, Cin- 
cinnati, Chicago, District of Columbia, and New York City 



Wood 


Tension, 
lb.sq.in. 


Com- 
pression 
With 
Grain, 
Ib.sq. in. 


Com- 
pression 
Across 
Grain, 
Ib.sq. in. 


Trans- 
verse 
Bending, 
Ib.sq. in. 


Shear 
With 
Grain, 
Ib.sq. in. 


Shear 
Across 
Grain, 
Ib.sq. in. 


Yellow pine 


1200 


1000 


600 


1200 


70 


500 


White pine 


800 


800 


400 


800 


40 


250 


Spruce and Va. pine. 
Oak 


800 
1000 


800 
900 


400 
800 


800 
1000 


50 
100 


320 
600 


Hemlock 


600 


500 


500 


600 


40 


275 


Chestnut 


600 


500 


1000 


800 




150 


Locust 




1200 


1000 


1200 


100 


720 

















Aa published in American Civil Engineers Pocket Book. 

Assuming 2-inch sheeting, the fiber stress is 1,300 pounds per 
square inch. This stress is too large. By reducing the ranger 
spacing slightly the stress can be brought within the required 
limits. 

Assuming a ranger spacing of 3 feet 9 inches the depth to the 
upper ranger is changed to 23 feet and the maximum stress in the 



278 CONSTRUCTION 

2-inch sheeting becomes 1,140 pounds per square inch, a satis- 
factory result. The results for the computations for the other 
ranger spacings are shown in Table 60. The spacing of the 
rangers at the sheeting junctions is controlled by convenience 
and is not computed so long as it is obviously safe. 

3. Size of Rangers. The rangers will be assumed as 16 feet 
long with two end cross braces and one intermediate cross brace 
for each ranger. Starting as before at the bottom of the trench. 

The area of the panel below the ranger and between 
cross braces is 24 square feet. 

The average intensity of pressure is 28.25X18 = 508.5 
pounds per square inch. 

The load transmitted to the ranger is 6,000 pounds. 

Similarly the load transmitted to the ranger from the 
panel above is 6,890 pounds. 

The total distributed load on the ranger is 12,890 pounds. 

If b is the vertical dimension of the ranger and d is the hori- 
zontal dimension in inches, then from the beam theory, using / as 

M 

1,200 pounds per square inch, bd?=-, in which M is expressed 



in inch pounds. The maximum bending moment is 

Wl 12,200X8X12 

-5- = - - Q - = 155,000 inch-pounds. 

o o 

Therefore, bd 2 = 775. 

An 8X10 inch beam will fulfill the conditions closely. Substi- 
tuting these dimensions in the beam formula 

Mc = 155,000X5X12 
*~ I '' 8X1000 

= 1,160 pounds per square inch tension in outer fiber. The results 
of the computations for other rangers are shown in Table 60. 

4. Size of Cross Braces. The cross braces act as columns. 
The dimensions of the cross braces are determined by trial in such 
a manner that the vertical dimension of the brace is equal to the 
vertical dimension of the ranger and the compressive stress in 
pounds per square inch is computed from the expression, 



Adopted by the Am. Ry. and Maintenance of Way Ass'n in 1907. 



DESIGN OF SHEETING AND BRACING 



279 



TABLE 60 

COMPUTATIONS FOR SHEETING AND BRACING FOR TRENCH SHOWN IN 

FIG. Ill 

Material is moist sand weighing 1 10 pounds per cubic foot, with an angle of repose of 30. 
Lumber is yellow pine, with working stresses as given in Table 59. Working stresses for 



columns given as S I 1 



Sheeting 2 Inches X 12 Inches 


Cross Braces 






Maxi- 








Actual 


Allow- 




Maxi- 


mum 








In- 


able 


Depth 


mum 
Bending 
Moment, 


Fiber 
Stress, 
Pounds 


Depth and 
Description 


Total 
Ix>ad, 
Pounds 


Size. 
Inches 


tensity, 
Pounds 
per 


In- 
tensity, 
Pounds 




Inch- 
Pounds 


per 

Square 








Square 
Inch 


per 

Square 






Inch 










Inch 


23'-26.75' 


9100 


1140 


end at 26' 9" 


6,445 


4X8 


202 


784 


iy-23' 


8800 


1100 


int. at 26' 9" 


12,890 


4X8 


403 


784 


13'-17.5' 


8550 


1070 


end at 23' 0" 


6,393 


4X8 


200 


784 


8'- 13' 


7160 


900 


int. at 23' 0" 


12,785 


4X8 


400 


784 


<y-6' 


3000 


375 


end at 19' 0" 


3,930 


4X8 


123 


784 








int. at 19 7 0" 


7,860 


4X8 


246 


784 








end at 17' 6" 


3,566 


4X8 


112 


684 








int. at 17' 6" 


7,132 


4X8 


224 


684 








end at 13' 0" 


4,385 


4X8 


137 


684 








int. at 13' 0" 


8,770 


4X8 


274 


684 








end at 8' 0" 


2,270 


4X6 


95 


687 








int. at 8' 0" 


4,540 


4X6 


189 


667 








end at 6' 0" 


1,344 


4X6 


56 


584 








int. at 6' 0" 


2,687 


4X6 


112 


584 








end at 0* 0" 


432 


4X6 


18 


584 








int. at ff 0" 


863 


4X6 


36 


584 



Rangers 















Maxi- 






Area 


In- 








mum 


Maxi- 




of 


tensity 




Load Transmitted to 




Bending 


mum 


Depth 


Panel 
Below 


of 
Pressure, 


Total 
Load 


the Ranger from the 


Size. 


Moment 
in 


Stress 
Pounds 




this 


Pounds 


in 




Inches 


Thou- 


per 




Depth, 


per 


Pounds 










sand 


Square 




Square 


Square 




Panel 


Panel 


Both 




Inch- 


Inch 




Feet 


Inch 




Below 


Above 


Panels 




Pounds 




26* 9" 


24 


508.5 


12,200 


6000 


6890 


12,890 


8X10 


155 


1160 


23' 0" 


30 


448 


13,440 


6545 


6240 


12,785 8X10 


153 


1150 


19' 0" 


32 


378 


12,100 


5860 


2000 


7,860 1 8X10 


94.3 


708 


17' 6" 


12 


328.5 


3,942 


1942 


5190 


7,132 


8X10 


85.6 


636 


13' 0" 


36 


274.5 


9,880 


4690 


4080 


8,770 


8X10 


105 


790 


8'0" 


40 


189 


7,560 


3480 


1060 


4,540 


6X 8 


54.4 


850 


6'0" 


16 


126 


2,020 


960 


1727 


2,687 


6X 8 


32.2 


503 


O'O" 


48 


54 


2,590 


863 





863 


6X 8 


10.4 


161 



280 CONSTRUCTION 

in which S permissible crushing across the grain in a 

column whose length is greater than 15 diam- 
eters ; 

Si =unit working compressive strength of wood; 
Z = length of the column; 
d = smallest dimension of the column; 
I and d are in the same units. 

The lower intermediate cross brace supports a length of 8 feet of 
the lower ranger on which the load has been found to be 12,890 
pounds. The load on the end cross brace for the same ranger is 
one-half of this or 6,445 pounds. The length of each brace is 
4 feet 4 inches. From Table 59, & is 1,000 pounds per square 
inch. From the column formula, S is 784 pounds per square 
inch. 

A 4X8 inch cross brace is the smallest that is feasible. This 
is stressed only 12,890 pounds or 403 pounds per square inch, 
which is well within the permissible limits. The results of the 
other computations for cross braces are shown in Table 60. 

158. Steel Sheet Piling. This is coming into more general 
use with the increased cost of lumber and better acquaintance 
with its superiority over wood under many conditions. Although 
its first cost is higher than that of wood, the fact that with proper 
care it can be used almost an indefinite number of tunes renders 
it economical to contractors who may have an opportunity to 
make repeated use of it. The life of good yellow pine sheeting 
with the best of care may be as much as three or four seasons. 
With no particular care it will be destroyed at the first using. 
Fig. 112 shows various sections of steel piling used for trench 
sheeting. These forms are practically water tight and aid mate- 
rially in maintaining dry trenches. The piling can be made water 
tight by slipping a piece of soft wood between the steel sections 
when they are being driven, or by pouring in between the piles 
some dry material which will swell when wet. The piling is gen- 
erally driven by a steam hammer and is pulled by attaching a 
ring through a bolt hole in the pile, or by grasping the pile with a 
clutch that tightens its grasp as the pull increases. An inverted 
steam hammer attached to the pile is sometimes used in pulling 
it. The impulses of the hammer together with a steady pull on 
the cable serve to drag out the most stubborn piece of piling. 



LOCATING THE TRENCH 



281 



LINE AND GRADE 

159. Locating the Trench. In order to locate a trench a line 
of stakes should be driven at about 50-foot intervals along the 
center line of the proposed sewer before excavation is commenced. 
Reference stakes or reference points to this line are located at 
some fixed offset or easily described point, or the stakes marking 




15" Arched- Wet Section (No.S.RH 15) 

Section Modulus, Smalt Il.tO 46.Slb. persa. ft of Wall 22702 kg. per sgmettr of Wall. 

,lnttrTocM'l4.ll S8.l2lt>.nlin.ft.ofBor. 86.49* - lin. n .. Bar. 




l4"Arched-WebSecHon(No5.REI4) 

Station Modului. Single - 7.6/ is Ik n*r *n ft of Wall 170.90 kg.ptrfg.mtttr of Wall. 

i 60.76 ?, tin. Bar. 




I5x TC" Center- Flanqe Section. 
' (No.S.RFlB) 

Section Modulus, Single - 5.71 48 Ib. persq. ft. of Wall. 

n n ,lnterlocid*62S 60 Id.. Ifn.ft. of Bar. 





234. 34 kg. per so. mgttr of Waif. 
89.29 S lin. ,. Bar. 



- yr ' v 1 ^*.^ > 

X ^-- X l2|xl"Straight-Web Section. " 

(No.S.PAIZ) 

Section Modulus, Single 4.12 40 /b'peno. ft. of Wall. 195. 32 kg. per so;, merer of Wall. 

n f Interlocked* 4.21 42. Sit. .. lin.fr. of Bar. 63.25 ?, l,n. a Bar. 

FIG. 112. Sections of Lackawanna Steel Sheet Piling. 

the center line of the trench may be driven at some constant 
offset distance one side of the trench, in order to avoid danger of 
loss or disturbance of the stakes. Grade or cut is seldom marked 
on the line of preliminary stakes, although the approximate cut 
may be indicated. 

For hand excavation the foreman lays out the trench from 
these stakes. In machine work the operator guides the machine 
so as to follow the line of the stakes. 



282 



CONSTRUCTION 



160. Final Line and Grade. After the excavation of the trench 
has proceeded to within a foot or two of the final depth, the grade 
and line are transferred to markers supported over the center of 
the trench. The markers are horizontal boards spanning the 
trench and held in position either by nails driven into stakes at 
the side of the trench, by nails driven into the sheeting, or by 
weights holding the boards on the ground. Two stakes driven in 
the ground at the side of the trench as shown in Fig. 113 are the 
common method of support. If the banks are too weak to stand 
under the jarring of the driving of the stakes, or pavement or 
other causes prevent their use the horizontal cross piece may be 
weighted down by bricks or a bank of earth. The cross pieces 
are located about every 25 feet along the trench and at any con- 



CenhrLine- 




Center', Line- 



FIG. 113. Methods for the Support of the Grade Line. 

venient distance above the surface of the ground. The nearer 
the ground the stronger the support but the greater the inter- 
ference with work in the trench. The center line of the sewer is 
marked on the cross pieces after they are set, and vertical struts 
are nailed on them with one edge of the strut straight, vertical, 
and on the center line as shown in Fig. 1. The corresponding 
edge should be used on all struts in order to avoid confusion. 
The edge is placed in a vertical position by means of a plumb 
bob or carpenter's level. 

The cut to the invert of the sewer is recorded to an even 
number of feet where practicable by driving a nail in the upright 
strut so that the top edge of the nail is at the desired elevation 
above the sewer, or the upright is nailed with its top at the proper 
number of feet above the sewer invert. The cut is marked on 
the upright in feet, tenths, and hundredths from the recorded 
point to the elevation of the invert. 

The inspector should watch these grade markers with care by 
sighting back along them to see that they are in line and have not 



TRANSFERRING GRADE AND LINE TO THE PIPE 283 



moved. In quicksand or caving material the marks may move 
during the setting of the pipes and the levelman should be on the 
job constantly. 

When excavation is being done by machine the depth of the 
excavation is controlled by the operator who maintains a sighting 
rod on the machine in line with the grade marks on the uprights. 

161. Transferring Grade and Line to the Pipe. In transfer- 
ring grade and line to the sewer a light strong string is stretched 
tightly from nail to nail on the 

uprights marking the line and 
grade. A rod with a right angle 
projection at the lower end, as 
shown in Fig. 114, is marked 
with chalk or a notch at such 
a distance from the end that 
when the mark is held on the 
grade cord the lower portion of 
the rod which projects into the 
pipe will rest on the invert. The 
pipe is placed in line by hang- 
ing a plumb bob so that the 
plumb bob string touches the 
grade and center line cord. These 
marks are taken only as fre- 
quently as may be necessary to 

keep the sewer in line. An experienced workman can maintain 
the line by eye for considerable distances. Measurements should 
never be taken to the top of the pipe in order to determine position 
and grade as the variations in the diameter of the pipe may cause 
appreciable errors. 

The position and elevation of the forms for brick, concrete, 
and unit block sewers are located by reference to the grade line, 
or they may be placed under the immediate direction of the survey 
party, or by specially located stakes. For large sewers requiring 
deep and wide excavation the grade and line stakes are driven in 
the bottom of the trench about a foot above the finished grade. 
This requires the constant presence of an engineer who is usually 
available on work of such magnitude. 

162. Line and Grade in Tunnel. In tunnels, line and grade 
are given by nails driven in the roof, the progress of excavation or 




FIG. 114. Diagram Showing the Use 
of the Grade Rod for Fixing the 
Elevation of a Sewer. 



284 CONSTRUCTION 

the shield being followed by eye and the forms set by direct 
measurement to the nails. 

TUNNELING 

163. Depth. The depth at which it becomes economical to 
tunnel depends mainly upon the character of the material to be 
excavated and on the surface conditions. In soft dry material 
with unobstructed working space at the surface, open cut may be 
desirable to depths as great as 35 or 40 feet. Tunnels are cut in 
rock at depths of 15 feet or less. In some very wet and running 
quicksand encountered in the construction of sewers for the Sani- 
tary District of Chicago it was found economical to tunnel at depths 
of 20 feet and less. Crowded conditions on the surface, expensive 
pavements, or extensive underground structures near the surface 
may make it advantageous to tunnel at shallower depths than 
would otherwise be economical. Winter is the best season for 
tunneling as the workmen are protected from the elements and 
labor is more plentiful. 

164. Shafts. In sinking a shaft in soft material, the excava- 
tion is usually done by hand, the material being thrown into a 
bucket which is hoisted to the surface and dumped. The size of 
the shaft is independent of the size of the sewer and depends princi- 
pally on the machinery which it is necessary to lower into the 
tunnel. Ordinarily a shaft 6 feet in the clear is satisfactory. A 
method of timbering a shaft is shown in Fig. 115. Because of 
the timbering the shaft must be started sufficiently large at the 
top to finish with the desired dimensions at the bottom. This 
excess size is sometimes obviated by driving the sheeting at an 
angle to maintain the same size of shaft from top to bottom. 

In timbering a shaft as shown in Fig. 115 the upper frame is 
staked securely in position at the surface of the ground. This 
frame is composed of timbers fastened together in the form of a 
square with the ends of the timbers extending about 12 inches on 
all sides. The protruding ends are used to hold the frame in 
position. Excavation is begun inside the frame, and sheeting is 
driven around the outside of it as excavation progresses. Only 
two or three men can work advantageously at one time in these 
small shafts. The second frame is made up of the same size tim- 
bers, but all are cut off flush with the outside of the square. The 



SHAFTS 



285 



v 2 "Jingle Sheeting 



outside dimensions of this frame are such as to allow sheeting to be 
slipped in between it and the sheeting already driven. The frame 
is lowered into position and supported from the upper frame by 
vertical struts nailed to it. The lower end of the sheeting already 
driven is held out from the lower frame by blocks of the thickness 
of the next length of sheeting. These blocks are removed as the 
next length of sheeting is placed 
and driven. The driving of the 
sheeting is facilitated by excavating 
beneath it as it descends. 

The sizes of sheeting and timber- 
ing should be computed on the same 
basis as that for trench sheeting 
except that for depths greater than 
30 to 35 feet Rankine's Theory is 
not applicable and judgment must 
be relied on for computing the sizes 
for deep shafts. In stiff dry ma- 
terial the pressures will change very 
little as the depth increases. Sheet- 
ing is needed in shaft excavation 
in rock only to protect the work- 
men from falling fragments, but in 
sand, particularly in quicksand 
and in wet ground, the pressures 
increase directly with the depth and 
the sheeting should be computed 
accordingly. Care must be taken 
to prevent the formation of cavities 
behind the sheeting, to fill them if 
formed, and to see that all pieces 
of the sheeting and bracing have a 

firm bearing. It is difficult to prevent the collapse of the shaft 
once the movement of earth against the sheeting has commenced. 

Shafts are also sunk in soft ground by constructing a concrete 
or metal shell resting on a cutting shoe on the surface. The 
material inside is dug out and the shell sinks of its own or added 
weight. The first section of the shell may be from 5 to 10 feet 
long. As this section sinks other sections are added. This is 
called the caisson method. It is advantageous in wet ground and 




Sectiorf'jS-j-'ip A-A. 

FIG. 115. Section of Shaft Tim- 
bering. 

Abbot, Journal Western Society of 
Engineers, Vol. 22. 



286 CONSTRUCTION 

when the shafts are to be left as a permanent manhole. If a 
permanent shaft is to be left in an excavation being braced with 
wood, the permanent lining should follow within 20 to 30 feet of 
the shaft excavation. This is done to avoid the difficulty of 
maintaining a great length of temporary wood shaft with the 
danger of collapse, or of blocks or other objects falling on the 
workers below. 

The distance between shafts is controlled by the depth and 
size of the tunnel, surface conditions, and the character of the 
material being tunneled. Except where surface conditions are 
crowded the shallower the cover to the tunnel the more frequent 
the shafts. The advantage of frequent shafts lies in the possi- 
bility of removing excavated material from the tunnel promptly, 
and in making ventilation of the tunnel easier. The saving made 
by the construction of numerous shafts must be balanced against 
the extra cost of the shafts. For the shallowest tunnels the 
shafts are seldom placed closer than every 500 feet. 

165. Timbering. After the shaft has been excavated to the 
proper grade the tunnel is struck out either by cutting through 
the wooden sheeting or by removing portions of the caisson 
lining. Practically all tunnels except those in solid rock must be 
framed to some extent. Some of the types of frames used in 
tunnel construction are shown in Fig. 116. Different combina- 
tions of these may be used in different classes of materials. In 
solid rock which remains firm on exposure no timbering is neces- 
sary. Where the roof only need be supported and the sides are 
strong enough to be used for support, a timber " hitch " or frame 
supported on the sides of the tunnel may be used. This is suit- 
able for loose rock roofs with solid rock sides. Timbering such as 
is shown in the lower left-hand corner of Fig. 116 becomes neces- 
sary in extremely soft, wet, or swelling material, where the bottom 
and sides as well as the roof tend to push in. The remaining 
frame in Fig. 116 shows a form frequently used and lying 
between the two extremes indicated. In wet tunnels a channel 
may be cut in the bottom below the sill for drainage purposes as 
shown in this form. The needle beam method of timbering is 
also shown in Fig. 116. This method of timbering is used mainly 
near the heading because of the speed and ease with which it can 
be installed, but it is undesirable because of the space occupied. 

The distance between frames is dependent on the size of the 



TIMBERING 



287 



tunnel and the character of the material. It is seldom greater 
than 6 feet and the frames are sometimes placed touching each 
other. The size of the timbering is a matter of experience and is 
generally determined by the judgment of the responsible person 
in charge of the construction as the result of observation during 
the progress of the work. 

The sheeting between frames is called poling boards, or spiling 
or lagging according as it is sharpened and driven ahead of the 
excavation or placed after the excavation has progressed. The 






Needle 




Beam 




\ 



'/v'-v :-if-'- i :-:.'----~'^^' r ' >ir4?ib,;- : 
'' ' '-:-,--'';:--.-; . '' ".-. ' '--''';" ' 

Longitudinal Section. 




Longitudinal Section 
Showing Poling Boards. 

Types of Frames and Timbering 
for Tunnels. 



Transverse Section, 



Tunnel Bracing Showing 
Needle Beam Support- for Roof. 



FIG. 116. Types of Frames and Timbering for Tunnels. 

horizontal strips placed between the frames to keep them apart 
are called wales. 

In cutting out from the shaft in soft materials requiring sup- 
port, where the width of the tunnel is the same or smaller than 
that of the shaft, a frame with a maximum width four thicknesses 
of sheeting less than the width of the tunnel is set up against the 
lining of the shaft. The vertical side pieces of the tunnel frame 
rest on the bottom frame of the shaft as a sill and are securely 
wedged into position. As the lining of the shaft at the top is cut 
away the top poling boards of the tunnel are slipped in between the 
cap of the first tunnel frame and the shaft frame immediately above 



288 



CONSTRUCTION 



it. The poling boards are driven with an upward pitch so that 
there may be room to slip the second length of boards between the 
next tunnel frame and the first length of boards. The placing of 
the side sheeting follows in a similar manner. Excavation is then 
started and the poling boards driven to keep pace with it. The 
next frame is placed in position and the previous sheeting or 
boards wedged out a sufficient distance to allow the advance 
lining to be slipped in when the wedges are removed. Waling 
pieces are nailed firmly between the frames to hold them in posi- 
tion. The various phases in the driving of a 12-foot sewer tunnel 
in Seattle are shown in Fig. 117. 

In soft or running material it may be necessary to protect the 
face of the tunnel by horizontal boards, called breast boards, 



Driving HeaaingTunnet , .-. A"x6"Po//nqs .-4t 6 Walfnq .:: ,,-... ,. ; 

^^a^^pii^iaia^^^ & ! &jtmaH 



'/2'i/ZCrdwnBars'*-''' 

Phase I. Phase 2 




Phase 
6. 

FIG. 117. Stages of Sewer Tunneling. 

Eng. Record, Vol. 69, 1914, p. 195. 

wedged back to the last frame placed. The excavation is per- 
formed by removing one board at a time, excavating behind it 
and then replacing it in the advance position. The advance is 
made from the top downwards. This represents the method 
pursued in the most difficult material where wooden sheeting 
without a shield is used. The timbering during the advance may 
be modified in any manner that the character of the material will 
permit. The timbering may lag behind the excavation a dis- 
tance of two or more frames, or it may be omitted altogether. 
Heavier timbering may be necessary in soft, slipping or shattered 
rock. 

166. Shields. Shields are used in tunneling in soft wet 
material and are particularly suitable for work under air pressure. 
They are used in rock tunnels where water is anticipated or air 



SHIELDS 



289 



pressure is used. The shields often save the expense and diffi- 
culty of timbering as the masonry of the sewer follows closely 
behind the shield. Fig. 118 shows the arrangement for a shield 
for tunneling in soft material in the construction of the Milwaukee 
sewers. The shield has an exterior diameter of 9 feet 4 inches 



/ Cast Iron Jack Sea t 
.-BJntPI 




Fio. 118. Shield for Driving Milwaukee Sewer Tunnel. 

Eng. News- Record, Vol. 80, 1918, p. 669. 

and an overall length of 9 feet 8| inches. The cutting edge sec- 
tion is 20 inches long. The shell is made of one inch plate to the 
back of the jack chambers and one-half inch plate in the tail. 
The shield is driven by ten 60-ton hydraulic jacks. The jacks 



290 CONSTRUCTION 

are shown in position in the figure. These jacks rest against the 
finished tunnel lining and serve to consolidate it at the same time 
that they push the shield into the material to be excavated. The 
face of the tunnel is cut with a pick and shovel while the jacks 
are removed one at a time and a new ring of lining is put in place. 
The lining may be temporary timbering to receive the thrust of 
the jacks, but it is usually desirable that the permanent lining 
follow immediately behind the shield. Since the shield is larger 
than the outside of the lining the space left by its passage should 
be grouted immediately after it has passed. 




FIG. 119. Method of Drilling and Loading Rock Tunnel Face. 

Courtesy, Aetna Power Co. 

167. Tunnel Machines. Tunnel machines have been used 
successfully on sewer tunnels in soft materials, but not in rock. 1 
The machines are of different types, but in general consist of a 
revolving cutting head, equipped with knives, and driven by an 
electric motor. The bearing on which the shaft for the cutting 
head rests is supported against the sides of the tunnel. The muck 
is carried away by means of a conveyor and dumped into muck 
cars without rehandling. Rapid progress can be made with these 
machines in suitable conditions. 

168. Rock Tunnels. Tunnels in rock are advanced by drilling 
into the face as shown in diagrammatic form in Fig. 119. The 

1 Tunneling Machines Successful on Detroit Sewers, Eng. News-Record, 
Vol. 84, 1920, p. 329. 



VENTILATION 291 

holes near the center are driven in at an angle towards the center 
and to depths from 6 to 15 feet. The harder the rock the greater 
the angle with the tunnel. This is called the center cut. Other 
holes are driven near the outer edge of the tunnel and parallel to 
its axis. When fired, the wedge of rock between the center cut 
holes is thrown back into the tunnel and a delayed explosion 
then throws the sides into the hole thus made. A final delay 
thrusting shot throws the muck so formed away from the face of 
the tunnel. For tunnels up to 6 or 8 feet in height the entire bore 
is cut out in this fashion. For larger tunnels, the upper portion 
called the heading, is taken out in this way, and the remainder, 
called the bench, is taken out by drilling and blowing holes normal 
to the axis of the tunnel. The amount of powder necessary in 
the bench holes is much less than that required in the heading. 

169. Ventilation. No tunnel more than 50 feet long should 
be built without ventilation. A fair amount of air for ordinary 
conditions is 75 cubic feet of free air per minute per person in the 
tunnel, and double this amount for each animal. Where explosive 
gases are met, or under conditions where the tunnel is hot, five or 
six times as much air may be needed in order to cool the tunnel or 
to dilute the gases. In order that the air may be fresh and cool 
at the face of the tunnel where work is going on it should be con- 
ducted to the tunnel face in a pipe and blown out into the tunnel. 
Immediately following a blast at the face the current should be 
reversed so as to draw the poisonous gases out of the tunnel 
through the duct. The high pressure air line leading to the drills 
should be opened at the same time to create a current towards the 
face in order to accelerate the clearing of the air at the heading. 
The capacity of the air machines should be sufficient to exhaust 
four times the volume of the gases created by the explosion, in 15 
minutes. This will ordinarily call for a capacity of about 4,000 
cubic feet of free air per minute. If the same blower is to be 
used for exhausting the gases as for ventilation while work is going 
on, it should have a high overload capacity to care for this situa- 
tion. The air line should be arranged to allow for reversal of flow. 

The diameter of the air pipe should be determined by a study 
of the saving of the cost and operation of the air equipment com- 
pared to the increased cost of a larger pipe line. Other factors 
affecting the size of the pipe line to be used are: the available 
space in the tunnel, the temporary character of the installation, 



292 CONSTRUCTION 

the use of the exhaust from high-pressure air machines for the 
purpose of ventilation, etc. Cast-iron, spiral-riveted galvanized 
sheet iron, and canvas pipes have been used for conducting low- 
pressure ventilating air. 

Ventilation in tunnels working under air pressure is supplied 
from the compressors, and the air is delivered near the face of 
the heading, except that being used in the locks. In tunnels 
using air drills, the air for the drills is conducted through a sep- 
arate pipe as it is not economical to compress the ventilating air 
to the pressure necessary to operate the drills. 

170. Compressed Air. Compressed air is used in tunnel work 
to prevent the entrance of water into the tunnel and to keep the 
work dry. The pressure of air used is closely that of the pressure 
of the ground water but in a large tunnel or a tunnel with a weak 
roof the pressure may be somewhat lower on account of the danger 
of blowing through the roof. It is evident that the water pres- 
sure cannot be balanced at the top and the bottom of the tunnel. 
To balance it at the bottom makes a blow out near the top more 
probable. To balance the pressure at the top may leave the 
bottom wet. Judgment and care must be exercised during con- 
struction and if the pressure is balanced at or near the bottom the 
roof must be carefully guarded by grouting and puddling with 
clay, or the surface, particularly if under water, may be covered 
with a clay bank. If the cavities in the tunnel lining are large, 
sawdust can be mixed with the grout to advantage, the mixture 
being pumped through holes in the roof by hand or power operated 
force pumps. " Blows " must be carefully guarded against as 
they endanger the lives of the workmen and threaten the loss of 
the tunnel. The pressure and volume of air supplied for some 
large subaqueous tunnels is shown in Table 61. 

Labor under compressed air is arduous and dangerous with 
the best of safeguards. 1 Pressure more than about 43 pounds 
per square inch cannot be used and at this high pressure men can- 
not work more than four hours at a time. Little or no distress 
is noted at pressures less than 15 pounds. 

Entrance and exit to the tunnel are gained through air locks. 
These are sheet-iron cylinders concreted into the lining of the 
tunnel or shaft. Air-tight iron doors are provided at both ends, 

1 Rules on Compressed-Air Work of N. Y. State Industrial Commission, 
Eng. News-Record, Vol. 85, 1920, p. 1225. 



COMPRESSED AIR 



29:; 



TABLE 61 

VOLUME AND PRESSURE OF COMPRESSED AIR IN TUNNELS 
(American Civil Engineers Pocket Book) 





Maxi- 




Maxi- 


Average 






mum 
Distance 


Mini- 


mum 
Air 


Air 




Tunnel 


High 
Water 


mum 
Cover 


Pressure, 
Pounds 


Pressure, 
Pounds 


Conditions and Cubic Feet of 
Free Air per Minute 






in 




per 






to 
Invert, 


Feet 


per 

Square 


Square 






Feet 




Inch 






City and South 


34 


42 


15 




In water bearing-sand. 1660 


London 










cubic feet per minute per face. 












When grouted 1000 to 1300 












cubic feet per minute per face 


Blackwall 


80 


5 


o7 


35 


10,000 cubic feet per minute per 












face in open ballast for some 












time 


Baker St. and 


70 


18 


35 


28 


In gravel, 3300 cubic feet of air 


Waterloo 










per minute per face. Parallel 












tunnel 1650 cubic feet per min. 












per face 


Greenwich 


70 


30 


28 


20 


Average 83.5 per man per minute. 












Never less than 66.7 


Battery, East 


94 


12 


42 


26 


In sand. Two working faces. 


Kivt-r. N Y. 










Maximum 32,000 


East River, N. Y.. 


93 


8 


42 


27 


Maximum for one face 25,000 


Penn. R.R. 










cubic feet per minute for 24 












hours. Capacity of plant for 












8 faces, 80,400 cubic feet per 












minute 


North River, 


98 


20 


37 


26 


Maximum in gravel 10,000 cubic 


N. Y., Penn. 










feet per man per hour. Gener- 


R.R. 










ally ranged between 1500 and 












5000 



which open inwards towards the tunnel. On entering the lock 
from the outside the door to the tunnel is found tightly closed. 
The outside door is then closed by hand, the ah- valve is opened 
and air is admitted to the lock until the pressure on the lock side 
of the tunnel door equalizes that on the tunnel side and the tunnel 
door is swung open by hand. When the lock is open to the 
tunnel the pressure in the tunnel keeps the outside door closed. 
In order to leave the tunnel the process is reversed. Materials 



294 CONSTRUCTION 

are passed through the lock by the lock tender or tenders who 
pass through the lock with the material if the pressure is low, or 
who manipulate the air outside of the lock if the pressure is high. 
If pressures of 30 to 40 pounds are being used, two or even three 
locks may be necessary. 

EXPLOSIVES AND BLASTING 1 

171. Requirements. The desirable features in an explosive 
to be used in trenching and tunneling in rock are: (1) stability 
in make up so as not to deteriorate in strength or to become 
dangerous during storage, (2) imperviousness to ordinary varia- 
tions in temperature and moisture, (3) insensibility to ordinary 
shocks received in transportation and handling, (4) not too diffi- 
cult of detonation, (5) convenient form for transportation and 
loading and for making up charges of different weights, (6) the 
non-formation of poisonous gases when fired, (7) imperviousness 
to water and usefulness in wet holes, (8) power without bulk, etc. 

172. Types of Explosives. Explosives which fill some or all 
these requirements can be divided into two classes, deflagrating 
and detonating. A deflagration is an explosion transmitted 
progressively from grain to grain. A detonation is a sudden dis- 
ruption caused by synchronous vibrations of a wave-like char- 
acter. The deflagrating explosives are represented by gun- 
powders and contractors' powders. They must be carefully 
tamped in the hole to develop their full power and they must be 
ignited by a fuse or flame. They are valueless in water or moist 
holes. These powders are used mainly for loosening frozen earth, 
soft sandstone, cemented gravels and similar materials where a 
thrusting action rather than a disruption is desired. The detonat- 
ing explosives are most commonly represented by the dynamites. 
These are exploded by a shock usually caused by another explosive 
which has been ignited by a fuse or electric spark, and which is 
known as the " detonator." Detonating explosives are more 
powerful than deflagrating explosives and are used in all but the 
softest materials. 

1 Taken mainly from the Engineer Field Manual of the U. S. Army; 
Safety Factors in the Use of Explosives by W. O. Snelling, Technical Paper 
No. 18, U. S. Bureau of Mines; and an article in Eng'g and Contracting, 
01.52,1919,?. 585. 



TYPES OF EXPLOSIVES 295 

Gunpowder. This is a mechanical mixture of sulphur, char- 
coal, and saltpeter generally in the proportions of 10 parts sulphur, 
15 parts charcoal, and 75 parts saltpeter (sodium nitrate). It 
weighs about 62^ pounds per cubic foot and produces about 280 
times its own volume in gas at a pressure of 4.68 tons per square 
inch at a temperature of 32 degrees F., which amounts to a pres- 
sure of approximately 38 tons per square inch at the temperature 
of explosion of 4,000 degrees F. 

Blasting Powder. This is a mixture of 19 parts sulphur, 15 
parts charcoal, and 66 parts saltpeter. These powders are made 
in different size angular polished grains, from the size of a pin 
head to sizes just passing a f to \ inch hole. The larger the grains 
the slower the action of the powder. 

Nitro-Substitution Compounds. These compounds are formed 
by the action of nitric acid on hydro-carbons. Triton, T.N.T., 
or trinitrotoluene, made famous during the war, is an example of 
these compounds. It is made by the successive nitration of 
toluene, a coal tar derivative. It melts at 80 degrees C., is very 
stable, and is of great explosive strength. It is manufactured in 
a convenient form, being compressed into blocks about 2 inches 
square by about 4 inches long with a specific gravity of about 1.5. 
The blocks are usually copper plated to protect the T.N.T. from 
moisture. The more dense it is the less its sensitiveness. It is 
also put up in crystalline form hi cartridges like dynamite, in 
which condition it is practically equal to 40 per cent dynamite. 
It can be cut with a knife, pounded with a hammer, and will burn 
freely and slowly in small quantities in the open air without 
exploding. It is suitable for all but the hardest rocks. It creates 
poisonous gases on detonation which are quickly dissipated in the 
open air but which render it unsuitable for use in tunnel work. 

Nitro-glycerine. This is formed by the action of nitric and 
sulphuric acids on animal compounds such as gelatine or glycerine. 
Nitro-glycerine is a yellowish, oily, highly unstable explosive 
liquid with a specific gravity of about 1.6. It will burn quietly 
when ignited in the open air. It will freeze at 41 degrees F., and 
will explode at 388 degrees F., or on concussion at a lower tempera- 
ture. It develops about 1,500 times its volume in gas, which due 
to the heat of combustion is increased to about 10,000 times its 
volume. It is a very dangerous explosive to handle, and is unsuit- 
able for use in the liquid form. 



296 CONSTRUCTION 

Blasting Gelatine. This is made by soaking guncotton in 
nitro-glycerine. Gelatine dynamite is a combination of blasting 
gelatine and an absorbent. For cite is a gelatine dynamite in 
which the blasting gelatine, forming 50 per cent of the compound, 
contains 90 per cent nitro-glycerine and 2 per cent guncotton; 
and the absorbent, forming the other 50 per cent of the compound, 
contains 76 per cent of sodium nitrate, 3 per cent sulphur, 20 per 
cent of wood tar, and 1 per cent of wood pulp. 

Blasting gelatine is packed in a jelly-like mass in metal lined 
wooden boxes. It is less sensitive than straight dynamite and is 
one of the most powerful explosives known. It can be made up 
to equal 100 per cent dynamite. It is suitable for use in the hard- 
est rocks and for subaqueous work as it is not affected by moisture. 
It is suitable for use in tunnels as the amount of carbon monoxide, 
peroxide of nitrogen, hydrogen sulphide and other dangerous 
gases is comparatively low when fully detonated. Gelatine 
dynamite J is sold as 30 per cent to 70 per cent dynamite, the 
actual percentage of nitro-glycerine being less than the nominal 
quantity given. 

Dynamite. The dynamites are made by soaking nitro-glycerine 
in some absorbent. If the absorbent is some neutral substance 
such as infusorial earth the combination is known as a true dyna- 
mite. The false or active dynamites are those in which the absorb- 
ent is also an explosive compound. The false dynamites form the 
best known contractors' explosives. Among the materials mixed 
with the nitro-glycerine are: magnesium carbonate, sulphur, 
wood meal, wood pulp, wood fiber, wood tar, nut galls, kieselguhr, 
sawdust, resin, pitch, sugar, charcoal, and guncotton. The 
strength of dynamites is noted by the per cent of nitro-glycerine 
and nitro substitutes contained. Dualin and Hercules powder 
both contain 40 per cent nitro-glycerine. Dualin contains 30 
per cent sawdust and 30 per cent potassium nitrate, but the 
Hercules powder, which is stronger, contains 16 per cent sugar, 
3 per cent potassium chlorate, 31 per cent potassium nitrate, and 
10 per cent magnesium carbonate. 

Dynamite is the most common explosive used on construction 

work. It is supplied in cylindrical sticks wrapped in paper, the 

diameter of the sticks varying between f and 2 inches. They are 

about 8 inches long. Forty per cent dynamite is the common 

> 1 See paper by C. T. Hall before Am. Inst. Chemical Engineers. 



PERMISSIBLE EXPLOSIVES 297 

strength found on the market. It is suitable for ordinary work 
in all but very hard rocks or very soft material. Direct contact 
with water separates the nitro-glycerine from the base and is 
dangerous when the explosive is used in wet places unless it is 
fired immediately after the hole is loaded. It freezes at about 42 
degrees F., or at even higher temperatures and in the frozen state 
it is highly dangerous, requiring powerful detonators for firing, 
but exploding spontaneously from a slight jar, or the breaking of 
the stick. Special low-freezing dynamites are made that will not 
freeze above 35 degrees F. 

Ammonia Compounds. Ammonia dynamite is a combination 
of nitro-glycerine, ammonium nitrate and such other ingredients 
as sodium nitrate, calcium carbonate and combustible material. 
This form of explosive is advantageous for underground work 
because, like gelatine dynamite, its explosion does not create large 
quantities of poisonous gases. It has a low freezing point and is 
relatively low in cost. It is seriously affected by moisture, however, 
and can not be used in wet places. Ammonium nitrate explosives 
which do not contain nitro-glycerine include 70 per cent to 95 per 
cent ammonium nitrate and some combustible material. Ammo- 
nal is a special type of 'this class formed by a mixture of ammo- 
nium nitrate, aluminum, and triton. All of these explosives are 
deliquescent, insensitive to shock, and are cheaper than the dyna- 
mites. 

173. Permissible Explosives. As specified by the United 
States Bureau of Mines explosives whose rapidity, detonation, 
and temperature of explosion will not ignite explosive mixtures 
of pit gases and air are known as permissible explosives. They 
include nitrate explosives, ammonia dynamite, and others. 

Gunpowder, triton, picric acid, blasting gelatine, dynamite, 
guncotton, etc., are not classed as permissible explosives. 

174. Strength. The relative weights for equal strength of 
various explosives are given in Table 62. 

175. Fuses and Detonators. The explosion of gunpowder and 
other deflagrating explosives is caused by the direct application 
of a flame led to the charge by a powder fuse, or they may be 
fired by a blasting cap which is itself exploded by the heat from 
a fuse or an electric spark. The powder fuse is a cord made up of 
a train of powder securely wrapped in a number of thicknesses of 
woven cotton or linen threads and usually made water-proof. 



298 



CONSTRUCTION 



TABLE 62 

RELATIVE WEIGHTS OF EXPLOSIVES WITH THE SAME STRENGTH AS A 
UNIT WEIGHT OF 40 PER CENT DYNAMITE 



Explosive 


Relative 
Weight 


Explosive 


Relative 
Weight 


Picric acid 


0.86 


Triton 


0.86 


Gun powder (well tamped) . 


3.10" 


Blasting gelatine 


0.43 


Straight dynamite, 15%. .. . 


1.45 


Gelatine dynamite, 30%. . . 


1.28 


Straight dynamite, 20 .... 


1.33 


Gelatine dynamite, 35 ... 


1.21 


Straight dynamite, 25 .... 


1.28 


Gelatine dynamite, 40 


1.14 


Straight dynamite, 30 .... 


1.18 


Gelatine dynamite, 50 


1.04 


Straight dynamite, 35 .... 


1.07 


Gelatine dynamite, 55 ... 


0.97 


Straight dynamite, 40 .... 


1.00 


Gelatine dynamite, 60 


0.90 


Straight dynamite, 45 .... 


0.93 


Gelatine dynamite, 70 ... 


0.83 


Straight dynamite, 50 .... 


0.86 






Straight dynamite, 55 .... 


0.83 


Ammonia dynamites are 




Straight dynamite, 60 .... 


0.78 


the same as gelatine 








dynamites. 




Low-freezing dynamites are 




Chlorates (sprengle) 




the same as straight 




Rack-a-rock 


1.33 


dynamites 




Guncotton 


0.72 


Smokeless powder, well 








tamped 


0.74 















Ordinary fuse burns at about 2 feet per minute but there may be 
wide variations from this rate due to the quality of the fuse, 
moisture, temperature, or pressure. Moisture tends to retard the 
rate, pressure to increase it. Instantaneous fuse will burn at 
about 120 feet per second. It is distinguished from the ordinary 
safety fuse both by eye and touch due to the rough red braid with 
which it is covered. It is used in firing a number of charges 
simultaneously. Powder fuses are lighted by the application of a 
flame or smoldering torch to the freshly cut or opened end expos- 
ing the powder grains. Cordeau Bickford is lead tubing filled with 
triton, in which the flame travels at about 17,000 feet per second. 
This is also used for igniting charges simultaneously. 

The detonation of an explosive is caused by the shock or heat 
of the explosion of a more sensitive substance which has been 
exploded by a powder fuse or electric spark. The common 
method of detonating explosive charges is by the firing of a blast- 



FUSES AND DETONATORS 



ing cap. These caps are copper cylinders, closed at one end, 
about 1% inches long and \ to | of an inch in diameter, or larger. 
They contain a mixture of about 85 per cent fulminate of mercury 
and 15 per cent potassium chlorate held in place by a wad of 
shellac, collodion, or paper. The strength of detonators is based 
on the weight of fulminate of mercury and is designated as shown 
in Table 63. 



TABLE 63 
STRENGTH OF BLASTING CAPS 



Blasting Cap, 
Commercial Grade 


Grains 
Fulminate 
of 
Mercury 


Electric Cap, 
Commercial Grade 


Grains 
Fulminate 
of 
Mercury 


3X or Triple 


8.3 


Single strength 


12 3 


4X or Quadruple 


10 


Double strength 


15 4 


5X or Quintiple .... 


12 3 


Triple strength 


23 1 


6X or Sextuple 


15.4 


Quadruple strength 


30 9 


7X or Number 20 


23.1 






8X or Number 30 


30 9 















The force of the explosion is markedly affected by the strength 
of the caps, the effect being greater for low-grade powders. For 
40 per cent dynamite the explosion caused by a 5X cap is 15 per 
cent stronger than that caused by a 3X cap. For 60 per cent 
dynamite the difference is only 6 per cent. The deterioration of 
the caps will reduce the strength of an explosion noticeably. 
With straight dynamite, 3X caps are generally used, but with 
gelatine dynamite 6X or heavier caps must be used. Caps may 
be tested by exploding thorn in a confined space and noting the 
report and the effect on the shell. A full strength cap will tear 
the shell into minute pieces, while a deteriorated cap will merely 
tear it into three or four large pieces. An ordinary blasting cap is 
shown in Fig. 120 together with other equipment for blasting. 

Firing by electricity is generally safer and more satisfactory 
than by the use of ordinary caps and powder fuses. The explosion 
is more certain and its exact time is under the control of the opera- 
tor. Fig. 121 shows a section through an electric blasting cap or 



300 



CONSTRUCTION 



detonator, commonly called an electric fuse. Delayed-action 
electric detonators are made by inserting a slow-burning sub- 
stance between the platinum bridge and the detonating substance. 
The time of delay is controlled by the depth of the slow -burning 
substance. Delayed-action detonators are useful in tunnel work 
where it is desired to explode the charge in three or four stages 
in order that the debris from one charge may be out of the 
way of the following, and that the forces of the explosions may 
not serve to nullify each other. 

176. Care in Handling. Some of the don'ts in the handling 






MAGNETO 




DYNAMlie, CARTRIDGES 
POWDEF 

ELECTRI 
CAPS 

ELECTRIC: 

FUSE 

BLASTING 
CAPS 



CAP CRIMPERS 



SAFETY RESISTANCE 
FUSE ____ COIL ........ 

FIG. 120. Blasting Supplies. 

' Courtesy, Aetna Powder Co. 



GALVANOMETER 



of explosives recommended by the U. S. Army Engineer Field 
Manual are : in the use of nitro-glycerine explosives of all kinds 

(a) Don't store detonators with explosives. Detonators 
should be kept by themselves. 

(6) Don't open packages of explosives in a store house. 

(c) Don't open packages of explosives with a nail puller, 
pick or chisel. Packages should be opened with a hard 
wood wedge and mallet, outside of the magazine and at 
some distance from it. 

(d) Don't store explosives in a hot or damp place. All 
explosives spoil rapidly if so stored. 

(e) Don't store explosives containing nitro-glycerine so 
that the cartridges stand on end. The nitro-glycerine is 
more likely to leak from the cartridges when they stand on 
end than it is when they lie on their sides. 



CARE IN HANDLING 



301 



-Electric Lead* 



i -Copper Shell 
.-Sulphur Filling 



Sulphur Plug 



(/) Don't use explosives that are frozen or partly 
frozen. The charge may not explode completely and seri- 
ous accidents may result. If the explosion is not complete 
the full strength of the charge is not exerted and larger 
quantities of harmful gases are given off. 

(g) Don't thaw frozen explo- 
sives in front of an open fire, nor in 
a stove, nor over a lamp, nor near 
a boiler, nor near steam pipes, nor 
by placing cartridges in hot water. 
Use a commercial or improvised 
thawer. 

(h) Don't put hot water or 
steam pipes in a magazine for 
thawing purposes. 

(t) Don't carry detonators and 
explosives in the same package. 
Detonators are extremely sensitive 
to heat, friction, or blows of any 
kind. 

(.7) Don't handle detonators or 
explosives near an open flame. 

(k) Don't expose detonators or 
explosives to direct sunlight for 
any length of time. Such exposure 
may increase the danger in their 
use. 

(0 Don't open a package of explosives until ready to 
use the explosive, then use it promptly. 

(m) Don't handle explosives carelessly. They are all 
sensitive to blows, friction, and fire. 

(ri) Don't crimp a detonator (blasting cap) around a 
fuse with the teeth. Use a cap crimper, which is supplied 
for this purpose. 

(o) Don't economize by using a short length of fuse. 

(p) Don't return to a charge for at least one-half hour 
after a miss fire. Hang fires are likely to happen. 

(q) Don't attempt to draw nor to dig out the charge in 
case of a miss fire. 

Some of the positive rules in connection with the handling of 
explosives are: build the magazine on an earth foundation remote 
from any other structures, protect it with earth embankments 
that will direct the force of the explosion upwards, and build it 
of materials that will supply as few missiles as possible. Hollow 
tile brick, double-walled galvanized iron filled with sand, and 
similar constructions are satisfactory. The magazine may be 



-Platinum Bridge 

Guncotton orLoose 
Mercury Fulminate 



Mercury Fulminaft 
Packed 



FIG. 121. Electric Fuse. 

Full size. 



302 



CONSTRUCTION 



heated by steam or hot-water pipes so located that explosives can- 
not come in contact with them, or by a cluster of incandescent 
bulbs, but if the explosives become frozen they must not be thawed 
out by turning on the steam or hot water. If powder or nitro- 
glycerine is dropped on the floor the magazine should be emptied, 
washed out with a hose and spots of nitro-glycerine scrubbed with 
a brush and a mixture of ^ gallon of wood alcohol, \ gallon of 
water and 2 pounds of sodium sulphide. Frozen explosives may 
be thawed by spreading out on special shelves in a warm thaw 
house not in the magazine proper, by burying in a manure pile 
so that the explosive may not become moistened, or more com- 
monly by heating slowly in a water bath. This is a dry kettle in 
which the explosives are placed and covered. The kettle is then 
put in another containing water which is heated gently to about 
120 degrees F. It should not be boiled. 

In case of a miss fire, instead of digging out the old charge put 
a new charge on top of the old and fire the two 
simultaneously. 

177. Priming, Loading, and Firing. Priming is 
the act of placing the cap or detonntor in the cart- 
ridge of explosive. The primer is either the cap or 
the cap and cartridge which are to be detonated by 
the fuse. If a cap and safety fuse are to be used the 
paper at the upper end of the cartridge is opened, a 
hole is poked in the explosive with the finger or a 
piece of wood, the cap and the attached fuse are 
pushed into the hole and gently embedded in the 
explosive so that the end of the cap is exposed 
sufficiently to prevent the fuse from igniting the 
dynamite directly. The paper is then folded up 
and tied firmly around the fuse with a piece of 
string. The result is shown in Fig. 122. 

In placing the fuse in the cap the end of the 
fuse is cut off square, and inserted in the open 
end of the cap, care being taken not to spill the 
Safety Fuse, l os e grains of powder or to grind the fuse down 
and Cap. on top of the cap. When the fuse is shoved 
firmly into place the upper portion of the copper 
cap is pressed or crimped with the cap crimpers shown in Fig. 120. 
The number of primers to be used is dependent on the size 



FIG. 122. 
Dynamite 
Cartridge, 



PRIMING, LOADING, AND FIRING 303 

and location of the charge, but in practically all sewer work only 
one primer is used to each hole. In bulky charges the primer 
should be placed near the center of the charge and the fuse so 
protected that it will not ignite the charge prematurely. In drill 
holes the primer is put in last with the cap end down. 

In loading a hole, it is first pumped and cleaned out. This 
can be done satisfactorily with the end of a stick frayed out into a 
broom. Cartridges which very nearly fill the hole are dropped in 
one at a time and are pressed firmly together, with a light wooden 
tamping bar. They should not be pounded. After the primer 
is placed, a wad of clay or similar material is pressed gently into 
the hole against it and the hole is then filled with well-tamped clay. 
In tunnel work tamping is not so essential as an overcharge of 
powder is usually used and the time of tamping, which is worth 
more than two or three sticks of dynamite, is saved. In handling 
bulk explosives, such as gunpowder, they are poured into the hole, 
the fuse is set in the upper portion and the remainder of the hole 
is tamped with clay as for dynamite cartridges. 

If a large number of charges are to be fired simultaneously 
with a safety fuse, the length of the fuse to each charge should be 
made equal or a safety 
fuse used to a common 
center and approxi- 
mately equal lengths 
of instantaneous fuse f 

n j TJ- i r J Vttuiiiiiiiiuiii....... 

or Cordeau Bickford ^ 

used from there to RG. 123. Methods for Cutting Safety Fuse for 
the charge. In splic- Splicing, 

ing the fuses for such 

connections they are cut diagonally as shown in Fig. 123 and bound 
together firmly with tape. Electric connections are particularly 
advantageous under such conditions as they avoid the dangers 
incidental to spliced fuses and are less expensive. In tunnel 
work simultaneous electric detonation is not desirable as the holes 
should be fired progressively: 1st, the cuts; 2nd, the relievers; 
3rd, the backs; 4th, the sides; and 5th, the lifters. Different 
lengths of safety fuse, or delayed action electric fuses can be used 
for these delay shots. 

In igniting a safety fuse an open flame such as that furnished 
by a match or candle is the most satisfactory. For electric fuses 




304 CONSTRUCTION 

the current is generated by a magneto shown in Fig. 120. 
Pressing vigorously down on the handle closes the circuit and 
generates an electric current which heats the platinum bridges 
and explodes the charges. For the small number of charges used 
in ordinary construction they are connected in series so that if 
there is a broken connection anywhere no charge will be exploded. 
If many charges are to be fired and a line circuit is to be used, the 
final connection should not be made until just before the charge 
is to be fired in order to obviate the danger of stray currents firing 
the charge prematurely. Care should be taken to see that all 
connections are good and that there are no broken wires on the line. 

178. Quantity of Explosive. The quantity of explosive to 
be used can be determined satisfactorily only by experience on 
the job in question, as the factors affecting the necessary quantity 
are so diverse. The figures in Table 64 indicate the relative 
amounts needed under different conditions. 

PIPE SEWERS 

179. The Trench Bottom. It is customary to dig the bottom 
of the trench to conform to the shape of the lower 45 degrees 
to 90 degrees of the sewer if the character of the material will 
allow such construction. In soft material which will not hold 
its shape the sewer may be encased in concrete or a concrete 
cradle may be prepared for the pipe. In rock the trench is 
excavated to about 6 inches below grade and refilled with well- 
tamped earth so as to form a cradle giving bearing to 60 to 90 
degrees of the pipe circumference. For large sewers to be con- 
structed in the trench special foundations are sometimes built. 

180. Laying Pipe. Before the pipe is lowered into the trench 
the sections which are to be adjacent should be fitted together 
on the surface and the relative positions marked by chalk so that 
the same position can be obtained in the trench. 

Small pipes are lowered into the trench and swung into posi- 
tion on a hook as shown in Fig. 124. Pipes up to 15 or 18 inches 
in diameter can be handled by the pipe layer and helper in the 
trench without assistance. Heavier pipes may be lowered into 
the trench by passing ropes around each end of the pipe. One 
end of the rope is fastened at the surface and the ropes are paid 
out by the men at the surface as the pipe is lowered. If the pipes 



QUANTITY OF EXPLOSIVE 



305 



Remarks 



Grade of 
Dynamite, 
Per Cent 



9 
9 







S3 5* 3 

3(2 



- 




0000000000000008 



OOQOOOOOlNiO 



ppt^ic 

i-KNO^-i 



OQ -000000000000011000 
O i-< OO OO O 1-1 IH O i-H O O >-H O 




d of R 







111 



306 CONSTRUCTION 

have been fitted together and marked at the surface it is undesir- 
able to use this method of lowering as the position in which the 
pipes arrive in the bottom of the trench can not be easily pre- 
dicted. A cradle may be used for shoving the pipe into position 
as is shown in Fig. 125. 

Pipes above 24 to 27 inches in diameter are too large to be 
handled from the side of the trench. A hook as shown in Fig. 
124 is placed in the pipe so that it will be in the proper position 
when lowered. It is raised by a rope passing through a block 
at the peak of a stiff-legged derrick which spans the trench, or 
by a crane. If a derrick is used the rope passes to a windlass 
on the opposite side of the trench from the pipe. Mechanical 





FIG. 124. Hook for Lowering and FIG. 125. Cradle for Placing 

Placing Sewer Pipe. Sewer Pipe. 

power may be used for raising pipes too heavy to be raised by hand. 
The pipe is then lowered and swung into position while sup- 
ported from the derrick. Excessive swinging is prevented by 
holding back on the guide rope as the pipe is raised and lowered. 
Pipes are usually laid with the bell end up grade as it is easier 
to fit the succeeding pipe into the bell so laid and to make the 
joint, particularly on steep grades. The Baltimore specifica- 
tions state: 

The ends of the pipe shall abut against each other in 
such a manner that there shall be no shoulder or unevenness 
of any kind along the inside of- the bottom half of the 
sewer or drain. Special care should be taken that the 

pipe are well bedded on a solid foundation The 

trenches where pipe laying is in progress shall be kept dry, 
and no pipe shall be laid in water or upon a wet bed unless 
especially allowed in writing by the Engineer. As the 
pipe are laid throughout the work they must be thoroughly 
cleaned and protected from dirt and water, no water being 
allowed to flow in them in any case during the construction 
except such as may be permitted in writing by the Engineer. 
No length of pipe shall be laid until the preceding length 
has been thoroughly embedded and secured in place, so as 
to prevent any movement or disturbance of the finished 
. joint. 



JOINTS 307 

The mouth of the pipe shall be provided with a board 
or stopper, carefully fitted to the pipe, to prevent all earth 
and any other substances from washing in. 

181. Joints. Pipes may be laid with open joints, mortar 
joints, cement joints, or poured joints. Open joints are used for 
storm sewers in dry ground close to the surface. Mortar and 
cement joints are commonly used on all sewers except in special 
cases. Cement joints are more carefully made than mortar 
joints and result in a greater percentage of water-tight joints. 
Poured joints are used in wet trenches where it is necessary to 
exclude ground water from the sewer. 

A specification used in some cities for open joints is: 

Pipes laid with open joints are to be laid with their 
inverts in the same straight line and shall be firmly bedded 
throughout their length on the bottom of the trench. No 
cement or mortar is to be used in the joints. Not more 
than | inch shall be left between the spigot end of the pipe 
and the shoulder of the hub of the pipe into which it fits. 
The joints shall be surrounded with cheese cloth, burlap, 
broken pipe, gravel or broken stone. 

The purpose of the cheese cloth, etc., is to prevent fine earth 
from sifting into the pipe until the cheese cloth or other material 
has rotted away, by which time the earth has become arched over 
the opening. 

Mortar joints are specified by Metcalf and Eddy as follows: 

Before a pipe is laid the lower part of the bell of the 
preceding pipe shall be plastered on the inside with stiff 
mortar of equal parts of Portland cement and sand, of 
sufficient thickness to bring the inner bottoms of the 
abutting pipe flush and even. After the pipe is laid the 
remainder of the bell shall be thoroughly filled with similar 
mortar and the joint wiped inside and finished to a smooth 
bevel outside. 

In some work a wood block or a stone is embedded in the mor- 
tar at the bottom of the joint to bring the spigot in place concen- 
tric with the next pipe. 

Cement joints are specified in the Baltimore specifications as 
follows: 

Cement joints shall be made with a narrow gasket of 
hemp or jute and cement mortar, and special care shall be 
taken to secure tight joints. The gasket shall be soaked 



308 CONSTRUCTION 

in Portland cement grout and then carefully inserted 
between the bell and the spigot, and well calked with 
suitable hardwood or iron calking tools. It shall be hi 
one continuous piece for each joint, an$ of such thickness 
as to bring the inverts of the two pipes smooth and even. 
The remainder of the joint shall be filled with cement mortar 
all around, on the bottom, top and sides, applied by hand 
with rubber mittens, well pressed into the annular space 
and beveled off from the outer edge of the bell to a dis- 
tance of two inches therefrom, or to an angle of 45 degrees. 
The inside of each joint shall be thoroughly cleansed of all 
surplus mortar that may squeeze out in making the joint; 
and to accomplish this some suitable scraper or follower, 
or form shall be provided and always used immediately 
after each joint is finished. 

Cement joints so made, form the most satisfactory joint 
for ordinary conditions and are the most frequently used. They 
are not always water-tight and can be penetrated by roots. Some 
roots are able to penetrate holes of almost microscopic size and 
to form growths in the sewer or to split the joints. 

Poured joints are made by pouring some jointing compound, 
while in a fluid state, into the joint in which it hardens, thus seal- 
ing the joint. Water-tightness in sewer lines to exclude ground 
water has also been attempted by using the ordinary cement joint 
and surrounding the pipe with a layer of cement or concrete. 
This has not always been successful as it is difficult to obtain the 
proper class of workmanship in wet sewer trenches. 

The requisite qualities of a poured jointing material are: 

(1) It should make a joint proof against the entrance 
of water and roots. 

(2) It should be inexpensive. 

(3) It should have a long life. 

(4) It should not deteriorate in sewage which may be 
either acid or alkaline. 

(5) It should adhere to the surface of the pipe. 

(6) It should run at a temperature below about 400 F., 
as too high temperatures will crack the pipe. 

(7) It should neither melt nor soften at temperatures 
below 250 F. in order to maintain the joint if hot liquids 
are poured into the sewer. 

(8) It should be elastic enough to permit slight move- 
ments of the pipes. 

(9) It should not require great skill in using as it must 
be handled ordinarily by unskilled workers. 



JOINTS 309 

The materials used for poured joints are: cement grout; 
sulphur and sand; and asphalt or some bituminous compound 
made of vulcanized linseed oil, clay, and other substances the 
resulting mixture having the appearance of vulcanized rubber 
or coal tar. The bituminous materials most nearly approach 
the ideal conditions. 

Cement grout is made up of pure cement and water mixed 
into a soupy consistency. Its main advantages are its cheapness 
and ease in handling in wet trenches or difficult situations. The 
result is no better than a well made cement joint. There is no 
elasticity to the joint and a movement of the pipe will 
break it. 

Sulphur and sand are inexpensive, comparatively easy to 
handle, and make an absolutely water-tight and rigid joint which 
is stronger than the pipe itself. It frequently results in the crack- 
ing of the pipe and is objected to by some engineers on that 
account. In making the mixture, powdered sulphur and very 
fine sand are mixed in equal proportions. It is essential that the 
sand be fine so that it will mix well with the sulphur and not 
precipitate out when the sulphur is melted. Ninety per cent 
of the sand should pass a No. 100 sieve and 50 per cent should pass 
a No. 200 sieve. The mixture melts at about 260 F. and dees 
not soften at lower temperatures. For making a joint in an 8 
inch pipe about \\ pounds of sulphur, \\ pounds of sand, \ 
pound of jute, and 0.4 pound of pitch are used. The pitch is 
used to paint the surface of the joint while still hot in order to 
close up any possible cracks. 

Among the better known of the bituminous joint compounds 
are: " G.K." Compound made by the Atlas Company, Mertz- 
town, Pa., Jointite and Filtite, manufactured by the Pacific Flush 
Tank Co., Chicago and New York, and some of the products of 
the Warren Brothers Co., Boston. These compounds fill nearly 
all of the ideal conditions except as to cost and ease in handling. 
They are somewhat expensive and if overheated or heated too 
long become carbonized and brittle. In cold weather they do 
not stick to the pipe well unless the pipe is heated before the 
joint is poured. On some work jbints have been poured under 
water with these compounds, but success is doubtful without 
skillful handling. An overheated compound will make steam 
in the joint causing explosions which will blow the joint clean, 



310 



CONSTRUCTION 



and an underheated compound will harden before the joint is 
completed. 

The materials should be heated in an. iron kettle over a gaso- 
line furnace or other controllable fire, until they just commence 
to bubble and are of the consistency of a thin sirup. Only a 
sufficient quantity of material for immediate use should be pre- 
pared and it should be used within 10 to 15 minutes after it has 
become properly heated. The ladle used should be large enough 
to pour the entire joint without refilling. There are other 
important points to be considered in pouring joints which can be 
learned best by experience. 

The quantity of material necessary for making these joints, 
as announced by the manufacturers, is shown in Table 65. 

TABLE 65 
QUANTITY OF COMPOUND NEEDED FOR POURED JOINTS 





Quantity of Material in Pounds per Joint 


Diameter 






of Pipe, 


Standard Socket 


Deep and Wide Socket 


in Inches 








Jointite 


Filtite 


O.K. 


Jointite 


Filtite 


O.K. 


6 


0.82 


0.72 


0.42 


1.46 


1.28 


0.72 


8 


1.06 


0.95 


0.73 


1.82 


1.60 


1.25 


10 


1.30 


1.15 


0.89 


2.26 


1.98 


1.52 


12 


2.08 


1.82 


1.42 


2.65 


2.32 


1.80 


15 


2.52 


2.20 


1.74 


3.20 


2.80 


2.20 


18 


3.02 


2.64 


2.58 


3.75 


3.29 


3.25 


20 


3.44 


3.00 


2.86 


4.30 


3.78 


3.60 


22 


3.62 


3.16 


3.13 


4.62 


4.07 


3.97 


24 


4.03 


3.50 


3.41 


4.91 


4.31 


4.27 



In making a poured joint the pipes are first lined up in posi- 
tion. A hemp or oakum gasket is forced into the joint to fill a 
space of about f of an inch. An asbestos or other non-combustible 
gasket such as a rubber hose smeared with clay is forced about 
% inch into the opening between the bell and the spigot and the 
compound is poured down one side of the pipe through a hole 
broken in the bell, until it appears on the other side, and the hole 



THE INVERT 311 

is filled. Occasionally the non-combustible gasket is wrapped 
tightly around the spigot of the pipe and pressed or tied firmly 
to the bell. In pouring cement grout joints a paper gasket 
is used which is held to the bell and spigot by draw strings. 
Greater speed in construction and economy in the use of materials 
are obtained by joining two or three lengths of pipe on the bank 
and lowering them into the trench as a unit. The pipes are set in 
a vertical position on the bank with the bell end up, one length 
resting in the other. The joint is calked with hemp and poured 
without the use of the gasket. The joint should always be poured 
immediately after being calked so that the hemp can not become 
water soaked. The asbestos gasket should be removed as soon 
as possible after the joint is poured in order to prevent sticking 
with resultant danger of breaking of the joint when attempting 
to pull the gasket free. 

One man can pour about 33 eight-inch joints, and two men 
can complete about 26 twelve-inch joints per hour on the bank 
where conditions are more or less fixed. 

182. Labor and Progress. The labor required for the laying 
of pipe sewers, exclusive of excavation, bracing and backfilling, 
consists of pipe layers and helpers. For pipes 24 to 27 inches 
in diameter or smaller one pipe layer and one or more helpers 
are necessary, dependent on the size of the pipe and the depth 
of the trench. For larger pipes two pipe layers can work econom- 
ically each working on one-half of the pipe and making hah" of 
the joint. The speed of pipe laying is ordinarily limited by the 
speed of the excavation, but on a job in Topeka, Kan., 1 where 
the average day's progress with a machine excavator was 200 
to 500 feet of trench per day, the pace was limited by the speed 
of the pipe laying gang. This gang consisted of two pipe layers 
in the trench and two helpers on the surface. The sizes of pipes 
handled were from 8 to 27 inches. 

BRICK AND BLOCK SEWERS 

183. The Invert. In good firm ground the excavation is 
cut to the shape of the sewer and the bricks are laid directly on 
the ground, being embedded in a thick layer of mortar. After 
the foundation has been prepared and before the bricks are laid, 

1 Eng. News, Vol. 75, 1916, p. 592. 




312 CONSTRUCTION 

two wooden templates, called profiles, are prepared, similar to that 
shown in Fig. 126, to conform to the shape of the inside and 
outside of the sewer. Each course of bricks is represented by 
a row of nails in the profile and each nail corresponds to a joint 
in the row. The two profiles are set true to line and grade. A 
cord is stretched tightly between the two lowest nails on opposite 
templates and a row of bricks is laid. The bricks are laid 
radially and on edge with their long dimension parallel to the 
axis of the sewer and with one edge just touching the string. 
As each one or two or three rows are completed the guide line is 
moved up to the next nails. When the bricks are laid on the 
ground all but large depressions are filled in with tamped sand or 
mortar by the masons. Approximately the 
same number of rows of bricks is kept com- 
pleted on either side of the center line. The 
FIG. 126. Profile for succeeding courses follow within three to five 

Brick Sewers. rows of each other, the only bond between 
courses being the mortar joint. This is 
called row lock bond and with few exceptions has been used on 
all brick sewers in the United States. As the sides of the sewer 
become higher during the construction, platforms must be built 
for the masons. These platforms are built of wood and rest 
directly on the green brickwork. They should be designed to 
spread the load as much as possible. The brickwork of the invert 
is continued up in this way to the springing line. As soon as 
one section is completed one profile is moved 10 to 20 feet ahead 
along the trench according to the standard length of sections, 
and set in position. The line is then strung from it to nails 
driven or pushed into the cement joints of the last completed 
section. Between work done on separate days the bricks are 
racked back in courses to provide a satisfactory bond. 

In ground too soft to support the brickwork directly a cradle 
is prepared by placing profiles in position in the sewer and nailing 
2-inch planks to these profiles, first firmly tamping earth under 
the planks. The bricks are laid in this cradle in a manner 
similar to that explained for sewers with a firm foundation. In 
still softer ground it may be necessary to construct a concrete 
cra'dle to support the bricks. 

184. The Arch. The arch centering consists of a wooden 
form made up of wooden ribs as shown in Fig. 127. The center 




BLOCK SEWERS '313 

conforms to the shape of the inside of the arch with allowance 
for the thickness of the lagging. The lagging is nailed on the ribs 
in straight strips parallel to the axis of the sewer. The center 
is supported on triangular struts resting against the sides and on 
the bottom of the sewer and is lifted into position by wedges 
driven between it and the support. The centers may be placed 
immediately after the completion of the invert, or a day or two 
may be allowed to pass to give the invert an opportunity to set. 
After the centers are fixed in place the arch brick are carried up 
evenly on each side and are pounded firmly into place. The center 
is usually, but not always 
" struck " immediately, and 
the arch brick are cleaned 
and pointed up from the 
inside. The outside is cov- 
ered with a layer of to f 
of an inch of cement mortar 
and may be backfilled to the 
top of the arch in order to 
maintain the moisture of the FIG. 127. Centering for Brick Sewer, 
mortar during setting and to 

press the bricks of the arch together firmly. The centers are some- 
times made collapsible so that they can be carried or rolled through 
the finished brickwork to the advanced position. In "striking" 
the centers the wedges are removed and the wings folded in. 

In tunneling, the invert of the sewer is constructed in the same 
fashion as for open-cut work. The arch centering is made in 
short sections and the bricks are put in position by reaching in 
over the end of the centering. All of the timbering of the tunnel 
is removed except the poling boards or lagging against which 
the bricks or mortar are tightly pressed, the boards being bricked 
in permanently. 

185. Block Sewers. Sewers made of unit blocks of concrete 
or vitrified clay are constructed in a similar manner to brick 
sewers. Fig. 128 shows the construction of a block sewer at 
Clinton, Iowa. In this sewer there are two rings; an inside one of 
solid blocks and an outside one of hollow blocks. Block sewers 
do not demand the skill in construction that is demanded by 
brick sewers, as the blocks are so cast that the joints are radial, 
whereas only experienced masons can lay bricks radially. 



314 



CONSTRUCTION 



186. Organization. The number of men employed on a 
brick or block sewer is proportioned according to the size of the 
sewer and the working conditions. The number of men working 
on different tasks usually bears the same ratio to the number 
of masons employed, regardless of the size of the work. These 

proportions are shown 
for different jobs, in 
Table 66. 

187. Rate of Progress. 
In a general way it can 
be assumed that the lay- 
ing of 1,000 bricks will 
require 3f hours of the 
time of one mason, 10 
man hours for helpers 
and laborers, 2 barrels of 
cement, 0.6 cubic yard of 
sand, and about 10 feet 
board measure of center- 
ing. One thousand bricks 
will make about 2 cubic 
yards of brickwork. To 
the costs, as estimated 
on the basis of materials 
and labor, must be added 
about 15 per cent for 
overhead and an addi- 
tional amount for the 

contractor's profit. The number of bricks required in various 
size sewers is shown in Table 67. A mason can lay more bricks 
per hour in a large sewer than in a small one as there is a smaller 
percentage of face work, there is more room to work, and it is 
easier to lay the bricks radially. The number of bricks laid and 
the rate of progress on various jobs are shown in Table 68. 




FIG. 128. Segmental Block Sewer at Clinton, 
Iowa, 



CONCRETE SEWERS 

188. Construction in Open Cut. In the construction of sewer 
pipe of cement and concrete one of two methods may be em- 
ployed; 1st, to manufacture the pipe in a plant at some distance 



CONSTRUCTION IN OPEN CUT 



313 



from the place of final use, or 2nd, to manufacture the pipe in 
place. The methods of the manufacture of cement and concrete 
pipe which are to be transported to the place of use are treated 
in Chapter VIII. The process of constructing the pipes in place 
is ordinarily used for pipes 48 inches or more in diameter. For 
smaller sizes, brick, vitrified clay, and precast cement pipes are 
usually more economical. 

TABLE 66 
ORGANIZATIONS FOR THE CONSTRUCTION OF BRICK AND BLOCK SEWERS 













84- to 




Type of Work 


General 
Ratio on 
Basis of 
Four Brick 
Layers 


15-foot, 
5-ring 
Brick, 
Chicago 


66-inch 
Circular 
Brick, 
Gary 


84-inch 
Circular 
Brick, 
Gary 


108-inch 
Sewer 
Brick in 
Detroit 
Tunnel 


42-inch 
Lock- 
Joint 
Tile 
Block 


Foreman 


1 


1 


1 


1 


1 


1 


Brick layers .... 


4 


12 


6 


6 


5 


2 


Helpers . 


2 


11 


3 


3 




1 


Scaffold men .... 


2 


21 


3 








Brick tossers .... 


2 


7 




15 




2 


Brick carriers. . . 


2 


2 








2 


Cement mixers . . 


2 


6 


6 


5 




1 


Cement carriers . 


2 


10 




8 






Form setters .... 


1 




3 


3 






Laborers . . 


1 


8 


19 


a 


14 


7 


Source of 
Information 1 


Municipal 
Engineering, 
Vol.54,p.228 


H. P. 


Gillette, Handbook of Cost Data 



The preparation of the foundation of a concrete sewer is 
similar to that for a brick sewer. If the ground is suitable the 
trench is shaped to the outside form of the sewer and the con- 
crete poured directly on it. In soft material which would give 
poor support to a sewer with a rounded exterior, the bottom of 
the trench is cut horizontal and a concrete cradle of poorer quality 
than that in the finished sewer is poured on the soft ground, on a 
board platform, on piles, or on cribbing supported on piles. 

If the invert of the sewer is so flat that the concrete will 
stand without an inside form the shape of the invert is obtained 



316 



CONSTRUCTION 



by a screed or straight-edge which is passed over the surface of 
the concrete and guided on two centers, or on one center and the 
face of the finished work. The construction of a flat invert 
sewer at Baltimore is shown in Fig. 1. The center for the con- 
crete is shown in the foreground. When the concrete for the 
next section is poured it will be smoothed to shape by a screed or 
straight-edge resting on the face of the finished concrete and the 
center. The center is shaped to conform to that of the finished 
concrete. It is firmly staked in position and acts as a bulk- 
head for the concrete as it is poured, as well as a guide for the 
screed. 

TABLE 67 

BRICK MASONRY IN CIRCULAR SEWERS. CUBIC YARDS PER LINEAR FOOT 
(From H. P. Gillette) 



Diameter, 
Feet and Inches 


One Ring 
(4 Inches) 


Two Ring 
(9 Inches) 


Three Ring 
(13| Inches) 


2 





0.103 


0.240 




2 


6 


0.125 


0.280 




3 





0.147 


0.327 




3 


6 


0.169 


0.371 




4 





0.191 


0.415 




4 


6 


0.213 


0.458 




5 





0.234 


0.501 


0.802 


5 


6 


0.256 


0.545 


0.867 


6 





0.278 


0.589 


0.933 


6 


6 




0.633 


1.000 


7 







0.677 


1.063 


7 


6 




0.720 


1.128 


8 







0.763 


1.193 


8 


6 




0.807 


1.260 


9 







0.851 


1.325 


9 


6 




0.895 


1.390 


10 







0.938 


1.456 





If inside forms are to be used they are made as units in lengths 
of 12 or 16 feet for wooden forms, and 5 feet for steel forms. 
The inside form is supported by precast concrete blocks placed 
under it and which are concreted into the sewer. It is held in 
position by cleats nailed to the outside form, to the sheeting, or 



RATE OF PROGRESS 



317 



Remarks 


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S?S ;- 2 ^ S c 5 

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Authority 


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1 rT -S 

oj O ^ 

o S o d oo o COOK 




1 

J 


2~ 
S3 | : | 1 1| | i 

* a * > ^S IS .^ 12 
O Q co O vi OO^O 


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O3 "3 

O O O CO (MiC <N O 

O CO i-l -t (N CO i-l CO 
I 1 


li 

I^ 1 


CO <O CO - o i-H 


oo C S? 

j3 r _ 


^4 C^ "^t* ^ C^ S2 C^ CO 
1C 


fe co 

.3 c 
So 
n 


O CO GO (N S0TJI <M CO(N (N 


III 


^3 3j3 S3 . . . g 


1 


t. ! I. t. U, fc. H t. fc, (H I. 

^ ? ^| ~ .."..... r~. .. 03 
O O O M)UU _ OUuiO -- 

' *O ' h.. 1- t- wr -- *- 

o o o w oo o bowo b 


"S 4> 

I" 1 


C^O 00 OS cbD OS OOtbtb O 



318 



CONSTRUCTION 



wedged against the outside of the trench. In some cases, 
particularly where steel forms are used, the inside form is hung 
by chains from braces across the trench as is shown in Fig. 129. 
The form is easily brought to proper grade by adjustment of the 
turnbuckles and is then wedged into position to prevent move- 
ment either sideways or upwards during the pouring of the 
concrete. It may be necessary to weight the forms down to 
prevent flotation. Cross bracing in the trench which interferes 
with the placing of the form is removed and the braces are placed 




FIG. 129. Blaw Standard Half -Round Sewer Form, Suspended from Overhead 

Support. 
Courtesy, Blaw Steel Form Co. 

against the form until the concrete is poured. They are removed 
immediately in advance of the rising concrete. 

The sewer section may be built as a monolith, in two parts, or 
in three parts. In casting the sewer as a monolith the complete 
full round inside form is fixed in place by concrete blocks and 
wires. The full round outside form is completed as far as pos- 
sible without interfering too much with the placing and tamping 
of the concrete. The concrete is poured from the top, being 
kept at the same height on each side of the form, and tamped 
while being poured. The remaining panels of the outside form 
are placed in position as the concrete rises to them. An open- 
ing is, left at the top of the outside arch forms which is of such a 



CONSTRUCTION IN OPEN CUT 319 

width that the concrete will stand without support. The casting 
of sewers as a monolith is difficult and is usually undesirable be- 
cause of the uncertainty of the quality of the work. It has the 
advantage, however, of eliminating longitudinal working joints 
in the sewers which may allow the entrance of water or act as a 
line of weakness. 

If the sewer is to be cast in two sections the invert is poured 
to the springing line or higher. A triangular or rectangular 
timber is set in the top of the ,A. 

wet concrete as shown in Fig. /~^~l c ^ r i 

130. When the concrete has set / / *- i I ,J 

the timber is removed and the ^V 

groove thus left forms a work- FIG. 130. Construction Joints for 

ing joint with the arch. After Concrete Sewers. 

the invert concrete has set, the 

arch centering is placed and the arch is completed. This is the 

most common method for the construction of medium-sized 

circular sewers. 

Large sewers with relatively flat bottoms are poured in two or 
three sections. First the invert is poured without forms and is 
shaped with a screed. About 6 inches of vertical wall is poured 
at the same tune. This acts as a support for the side-wall forms. 
The side walls reach to the springing line of the arch and are 
poured after the invert has set. At the third pouring the arch 
is completed. The sewer shown in Fig. 1 is being poured in two 
steps, as the side walls are so low that they are poured at the 
same time as the invert. A transverse working joint similar to 
one of the types used in Fig. 130 is set between each day's work. 

The length of the form used and the capacity of the plant 
should be adjusted so that one complete unit of invert, side wall, 
or arch can be poured in one operation. The forms are left in 
place until the concrete has set. Invert and side wall-forms are 
generally left in position for at least two days, and in cold weather 
longer. The arch forms are left in place for double this time. 
For example if 20 feet of invert and arch can be poured in a day, 
60 feet of invert form and 100 feet of arch form will be required. 
As the forms are released they must be moved forward through 
those in place. For this reason collapsible or demountable 
forms are necessary and steel forms are advantageous. Wooden 
arch forms are sometimes dismantled and carried forward in 



320 



CONSTRUCTION 



sections, but are preferably designed to collapse as shown in 
Fig. 131, so that they can be pulled through on rollers or a carriage. 
189. Construction in Tunnels. In tunnels the invert and 
side walls are constructed in the same manner as for open cut 
work. The tunneling, which acts as the outside form, is 
concreted permanently in place. The concreting of a tunnel 
by hand is shown in Fig. 132. If the work is to be done by hand 
the concrete is thrown in between the ribs of the arch centering 
and behind the plates or lagging, which are set in advance of the 
rising concrete, The lagging plates are 5 feet long which makes 



.-I x3 Lagging 
Covered with 
Galvanized 
Sheet Iron. 




FIG. 131. Section through a Collapsible Wood Form. 

it possible to throw the concrete in place at the arch, and to tamp 
it in place from the end. A bulkhead and a well-greased joint 
timber are placed in position as the concrete rises. 

Pneumatic transmission of concrete is also used for filling 
the arch forms as well as the side walls and invert forms. In 
using this method the mixer may be placed at the surface or at 
the bottom of the shaft or other convenient permanent location 
which may be some distance from the form. The mixture is 
discharged into a pipe line through which it is blown by air to 
the forms. The starting pressure of about 80 pounds per 
square inch can be reduced after flow has commenced. In con- 
structing the St. Louis Water Works tunnel the compressor 
equipment for moving the concrete had a capacity of 1,600 



MATERIALS FOR FORMS 321 

cubic feet per minute at a pressure of 110 pounds. The tunnel 
is horse-shoe shaped, 8 feet in height and with walls varying from 
9 to 20 inches in thickness. The extreme travel of the concrete 
was 1,100 feet in an 8 inch pipe. The amount of air consumed at 
110 pounds varied from 1.2 to 1.7 cubic feet of free air per linear 
foot of pipe. By the time the batch had been discharged the 
pressure had reduced to 25 to 40 pounds, depending on the length 
of the pipe. It is reported that a 6-inch pipe line would probably 
have given better results. 




FIG. 132. Ogier's Run Intercepting Storm-Water Drain, Baltimore, Mary- 
land. 

Placing concrete in Arch. T..r steel lagging of the forms is carried up in sections as the 
concrete is deposited. The drain is horse-shoe shaped, and is 12 feet 3 inches high and 12 
feet 3 inches wide. 



The end of the concrete conveying pipe is provided with a 
flexible joint the simplest form of which can be made by slipping 
a section of pipe of larger diameter over the end of the trans- 
mission line. The concrete is deposited directly on the invert 
or into the side-wall forms and can be blown into the arch forms 
for 20 to 25 feet. 

190. Materials for Forms. The materials used in forms for 
concrete sewers are: wood, wood with steel lining, and steel 
alone. The first cost of wood forms is lower than that of steel 
but their life is relatively short. If the forms are to be used 
a number of times steel is more economical. With proper care 



322 CONSTRUCTION 

and repairs steel forms will outlast any other material. Because 
of the increasing price of lumber and improvements in steel 
forms, wood forms are not frequently used. A common type 
of specification under which forms are used is: 

The material of the forms shall be of sufficient thick- 
ness and the frames holding the forms shall be of sufficient 
strength so that the forms shall be unyielding during the 
process of filling. The face of the form next to the concrete 
shall be smooth. If wooden forms are used the planking 
forming the lining shall invariably be fastened to the stud- 
ding in horizontal lines, the ends of these planks shall be 
neatly butted against each other, and the inner surface of 
the form shall be as nearly as possible perfectly smooth, 
without crevices or offsets between the ends of adjacent 
planks. Where forms are used a second time, they shall 
be freshly jointed so as to make a perfectly smooth finish 
to the concrete. All forms shall be water-tight and shall 
be wetted before using. 

Any material in contact with wet concrete should be oiled or 
greased beforehand in order to prevent adherence to the concrete. 

191. Design of Forms. The design of forms for reinforced 
concrete work requires some knowledge of the strength of materials 
and the theories of beams, columns, and arches. Forms can be 
constructed without such knowledge but that they will be both 
economical and adequate is an improbability. The ordinary 
beam and column formulas are applicable to the design of forms. 
The maximum bending moment for sheeting and ribs is taken as 

wl 2 

x-, where w is the load per unit length, and I is the length between 
o 

supports. Sanford Thompson recommends that the deflection be 

wl 3 
calculated as Fr> in which E is the modulus of elasticity of 

' - 



the material, and I is the moment of inertia of the cross-section 
referred to the neutral axis. The horizontal pressure of the con- 
crete against the forms has been expressed empirically by E. B. 
Smith, 1 as 



1 Pressure of Concrete on Forms Measured in Tests, by E. B. Smith, 
before Am. Concrete Institute, Feb. 15, 1920. Abstracted in Eng. News- 
Recojd, Vol. 84, 1920, p. 665. 



DESIGN OF FORMS 



323 



in which P = lateral pressure in pounds per square inch; 
R = rate of filling forms in feet per hour; 
H = head of fill. Ordinarily taken as %R, but in cold 

weather or when continuously agitated it may be 

as high as f/; 

C = ratio, by volume, of cement to aggregate; 
S = consistency in inches of slump. 

Earlier investigators have usually concluded that the pressures 
were equal to those caused by a liquid weighing 144 pounds 
per cubic foot, but the tests of the United States Bureau of 
Public Roads, from which the above formula was devised, show 
the pressures to be decidedly below this amount under certain 
conditions. 

With these units and formulas the design of the lagging becomes 
a matter of substitution in, and the solution of, the equations 
produced. 1 The forces acting on the ribs are indeterminate. 
No more satisfactory design can be made 
for the ribs than to follow successful prac- 
tice, or what is seldom done, to determine 
the stresses in the forms by the application 
of one of the theories for the solution of 
arch stresses. The sizes of the lumber 
used in the ribs varies from 1^X6 inches 
to 2X10 inches, depending on the size 
of the sewer. If vertical posts are used 
at the ends to support the arch forms p IG 133 .Centering for 
they are computed as columns taking the Large Forms, 

full weight of the arch. If the span is 

so wide that radial supports are used as shown in Fig. 133 
the load at the center is assumed as one-fourth of the weight of 
the arch. 

192. Wooden Forms. Norway and Southern pine, spruce, 
and fir are satisfactory for form construction. White pine is 
satisfactory but is generally too expensive. The hard woods 
are too difficult to work. The lumber should be only partly 
dried as kiln-dried lumber swells too much when it is moistened, 
warping the forms out of shape or crushing the lagging at the 

1 See, also, Concrete Form Design, by E. F. Rockwood, Eng. and Con- 
tracting, Vol. 55, 1921, p. 528. 




324 



CONSTRUCTION 



joints. Green lumber must be kept moist constantly to prevent 
warping before use and when it is used it does not swell enough 
to close the cracks. The lumber should be dressed on the face 
next to the concrete and at the ends. Either beveled or matched 
lumber may be used for lagging. The joint made by beveled 




FIG. 134. FIG. 135. FIG. 136. 

FIG. 134. Beveled Joint for Wood Fords. 

FIG. 135. Collapsible Wooden Invert Form for Concrete Sewers. 
FIG. 136. Support for Arch Centering. 




FIG. 137. Wooden Forma Used in Tunnel, North Shore Sewer, Sanitary Dis- 
trict of Chicago. 
Journal Western Society of Engineers, Vol. 22, p. 385. 

lumber shown in Fig. 134 is cheaper but less satisfactory than a 
tongued and grooved joint. 

Types of wooden forms are shown in Figs. 135 and 136 for 
use in sewers to be built as monoliths or in two portions. Fig. 
137 shows the details of a built-up wooden form used in tunnel 
work for a 42| inch egg-shaped sewer. 



STEEL FORMS 



325 



193. Steel-lined Wooden Forms. Sheet metal linings are 
sometimes used on wooden forms. They permit the use of 
cheaper undressed lumber, demand less care in the joining of the 
lagging, and when in good condition give a smooth surface to 
the finished concrete. Their use has frequently been found 
unsatisfactory and more expensive than well-constructed wooden 
forms because of the difficulty of preventing warping and 
crinkling of the metal lining and in keeping the ends fastened 
down so that they will not curl. Sheet steel or iron of No. 18 or 
20 gage (0.05 to 0.0375 of an inch) weighing 2 to 1| pounds 
per square foot is ordinarily used for the lining . 




FIG. 138. Blaw Standard Full Round Telescopic Sewer Forms, Showing 
Knocked-Down Sections Loaded on a Truck. 
Courtesy, Blaw Steel Form Co. 

194. Steel Forms. These are simple, light, durable, and easy 
to handle. The engineer is seldom called upon to design these 
forms as the types most frequently used are manufactured by the 
patentees and are furnished to the contractor at a fixed rental 
per foot of form, exclusive of freight and hauling from the point 
of manufacture. The forms can be made in any shape desired, 
the ordinary stock shapes such as the circular forms being the 
least expensive. The smaller circular forms are adjustable 
within about 3 inches to different diameters so that the same 
form can be used for two sizes of sewers. The same form can be 
used for arch and invert in circular sewers. Fig. 138 shows the 



326 



CONSTRUCTION 



collapsible circular forms and the manner in which they are 
pulled through those still in position. Fig. 129 shows a half 
round steel form swung in position by chains and turnbuckles 
from the trench bracing, and Fig. 139 shows the free unobstructed 
working space in the interior of some large steel forms. 

195. Reinforcement. It is essential that the reinforcement 
be held firmly in place during the pouring of the concrete. A 
section of reinforcement misplaced during construction may 
serve no useful purpose and result in the collapse of the sewer. 




FIG. 139. Interior of .Steel Forms for Calumet Sewer, Chicago. 
Sewer is 16 feet wide. Note absence of obstructions. Courtesy, Hydraulic Steelcraft Co. 

In sewer constructipn a few longitudinal bars may be laid in 
order that the transverse bars may be wired to them and held 
in position by notches in the centering and in fastenings to bars 
protruding from the finished work. This construction is shown 
in Fig. 1. The network of reinforcement is held up from the 
bottom of the trench by notched boards which are removed as 
the concrete reaches them, or better by stones or concrete 
blocks which are concreted in. Sometimes the reinforcement 
is laid on top of the freshly poured portion of the concrete the 
surface of which is at the proper distance from the finished face 



COSTS OF CONCRETE SEWERS 327 

of the work. This method has the advantage of not requiring 
any special support for the reinforcement, but it is undesirable 
because of the resulting irregularity in the reinforcement spacing 
and position. 

In the side walls the position of the reinforcement is fixed by 
wires or metal strips which are fastened to the outside forms or 
to stakes driven into the ground. Wires are then fastened to the 
reinforcement bars and are drawn through holes in the forms 
and twisted tight. When the forms are removed the wires or 
strips are cut leaving a short portion protruding from the face 
of the wall. The reinforcing steel from the invert should pro- 
trude into the arch or the side walls for a distance of about 40 
diameters in order to provide good bond between the sections. 
The protruding ends are used as fastenings for the new reinforce- 
ment. The arch steel may be supported above the forms by 
specially designed metal supports, by small stones or concrete 
blocks which are concreted into the finished work; or by notched 
strips of wood which are removed as the concrete approaches 
them. Strips of wood are not satisfactory because thay are 
sometimes carelessly left in place in the concrete resulting in a 
line of weakness in the structure. Metal chairs are the most 
secure supports. They are fastened to the forms and the bars 
are wired to the chairs. In some instances the entire rein- 
forcement has been formed of one or two bars which are fastened 
into position as a complete ring. This results in a better bond in 
the reinforcement, requires less fastening and trouble in handling, 
but is in the way during the pouring of the concrete and inter- 
feres with the handling of the forms. 

196. Costs of Concrete Sewers. Under present day conditions 
a general statement of the costs of an engineering structure can 
not be given with accuracy. Only the items of labor, materials, 
and transportation that go to make up the cost can be estimated 
quantitively, and the total cost computed by multiplying the 
amount of each item by its proper unit cost obtained from the 
market quotations. 

A summary of some of the items that go to make up the cost 
of a concrete sewer and the relative amount of these items on 
different jobs is given in Tables 69 and 70. 



328 CONSTRUCTION 

TABLE 69 

DIVISION OF LABOR COSTS FOR THE CONSTRUCTION OP 
96-INCH CIRCULAR CONCRETE SEWER 



Classification of Labor 


Classification of Work 


Task or Title 


Number 
of 
men 


Total 
dollars 
per day 


Type of Work 


Dollars 
per foot 




1 
1 
1 

2 
10 
2 
1 
2 
2 
2 
3 
16 
3 
1 


6.00 
3.50 
2.00 
3.30 
16.50 
3.30 
3.00 
3.30 
3.30 
3.30 
5.25 
26.40 
5.25 
1.00 
5.00 


Excavation 


1.80 

0.58 
0.17 
0.45 
0.33 

1.17 
1.54 
0.29 

0.20 

0.09 
0.62 
0.62 






Hoister (engineman) 












Making and placing invert .... 


Carpenter on bracing 


Laying brick in invert 
Bending and placing steel in 
arch 








Bending and placing steel in 
invert 


Mixing and placing concrete. 
On steel forms 


Moving forms and centers. . . . 
Watchmen, water boy, etc. . . . 

Total .... 






Total . 


90.40 


7.86 







NOTES. Trench was 12j feet wide and of various depths. At depth of 12 feet the 
cost of excavation was $1.61 per foot. From Engineering and Contracting, Vol. 47, p. 157. 

BACKFILLING 

197. Methods. Careful backfilling is necessary to prevent the 
displacement of the newly laid pipe and to avoid subsequent 
settlement at the surface resulting in uneven street surfaces and 
dangers to foundations and other structures. 

The backfilling should commence as soon as the cement in the 
joints or in the sewer has obtained its initial set. Clay, sand, 
rock dust, or other fine compactible material is then packed by 
hand under and around the pipe and rammed with a shovel and 
light tamper. This method of filling is continued up to the top 
of the pipe. The backfill should rise evenly on both sides of the 
pipe and tamping should be continuous during the placing of the 
backfill. For the next 2 feet of depth the backfill should be placed 
with a shovel so as not to disturb the pipe, and should be tamped 
while being placed, but no tamping should be done within 6 
inches of the crown of the sewer. The tamping should become 



BACKFILLING METHODS 



329 



progressively heavier as the depth of the backfill increases. 
Generally one man tamping is provided for each man shoveling. 

TABLE 70 

DIVISION OP COSTS FOR THE CONSTRUCTION OF CONCRETE SEWERS 

Gillette's Handbook of Cost Data. 



Item 


Location 


Fond 
du Lac 


South 
Bend 


Wilming- 
ton 


Richmond, Indiana 


Diameter in inches 
Shape 


30 
circular 
plain 
0.11 
47 
1.20 

39.0* 
1.5 
12.4 
0.9 
0.7 
0.0 
23.0 
2.0 
12.5 
8.0 
8 
1908 


66 
circular 
rein. 
0.594 
24 to 36 
4.40 

33.5 
11.5 
15.5 

11.5 
20.0 
8.0 

10 
1906 


53 
horseshoe 
rein. 
0.37 

2.97 
33.0 
18.9 

14.5 
27.5 

6.1 


54 48 
circular circular 
rein. rein. 
5" shell 5" shell 

1.35 1.08 
17.1 
19.3 

28.3 
32.0T 

0.3 

Prewar conditions 


42 
circular 
rein. 
4" shell 

0.91 


Plain or reinforced 


Cubic yards per foot. . . . 


Cost per foot, dollars .... 
Per cent of total coat: 
Labor 


Tools 






Water 




Cement 


Frost prevention 
Forms 




Length of day, hours .... 
Year of construction . . . . 



* Includes 6 cents per foot for excavation. Labor for this was 58 per cent of the total 
labor cost. 

t Cement at $1.25 per barrel. 

Above a point 2 feet above the top of the sewer the method 
pursued and the care observed in backfilling will depend on the 
character of the backfilling material and the location of the 
sewer. If the sewer is in a paved street the backfill is spread in 
layers 6 inches thick and tamped with rammers weighing about 
40 pounds with a surface of about 30 square inches. One man 
tamping for each man shoveling is frequently specified. If no 
pavement is to be laid but it is required that the finished surface 
shall be smooth, slightly less care need be taken and only one 
man tamping is specified for each two men shoveling. On paved 
streets a reinforced concrete slab with a bearing of at least 12 
inches on the undisturbed sides of the trench may be designed 



330 CONSTRUCTION 

to support the pavement and its loads. This is of great help 
in preventing the unsightly appearance and roughness due to 
an improperly backfilled trench. On unpaved streets the 
backfill is crowned over the trench to a depth of about 6 inches 
and then rolled smooth by a road roller. In open fields, in side 
ditches, or in locations where obstruction to traffic or unsight- 
liness need not be considered, after the first 2 feet of backfill have 
been placed with proper care, the remainder is scraped or thrown 
into the trench by hand or machine, care being taken not to 
drop the material so far as to disturb the sewer. 

If the top of the sewer, manhole, or other structure comes 
close to or above the surface of the ground, an earth embank- 
ment should be built at least 3 feet thick over and around the 
structure. The embankment should have side slopes of at least 
1^ on 1 and should be tamped to a smooth and even finish. 

If sheeting is to be withdrawn from the trench it should be 
withdrawn immediately ahead of the backfilling, and in trenches 
subject to caving it may be pulled as the backfilling rises. 

Puddling is a process of backfilling in which the trench is 
filled with water before the filling material is thrown in. It 
avoids the necessity for tamping and can be used satisfactorily 
with materials that will drain well and will not shrink on drying. 
Sand and gravel are suitable materials for puddling, heavy 
clay is unsatisfactory. Puddling should not be resorted to before 
the first 2 feet of backfill has been carefully placed. More 
compact work can be obtained by tamping than with puddling. 

Frozen earth, rubbish, old lumber, and similar materials 
should not be used where a permanent finished surface is desired 
as these will decompose or soften resulting in settlement. Rocks 
may be thrown in the backfill if not dropped too far and the 
earth is carefully tamped around and over them. In rock 
trenches fine materials such as loam, clay, sand, etc., must be 
provided for the backfilling of the first portion of the trench for 
2 feet over the top of the pipe. More clay can generally be packed 
in an excavation than was taken out of it, but sand and gravel 
occupy more space than originally even when carefully tamped. 

Tamping machines have not come into general use. One 
type of machine sometimes used consists of a gasoline engine 
which raises and drops a weighted rod. The rod can be swung 
back and forth across the trench while the apparatus is being 



BACKFILLING METHODS 331 

pushed along. It is claimed that two men operating the machine 
can do the work of six to ten men tamping by hand. The machine 
delivers 50 to 60 blows per minute, with a 2 foot drop of the 80 
to 90 pound tamping head. 

Backfilling in tunnels is usually difficult because of the small 
space available in which to work. Ordinarily the timbering is 
left in place and concrete is thrown in from the end of the pipe 
between the outside of the pipe and the tunnel walls and roof. 
If vitrified pipe is used in the tunnel, the backfilling is done with 
selected clayey material which is packed into place around the 
pipe by workmen with long tamping tools. The backfilling 
should be done with care under the supervision of a vigilant 
inspector in order that subsequent settlement of the surface may 
be prevented. 



CHAPTER XII 
MAINTENANCE OF SEWERS 

198. Work Involved. The principal effort in maintaining 
sewers is to keep them clean and unobstructed. A sewerage 
system, although buried, cannot be forgotten as it will not care 
for itself, but becoming clogged will force itself on the attention 
of the community. Besides the cleaning and repairing of 
sewers and the making of inspections for determining the neces- 
sity for this work, ordinances should be prepared and enforced 
for the purpose of protecting the sewers from abuse. Inspec- 
tions to determine the amount of the depreciation of sewers 
with a view towards possible renewal, or to determine the 
capacity of a sewer in relation to the load imposed upon it are 
sometimes necessary. The valuation of the sewerage system 
as an item in the inventory of city property may be assigned to 
the engineer in charge of sewer maintenance. 

The work involved in the inspection and cleaning of sewers 
in New York City for the year ending May, 1914, included the 
removal of 22,687 cubic yards of material from catch basins, 
and 14,826 catch-basin cleanings. This made an average of 
two and one-half cleanings per catch-basin per year, or 1^ cubic 
yards removed at each cleaning. The 6,432 catch-basins were 
inspected 71,890 times. There were 4,112 cubic yards of material 
removed from 517 miles of sewers, or about 8 cubic yards per 
mile. Inspection of 194 miles of brick sewers were made, 4.4 
miles were flushed, and 27 miles were cleaned. Inspections of 198 
miles of pipe sewers were made, 80 miles were examined more 
closely, 37 miles were flushed, and 91 miles were cleaned. The 
field organization for this work consisted of 17 foremen, 8 
assistant foremen, 29 laborers, 71 cleaners, 13 mechanics, 7 
inspectors of construction, 3 inspectors of sewer connections, 
13 horses and wagons, and 28 horses and carts. 1 
1 Mun. Journal, Vol. 36, 1914, p. 736. 
332 



CAUSES OF TROUBLES 333 

199. Causes of Troubles. The complaints most frequently 
received about sewers are caused by clogging, breakage of pipes, 
and bad odors. Sewers become clogged by the deposition of sand 
and other detritus which results in the formation of pools in 
which organic matter deposits, aggravating the clogged condi- 
tion of the sewers and causing the odors complained of. Grease 
is a prolific cause of trouble. It is discharged into the sewer 
in hot wastes, and becoming cooled, deposits in thick layers 
which may effectively block the sewer if not removed. It can 
be prevented from entering the sewers by the installation of 
grease traps as described in Chapter VI. The periodic cleaning 
of these traps is as important as their installation. 

Tree roots are troublesome, particularly in small pipe sewers 
in residential districts. Roots of the North Carolina poplar, 
silver leaf poplar, willow, elm, and other trees will enter the 
sewer through minute holes and may fill the sewer barrel com- 
pletely if not cut away in time. Fungus growths occasionally 
cause trouble in sewers by forming a network of tendrils that 
catches floating objects and builds a barricade across the sewer. 
Difficulties from fungus growths are not common, but constant 
attention must be given to the removal of grit, grease, and roots. 
Tarry deposits from gas-manufacturing plants are occasionally 
a cause of trouble, as they cement the detritus, already deposited 
into a tough and gummy mass that clings tenaciously to the 
sewer. 

Broken sewers are caused by excessive superimposed loads, 
undermining, and progressive deterioration. The changing char- 
acter of a district may result in a change of street grade, an increase 
in the weight of traffic, or in the construction of other structures 
causing loads upon the sewer for which it was not designed. 
The presence of corrosive acids or gases may cause the deteriora- 
tion of the material of the sewer. 

200. Inspection. The maintenance of a sewerage system 
is usually placed under the direction of a sewer department. In 
the organization of the work of this department no regular 
routine of inspection of all sewers need be followed ordinarily. 
Attention should be given regularly to those sewers that are 
known to give trouble, whereas the less troublesome sewers 
need not be inspected more frequently than once a year, pref- 
erably during the winter when labor is easier to obtain. 



334 



MAINTENANCE OF SEWERS 



The routine inspection of sewers too small to enter is made by 
an examination at the manhole. If the water is running as freely 
at one manhole as at the next manhole above, it is assumed 
that the sewer between the manholes is clean and no further 
inspection need be given unless there is some other reason to 
suspect clogging between manholes. If the sewage is backed 
up in a manhole it indicates that there is an obstruction in the 
sewer below. If the sewage in a manhole is flowing sluggishly 
and is covered with scum it is an indication of clogging, slow 
velocity and septic action in the sewer . Sludge banks on the slop- 
ing bottom of the manhole or signs of sewage high upon the 



-Mirror 




FIG. 140. Inspecting Sewers with Reflected Sunlight. 

walls indicate an occasional flooding of the sewer due to inade- 
quate capacity or clogging. 

If any of the signs observed indicate that the sewer is clogged, 
the manhole should be entered and the sewer more carefully 
inspected. Such inspection may be made with the aid of mirrors 
as shown in Fig. 140 or with a periscope device as shown in Fig. 
141. Sunlight is more brilliant than the electric lamp shown 
in Fig. 141, but the mirror in the manhole directs the sunlight 
into the eyes of the observer, dazzling him and preventing a good 
view of the sides of the sewer. The observers' eyes can be pro- 
tected against the direct rays of the electric light, which can be 
projected against the sides of the pipe by proper shades and 
reflectors. It is possible with this device to locate house con- 



INSPECTION 



335 



nection, stoppages, breaks of the pipe, and to determine fairly 
accurately the condition of the sewer without discomfort to the 
observers. 

Sewers that are large enough to enter should be inspected by 
walking through them where possible. The inspection should be 
conducted by cleaning off the sewer surface in spots with a 
small broom, and examining the brick wall for loose bricks, loose 
cement or cement lost from the joints, open joints, broken bond, 
eroded invert, and such other items as may cause trouble. An 
inspection in storm sewers is sometimes of value in detecting the 
presence of forbidden house connections. 




FIG. 141. Inspecting Sewers with Periscope and Electric Light. The G-K 

System. 

Certain precautions should be taken before entering sewers 
or manholes. If a distinct odor of gasoline is evident the sewer 
should be ventilated as well as possible by leaving a number of 
manhole covers open along the line until the odor of gasoline 
has disappeared. The strength of gasoline odor above which 
it is unsafe to enter a sewer is a matter of experience possessed 
by few. A slight odor of gasoline is evident in many sewers 
and indicates no special danger. A discussion of the amount of 
gasoline necessary to create explosive conditions is given in 
Art. 206. In making observations of the odor it should also be 
noted whether air is entering or leaving the manhole. The 
presence of gasoline cannot be detected at a manhole into which 
ah* is entering. 



336 MAINTENANCE OF SEWERS 

As soon as it is considered that the odors from a sewer indicate 
the absence of an explosive mixture, a lighted lantern or other 
open flame should be lowered into the manhole to test the 
presence of oxygen. Carbon monoxide or other asphyxiating 
gases may accumulate in the sewer, and if present will extinguish 
the flame. If the flame burns brilliantly the sewer is probably 
safe to enter, but if conditions are unknown or uncertain, the man 
entering should wear a life belt attached to a rope and tended 
by a man at the surface. Asphyxiating or explosive gases are 
sometimes run into without warning due to their lack of odor, 
or the presence of stronger odors in the sewer. Breathing masks 
and electric lamps are precautions against these dangers, the 
masks being ready for use only when actually needed. More 
deaths have occurred in sewers due to asphyxiating gases than 
by explosions, as the average sewer explosion is of insufficient 
violence to do great damage, although on occasion, extremely 
violent explosions have occurred. During inspections of sewers 
there should always be at least one man at the surface to call 
help in case of accident and the inspecting party should consist 
of at least two men. 

It must not be felt that entering sewers is fraught with great 
danger, as it is perfectly safe to enter the average sewer. The 
air is not unpleasant and no discomfort is felt, but conditions 
are such that unexpected situations may arise for which the man 
in the sewer should be prepared. It is therefore wise to take 
certain precautions. These may indicate to the uninitiated, a 
greater danger than actually exists. 

The inspection of sewers should include the inspection of the 
flush-tanks, control devices, grit chambers, and other appurte- 
nances. A common difficulty found with flush-tanks is that the 
tank is " drooling," that is to say the water is trickling 
out of the siphon as fast as it is entering the tank, and the inter- 
mittency of the discharge has ceased. If, when the tank is first 
inspected the water is about at the level of the top of the bell it 
is probable that the siphon is drooling. A mark should be made 
at the elevation of the water surface and the tank inspected again 
in the course of an hour or more. If the water level is unchanged 
the siphon is drooling. This may be caused by the clogging of 
the snif t hole or by a rag or other obstacle hanging over the siphon 
which permits water to pass before the air has been exhausted, 



CLEANING SEWERS 337 

or a misplacement of the cap over the siphon, or other difficulty 
which may be recognized when the principle on which the siphon 
operates is understood. Occasionally it is discovered that an 
over zealous water department has shut off the service. 

Control devices, such as leaping or overflow weirs, automatic 
valves, etc., may become clogged and cease to operate satis- 
factorily. They should be inspected frequently, dependent upon 
their importance and the frequency with which they have been 
ound to be inoperative. An inspection will reveal the obstacle 
which should be removed. Floats should be examined for loss 
of buoyancy or leaks rendering them useless. Grit and screen 
chambers should be examined for sludge deposits. 

Catch-basins on storm sewers are a frequent cause of trouble 
and need more or less frequent cleaning. Cleanings are more 
important than inspections for catch-basins for if they are 
operating properly they are usually in need of cleaning after every 
storm of any magnitude, and a regular schedule of cleaning should 
be maintained. 

A record should be kept of all inspections made. It should 
include an account of the inspection, its date, the conditions 
found, by whom made and the remedies taken to effect repairs. 

201. Repairs. Common repairs to sewerage systems consist 
in replacing street inlets or catch-basin covers broken by traffic; 
raising or lowering catch-basin or manhole heads to compensate 
for the sinking of the manhole or the wear of the pavement; 
replacing of broken pipes, loosened bricks or mortar which has 
dropped out; and other miscellaneous repairs as the necessity 
may arise. Connections from private drains are a source of 
trouble because either the sewer or the drain has broken due 
to careless work or the settlement of the foundation or the 
backfill. 

202. Cleaning Sewers. Sewers too small to enter are cleaned 
by thrusting rods or by dragging through them some one of the 
various instruments available. The common sewer rod shown 
in Fig. 142 is a hickory stick, or light metal rod, 3 or 4 feet long, 
on the end of which is a coupling which cannot come undone in 
the sewer. Sections of the rod are joined in the manhole and 
pushed down the sewer until the obstruction is reached and 
dislodged. Occasionally pieces of pipe screwed together are 
used with success. The end section may be fitted with a special 



338 



MAINTENANCE OF SEWERS 



cutting shoe for dislodging obstructions. In extreme cases these 
rods may be pushed 400 to 500 feet, but are more effective at 
shorter distances. Obstructions may be dislodged by shoving a 
fire hose, which is discharging water under high pressure through 
a small nozzle, down the sewer toward the obstruction. The 
water pressure stiffens the hose, which, together with the support 
from the sides of the conduit, make it possible to push the hose 
in for effective work 100 feet or more from the manhole. A 
strip of flexible steel about inch thick and 1^ to 2 inches wide 
is useful for " rodding " a short length of crooked sewer. 

Sewers are seldom so clogged that no channel whatever remains. 
As a sewer becomes more and more clogged, the passage becomes 
smaller, thereby increasing the velocity of flow of the sewage 




FIG. 142. Sewer Rods 



around the obstruction and maintaining a passageway by erosion. 
This phenomenon has been taken advantage of in the cleaning of 
sewers by " pills." These consist of a series of light hollow balls 
varying in size. One of the smaller balls is put into the sewer 
at a manhole. When the ball strikes an obstruction it is caught 
and jammed against the roof of the sewer. The sewage is backed 
up and seeks an outlet around the ball, thus clearing a channel 
and washing the ball along with it. The ball is caught at the 
next manhole below. A net should be placed for catching the ball 
and a small dam to prevent the dislodged detritus from passing 
down into the next length of pipe. The feeding of the balls into 
the sewer is continued, using larger and larger sizes, until the sewer 
is clean. This method is particularly useful for the removal of 
sludge deposits, but it is not effective against roots and grease. 



CLEANING SEWERS 



339 



The balls should be sufficiently light to float. Hollow ir.et.Ll 
balls are better than heavier wooden ones. 

Plows and other scraping instruments are dragged through 
pipe sewers to loosen banks of sludge and detritus and to cut 
roots or dislodge obstructions. One form of plow consists of 
a scoop l similar to a grocer's sugar scoop, which is pushed or 




FIG. 143. Cable and Windlass Method of Cleaning Sewers. 

The cable is held to the bottom of the sewer by bracing a 2x4 upright in the sewer, with a 
snatch block attached. A trailer is attached to the scoop to prevent loss of material. 

dragged up a sewer against the direction of flow. As fast as the 
scoop is filled it is drawn back and emptied. The method of 
dragging this through a sewer is indicated in Fig. 143. At 
Atlantic City the crew operating the scoop comprises five men, 
two are at work in each manhole and one on the surface to warn 
traffic and wait on the men in the manholes. The outfit of 




FIG. 144. Sewer Cleaning Device. 
Eng. News, Vol. 42, 1899, p. 328. 

tools is contained in a hand-drawn tool box and includes sewer 
rods, metal scoops for all sizes of sewers, picks, shovels, hatchets, 
chisels, lanterns, grease and root cutters, etc., and two winches 
with from 400 to 600 feet of f-inch wire cable. 

Another form of plow or drag consists of a set of hooks or 
teeth hinged to a central bar as shown in Fig. 144. A root cutter 
1 Mun. Journal, Vol. 39, 1915, p. 911. 



340 



MAINTENANCE OF SEWERS 



and grease scraper in the form of a spiral spring with sharpened 
edges, and other tools for cleaning sewers are shown in Fig. 145. 
A turbine sewer cleaner shown in Fig. 146 consists of a set of 
cutting blades which are revolved by a hydraulic motor of about 






FIG. 145. Tools for Cleaning Sewers. 

3 horse-power under an operating pressure of about 60 pounds 
per square inch. The turbine is attached to a standard fire 
hose and is pushed through the sewer by utilizing the stiffness 
of the hose, or by rods attached to a pushing jack as shown in 
the figure. This machine was invented and patented by W. A. 




FIG. 146. Turbine Sewer Machine Connected to Forcing Jack. 

The forcing jack is used when windlass and cable cannot be used. 
Courtesy, The Turbine Sewer Machine Co. 

Stevenson in 1914. Its performance is excellent. The blades 
revo>ve at about 600 R.P.M., cutting roots and grease. The 
revolving blades and the escaping water also serve to loosen 
and stir up the deposits and the forward helical motion imparted 
to the water is useful in pushing the material ahead of the machine 



FLUSHING SEWERS 341 

and in scrubbing the walls of the sewer. In Milwaukee four men 
with the machine cleaned 319 feet of 12-inch sewer in 16 hours, 
and in Kansas City 7,801 feet of sewers were cleaned in 14 days. 

Sewers large enough to enter may be cleaned by hand. The 
materials to be removed are shoveled into buckets which are 
carried or floated to manholes, raised to the surface and dumped. 
In very large sewers temporary tracks have been laid and small 
cars pushed to the manhole for the removal of the material. 
Hydraulic sand ejectors may also be used for the removal of 
deposits, similar to the steam ejector pump shown in Fig. 97. 
The water enters the apparatus at high velocity, under a pressure 
of about 60 pounds per square inch, leaps a gap in the machine 
from a nozzle to a funnel-shaped guide leading to the discharge 
pipe. The suction pipe of the machine leads to the chamber 
in which the leap is made. In leaping this gap the water creates 
a vacuum that is sufficient to remove the uncemented detritus 
large enough to pass through the machine, and will lift small 
stones to a height of 10 to 12 feet. Occasionally barricades of 
logs, tree branches, rope, leaves, and other obstructions which have 
piled up against some inward projecting portion of the sewer, 
must be removed by hand either by cutting with an axe or by 
pulling them out. Projections from the sides of sewers are 
objectionable because of their tendency to catch obstacles and 
form barricades. 

Little authentic information on the cost of cleaning sewers 
is available. A permanent sewer organization is maintained 
by many cities. The division of their time between repairs, 
cleaning, and other duties is seldom made a matter of record. 
From data published in Public Works 1 it is probable that the 
cost varies from $3 to $15 per cubic yard of material removed. 
From the information in Vol. II of "American Sewerage Practice" 
by Metcalf and Eddy the combined cost of cleaning and flushing 
will vary between $10 and $40 per mile; the expense of 
either flushing or cleaning alone being about one-half of this. 

203. Flushing Sewers. Sewers can sometimes be cleaned or 
kept clean by flushing. Flushing may be automatic and frequent, 
or hand flushing may be resorted to at intervals to remove 
accumulated deposits. Automatic flush-tanks, flushing man- 
holes, a fire hose, a connection to a water main, a temporary 
1 Formerly the Municipal Journal. 



342 MAINTENANCE OF SEWERS 

fixed dam, a moving dam, and other methods are used in flushing 
sewers. The design, operation, and results obtained from the 
use of automatic flush-tanks and flushing manholes are discussed 
in Chapter VI. 

The method in use for cleaning a sewer by thrusting a fire 
hose down it can also be used for flushing sewers. It is an 
inexpensive and fairly satisfactory method. There is, however, 
some danger of displacing the sewer pipe because of the high 
velocity of the water. An easier and safer but less effective 
method is to allow water to enter at the manhole and flow down 
the sewer by gravity. Direct connections to the water mains 
are sometimes opened for the same purpose. 

Sewers are sometimes flushed by the construction of a tempo- 
rary dam across the sewer, causing the sewage to back up. When 
the sewer is half to three-quarters full the dam is suddenly removed 
and the accumulated sewage allowed to rush down the sewer, thus 
flushing it out. The dam may be made of sand bags, boards fitted 
to the sewer, or a combination of boards and bags. The expense of 
equipment for flushing by this method is less than that by any 
other method, but the results obtained are not always desirable. 
Below the dam the results compare favorably with those obtained 
by other methods, but above the dam the stoppage of the flow 
of the sewage may cause depositions of greater quantities of 
material than have been flushed out below. A time should be 
chosen for the application of this method when the sewage is 
comparatively weak and free from suspended matter. The 
most convenient place for the construction of a dam is at a man- 
hole in order that the operator may be clear of the rush of sewage 
when the dam is removed. 

Movable dams or scrapers are useful in cleaning sewers of a 
moderate size, but are of little value in small sewers. The 
scraper fits loosely against the sides of the sewer and is pushed 
forward by the pressure of the sewage accumulated behind it. 
The iron-shod sides of the dam serve to scrape grease and 
growths attached to the sewer and to stir up sand and sludge 
deposited on the bottom. The high velocity of the sewage 
escaping around the sides of the dam aids in cleaning and 
scrubbing the sewer. 

A natural watercourse may be diverted into the sewer if 
topographical conditions permit, or where sewers discharge 



CLEANING CATCH-BASINS 343 

into the sea below high tide a gate may be closed during the 
flood and held closed until the ebb. The rush of sewage on the 
opening of the gate serves to flush the sewers and stir up the 
sludge deposited during high tide. Other methods of flushing 
sewers may be used dependent on the local conditions and the 
ingenuity of the engineer or foreman in charge. 

In some sewers it is not necessary to remove the clogging 
material from the sewer. It is sufficient to flush and push it 
along until it is picked up and carried away by higher velocities 
caused by steeper grades or larger amounts of sewage. 

204. Cleaning Catch-basins. 1 Catch-basins have no reason 
for existence if they are not kept clean. Their purpose is to 
catch undesirable settling solids and to prevent them from 
entering the sewers, on the theory that it is cheaper to clean a 
catch-basin than it is to clean a sewer. If the cleaning of storm 
sewers below some inlet to which no catch-basin is attached 
becomes burdensome, the engineer in charge of maintenance 
should install an adequate catch-basin and keep it clean. Catch- 
basins are cleaned by hand, suction pumps, and grab buckets. 
In cleaning by hand the accumulated water and sludge are 
removed by a bucket or dipper and dumped into a wagon from 
which the surplus settled water is allowed to run back into the 
sewer. The grit at the bottom of the catch-basin is removed 
by shoveling it into buckets which are then hoisted to the 
surface and emptied. 

Suction pumps in use for cleaning catch-basins are of the 
hydraulic eductor type. The eductor works on the principle of 
the steam pump shown in Fig. 97, except that water is used 
instead of steam. The material removed may be discharged 
into settling basins constructed in the street, or may be dis- 
charged directly into wagons. 2 In Chicago a special motor- 
driven apparatus is used. This consists of a 5-yard body on a 
5-ton truck, and a centrifugal pump driven by the truck motor. 
In use, the truck, about half filled with water, drives up to the 
catch-basin, the eductor pipe is lowered and water pumped from 
the truck into the eductor and back into the truck again, together 
with the contents of the catch-basin. The surplus water drains 

1 See Eng. Record, Vol. 75, 1917, p. 463. 

1 Eng. Record, Vol. 73, 1916, p. 141, and Eng. News-Record, Vol. 79, 
1917, p. 1019. 



344 MAINTENANCE OF SEWERS 

back into the sewer. The Chicago Bureau of Sewers reports 
a truck so equipped to have cleaned 1013 catch-basins, removing 
1763 cubic yards of material, and running 1380 miles, during the 
months of August, September and October, 1917. The cost, 
including all items of depreciation, wages, repairs, etc., was 
$1,393.89. Orange-peel buckets, about 20 inches in diameter, 
operated by hand or by the motor of a 3| to 5-ton truck with a 
water-tight body, are used for cleaning catch-basins in some cities. 

Catch-basins in unpaved streets and on steep sandy slopes 
should be cleaned after every storm of consequence. Basins 
which serve to catch only the grit from pavement washings 
require cleaning about two or three times per year, and from one to 
three cubic yards of material are removed at each cleaning. The 
cost of cleaning ordinary catch-basins by hand may vary from $15 
to $25, but with the use of eductors or orange-peel buckets the 
cost is somewhat lower. In Seattle the cost of cleaning large 
detritus basins by hand is said 1 to vary from $45 to $60. With 
the use of eductors this cost has been reduced to one-third or 
one-fifth the cost of cleaning by hand. 

205. Protection of Sewers. 2 City ordinances should be 
wisely drawn and strictly enforced for the protection of sew- 
ers against abuse and destruction. The requirements of some 
city ordinances are given in the following paragraphs. 

Washington, D. C., 3 sewer ordinances provide that: 

No person shall make or maintain any connection with 
any public sewer or appurtenance thereof whereby there 
may be conveyed into the same any hot, suffocating, 
corrosive, inflammable or explosive liquid, gas, vapor, 
substance or material of any kind . . . provided that 
the provisions of this act shall not apply to water from 
ordinary hot water boilers or residences. 

The following extracts from the ordinances of Indianapolis 
are typical of those from many cities: 

2950. No connection shall be made with any public 
sewer without the written permission of the Committee 
on Sewers and the Sewerage Engineer. 

1 Eng. Record, Vol. 72, 1915, p. 690. 

2 Eng. Record, Vol. 71, 1915, p. 256. 

3 Eng. and Contr., Vol. 41, 1914, p. 250. 



PROTECTION OF SEWERS 345 

2953. No person shall be authorized to do the work 
of making connections until he has furnished a satisfactory 
certificate that he is qualified for the duties. He shall 
also file bond for not less than SI, 000 that he will indemnify 
the City from all loss or damage that may result from his 
work and that he will do the work in conformity to the 
rules and regulations established by the City Council. 

2955. It shall be unlawful for any person to allow 
premises connected to the sewers or drains to remain 
without good fixtures so attached as to allow a sufficiency 
of water to be applied to keep the same unobstructed. 

2956. No butcher's offal or garbage, or dead animals, 
or obstructions of any kind shall be thrown in any receiving 
basin or sewer in penalty not greater than $100. Any 
person injuring, breaking, or removing any portion of 
any receiving basin, manhole cover, etc., shall be fined 
not more than $100. 

2962. No person shall drain the contents of any cess- 
pool or privy vault into any sewer without the permission 
of the Common Council. 

The Cleveland ordinances are similar and contain the 
following in addition : 

1251. Rule 4. All connections with the main or branch 
sewers shall be made at the regular connections or junctions 
built into the same, except by special permit. 

Rule 16. No stean pipe, nor the exhaust, nor the blow 
off from any steam engine shall be connected with any 
sewer. 

Evanston, Illinois, protects its sewers against the additions of 
grease and other undesirable substances as follows: 

1444. It is unlawful for any person to use any sewer 
or appurtenance to the sewerage system in any manner 
contrary to the orders of the Commissioner of Public Works. 

1446. Wastes from any kitchen sinks, floor drains, or 
other fixtures likely to contain greasy matter from hotels, 
certain apartment houses, boarding houses, restaurants, 
butcher shops, packing houses, lard rendering establish- 
ments, bakeries, 'launderies, cleaning establishments, gar- 
ages, stables, yard and floor drains, and drains from 
gravel roofs shall be made through intervening receiving 
basins constructed as prescribed in par. VIII of this code. 

Receiving basins suitable for the work required in the code 
are illustrated in Chapter VI. 



346 MAINTENANCE OF SEWERS 

206. Explosions in Sewers. Disastrous explosions in sewers 
were first recorded about 1886. 1 Up to about 1905 explosions 
were infrequent and were considered as unavoidable accidents 
and so rare as to be unworthy of study. For a decade or more 
after 1905 explosions occurred with increasing violence and 
frequency causing destruction of property, but by some freakish 
chance, but little loss of life. A violent and destructive explosion 
occurred in Pittsburgh on Nov. 25, 1913, 2 and another on March 
12, 1916. The property damage amounted to $300,000 to 
$500,000 on each occasion, but there was no loss of life. Two 
miles of pavement were ripped up, gas, water, and other sewer 
pipes were broken, buildings collapsed and the streets were 
flooded. The streets were rendered unserviceable for long 
periods during the expensive repairs that were necessary. In 
recent years the number of explosions in sewers has been smaller, 
due probably to the gain in knowledge of the causes and intelli- 
gent methods of prevention. 

The three principal causes of explosions in sewers are: gasoline 
vapor, illuminating gas, and calcium carbide. It is probable 
that gasoline vapor is by far the most troublesome. Explo- 
sions caused by these gases are not so violent as those caused 
by dynamite or other high explosives, as the volume of gas and 
the temperature generated are much less. The violence of sewer 
explosions may be increased somewhat by the sudden pressures 
that are put upon them. 

Gasoline finds its way into sewers from garages and cleaning 
establishments. A mixture of 1| per cent gasoline vapor and 
ah* may be explosive. It needs only the stray spark of an 
electric current, a lighted match, or a cigar thrown into the sewer 
to cause the explosion. As the result of a series of experiments 
on 2,706 feet of 8-foot sewer, Burrell and Boyd conclude: 3 

One gallon of gasoline if entirely vaporized produces 
about 32 cubic feet of vapor at ordinary temperature 
and pressure. If \\ per cent be adopted as the low explo- 
sive limit of mixtures of gasoline vapor and air, 55 gallons 
or a barrel of gasoline would produce enough vapor to 
render explosive the mixture in 1,900 feet of 9 foot sewer 

1 H. J. Kellogg in Journal Connecticut Society of Civil Engineers, 1914, 
and Technical Paper 117, U. S. Bureau of Mines. 
2 Eng. News, Vol. 70, 1913, p. 1157. 
Technical Paper No. 117, U. S. Bureau of Mines. 



EXPLOSIONS IN SEWERS 347 

provided the gasoline and the air were perfectly mixed. 
Many different factors, however, govern explosibility, 
such as: size of the sewer, velocity of the sewage, tem- 
perature of the sewer, volatility and rate of inflow of the 
gasoline. Only under identical conditions of tests would 
duplicate results be obtained. A large amount of gaso- 
line poured in at one time is less dangerous than the same 
amount allowed to run in slowly. With a velocity of 
flow of about 6? feet per second it was evident that 55 
gallons of gasoline poured ah 1 at once into a manhole 
rendered the air explosive only a few minutes (less than 
10) at any particular point. With the same amount of 
gasoline run in at the rate of 5 gallons per minute, an 
explosive flame would have swept along the sewer if ignited 
15 minutes after the gasoline had been dumped. With 
a slow velocity of flow and a submerged outlet the gasoline 
vapor being heavier than air accumulated at one point 
and extremely explosive conditions could result from a 
small amount of gasoline. Comparatively rich explosive 
mixtures were found 5 hours after the gasoline had been 
discharged. High-test gasoline is much more dangerous 
than the naphtha used in cleaning establishments, yet 
on account of the large quantity of waste naphtha the 
sewage from cleaning establishments may be very dan- 
gerous. 

Illuminating gas is not so dangerous as gasoline vapor as it is 
lighter than air and it is more likely to escape from the sewer 
than to accumulate in it. It requires about one part of 
illuminating gas to seven parts of air to produce an explosive 
mixture. 

Calcium carbide is dangerous because it is self igniting. The 
heat of the generation of gas is sufficient to ignite the explosive 
mixture. The gases are highly explosive and cause a relatively 
powerful explosion. Fortunately large amounts of this material 
seldom reach a sewer, the gas being generated in garage drains 
or traps and escaping in the atmosphere. 

A hydrocarbon oil used by railroads in preventing the freezing 
of switches, if allowed to reach the sewers, may cause explosions 
therein. 1 The oil crystallizes and in this form it is soluble in 
water. It will thus pass traps and on volatilization will pro- 
duce explosive mixtures. 

Methane, generated by the decomposition of organic matter, 

1 Eng. News, Vol. 71, 1914, p. 84. 



348 MAINTENANCE OF SEWERS 

is a feebly explosive gas occasionally found in sewers. Its 
presence may add to the strength of other explosive mixtures. 

Sewer explosions may be prevented by the building of proper 
forms of intercepting basins to prevent the entrance of gasoline 
and calcium carbide gases, and by ventilation to dilute the 
explosive mixtures which may be made up in the sewer. There 
are no practical means to predict when an explosion is about' to 
occur, and after an explosion has occurred it is difficult to deter- 
mine the cause as all evidence is usually destroyed. 

207. Valuation of Sewers. The necessity for the valuation 
of a sewerage system may arise from the legal provisions in some 
states limiting the amount of outstanding bonds which may be 
issued by a municipality to a certain percentage of the present 
worth of municipal property. The investment in the sewerage 
system is usually great and forms a large portion of the City's 
tangible property. It may be desirable also to determine the 
depreciation of the sewers with a view towards their renewal. 

The most valuable work on the valuation of sewers has been 
done in New York City l by the engineers of the Sewer Depart- 
ment. The committee of engineers appointed to do the work 
recommended: (1) that the original cost be made the basis of 
valuation, and that (2), in fixing this cost the cost of pavement 
should be omitted or at most the cost of a cheap (cobblestone) 
pavement should be included. Trenches previously excavated 
in rock were considered as undepreciated assets. 

The present worth of sewers depends on many factors aside 
from the effects of age, such as the care exercised in the original 
construction, the material used, the kind and quantity of sewage 
carried, the care taken in maintenance, and finally the injury 
caused by the careless building of adjoining substructures. Dur- 
ing the progress of the inspections the examination of brick sewers, 
due to their accessibility, yielded better results than the examina- 
tion of pipe sewers. The routine of the examination of the 
brick sewers consisted in cleaning off the bricks with a short 
broom, tapping the brick with a light hammer to determine 
solidity, and testing the cement joints by scraping with a chisel. 
In addition, measurements of height and width were taken every 
30 feet. The bricks in the invert at and below the flow line 
were examined for wear. 

1 Eng. News, Vol. 71, 1914, p. 82.' 



VALUATION OF SEWERS 



349 



A study of the reports of these examinations disclosed that the 
following defects were noticeable: 

1. Cement partly out at water line. 

2. Cement partly out above water line. 

3. Depressed arch and sewer slightly spread. 

4. Large open joints. 

5. Loose brick. 

6. Bond of brick broken. 

7. Distorted sides, uneven bottom, joints out of line. 



Age in Years. 



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Date of Construction 
a. 6. 

FIG. 147. Diagrams used in Estimating Depreciation of Brick Sewers Due to 
Age, Manhattan Borough, New York City. 

o. Proportionate deterioration from various causes. 

b. Percentage of depreciation based on examination of sewers, use of deterioration curve 
(Fig. a), and age of sewers examined. 

Eng. News, Vol. 71, p. 84. 

Inspection of pipe sewers from manholes, the pipe being illu- 
minated by floating candle?, was found to be unsatisfactory. 
Reliance was placed on the reports of men experienced in making 
connections and repairs to the sewers. Early pipe sewers in New 
York were laid directly on the bottom of the trench. Under these 
circumstances a small leak at a joint was sufficient to wash the 
earth away and to drop the pipe, causing serious conditions 
along the line. No wear or deterioration of pipe sewers were 
noted, the only defects being cracking of the pipes at the center 
line due to poor foundation and to defects in the pipe itself. 



350 



MAINTENANCE OF SEWERS 



The depreciation of brick sewers as studied in New York, 
is shown graphically in Fig. 147. At zero the sewer is in good 
condition and at 100 it is in such a state of dilapidation as to 
require instant rebuilding. Repairs are not considered economical 
in this condition. In the preparation of this diagram each 
condition on the list above was given a certain number of points, 

Age in Years. 





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FIG. 148. Diagram Showing Rate of Depreciation of Pipe Sewers. 

Eng. News, Vol. 71, p. 86. 



which when added together represented the state of depreciation 
of the sewer. These sums were plotted as ordinates and the 
corresponding ages of the sewer were plotted as abscissas. The 
various points were taken cumulatively, and where the bond 
of the brickwork was broken (given a value of 72) plus other 
defects gave a total of 164 the sewer was considered as valueless 
and not worth repair. The scale of 164 was later reduced to 
a percentage basis as shown on the right of the figure. Fig. 
148 shows a similar diagram for the depreciation of pipe sewers. 



VALUATION OF SEWERS 351 

It was concluded that the life of a brick sewer in New York 
is 64 years. Some of the sewers examined were over 200 years 
old. The total original cost of 483 miles of brick, pipe and wood 
sewers was figured as $23,880,000 with a present worth of 
$18,665,000 and an average annual depreciation cf 2.2 per cent. 
In figuring these amounts no account was taken of obsolescence. 
The deterioration of catch-basins proceeded at about the same 
rate as for brick sewers. 



CHAPTER XIII 
COMPOSITION AND PROPERTIES OF SEWAGE 

208. Physical Characteristics. Sewage is the spent water 
supply of a community containing the wastes from domestic, 
industrial, or commercial use, and such surface and ground water 
as may enter the sewer. 1 Sewages are classed as: domestic 
sewage, industrial waste, storm water, surface water, street 
wash, and ground water. Domestic sewage is the liquid dis- 
charged from residences or institutions and contains water 
closet, laundry, and kitchen wastes. It is sometimes called sani- 
tary sewage. Industrial sewage is the liquid waste resulting 
from processes employed in industrial establishments. Storm 
water is that part of the rainfall which runs over the surface of 
the ground during a storm and for such a short period following 
a storm as the flow exceeds the normal and ordinary run-off. 
Surface water is that part of the rainfall which runs over the 
surface of the ground some time after a storm. Street wash 
is the liquid flowing on or from the street surface. Ground 
water is water standing in or flowing through the ground below 
its surface. 

Ordinary fresh sewage is gray in color, somewhat of the appear- 
ance of soapy dish water. It contains particles of suspended 
matter which are visible to the naked eye. If the sewage is 
fresh the character of some of the suspended matter can be dis- 
tinguished as: matches, bits of paper, fecal matter, rags, etc. 
The amount of suspended matter in sewage is small, so small as 
to have no practical effect on the specific gravity of the liquid 
nor to necessitate the modification of hydraulic formulas 
developed for application to the flow of water. The total 
suspended matter in a normal strong domestic sewage is about 
500 parts per 1,000,000. It is represented graphically in Fig. 
149. The quantity of organic or volatile suspended matter 
1 Similar to definition proposed by the Am. Public Health Ass'n. 

352 



PHYSICAL CHARACTERISTICS 



353 



is about 200 parts per 1,000,000. It is shown graphically in the 
smaller cube in Fig. 149. 

The odor of fresh sewage is faint and not necessarily unpleasant. 
It has a slightly pungent odor, somewhat like a damp unven- 
tilated cellar. Occasionally the odor of gasoline, or some other 
predominating waste matter may hide all other odors. Stale 
sewage is black and gives off 
nauseating odors of hydrogen 
sulphide and other gases. If 
the sewage is so stale as to 
become septic, bubbles of gas 
will be seen breaking the sur- 
face and a black or gray scum 
may be present. Before the 
South Branch of the Chicago 
River was cleaned up and 
flushed this scum became so 
thick in places, particularly in 
that portion of the Stock Yards 
where the river became known 
as Bubbly Creek, that it is said 
that weeds and small bushes 
"sprouted in it, and chickens 
and small animals ran across 
its surface. 

A physical analysis of sewage should include an observation 
of its appearance, and a determination of its temperature, tur- 
bidity, color, and odor, both hot and cold. The temperature is 
useful in indicating certain of the antecedents of the sewage, 
its effect on certain forms of bacterial life, and its effect on the 
possible content of dissolved gases. Temperatures higher than 
normal are indicative of the presence of trades wastes discharged 
while hot into the sewers. A low temperature may indicate 
the presence of ground water. If the temperature is much over 
40 C. bacterial action will be inhibited and the content of dis- 
solved gases will be reduced. Turbidity, color, and odor deter- 
minations may be of value in the control of treatment devices, 
or to indicate the presence of certain trades wastes, which give 
typical reactions. Since all normal sewages are high in color 
and turbidity, the relative amounts of these two constituents 




149. Graphical Representation 
of Relative Volumes of Liquids and 
Solids in Sewage. 



354 COMPOSITION AND PROPERTIES OF SEWAGE 

in two different sewages has little significance regarding the 
relative strengths of the two sewages or the proper method of 
treating them. A fresh domestic sewage should have no highly 
offensive odor. The presence of certain trades wastes can be 
detected sometimes in fresh sewages, and a stale sewage may 
sometimes be recognized by its odor. 

Sewage is a liability to the community producing it. Although 
some substances of value can be obtained from sewage x the cost 
of the processes usually exceed the value of the substances 
obtained. Where it becomes necessary to treat sewage the value 
of these substances may be helpful in defraying the cost of 
treatment. 

209. Chemical Composition. Sewage is composed of mineral 
and organic compounds which are either in solution or are sus- 
pended in water. In making a standard chemical analysis of 
sewage only those chemical radicals and elements are determined 
which are indicative of certain important constituents. Neither 
a complete qualitative nor quantitative analysis is made. A 
sewage analysis will not show, therefore, the number of grams of 
sodium chloride present or any other constituent. A complete 
standard sanitary chemical analysis will report the constituents 
as named in the first column of Table 71. The quantities of 
these materials found in average strong, medium and weak 
sewages are also shown in this table. These values are not 
intended as fixed boundaries between sewages of different 
strengths. They are presented merely as a guide to the inter- 
pretation of sewage analyses. 

The principal objects of a chemical analysis of sewage are to 
determine its strength and its state of decomposition. The 
influents and effluents of a sewage treatment device are analyzed 
to aid in the control of the device and to gain information con- 
cerning the effect of the treatment. Chemical and other analyses, 
in connection with the desired conditions after disposal, will 
indicate the extent of treatment which may be required. The 
standard methods of water and sewage analysis adopted by the 
American Public Health Association have been generally accepted 
by sanitarians. These uniform methods make possible com- 

1 Economic Values in Sewage and Sewage Sludge, by Raymond Wells, 
Proceedings Am. Society Municipal Improvements, Nov. 12, 1919. Eng. 
News-Record, Vol. 83, 1919, p. 948. 



CHEMICAL COMPOSITION 



355 



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356 COMPOSITION AND PROPERTIES OF SEWAGE 

parisons of the results obtained by laboratories working according 
to these standards. 

210. Significance of Chemical Constituents. Organic nitro- 
gen and free ammonia taken together are an index of the organic 
matter in the sewage. Organic nitrogen includes all of the 
nitrogen present with the exception of that in the form of ammonia, 
nitrites, and nitrates. Free ammonia or ammonia nitrogen is 
the result of bacterial decomposition of organic matter. A fresh 
cold sewage should be relatively high in organic nitrogen and 
low in free ammonia. A stale warm sewage should be relatively 
high in free ammonia and low in organic nitrogen. The sum of 
the two should be unchanged in the same sewage. 

Nitrites (RNO2) and nitrates (RNOs) 1 are found in fresh 
sewages only in concentrations of less than one part per million. 
In well-oxidized effluents from treatment plants the concen- 
tration will probably be much higher. Nitrates contain one more 
atom of oxygen than nitrites. They represent the most stable 
form of nitrogenous matter in sewage. Nitrites are not stable 
and are reduced to ammonias or are oxidized to nitrates. Their 
presence indicates a process of change. They are not found in 
large quantities in raw sewage because their formation requires 
oxygen which must be absorbed from some other source than 
the sewage. In an ordinary sewer or sluggishly flowing open 
stream this absorption cannot take place from the atmosphere 
with sufficient rapidity to supply the necessary oxygen. 

Oxygen consumed is an index of the amount of carbonaceous 
matter readily oxidizable by potassium permanganate. It does 
not indicate the total quantity of any particular constituent, 
but it is the most useful index of carbonaceous matter. Car- 
bonaceous matter is usually difficult of treatment and a high 
oxygen consumed is indicative of a sewage difficult to care for. 
The amount of oxygen consumed, as expressed in the analysis, 
is dependent on the amount of oxidizable carbonaceous matter 
present, the oxidizing agent used, and the time and temperature 
of contact of the sewage and the oxidizing agent. It is essential 
therefore that the test be conducted according to some standard 
method, since the results are of value only as compared with 
results obtained under similar conditions. 

Total solids (residue on evaporation) are an index of the 
1 R represents any chemical element such as K, Na, etc. 



SIGNIFICANCE OF CHEMICAL CONSTITUENTS 357 

strength of the sewage. They are made up of organic and 
inorganic substances. The inorganic substances include sand, 
clay, and oxides of iron and aluminum, which are usually insolu- 
ble, and chlorides, carbonates, sulphates and phosphates, which 
are usually soluble. The insoluble inorganic substances are 
undesirable in sewage because of their sediment forming prop- 
erties which result in the clogging of sewers, treatment plants, 
pumps, and stream beds. The soluble inorganic substances are 
generally harmless and cause no nuisance, except that the 
presence of sulphur may permit the formation of hydrogen 
sulphide, which has a highly offensive odor. The organic sub- 
stances are: carbo-hydrates, fats, and soaps, which are car- 
bonaceous and are difficult of removal by biological processes; 
and the nitrogenous substances such as urea, proteins, amines, 
and amino acids. The inorganic and organic substances may be 
either in solution or suspension or in a colloidal condition. 

Volatile solids are used as an index of the organic matter 
present, as it is assumed that the organic matter is more easily 
volatilized than the inorganic matter. The amount of volatile 
inorganic matter present is usually so small as to be negligible. 

Fixed solids are reported as the difference between the total 
and volatile solids. They are therefore representative of the 
amount of inorganic matter present. 

Suspended matter is the undissolved portion of the total 
solids.. High volatile suspended matter is an indication of 
offensive qualities in the nature of putrefying organic matter, 
whereas fixed suspended matter is indicative of inoffensive 
inorganic matter. It is difficult to obtain a sample of sewage 
which will represent the amount of suspended matter in the 
sewage, since a sample taken from near the surface will contain 
less inorganic matter and grit than a sample taken near the bottom. 

Settling solids are indicative of the sludge forming properties 
of the sewage and of the probable degree of success of treatment 
by plain sedimentation. Volatile settling solids indicate the 
property of the formation of offensive putrefying sludge banks. 
There is no chemical test which will indicate the scum-forming 
properties of sewage. Fixed settling solids indicate the presence 
of inorganic matter, probably gritty material such as sand, clay, 
iron oxide, etc. 

Colloidal matter is material which is too finely divided to be 



358 COMPOSITION AND PROPERTIES OF SEWAGE 

removed by filtration or sedimentation, yet is not held in solu- 
tion. It can sometimes be removed by violent agitation in the 
presence of a flocculent precipitate, as in the treatment with 
activated sludge, or by the flocculent precipitate alone, as in 
chemical precipitation, or by the acidulation of the sewage so 
as to precipitate the colloids. Colloidal matter is probably the 
result of the constant abrasion of finely divided suspended matter 
while flowing through the sewer or other channel. High colloidal 
matter may therefore indicate a stale sewage, or the presence of a 
particular trades waste. Colloids are difficult of removal. For 
this reason, where sewage is to be treated, turbulence in the 
tributary channels should be avoided. 

Alkalinity may indicate the possibility of success of the 
biologic treatment of sewage, since bacterial life flourishes better 
in a slightly alkaline than in a slightly acid sewage. Within 
the normal limits of the amount of alkalinity in sewage the exact 
amount has little significance in sewage analyses. Sewages are 
normally slightly alkaline. An abnormal alkalinity or acidity 
may indicate the presence of certain trades wastes necessitating 
special methods of treatment. A method of sewage treatment 
may be successful without changing the amount of alkalinity 
in the sewage since the amount of alkalinity is not inherently 
an objection. 

Chlorine, in the form of sodium chloride, is an inorganic sub- 
stance found in the urine of man and animals. The amount of 
chlorine above the normal chlorine content of pure waters in the 
district is used as an index of the strength of the sewage. The 
chlorine content may be affected by certain trades wastes such 
as ice-cream factories, meat-salting plants, etc., which will increase 
the amount of chlorine materially. Since chlorine is an inorganic 
substance which is in solution it is not affected by biological 
processes nor sedimentation. Its diminution in a treatment 
plant or in a flowing stream is indicative of dilution and the 
reduction of chlorine will be approximately proportional to the 
amount of dilution. 

Fats have a recoverable market value when present in 
sufficient quantity to be skimmed off the surface of the sewage. 
Ordinarily fats are an undesirable constituent of sewage as they 
precipitate on and clog the interstices in filtering material, they 
form objectionable scum in tanks and streams, and they are acted 



SIGNIFICANCE OF CHEMICAL CONSTITUENTS 



359 



on very slowly by biological processes of sewage treatment. 
Although fats are carbonaceous matter they are not indicated 
by the oxygen consumed test because they are not easily oxidized. 
They are therefore determined in another manner; by evapora- 
tion of the liquid and extracting the fats from the residue by 
dissolving them in ether. 

Relative stability and bio-chemical oxygen demand are 
the most important tests indicating the putrefying character- 
istics of sewage. Since stability and putrescibility have opposite 
meanings the relative stability test is sometimes called the 
putrescibility test. The relative stability of a sewage is an 
expression for the amount of oxygen present in terms of the 
amount required for complete stability. 

A relative stability of 75 signifies that the sample 
examined contains a supply of available oxygen equal 
to 75 per cent of the amount of oxygen which it requires 
in order to become perfectly stable. The available 
oxygen is approximately equivalent to the dissolved oxygen 
plus the available oxygen of nitrate and nitrite. 1 

TABLE 72 
RELATIVE STABILITY NUMBERS 



Time Required for 
Decolorization at 
20 C. 
Days 


Relative 
Stability 
Number 


Time Required 
for Decoloriza- 
tion at 20 C. 
Days 


Relative 
Stability 
Number 


0.5 


11 


8.0 


84 


1.0 


21 


90 


87 


1.5 


30 


10.0 


90 


2.0 


37 


11.0 


92 


2.5 


44 


12.0 


94 


3.0 


50 


13.0 


95 


4.0* 


60 


14.0 


96 


5.0 


68 


16.0 


97 


6.0 


75 


18.0 


98 


7.0 


80 


20.0 


9& 



* Routine testa are ordinarily incubated for this period only, and if not decolorised in 
this time are recorded as stable. 

1 Standard Methods of Water Analysis, American Public Health Asso- 
ciation, 1920. 



360 COMPOSITION AND PROPERTIES OF SEWAGE 

The relative stability numbers, given in Table 72, are computed 
from the expression, S = 100(1 0.7940 in which S is the 
stability number and t is the time in days that the sample has 
been incubated at 20 C. The bio-chemical oxygen demand is 
more directly an index of the consumption of available oxygen 
by the biological and chemical changes which take place in the 
decomposition of sewage or polluted water. As such it is a 
more valuable, though less easily performed test than the test 
of relative stability. 

The methods for the determination of the relative stability 
and the bio-chemical oxygen demand are given to show more 
clearly what these tests represent. The procedure in the 
relative stability test is to add 0.4 c.c. of a standard solution 
of methylene blue to 150 c.c. of the sample. The mixture is then 
allowed to stand in a completely filled and tightly stoppered bottle 
at 20 C. for 20 days or until the blue fades out due to the ex- 
haustion of the available oxygen. There are three methods 
in use for the determination of the biochemical oxygen demand; 1 
the relative stability method, the excess nitrate method, and the 
excess oxygen method In the relative stability method the 
sample to be treated should have a relative stability of at least 
50. If it is lower than this the sample should be diluted with 
water containing oxygen until the relative stability has been 
raised to or above this point. The oxygen demand in parts 
per million is then expressed as 

0' = 



RP 

in which 0' is the oxygen demand, is the initial oxygen in parts 
per million (p. p.m.) in the diluting water or sewage, P is the 
proportion of sewage in the mixture expressed as a ratio, and 
R is the relative stability of the mixture expressed as a decimal. 
For the effluents from sewage treatment plants, polluted waters, 
and similar liquids, the total available oxygen expressed as the 
sum of the dissolved oxygen, nitrites, and nitrates, divided by 

1 Determination of the Biochemical Oxygen Demand of Sewage and 
Industrial Wastes, by E. J. Theriault, Report of the U. S. Public Health 
Service, Vol. 35, May 7, 1920, No. 19, p. 1087. 

2 Standard Methods of Water Analysis, American Public Health Asso- 
ciation, 1920. 



SIGNIFICANCE OF CHEMICAL CONSTITUENTS 361 

the relative stability expressed as a decimal will give the bio- 
chemical oxygen demand. The excess nitrate method requires 
the determination of the total oxygen available as dissolved 
oxygen, nitrites, and nitrates and the addition of a sufficient 
amount of oxygen in the form of sodium nitrate to prevent the 
exhaustion of oxygen during a 10-day period of incubation. At 
the end of the period the total available oxygen is again deter- 
mined. The difference between the original and the final oxygen 
content represents the bio-chemical oxygen demand. The 
excess oxygen test requires the determination of the total avail- 
able oxygen as before, and the addition of a sufficient amount of 
oxygen, in the form of dissolved oxygen in the diluting water, 
to prevent exhaustion of the oxygen in a 10-day period of incu- 
bation. The difference between the original and final oxygen 
content represents the bio-chemical oxygen demand. Theriault 
concludes as a result of his tests, that the relative stability and 
excess nitrate methods are open to objections but that the excess 
oxygen method yields very accurate and consistent results with 
as little or less labor than is required by other methods. 

Dissolved oxygen represents what its name implies, the 
amount of oxygen (0%) which is dissolved in the liquid. Normal 
sewage contains no dissolved oxygen unless it is unusually fresh. 
It is well, if possible, to treat a sewage before the original dis- 
solved oxygen has been exhausted. Normal pure surface water 
contains all of the oxygen which it is capable of dissolving, as 
shown in Table 73. The presence of a smaller amount of oxygen 
than is shown in this table indicates the presence of organic 
matter in the process of oxidation, which may be in such quanti- 
ties as ultimately to reduce the oxygen content to zero. Normal 
pure ground waters may be deficient in dissolved oxygen because 
of the absence of available oxygen for solution. The presence 
of certain oxygen-producing organisms in polluted or otherwise 
potable surface waters may cause a supersaturation with oxygen 
however. 

The dissolved-oxygen test for polluted water is probably the 
most significant of all tests. If dissolved oxygen is found in a 
polluted water it means that putrefactive odors will not occur, 
since putrefaction cannot begin in the presence of oxygen. It 
is possible for different strata in a body of water to have different 
quantities of dissolved oxygen, and putrefaction may be proceeding 



362 



COMPOSITION AND PROPERTIES OF SEWAGE 



in the lower strata before the oxygen is exhausted from the upper 
strata. The oxygen content of a river water will indicate the 
ability of the river to receive sewage without resulting in a 
nuisance. 

TABLE 73 
SOLUBILITY OF OXYGEN IN WATER 

Under an atmospheric pressure of 760 mm. of mercury, the atmosphere 
containing 20.9 per cent of oxygen. 



Temperature, degrees C 


0.00 


1 


2 


3 


4 


5 


6 


7 




















Oxygen in parts per million . . . 


14.62 


14.23 


13.84 


13.48 


13.13 


12.80 


12.48 


12.17 


Temperature, degrees C 


8 


9 


10 


11 


12 


13 


14 


15 




















Oxygen in parts per million. . . 


11.87 


11.59 


11.33 


11.08 


10.83 


10.60 


10.37 


10.15 


Temperature, degrees C 


16 


17 


18 


19 


20 


21 


22 


23 


Oxygen in parts per million. . . 


9.95 


9.74 


9.54 


9.35 


9.17 


8.99 


8.83 


8.68 


Temperature, degrees C 


24 


25 


26 


27 


28 


29 


30 






















Oxygen in parts per million. . . 


8.53 


8.38 


8.22 


8.07 


7.92 


7.77 


7.63 





211. Sewage Bacteria. A slight knowledge of the nature 
of bacteria is necessary in order that the biological changes 
which occur in the treatment of sewage may be understood. 
Bacteria are living organisms which are so small that it is difficult 
or impossible to study them either with the eye alone or with 
the aid of powerful microscopes. They are studied by means 
of cultures, stains, and certain characteristic phenomena such 
as the production of a gas, the production of a red colony on 
litmus lactose agar, etc. Bacteria occur in three forms: 
spherical, called coccus; cylindrical, called bacillus; and spiral, 
called spirillum. In size they vary from the largest at about 
1/10,000 of an inch to sizes so small as to be invisible under the 
most powerful microscope. An ordinary size is 1/25,000 of an 
inch. The cylindrical or rod bacteria are about four times as long 
as they are wide. Some bacteria possess the power of motion 
due to the presence of flagella or hairs which can be moved and 



ORGANIC LIFE IN SEWAGE 363 

cause the cell to progress at a rate as high as 18 cm. per hour, 
but usually the rate is very much less than this. The compo- 
sition of the bacterial cell has never been definitely determined. 

Bacteria are unicellular plants. They possess no digestive 
organs and apparently obtain their food by absorption from the 
surrounding media. Reproduction is by the division of the cell 
in f o two approximately equal portions. This reproduction may 
occur as frequently as once every half hour and if unchecked 
would quickly mount to unimaginable numbers. The natural 
cause limiting the growth of bacteria is the generation by the 
bacterium of certain substances such as the amino acids, which 
are injurious to cell life. The exhaustion of the food supply is 
not considered as an important cause of inhibition of multipli- 
cation. The products of growth of one species of bacteria may be 
helpful or harmful to other forms. Where the products are 
helpful the effect is known as symbiosis, and where harmful the 
effect is known as antibiosis. In sewage the presence of both 
aerobic and anaerobic bacteria is usually mutually helpful and 
the condition is an example of symbiosis. The aerobes, some- 
times called obligatory aerobes, are bacteria which demand 
available oxygen for their growth. The anaerobes, or obligatory 
anaerobes, can grow only in the absence of oxygen. There are 
other forms that are known as facultative anaerobes (or aerobes) 
whose growth is independent of the presence or absence of 
oxygen. 

Spores are formed by some bacteria when they are subjected 
to an unfavorable environment such as high temperatures, the 
absence of food, the absence of moisture, etc. Spores are cells- 
in which growth and animation are suspended but the life of 
the cell is carried on through the unsuitable period, somewhat 
similar to the condition ir a plant seed. 

212. Organic Life in Sewage. Living organisms, both plants 
and animals, exist in sewage. Bacteria are the smallest of these 
organisms. Others, which can be studied easily under the 
microscope or can be seen with difficulty by the naked eye but 
which do not require special cultures for their study, are classed 
as microscopic organisms or plankton. Organisms which are 
large enough to be studied without the aid of a microscope or 
special cultures are classed as macroscopic. The part taken in 
the biolysis of sewage by macroscopic organisms belonging to 



364 COMPOSITION AND PROPERTIES OF SEWAGE 

the animal kingdom, such as birds, fish, insects, rodents, etc., 
which feed upon substances in the sewage is so inconsequential 
as to be of no importance. Both plants and animals are found 
among the macroscopic organisms. 

Organisms in sewage may be either harmful, harmless, or 
beneficial. From the viewpoint of mankind the harmful organ- 
isms are the pathogenic bacteria. Their condition of life in sewage 
is not normal and in general their existence therein is of short 
duration. It may be of sufficient length, however, to permit 
the transmission of disease. The diseases which can be trans- 
mitted by sewage are only those that are contracted through 
the alimentary canal, such as typhoid fever, dysentery, 
cholera, etc. Diseases are not commonly contracted by contact 
of sewage with the skin nor by breathing the air of sewers. It 
is safe to work in and around sewage so long as the sewage is 
kept out of the mouth, and asphyxiating or toxic gases are 
avoided. 

The beneficial organisms in sewage are those on which 
dependence is placed for the success of certain methods of treat- 
ment. These organisms have not all been isolated or identified. 

The total number of bacteria in a sample of sewage has little 
or no significance. In a normal sewage the number may be 
between 2,000,000 and 20,000,000 per c.c. and because of the 
extreme rapidity of multiplication of bacteria a sample showing 
a count of 1,000,000 per c.c. on the first analysis may show 4 to 
5 times as many 3 or 4 hours later. A bacterial analysis of 
sewage is ordinarily of little or no value, since pathogenic organ- 
isms are practically certain to be present, there is no interest 
in the harmless organisms, and the helpful nitrifying and aerobic 
bacteria will not grow on ordinary laboratory media. Occa- 
sionally the presence of certain bacteria may indicate the presence 
of certain trades wastes. In general, the total bacterial count, 
as sometimes reported, represents only the number of bacteria 
which have grown under the conditions provided. It bears no 
relation to the total number of bacteria in the sample. 

The presence of bacteria in sewage is of great importance 
however, as practically all methods of treatment depend on 
bacterial action, and all sewages which do not contain deleterious 
trades wastes, contain or will support the necessary bacteria 
for their successful treatment, if properly developed. 



DECOMPOSITION OF SEWAGE 365 

213. Decomposition of Sewage. If a glass container be filled 
with sewage and allowed to stand, open to the air, a black sedi- 
ment will appear after a short time, a greasy scum may rise to 
the surface, and offensive odors will be given off. This con- 
dition will persist for several weeks, after which the liquid will be- 
come clear and odorless. The sewage has been decomposed and is 
now in a stable condition. The decomposition of sewage is brought 
about by bacterial action the exact nature of which is uncertain. 

It 1 is well established that many of the chemical 
effects wrought by bacteria, as by other living cells, are 
due, not to the direct action of the protoplasm, but to 
the intervention of soluble ferments or enzymes. 

Enzymes are soluble ferments produced by the growth of the 
bacterial cell. 

In 2 many cases the enzymes diffuse out from the 
cell and exert their effort on the ambient substances . . . 
in others the enzyme action occurs within the cell and 
the products pass out, (for example) . . . the alcohol- 
producing enzymes of the yeast cell act upon sugar within 
the cell, the resulting alcohol and carbon dioxide being 
ejected. 

Other chemical effects may be brought about by the direct action 
of the living cells, but this has never been well established. 

Metabolism is the life process of living cells by which they 
absorb their food and convert it into energy and other products. 
It is the metabolism of bacterial growth that in itself or by the 
production of enzymes hastens the putrefactive or oxidizing 
stages of the organic cycles in sewage treatment. Bacteria can 
assimilate only liquid food since they have no digestive tract 
through which solid food can enter. The surrounding solids 
are dissolved by the action of the enzymes, the resulting solution 
diffusing through the chromatin or outer skin, and being digested 
throughout the interior cytoplasm. 

Bacteria are sometimes classified as parasites and saprophytes. 
The parasites live only on the growing cells of other plant or 
animal life. The saprophytes obtain their food only from the 

1 Jordan, General Bacteriology, 1909, p. 91. 
* Ibid. 



366 COMPOSITION AND PROPERTIES OF SEWAGE 

life products of living organisms and do not exist at the expense 
of the organisms themselves. Facultative saprophytes (or 
parasites) may exist on either living or dead tissue. 

The decomposition of sewage may be divided into anaerobic 
and aerobic stages. These conditions are usually, but not 
always, distinctly separate. The growth of certain forms of 
bacteria is concurrent, while the growth of other forms is dependent 
on the results of the life processes of other bacteria in the early 
stages of decomposition. 

When sewage is very fresh it contains some oxygen. This 
oxygen is quickly exhausted so that the first important step in 
the decomposition of sewage is carried on under anaerobic condi- 
tions. This is accompanied by the creation of foul odors of 
organic substances, ammonia, hydrogen sulphide, etc.; other 
odorless gases such as carbon dioxide, hydrogen, and marsh 
gas, the latter being inflammable and explosive; and other com- 
plicated compounds. An exception to the rule that putrefac- 
tion takes place only in the absence of oxygen is the production 
of other foul-smelling substances by the putrefactive activity 
of obligatory and facultative aerobes. Hydrogen sulphide may 
be produced apparently in the presence of oxygen the action 
which takes place not being thoroughly understood. 

The biolysis of sewage is the term applied to the changes 
through which its organic constituents pass due to the metab- 
olism of bacterial life. Organic matter is composed almost 
exclusively of the four elements: carbon, oxygen, hydrogen, 
and nitrogen (COHN) and sometimes in addition sulphur and 
phosphorus. The organic constituents of sewage can be divided 
into the proteins, carbohydrates, and fats. The proteins are 
principally constituents of animal tissue, but they are also found 
in the seeds of plants. The principal distinguishing character- 
istic of the proteins is the possession of between 15 and 16 per cent 
of nitrogen. To this group belong the albumens and casein. 
The carbohydrates are organic compounds in which the ratio 
of hydrogen to oxygen is the same as in water, and the number 
of carbon atoms is 6 or a multiple of 6. To this group belong 
the sugars, starches and celluloses. The fats are salts formed, 
together with water, by the combination of the fatty acids with 
the tri-acid base glycerol. The more common fats are stearin, 
palmatin, olein, and butyrine. The soaps are mineral salts of the 



THE NITROGEN CYCLE 367 

fatty acids formed by replacing the weak base glycerol with some 
of the stronger alkalies. 

The first state in the biolysis of sewage is marked by the 
rapid disappearance of the available oxygen present in the water 
mixed with organic matter to form sewage. In this state the 
urea, ammonia, and other products of digestive or putrefactive 
decomposition are partially oxidized and in this oxidation the 
available oxygen present is rapidly consumed, the conditions 
in the sewage becoming anaerobic. The second state is putre- 
faction in which the action is under anaerobic conditions. The 
proteins are broken down to form urea, ammonia, the foul- 
smelling mercaptans, hydrogen sulphide, etc., and fatty and 
aromatic acids. The carbohydrates are broken down into their 
original fatty acid, water, carbon dioxide, hydrogen, methane, 
and other substances. Cellulose is also broken down but much 
more slowly. The fats and soaps are affected somewhat similarly 
to the hydrocarbons and are broken down to form the original 
acids of their make up together with carbon dioxide, hydrogen, 
methane, etc. The bacterial action on facts and soaps is much 
slower than on the proteins, and the active biological agents 
in the biolysis of the hydrocarbons, fats, and soaps are not so 
closely confined to anaerobes as in the biolysis of the proteins. 
The third state in the biolysis of sewage is the oxidation or 
nitrification of the products of decomposition resulting from 
the putrefactive state. The products of decomposition are 
converted to nitrites and nitrates, which are in a stable condition 
and are available for plant food. It must be understood that 
the various states may be coexistent but that the conditions 
of the different states predominate approximately in the order 
stated. In the biolysis of sewage there is no destruction of matter. 
The same elements exist in the same amount as at the start of the 
biolytic action. 

214. The Nitrogen Cycle. Nitrogen is an element that is 
found in all organic compounds. Its presence is necessary to all 
plant and animal life. The nitrogenous compounds are most 
readily attacked by bacterial action in sewage treatment. The 
non-nitrogenous substances such as soaps and fats, and the inor- 
ganic compounds are more slowly affected by bacterial action 
alone. The element nitrogen passes through a course of events 
from life to death and back to life again that is known as the 



368 COMPOSITION AND PROPERTIES OF SEWAGE 

Nitrogen Cycle. It is typical of the cycles through which all 
of the organic elements pass. 

Upon the death of a plant or animal, decomposition sets in 
accompanied by the formation of urea which is broken down 
into ammonia. This is known as the putrefactive stage of the 
Nitrogen Cycle. The next state is nitrification in which the 
compounds of ammonia are oxidized to nitrites and nitrates, 
and are thus prepared for plant food. In the state of plant life 
the nitrites and nitrates are denitrified so as to be available as a 
plant or animal food. The highest state of the Nitrogen Cycle 
is animal life, in which nitrogen is a part of the living animal 
substance or is charged from protein to urea, ammonia, etc., by 
the functions of life in the animal. Upon the death of this 
animal organism the cycle is repeated. The Nitrogen Cycle, 
like the cycle of Life and Death, is purely an ideal condition as 
in nature there are many short circuits and back currents which 
prevent the continuous progression of the cycle. The con- 
ception of this cycle is an aid, however, in understanding the 
processes of sewage treatment. 

215. Plankton and Macroscopic Organisms. In general 
the part played by these organisms in the biolysis of sewage is 
not sufficiently well understood to aid in the selection of methods 
of sewage treatment involving their activities. The presence 
in bodies of water receiving sewage, of certain plankton which 
are known to exist only when putrefaction is not imminent, 
indicates that the body of water into which the discharge of 
sewage is occurring is not being overtaxed. The control of sewage 
treatment plant effluents so as to avoid the poisoning of fish life 
or the contamination of shell fish is likewise important. The 
study of plankton and macroscopic life in the treatment of sewage 
is an open field for research. 

216. Variations in the Quality of Sewage. The quality of 
sewage varies with the hour of the day and the season of the 
year. Some of the causes of these variations are: changes in the 
amount of diluting water due to the inflow of storm water or 
flushing of the streets or sewers; variations in domestic activities 
such as the suspension of contributions of organic wastes during 
the night, Monday's wash, etc.; characteristics of different 
industries which discharge different kinds of wastes according 
to the stage of the manufacturing process, etc. In general 



VARIATIONS IN THE QUALITY OF SEWAGE 



369 



TABLE 74 

SEWAGE ANALYSES SHOWING HOURLY, DAILY, AND SEASONAL VARIATIONS 

IN QUALITY 



Place 


Time 


Total 
Nitro- 
gen 


Chlo- 
rine 


Sus- 
pended 
Matter 


Remarks 


Refer- 
ence 


Marion Ohio 


Mid't-noon, 5-21-06. 


45 


53 


190 


Industrial 


1 




Noon-mid't 5-21-O6. 


37 


94 


133 


Domestic 


1 


Westerville, Ohio 


Day 


10.2 


76 


118 


\ college 


1 




Night 


2.6 


74 


41 


\ town 


1 


Columbus, Ohio 


1904-1905 














Mid't to 2 a.m. 


4.6 


50 


131 




2 




2 a.m. to 4 a.m. 


3.0 


52 


95 




2 




4 a.m. to 6 a.m. 


2.3 


51 


83 




2 




6 a.m. to 8 a.m. 


2.7 


48 


83 




2 




8 a.m. to 10 a.m. 


16.3 


66 


476 




2 




10 a.m. to noon 


11.4 


100 


324 




2 




Noon to 2 p.m. 


11.3 


86 


246 




2 




2 p.m. to 4 p.m. 


12.3 


78 


246 




2 




4 p.m. to 6 p.m. 


22.0 


78 


368 




2 




6 p.m. to 8 p.m. 


8.2 


71 


209 




2 




8 p.m. to 10 p.m. 


7.8 


80 


120 




2 




10 p.m. to mid't 


6.2 


56 


117 




2 


Center Ave., Chicago . 


Mid't to 3 a.m. 






123 




3 




4 a.m. to 7 a.m. 






316 




3 




8 a.m. to 11 a.m. 






608 




3 




Noon to 3 p.m. 






785 




3 




4 p.m. to 7 p.m. 






717 




3 




8 p.m. to 11 p.m. 






287 




3 


Columbus, Ohio 


Sunday 


6.7 


55 


858 




2 




Monday 


9.1 


66 


1048 




2 




Tuesday 


9.4 


69 


1024 




2 




Wednesday 


9.6 


68 


1005 




2 




Thursday 


9.2 


66 


990 




2 




Friday 


9.2 


67 


1018 




2 




Saturday 


9.3 


67 


1016 




2 


Baltimore, 1907-1908 


Aug. 1 to Sept. 1 


16.0 




246 




4 




Sept. 4 to Oct. 3 


19.0 




190 




4 




Oct. 6 to Nov. 4 


20.0 




188 




4 




Nov. 15 to Nov. 29 


20.0 




164 




4 




Dec. 3 to Dec. 29 


20.0 




123 




4 




Jan. 6 to Jan. 21 


19.0 




127 




4 




Feb. 2 to Feb. 26 


20.0 




149 




4 




Feb. 29 to Mar. 24 


28.0 




274 




4 




Mar. 27 to April 29 


25.0 




165 




4 




April 30 to May 26 


19.0 




104 




4 




June 8 to July 1 1 


15.0 




88 




4 




July 13 to Aug. 8 


9.5 




124 




4 



References: 

1. 1908 Report of the Ohio State Board of Health. 

2. Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905. 

3. Report on Industrial Wastes from the Stock Yards and Packingtown in Chicago, by 

the Sanitary District of Chicago. 1921. 

4. Report of the Baltimore Sewerage Commission, 1911. 



370 COMPOSITION AND PROPERTIEvS OF SEWAGE 

night sewage is markedly weaker than day sewage in both domestic 
and industrial wastes, but in specific cases the varying strength 
depends entirely upon the characteristics of the district. Some 
analyses are given in Table 74, which emphasize these 
points. 

217. Sewage Disposal. Previous to the development of the 
water-carriage method for removing human excreta and other 
liquid wastes the solid matter was disposed of by burial and the 
liquid wastes were allowed to seep into the ground or to run 
away over its surface. Following the development of the water- 
carriage system, which necessitated the development of sewers, 
the problem of ultimate disposal was rendered more serious by 
the concentration of human excreta together with a large volume 
of water. The unthinking citizen believes the problem of sewage 
disposal is solved when the toilet is flushed or the bath tub is 
drained. The problem may more truly be said to commence 
at this point. 

It would appear that the simplest method of disposal of 
sewage would be to discharge it into the nearest water course. 
Unfortunately the nature of sewage is such that it may be either 
highly offensive to the senses or dangerous to health or both, 
when discharged in this manner. Only the most fortunate, 
communities are favored with a body of water of sufficient size 
to receive sewage without creating a nuisance. 

The problems of sewage disposal are to prevent nuisances 
causing offense to sight and smell; to prevent the clogging of 
channels; to protect pumping machinery; to protect public 
water supplies; to protect fish life; to prevent the contamination 
of shell fish; to recover valuable constituents of the sewage; 
to enrich and to irrigate the soil; to safeguard bathing and boat- 
ing; for other minor purposes; and in some cases to comply with 
the law. Sewage may be treated to attain one or more of these 
objects by methods of treatment varying as widely as the objects 
to be attained. 

218. Methods of Sewage Treatment. In studying the sub- 
ject of sewage treatment it must be borne in mind that it is 
impossible to destroy any of the elements present. They may be 
removed from the mixture only by gasification, straining or 
sedimentation. Their chemical combinations may be so changed, 
however, as to result in different substances than those intro- 



METHODS OF SEWAGE TREATMENT 371 

duced to the treatment plant. It is with these chemical changes 
that the student of sewage treatment is interested. 

The methods of sewage treatment can be classified as mechan- 
ical, chemical and biological. These classifications are not sepa- 
rated by rigid lines but may overlap in certain treatment devices 
or methods. Mechanical methods of treatment are exemplified 
by sedimentation, and screening. Chemical precipitation and 
sterilization are examples of chemical methods. The biological 
methods, the most important of all, include dilution, septiciza- 
tion, filtration, sewage farming, activated sludge, etc. If for 
any reason it is desired to treat sewage by more than one of these 
methods the procedure should follow as nearly as possible the 
order of the occurrence of the phenomena in the natural biolysis 
of sewage. For example, in one treatment plant the sewage 
would first pass through a grit chamber where the coarse sediment 
would be removed, then through a screen where the floating 
matter and coarse suspended matter would be removed, then 
to a sedimentation basin where some finer suspended matter 
might settle out, then to a digestive tank where the solid matter 
deposited would be worked upon by bacterial action and partially 
liquefied. Simultaneous to the liquefaction of the deposited 
solid matter the liquid effluent from the digestive tank might 
proceed to an aerating device to expedite oxidation, then to an 
aerobic filter, and finally to disposal by dilution. 



CHAPTER XIV 
DISPOSAL BY DILUTION 

219. Definition. Disposal of sewage by dilution is the dis- 
charge of raw sewage or the effluent from a treatment plant into 
a body of water of sufficient size to prevent offense to the senses 
of sight and smell, and to avoid danger to the public health. 

220. Conditions Required for Success. Among the desired 
conditions for successful disposal by dilution are: adequate 
currents to prevent sedimentation and to carry the sewage 
away from all habitations before putrefaction sets in, or sufficient 
diluting water high in dissolved oxygen to prevent putrefaction; 
a fresh or non-septic sewage; absence of floating or rapidly 
settling solids, grease or oil; and absence of back eddies or 
quiet pools favorable to sedimentation in the stream into which 
disposal is taking place. The conditions which should be pre- 
vented are: offensive odors due to sludge banks, the rise of septic 
gases, and unsightly floating or suspended matter. In some 
instances the pollution of the receiving body of water is undesirable 
and the sewage must be freed from pathogenic organisms and the 
danger of aftergrowths minimized before disposal. Such con- 
ditions are typified at Baltimore, where the sewage is discharged 
into Back Bay, an arm of Chesapeake Bay. One of the impor- 
tant industries of the state of Maryland is the cultivation of oysters. 
The pollution of the Bay was therefore so objectionable that care- 
ful treatment of the Baltimore sewage has been a necessary 
preliminary to final disposal by dilution. It is unwise to draw 
public water supplies, without treatment, from a stream receiving 
a sewage effluent, no matter how careful or thorough the treat- 
ment of the sewage. The treatment of the sewage is a safe- 
guard, and lightens the load on the water purification plant, 
but under no considerations can it be depended upon to protect 
the community consuming the diluted effluent. 

372 



SELF-PURIFICATION OF RUNNING STREAMS 373 

The sewer outlet should be located well out in the current of 
the stream, lake, or harbor. Deeply submerged outlets are 
usually better than an outlet at the surface, as a better mixture 
of the sewage and water is obtained. The discharge of sewage 
into a body of water of which the surface level changes, alternately 
covering and exposing large areas of the bottom is unwise, as the 
ludge which is deposited during inundation will cause offensive 
odors when uncovered. Such conditions must be carefully 
guarded against when selecting a point of disposal in tidal 
estuaries because of the frequent fluctuations in level. 

221. Self-Purification of Running Streams. The self -purifi- 
cation of running streams is due to dilution, sedimentation, and 
oxidation. The action is physical, chemical, and biological. 
When putrescible organic matter is discharged into water the 
offensive character of the organic matter is minimized by dilution. 
If the dilution is sufficiently great, it alone may be sufficient to 
prevent all nuisance. The oxidation of the organic matter 
commences immediately on its discharge into the diluting water 
due to the growth and activity of nitrifying and other oxidizing 
organisms and to a slight degree to direct chemical reaction. 
So long as there is sufficient oxygen present in the water septic 
conditions will not exist and offensive odors will be absent. 
When the organic matter is completely nitrified or oxidized 
there will be no further demand on the oxygen content of the 
stream and the stream will be said to have purified itself. At 
the same time that this oxidation is going on some of the organic 
matter will be settling due to the action of sedimentation. If 
oxidation is completed before the matter has settled on the 
bottom the result will be an inoffensive silting up of the river. 
If oxidation is not complete, however, the result will be offensive 
putrefying sludger banks which may send their stinks up through 
the superimposed layers of clean water to pollute the surrounding 
atmosphere. 

The most important condition for the successful self-purifi- 
cation of a stream is an initial quantity of dissolved oxygen to 
oxidize all of the organic matter contributed to it, or the addition 
of sufficient oxygen subsequent to the contribution of sewage to 
complete the oxidation. Oxygen may be added through the dilu- 
tion received from tributaries, through aeration over falls and 
rapids, or by quiescent absorption from the atmosphere. The 



374 DISPOSAL BY DILUTION 

rapidity of self-purification is dependent on the character of the 
organic matter, the presence of available oxygen, the rate of 
reaeration, temperature, sedimentation, and the velocity of the 
current. Sluggish streams are more likely to purify themselves 
in a shorter distance and rapidly flowing turbulent streams 
are more likely to purify themselves in a shorter time, other 
conditions being equal. Although the absorption of oxygen by 
a stream whose surface is broken is more rapid than through a 
smooth unbroken surface, the growth of algse, biological activity, 
the effect of sunlight, and sedimentation are more potent factors 
and have a greater effect in sluggish streams than the slightly 
more rapid absorption of oxygen in a turbulent stream. It is 
frequently more advantageous to discharge sewage into a 
swiftly moving stream, however, regardless of the conditions 
of self-purification, as the undesirable conditions which may 
result occur far from the point of disposal and may be offensive 
to no one. 

The sewage from a population of about 3,000,000 persons 
residing in and about Chicago is discharged into the Chicago 
Drainage Canal. It ultimately reaches tide water through 
the Des Plaines, the Illinois, and the Mississippi Rivers. The 
action occurring in these channels is one of the best illustrations 
known of the self-purification of a stream. In Table 75 are 
shown the results of analyses of samples taken at various points 
below the mouth of the Chicago River where the diluting water 
from Lake Michigan enters, to Grafton, Illinois, at the junction 
of the Illinois and Mississippi Rivers about 40 miles above St. 
Louis. The effect of the physical characteristics of the stream 
on its chemical composition is well illustrated in this table. 
The rise in the chlorine content between Lake Michigan and the 
entrance to the Drainage Canal is a measure of the addition 
of sewage. Since the chlorine is an inorganic substance which 
is not affected by biologic action, its loss in concentration in the 
lower reaches of the rivers is due to dilution by tributaries and 
sedimentation, e.g., between the end of the canal at Lockport 
and the sampling point at Joliet, the entrance of the Des Plaines 
River reduces the concentration of chlorine from 124.5 to 41.5 
parts per million. The entrance of the Kankakee River at 
Dresden Heights further reduces the chlorine to 24.5 p. p.m. 
The - increase of albuminoid and ammonia nitrogen accompanied 



SELF-PURIFICATION OF RUNNING STREAMS 



375 



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376 DISPOSAL BY DILUTION 

by a decrease in nitrites and nitrates, between the upper end 
of the canal at Bridgeport and its lower end at Lockport indicates 
the reducing action proceeding therein. The oxidizing action 
over the various dams and the effect of dilution with water 
containing oxygen is shown between miles 34 and 38, at mile 79, 
and at mile 294. The excellent effect of quiescent sedimentation 
and aeration in Peoria Lakes is shown between miles 145, 161 and 
165. 

222. Self-Purification of Lakes. Sewage may be disposed of 
into lakes with as great success as into running streams if condi- 
tions exist which are favorable to self-purification. Lakes and 
rivers purify themselves from the same causes; oxidation, sedi- 
mentation, etc., but in the former the currents are much less 
pronounced and may be entirely absent. In shallow lakes 
(20 feet or less in depth) dependence must be placed on horizontal 
currents and the stirring action of the wind to keep the water 
in motion in order that the sewage and the diluting water may 
be mixed. In deeper bodies of water, currents induced by the 
wind are helpful but entire dependence need not be placed upon 
them. Vertical currents, and the seasonal turnovers in the spring 
and fall completely mix the waters of the lake above those layers 
of water whose temperature never rises higher than 4 C. 

In the early winter the cold air cools the surface waters of 
a lake. The cooling increases the density of the surface water 
causing it to sink, and allowing the warmer layers below to rise 
and become cooled. After the temperature of the entire lake 
has reached 4 C. the vertical currents induced by temperature 
cease, as continued cooling decreases the density of the surface 
water maintaining the same layer at the surface. In the spring 
as the temperature of the surface water rises to 4 C. and above 
it becomes heavier and drops through the colder water below 
causing vertical currents. These phenomena are known as the 
fall and spring turnovers. The former is more pronounced. 
These turnovers are effective in assisting in the self-purification 
of lakes. 

223. Dilution in Salt Water. The oxygen content in salt 
water is about 20 per cent less than in fresh water at the same 
temperature. The greater content of matter in solution in 
salt- water reduces its capacity to absorb many sewage solids. 
This, together with the chemical reaction between the constitu- 



QUANTITY OF DILUTING WATER NEEDED 377 

ents of the salt water and those of the sewage,, serve to precipitate 
some of the sewage solids and to form offensive sludge banks. 
The evidence of the action which takes place in the absorption 
of oxygen from the atmosphere by salt water and its effect on 
dissolved sewage solids is conflicting, but in general fresh water 
is a better diluting medium than salt water. 

Black and Phelps have made valuable studies of the relative 
rates of absorption of oxygen from the air by fresh and salt water. 
The results of their experiments are published in a Report to the 
Board of Estimate and Apportionment of N. Y. City, made 
March 23, 19 II. 1 Concerning these rates they conclude: 

Therefore there is no reason to believe that the 
reaeration of salt water follows any other laws than 
those we have determined mathematically and experi- 
mentally for fresh water. In the absence of fuller infor- 
mation on the effect of increased viscosity upon the 
diffusion coefficient, it can only be stated that the rate 
of reaeration of salt water is less than that of fresh water, 
in proportion to the respective solubilities of oxygen in 
the two waters, and still less, but to an unknown extent, 
by reason of the greater viscosity and consequent small 
value of the diffusion coefficient. 

224. Quantity of Diluting Water Needed. In a large majority 
of the problems of disposal of sewage by dilution it is not neces- 
sary to add sufficient diluting water to oxidize completely all 
organic matter present. Ordinarily it is sufficient to prevent 
putrefactive conditions until the flow of the stream, lake, or 
tidal current, has reached some large body of diluting water 
or where putrefaction is no longer a nuisance. It is never desirable 
to allow the oxygen content of a stream to be exhausted as putres- 
cible conditions will exist locally before exhaustion is complete. 
The exact point to which oxygen can be reduced in safety is in 
some dispute. Black and Phelps have assumed 70 per cent of 
saturation as the allowable limit; Fuller has placed it at 30 
per cent; Kinnicutt, Winslow, and Pratt have placed it at 50 
per cent. Since the reaction between the oxygen and the organic 
matter is quantitative, others have placed the limit in terms of 
parts per million of oxygen. Wisner, 2 has recommended a mini- 

1 Reprinted in Vol. Ill of Contributions from the Sanitary Research 
Laboratory of Massachusetts Institute of Technology. 

1 Formerly Chief Engineer of the Sanitary District of Chicago. 



378 DISPOSAL BY DILUTION 

mum of 2.5 p. p.m. as the limit for the sustenance of fish life, 
which is not far from Fuller's limit for hot-weather conditions. 

Formulas of various types have been devised to express the 
rate of absorption of oxygen with a given quantity of diluting 
water which is mixed with a given quantity and quality of sewage. 
The quantity of sewage is sometimes expressed in terms of the 
tributary population or in other ways. Knowing the rate at 
which oxygen is exhausted and the velocity of flow of the stream, 
the point at which the oxygen will be reduced to the limit allowed 
is easily determined. The accuracy of none of these formulas 
has been proven, and their use, without an understanding of the 
effect of local conditions, may lead to error. They may be used 
as a check on the bio-chemical oxygen demand determinations, 
which should be conclusive. 

The following formula, based on the work of Black and 
Phelps, is a guide to the amount of sewage which can be added 
to a stream without causing a nuisance. It is: 

log o~ 

r< 

kt > 

in which C =per cent of sewage allowed in the water; 

O'=per cent of saturation or the p.p.m. of oxygen in 

the mixture at the time of dilution; 
0=per cent of saturation or the p.p.m. of oxygen in 
the stream after period of flow to point beyond 
which no nuisance can be expected; 
J=time in hours required for the stream to flow to 

this point; 

k= constant determined by test determinations of 
the factors in the following expression: 

i 'i 
log 7^- 

, _ 

= 



in which 0'\ = per cent of saturation or the p.p.m. of oxygen in 

the diluting water before mixing with the sewage; 

Oi=per cent of saturation or the p.p.m. of oxygen 

in an artificial mixture made in the laboratory, 

after t\ hours of incubation; 



QUANTITY OF DILUTING WATER NEEDED 379 

Ci =per cent of sewage in the mixture; 
ti = number of hours of incubation of the mixture of 

sewage and diluting water under laboratory 

conditions. 

In the solution of these formulas it is desired to determine 
the permissible amount of sewage to discharge into a given 
quantity of diluting water. This value is expressed by C in the 
first equation. In solving this equation: 

0' is determined by laboratory tests and should repre- 
sent the conditions to be expected during 
various seasons of the year; 

is determined by judgment. It may be 30 per cent 

or 50 per cent or more as previously explained; 

t is determined by float tests or other measurements 
of the stream flow; 

k is determined by laboratory tests in which mix- 
tures of various strengths are incubated for vari- 
ous periods of time. Different values of k will 
be obtained for different characteristics of the 
sewage; but for the same sewage the value of k 
should be unchanged for different periods of 
incubation. 

Rideal devised the formula: 1 

XO = C(M-N)S 

in which X = flow of the stream expressed in second feet; 

= grams of free oxygen in one cubic foot of water; 
S =rate of sewage discharge in second feet; 
M = grams of oxygen required to consume the organic 

matter in one cubic foot of diluted sewage as 

determined by the permanganate test with 4 

hours boiling; 
N = grams of oxygen available in the nitrites and 

nitrates in one cubic foot of diluted sewage; 
C = ratio between the amount of oxygen in the stream 

and that required to prevent putrefaction. Where 

C is equal to or greater than one, satisfactory 

conditions have been attained. 

1 From " Sewage," by Samuel Rideal, 1900, p. 16. 



380 DISPOSAL BY DILUTION 

In using this formula it is necessary to make analyses of trial 
mixtures of sewage and water until the correct mixture has been 
found. 

Hazen's formula is: 1 

x 4m 
D= S = ~0~> 

in which D = dilution ratio; 

x = volume of water; 

S= volume of sewage; 

m= result of the oxygen consumed test expressed in 

p.p.m. after 5 minutes, boiling with potassium 

permanganate; 
= amount of dissolved oxygen in the diluting water 

expressed in p.p.m. 

For comparison with RideaFs formula the factor of 7 should be 
used instead of 4 to allow for the increased time of boiling. 

Since the amount of oxygen needed is dependent on the amount 
of organic matter in the sewage rather than the total volume of 
the sewage, and since the amount of organic matter is closely 
proportional to the population, the amount of diluting water has 
sometimes been expressed in terms of the population. Hering's 
recommendation for the quantity of diluting water necessary for 
Chicago sewage was 3.3 cubic feet of water per second per 
thousand population. Experience has proven this to be too small. 
Between a minimum limit of 2 second-feet and a maximum of 8 
second-feet of diluting water per thousand population the success 
of dilution is uncertain. Above this limit success is practically 
assured and below this limit failure can be expected. 

Even with these carefully devised formulas and empirical 
guides, the factors of reaeration, dilution, sedimentation, tem- 
perature, etc., may have so great an effect as to vitiate the con- 
clusions. As shown in Table 75 dilution in winter is far more 
successful than in summer. The lower temperatures so reduce 
the activity of the putrefying organisms that consumption of 
oxygen is greatly retarded. 

225. Governmental Control. A comprehensive discussion 
of the legal principles governing the pollution of inland waters 

1 See Am. Civil Engineers' Pocket Book, Second Edition, p. 982. 



PRELIMINARY TREATMENT 381 

is contained in " A Review of the Laws Forbidding the Pollution 
of Inland Waters," by E. B. Goodell, published by the United 
States Geological Survey in 1905, as Water Supply Paper No. 152. 
The disposal of sewage by dilution is subject to statutory 
limitations in many states. The enforcement of these laws is 
usually in the hands of the state board of health, which is fre- 
quently given discretionary powers to recommend and some- 
times to enforce measures for the abatement of an actual or 
potential nuisance. Such recommendations usually take the 
form of a specification of certain forms of treatment preliminary 
to disposal by dilution. No project for the disposal of sewage 
by dilution should be consummated until the local, state, 
national, and in the case of boundary waters, international laws 
have been complied with. The attitude of the courts in dif- 
ferent states has not been uniform. Little guidance can be taken 
from the personal feeling of the persons immediately interested. 
The opinion of the riparian owner 5 miles down stream may differ 
materially from the popular will of the voters of a city, and it is 
likely to receive a more favorable hearing from the court. 
Statutes and legal precedents are the safest guides. 

226. Preliminary Treatment. If the sewage to be disposed 
of by dilution contains unsightly floating matter, oil, or grease, 
no amount of oxygen in the diluting water will prevent a nuisance 
to sight, or the formation of putrefying sludge banks. Under 
such conditions it will be necessary to introduce screens or sedi- 
mentation basins, or both, in order to remove the floating and the 
settling solids. Biologic tanks, filtration, or other methods of 
treatment may be necessary for the removal of other undesirable 
constituents. 

227. Preliminary Investigations. Before adopting disposal 
of sewage by dilution without preliminary treatment, or before 
considering the proper form of treatment necessary to render 
disposal by dilution successful, a study should be made of the 
character of the body of water into which the sewage or effluent 
is to be discharged. This study should include: measurements 
of th" 1 quantity of water available at all seasons of the year: 
analyses of the diluting water to determine particularly the 
available dissolved oxygen: observations of the velocity and 
direction of currents, and the effect of winds thereon: a study 
of the effect on public water supplies, bathing beaches, fish life, 



382 DISPOSAL BY DILUTION 

etc. Good judgment, aided by the proper interpretation of 
such information should lead to the most desirable location for 
the sewer outlet. If preliminary treatment is found to be neces- 
sary tests should be made to determine the necessary extent and 
thoroughness of the treatment. 



CHAPTER XV 
SCREENING AND SEDIMENTATION 

228. Purpose. The first step in the treatment of sewage 
is usually that of coarse screening in order to remove the larger 
particles of floating or suspended matter. Screens and sedi- 
mentation basins are used to prevent the clogging of sewers, 
channels, and treatment plants; to avoid clogging of and injuries 
to machinery; to overcome the accumulation of putrefying 
sludge banks; to minimize the absorption of oxygen in diluting 
water; and to intercept unsightly floating matter. 

By the plain sedimentation of sewage is meant the removal 
of suspended matter by quiescent subsidence unaffected by 
septic action or the addition of chemicals or other precipitants. 
In order to prevent septic action plain sedimentation tanks must 
be cleaned as frequently as once or twice a week in warm weather 
but not quite so often in cold weather. 

Fine screening may take the place of sedimentation where 
insufficient space is available for sedimentation tanks, and it is 
desired to remove only a small portion of the suspended matter. 
Recent American practice has tended to restrict the field of fine 
screening to treatment requiring less than 10 per cent removal 
of suspended matter, thus eliminating screens from the field 
covered by plain sedimentation tanks. The practice is well 
expressed by Potter, who states: 1 

Where a high degree of purification is sought, the use 
of fine screens is of doubtful value. A modern settling 
tank will give better results and at a less cost for a given 
degree of purification. A settled liquid is also superior 
to a screened liquid for subsequent biological treatment 
in filters. . . . Again the storing of large quantities of 
screenings must necessarily be more objectionable than 
the storing of the digested sludge of a modern settling 
tank. 

1 Trans. Am. Society Civil Engineers, Vol. 58, 1907, p. 988. 
383 



384 



SCREENING AND SEDIMENTATION 



229. Types of Screens. The definitions of some types of 
screens as proposed by the American Public Health Association 
follow: A bar screen is composed of parallel bars or rods. A 
mesh screen is composed of a fabric, usually wire. A grating 
consists of 2 sets of parallel bars in the same plane in sets inter- 
secting at right angles. A band screen consists of an endless 
perforated band or belt which passes over upper and lower 
rollers. A perforated plate screen is made of an endless band 
of perforated plates similar to a band screen. A wing screen 
has radial vanes uniformly spaced which rotate on a horizontal 
axis. A disc screen consists of a circular perforated disc with 
or without a central truncated cone of similar material mounted 





(.-Band Screen. 2.- Wing Screen. 



3.- Shovel -Vane 
Screen. 





4.- Drum Screen. 



5-Riensch-Wurl 
Screen. 



FIG. 150. Types of Moving Screens. 
Trans. Am. Society Civil Engineers, Vol. 78, 1915, p. 893. 

in the center. The Reinsch Wurl screen is the best known type 
of disc screen. A cage screen 1 consists of a rectangular box 
made up of parallel bars with the upstream side of the box or cage 
omitted. Allen 2 gives the following definitions: A drum 
screen is a cylinder or cone of perforated plates or wire mesh 
which rotates on a horizontal axis. A shovel vane screen is 
similar to a wing screen with semicircular wings and a different 
method of removing the screenings. Examples of a band screen, 
a wing screen, a shovel vane screen, a drum screen and a disc 



1 Not defined by the American Public Health Association. 

2 Trans. Am. Society Civil Engineers, Vol. 78, 1915, p. 892. 



TYPES OF SCREENS 



screen are shown in Fig. 150. A bar screen is shown in Fig. 
151 and a cage screen is shown in Fig. 152. 




Direction 
of Flow 



Side View. 



I 




I 


1 




1 






^ 


! 




1 






i 



Front View 
Looking down Stream. 



FIG. 151. Sketch of a Bar Screen. 



Screens can be classed as fixed, movable, or moving. Fixed 
screens are permanently set in position and -must be cleaned 
by rakes or teeth that are pulled between the bars. Movable 
screens are stationary when in 
operation, but are lifted from 
the sewage for the purpose of 
cleaning. Moving screens are 
in continuous motion when in 
operation and are cleaned while 
in motion. Fixed bar screens 
may be set either vertical, in- 
clined, or horizontal. 

Movable screens with a 
cage or box at the bottom 
are sometimes used. Ihe box 
should be of solid material 
to prevent the forcing of 




FIG. 152. Sketch of a Cage Screen. 



screenings through it \vhcii 

the screen is being raised for 

cleaning. A mesh screen should be used only under special 

circumstances because of the difficulty in cleaning. Screens 

which must be raised from the sewage for cleaning should be 



386 SCREENING AND SEDIMENTATION 

arranged in pairs in order that one may be working when the 
other is being cleaned. Movable screens are undesirable for 
small plants because the labor involved in raising and lowering 
is greater than in cleaning with a rake and the screens are more 
likely to be neglected. In a large plant rakes operated by hand 
are too small for cleaning the screens. A fixed screen is sometimes 
used with moving teeth fastened to endless chains. The teeth 
pass between the parallel bars and comb out the screenings. If 
the screen chamber in a small plant is too deep for accessibility 
a movable cage or box screen may be desirable. 

Moving screens are generally of fine mesh or perforated plates. 
They are kept moving in order to allow continuous cleaning. 
They are cleaned by brushes or by jets of air, water, or steam. 

230. Sizes of Openings. The area or size of the opening of 
a screen is dependent upon the character of the sewage to be 
treated and upon the object to be attained. 

Large screens, with openings between 1^ inches and 6 inches 
are used to protect centrifugal pumps, tanks, automatic dosing 
devices, conduits, and gate valves from large objects such as 
pieces of timber, dead animals, etc., which are found in sewage. 
The quantity of material removed is variable, and is usually 
small. 

Medium-size screens with openings from \ inch to 1^ inches 
are used to prepare sewage for passage through reciprocating 
pumps, complex dosing apparatus, contact beds, and sand filters. 
The amount of material removed varies from 0.5 to 10 cubic 
feet per million gallons of sewage treated, dependent on the 
character of the sewage and the size of the screen. Screenings 
before drying contain 75 to 90 per cent moisture and weigh 
40 to 50 pounds per cubic foot. At times the amount removed 
may vary widely from the limits stated. Schaetzle and Davis l 
state : 

Screenings differ greatly both in amount and character. 
. . . The amount varies with the days of the week as 
well as during the course of the day. It reaches its 
maximum about noon or shortly before and commences 
to disappear about midnight, reaching a mimimum about 
4 or 5 A.M. The material is almost wholly organic and 

1 Removal of Suspended Matter by Sewage Screens, Cornell Civil En- 
gineer, 1914. Abstracted in Engineering and Contracting, Vol. 41, 1914, 
p. 451. 



SIZES OF OPENINGS 387 

consists of scraps of vegetables or fruit, cloth, hair, wood, 
paper and lumps of fecal matter. The amount varies so 
widely that it is impossible to state just what to expect 
any definite size screen to remove. The amount of 
water contained is small compared with that in the 
sludge in sedimentation basins and amounts to from 70 
per cent to 80 per cent. On account of its organic origin 
it is highly putrescible. 

Medium-size screens are sometimes placed close together with 
the bars of the one opposite the openings in the other, thus 
approaching a fine screen. 

Fine screens vary in size of opening from f- inch to 50 open- 
ings per linear inch or 2,500 per square inch. They are used for 
removing solids preparatory to disposal by dilution, to protect 
sprinkling filters, complex dosing apparatus, sand filters, sewage 
farms, and to prevent the formation of scum in subsequent 
tank treatment. In general, fine screens will remove from 0.1 
to 1 cubic yard of wet material per million gallons of sewage 
treated. The wet screenings will contain about 75 per cent 
moisture and will weigh about 60 pounds per cubic foot. The 
dry weight of the screenings will therefore be about 10 to 400 
pounds per million gallons of sewage treated. The effect of the 
removal of this amount of material is usually not detectable by 
methods of chemical analysis, the amount of suspended matter 
before and after screening being found unchanged. 

In his conclusions on the discussion of the results to be 
expected from fine screens, Allen states: 1 

With openings not more than 0.1 inch in size, fine 
screening should remove at least 30 per cent of the sus- 
pended solids and 20 per cent of the suspended organic 
solids from ordinary domestic sewage, or 0.1 cubic yard 
of screenings, containing 75 per cent water per thousand 
population daily. 

The effect of the use of different size openings under the same 
conditions is shown in Fig. 153. 2 Some data covering the amount 
of material removed by screening are given in Table 76. More 

* " The Clarification of Sewage by Fine Screens," Trans. Am. Society 
Civil Engineers, Vol. 78, 1915, p. 1000. 

J Langdon Pearse, Trans. Am. Society Civil Engineers, Vol. 78, 1915, 
p. 1000. 



388 



SCREENING AND SEDIMENTATION 



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DESIGN OF FIXED AND MOVABLE SCREENS 



389 



extensive data are given in Volume III of "American Sewerage 
Practice" by Metcalf and Eddy. 

231. Design of Fixed and Movable Screens. The determina- 
tion of the size of the opening is the first step ,in the design of 
a sewage screen. This is followed by the computation of the net 
area of openings in the screen. The final steps are the deter- 



0.20 



Removal by Screening. 
Individual Resu/fc on Stockyards Sewage. 
+ Average Results o 




Screen 



No. 6 Screen 



400 800 1200 1600 2000 2400 
Dry Screenings in Pounds per Million Gallons. 



-No. 10 Screen 



-No. 16 Screen 
No. 20 Screen 
-No. 24 Screen 

- No. 50 Screen 

- No AO Screen 



2800 



FIG. 153. Screenings Collected on Different Sized Opening. 

1921 Report on Industrial Wastes Disposal, Union Stock Yards District, Chicago, Illinois, 
to the Sanitary District of Chicago. 

mination of the overall dimensions of the screen; the size of the 
bar, wire, or support ; and the dimensions of the screen chamber. 
The net area of openings is fixed by the permissible velocity of 
flow through the screen and the quantity of sewage to be treated. 
In determining the velocity of flow the general principle should 
be followed that the velocity should not be reduced sufficiently 



390 SCREENING AND SEDIMENTATION 

to allow sedimentation in the screen chamber. The velocity 
of grit bearing sewage in passing through coarse screens should 
not be reduced below 2 or 3 feet per second. If the sewage con- 
tains no grit, or the screen is placed below a grit chamber the 
velocity through a medium or fine screen should be from \ to \\ 
feet per minute. The velocity through the screen in a direction 
normal to the plane of the screen can be reduced without reducing 
the horizontal velocity of the sewage by placing the screen in a 
sloping position. 

The final steps are the design of the screen bar and the deter- 
mination of the dimensions of the screen and of the screen chamber. 
The size of the bar in a bar screen, or as a support to a wire mesh, 
is dependent on the unsupported length of the bar. The stresses 
in the bars are the results of impact and bending, caused by clean- 
ing, and of the load due to the backing up of the sewage when the 
screen is clogged. Allowance should be made for a head of 
2 or 3 feet of sewage against the screen. A generous allowance 
should be made in addition for the indeterminate stresses due to 
cleaning. The screen should be supported only at the top and 
bottom, as intermediate supports in a bar screen are undesirable 
unless they are so arranged as not to interfere with the teeth 
of the cleaning devices. 

Fixed screens should be placed at an angle between 30 and 
60 with the horizontal, with the direction of slope such that the 
screenings are caught on the upper portion of the screen. A 
small slope is desirable in order to obtain a low velocity through 
the screen. The slope is limited since the smaller the slope the 
longer the bars of the screen and the greater the difficulty of hand 
cleaning. Small slopes will tend to make the screens self cleaning. 
As the screen clogs, the increasing head of sewage will push the 
accumulated screenings up the screen. The use of flat screens 
in a vertical position is not desirable because of the difficulty of 
cleaning and the accumulation of material at inaccessible points. 
If a flat screen is placed in a horizontal position with the flow of 
sewage downward difficulties are encountered in cleaning and 
solid matter is forced through the screen as clogging increases. 
An upward flow through a horizontal screen is undesirable as the 
material is caught in a position inaccessible for cleaning. Movable 
screens are more easily handled when placed in a vertical position. 

In the construction of small screens, round bars are sometimes 



THEORY OF SEDIMENTATION 391 

used where the unsupported length of the bar is less than 3 or 
4 feet. They are not recommended, however, as the efficient 
area and the amount of material removed by the screen are 
diminished. Bars which produce openings with the larger 
end upstream are undesirable, as particles become wedged in 
the screen, and are either forced through or become difficult 
to remove. 1 Rectangular bars are easily obtained and give 
satisfactory service except where they are of insufficient strength 
laterally. For greater lateral thickness a pear-shaped bar is 
sometimes used, with the thicker side upstream. Fine mesh 
screens or perforated plates are supported on grids or parallel 
bars of stronger material designed to take up the heavy stresses 
on the screen. 

The dimensions of the bar may be selected arbitrarily. The 
length and width of the screen are fixed to give desirable dimen- 
sions to the screen chamber and to give the necessary net opening 
in the screen. The width of the screen chamber and the screen 
should be the same. The screen chamber should be sufficiently 
long to prevent swirling and eddying around the screen. If the 
dimensions thus fixed permit an undesirable, velocity in the screen 
chamber they should be changed. A sufficient length of screen 
should be allowed to project above the sewage for the accumula- 
of screenings. The bars may be carried up and bent over at 
the top as shown in Fig. 151 to simplify the removal of screenings. 

Coarse screens are usually placed above all other portions 
of a treatment plant. They may be followed by grit chambers 
or finer screens. Coarse screens are occasionally placed as a 
protection above medium or fine screens. In sewage containing 
grit the smaller screens are sometimes placed below the grit cham- 
ber. It is desirable to provide some means of diverting the sewage 
from a screen chamber to allow of repairs to the screen and the 
cleaning of the chamber. Screen chambers are sometimes 
designed in duplicate to allow for the cleaning of one while the 
other is operating. 

PLAIN SEDIMENTATION 

232. Theory of Sedimentation. Sedimentation takes place in 
sewage because some particles of suspended matter have a 
greater specific gravity than that of water. All particles do 
1 See article by Henry Ryon in Cornell Civil Engineer, 1910. 



392 SCREENING AND SEDIMENTATION 

not settle at the same rate. Since the weights of particles vary 
as the cubes of their diameters, whereas the surface areas (upon 
which the action of the water takes place) vary only as the 
squares of the diameters, the amount of the skin friction on 
small particles is proportionally greater than that on large 
particles, because of the relatively greater surface area compared 
to their weight. As a result the smaller particles settle more 
slowly. The velocity of sedimentation of large particles has 
been found to vary about as the diameter and of small particles 
as the square root of the diameter. The change takes place at 
a size of about 0.01 mm. 

Sedimentation is accomplished by so retarding the velocity 
of flow of a liquid that the settling particles will be given the 
opportunity to settle out. The slowing down of the velocity 
is accomplished by passing the sewage through a chamber of 
greater cross sectional area than the conduit from which it came. 
The time that the sewage is in this chamber is called the period 
of retention. Although the shape of a basin, the arrangement 
of the baffles and other details have a marked effect on the 
results of sedimentation, the controlling factors are the period 
of retention and the velocity of flow. Another factor affecting 
the efficiency of the process is the quality of the sewage. Usually 
the greater the amount of sediment in the sewage the greater the 
per cent of suspended matter removed. A method for the 
determination of the proper period of sedimentation has been 
developed by Hazen in Transactions of the American Society of 
Civil Engineers, Volume 53, 1904, page 45. The results of 
his studies are summarized in Fig. 154 which shows the per cent 
of sediment remaining in a treated water after a certain period 
of retention. This period of retention is expressed in terms of 
the hydraulic coefficient 1 of the smallest size particle to be 
removed. Table 77 shows the hydraulic coefficients of various 
particles. In Fig. 154 a represents the period of retention and 
t the time that it would take a particle to fall to the bottom of 
the basin. The different lines of the diagram represent the 
results to be expected by various arrangements of settling basins. 
The meaning of these lines is given in Table 78. 



1 The hydraulic coefficient is denned as the rate of settling in mm. per 
second. 



THEORY OF SEDIMENTATION 



393 



TABLE 77 
HYDRAULIC VALUES OF SETTLING PARTICLES IN MILLIMETERS PER SECOND 















Diam- 




Diameter 
in 


Hy- 

draulic 


Diameter 
in 


Hy- 

draulic 


Diameter 
in 


Hy- 

draulic 


eter 


Hydraulic 


mm. 


Value 


mm. 


Value 


mm. 


Value 


in 


Value 














mm. 




1.00 


100 


0.15 


15 


0.02 


0.62 


0.003 


0.0138 


0.80 


83 


0.10 


8 


0.015 


0.35 


0.002 


0.0062 


0.60 


63 


0.08 


6 


0.010 


0.154 


0.0015 


0.0035 


0.50 


53 


0.06 


3.8 


0.008 


0.098 


0.001 


0.00154 


0.40 


42 


0.05 


2.9 


0.006 


0.055 


0.0001 


0.0000154 


0.30 


32 


0.04 


2.1 


0.005 


0.0385 






0.20 


21 


0.03 


1.3 


0.004 


0.0247 








|l 




i 









An example will be given to illustrate the method of using the 
diagram and tables to determine the size of a sedimentation 
basin to perform certain required work. 

Let it be required to determine the period of retention 
in a continuously operated sedimentation basin with good 
baffling, corresponding to two properly baffled sedimentation 
basins in series. The basins are to remove 60 per cent of 
the finest particles which are to have a size of 0.01 mm. 
The quantity to be treated daily is 3,000,000 gallons. 

1st. Entering Table 77, we find that the hydraulic 
value of the finest particles is 0.154 mm. per second. 

2d. Since we wish to remove 60 per cent of the finest 
particles, 40 per cent will remain. Since Fig. 154 shows 
the per cent remaining after the time alt we enter Fig. 
154 at 40 per cent on the ordinates and run horizontally 
until we encounter Line 4 corresponding to good baffling 
in Table 78. We then run down vertically from this 
intersection and find that the ratio of a/t is 1.0. 

Then a equals t, which means that the period of reten- 
tion should equal the time that it takes a particle 0.01 mm. 
in diameter to drop from the top to the bottom of the 
basin. Since this depends on the depth of the basin it 
is necessary to determine the depth before the other 
dimensions of the basin can be fixed. 

Although this method is seldom used in practice for the final 
design of a sedimentation basin, it is a guide to judgment and 
can be used to supplement the data obtained from tests. 



394 



SCREENING AND SEDIMENTATION 




0.5 1.0 1.5 2.0 2.5 3.0 3.5 

^, or Time of Settling,in Terms of Time Required for One ParficletpSettlefrornToptoBottom. 

FIG. 154. Hazen's Diagram, Showing the Relation between the Time of Set- 
tling and the Period of Retention in Various Types of Sedimentation 
Basins. 

Trans. Am. Society Civil Engineers, Vol. 53, 1904, p. 45. 

TABLE 78 

COMPARISON OF DIFFERENT ARRANGEMENTS OF SETTLING BASINS 
(From Hazen) 



Description of Basins 


Line 
in 
Fig. 
154 


Values of a/t. 


Per Cent of Matter 
Removed 


50 


74 


87.5 


Theoretical maximum. Cannot be reached .... 
Surface skimming. Rockner Roth system 
Intermittent basins, reckoned on time of service 
only 


A 
B 

C 
D 

16 
8 
4 
2 
1.5 
E 
1 


0.50 
0.54 

0.63 
0.69 
0.71 
0.73 
0.76 
0.82 
0.90 
1.26 
1.00 


0.75 
0.98 

1.26 
1.38 
1.45 
1.62 
1.66 
2.00 
2.34 
2.50 
3.00 


0.875 
1.37 

1.89 
2.08 
2.23 
2.37 
2.75 
3.70 
4.50 
3.80 
7.00 


Continuous basin Theoretical limit .... 


Close approximation to the above 


Very well baffled basin 


Good baffling 


Two basins tandem 


Ono long basin well controlled 


Intermittent basin in service half time 


One basin continuous 





TYPES OF SEDIMENTATION BASINS 395 

The design of sedimentation basins should be based on 
experimental observations made upon the quantity of sediment 
removed at certain rates of flow and periods of retention in 
different types of basins. Hazen's mathematical analysis is service- 
able in making preliminary estimates and in checking the results. 
The shape of the tank, period of retention and rate of flow pro- 
ducing the most desirable results should be duplicated with the 
expectation of obtaining similar results or results but slightly 
modified from those obtained in the tests. This is the most 
satisfactory method of determining the proper period of retention. 

233. Types of Sedimentation Basins. A sedimentation basin 
is a tank for the removal of suspended matter either by quiescent 
settlement or by continuous flow at such a velocity and time of 
retention as to allow deposition of suspended matter. 1 The 
difference between sedimentation tanks and other forms of tank 
treatment is that no chemical or biological action is depended 
on for the successful operation of the tank. Sedimentation 
tanks may be divided into two classes, grit chambers and plain 
sedimentation basins. 

A grit chamber is a chamber or enlarged channel in which the 
velocity of flow is so controlled that only heavy solids, such as 
grit and sand, are deposited while the lighter organic solids are 
carried forward in suspension. If the velocity of flow is more 
than about one foot per second, the tank is a grit chamber and 
below this velocity it is a plain sedimentation basin. 

There are sixth general types of plain sedimentation 
basins : 

1st. Rectangular flat-bottom tanks operated on the 
continuous-flow principle. 

2nd. Rectangular flat-bottom tanks operated on the 
fill and draw principle. 

3rd. Rectangular or circular hopper-bottom tanks 
operated on the continuous-flow principle, with hori- 
zontal flow. 

4th. Rectangular or circular hopper-bottom tanks 
operated on the fill and draw principle, with horizontal 
flow. 

5th. Rectangular or circular hopper-bottom tanks 
operated on the continuous-flow principle with vertical flow. 

6th. Circular hopper-bottom tanks operated on the 
continuous-flow principle with radial flow. 

1 Definition suggested by the American Public Health Association. 



396 



SCREENING AND SEDIMENTATION 



TABLE 79 

CRITICAL VELOCITIES FOR THE TRANSPORTATION OP DEBRIS 
Sedimentation will not Occur at Higher Velocities 



Diameter 
of Particle 


Critical Velocity, Feet per Second. 


Size of Screen 
or Number of 


Specific Gravity 


in 
Millimeters 


1.5 


2.0 


3.0 


5.0 


Meshes per Inch 


0.010 


0.13 


0.20 


0.22 


0.28 




0.050 


0.23 


0.34 


0.39 


0.50 


More than 200 


0.100 


0.30 


0.42 


0.50 


0.65 


More than 150 


0.500 


0.55 


0.73 


0.91 


1.15 


More than 28 


1.0 


0.71 


0.92 


1.18 


1.50 


More than 14 


1.25 


0.77 


1.00 


1.30 


1.60 




2.0 


0.92 


1.20 


1.50 


1.90 


More than 10 


5.0 


1.30 


1.70 


2.20 


2.60 


More than 4 


10 


1.70 


2.20 


2.8 


3.4 





Diameter in Millimeters for a Velocity of 1 Foot per Second 



2.5 



1.25 



0.65 



0.32 



234. Limiting Velocities. Sand, clay, bits of metal and other 
particles of mineral matter will commence to deposit in appreci- 
able quantities when the velocity of flow falls below 3 feet per 
second. The amount deposited will increase as the velocity 
decreases. In Table 79 are given the approximate horizontal 
velocities at which certain size particles of mineral matter will 
deposit. At a velocity of about one foot per second organic 
matter will commence to deposit. It will be noticed by inter- 
polation in Table 79, 1 that particles with the same specific 
gravity as sand (2.6), larger than one mm. in diameter will 
deposit at a velocity of about one foot per second or less, and 
that smaller and lighter particles will not deposit at velocity 
of one foot per second or greater. It will also be noticed that a 

1 Computed from formula by Gilbert in " Transportation of Debris by 
Running Water," U. S. Geological Survey, Professional Paper No. 86, 1914. 
1.28 (velocity) 2 ' 7 



Diameter in mm. = - 



Sp. gv.-l 



QUANTITY AND CHARACTER OF GRIT 



397 



velocity of one foot per minute is sufficiently slow to permit the 
deposit of the smallest and lightest particles. For this reason 
velocities of 1 or 2 or even 3 feet per second have been adopted 
as the velocities in grit chambers and velocities less than 1 foot 
per minute in plain sedimentation basins. 

235. Quantity and Character of Grit. The amount of 
material deposited in grit chambers varies approximately between 
0.10 and 0.50 cubic yard per million gallons. It is to be noted 
that grit chambers are used only for combined and storm sewage 
and for certain industrial wastes. They are unnecessary for 
ordinary domestic sewage. The material Heposited in grit cham- 
bers operating with a velocity greater than one foot per second 
is nonputrescible, inorganic, and inoffensive. It can be used for 
filling, for making paths and roadways, or as a filtering material 
for sludge drying beds. An analysis of a typical grit chamber 
sludge is shown in Table 80. 

TABLE 80 
ANALYSIS OF GRIT CHAMBER SLUDGE 



Velocity 
Feet per 
Second 


Specific 
Gravity 


Per Cent 
Moisture 


Calculated to Dry Weight. Per Cent 


Nitrogen 


Fixed Matter 


Miscellaneous 


1.0 


1.5 


45 


20 


78 


2 



236. Dimensions of Grit Chambers. The quantity of sewage 
to be treated and the amount and character of the settling solids 
which it contains should be determined by measurement and 
analysis, and the amount of settling solids to be removed should be 
determined by a study of the desired conditions of disposal, in 
order that a grit chamber that will accomplish the desired results 
may be designed. The period of retention and the velocity of 
flow are the controlling features in the successful operation of 
any grit chamber. These should be determined by experiment 
or as the result of experience. Where neither are available, 
Hazen's method can be followed or a decision made based on a 
study of other grit chambers. In general, the period of retention 



398 SCREENING AND SEDIMENTATION 

in grit chambers is from 30 to 90 seconds, and the velocity of flow 
is about one foot per second. 

After having determined the quantity of sewage to be treated, 
the quantity of grit to be stored between cleanings, the period 
of retention, the arrangement of the chambers, and the velocity 
of flow to be used, the overall dimensions of the chambers are 
computed. The capacity of the chamber is fixed as the sum of 
the quantity of sewage to be treated during the period of reten- 
tion and the required storage capacity for grit accumulated 
between cleanings. The length of the chamber is fixed as the 
product of the velocity! of flow and the period of retention. The 
cross-sectional area of the portion of the chamber devoted to 
sedimentation is fixed as the quotient of the quantity of flow of 
sewage per unit time and the velocity of flow. Only the relation 
between the width and depth of the portion devoted to sedi- 
mentation and the portion devoted to the storage of grit remain 
to be determined. These should be so designed as to give the 
greatest economy of construction commensurate with the required 
results. They will be affected by the local conditions such as 
topography, available space, difficulties of excavation, etc. Com- 
mon depths in use lie between 8 and 12 feet, although wide varia- 
tions can be found. A study of the proportions of existing 
grit chambers will be of assistance in the design of other 
basins. 

237. Existing Grit Chambers. The details of some typical 
grit chambers are shown in Figs. 155 and 156. The grit chamber 
at the foot of 58th Street, in Cleveland, Ohio, is shown in Fig. 
155. The special feature of this structure is the shape of the 
sedimentation basin, the bottom of which is formed by sloping 
steel plates forming a 6-inch longitudinal slot above the grit 
storage chamber. Flows between 8,000,000 and 16,000,000 
gallons per day are controlled by the outlet weir so that the 
velocity of flow remains at one foot per second. This is accom- 
plished by increasing the depth of flow in the same ratio as the 
increase in the rate of flow. The bottoms of the two chambers 
differ, one having a special hopper for grit and the other a flat 
bottom. This is due to the method of cleaning the chambers, 
it being necessary in the one with a flat bottom to shut off the 
flow when removing the grit while in the one with the hopper 
bottqm it is hoped to remove the grit by the use of sand ejectors 



EXISTING GRIT CHAMBERS 

--^-C--> 50'-0" -| A 



399 



. SO'-O" - 



M W^fc S-l) H H W V JTJ 

IJ j- jjj! j JJJJMHMJ i \& 




FIG. 155. Grit Chamber at Cleveland, Ohio. 

En'g. Record, Vol. 73, 1916, p. 409. 



Weir .-12'Steej C;30lbs. 
Penstocks , Screen J^ Contr _ 
Sewer 




Cross Section. 
FIG. 156. Grit Chamber at Hamilton, Ontario. 

Eng. News, Vol. 73, 1915, p. 425. 



400 SCREENING AND SEDIMENTATION 

without stopping the sewage flow. The details of the chamber 
at Hamilton, Ontario, are shown in Fig. 156. In studying these 
drawings the following features should be noted : 1st, the smooth 
curves in the channel to prevent eddies, undue deposition of 
organic matter, and difficulties in cleaning; 2nd, the hopper in the 
upper end of the grit storage chamber and the slope of the bot- 
tom of at least 1 : 20 ; and 3rd, the simplicity of the inlet and out- 
let devices which may be either stop planks or cast iron sluice 
gates. 

The drawings shown are merely representative of some sat- 
isfactory types. The number and variety of grit chambers 
in existence is great. In designing grit chambers consideration 
must be given to the method of cleaning. They are ordinarily 
cleaned by such methods as have been described for the cleaning 
of catch-basins in Chapter XII. Continuous bucket scrapers 
similar to excavating machines are sometimes used for the clean- 
ing of large grit chambers. The period between cleanings is 
variable. The design should be such as not to require more 
frequent cleanings than twice a month under the worst conditions. 
The fluctuations in quality and quantity of grit will vary the period 
between cleanings. 

238. Number of Grit Chambers. The period of retention 
in grit chambers is so short and the velocity of flow so near the 
maximum and minimum limitations that the wide fluctuations 
in the rate of discharge in storm and combined sewers necessi- 
tates the construction of a number of chambers which should 
be operated in parallel in order to maintain the velocity between 
th<? proper limits. Unless arrangements are made permitting 
the cleaning of grit chambers during operation, more than one 
grit chamber should be installed in order that when one is being 
cleaned the others may be in operation. The number of grit 
chambers must be determined by the desired conditions of 
operation and the cost of construction. The larger the number 
of basins the more nearly the flow in any one basin can be 
maintained constant, but the more expensive the construction. 
The increase in velocity of flow with increasing quantity is 
dependent on the outlet arrangements. In a shallow chamber 
with vertical sides and a standard sharp-crested rectangular 
weir at the outlet the velocity will vary approximately as the cube 
root of the rate of flow. Similarly if the outlet is a V notch the 



QUANTITY AND CHARACTERISTICS OF SLUDGE 401 

velocity will vary as the fifth root of the rate of flow. In all 
cases the deeper the basin the more nearly the velocity varies 
directly as the rate of flow. The outlet weir can be arranged 
as at Cleveland, so that the velocity remains constant for all 
rates of flow within certain limits. It is seldom that more than 
three grit chambers are necessary to care for the fluctuations 
in flow. 

239. Quantity and Characteristics of Sludge from Plain 
Sedimentation. The sludge removed from plain sedimentation 
basins is slimy, offensive, not easily dried, and is highly putres- 
cible and odoriferous. It contains about 90 per cent moisture 
and has a specific gravity from 1.01 to 1.05. The amount 
removed varies between 2 and 5 cubic yards per million gallons 
of sewage. The percentage of suspended matter removed 
varies between 20 and 60. The total amount removed and the 
percentage removal depend on the character of the sewage, 
the type of basin, and the period of detention. 

240. Dimensions of Sedimentation Basins. The dimensions 
of a sedimentation basin are determined by a method similar 
to the one given for the determination of the dimensions of a 
grit chamber in Art. 236. The capacity of the basin is first 
fixed upon to give the required period of sedimentation and 
sludge storage capacity. The length of the basin is the product 
of the velocity and the period of retention. The length, width, 
and depth of the basin are normally fixed by considerations of 
economy and the limitations of the local conditions, such as 
available area, topography, foundations, etc., and examples of 
good practice. A study of basins in use shows the relation 
between length and width to vary normally between 2:1 and 4:1. 
Widths greater than 30 to 50 feet are undesirable because of the 
danger of cross currents and back eddies which will reduce the 
efficiency of the sedimentation. Depths used in practice vary 
too widely to act as guides for any particular design. Theo- 
retically the shallower the basin the better the result. Tanks 
abroad have been built as shallow as 3 feet and some in this 
country as deep as 16 feet. The economical dimensions can be 
determined by trial or by calculus. They will serve as a guide 
in the adoption of the final dimensions. 

The method to be pursued in determining the economical 
dimensions of any engineering structure are: 



402 



SCREENING AND SEDIMENTATION 



I. Express the total cost of the structure in terms of 
as few variables as possible. 

II. Express all of the variables in terms of any one and 
rewrite the expression for the total cost in terms of this 
one variable. 

III. Equate the first derivative of the expression with 
regard to this variable to zero and solve for the variable. 
The result will be the economical value of the variable. 
The values of the other variables can be computed from the 
relations already expressed. 

For example, let it be desired to determine the dimen- 
sions of two continuous-flow sedimentation basins as shown 
in Fig. 157, in which the 
period of retention in each 
is to be 2 hours, the veloc- 
ity of flow is not to ex- 
ceed one foot per second, 
and the sludge accumula- I 
ted will be 3 cubic yards 
per million gallons of sew- I 
age treated. The quantity 



-L 



b - 



of sewage to be treated 

is 18,000,000 gallons per FlG - 157. Diagram for the Compu- 

day. The shortest time tation of Economical Basin Di- 

between cleanings will be mensions. 

2 weeks. 

The capacity of each basin must be 2/24 of 18,000,000 
gallons, or 200,000 cubic feet in order to allow a period 
of retention of 2 hours. To this volume should be added 
sufficient capacity to allow for the 2 "weeks of sludge stor- 
age between cleanings. When a basin is being cleaned 
the load must be put on the remaining basins. Then if 
Q represents the rate of accumulation of sludge per day, 
n represents the number of days between cleanings, ra 
represents the number of basins, and s the sludge capacity 
of one basin, then 



m 






Q 



m1' 



The sludge storage capacity for the example given 
will be approximately 11,000 cubic feet. 

In expressing the total cost of the basins let 

h = the depth in feet. 
I =the length in feet. 
6 = the width in feet. 



DIMENSIONS OF SEDIMENTATION BASINS 403 

The cost of land, floor, etc., per square foot =p dollars. 
The cost of wall per foot length =qh 2 dollars. 

The cost of pipes, valves and appurtenances = P dollars. 

Then the total cost C = (3Z+46) qh 2 +2plb+ P. 

It is now necessary to express the three variables b, I, 
and h, in terms of one of them. From the relation Q = 
2blh it is possible to rewrite the expression for the total c\ 
cost as: 

' 



C = 






Q- ** 



Holding h constant and differentiating with regard to 
b in the first expression and with regard to I in the second 
expression, equating to zero and solving we get: 

J3Q 



The economical relation between 6 and I is therefore 
6 =0.7 51 

regardless of the value of h. 

Substituting these values of I and 6 in the original 
expression for the total cost, it becomes 



^+4. ^ a tf+^+P. 




Differentiating with regard to h, equating to zero, and 
solving 



9 / 

In the example given if q = . 2 and p = 1 . then 

h = 11 . 6 feet, b = 120 feet and I = 160 feet. 

Since these are reasonable dimensions and in accord with good 
practice they should be used, unless other conditions are unsuit- 
able or the velocity of flow is too great. A width of channel of 
120 feet as compared to a length of 160 feet is conducive to a 
poor distribution of velocity across the basin. A ratio of width 
to length of about 1:4 is desirable. In this case, by the use 
of three baffles parallel to the length of the basin, thus dividing 
it into channels 40 feet wide and 11.6 feet deep, the ratio of 
width to length is changed to 1 : 4 and the velocity will be 



404 



SCREENING AND SEDIMENTATION 



increased only to 0.06 foot per second or 3.6 feet per minute, 
which is a reasonable velocity. It could be reduced by increasing 
the spacing of the baffles or the depth of the chamber. 

Complicated baffling is undesirable. Two or three overflow 
baffles may be used to permit quiescent sedimentation in the 
space thus formed, and hanging baffles may be placed before 
the inlet and outlet to break up surface currents, or to prevent 
the movement of scum. The hanging baffles should not extend 
more than 12 to 18 inches below the water surface. The inlet 
and outlet are sometimes arranged to permit the reversal of flow, 
and the connecting channels between basins to allow the opera- 
tion of any number of basins in series or in parallel, although 
such arrangements are more important in water purification. 
Sewage should enter and leave at the top of the basin. 

Cleaning is facilitated by the location of a central gutter in 
the bottom of the basin with the slope of the bottom of the basin 
towards the gutter from 1 : 25 to 1 : 80 or steeper. A pipe, 
2 inches or larger in diameter, containing water under pres- 
sure with connections for hose placed at frequent intervals is a 
useful adjunct in flushing the sludge from the sedimentation 
basins. For equal capacity, deer- vertical flow tanks are more 

expensive and difficult to con- 
struct than the shallower rect- 
angular type. Deep tanks are 
advantageous, however, in that 
sludge can sometimes be re- 
moved by gravity or by pump- 
ing without stopping the opera- 
tion of the tank. They will 
also operate successfully with 
shorter periods of detention 
and higher velocities. The up- 
ward velocity should not be 
greater than the velocity of 
sedimentation of the smallest 
particle to be removed. The 

efficiency of sedimentation in them will be increased by the 
sedimentation of the larger particles which drag some of the 
smaller particles down with them. The Dortmund tank shown 
in Fig. 158 is an example of this type. 




FIG. 158. Section through a Dort- 
mund Tank. 
Depth 20 to 30 feet. 



CHEMICALS 405 

Ordinarily it is not necessary to roof sedimentation basins 
as the odors created are not strong, and difficulties with ice are 
seldom serious. 

CHEMICAL PRECIPITATION 

241. The Process. Chemical precipitation consists in adding 
to the sewage such chemicals as will, by reaction with each other 
and the constituents of the sewage, produce a flocculent precipi- 
tate and thus hasten sedimentation. The advantages of this 
process over plain sedimentation are a more rapid and thorough 
removal of suspended matter. Its disadvantages include the 
accumulation of a large amount of sludge, the necessity for 
skilled attendance, and the expense of chemicals. The process 
is not in extensive use as the conditions under which the 
advantages outweigh the disadvantages are unusual. Sewage 
containing large quantities of substances which will react with a 
small amount of an added chemical to produce the required 
precipitate are the most favorable for this method of treatment. 

Chemical precipitation accomplishes the same result as plain 
sedimentation, although the effluent from the chemically pre- 
cipitated sewage may be of better quality than that from a plain 
sedimentation basin. 

242. Chemicals. Lime is practically the only chemical used 
for the precipitation of the solid matter in sewage. Commercial 
lime used for precipitation consists of calcium oxide (CaO), 
with large quantities of impurities. It should be stored in a dry 
place and protected from undue exposure to the air to prevent 
the formation of calcium carbonate (CaCOs), the formation of 
which is commonly known as air slacking. The active work 
in the formation of the precipitate is performed by the lime (CaO) 
or calcium hydroxide (Ca(OH)2). The lime should therefore 
be purchased on the basis of available CaO, which may be as 
low as 10 to 15 per cent in some commercial products. The 
amount of lime necessary depends on the quality of the sewage, 
the period of retention in the sedimentation basin, the method 
of application, the required results, and other less easily measured 
factors. Full scale tests for the amount of lime needed to 
produce certain results are the most satisfactory. In practice 
the amount of lime necessary when lime alone is used as a pre- 



406 SCREENING AND SEDIMENTATION 

cipitant has been found to be about 15 grains per gallon. This 
may be markedly different, dependent on the quality of the 
sewage. For acid sewages, lime alone is not suitable as a pre- 
cipitant since it is necessary to add sufficient lime to neutralize 
the sewage before the calcium carbonate will be precipitated. 

The use of copperas (FeSO4) together with lime, leads to 
economy in the use of chemicals as the flocculent precipitate of 
ferrous hydroxide (Fe(OH)2) is more voluminous than the 
precipitate of calcium carbonate. This is commonly known as 
the lime and iron process. The presence of iron in certain trade 
wastes may reduce the cost of chemical precipitation, as the 
necessary amount of copperas is reduced.- Where 15 grains of 
lime alone will be needed per gallon of sewage, the total amount 
of chemicals used will be reduced to 8 to 10 grains per gallon 
with the use of lime and iron. This combination is less expensive 
than the use of lime alone, and is even cheaper where the iron is 
already present in the sewage. Such a condition is well illus- 
trated by the sewage at Worcester, Mass., where the oldest and 
best known chemical precipitation plant in the United States is 
located. The amount of lime used at this plant has varied between 
6 and 10 grains per gallon of sewage, the normal amount being 
about 7 grains. No iron is added because of the amount already 
in solution. 

The results of a series of experiments on the chemical precipi- 
tation of sewage by Allen Hazen, are given in the 1890 Report 
of the Massachusetts State Board of Health, on p. 737 of the 
volume on the Purification of Water and Sewage. Hazen con- 
cludes as the result of his experiments: concerning lime, 

There is a certain definite amount of lime . . . 
which gives as good or better results than either more or 
less. This amount is that which exactly suffices to form 
normal carbonates with all the carbonic acid of the 
sewage. This amount can be determined in a few minutes 
by simple titration. 

Concerning lime and iron (copperas) he states: 

Ordinary house sewage is not sufficiently alkaline 

to precipitate copperas, and a small amount of lime must 

be added to obtain good results. The quantity of lime 

required depends both upon the composition of the sewage 

, and the amount of copperas used, and can be calculated 



PREPARATION AND ADDITION OF CHEMICALS 407 

from titration of the sewage. Very imperfect results are 
obtained from too little lime, and, when too much is 
used, the excess is wasted, the result being the same as 
with a smaller quantity. 

In precipitation by ferric sulphate and crude alum, 
the addition of lime was found unnecessary, as ordinary 
sewage contains enough alkali to decompose these salts. 
Within reasonable limits the more of these precipitants 
used the better is the result, but with very large quanti- 
ties the improvement does not compare with the increased 
cost. 

Using equal values of different precipitants, applied 
under the most favorable conditions for each, upon the 
same sewage, the best results were obtained from ferric 
sulphate. Nearly as good results were obtained from 
copperas and lime used together, while lime and alum 
each gave somewhat inferior effluents. . . . When lime 
is used there is always so much lime left in solution that 
it is doubtful if its use would ever be found satisfactory 
except in case of an acid sewage. 

It is quite impossible to obtain effluents by chemical 
precipitation which will compare in organic purity with 
those obtained by intermittent filtration through sand. 

It is possible to remove from one-half to two-thirds 
of the organic matter by precipitation . . . and it seems 
probable that ... a result may be obtained which will 
effectually prevent a public nuisance. 

243. Preparation and Addition of Chemicals. Lime is not 
readily soluble in water. Therefore, it is not best to add the lime 
as a powder to the sewage, but to form a milk of lime, that is, 
a supersaturated solution containing from 2,000 to 4,000 grains 
per gallon, although dry slaked lime has sometimes been applied 
directly. The solution is prepared in tanks in a quantity sufficient 
for some part of the day's run, commonly sufficient to last through 
one shift of 8 or 10 hours. The lime is prepared by placing the 
amount necessary to fill one storage tank into a slaking tank 
containing some cold water. Sufficient water is added to keep 
the solution just at the boiling point, or steam may be added to 
make it boil. After slaking, it is run into the milk-of-lime solu- 
tion tank and sufficient water added to bring to the proper 
strength. The milk of lime is added in measured quantities, 
being controlled by a variable head on a fixed orifice or weir, 
so that it may be varied with the amount of sewage flowing through 
the plant. The amount of lime to be added is determined by 



408 



SCREENING AND SEDIMENTATION 



titration with phenolphthalein, experience indicating the color 
to be obtained when the proper amount of lime has been added. 
The use of either copperas or alum has been so rare, for the 
precipitation of sewage, that a description of the methods of 
handling these chemicals as a sewage precipitant is not war- 
ranted. An excellent description of the methods of handling 
these chemicals in water purification will be found in " Water 
Purification " by Ellms. 

TABLE 81 
RESULTS OP CHEMICAL PRECIPITATION AT WORCESTER, MASSACHUSETTS* 





1900 


1910 


1920 


Amount of sewage treated, million 
gallons 


4,781 


5,317 


8893 


Amount of sewage chemically 
treated, million gallons 


3,650 


3,574 


7,300 


Gallons of wet sludge per million 
gallons of sewage treated . . . 




4,450 


4,185 


Per cent of solids in sludge 


4.42 


8.20 


4 64t 


Tons of solids 


7,294 


4,182 


6,43 It 


Pounds of lime added per million 
gallons of sewage pumped 


999 


762 f 


534 


Per cent of organic matter removed : 
By albuminoid ammonia: 
Total 


52. 7t 


58.4 


51 9 


Suspended 


90. OJ 


88.7 


83 6 


By oxygen consumed: 
Total 


62. 8J 


61.1 


62 5 


Suspended 


86. 6 J 


89.7 


86 2 











* Computed from Annual Report of the Superintendent of Sewers, Nov. 30, 1919, 
and 1920. 

t These figures are for 1919. J These figures are for 1902. These figures are for 1905. 



244. Results. The results of Hazen's experiments indicate 
that a greater amount of suspended matter can be removed in 
the same time by chemical precipitation than by plain sedimen- 
tation. The percentage of removal of suspended matter may be 
as high as 80 to 90 per cent with a period of retention of 6 to 8 
hours and the addition of a proper amount of chemical. That 



RESULTS 409 

the method is not always a success is shown by the results of 
some tests at Canton, Ohio. 1 The report states: 

.... lime treatment removes about 50 per cent of 
the suspended matter, and in the main about 50 per cent 
of the organic matter. . . . These data are instructive 
as indicating that the addition of lime to the Canton 
sewage in quantities as previously stated does not materi- 
ally improve the character of the resulting effluent over 
and above that which could be produced by plain sedi- 
mentation alone. 

The plant at Worcester, Mass., is the largest in the United 
States and information from it is of value. A summary of the 
results at Worcester for 1900, 1910, and 1920 are shown in Table 
81. 

1 Report of the Ohio State Board of Health, 1908, page 425. 



CHAPTER XVI 
SEPTICIZATION 

245. The Process. Septic action is a biological process caused 
by the activity of obligatory or facultative anaerobes as the result 
of which certain organic compounds are reduced from higher to 
lower conditions of oxidation, some of the solid organic substances 
are rendered soluble, and a quantity of gas is given off. Among 
these gases are: methane, hydrogen sulphide, and ammonia. 
The biologic process in the septic tank represents the downward 
portion of the cycle of life and death, in which complex organic 
compounds are reduced to a more simple condition available 
as food for low forms of plant life. The disposal of sewage by 
septic action, when introduced, promised the solution of all 
problems in sewage treatment. Septic action is now better 
understood, and it is known that some of the early claims were 
unfounded. 

The principal advantage of septic action in sewage treatment 
is the relatively small amount of sludge which must be cared 
for compared to that produced by a plain sedimentation tank. 
The sludge from a septic tank may be 25 to 30 per cent and in 
some cases 40 per cent less in weight, and 75 to 80 per cent less 
in volume than the sludge from a plain sedimentation tank. 
The most important results of septic action and the greatest 
septic activity occur in the deposited organic matter or sludge. 
The biologic changes due to septic action which occur in the 
liquid portion of the tank contents are of little or no importance. 
The installation of a septic tank, although it may fail to prevent 
the nuisance calling for abatement, has a remarkable psycho- 
logical effect in stilling complaints. Among other advantages 
are the comparative inexpensiveness of the tanks and the small 
amount of attention and skilled attendance required. The 
tanks need cleaning once in 6 months to a year. If properly 
designed no other attention is necessary. 

410 



THE SEPTIC TANK 411 

The septic tank has fallen into some disrepute because of the 
better results obtainable by other methods, the occasional dis- 
charge of effluents worse than the influent, the occasional dis- 
charge of sludge in the effluent caused by too violent septic boiling, 
and on account of patent litigation. This last difficulty has been 
overcome as the Cameron patents expired in 1916. Occasionally 
the odors given off by the septic process are highly objectionable 
and are carried for a long distance. These odors can be controlled 
to a large extent by housing the tanks. Over-septicization must 
be guarded against as an over-septicized effluent is more difficult 
of further treatment or of disposal than a comparatively fresh, 
untreated sewage. An over-septicized or stale sewage is indi- 
cated by the presence of large quantities of ammonias, either 
free or albuminoid, frequently accompanied by hydrogen sul- 
phide and other foul-smelling gases. The oxygen demand in 
an over-septicized sewage is greater than that in a fresh or more 
carefully treated sewage. 

246. The Septic Tank. A septic tank is a horizontal, continu- 
ous-flow, one-story sedimentation tank through which sewage 
is allowed to flow slowly to permit suspended matter to settle 
to the bottom where it is retained until anaerobic decomposition 
is established, resulting in the changing of some of the suspended 
organic matter into liquid and gaseous substances, and a conse- 
quent reduction in the quantity of sludge to be disposed of. 1 
It is to be noted that a continuous flow is essential to a septic tank. 
Small tanks containing stagnant household sewage are called cess- 
pools, although sometimes erroneously spoken of as septic tanks. 

Septic and sedimentation tanks differ in their method of opera- 
tion only in the period of storage and the frequency of cleaning. 
The period of flow in a septic tank is longer and it is cleaned less 
frequently. The results obtained by the two processes differ 
widely. A septic tank can be converted into a sedimentation 
tank, or vice versa, by changing the method of operation, no 
constructional features requiring alteration. The purpose of 
the tank is to store the sludge for such a period of time that 
partial liquefaction of the sludge may take place, and thus 
minimize the difficulty of sludge disposal. For this reason the 
sludge storage capacity of a septic tank is sometimes greater 
than would be necessary for a plain sedimentation tank. 
1 Definition proposed by the Am. Public Health Assn. 



412 



SEPTICIZATION 





1 


00 t-i-l 
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^i co i i -ir-i*Hi ^t coco* looco ic^fic 

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V 
13 

>-5 


t-00 OCOOO CO OS 0000") OiCCN IN'fi-i CO-^IN i-l COiOt O 
1 OSOO i-l i-i t-co 




1 


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COCO COCO i CO CN ^-iOi-< CNOi OO CO 00 CO * 
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O J3 


a 


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t-t COOCO t C C O) -^ H t- IO CO CM t -l O 00 1-1 O CO CN CO *}< 
i OOO tNi-i OOt 


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lo- 


March 


CO 
^j* >O OS t CO 

1 OOt rti-i COCO 


8 S 



s s" 


xi 

6 


iO 

oo>c ret co comes oooot ocoio COI"T)< ^jigsco o ojt-os 
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HOO ocuo 



RESULTS OF SEPTIC ACTION 413 

247. Results of Septic Action. The results obtained from 
the septic tanks at the Columbus Sewage Experiment Station 
are given in Table 82. The effluent is higher than the influent 
in free ammonia, but the reduction of other constituents, par- 
ticularly suspended matter, is marked. 

Septic action is sensitive to temperature changes, and to cer- 
tain constituents of the incoming sewage. Cold weather or an 
acid influent will inhibit septicization. In winter the liquefaction 
of sludge may practically cease, whereas in summer liquefaction 
may exceed deposition. The amount of gas generated is a 
measure of the relative amount of septic action. The rapid 
generation of gas in warrii weather disturbs the settled sludge 
and may cause a deterioration of the quality of the effluent because 
of the presence of decomposed sludge. The results in Table 82 
show the effect of cold weather on the process. In warm weather 
the violent ebullition of gas sometimes causes the discharge of 
sludge in the effluent, resulting in a liquid more difficult of 
disposal than the incoming sewage. Since septic action is 
dependent on the presence of certain forms of bacteria, where 
these are absent there will be no septic action. Sewage generally 
contains the forms of bacteria necessary for this action but it 
has occasionally been found necessary to seed new tanks in order 
to start septic action. 

The sludge from septic tanks is usually black, with a slight 
odor, though in some cases this odor may be highly offensive. 
The sludge will flow sluggishly. It can be pumped by centrif- 
ugal pumps and it will flow through pipes and channels. It 
has a moisture content of about 90 per cent and a specific gravity 
of about 1.03. It is dried with difficulty on open-air drying beds, 
and it is worthless as a fertilizer. The composition of some 
septic sludges are shown in Table 83. 

248. Design of Septic Tanks. The sedimentation chambers 
of a septic tank are designed on the same principles as the sedi- 
mentation basins described in Art. 240. The velocity of flow 
should not exceed one foot per minute. The channels should bo 
straight and free from obstructions causing back eddies. The 
ratio of length to width of channel should be between 2 : 1 to 4 : 1 
with a width not exceeding 50 feet, and desirably narrower. 
The depths used vary between 5 and 10 feet, exclusive of the 
sludge storage capacity. Hanging baffles should be placed, one 



414 



SEPTICIZATION 



before the inlet and the other in front of the outlet, so as to 
distribute the incoming sewage over the tank, and to prevent 
scum from passing into the outlet. The baffles should hang 
about 12 inches below the surface of the sewage. Intermediate 
baffles are sometimes desirable to prevent the movement of 
sludge or scum towards the outlet. The placing of baffles must 
be considered carefully as injudicious baffling may lessen the 
effectiveness of a tank by so concentrating the currents as to pre- 
vent sedimentation or the accumulation of sludge. Baffles 
should be built of concrete or brick, as wood or metal in contact 
with septic sewage deteriorates rapidly. In designing the sludge 
storage chambers it may be assumed that one-half of the organic 
matter and none of the mineral matter will be liquefied or gasi- 
fied. The net storage volume allowed is about 2 to 3 cubic yards 
per million gallons of sewage treated. Variations between 0.1 
and 10.0 cubic yards have been recorded, however. If grit is 
carried in the sewage to be treated, it should be removed by 
the installation of a grit chamber before the sewage enters the 
septic tank. 

TABLE 83 
ANALYSIS OF TANK SLUDGES 







V 




, 


. 










. 


H 


Per Cent in Terms 


a 


8 










*j 


"S 


of Dry Matter 


JjpS 













> 


"o 




D 03 


a! 










03 
ft 


s 


1 . 






H 


W 


Kind of 






Place 


O 




a 









V 




= a 


.2 


Sludge 




Reference 












B 






















T3 


o 




















J3 


<u 


















a 


V 


"o 


X 


.t2 


"5 


3 ^ 


o 










03 


PH 


> 





* 





O 

































1908 Report, 


Mansfield, O... 


1.11 


80.8 














Septic 




State Board of 






















[ 


Health 


Chicago, 111. ... 


1.03 


90 


40 


60 


1.9 


7.0 


1.0 


200 


Septic 




















1.5 


300 








Columbus, O. . . 


1.09 


83.3 


4.4 


16.7 


0.25 




0.94 




Septic < 


f 

, 


G. A. Johnson 
1905 Report 


Atlanta, Ga 


1.02 


87.1 


39 1 


60.9 


1.25 


6.11 






Imhoff < 


f 
I 


Eng. Rec., V. 72, 
1915 p. 4 


Baltimore, Md. 


1.02 


91.9 


66.2 




2.45 


4 0? 






Diges- ' 
tion 
Tank , 




Eng. News-Rec., 
V. 87, 1921, p. 98 










1 


Baltimore, Md . 


1.02 


92.4 


62.7 




2.75 








Imhoff 




do. 




















Raw 






Baltimore, Md. 




79.2 


73.8 




2.64 


9.00 






Sludge 




do. 


Baltimore, Md. 




92.4 


58.0 


3.19 










Settling 




do. 


, 


















Basin 







DESIGN OF SEPTIC TANKS 415 

Two or more tanks should be constructed to allow for the shut 
down of one for cleaning and to increase the elasticity of the 
plant. The number of tanks to be used is dependent on the 
total quantity of sewage and the fluctuations in rate of flow. An 
average period of retention of about 9 to 10 hours with a mini- 
mum period of 6 hours during maximum flow is a fair average 
to be assumed for design. The period of retention should not 
exceed about 24 hours, as the sewage may become over-septi- 
cized. The sludge storage period should be from 6 to 12 months. 

A cover is not necessary to the successful operation of a septic 
tank. Covers are sometimes used with success, however, in 
reducing the dissemination of odors from the tank. They are 
also useful in retaining the heat of the sewage in cold weather 
and thus aid in promoting bacterial activity. Types of covers 
vary from a building erected over the tank to a flat slab set close 
to the surface of the sewage. In the design of a cover, good 
ventilation should be provided to permit the escape of the gases, 
and easy access should be provided for cleaning. Tightly 
covered tanks or tanks with too little ventilation have resulted 
in serious explosions, as at Saratoga Springs in 1906 and at 
Florenceville, N. C., in 1915. 1 

The sludge may be removed through drains in the bottom of 
the tank as described for sedimentation basins, or where such 
drains are not feasible the sludge and sewage are pumped out. 
For this purpose a pump may be installed permanently at the 
tank, or for small tanks portable pumps are sometimes used. 
Septic tanks should be cleaned as infrequently as possible without 
permitting the overflow of sludge into the effluent. The less 
frequent the cleaning the less the amount of sludge removed 
since digestion is continuous throughout the sludge. It is 
necessary to clean when the tank becomes so filled with sludge, 
that the period of retention is materially reduced, or sludge is 
being carried over into the effluent. 

The details of the septic tank at Champaign, Illinois, are 
shown in Fig. 159. This tank was designed by Prof. A. N. 
Talbot, and was put in service on Nov. 1, 1897. It was among 
the first of such tanks to be installed in the United States. The 
tank shown in Fig. 159 is an example of present day practice 
in single-story septic tank design. 

1 See Eng. News. Vol. 73, 1915, p. 410. 



416 



SEPTICIZATION 




Vain 
Cfiarrii 



her 



Plan, with Roof Removed 
FIG. 159. Septic Tank at Champaign, Illinois. 

^Sludge Outlet 









1 1 










1 1 


1 II 






1 


-/ 


3 


\_J<- Sump 


T 


TT 


1 


1 


J 




1 


JO 0) 












^/ 






























K .- 


8-0"- sJ 







Plan 



6"lnlet 






Sludge Ou-Het-. 




Section . 

FIG. 160. Design for a Residential Septic Tank for a Family of Ten. Illinois 
State Board of Health. 



IMHOFF TANKS 



417 



Small septic tanks for rural homes of 5 to 15 persons, or on 
a slightly larger scale for country schools and small institutions, 
are little more than glorified cesspools. Nevertheless much 
attention has been given to the construction of such tanks by 
the National Government and by state boards of health. 1 The 
recommendations of some of these boards have been compiled 
in Table 84. A typical method for the construction of such tanks, 
as recommended by the Illinois State Board of Health, is shown 
in Fig. 160. A subsurface filter, into which the effluent is dis- 
charged, is an important adjunct where no adequate stream is 
available to receive the discharge from the tank. 



Rule Recommended by 
State Board of Health 


Number, 
Persons 


Capacity, 
Gallons 
per Person 


Period of 
Retention 


Remarks 


Wisconsin . 




30 
50 


24 hours 


Not less than 560 gallons 
Not more than 5 feet deep 

25 per cent additional 
capacity for sludge . 

Not less than 500 gallons 


Ohio 


4 to 10 


Kentucky 


24 to 48 hours 
24 hours 
24 hours 
24 hours 


Texas 






Illinois 




45 

40 

} - 

25 
1 20 
25 to 30 


U. S. Dept. Agriculture. 
North Carolina 




Large 
Schools 
20 pupils 
Medium 
School 
Homes 






North Carolina 


North Carolina 





249. Imhoff Tanks. In the discussion of septic tanks it has 
been brought out that one of the objections to their use is the 
unloading of sludge into the effluent which occasionally causes 
a greater amount of suspended matter in the effluent than in the 
influent. The Imhoff tank is a form of septic tank so arranged 
that this difficulty is overcome. It combines the advantages 
of the septic and sedimentation tanks and overcomes some of their 
disadvantages. An Imhoff tank is a device for the treatment 
of sewage, consisting of a tank divided into 3 compartments. 
The upper compartment is called the sedimentation chamber. In 

1 Sewage Treatment from Single Houses and Small Communities, by L. 
C. Frank. U. S. Public Health Service, Bulletin 101, 1920. 



418 



SEPTICIZATION 



it the sedimentation of suspended solids causes them to drop 
through a slot in the bottom of the chamber to the lower com- 
partment called the digestion chamber. In this chamber the solid 
matter is humified by an action similar to that in a plain septic 
tank. The generated gases escape from the digestion chamber to 
the surface through the third compartment called the transition or 
scum chamber. Sections of Imhoff tanks are shown in Fig. 161. 
It is essential to the construction of an ImhorT tank that the slot 
in the bottom of the sedimentation chamber does not permit 



Effect o-f 
Design on 
Sludge-Storage 
Capacity. 
(AandB) 




Downward and Upward 
FlowTank 

FIG. 161. Typical Sections through Imhoff Tanks. 

Eng. News, Vol. 75, p. 15. 

the return of gases through the sedimentation chamber, and 
that there be no flow in the digestion chamber. 

The Imhoff tank was invented by Dr. Karl ImhorT, director 
of the Emscher Sewerage District in Germany. Its design is 
patented in the United States, the control of the patent being 
in the hands of the Pacific Flush Tank Co. of Chicago, which 
collects the royalties which are payable when construction work 
begins. The fee for a tank serving 100 persons is $10, for 1,000 
persons is $80 and for 100,000 persons is $2550. The rate of 
the royalty reduces in proportion as the number of persons served 
increases. 1 As designed by ImhorT and used in Germany the tanks 
were of the radial flow type and quite deep. The depth, as 
1 Eng. News Record, Vol. 78, 1917, p. 566. 



DESIGN OF IMHOFF TANKS 



419 



explained by Imhoff, is one of the chief requirements for the 
successful operation of the tank. As adapted to American 
practice the tanks are generally of the longitudinal flow type 
and are not made so deep. An isometric view of a radial flow 
Imhoff tank is shown in Fig. 162. The sewage enters at the 
center of the tank near the surface and flows radially outward 
under the scum ring and over a weir placed near the circum- 
ference of the tank. One type of longitudinal flow tank is shown 
in isometric view in Fig. 163. 




Sludge Bed 
\jSedimenMion Compt. 

tV^_ 
- Sludge Corfip't 

~El.'JeJ'~ 
FIG. 162. Sketch of Radial Flow Imhoff Tank at Baltimore, Maryland. 

Eng. Record, Vol. 70, p. 5. 

250. Design of Imhoff Tanks. The velocity of flow, period 
of retention, and the quantity of sewage to be treated determine 
the dimensions of the sedimentation chamber as in other forms 
of tanks. The velocity of flow should not exceed one foot per 
minute, with a period of retention of 2 to 3 hours. A greater 
velocity than one foot per minute results in less efficient sedi- 
mentation. A longer period of retention than the approximate 
limit set may result in a septic or stale effluent, and a shorter 
period may result in loss of efficiency of sedimentation. The 
bottom of the sedimentation chamber should slope not less 
than \% vertical to 1 horizontal, in order that deposited material 
will descend into the sludge digestion chamber. Provision 
should be made for cleaning these sloping surfaces by placing 
a walk on the top of the tank from which a squeegee can be 
handled to push down accumulated deposits. It is desirable 
to make the material of the sides and bottom of the sedimenta- 



420 



SEPTICIZATION 



tion chamber as smooth as possible to assist in preventing the 
retention of sludge in the sedimentation chamber. Wood, glass, 
and concrete have been used. The latter is the more common 
and has been found to be satisfactory. The length of the sedi- 
mentation chamber is fixed by the velocity of flow and the 
period of retention. Tanks are seldom built over 100 feet in 

Stop Plank ^Outlet Channel for Reverse Flow f~~$f Q n pj a nk 



InletChannel, 
Direct Flow or 
Outlet Channel, 
Reverse Flow-- 



General ' Arrangement 




t 10-0" 
C.toC. 



AJv 

>jj^' Reinforced 
'\i Concrete 
^ Walls and 
Partitions. 



Reducer 
Iron Suppoi 



FIG. 163. Isometric View of Longitudinal Flow Imhoff Tank at Cleburne, 

Texas. 

Eng. News, Vol. 76, p. 1029. 

length, however, because of the resulting unevenness in the accu- 
mulation of sludge. Where longer flows are desired two or more 
tanks may be operated in series. The width of the chamber 
is fixed by considerations of economy and convenience. It 
should not be made so great as to permit cross currents. In 
general a narrow chamber is desirable. Satisfactory chambers 
have been constructed at depths between 5 and 15 feet. The 



DESIGN OF IMHOFF TANKS 421 

depth of the sedimentation chamber and the depth of the diges- 
tion chamber each equal about one-half of the total depth of the 
tank. This should be made as deep as possible up to a limit 
of 30 to 35 feet, with due consideration of the difficulties of 
excavation. C. F. Mebus states: 1 

In 9 of the largest representative United States 
installations, the depth from the flow line to the slot 
varies from 10 feet 10 inches to 13 feet 6 inches. 

Imhoff states, concerning the depth of tanks: 

Deep tanks are to be preferred to shallow tanks 
because in them the decomposition of the sludge is 
improved. This is so because in the deeper tanks the 
temperature is maintained more uniformly and because 
the stirring action of the rising gas bubbles is more 
intense. 

The stirring action of the gas bubbles is desirable as it brings the 
fresh sludge more quickly under the influence of the active bac- 
terial agents. The greater pressure on the sludge in deep tanks 
also reduces its moisture content. 

Two or more sedimentation chambers are sometimes used over 
one sludge digestion chamber in order to avoid the depths called 
for by the sloping sides of a single sedimentation chamber. An 
objection to multiple-flow chambers is the possibility of interchange 
of liquid from one chamber to another through the common 
digestion chamber. 

The inlet and outlet devices should be so constructed that 
the direction of flow in the tank can be reversed in order that the 
accumulated sludge may be more evenly distributed in the hop- 
pers of the digestion chamber. The sewage should leave the 
sedimentation chamber over a broad crested weir in order to 
minimize fluctuations in the level of sewage in the tank. The 
gases in the digesting sludge are sensitive to slight changes in 
pressure. A lowering of the level of sewage will release com- 
pressed gas and will too violently disturb the sludge in the 
digestion chamber. Hanging baffles, submerged 12 to 16 inches 
and projecting 12 inches above the surface of the sewage, should 
be placed in front of the inlet and outlet, and in long tanks inter- 
mediate baffles should be placed to prevent the movement of 
1 Municipal Engineering, Vol. 54, p. 149. 



422 SEPTICIZATION 

scum or its escape into the effluent. An Imhoff tank which is 
operating properly should not have any scum on the surface of 
the sewage in the sedimentation chamber. 

The slot or opening at the bottom of the sedimentation 
chamber should not be less than 6 inches wide between the lips. 
Wider slots are preferable, but too wide a slot will involve too 
much loss of volume in the digestion chamber. One lip of the 
slot should project at least 3 inches horizontally under the other 
so as to prevent the return of gases through the sedimentation 
chamber. A triangular beam may be used as shown in Fig. 161 A. 
This method of construction is advantageous in increasing the 
available capacity for sludge storage. 

The digestion chamber should be designed to store sludge from 
6 to 12 months, the longer storage periods being used for smaller 
installations. In warm climates a shorter period may be used 
with success. The amount of sludge that will be accumulated 
is as uncertain as in other forms of sewage treatment. A widely 
quoted empirical formula, presented in " Sewage Sludge " by 
Allen, states: 

C = 10 . 5 P D f or combined sewage; 
C = 5 . 25 P D for separate sewage, 

in which C = the effective capacity of the digestion chamber 

in cubic feet ; 

P = the population served, expressed in thousands; 
D = the number of days of storage of sludge. 

The effective capacity of the chamber is measured as the entire 
volume of the chamber approximately 18 inches below the 
lower lip of the slot. The capacity as computed from the above 
formula is assumed as satisfactory for a deep tank. Frank 
and Fries l recommend the increase of the capacity for shallow 
tanks to compensate for the decreased hydrostatic pressure. 
In any event the formula can be no more than a guide to design. 
No formula can be of equal value to data accumulated from 
tests on the sewage to be treated. The Illinois State Board of 
Health requires 3 cubic yards of sludge digestion space per 
million gallons of sewage treated. Frank and Fries recommend 
an allowance of 0.007 cubic foot of storage per inhabitant per day 
for combined sewage and one-half that amount for separate 

1 Eng. Record, Vol. 68, 1913, p. 452. 



DESIGN OF IMHOFF TANKS 423 

sewage. If this is based on 80 per cent moisture content, the 
volume for other percentages of moisture can be easily com- 
puted. An average figure used in the Emscher District is one 
cubic foot capacity for each inhabitant for the combined system, 
and three-fourths of this for the separate system. Metcalf 
and Eddy 1 recommend the following method for the deter- 
mination of the sludge storage capacity: (1) From analyses of 
the sewage or study of the sources ascertain the amount of sus- 
pended matter. (2) Assume, or determine by test, the amount 
which will settle in the period of detention selected, say 60 per 
cent in 3 hours. (3) Estimate the amount which will be digested 
in the sludge chamber at about 25 per cent, leaving 75 per cent 
to be stored. (4) Estimate the percentage moisture in the sludge 
conservatively, say 85 per cent. The total volume of sludge 
can then be computed. This method is more rational than 
the use of empirical formulas, but because of the estimates 
which must be made its results will probably be of no greater 
accuracy than those obtained empirically. 

The digestion chamber is made in the form of an inverted 
cone or pyramid with side slopes at most about 2 horizontal to 
1 vertical and preferably much steeper without necessitating too 
great a depth of tank. The purpose of the steep slope is to con- 
centrate the sludge at the bottom of the hopper thus formed. 
Concrete is ordinarily used as the material of construction as 
a smooth surface can be obtained by proper workmanship. 
Where flat slopes have been used, a water pipe perforated at 
intervals of 6 to 12 inches may be placed at the top of the slopes, 
and water admitted for a short time to move the sludge when the 
tank is being cleaned. 

A cast-iron pipe, 6 to 8 inches in diameter, is supported in an 
approximately vertical position with its open lower end supported 
about 12 inches above the lowest point in the digestion chamber. 
This is used for the removal of sludge. A straight pipe from the 
bottom of the tank to a free opening in the atmosphere is desir- 
able in order to allow the cleaning of the pipe or the loosening 
of sludge at the start, and to prevent the accumulation of gas 
pockets. The sludge is led off through an approximately hori- 
zontal branch so located that from 4 to 6 feet of head are available 
for the discharge of the sludge. A valve is placed on the hori- 
1 Am. Sewerage Practice, Vol. Ill, p. 437. 



424 SEPTICIZATION 

zontal section of the pipe. A sludge pipe is shown in Fig. 162 
and 163. Under such conditions, when the sludge valve is opened 
the sludge should flow freely. The hydraulic slope to insure 
proper sludge flow should not be less than 12 to 16 per cent. 
Where it is not possible to remove the sludge by gravity an air 
lift is the best method of raising it. 

The volume of the transition or scum chamber should equal 
about one-half that of the digestion chamber. The surface area 
of the scum chamber exposed to the atmosphere should be 25 
to 30 per cent of the horizontal projection of the top of the 
digestion chamber. Some tanks have operated successfully 
with only 10 per cent, but troubles from foaming can usually 
be anticipated unless ample area for the escape of gases has 
been provided. 

All portions of the surface of the tank should be made 
accessible in order that scum and floating objects can be broken 
up or removed. The gas vents should be made large enough 
so that access can be gained to the sludge chamber through 
them when the tank is empty. 

Precautions should be taken against the wrecking of the tank 
by high ground water when the tank is emptied. With an empty 
tank and high ground water there is a tendency for the tank to 
float. The flotation of the tank may be prevented by building 
the tank of massive concrete with a heavy concrete roof, by 
underdraining the foundation, or by the installation of valves 
which will open inwards when the ground water is higher than 
the sewage in the tank. Dependence should not be placed on the 
attendant to keep the tank full during periods of high ground 
water. 

Roofs are not essential to the successful operation of Imhoff 
tanks. They are sometimes used, however, as for septic tanks, 
to assist in controlling the dissemination of odors, to minimize 
the tendency of the sewage to freeze, and to aid in bacterial 
activity. In the construction of a roof, ventilation must be 
provided as well as ready access to the tank for inspection, 
cleaning, and repairs. 

251. Imhoff Tank Results. The Imhoff tank has the 
advantage over the septic tank that it will not deliver sludge 
in the effluent, except under unusual conditions. The Imhoff 
tank serves to digest sludge better than a septic tank and it 



STATUS OF IMHOFF TANKS 425 

will deliver a fresher effluent than a plain sedimentation tank. 
Imhoff sludge is more easily dried and disposed of than the 
sludge from either a septic or a sedimentation tank. This is 
because it has been more thoroughly humified and contains 
only about 80 per cent of moisture. As it comes from the tank 
it is almost black, flows freely and is filled with small bubbles 
of gas which expand on the release of pressure from the bottom 
of the tank, thus giving the sludge a porous, sponge-like consist- 
ency which aids in drying. When dry it has a inoffensive odor 
like garden soil, and it can be used for filling waste land, without 
further putrefaction. It has not been used successfully as a 
fertilizer. 

Offensive odors are occasionally given off by Imhoff tanks, 
even when properly operated. They also have a tendency to 
" boil " or foam. The boiling may be quite violent, forcing scum 
over the top of the transition chamber and sludge through the 
slot in the sedimentation chamber, thus injuring the quality 
of the effluent. The scum on the surface of the transition chamber 
may become so thick or so solidly frozen as to prevent the escape 
of gas with the result that sludge may be driven into the sedi- 
mentation chamber. 

Some chemical analyses of Imhoff tank influents and efflu- 
ents are given in Table 86 and the analyses of some sludges from 
Imhoff tanks are given in Table 83. It is to be noted that the 
nitrites and nitrates are still present in the effluent, whereas 
they are seldom present in the effluent from septic tanks. The 
per cent of moisture in the Imhoff sludge is less than that in the 
septic tank sludge, and its specific gravity is higher. It is heavier 
and more compact because of the longer time and the greater 
pressure it has been subjected to in the digestion chamber of the 
Imhoff tank. 

252. Status of Imhoff Tanks. The introduction of the 
Imhoff tank into the United States, like the introduction of the 
Burkli-Ziegler Run-Off Formula, and Kutter's Formula, is to be 
credited to Dr. Rudolph Bering. He advised Dr. Imhoff to 
come to the United States to introduce his tank and gave him 
material aid through recommendations and introductions to 
engineers. Shortly after its introduction, in 1907, the tank 
became very popular and installations were made in many 
cities. This popularity was caused by a growing dissatisfaction 



426 SEPTICIZATION 

with the septic tank, the litigation then progressing over septic 
patents, the production of inoffensive sludge, and the promising 
results which had been obtained in Germany. As a result of the 
extended experience obtained in the use of Imhoff tanks American 
engineers have learned that, like all other sewage treatment 
devices introduced up to the present time, the Imhoff tank 
requires experienced attention for its successful operation. These 
tanks are now being installed in the place of septic tanks, and 
they are frequently used in conjunction with sprinkling filters. 

253. Operation of Imhoff Tanks. The important feature 
in the successful operation of an Imhoff tank is the proper 
control of the sludge and transition chambers. During the 
ripening process, which may occupy 2 weeks to 3 months after 
the start of the tank, offensive odors may be given off, the tank 
may foam violently, and scum may boil over into the sedi- 
mentation chamber. This is usually due to an acid condition 
in the digestion chamber which may possibly be overcome by 
the addition of lime. A very fresh influent will have a similar 
effect. Too violent boiling is not likely to occur where the 
area for the escape of gas has been made large and the gas is 
not confined. Any accumulation of scum should be broken up 
and pushed down into the digestion chamber, or removed from 
the tank. The stream from a fire hose is useful in breaking up 
scum. The side walls of the sedimentation chamber should be 
squeegeed as frequently as is necessary to keep them free from 
sludge, which may be as often as once or twice a week. Material 
floating on the surface of the sedimentation chamber should be 
removed from the tank or sunk into the digestion chamber 
through the gas vents in the transition chamber. 

No sludge should be removed, except for the taking of samples, 
until the tank is well ripened. The ripening of the sludge can be 
determined by examining a sample and observing its color and odor. 
An odorless, black, granular, well humified sludge is indicative 
of a ripened tank. After the tank has ripened, sludge should be 
removed in small quantities at 2 to 3-week intervals, except in 
cold or rainy weather. The sludge should be drawn off slowly 
to insure the removal of the oldest sludge at the bottom of the 
digestion chamber. After the drawing off of the sludge has 
ceased the pipe should be flushed with fresh water to prevent 
its clogging with dried sludge in the interim until the next 



OTHER TANKS 427 

removal. Under no circumstances should all the sludge in the 
tank be removed at any time. The removal of some sludge 
during foaming after ripening may reduce or stop the foaming. 
The ripening of a tank can be hastened by adding some sludge 
from a tank already ripened. 

Sludge should not be allowed to accumulate within 18 inches 
of the slot at the bottom of the digestion chamber. The elevation 
of the surface of the sludge can be located by lowering into the 
tank, a stoppered, wide-mouthed bottle on the end of a stick. 
The stopper is pulled out by a string when the bottle is at some 
known elevation. The bottle is then carefully raised and 
observed for the presence of sludge. The process is repeated 
with the bottle at different elevations until the surface of the 
sludge has been discovered. Another method is to place the 
suction pipe of a small hand pump at known points, successively 
increasing in depth, and to pump in each position until one posi- 
tion is found at which sludge appears in the pump. When the 
sludge in one portion of the digestion chamber has risen higher 
than in another portion, the direction of flow in the sedimentation 
chamber should be reversed if possible. In the ordinary routine 
of operation it is never necessary to shut down an Imhoff tank. 
Sludge is removed while the tank is operating. The shut down 
of a tank will be caused by accidents and breaks to the structure 
or control devices. 

254. Other Tanks. The Travis Hydrolytic Tank represents 
a step in the development from the septic tank to the Imhoff 
tank. The Doten tank and the Alvord tank are recent develop- 
ments, and are somewhat similar in operation to the Imhoff 
tank. 

The Travis Hydrolytic Tank when first designed differed 
from the later design of the Imhoff tank in the slot between 
the sedimentation chamber and the digestion chamber which 
was not trapped against the escape of gas from the latter to the 
former, and in operation a small quantity of fresh sewage was 
allowed to flow through the digestion chamber. The tank is 
called a hydrolytic tank because some solids are liquefied in it. 
The tank is mainly of historic interest as designs similar to it are 
rarely made to-day. Better results are obtained from the use 
of the Imhoff tank. Recent developments have altered the 
original design of the Travis tank so that it is hardly recognizable. 



428 



SEPTICIZATION 



The Travis tank at Luton, Eng., is shown in Fig. 164. The 
detailed description given in the Engineering News in connection 
with this illustration shows that the governing object of the 
design is to separate as quickly as possible the sludge deposited 



HYDROLIZING 
> CHAMBER-. 
ft -t-~ 



,'18 "Outlet- Carrier 
to Filter Beds 
,9"fnler 

^. ,InM 
flT^k Chamber 




'I5"CJ. 



^^f^^fy^f^ 




Section through 
Inlet Chamber. 



Cross 
Section.. 



FIG. 164. Plan and Section of Hydrolytic Tank at Luton, England. 
Eng. News, Vol. 76, 1916, p. 194. 

by the sewage without septic action being set up. To aid in the 
collection and settlement of flocculent matter vertical wooden 
grids or colloiders are used. The suspended matter strikes these 
and forms a slimy deposit on them that in a short time slips off 
in pieces large enough to settle readily. 



OTHER TANKS 



429 



The Doten tank 1 is a single-storied, hopper-bottomed septic 
tank, views of which are shown in Fig. 165. It was devised by 
L. S. Doten for army cantonments during the War. Its chief 
purpose was to avoid the foaming and frothing so common to 
Imhoff tanks when overdosed with fresh sewage. The first 
Alvord tank was constructed in Madison, Wis., in 1913. 2 As 
now constructed the tank consists of three deep, single-story com- 
partments with hopper bottoms. These compartments are 
arranged side by side in any one unit. Sewage enters at the sur- 



IS'C.f. $ 

Slope /.4% 8"C.I.SoilPipe,6ludgeDrain^ Outlet*. t* g. 

^10'Gate Valve '%< 8"6a/efo/V8S V ) \ ^i" 5 " 

" ^f^r i \m i "TUT *'s 




-4-0*4-0 "Trap Doors 

L 



i 

Slope Gutter to End of Building 



IO"b.I.Soil'Pipe 

A 



Trap 

, , r ] Doors'** 

i \-~97-9 \--\ 

-23-6- )\'< 23-6- 

-8"6afe Valve Z, 



Plan of Septic Tank 




.-2"x6'Plate &&' 




Section B-B 



FIG. 165. Doten Tank for Army Cantonment Sewage Disposal. 

Eng. News-Record, Vol. 79, 1917, p. 931. 

face of one of the compartments and is retained here during 
one-half of the total period of retention. It leaves the first 
compartment over a weir and passes in a channel over the top 
of the intermediate compartment to the third or effluent com- 
partment, where it is held for the remainder of the period of 
detention. Accumulated scum and sludge are drawn off into the 
intermediate compartment at the will of the operator, this 

1 Trans. Am. Society Civil Engineers, Vol. 83, 1920, p. 337. 
1 Eng. News Record, Vol. 83, 1919, p. 510. 



430 



SEPTICIZATION 



compartment being used for sludge digestion only. Such tanks 
as the Doten and the Alvord have been used for plants receiving 
very fresh sewages such as is discharged from military canton- 
ments, in order to assist in the prevention of the foaming to be 
expected from an Imhoff tank receiving such a fresh influent. 
The tanks are suitable for small installations, or where excavation 
to the depth required for an Imhoff tank is not practicable. 



CHAPTER XVII 
FILTRATION AND IRRIGATION 

255. Theory. The cycle through which the elements forming 
organic matter pass from life to death and back to life again 
has been described in Chapter XIII. It has been shown in 
Chapter XVI that septic action occupies that portion of the 
cycle in which the combinations of these elements are broken 
down or reduced to simpler forms and the lower stages of the cycle 
are reached. The action in the filtration of sewage builds up 
the compounds again in a more stable form and almost complete 
oxidation is attained, dependent on the thoroughness of the 
filtration. In the filtration of sewage only the coarsest particles 
of suspended matter are removed by mechanical straining. The 
success of the filtration is dependent on biologic action. The 
desirable form of life in a filter is the so-called nitrifying bacteria 
which live in the interstices of the filter bed and feed upon the 
organic matter in the sewage. Anything which injures the 
growth of these bacteria injures the action of the filter. In a 
properly constructed and operated filter, all matter which enters 
in the influent, leaves with the effluent, but in a different molec- 
ular form. A slight amount may be lost by evaporation and 
gasification but this is more than made up by the nitrogen and 
oxygen absorbed from the atmosphere. The nitrifying action 
in sewage filtration is shown by the analysis of sewage passing 
through a trickling filter, as given in Tables 86 and 87. It is 
shown by the reduction of the content of organic nitrogen, free 
ammonia, oxygen consumed, and the increase in nitrites, nitrates, 
and dissolved oxygen. The reduction of suspended matter is 
interrupted periodically when the filter " unloads." The sus- 
pended matter in the effluent is then greater than in the influent. 

The nitrifying organisms have been isolated and divided 
into two groups nitrosom&nas, the nitrite formers, and nitrobacter, 
the nitrate formers. Experiments indicate that the growth of the 

431 



432 FILTRATION AND IRRIGATION 

nitrobacter organisms is dependent on the presence of the 
nitrosomonas organisms, which are in turn dependent on the 
presence of the putrefactive compounds resulting from the action 
of putrefying bacteria. The existence of these organisms is an 
example of symbiotic action in bacterial growth. The organisms 
have been found to grow best on rough porous material on which 
their zocgleal jelly can be easily deposited and affixed. Sewage 
filters were constructed to provide these ideal conditions before 
the action of a filter was thoroughly understood. 

The action in irrigation is similar to that in filtration. 
Although more strictly a method of final disposal rather than 
preliminary treatment, the similarity of the actions which take 
place, and the grading of sand filtration into broad irrigation 
with no distinct line of difference has resulted in the inclusion of the 
discussion of irrigation in the same chapter with filtration. 

256. The Contact Bed. A contact bed is a water-tight basin 
filled with coarse material, such as broken stone, with which 
sewage and air are alternately placed in contact in such a manner 
that oxidation of the sewage is effected. A contact bed has some 
of the features of a sedimentation tank and an oxidizing filter. 
As such it marks a transitory step from anaerobic to aerobic 
treatment of sewage. A plan and a section of a contact bed are 
shown in Fig. 166. 

Because of its dependence on biologic action a contact bed 
must be ripened before a good effluent can be obtained. The 
ripening or maturing occurs progressively during the first few 
weeks of operation, the earlier stages being more rapidly 
developed. The time required to reach such a stage of maturity 
that a good effluent will be developed will vary between one and 
six or eight weeks, dependent on the weather and the character 
of the influent. During the period of maturing the load on the 
bed should be made light. 

The use of contact beds has been extensive where a more 
stable effluent than could be obtained from tank treatment has 
been desired, yet the best quality of effluent was not required. 
The sewage to undergo treatment in a contact bed should be given 
a preliminary treatment to remove coarse suspended matter. 
The efficiency of the contact treatment can be increased by 
passing the sewage through two or three contact beds in series. 
In dpuble contact treatment the primary beds are filled with 



THE CONTACT BED 



433 



coarser material and operate at a more rapid rate than the 
secondary beds. Double contact gives better results than 
single contact, but triple-contact treatment, though showing 
excellent results, is hardly worth the extra cost. An advantage 
which contact treatment has over all other methods of sewage 
filtration is that the bed can be so operated that the sewage is 
never exposed to view. As a result the odors from well-operated 
contact beds are slight or are entirely absent and there should be 



Sand .Filters ; Contact Beds 

151' *K- 148-- 



,Hor. Scale. 
20' 40' 60' 




" ..' . TopofSond,Et.3l.23 

Longitudinal Section. 

FIG. 166. Plan and Section of Treatment Plant at Marion, Ohio, Showing 

Septic Tank, Contact Bed, and Sand Filter. 

1908 Report Ohio State Board of Health. 

no trouble from flying insects. Such a method of treatment is 
favorable to plants located in populous districts and to the fancies 
of a landscape architect. Another advantage of the contact 
bed is the small amount of head required for its operation, 
which may be as low as 4 to 5 feet. This low head consumption 
by a sewage filter is equaled only by the intermittent sand 
filter. 

The quality of the effluent from some contact beds is shown 
in Table 85. It is to be noted that nitrification has been carried 
to a fair degree of completion, and that the reduction of oxygen 
consumed has been marked. In comparison with the effluent 



434 



FILTRATION AND IRRIGATION 



from filters, contact effluent contains a smaller amount of nitro- 
gen as nitrites and nitrates, and suspended solids. Contact 
effluent is usually clear and odorless, but it is not stable without 
dilution. The absence of nitrites and nitrates is sometimes 
advantageous as the effluent will not support vegetable growths 
dependent on this form of nitrogen. The absence of suspended 
solids obviates the use of secondary sedimentation basins which 
are needed with trickling filters. The head of 5 to 8 feet 
required for contact treatment is low in comparison to the 10 
to 15 feet required for trickling filters, but is slightly higher than 
the head required for intermittent sand filtration. The cost 
of contact treatment is higher than the cost of trickling filters 
but is lower than the cost of intermittent sand filtration, as 
shown in Table 90. 

TABLE 85 

QUALITY OF EFFLUENTS FROM CONTACT BEDS 
Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905. 













i 










"R o+o 


1 


Nitrogen as 


Suspended 
Matter 


a 
i 







Size of 


rtate, 
Million 


1 

















O 







Material 


Gallons 







'3 
















fe 


in Inches 


per Acre 


a 


B 





I 


2 









> 


C 


J3 
"ft 




per Day 


& 


a 
B 




V 

'-S 


2 


3 


_C8 


| 


8 




E 






D 










o 




M 







P 






O 


o 





2 


* 




> 





a 
















Parts 


per Million 








A 


5 


0.25-1.00 


0.953 


23 


3.5 


8.7 


0.20 


1.6 


832 


94 


737 


0.3 


B 


5 


0.25-2.00 


1.514 


21 


4.0 


8.4 


0.15 


1.4 


831 


85 


746 


0.1 


C 


5 


0.25-1.50 


1.222 


24 


3.5 


10.8 


0.11 


0.6 


826 


92 


734 


0.8 


D 


5 


0.50-1.50 


1.405 


22 


3.3 


9.5 


0.13 


0.9 


810 


91 


717 


0.9 










Per Cent Removal of Constituer 


its of Appli( 


;d Sew 


age 


A 


5 


0.25-1.00 


0.953 


48 


49 


10 






73 


70 


76 




B 


5 


0.25-2.00 


1.514 


52 


40 


11 






80 


77 


83 




C 


5 


0.25-1.50 


1.222 


47 


31 


12 






70 


70 


70 




D 


5 


0.50-1.50 


1.405 


46 


37 


19 






67 


61 


72 









The depth of the contact bed is generally made from 4 to 
6 feet. The deeper beds are less expensive per unit of volume, 
to construct, as the cost of the underdrains and the distribution 
system is reduced in relation to the capacity of the filter. The 
increased depth reduces the aeration, and the periods of filling 



THE CONTACT BED 435 

and emptying are so increased as to limit the depths to the figures 
stated. The other dimensions of the bed are controlled by 
economy and local conditions, as the success of the contact 
treatment is not affected by the shape of the bed. Contact 
units are seldom constructed larger than one-half an acre in area, 
as larger beds require too much time for rilling and emptying. 
A large number of small units is also undesirable because of the 
increased difficulty of control. In general it is well to build as 
large units as are compatible with efficient operation, elasticity 
of plant, and which can be filled within the time allowed at the 
average rate of sewage flow, or from dosing tanks in which the 
storage period is not so long as to produce septic conditions. 

The interstices in a contact bed will gradually fill up, due to 
the deposition of solid matter on the contact material, the dis- 
integration of the material, and the presence of organic growths. 
The period of rest allowed every five or six weeks tends to restore 
partially some of this lost capacity through the drying of the 
organic growths. It is occasionally necessary to remove the 
material from the bed and wash it in order to restore the original 
capacity. It may be necessary to do this three or four times a 
year, in an overloaded plant, or as infrequently as once in five or 
six years in a more lightly loaded bed. The period is also 
dependent on the character of the contact material and the quality 
of the influent. This loss of capacity may reduce the voids from an 
original amount of 40 to 50 per cent of voids to 10 to 15 per 
cent. If the bed is not overloaded the loss of capacity will not 
increase beyond these figures. 

The rate of filtration depends on the strength of the sewage, 
the character of the contact material, and the required effluent. 
It should be determined for any particular plant as the result 
of a series of tests. For the purposes of estimation and com- 
parison the approximate rate of filtration should be taken at 
about 94 gallons per cubic yard of filtering material per day on 
the basis of three complete fillings and emptyings of the tank. 
This is equivalent to 150,000 gallons per acre foot of depth per 
day, or for a bed 5 feet deep to a rate of 750,000 gallons per acre 
per day. The net rate for double or triple filtration is less than 
these figures, but on each filter the rates are higher. 

The material of the contact bed should be hard, rough, and 
angular. It should be as fine as possible without causing clogging 



436 FILTRATION AND IRRIGATION 

of the bed. Materials in successful use are: crushed trap rock 
or other hard stone, broken bricks, slag, coal, etc. Soft crumbling 
materials such as coke are not suitable as the weight of the 
superimposed material and the movement of the sewage crushes 
and breaks it into fine particles which accumulate in the lower 
portion of the filter and clog it. Roughness, porosity, and small 
size are desirable, as the greater the surface area the more rapid 
the deposition of material. After a short time, however, the 
advantages of roughness and porosity are lost, as the sediment 
soon covers all unevenness alike. The minimum size of the 
material is limited by the tendency towards clogging. The sizes 
in successful use vary between \ and f of an inch, \ inch being 
a common size. The same size of material is used throughout 
the depth of the bed except that the upper 6 inches may be 
composed of small white pebbles or other clean material, which 
does not come in contact with the sewage and which will give 
an attractive appearance to the plant. In double or triple con- 
tact beds 3 or 4-inch material is sometimes used for the primary 
beds, and j-inch material in the final bed. 

Sewage may be applied at any point on or below the surface. 
The sewage is withdrawn from the bottom of the bed. It is 
undesirable to have too few inlet or outlet openings as the 
velocity of flow about the openings will be so great as to disturb 
the deposit on the contact material. The distribution system 
and the underdrains for the bed at Marion, Ohio, are shown in 
Fig. 166. 

The cycle of operation of a contact bed is divided into four 
periods. A representative cycle might be: tune of filling, one 
hour; standing full, 2 hours; emptying, one hour; standing 
empty, 4 hours. The length of these periods is the result of long 
experience based on many tests and are an average of the conclu- 
sions reached. Wide variations from them may be found in 
different plants, and tests may show successful results with 
different periods. The combination of these four periods is known 
as the contact cycle. 

The period of filling should be made as short as possible 
without disturbing the material of the bed nor washing off the 
accumulated deposits. The sewage should not rise more rapidly 
than one vertical foot per minute. During the contact or stand- 
ing full period sedimentation and adsorption of the colloids are 



THE TRICKLING FILTER 437 

occurring on the area of surface exposed to the sewage. This 
period should be of such length that septic action does not become 
pronounced, and long enough to permit of thorough sedimenta- 
tion. The period of emptying should be made as short as possible 
without disturbing the bed, on the same basis that the period 
of filling is determined. During the period of standing empty, 
air is in contact with the sediment deposited in thin layers on the 
contact material, and the oxidizing activities of the filter are taking 
place. The filter is given a rest period of one or two days 
every five or six weeks, in order that it may increase its 
capacity and it biologic activity. 

The control of a contact bed may be either by hand or auto- 
matic, the latter being the more common. Hand control requires 
the constant attention of an operator and results in irregularity 
of operation, whereas automatic control will require inspection 
not more than once a day and insures regularity of operation. 
A number of automatic devices have been invented which give 
more or less satisfaction. The air-locked automatic siphons, 
without moving parts, have proven satisfactory and are practi- 
cally " fool-proof." The operation of these devices is explained 
in Chapter XXI. 

257. The Trickling Filter. A trickling or sprinkling filter 
is a bed of coarse, rough, hard material over which sewage is 
sprayed or otherwise distributed and allowed to trickle slowly 
through the filter in contact with the atmosphere. A general 
view of a trickling filter in operation at Baltimore is shown in 
Fig. 167. The action of the trickling filter is due to oxidation 
by organisms attached to the material of the filter. The solid 
organic matter of the sewage deposited on the surface of the 
material, is worked over and oxidized by the aerobic bacteria, 
and is discharged in the effluent in a more highly nitrified con- 
dition. At times the discharge of suspended matter becomes 
so great that the filter is said to be unloading. The action differs 
from that in a contact bed in that there is no period of septic 
or anaerobic action and the filter never stands full of sewage. 

The effluent from a trickling filter is dark, odorless, and is 
ordinarily non-putrescible. Analyses of typical effluents are 
given in Tables 86 and 87. The unloading of the filter may occur 
at any time, but is most likely to occur in the spring or in a 
warm period following a period of low temperatures. It causes 



438 



FILTRATION AND IRRIGATION 



higher suspended matter in the effluent than in the influent 
and may render the effluent putrescible. The action is marked 
by the discharge of solid matter which has sloughed off of the 
filter material and which increases the turbidity of the effluent. 
Where the diluting water is insufficient to care for the solids so 
carried in the effluent, they can be removed by a 2-hour period 
of sedimentation. The effluent may become septic during this 
time, however. The nitrogen in the effluent is almost entirely 
in the form of nitrates, and the percentage of saturation with 
dissolved oxygen is high. The effluent is more highly nitrified 
than that from a contact bed, and its relative stability is also 
higher, thus demanding a smaller volume of diluting water. 




FIG. 167. Sprinkling Filter in Operation in Winter at Baltimore. 

The principal advantage of a trickling filter over other methods 
of treatment is its high rate which is from two to four times faster 
than a contact bed, and about seventy times faster than an inter- 
mittent sand filter. The greatest disadvantage is the head of 12 to 
15 feet or more necessary for its operation. Sedimentation of the 
effluent is usually necessary to remove the settleable solids. 
During the period of secondary sedimentation the quality of the 
filter effluent may deteriorate in relative stability. In winter the 
formation of ice on the filter results in an effluent of inferior 
quality, but as the diluting water can care for such an effluent 
at this time the condition is not detrimental to the use of the 
trickling filter. In summer the filters sometimes give off offen- 
sive odors that can be noticed at a distance of half a mile, and 
flying insects may breed in the filter in sufficient quantities to 



THE TRICKLING FILTER 



439 



become a nuisance if preventive steps are not taken. The dis- 
semination of odors is especially marked when treating a stale 
or septic sewage. The treatment of a fresh sewage seldom results 
in the creation of offensive odors. 

TABLE 86 

ANALYSIS OF CRUDE SEWAGE, IMHOFF TANK, AND SPRINKLING FILTER 
EFFLUENTS AT ATLANTA, GEORGIA 

(Engineering Record, Vol. 72, p. 4) 





. 

-1 
a 


Parts per Million 


turation, 
Oxygen 


X 

IS 


Nitrogen as 




Suspended Matter 




i t 










o 








OQ * 






























a 



Organic 


Free Am 
monia 


Nitrites 


Nitrates 


if 

Si 

x o 


"3 
e 
E-i 


Volatile 


1 

R 


Per Cent 
Dissol 


Relative 



Crude Se-vage 



1913 
Maximum 


77 
61 


15.6 
10 4 


21.8 
16.5 


0.1 
0.1 


3.0 
1.4 


100.0 
78 3 


371 
222 


154 
98 


163 
112 


47 
11 






70 


12.8 


18.8 


0.1 


2.2 


90.6 


285 


126 


138 


28 




1914 (7 months) 


74 


16.9 


33.4 




2.3 




431 






48 






60 


9.5 


18.1 




1.6 




279 






12 






60 


13.4 


27.1 




2.0 




351 






30 





























Imhoff Effluent 



1913 
Maximum 


78 
58 


13.2 
6.5 


21.9 
16.8 


0.2 
1 


3.1 
1 1 


68.0 
53 1 


90 
35 


50 
42 




41 

21 






68 


9 


20 


2 


2 1 


60 1 


68 


46 




33 




1914 (7 months) 
Maximum 
Minimum 


77 
59 


10.3 
4.1 


30.3 
18.0 




2.0 
1.5 




73 
49 






48 
34 






65 


7.7 


25.9 




1.8 




65 






43 





























Sprinkling Filter Effluent 



1913 
























Maximum 


79 


5.6 


14.2 


0.8 


11.3 


32.1 


60 


31 


28 


76 


99 


Minimum 


55 


2.6 


6.2 


0.5 


5.8 


23.6 


33 


26 


28 


52 


88 




M 


3.8 


9.9 


0.7 


8.2 


28 2 


49 


28 


28 


64 


89 


1914 (7 months) 
























Maximum 


77 


8.5 


20.7 




11.2 




106 






79 


99 


Minimum 


55 


4.4 


8.8 




3.6 




40 






55 


89 




63 


5.7 


15.2 




7.2 




62 






65 


95 



























440 



FILTRATION AND IRRIGATION 



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THE TRICKLING FILTER 441 

Raw sewage cannot be treated successfully on a trickling 
filter. Coarse solid particles should be screened and settled out, 
in order that the distributing devices or the filter may not become 
clogged. The effluent from an Imhoff tank has proven to be a 
satisfactory influent for a trickling filter. A septic tank effluent 
may be so stale as to be detrimental to the biologic action in the 
filter. 

In the operation of a trickling filter the sewage is sprayed 
or otherwise distributed as evenly as possible in a fine spray or 
stream, over the top of the filtering material. The sewage then 
trickles slowly through the filter to the underdrains through 
which it passes to the final outlet. The distribution of the 
sewage on the bed is intermittent in order to allow air to enter 
the filter with the sewage. The cycle of operation should be 
completed in 5 to 15 minutes, with approximately equal periods 
of rest and distribution. Cycles of too great length will expose 
the filter to drying or freezing and will give poorer distribution 
throughout the filter. Cycles which are too short will operate 
successfully only with but slight variation in the rate of sewage 
flow. In some plants it has been found advantageous to allow 
the filters to rest for one day in 3 to 6 weeks or longer, dependent 
on the quality of the effluent. 

The rate of filtration may be as high as 2,000,000 gallons per 
acre per day, which is equivalent to 200 gallons per cubic yard 
of material per day in a bed 6 feet deep. This is more than 
double the rate permissible in a contact bed. The exact rate 
to be used for any particular plant should be determined by 
tests. It is dependent on the quality of the sewage to be treated, 
on the depth of the bed, the size of the filling material, the 
weather, and other minor factors. 

The filtering material is similar to that used in a contact bed. 
It should consist of hard, rough, angular material, about 1 to 
2 inches in size. Larger sizes will permit more rapid rates of 
filtration, but will not produce so good an effluent. Smaller 
sizes will clog too rapidly. 

The depth of the filter is limited by the possibility of ventila- 
tion and the strength of the filtering material to withstand crush- 
ing. The deeper the bed the less the expense of the distribution 
and collecting system for the same volume of material, and the 
more rapid the permissible rate of filtration. The depths in 



442 FILTRATION AND IRRIGATION 

use vary between 6 and 10 feet, with 6 to 8 feet as a satisfactory 
mean. From a biologic standpoint the action of the filter seems 
to be proportional to the volume of the filtering material and 
therefore proportional to the depth of the bed, being limited to 
a minimum depth of about 5 feet, below which sewage may pass 
through the filter without treatment. The shape and other 
dimensions of the filter depend on the local conditions and the 
economy of construction. The filters need not be broken up 
into units by water-tight dividing walls. One filter can be 
constructed sufficient for all needs and various portions of it can 
be isolated as units by the manipulation of valves in the dis- 
tribution system. Ventilation is provided by the air entrained 
with the sewage as it falls upon the surface. If the sides of the 
filter are built of open stone crib work the ventilation will be 
greatly improved, but it will not be possible to flood the filters 
to keep down flies, and in cold climates these openings must be 
covered in winter to prevent freezing. Filters have been con- 
structed without side walls, the filtering material being allowed 
to assume its natural angle of repose. This has usually been 
found to be more expensive than the construction of side retaining 
walls, due to the unused filling material and the extra under- 
drains required. 

The distribution of sewage is ordinarily effected by a system 
of pipes and spray nozzles as shown in Fig. 168 and 169. Other 
methods of distribution have been used. At Springfield, Mo., 1 
a moving trough from which the sewage flows continuously is 
drawn back and forth across the bed by means of a cable. In 
England circular beds have been constructed and the sewage 
distributed on them through revolving perforated pipes. At 
the Great Lakes Naval Training Station 2 the distributing pipes 
in the plant, now abandoned, were supported above the surface 
of the filter. The sewage fell from holes in the lower side of these 
pipes on to brass splash plates 14 inches above the filter. It 
was deflected horizontally from these plates over the filter sur- 
face. Pipes and spray nozzles have been adopted almost univer- 
sally in the United States. Splash plates, traveling distributors, 
and other forms of distribution have been used only in excep- 

1 See Eng. News, Vol. 70, 1913, p. 1112; Eng. Record, Vol. 68, 1913, p. 
440, and Eng. News, Vol. 75, 1916, p. 1028. 
*See Eng. Record, Vol. 67, 1913, p. 232. 



THE TRICKLING FILTER 



443 



tional cases. In a distributing system consisting of pipes and 
nozzles, a network of pipes is laid out somewhat as shown in 
Fig. 168, in such a manner that the head loss to all points is 
approximately equal. The number of valves required should 
be reduced to a minimum. The pipes may be laid out with the 
main feeders leading from a central point and branches at right 
angles to them, somewhat on the order of a spider's web, or they 
may be laid out on a rectangular or gridiron system. The 
radial system is advantageous because of the central location 



16'C.I. Pipe Distributors 




''^Crushed Stone Filling I 'to 2 

.-Port Hole 
fel 

4 




tyW!'-"W / p, 

^-Flushing 

Gallery 
Cross Section A-A. 

Grade 0.00 ?5 '-, 




21 "Side Drain 



... , 



^f 12" C.I. Pipe 



No:7-l -?''- 



6"C.f.H'aferMa/n^ 6"Disch Pipe 



-V 



Grade 0.0028- 



A '-W'By-rtiss 

Drain 
Plan 



FIG. 168. Section through Sprinkling Filter at Fitchburg, Mass., Showing 

Distribution System. 
Eng. Record, Vol. 07, p. 634. 

of the control house, but it does not always lend itself favorably 
to the local conditions, and the piping and nozzle location are 
not so simple. The gridiron system lends itself favorably to 
the equalization of head losses. The pipes used should be larger 
than would be demanded by considerations of economy alone, 
both for the purpose of reduction of head loss and ease in cleaning. 
No pipe less than 6 inches in diameter should be used, and the 
average velocity of flow should not exceed one foot per second. 
Cast iron, concrete, or vitrified clay pipe may be used, but cast 



444 FILTRATION AND IRRIGATION 

iron is the material commonly used. The system should be 
arranged for easy flushing and cleaning and the pipes so sloped 
that the entire system can be drained in case of a shut down 
in cold weather. 

The pipes are placed far enough below the surface of the 
filling material so that the top of the spraying nozzle is 6 to 
12 inches above the surface of the filter. If the pipes are placed 
near the surface they are accessible for repairs, but are exposed 
to temperature changes. If the pipes are large their presence 
near the surface of the filter may seriously affect the distribution 
of the sewage through the filter. If the distributing pipes are 
placed near the bottom of the filter they are inaccessible for 
repairs and the nozzles must be connected to them by means 
of long riser pipes. The distributing pipes should be supported 
by columns extending to the foundation of the filter bed, there 
being a column at every pipe joint with such intermediate sup- 
ports as may be required. In some plants the pipes have been 
supported by the filtering material. Although slightly less 
expensive in first cost the practice of so supporting the pipes is 
poor, as settling of the material may break the pipe or cause 
leaks, and if the bed becomes clogged, removal of the material 
is made more difficult. Valves should be placed in the distribut- 
ing system in such a manner that different sets of nozzles can 
be cut out at will, thus resting those portions of the filter 
and permitting repairs without shutting down the entire 
filter. 

The spacing of the nozzles is fixed by the type and size of the 
nozzle, the available head, and the rate of filtration. Various 
types of sprinkler nozzles are shown in Fig. 169 and the dis- 
charge rates, head losses, and distances to which sewage is 
thrown for the Taylor nozzles, are shown in Fig. 170. Nozzles 
are available which will throw circular, square, or semicircular 
sprays. In the use of circular sprays there is necessarily some 
portion of the filter which is underdosed if the nozzles are placed 
at the corners of squares with the sprays tangent, and there is 
an overdosing of other portions if the sprays are allowed to 
overlap so that no portion of the filter is left without a dose. 
Rectangular sprays will apparently overcome these difficulties, 
but studies have shown that circular sprays with some over- 
lapping, and the nozzles placed at the apexes of equilateral tri- 



THE TRICKLING FILTER 



445 



A'Dfamefer 
of Orifice. 
B* Diameter 
ofSpindle 
at Orifice. 




Prieslman-Beddoes Weand, Atlantic Type 
Round Nonle. Round Noiile. Worcester 1 Round 

FIG. 169. Sprinkling Filter Nozzles. 

Bulletin No. 3, Engineering Experiment Station, Purdue University. 



30 28 



Discharge, Gallons per Mirru+e 
26 24 22 20 18 16 




10 



VL 14 16 

Nozzle Spacing, Feet., 



FIG. 170. Diagram Showing the Discharge and Spacing of Taylor Nozzles. 



446 



FILTRATION AND IRRIGATION 



angles as shown in Fig. 172 will give as satisfactory distribution 
as other forms. 

The nozzles should be selected to give the best distribution, 
to consume all of the head available, and to give the proper 
cycle of operation. The entire head available should be consumed 
in order that the fewest number of nozzles may be used. An 
excellent study of the characteristics of various types of nozzles 
has been published in Bulletin No. 3 of the Engineering Experi- 
ment Station at Purdue University, 1920. As a result of the 
tests on the nozzles shown in Fig. 169, it was determined for all 
nozzles, except No. 8, that 

Q=CaV2gh; 

in which Q =the rate of discharge in cubic feet per second; 
C = a coefficient shown in Table 88 ; 
a=the net cross-sectional opening of the nozzle in 

square feet; 
h =the pressure on the nozzle in feet of water. 

TABLE 88 
COEFFICIENTS OF DISCHARGE FOR SPRINKLER NOZZLES SHOWN IN FIG. 169 



Nozzle Number 


1 


2 


3 


4 


5 


6 


7 


















Coefficient 


648 


.756 


.696 


.666 


.675 


.598 


.569 



















It is evident that if the head on the nozzles is constant and the 
nozzle throws a circular spray, the intensity of dosing at the 
circumference will be greater than nearer the center. This 
difficulty is overcome by so designing the dosing tank from which 
the sewage is fed that the head on the nozzle and the quantity 
thrown will vary in such a manner that the distribution over 
the bed is equalized. Intermittent action is obtained by an 
automatic siphon which commences to discharge when the tank 
is full and empties the tank in the period allowed for dosing. 
Under such conditions the tank should discharge for a longer 
time at the higher heads than at the lower heads as there is 
more territory to be covered at the higher heads. The design 
of the tank to do this with exactness is difficult, and the con- 
struction of the necessary curved surfaces is expensive. Where 



THE TRICKLING FILTER 



447 



a dosing tank is used for such conditions it has been found satis- 
factory to construct the tank with plane sides sloping at approxi- 
mately 45 degrees from the vertical (or horizontal). A tank 
with curved surfaces is shown in Fig. 171. The dosing siphon 
is usually placed in the tank as shown in the figure. The head 
and quantity of discharge through the nozzles can be varied 
also by maintaining a constant depth in a dosing tank by means 




FIG. 171. Section of 12-inch Siphon and Dosing Tank, for King's Park, Long 

Island. 

of a float feed valve, and varying the head and quantity dis- 
charged to the nozzles by a butterfly valve in the main feed line, 
or by the use of a Taylor undulating valve designed for this 
purpose. The butterfly valve is opened and closed by a cam 
so designed and driven at such a rate that the required distribu- 
tion is obtained. The Taylor undulating valve is opened and 
closed at a constant rate, the shape of the valve giving the 
required variations in head and discharge Other methods 
of control have been attempted but have not been used exten- 
sively. 

An example of the design of the nozzle layout and dosing 
tank for a sprinkling filter follows: 



448 



FILTRATION AND IRRIGATION 



Let it be required to determine the nozzle layout 
for one acre of sprinkling filters with 5 feet available head 
on the nozzles. 

The selection of the type of nozzle and the size of 
opening is a matter of judgment and experience. Noz- 
zles with large openings are less liable to clog and fewer 
nozzles are needed than where small nozzles are used, 
but the distribution of sewage is not so even as with the 
use of small nozzles. In this example Taylor circular 
spray nozzles will be selected. Fig. 170 shows that a 
Taylor circular spray nozzle will discharge 22.3 g.p.m. 
under a head of 5 feet, and that the economical nozzle 
spacing will be 15.3 feet. The least number of nozzles 
at this spacing required for a bed of one acre in area is 
found as follows: In Fig. 172, let n equal the number of 




FIG. 172. Typical Sprinkler Nozzle Layout. 

nozzles in a horizontal row, counting half-spray nozzles as |, 
and let ra equal the number of rows counting rows of half- 
spray nozzles as half rows. 1 Then the number of nozzles, 
N, equals ran, and 15.3raXl3.2n equals 43,560 or ran 
equals 215. 

The next step should be the design of the dosing tank and 
siphon. It is possible to design a tank which will give equal 
distribution over equal areas of filter surface. It has been 

1 The use of half-spray nozzles is not always advocated as it is consid- 
ered that their use does not markedly improve the distribution. Where hah" 
nozzles are used, a margin of 18 inches to 2 feet should be allowed between 
the edge of the filter and the nozzle, to prevent the blowing of raw sewage 
from the filter. 



THE TRICKLING FILTER 449 

found, however, that the expense of this refinement is unwar- 
ranted as there are a number of outside factors which tend to 
overcome the theoretical design. The effect of wind, unequal 
spacing, and irregularities in the elevation of the nozzles have a 
tendency to offset refinements in the design of a dosing tank. 
It is therefore the general practice to slope the sides of the tank 
at an angle of about 45 degrees as previously stated. The 
dosing tank is generally designed to have a capacity which will 
give a complete cycle of operation once in 15 minutes. In the 
ordinary design the factors given are the rate of inflow and the 
given time of filling. In the following example the time of filling 
will be taken as 10 minutes, the time of emptying as 5 minutes, 
and the rate of flow as 1,000,000 gallons per day. The capacity 

1 000 000 
of the tank will therefore be ' ' =7,000 gallons. The 

diameter of the siphon to be selected can be computed as follows : 

Let Q =the capacity of the tank in cubic feet; 

q\ = the rate of discharge of the siphon in cubic feet per second ; 
Q2 = the rate of inflow to the tank in cubic feet per second ; 
q = the rate of emptying the tank in cubic feet per second = 

(91-92); 
A =the cross-sectional area of the free surface of the water 

in the tank at any instant, in square feet; 
a =the cross-sectional area of the siphon in square feet; 
6 =the small dimension of the base of the tank in feet; 
h =the head of water, in feet, on the discharge siphon; 
hi =the initial head of water, in feet, on the siphon; 
/i2 = the final head of water in feet, on the siphon ; 
t =the time, in seconds, required to empty the tank, 

then 
and 

but 

Adh 
therefore t= 



but A =4fc 2 +46/i+& 2 , 

r h *(b 2 
= I 

J h , Q 



AU f +4bh+4h 2 )dh 

therefore t 



AaV2gh-q 2 

1 From paper by E. G. Bradbury in Proceedings of the Ohio Eng. Society, 
1910, p. 79. 



450 



FILTRATION AND IRRIGATION 



The integration of this expression is tedious. Its solution 
for siphons between 6 inches and 12 inches operating under 
heads commencing from 3 feet to 6 feet, with a time of emptying 
of 5 minutes and time of filling of 10 minutes is given in Fig. 
173. In the example given the rate of inflow is 1.55 sec. feet 
and the head is 5 feet. Then from Fig. 173 the size of the siphon 
to be used is 12 inches. Where a siphon of the size required 

Rate of Inflow; Cubic Feet per Second 
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 .1.7 




2000 



7000 



8000 



3000 4000 5000 6000 

Capacity of Tank in Gallons. 

FIG. 173. Diagram for the Determination of the Capacities of Dosing Tanks 
for Trickling Filters. 

Time of emptying, 5 minutes. Time of filling, 10 minutes. Shape of tank is a right pyramid 
or a truncated right pyramid with all four sides making an angle of 45 degrees with the ver- 
tical. All horizontal cross-sections are squares. 

to empty the tank in the time fixed is not available, combinations 
of available sizes can sometimes be used. 

For example, if the given head is 6 feet, and the rate 
of inflow is 1.4 sec. feet, it is evident from Fig. 173 that 
a 6,300-gallon dosing tank and two 8-inch siphons will 
give the required cycle. 

The method used for the design of the setting of Taylor 
nozzles by the Pacific Flush Tank Co., is less rational but more 
simple and probably as satisfactory. In this method the steps 
are as follows: 

(1) Divide the maximum daily rate of sewage flow by 
. 1,000 to get the maximum minute inflow. 






THE TRICKLING FILTER 451 

(2) The number of nozzles required is determined by 
dividing the preceding figure by 6. Generally a Taylor 
nozzle with an orifice of | of an inch will discharge about 
20 g.p.m. at the high head and about 8 g.p.m. at the low 
head, and as the nozzles must have a capacity which 
will take care of the inflow at the low head, the divisor 6 
is used as a factor of safety instead of using 8 as the 
divisor. 

(3) The type of nozzle to be used is selected from 
experience or as a matter of judgment. Circular-spray 
nozzles are more generally used. 

(4) The spacings are determined from Fig. 170. 

(5) The dosing tank of the shape described is then 
designed. The capacity is such as to give a complete 
cycle once every 15 minutes. The method of this design 
is similar to that followed previously. 

(6) The dosing siphons are designed so that they will 
have a capacity at the minimum head of from 40 to 50 
per cent in excess of the maximum minute inflow, and the 
draining depth of the siphon will be limited to a maximum 
of 5 to 5^ feet. The siphons are all made adjustable with 
a variation of 6 inches or more on either side of the normal 
discharge line so that the spraying area and cycle can be 
varied to secure the best results. 

The underdrainage of a trickling filter should consist of some 
form of false bottom such as the types shown in Fig. 174. Where 
possible the underdrains should be open at both ends for the 
purpose of ventilation and flushing. It is desirable that the 
drains be so arranged that a light can be seen through them in 
order that clogging can be easily located. The drains should be 
placed on a slope of approximately 2 in 100 towards a main 
collector. The length of the drains is limited by their capacity 
to carry the average dose from the area drained by them. 
The main collecting conduits must be designed in accordance 
with the hydraulic principles given in Chapter IV. No valves, 
or other controlling apparatus, are placed on the underdrains 
or outlets from the filter. 

Covers have been provided in winter for some trickling 
filters in cold climates. The Taylor sprinkling nozzle has been 
found to work successfully in extremely cold weather, and it is 
generally accepted that the covering of filters is unnecessary, if 
the filter is not to be shut down for any length of time in cold 
weather. 



452 



FILTRATION AND IRRIGATION 



The operation of devices for automatically controlling the 
operation of a trickling filter is explained in Chapter XXI. 

258. Intermittent Sand Filter. An intermittent sand filter 
is a specially prepared bed of sand, or other fine grained material, 
on the surface of which sewage is applied intermittently, and 
from which the sewage is removed by a system of underdrains. 
It differs from broad irrigation in the character of the material, 
the care and preparation of the bed, and the thoroughness of the 
underdrainage. A distinctive feature of the intermittent sand 



Slotted 
Vitrified 
Half 
Tile. 




Type C. Type D7 Type C-Z. 

FIG. 174. Types of False Bottoms for Trickling Filters. 
Eng. News, Vol. 74, p. 5. 

filter is the quality of the effluent delivered by it. In a properly 
designed and operated plant the effluent is clear, colorless, odor- 
less, and sparkling. It is completely nitrified, is stable and con- 
tains a high percentage of dissolved oxygen. It contains no 
settleable solids except at widely separated periods when a small 
quantity may appear in the effluent. The percentage removal 
of bacteria may be from 98 to 99 per cent. Some analyses of 
sand filter effluents are given in Table 89. The dissolved solids, 
the remaining bacteria, and the antecedents of the effluent are 
the only differences between it and potable water. An effluent 
from an intermittent sand filter is the most highly purified 



INTERMITTENT SAND FILTER 



453 







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454 FILTRATION AND IRRIGATION 

effluent delivered by any form of sewage treatment. The 
effluent can be disposed of without dilution, on account of its 
high stability. The treatment of sewage to so high a degree is 
seldom required, so that the use of intermittent filters is not 
common. Other drawbacks to their use are the relatively large 
area of land necessary and the difficulty of obtaining good filter 
sand in all localities. 

The action in an intermittent sand filter is more complete 
than in other forms of filters because a greater surface is exposed 
to the passage of sewage by the fine sand particles, and the 
sewage is in contact with the filtering material a longer time 
due to the lower rate of filtration and the slow velocity of flow 
through the filter. It is essential that the sewage be applied 
to the bed intermittently in order that air shall be entrained in 
the filter. The period between doses should not be so long that 
the filter becomes dry. 

In the operation of an intermittent sand filter one dose per 
day is considered an ordinary rate of application, although 
some plants operate with as many as four doses per day per filter, 
and others on one dose at long and irregular intervals. It is 
not always necessary to rest the filter for any length of time unless 
signs of overloading and clogging are shown. The intermittent 
dosing action may be obtained by the action of an automatic 
siphon as is described in Chapter XXI. The sewage is distributed 
on the beds through a number of openings in the sides of distribut- 
ing troughs resting on the surface of the filter. The sewage is 
withdrawn from the bottom of the filter through a system of 
underdrains, into which it enters after its passage through the 
bed. There are no control devices on the outlet, as the rate of 
filtration is controlled by the action of the dosing apparatus 
and the rate at which sewage is delivered to it. The action 
of the dosing apparatus should respond quickly to variations 
in sewage flow. As the doses are applied to a sand filter, a mat 
of organic matter or bacterial zooglea is formed on the surface 
of the bed. The mat is held together by hair, paper, and the 
tenacity of the materials. It may attain a thickness of j to \ 
an inch before it is necessary to remove it. So long as the filter 
is draining with sufficient rapidity this mat need not be removed, 
but if the bed shows signs of clogging, the only cleaning that 
may be necessary will be the rolling up of this dried mat. It 



INTERMITTENT SAND FILTER 455 

is believed that the greater portion of the action in the filter 
occurs in the upper 5 to 8 inches of the bed, but occasionally the 
beds become so clogged that it is necessary to remove f of an 
inch to 2 inches of sand in addition to the surface mat, or to 
loosen up the surface by shallow plowing or harrowing. The 
necessity for such treatment may indicate that the filter is being 
overloaded as a result of which the rate of filtration should be 
decreased or the preliminary treatment should be improved. 
The plowing of clogging material into the bed should be avoided 
as under these conditions the final condition of the bed will be 
worse than its condition when trouble was first observed. 

In winter the surface of the bed should be plowed up into 
ridges and valleys. The freezing sewage forms a roof of ice 
which rests on the ridges and the subsequent applications of 
sewage find their way into the filter through the valleys under 
the ice. In a properly operated bed the filtering material will 
last indefinitely without change. If a filter is operated at too 
high a rate, however, although the quality of the effluent may be 
satisfactory, it will be necessary at some time to remove the sand 
and restore the filter. 

The rate of filtration depends on the character of the influent, 
the desired quality of the effluent, and the depth and character 
of the filtering material. Filters can be found operating at rates 
of 50,000 gallons per acre per day and others at eight times this 
rate. For sewage which has had some preliminary treatment, the 
rate should not exceed 100,000 gallons per acre per day, whereas 
the rate for raw sewage should be less than this. For rough 
estimates made without tests of the sewage in question, the rate 
should not be taken at more than 1,000 persons per acre. If the 
preliminary treatment of the sewage has been thorough and the 
material of the sand filter is coarser than ordinary the rate of 
filtration can be high. For less careful preliminary treatment 
and fine filtering material the rates must be reduced. The 
sewage must undergo sufficient preliminary treatment to remove 
large particles of solid matter which would otherwise clog the 
dosing apparatus and the filter. This treatment should include 
grit removal, screening, and some form of tank treatment. 
Some plants have operated successfully with a stale sewage and 
no preliminary treatment, as at Brockton, Mass. Septic tank 
effluent can be treated successfully on an intermittent sand 



456 FILTRATION AND IRRIGATION 

filter, but not so satisfactorily as the effluent from a tank 
delivering a fresh sewage. 

The material of the filter should consist of clean, sharp, 
quartz or silica sand with an effective size l of 0.2 to 0.4 mm., 
preferably about 0.25 to 0.35 mm., and a uniformity coefficient 2 
of 2 to 4. Within the limits mentioned no careful attention 
need be given to the size of the material. Natural sand found 
in place has been underdrained and used successfully for sewage 
treatment. The size of the sand is fixed by the rate of filtra- 
tion rather than the bacteriological action of the filter. A 
coarse sand will permit the sewage to pass through the bed too 
rapidly, and a fine sand will hold it too long or will become 
clogged. The same size of material should be used throughout 
the bed, except that a layer of gravel from 6 to 12 inches thick, 
graded from very small sizes to stones just passing a 2-inch 
ring should be placed at the bottom to facilitate the drainage of 
the bed. 

The thickness of the sand layer should not be less than 30 
inches to insure complete treatment of the sewage. In shallower 
beds the sewage might trickle through without adequate treat- 
ment. Beds are ordinarily made from 30 to 36 inches deep, 
but when deeper layers of sand are found in place there is no set 
limit to the depth which may be used. The shape and overall 
dimensions of the bed should conform to the topography of the 
site and the rate of filtration adopted. A plan and cross-section 
of an intermittent sand filter showing the distribution and 
underdrainage systems are given in Fig. 166 and 175. 

The distribution system consists of a system of troughs on 
the surface of the filter, laid out in a branching form, as shown 
in the figure. The openings in the troughs should be so 
located that the maximum distance from any point on the bed 
to the nearest opening should not exceed 20 to 30 feet. If the 
filters are small enough, troughs need not be used, the sewage 
being distributed from one corner, or from mid-points on the 
sides. Where troughs are used they should be supported from 

1 The effective size of sand is the diameter in millimeters of the largest 
grain in that 10 per cent, by weight, of the material which contains the 
smallest grains. 

2 The uniformity coefficient is the ratio of the diameter of the largest 
particle of the smallest 60 per cent, by weight, to the effective size. 



INTERMITTENT SAND FILTER 



457 



the bottom of the filter in order to prevent uneven settling due 
to the washing of the sand. The openings in the troughs are made 
adjustable by swinging gates as shown in Fig. 176, or by other 
means so that after the filter is in operation the intensity of the 
dose on any portion of the filter can be changed. The troughs 
may be placed with their bottoms level with the surface of the 
sand and with sides of sufficient height to give the required gra- 
dient to the water surface, or they may be built up above the sur- 




^ : Sand Filter 
Section on Line A-B. 

FIG. 175. Plan and Section of an Intermittent Sand Filter Showing Central 
Location of Control House. 

face of the filter and given the required slope so that the surface 
of the flowing water is parallel to the bottom of the trough. 
In either case a splash plate should be placed at each opening, 
so that not less than 2 feet of the surface of the sand is protected 
in all directions from the opening. A stone or concrete slab 
2 to 4 inches thick makes a satisfactory splash plate. Either 
wood or concrete may be used for the construction of the 
troughs. The former is less durable, but also less expensive 



458 



FILTRATION AND IRRIGATION 



in first cost. The capacity of the troughs may be computed 
by Kutter's formula with the quantity to be carried equal to the 
maximum rate of discharge of the feeding siphon, with a reduc- 
tion in size below each branch or outlet proportional to the 
amount which will be discharged above this point. 

The operation of automatic devices for dosing the bed is 
explained in Chapter XXI. The dosing tank should have a 
capacity sufficient to cover the bed to a depth of about 1 to 3 
inches at one dose, and the siphon should discharge at a rate of 
about one second-foot for each 5,000 square feet of filter area. 
A dose should disappear within 20 minutes to half an hour after 
it is applied to the filter. With the rate stated and four appli- 




FIG. 176. Distributing Trough with Adjustable Openings. 

cations per day to a depth of 1 inch at each dose, the rate per 
acre per day will be 109,000 gallons. 

The filtration of sewage through sand in a manner similar 
to the rapid sand filtration of water is being attempted at the 
Great Lakes Naval Training Station. No results of this treat- 
ment have been published and the practical success of the method 
has not been assured. 

259. Cost of Filtration. Only comparative figures can be 
given in stating the costs of filtration, as most data available 
are based on pre-war conditions, and are therefore unreliable 
for present conditions. The variations from the figures given 
may be very large but in general the relative costs have not 
changed. The figures given in Table 90 are suggestive of the rela- 
tive costs of the different forms of filtration. 



THE PROCESS 



459 



TABLE 90 

RELATIVE COSTS OF DIFFERENT MKTIIODS OF SEWAGE TREATMENT 
Costs in Dollars per Million Gallons per Day 



Form of Treatment 


First Cost * 


Operation 
and 
Maintenance 


Total 


Coarse screens 






0.20 


Fine screens 






3.00 


Plain sedimentation 


7.00 


3.00 


10 00 


Chemical precipitation 






22.00f 


Septic tank 


7.00 


1.00 


8.00 


Imhoff tank 


10.00 


1.00 


11.00 


Contact bed 


8.00 


2.00 


10.00 


Trickling filter 


4 00 


2 00 


6 00 


Intermittent sand filter 


15 00 


10 00 


25 00 


Activated sludge 


6 50 


8 50 


15 OOJ 











* Interest at 6 per cent, 
the sale of sludge. 



t Worcester figures. t This method may show a profit from 



IRRIGATION 

260. The Process. Broad irrigation is the discharge of 
sewage upon the surface of the ground, from which a part of 
the sewage evaporates and through which the remainder perco- 
lates, ultimately to escape in surface drainage channels. Sewage 
farming is broad irrigation practiced with the object of raising 
crops. Broad irrigation can be accomplished successfully with- 
out the growing of crops, but it is seldom attempted as some 
return and sometimes even a profit can be obtained from the 
crops raised. Broad irrigation and sewage farming differ from 
intermittent sand filtration in the intensity of the application 
of the sewage, the method of preparing the area on which the 
sewage is to be treated, and the care in operation. In broad 
irrigation and intermittent sand filtration the paramount con- 
sideration is successful disposal of the sewage. In sewage farming 
the paramount consideration is the growing of crops. The 
growing of crops may be combined with irrigation and filtration, 
however, but the crop should be sacrificed to the successful 
disposal of the sewage. 



460 FILTRATION AND IRRIGATION 

The change which occurs in the characteristics of the sewage 
due to its filtration through the ground is the same as occurs in 
aerobic filtration. The effect on the crops is mainly that of an 
irrigant, as the manurial value of the sewage is small. 

261. Status. The disposal of sewage by broad irrigation was 
practiced in England previous to the development of any of the 
more intensive biologic methods of treatment. It was con- 
sidered the only safe and sanitary method for the disposal of 
sewage, and as a result, areas irrigated by sewage were common 
throughout England. Crops were grown on these areas as a 
minor consideration, and sewage farming gained some of its 
popularity from the apparent success of these disposal areas. 
The success of sewage farms is due more to generous irrigation 
in dry years than to fertilization by sewage. 

The sewage farms of Paris and Berlin are frequently cited 
as examples of the successful and remunerative disposal of 
sewage by farming in connection with broad irrigation. Kinni- 
cutt, Winslow, and Pratt l state: 

The Berlin Sewage farms offer examples of broad 
irrigation under better conditions ... of 21,008 acres 
receiving sewage, 16,657 acres were farmed by the city, 
3,956 acres were leased to farmers, and only 395 acres 
were unproductive. The contributing population at this 
time was 2,064,000 and the average amount of sewage 
treated was 77,000,000 gallons, giving a daily rate of 
treatment of about 3,700 gallons per acre of prepared 
land. The soil is sandy and of excellent quality. A quarter 
of the area operated by the authorities is devoted to 
pasturage, and about a third to the cultivation of cereals, 
of which winter rye and oats are the most important. 
Potatoes and beets are grown in considerable amounts 
and a wide variety of other crops in smaller proportions. 
. . . Even fish ponds are made to yield a part of the 
revenue, and the drains on some of the farms have been 
successfully stocked with breed trout. 

The cost of the Berlin farms to March 31, 1910, 
was $17,470,000, somewhat more than half being the 
purchase price of the land. The expenses for this year 
amounted to $1,300,385 for maintenance, and $741,818 
for interest charges. The receipts were $1,240,773 and 
there was an estimated increase of $122,593 in value 
of live stock and other property. 

1 Sewage Disposal, 1919, p. 223. 



PREPARATION AND OPERATION 461 

The conditions at Berlin are quoted at length to indicate the 
success which can accompany broad irrigation, and as an 
example of what is being done abroad, where the rainfall is light 
and the soil is suitable. 

In the United States success in sewage farming has not been 
marked. This may be due partially to the relative weakness of 
American sewages, to the cost of labor, to lack of satisfactory 
irrigation areas, and to inattention to details. An attempt was 
made to grow crops on the sand filters at Brockton, Mass., but 
it was finally abandoned as the interests of the crops and the 
successful treatment of the sewage could not both be satisfied. 
At Pullman, Illinois, 1 in 1880, there was commenced probably 
the most extensive attempt at sewage farming in eastern 
United States. The farm was a failure from the start, because 
of the clay soil, and it was subsequently abandoned. Sewage 
farming, mainly as a subsidiary consideration to the filtration 
of sewage, is practiced in a few cities in the eastern portion of 
the United States to-day. Among the cities mentioned by 
Metcalf and Eddy 2 are Danbury, Conn., and Fostoria, Ohio. 
In the western portion of the United States where water is 
scarce and the ground is porous, sewage has been used as an irri- 
gant with some success. Such use of sewage cannot be considered 
as a method of treatment since the prime consideration is the 
growing of crops. In this process all sewage not used as an 
irrigant is discharged without treatment into water courses. 
According to Metcalf and Eddy there were 35 cities in Cali- 
fornia in 1914 that were operating sewage farms. Among 
these are Pasadena, Fresno, and Pomona. Other farms, notably 
the pioneer farm at Cheyenne, Wyo., have been abandoned 
because of the local nuisance created and the lack of financial 
success. 

262. Preparation and Operation. A porous sandy soil on 
a good slope and with good underdrainage is most suitable for 
broad irrigation. Impervious clay or gumbo soils are unsuitable 
and should not be used. They become clogged at the surface, 
forming pools of putrefying sewage, or in hot weather form cracks 
which may permit untreated sewage to escape into the underdrains. 

The sewage may be distributed to the irrigated area in any 

1 See Eng. News, Vol. 9, 1883, p. 203, and Vol. 29, 1893, p. 27. 
1 American Sewerage Practice, Vol. III. 



462 FILTRATION AND IRRIGATION 

one of five ways which are known as: flooding, surface irriga- 
tion, ridge and furrow irrigation, filtration, and sub-surface 
irrigation. In flooding, sewage is applied to a level area 
surrounded by low dikes. The depth of the dose may be from 
1 inch to 2 feet. In surface irrigation the sewage is allowed to 
overflow from a ditch over the surface of the ground into which 
it sinks or over which it flows into another ditch placed lower 
down. This ditch conducts it to a point of disposal or to another 
area requiring irrigation. Ridge and furrow irrigation consists 
in plowing a field into ridges and furrows and filling the furrows 
with sewage while crops are grown on or between the ridges. 
In filtration the sewage is distributed in any desired fashion on 
the surface and is collected by a system of underdrains after it 
has filtered through the soil. In subsurface irrigation the sewage 
is applied to the land through a system of open joint pipes laid 
immediately below the surface, similarly to a system of under- 
drains. Combinations of and modifications to these methods 
are sometimes made. Underdrains may be used hi connection 
with any of these forms of distribution. 

The preparation of the ground consists in: the construction 
of ditches or dikes to permit of any of the above described 
methods of application, grading of the surface to prevent 
pooling, the laying of underdrains, and the grubbing and clearing 
of the land. The main carriers may be excavated in open earth 
or earth lined with an impervious material. The distribution 
of the sewage from the main carriers to groups of laterals may 
be controlled by hand-operated stop planks. If the soil has a 
tendency to become waterlogged it may be relieved by install- 
ing underdrains at depths of 3 to 6 feet, and 40 to 100 feet 
apart. The tile underdrains may discharge into open ditches 
excavated for the purpose which serve also to drain the land. 
Drains should be used where the ground water is within 4 feet 
of the surface, and the open ditches should be cut below the 
drains to keep the ground water out of them. Four or 6-inch 
open-joint farm tile may be used for underdrains. The porosity 
of the soil will be increased by cultivation. Where particular 
care is taken in the cultivation of the soil so that sewage can 
be applied at a high rate, broad irrigation merges into the more 
intensive intermittent filtration through sand. 

Before being turned on to the land, sewage should be screened 



SANITARY ASPECTS 463 

and heavy-settling particles should be removed. The rate of 
application may be increased as the intensity of the preliminary 
treatment is increased. The rate at which sewage may be 
applied is dependent also on the character of the soil, and may 
vary between 4,000 and 30,000 gallons per acre per day, although 
higher rates have been used with the effluent from treatment 
plants and on favorable soil. The sewage should be applied 
intermittently in doses, the time between doses varying between 
one day and two or three weeks or more, dependent on the 
weather and the condition of the soil. The methods of dosing 
vary as widely as the rates. The dose may be applied con- 
tinuously for one or two weeks with correspondingly long rests, 
or it may be applied with frequent intermittency alternated 
with short rests, interspersed with long rest periods at longer 
intervals of time. When applying the sewage to the land the 
rate of application of the dose is about 10,000 to 150,000 gallons 
per acre per day. The area under irrigation at any one time 
may be as much as 10 to 15 acres. The rate of the application 
of the sewage is also dependent on the weather and may vary 
widely between seasons. It is obvious that a rain-soaked 
pasture cannot receive a large dose of sewage without danger 
of undue flooding. One of the principal difficulties with the 
treatment or disposal of sewage by broad irrigation is that the 
greatest load of sewage must be cared for in wet seasons when the 
ground is least able to absorb the additional moisture. 

263. Sanitary Aspects. A well-operated sewage farm should 
cause no offense to the eye or nose, and is not a danger to the 
public health. In Berlin, a portion of the sewage farms are 
laid out as city parks. The liquid in the drainage ditches or 
underdrains may be clear, odorless, and colorless, high in nitrates 
and non-putrescible. Where the farm has been improperly 
managed or overdosed the condition may be serious from both 
esthetic and health considerations. Sewage may be spread 
out to pollute the atmosphere and to supply breeding places for 
flying insects which will spread the filth for long distances sur- 
rounding the farm. The character of the crop is also a sani- 
tary consideration. 

264. The Crop. From a sanitary viewpoint no crops which 
come in contact with the sewage should be cultivated on a 
sewage farm. Such products as lettuce, strawberries, asparagus, 



464 



FILTRATION AND IRRIGATION 



potatoes, radishes, etc., should not be grown. Grains, fruits, 
and nuts are grown successfully and as they do not come in con- 
tact with the sewage there is no sanitary objection to their 
cultivation in this manner. Italian rye grass and other forms 
of hay are grown with the best success as they will stand a 
large amount of water without injury. The raising of stock 
is also advisable for sewage farms where hay and grain are culti- 
vated. The stock should be fed with the fodder raised on the 
irrigated lands and should not be allowed to graze on the crops 
during the time that they are being irrigated. This is due as 
much to the danger of injury to the distributing ditches and 
the formation of bogs by the trampling of the cattle, as to the 
danger to the health of the cattle. 



ACTIVATED SLUDGE 

265. The Process. In the treatment of sewage by the 
activated sludge process the sewage enters an aeration tank after 
it has been screened and grit has been removed. As it enters 
the aeration tank it is mixed with about 30 per cent of its 
volume of activated sludge. The sewage passes through the 
aeration tank in about two to four hours during which time air 
is blown through it in finely divided bubbles. The effluent 
from the aeration tank passes to a sedimentation tank where it 
remains for one-half an hour to an hour to allow the sedimen- 
tation of the activated sludge. The supernatant liquid from the 
sedimentation tank is passed to the point of final disposal. A 
portion of the sludge removed from the tank is returned to the 
influent of the aeration tank. The remainder may be sent to 
any or all of the following: the sludge drying process, the reae'ra- 
tion tanks, or to some point for final disposal. Sections of the 
activated sludge plant at Houston, Texas, are shown in Fig. 177. 

The biological changes in the process occur in the aeration 
tank. These changes are dependent on the aerobic organisms 
which are intensively cultivated in the activated sludge. When 
placed in intimate contact with fresh sewage, brought about by 
the agitation caused by the rising air, and in the presence of an 
abundance of oxygen, the organic matter is partially oxidized. 
The putrefactive stage of the organic cycle is avoided. Col- 
loids and bacteria are partially removed probably by the agita- 
tion effected in the presence of activated sludge but the exact 
action which takes place is not well understood. 

266. Composition. Activated sludge is the material obtained 
by agitating ordinary sewage with air until the sludge has assumed 
a flocculent appearance, will settle quickly, and contain 
aerobic and facultative bacteria in such numbers that similar 
characteristics can be readily imparted to ordinary sewage 

465 



466 



ACTIVATED SLUDGE 



sludge when agitated with air in the presence of activated sludge. 
Copeland described activated sludge as follows: 1 

The sludge embodied in sewage and consisting of 
suspended organic solids, including those of a colloidal 
nature, when agitated with air for a sufficient period 
assumes a flocculent appearance very similar to small 
pieces of sponge Aerobic and facultative bacteria gather 
inr these flocculi in immense numbers from 12 to 14 



8"6luice 
Gate-. 



/Sludge Return Channel 




l2"AirP/pe. 



Part Plan of Outside Unit Aeration and Settling Tank. 

- -- '-' 



58-6",Approx,Posif/onof Drain' 

29-6 H ; Sludge Blankets. Lonqitudi- 

^1 A*! if, V, J^IA| na ig ection 

A-A. 
4"W.[.Pipe 




Half Section through Settling Tanks. 



58 '- 6 " fo r North Side PI a nk- 
sg'-o" > South _ * 

Half Section through Aeration Tanks 



FIG. 177. Activated Sludge Plant at Houston, Texas. 
Eng. News, Vol. 77, p. 236. 

million per c.c. some having been strained from the 
sewage and others developed by natural growth. Among 
the latter are species that have the power to decompose 
organic matter, especially of an albuminoid or nitrog- 
enous nature, setting the nitrogen free; and others 
absorbing the nitrogen convert it into nitrites and 
nitrates. These biological processes require time, air, 
and favorable environment such as suitable temperature, 

Reference 11, at end of this chapter. 



COMPOSITION 



467 



food supply and sufficient agitation to distribute them 
throughout all parts of the sewage. 

Ardern states that the sludge differs entirely from the usual 
tank sludge. It is inoffensive and flocculent in character. The 
percentage of moisture is from 95 to 99 per cent. American 
experience has generally been that the sludge does not readily 
separate from its moisture by treatment on fine-grain filters, 
but the results in England and at Milwaukee, Wisconsin, are in 
conflict with this general experience. Upon standing 24 hours 
or more partially dried activated sludge may start to decompose 
accompanied by the production of offensive odors. 

Duckworth states : 

The activated sludge at Salford contained three times 
as much nitrogen, twice as much phosphoric acid and 
one-half as much fa ,y matter as ordinary sludge. 

TABLE 91 
COMPOSITION OF SEWAGE, IMHOFP SLUDGE, AND ACTIVATED SLUDGE AND 

EFFLUENT AT MILWAUKEE 
(W. R. Copeland, Eng. News, Vol. 76, p. 665) 







Parts per Million 






I 


Nitrogen as 









5* 




Nitrogen Reported 


Period of 


Source of 









g 






as Ammonia on a 


Test 


Sample 


3 


09 

'5 
o 


"3.2 

'o 

a 2 


1 






Basis of Sludge 
Dried to 10 Per 






3 

2 







O ~ 


s 


8 


Cent Moisture. 






1 




B i 

s< 


- - 

Z< 


I 2 


i 


I 


Three samples of 
Sludge 






J. 





< 





2 


2 




Aug., I'.ll"). 


Sewage .... 


253 


14.6 


7.88 


29 


0.15 


0.13 










Imhoff effluent. . 


105 


16.2 


6.10 


27 


0.19 


0.13 


2.87 


3.82 






Activated sludge 






















effluent 


14 


3.8 


3.19 


6 


0.29 


6.00 


5.71 


4.97 


7.04 
























Sept., 1915. 


Sewage . . 


300 


13.5 


8.81 


29 


0.25 


0.14 










Imhoff effluent. . 


116 


15.4 


7.10 


27 


0.12 


0.09 


3.88 








Activated sludge 






















effluent. 


8 


5.7 


2.22 


9 


0.24 


5.01 


8.69 


9.00 







These results have been roughly checked by American experi- 
menters as shown in Table 91. ' In the recovery of nitrogen 
from sewage the activated sludge process is the most promising 
for satisfactory results. In all other processes of sewage treat- 

1 Reference 15. 



468 



ACTIVATED SLUDGE 



ment the sludge is digested to some extent and nitrogen lost in 
the gases or in the soluble matter which passes off with the 
effluent. In the activated sludge process a negligible amount 
of gasification and liquefaction take place and only a small 
amount of nitrogen passes off with the effluent as compared with 
the loss from the Imhoff process as shown in Table 91. The 
percentage of nitrogen in dried activated sludge is shown in 
Table 92, 

TABLE 92 

NITROGEN CONTENT OF DRY ACTIVATED SLUDGE AND SLUDGE 
FROM OTHER PROCESSES 

(G. W. Fuller, Eng. News, Vol. 76, p. 667) 



Source 


Per Cent 
Nitrogen 


Milwaukee (Copeland) 


4.40 


Manchester, England (Ardern) 


4.60 


Salford, England (MeUing) 


3 75 


Urbana, Illinois (Bartow) 


3 5 to 6 4 


Armour and Co. (Noble) 


4.6 


Approximate range of all other processes . 


l.Oto 3.0 



These figures are expressed in terms of nitrogen and not of ammonia. Nitrogen is only 
82 per cent of the ammonia content. 

Nitrifying bacteria and other species which have the power 
of destroying organic matter have been isolated from the sludge. 
An analysis of the dried sludge at Urbana 1 showed the following 
results after the weight had been reduced 95.5 per cent by drying: 
6.3 per cent nitrogen, 4.00 per cent fat, 1.44 per cent phosphorus, 
and 75 per cent volatile matter or loss on ignition. Analyses 
of other domestic sewages have not shown such high contents 
of these desirable constituents. 

The dewatering of activated sludge is a problem which offers 
serious obstacles to the successful operation of the process. It 
is its greatest disadvantage. Five to ten times the volume of 
sludge may be produced by the activated sludge process as by 
an Imhoff tank, and the activated sludge contains a greater 
percentage of water. According to Copeland : 

1 Reference 2. 



ADVANTAGES AND DISADVANTAGES 469 

The best information now available points to a 
combination of settling and decantation as a preliminary 
dewatering process. By this means the water will be 
cut down from about 99 per cent to 96 per cent. On 
passing the concentrated residue through a pressure 
filter the moisture can be cut down to 75 per cent. The 
press cake can be dewatered in a heat drier to 10 per cent 
moisture or less. 1 

The quantity of sludge produced at Milwaukee 2 is about 15 
cubic yards per million gallons of sewage, the sludge having 
about 98 per cent moisture. On the basis of 10 per cent mois- 
ture it produces \ ton of dry sludge per million gallons of sewage 
treated. At Cleveland, 3 20 cubic yards per million gallons 
at 97.5 per cent moisture are produced. Methods of drying 
sludge are discussed in Chapter XX. 

Chemical analyses and biological tests indicate that the 
fertilizing value of the sludge is appreciable. Professor C. B. 
Lipman states, as the result of a series of tests in which a sludge 
and a soil were incubated for one month, as follows : 4 

The amounts of nitrates produced in one month's 
incubation from the soil's own nitrogen and from the 
nitrogen from the sludge mixed with the soil in the ratio 
of one part of sludge to 100 of soil is, in milligrams of 
nitrate, as follows: Anaheim soil without sludge 6.0, 
with sludge 10.0; Davis soil without sludge 4.2, with 
sludge 14.0; Oakley soil without sludge 2.2, with sludge 
4.0. 

The effect of the sludge on plant growth is shown in Table 93. 5 
The results represent the growth obtained after fifteen weeks 
from the planting of 30 wheat seeds in each pot. 

267. Advantages and Disadvantages. Some of the advantages 
of the process are : a clear, sparkling, and non-putrescible effluent 
is obtained; the degree of nitrification is controllable within 
certain limits; the character of the effluent can be varied to 
accord with the quantity and character of the diluting water 

1 For mechanical methods of drying sludge, see Reference 22, p. 1127, 
and No. 33, p. 843. 

2 Reference 10. 
1 Reference 13. 

4 University of California, Bulletin 251, 1915. 
4 Reference 25. 



470 



ACTIVATED SLUDGE 



available ; more than 90 per cent of the bacteria can be removed ; 
the cost of installation is relatively low; and the sludge has some 
commercial value. 

TABLE 93 

FEKTILIZING VALUE OP ACTIVATED SLUDGE 
(E. Bartow, Journal Am. Water Works Ass'n, Vol. 3, p. 327) 



Cultivating Medium 


Grams Contained in Experimental Pot 


1 


2 


3 


4 


White sand 


19,820 
60 
6 
3 






19,820 
60 
6 
3 



8.61 


19,820 
60 
6 
3 
20 





19,820 
60 
6 
3 


20 



Dolomite . 


Bone meal 


Potassium sulphate 


Activated sludge 


Activated sludge extracted 
with Ligroin 


Dried blood 


Number of heads of wheat. 
Number of seeds 


14 
85 
2.38 
6.20 

19.40 
2.25 
0.18 


15 
189 
5.29 
13.6 

23.0 

8.25 
0.68 


22 
491 
13.748 
35.9 

35.4 
26.75 
2.23 


23 
518 
14.504 
38.7 

37.1 
26.21 
2.18 


Weight of seeds, grams .... 
Bushels per acre, calculated. 
Average length of stalk, 
inches 


Weight of straw, grams. . . 
Tons per acre, calculated . . . 



Among the disadvantages of the process can be included, 
uncertainty due to the lack of information concerning the 
results to be expected under all conditions, high cost of operation 
under certain conditions, the necessity for constant and skilled 
attendance, and the difficulty of dewatering the sludge. 

268. Historical. The most notable work in the aeration of 
sewage within recent years was that performed by Black and 
Phelps for the Metropolitan Sewerage Commission of New York, 
in 19 10, 1 and by Clark and Gage at the Lawrence, Massachusetts, 
Sewage Experiment Station in 1912 and 1913. 2 The results of 

1 See Report by Black & Phelps of Metropolitan Sewerage Commission, 
1911, reprinted as Vol. VII of Contributions from the Sanitary Research 
Laboratory of the Massachusetts Institute of Technology. 

2 See Reports, Mass. State Board of Health. 






AERATION TANK 471 

these investigations showed that the treatment of sewage by 
forced aeration might give a satisfactory effluent, but that the 
time and expense in connection thereto rendered the method 
impractical. 

It remained for Messrs. Ardern and Lockett of Manchester, 
England, to introduce the process of the aeration of sewage in 
the presence of activated sludge, as a result of their connection 
with Dr. Fowler, who attributes his inspiration to his visit to 
the Lawrence Experiment Station and observing the work of 
Clark and Gage. Ardern and Lockett commenced their experi- 
ments in 1913. Their results were published in the Journal 
of the Society of Chemical Industry, May 30, 1914, Vol. 33, p. 523. 
Shortly thereafter experiments were started at the University 
of Illinois by Dr. Edw. Bartow and Mr. F. W. Mohlmann of the 
Illinois State Water Survey. At about the same time an experi- 
mental plant was started at Milwaukee, by T. C. Hatton, Chief 
Engineer of the Milwaukee Sewerage Commission. The United 
States Public Health Service became actively interested in 
December, 1914, and on February 20, 1915, announced its 
intention to co-operate with the Baltimore Sewerage Commission 
in the conduct of experiments. In May, 1915, patent number 
1,139,024 was granted to Leslie C. Frank, Sanitary Engineer 
of the U. S. Public Health Service, covering certain features of 
the process. Mr. Frank generously donated this patent to the 
public for the use of municipalities. 

The first full sized plant for the treatment of sewage by 
this method was erected in Milwaukee in December, 1915. This 
plant had a capacity of 1,600,000 gallons per day. It was used 
for experimental purposes and is not now in use. The Champaign, 
Illinois, septic tank, among the first of its kind in the country, 
was converted into an activated sludge tank on April 13, 1916. 
The changes, developments, and the results obtained from these 
and other plants have been reported in the technical press from 
time to time. 

269. Aeration Tank. The sewage on leaving the screen and 
grit chamber enters the aeration tank, which is usually operated 
on the continuous-flow principle, although in the early days of 
experimentation the fill-and-draw method was practiced. This 
tank should be rectangular with a depth of about 15 feet and a 
width of channel not to exceed 6 to 8 feet. Such proportions 



472 ACTIVATED SLUDGE 

allow better air and current distribution than larger tanks. 
The bottom should be level to insure an even distribution of 
air. The velocity of flow of sewage through the tank is usually 
in the neighborhood of 5 feet per minute, dependent on the 
length of the tank and the period of retention. The period of 
retention is in turn dependent on the desired quality of the 
effluent. The process is flexible and the quality of the effluent 
can be changed by changing the period of retention or by changing 
the rate of application of the air, or both. The period of retention 
in the aeration tank is usually about 4 hours. 

The bottom of the aeration tank is usually made of concrete 
arranged in ridges and valleys, or small shallow hoppers, at the 
bottom of which the air-diffusing devices are located, as shown 
in Fig. 177. The inlet and outlet devices are similar to those 
in a plain sedimentation tank. 

270. Sedimentation Tank. It is evident that as no sedi- 
mentation is permitted in the aeration tank, the settleable parti- 
cles will be discharged in the effluent unless some provision is 
made for their detention. The effluent from the aeration tank 
is therefore run through a plain sedimentation tank, usually 
with a hopper bottom, which has been arranged to permit fre- 
quent and easy cleaning. An air lift or a centrifugal sludge 
pump is satisfactory for this purpose. Another type of sedi- 
mentation tank which has been used has a smooth bottom with 
a slight slope towards the center. A revolving scraper collects 
the sludge continuously, scraping it towards the center of the 
tank. Although this arrangement gives better results than the 
hopper-bottom tank, its expense has usually prevented its instal- 
lation. 1 

The period of sedimentation in different plants varies from 
30 minutes to one hour, although the longer periods usually give 
the better results. Approximately 65 per cent of the sludge will 
settle in the first 10 minutes, 80 per cent in the first 30 minutes, 
and about 5 per cent more in the next half hour. 

The effluent from the sedimentation tank is ready for final 
disposal or if desired, for further treatment by some other 
method. The sludge, or a portion of it, is pumped back into the 
influent of the aeration tank, provided the sludge is in a satis- 
factory state of nitrification. Otherwise it should be pumped 

1 Reference 47. 



THE REAERATION TANK 473 

to the reaeration tanks. The remainder of the sludge which is 
not to be used in the process is ready for drying and final 
disposal. 

271. Reaeration Tank. The purpose of the reaeration or 
sludge aeration tank is to reactivate the sludge which has gone 
through the aeration tank. During the process of the aeration 
of the sewage in the aeration tank the activated sludge may lose 
some of its qualities because of the deficiency of oxygen to 
maintain aerobic conditions. By blowing air through the sludge 
in the reaeration tank these properties are returned and the sludge 
made available to be pumped back into the aeration tank. The 
reactivation of the sludge obviates the necessity for supplying 
sufficient air to the entire mass of the sewage to maintain aerobic 
conditions, and results in an economy in the use of air. The 
use of mechanical agitators has also been attempted both in the 
reaeration and the aeration tanks with the expectation of saving 
in the use of air, but with indifferent success. 

It is difficult to say, without experimentation, what the size 
of the reaeration tank should be, as the necessary amount or 
reactivation is uncertain. In the experimental plant at Mil- 
waukee, there were eight units of aeration tanks, one sedimenta- 
tion tank, and two reaeration tanks, all of the same capacity and 
general design. This represents a ration of about one reaeration 
tank to four aeration tanks. 

272. Air Distribution. Air is applied to the sewage at the 
bottom of the aeration tank at a pressure in the neighborhood 
of J>.5 to 6.0 pounds per square inch, dependent on the depth of 
the sewage, the loss of head through the distributing pipes, 
and the rate of application. In different experimental plants 
the pressure has varied from 3 to 30 pounds per square inch. 
Such pressures are on the line which divides the use of direct 
blowers for low pressures from turbo and reciprocating pressure 
machines for pressures above 10 pounds per square inch. Posi- 
tive-pressure blowers or direct blowers operate on the principle 
of a centrifugal pump and because of the lighter specific gravity 
of air they rotate at a very high speed. The Nash Hytor Turbo 
Blower consists of a rotor with a large number of long teeth 
slightly bent in the direction of rotation. The rotor, which has 
a circular circumference, revolves in an elliptical casing. At 
the commencement of operation the rotor and casing are partially 



474 



ACTIVATED SLUDGE 



filled with water. The revolution of the rotor throws the water 
to the outside of the elliptical casing thus forming a partial 
vacuum between any two teeth as the water is thrown from near 
the center of the short diameter of the casing to the extremity 
of the long diameter of the casing. Air is allowed to enter 
through the inlet port to relieve the vacuum. As the teeth pass 
from the long diameter to the short diameter of the ellipse, the 
water again approaches the center of the rotor compressing the 

air trapped between the 
teeth and forcing ic out 
under pressure into tne ex- 
haust pipe. Among the ad- 
vantages of this compressor 
are the washing of the air, 
cooling, and ease in opera- 
tion. Reciprocating air com- 
pressors operate similarly to 
direct-acting steam pumps 
or crank - and - fly - wheel 
pumps but at much higher 
speeds, and they require 
more floor space than either 
of the other types. Fig. 




30 



FIG. 178. Economic Range of Air Com- 
pressors. 
From Eng. News, Vol. 74, p. 906. 



5 ,10 15 20 Z5 
Volum* of Frea Air in Thousands 
Cu. Ft per Minu+e 

178 shows the field of 
serviceability of various 
types of air compression 
machinery. 

For pressures up to about 10 pounds per square inch the posi- 
tive blower seems most desirable. It has a low first cost and a 
relatively high efficiency of about 75 to 80 per cent of the power 
input. No oil or dirt is added to the air to clog the distributing 
plates, as in the reciprocating machine. A disadvantage is the 
difficulty of varying the pressure or quantity of the output of 
the machine. As the required pressure and volume of air 
increases the turbo blower becomes more and more desirable 
within the limits of pressure which are ordinarily used in this 
process. For small installations the best form of power is 
probably the electric drive, but when the capacity becomes 
such as to make turbo blowers advisable they should be driven 
by directly connected steam turbines. 



AIR DISTRIBUTION 475, 

The quantity of air required varies between 0.5 to 6.0 cubic 
feet per gallon of sewage, with from 3 to 6 hours of aeration. 
The quantity of air depends on the degree of treatment required, 
the strength of the sewage, the depth of the tank, and the period 
of aeration. The deeper the tank the less the amount of air 
needed because of the greater travel of the bubble in passing 
through the sewage, but the higher the pressure at which the 
air must be delivered. Shallow tanks usually require a longer 
period of retention. The depth of the tank then has very little 
to do with economy in the use of air. Hatton states: 1 

The purification of sewage obtained varies decidedly 
with the volume of air applied. Small volumes applied 
for 5 or 6 hours do as well as larger volumes applied for 
3 or 4 hours, but the tune of aeration required to obtain 
a like effluent does not vary directly with the volume of 
air applied per unit of time. For instance air applied 
at a rate of 2 cubic feet per minute purifies the sewage 
in less time than one cubic foot of air per minute, but will 
not accomplish an equal degree of purification in half the 
time. 

It has been found that although a low temperature has a dele- 
terious effect on the process, by the use of an additional quantity 
of air good results can be maintained. The effect of changing 
the quantity of air and the period of aeration are shown in Table 
94 taken from Hatton. 

The velocity of the air in the pipes should be about 1,000 
feet per minute. There should be relatively few sharp turns 
in the line, and the distributing mains should be arranged with- 
out dead ends. It is desirable to use as little piping as possible 
and at the same time to make the travel of the sewage long in 
order to maintain a non-settling velocity and intimate contact 
with the air. The piping should be accessible and well provided 
with valves. It should be non-corrodible, particularly on the 
inside, as flakes of rust will quickly clog the air diffusers. It 
should drain to one point in order that it can be emptied when 
flooded, as occasionally happens. 

It is desirable to diffuse the air in small bubbles as by this 
means the greatest efficiency seems to be obtained from the 
amount of air added. A diameter ^ to $ of an inch is approxi- 

1 Reference 10. 



476 



ACTIVATED SLUDGE 



w 
*/: 



Pi OH 



W W 

t-3 So 



11 

^ H 

S as 

J H 



K 

B 

c 

s 
^ 




fc 
W 






-O V 



O 



Nitrite 



5^3 | 



^'-D 3 



O Oi-i * to CO 



OOOT-KNdOOOOOOOOO 



O CO O Ci *- O5 



t^co eor>- M r~- 

GO Ci C~. CZ 



73, 
HOOOOO 



OOOOOiCOOO 



AIR DISTRIBUTION 



477 



mately the maximum limit for the size of an effective bubble. 
Monel metal cloth, porous wood blocks, open jets, paddles, and 
other forms of diff users have been tried, but none have given 
the satisfaction of the filtros plate. The relative value of dif- 
ferent types of diffusers is shown in Table 95 taken from Hatton. 1 
The Filtros plates are a proprietary article manufactured by 
the General Filtration Company of Rochester, N. Y. They 
are made of a quartz sand firmly cemented together and can be 

TABLE 95 

COMPARATIVE RESULTS FROM THE AERATION OF SEWAGE IN THE PRESENCE 
OF ACTIVATED SLUDGE WITH THE USE OF DIFFERENT DISTRIBUTING 
MEDIA 

(T. C. Hatton, Eng. Record, Vol. 73, p. 255) 







Pounds 


Air, 


Per 


Nitrates, 


Stability 


Diff users 


Months in 1915 


per 
Square 


Cubic 
Feet per 


Cent 
Bacteria 


Parts 
per 


Effluent 
in 






Inch 


Gallon 


Removed 


Million 


Hours 


Filtros plate 


June 1 to Aug. 15 


4.3 


2.06 


91 


3.4 


78 






3.5 


1.94 


91 


2 2 


52 


Filtros plate 


Nov. 18 to Dec. 7 


4.6 


1.71 


90 


0.3 


113 


Monel metal 


Nov. 18 to Dec. 7 


3.0 


1.71 


80 


0.2 


63 



obtained with practically any degree of porosity, size of pore 
opening or dimension of plate, but they are made in a standard 
size 12 inches square by \\ inches thick. The frictional loss 
through the plate is not very great for the amount of air ordi- 
narily used. The plates are classified in accordance with the 
volume of air which will pass through them, when dry, per 
minute when under a pressure of 2 inches of water. These 
classes run from \ to 12 cubic feet of air per minute. The type 
usually specified passes about 2 cubic feet of air per minute. 
The loss of head through these plates as tested at Milwaukee 
showed an initial loss of f of a pound and an additional loss 
of about \ of a pound for every cubic foot of air per minute per 
square foot of surface. It is necessary to screen and wash the 
air before blowing it through the filtros plate as ordinary air 
is so filled with dirt as to clog the pores of the diffuser quite 
rapidly. 

1 Reference 10. 



478 ACTIVATED SLUDGE 

The area of filtros plates required in the bottom of the tank 
is usually expressed in terms of the free surface of the tank or 
as a ratio thereto. In the Urbana tests the best ratio was 
found. to be less than 1 : 3 and more than 1:9. In Milwaukee 1 
the ratio adopted is in the neighborhood of 1 : 4 or 1 : 5. At 
Fort Worth the ratio will be about 1 : 7 and at Chicago it will 
be 1 : 8. The exact ratio should be determined by experiment 
and will depend on the construction of the tank and the char- 
acter of the raw sewage and the desired effluent. It is essential 
that the filtros plates be placed level and at the same elevation 
as otherwise the distribution of air will be uneven. 

273. Obtaining Activated Sludge. After a plant is once 
started activated sludge is generated during the process of treat- 
ment and with careful management a stock of activated sludge 
can be kept on hand. When a plant is new, or if shut down for 
such a length of time that the sludge loses its activation, it is 
necessary to activate some new sludge. This is done by blowing 
air continuously through sewage either on the fill and draw 
method with periodic decantations of the supernatant liquid, 
or by the continuous-flow process, but more preferably by the 
latter. Where activated sludge is to be obtained from fresh 
sewage alone the time required is in the neighborhood of 10 to 
14 days, and purification begins at the start. An estimate of 
the quantity which will be obtained can not be made with 
accuracy. After the initial quantity of sludge has been obtained 
activated sludge can be maintained during the process of aeration 
of the raw sewage, or by means of the reaeration tanks previously 
described. 

The volume of activated sludge present in the aeration tank 
should be about 25 per cent of the volume of the tank. The 
volume of the sludge is measured in a somewhat arbitrary manner 
as the amount by volume which will settle in 30 minutes in an 
ordinary test tube. It is found that this is almost 90 per cent 
of the solids settling in 4 to 6 hours. 

274. Cost. The available information on the cost of the 
activated sludge process is meager and unreliable. The factors 
entering into the cost are: the price of fuel, the size of the plant, 
the period of sedimentation, the amount of air per gallon of sewage, 
the air pressure, and the percentage of sludge to be aerated in the 

1 Reference 10. 



COST 



479 



mixture. In Milwaukee l the cost of construction is estimated 
at $44,000 per million gallons, and $4.75 per million gallons for 
operation. At Houston, Texas, the cost is estimated at $24,000 
per million gallons, exclusive of the sludge-drying plant, which 
may cost $40,000 per million gallons. At Milwaukee, the cost 
of pressing the sludge is $4.82 per dry ton and of drying is $3.93 
per dry ton. The sludge may be sold at the normal rate of $2.50 
per unit of nitrogen. Based on the normal value the evident 
profit will be $3.75 per ton. The net cost of disposing of Mil- 
waukee sewage is estimated at $9.64 per million gallons of which 
$4.89 is chargeable to overhead and $4.75 to repairs, operation 
and renewal. In a comparison of the costs of activated sludge 
and Imhoff tanks with sprinkling filters, 2 the information given 
by Eddy has been summarized in Table 96. In comparing the 

TABLE 96 

COMPARATIVE COSTS OF ACTIVATED SLUDGE, AND OF IMHOFF TANKS 
FOLLOWED BY SPRINKLING FILTERS 

(H. P. Eddy, Eng. Record, Vol. 74, p. 557) 









Total Annual Cost at 


, 






4 Per Cent with 


Process 


First 
Cost per 
Million 


Operation 
per 
Million 


Sinking Fund at 
2.5 Per Cent per 




Gallons, 


Gallons, 








Dollars 


Dollars 


Million 
Gallons, 


Capita, 
Dollars 








Dollars 




Activated sludge 


57,100 


20.00 


29.85 


1 09 


Imhoff tank and sprinkling filter. 


78,500 


8.50 


21.84 


0.80 



relative areas required for different methods of sewage treatment, 
activated sludge should be allowed about 15 million gallons 
per acre per day on the basis of aeration tanks 15 feet deep. 
This figure represents approximately the gross area of the plants 
at Milwaukee and at Cleveland. 



1 Hatton, reference 33. 



Reference 18. 



480 



ACTIVATED SLUDGE 



REFERENCES AND BIBLIOGRAPHY ON ACTIVATED 

SLUDGE 

The following abbreviations will be used: A.S. for Activated Sludge, 
E.G. for Engineering and Contracting, E.N. for Engineering News, E.R. 
for Engineering Record, E.N.R. for Engineering News-Record, p. for page, 
and V. for volume. 

No. 

1. Cooperation Sought in Conducting A.S. Experiments at Baltimore, 

by Franks and Hendrick. E.R. V. 71, 1915, pp. 521, 724, and 784. 
V. 72, 1915, pp. 23, and 640. 

2. Sewage Treatment Experiments with Aeration and A.S., by Bartow 

and Mohlman. E.N. V. 73, 1915, p. 647, and E.R. V. 71, 1915, p. 421. 

3. A.S. Experiments at Milwaukee, Wisconsin, by Hatton. E.N. V. 74, 

1915, p. 134. 

4. A.S. in America, An Editorial Survey, by Baker. E.N. V. 74, 1915, 

p. 164. 

5. Choosing Air Compressors for A.S., by Nordell, E.N. V. 74, 1915, p. 904. 

6. A Year of A.S. at Milwaukee, by Fuller. E.N. V. 74, 1915, p. 1146. 

7. A.S. Experiments at Urbana. E.N. V. 74, 1915, p. 1097. 

8. Experiments on the A.S. Process, by Bartow and Mohlman. E.G. V. 

44, 1915, p. 433. 

9. Milwaukee's A.S. Plant, the Pioneer Large Scale Installation, by Hat- 

ton. E.R. V. 72, 1915, p. 481 and E.G. V. 44, 1915, p. 322. 

10. A.S. Experiments at Milwaukee, by Hatton. Journal American Water- 

works Association and Proceedings Illinois Society of Engineers, 1916. 
Also E.R. V. 73, 1916, p. 255. E.G. V. 45, 1916, p. 104, and E.N. 
V. 75, 1916, pp. 262 and 306. 

11. A.S. Defined. E.N. V. 75, 1916, p. 503, and E.N.R. V. 80, 1918, p. 205. 

12. Status of A.S. Sewage Treatment, by Hammond. E.N. V. 75, 1916, 

p. 798. 

13. Trial A.S. Unit at Cleveland, by Pratt. E.N. V. 75, 1916, p. 671. 

14. Air Diffuser Experience with A.S. E.N. V. 76, 1916, p. 106. 

15. Nitrogen from Sewage Sludge, Plain and Activated, by Copeland, 

Journal American Chemical Society, Sept. 28, 1916. E.N. V. 76, 

1916, p. 665. E.R. V. 74, 1916, p. 444. 

16. Tests Show A.S. Process Adapted to Treatment of Stock Yards Wastes. 

E.R. V. 74, 1916, p. 137. 

17. Aeration Suggestions for Disposal of Sludge, by Hammond. Journal 

American Chemical Society, Sept. 25, 1916. E.R. V. 74, 1916, p. 448. 

18. Cost Comparison of Sewage Treatment. Imhoff Tank and Sprinkling 

Filters vs. A.S., by Eddy. E.R. V. 74, 1916, p. 557. 

19. Large A.S. Plant at Milwaukee. E.N. V. 76, 1916, p. 686. 

20. A.S. Novelties at Hermosa Beach, Cal. E.N. V. 76, 1916, p. 890. 

21. A.S. Experiments at University of Illinois, by Bartow, Mohlman, and 

Schnellbach. E.N. V. 76, 1916, p. 972. 



REFERENCES AND BIBLIOGRAPHY 481 

No. 

22. A.S. Results at Cleveland Reviewed, by Pratt and Gascoigne. E.N. 

V. 76, 1916, pp. 1061 and 1124. 

23. Sewage Treatment .by Aeration and Activation, by Hammond. Pro- 

ceedings American Society Municipal Improvements, 1916. 

24. A.S., by Bartow and Mohlman, Proceedings Illinois Society of Engineers, 

1916. 

25. The Latest Method of Sewage Treatment, by Bartow. Journal Ameri- 

can Waterworks Association, V. 3, March, 1916, p. 327. 

26. Winter Experiences with A.S., by Copeland. Journal American Society 

of Chemical Engineers, April 21, 1916. E.G. V. 45, 1916, p. 386. 

27. A.S. Process Firmly Established, by Hatton. E.R. V. 75, 1917, p. 16. 

28. Operate Continuous Flow A.S. Plant, by Bartow, Mohlman, and 

Schnellbach. E.R. V. 75, 1917, p. 380. 

29. Chicago Stock Yards Sewage and A.S., by Lederer. Journal American 

Society of Chemical Engineers, April 21, 1916. E.G. V. 45, 1916, p. 388. 

30. The Patent Situation Concerning A.S. E.G. V. 45, 1916, p. 208. 

31. " Sewage Disposal " by Kinnicutt, Winslow, and Pratt, published by 

John Wiley & Sons. 2d Edition, Chapter 12. 

32. A.S. Tests Made by California Cities. E.N.R. V. 79, 1917, p. 1009. 

33. Conclusions on the A.S. Process at Milwaukee. Journal American 

Public Health Association, 1917. E.N.R. V. 79, 1917, p. 840. 

34. Dewatering A.S. at Urbana, by Bartow. Journal American Institute 

of Chemical Engineers, 1917. E.N.R. V. 79, 1917, p. 269. 

35. Milwaukee Air Diffusion Studies in A.S. E.N.R. V. 78, 1917, p. 628. 

36. A.S. Bibliography (up to May 1, 1917) by J. E. Porter. 

37. Air Diffusion in A.S. E.N.R. V. 78, 1917, p. 255. 

38. A.S. Plant at Houston, Texas. E.N. V. 77, 1917, p. 236, E.N.R. 83, 

1919, p. 1003, and V. 84, 1920, p. 75. 

39. A.S. Power Costs, by Requardt. E.N. V. 77, 1917, p. 18. 

40. A.S. at San Marcos, Texas, by Elrod. E.N. V. 77, 1917, p. 249. 

41. Filtros Plates Made the Best Showing in Air Diffuser Tests. E.N.R. 

V. 79, 1917, 269. 

42. Results of Experiments on A.S., by Ardern and Lockett. Journal 

Society for Chemical Research, V. 33, May 30, 1914, p. 523. 

43. Final Plans at Milwaukee. E.N.R. V. 84, 1920, p. 990. 

44. A.S. Bibliography, published by General Filtration Co., Rochester, 

N. Y., 1921. 

45. A.S. at Manchester, Eng. by Ardern. Journal Society Chemical Indus- 

try, 1921. E.G. V. 55, 1921, p. 310. 

46. The Des Plaines River A.S. Plant, by Pearse. E.N.R. V. 88, 1920, 

p. 1134. 

47. Sewage Treatment by the Dorr System, by Eagles. Proceedings, 

Boston Society of Engineers, 1920. Public Works V. 50, 1920, P. 53. 



CHAPTER XIX 

ACID PRECIPITATION, LIME AND ELECTRICITY, AND 
DISINFECTION 

275. The Miles Acid Process. The Miles Acid Process for 
the treatment of sewage was devised and patented by G. W. 
Miles. It was tried experimentally at the Calf Pasture sewage 
pumping station, Boston, Mass., 1911 to 1914. In 1916 it was 
tried experimentally at the Massachusetts Institute of Tech- 
nology, and it has been tested subsequently at other places, nota- 
bly at New Haven, Conn., in 1917 and 1918. It is one of the 
most recent developments in sewage treatment and no extensive 
experience has been had with it. The process consists in the 
acidification of sewage with sulphuric or sulphurous acid, as the 
result of which the suspended matter and grease are precipitated 
and bacteria are removed. The equipment required for the 
process consists of devices for the production of sulphur dioxide 
(802), and for feeding niter cake or other forms of acid; sub- 
siding basins; sludge-handling apparatus; sludge driers; grease 
extractors; grease stills; and tankage driers and grinders. 

The first step is the acidification of the sewage. The period 
of contact with the acid is about 4 hours. Sulphurous acid 
seems to give better results than sulphuric because of the ease 
in which it can be manufactured on the spot. It seems also to 
be more virulent in attacking bacteria than an equal strength 
of sulphuric acid. In experimental plants the acidulation has 
been accomplished in different ways such as: by the addition 
of compressed sulphur dioxide from tanks; by the addition of 
sulphur dioxide made from burning sulphur; or by the roasting 
of iron pyrite (FeS2). The acidulation precipitates most of the 
grease as well as the suspended matter and results in a sludge 
which gives some promise of commercial value. In referring 
to the process R. S. Weston states: 1 

1 Reference 1, at end of this chapter. 
482 



THE MILES ACID PROCESS 483 

(1) It disinfects the sewage by reducing the numbers 
of bacteria from millions to hundreds per c.c. 

(2) If the drying of the sludge and the extraction of 
the grease can be accomplished economically, it is possible 
that a large part, if not all, of the cost of the acid treat- 
ment may be met by the sale of the grease and fertilizer 
recovered from the sewage. 

(3) The use of so strong a deodorizer and disinfectant 
as sulphur dioxide would prevent the usual nuisances of 
treatment works. 

(4) The addition of sulphur dioxide to the sewage 
also avoids any fly nuisance, which is a handicap to the 
operation of Imhoff tanks and trickling filters. 

The amount of acid used varies with the quality of the 
sewage and the desired character of the effluent. At Bradford, 
England, 1 5,500 pounds of sulphuric acid are used per million 
gallons, producing about 2,340 pounds of grease or 0.43 pound of 
grease per pound of sulphuric acid. At Boston only 0.215 pound 
of grease were produced per pound of sulphuric acid. The dif- 
ference is probably due to the great difference in the amount of 
grease in the raw sewage. In the East Street sewer at New Haven, 
Conn., 2 only 700 pounds of acid are used per million gallons of 
sewage as the alkalinity is only 50 p. p.m. This amount of acid 
secures an acidity of 50 p.p.m. whereas in the Boulevard sewer 
1,130 pounds of acid had to be added to produce the same result. 
The results obtained by the experiments conducted by the 
Massachusetts State Board of Health in 1917 are shown in 
Table 97. The character of the sludge from the same tests 
is shown in Table 98. After acidification 3 the sewage contains 
bisulphites and some free sulphurous acid, with some lime and 
magnesium soaps which are attacked by the acid liberating the 
free fatty acids. Part of the bisulphites and sulphurous acid 
are oxidized to bisulphates and sulphuric acid. It was found 
as a result of the New Haven 3 experiments that the presence 
of sulphur dioxide in the effluent caused an abnormal oxygen 
demand from the diluting water and that this difficulty could be 
partly overcome by the aeration of the effluent after acidulation 
and sedimentation, without prohibitory expense. The effluent 
and sludge are both stable for appreciable periods of time and 
are suitable for disposal by dilution. The character of the 

1 Reference 2. * Reference 6. * Reference 5. 



484 ACIDIFICATION, ELECTROLYSIS, DISINFECTION 

sludge as determined by the New Haven tests l is shown in Table 
99. 

TABLE 97 



(Eng. News-Record, Vol. 80, p. 319) 



Sample 


Parts per Million 


Bacteria, 
Millions 


Ammonia 


Kjeldahl 
Nitrogen 


Chlor- 
ine 


Oxy- 
gen 
Con- 
sumed 


Free 


Albuminoid 


Total 


Total 


Diss. 


Total 


Diss. 


20 


37 


Paddock's Island 




14.0 
12.2 

20.9 


3.3 
1.6 

5.2 


1.8 
1.1 

3.9 


6.8 
3.5 

10.0 


3.6 

2.2 

7.5 


134 


23.1 
15.4 


1.86 

units 
94 


4.15 

units 
91 




Acidified and settled 










Deer Island 




23.3 
21.1 

20.9 


8.2 
5.6 

5.2 


4.8 
3.9 

3.9 


16.8 
10.7 

10.0 


8.9 
7.3 

7.5 


3100 


87.3 
62.2 


2.63 

units 
147 


1.50 

units 
85 


Settled sewage 
Acidified and settled 








Calf Pasture 




18.0 
19.1 

17.8 


4.5 
2.3 

2.4 


2.0 
1.4 

1.6 


9.7 
4.9 

4.9 


4.1 
3.3 

3.3 


3254 


41.2 
25.8 


1.89 

units 
277 


0.98 

units 
149 




Acidified and settled 












The success of the Miles Acid Process in comparison with other 
processes is dependent on the commercial value of the sludge 
produced. The New Haven experiments indicate that 16 to 21 
per cent of the grease in the sludge is unsaponifiable and seri- 
ously impairs the value of the process. 

1 Reference 6. 



THE MILES ACID PROCESS 



TABLE 98 



485 



AVERAGE AMOUNT OF SLUDGE AND FATS OBTAINED FROM SEWAGE ENTER- 
ING BOSTON HARBOR AFTER EIGHTEEN HOURS SEDIMENTATION 
WITH AND WITHOUT ACIDIFICATION 

(Eng. News-Record, Vol. 80, p. 319) 





Paddock's Island 


Deer Island 


Calf Pasture 


Sedimentation 


Sedimentation 


Sedimentation 


Plain 


Acidu- 
lated 


Plain 


Acidu- 
lated 


Plain 


Acidu- 
lated 


Pounds of SC>2 used per million 




818 
959 
3.38 
27.30 




1513 
1939 
3.45 
19.40 




1189 
1427 
2.83 
26.30 


Dry sludge per million gallons. . 
Per cent Nitrogen in sludge. . . . 


762 
3.10 
27.30 


1709 
3.57 
24.60 


1208 
3.18 
24.30 





TABLE 99 

CHARACTER OF MILES ACID SLUDGE AT NEW HAVEN 
(Eng. News-Record, Vol. 81, p. 1034) 





East Street Sewer 


Boule- 
. vard 
Sewer 


Length of run in days 


25 
260 

3750 
1.067 
86.6 

503 
23.7 
119 

47.2 
1.6 


24 
239.4 

4025 
1.048 

88 

483 
24.0 
116 

51.2 
1.6 


44 
407.8 

3200 
1.054 
86.3 

439 
29 
127 

57.3 
2.4 


70 
602.2 

2600 
1.061 

85.7 

368 
32.6 
120 

63.8 
2.0 


29 
145.5 
5375 


Total sewage treated, thou- 
sand gallons 


Gallons wet sludge per mil- 
lion gallons sewage . . . 


Specific gravity 


Per cent moisture 


92.5 
403 
30.9 
124 

78.5 
3.0 


Pounds of dry sludge per 
million gallons sewage 
Ether extract, per cent dry 
sludge ... 


Ether extract, pounds per 
million gallons 


Volatile matter, per cent dry 
sludge 


Nitrogen, per cent dry sludge 



486 ACIDIFICATION, ELECTROLYSIS, DISINFECTION 

The conclusions reached as a result of the New Haven experi- 
ments are: 1 

Our experience with New Haven sewage lends no 
color to the hope that a net financial profit can be obtained 
by the use of the Miles Acid Process, except with sewage 
of exceptionally high grease content and low alkalinity. 
They do, however, suggest that for communities where 
clarification and disinfection are desirable where screening 
would be insufficient and nitrification unnecessary the 
process of acid treatment comes fairly into competition 
with the other processes of tank treatment, and that it 
is particularly suited to dealing with sewages that contain 
industrial wastes, and to use in localities where local 
nuisances must be avoided at all costs and where sludge 
disposal could be provided for only with 'difficulty. 

The conclusions reached as a result of the Chicago experi- 
ments are: 2 

The results on hand indicate that treatment of this 
sewage with acid results in a somewhat greater retention 
of fat. An apparent reduction in the oxygen demand 
over that resulting from plain sedimentation, while remark- 
able, is probably not real, being simply due to a retarda- 
tion of decomposition by the sterilization of the bacteria 
present, the organic matter being left in solution. . . . 
However, there appears the added cost of acid treatment 
and the cost of recovery of the grease, as well as the 
uncertainty of the price to be received for the grease 
recovered. 

The cost of the treatment is estimated by Dorr to be $18 per 
million gallons, and the value of the sludge obtained from the 
Boston sewage as $24 per million gallons, giving a net margin 
of profit of $6 per million gallons. At New Haven, the total 
return is estimated at $7.09 per million gallons. Based on the 
production of sulphur dioxide by burning sulphur (assumed to 
cost $36 per long ton) and on drying from 85 per cent to 10 
per cent moisture with coal assumed to cost $7.50 per ton, it 
appears that the acid treatment of sewage should be materially 
cheaper than either the Imhoff treatment or fine screening under 
the local conditions. A comparison of the cost of the treatment 
of the East Street and the Boulevard sewage at New Haven 

1 Reference 6. 2 Reference 8. 



THE MILES ACID PROCESS 



487 



and the Calf Pasture sewage in Boston is given in Table 100. 
The cost of construction was estimated by Dorr and Weston 
in 1919 as greater than $15,000 per million gallons of sewage 
per day capacity. 

TABLE 100 

ESTIMATED COST OP SEWAGE TREATMENT AT NEW HAVEN AND BOSTON 
BY THREE DIFFERENT PROCESSES 

Cost in Dollars per Million Gallons Treated 
(Engineering and Contracting, Vol. 51, p. 510) 





Miles Acid Process 


Imhoff Tank and 
Chlori nation 


Fine Screens 
and Chlorination 


East 
Street 


Boule- 
vard 


Calf 
Pasture 


East 
Street 


Boule- 
vard 


Calf 
Pasture 


East 
Street 


Boule- 
vard 


Tanks and Buildings 
Int and Dep .... 


2.47 
6.93 
2.09 
1.78 
0.17 
1.06 

1.00 


2.47 
10.74 
2.04 
1.91 
0.17 
2.65 

1.00 


2.47 
18.65 
10.34 
9.12 
0.10 
1.06 

1.00 


5.28 

0.46 
1.20 

1.00 
4.05 
11.99 

11.99 


4.44 

1.15 
1.50 

1.00 
4.05 
12.14 

12.14 




4.60 

0.47 
1.42 

0.50 
4.05 
11.03 

11.03 


4.60 

1.15 
2.05 

0.50 
4.05 
12.35 

12.35 






Drying sludge 


Degreftsing sludge. . . . 
Redrying tankage. . . . 


Labor on tanks and 




Disposal of sludge or 
















15.50 
6.57 
8.93 


20.98 
10.66 
10.32 


42.75 
47.59 
4.84 




Revenue 




Net cost 







ELECTROLYTIC TREATMENT 

276. The Process. This process has been generally unsuc- 
cessful in the treatment of sewage and has grown into disrepute. 
In the words of the editor of the Engineering News-Record: l 

Thirty years of experiments and demonstrations with 
only a few small working plants built and most of them 
abandoned such in epitome is the record of the electro- 
lytic process of sewage treatment. 

It is probably true that the process has never received a thorough 
and exhaustive test on a large scale, but the small-scale tests have 

1 Reference 20. 



488 ACIDIFICATION, ELECTROLYSIS, DISINFECTION 

not been promising of good results. Among the most extensive 
tests have been those at Elmhurst, Long Island, 1 Decatur, 111., 2 
and Easton, Pa. 3 

Whatever degree of popularity the method has possessed 
has been due possibly to the mystery and romance of " elec- 
tricity " and to the personality of its promoters. The process 
should, nevertheless, be understood by the engineer in order 
that it may be explained satisfactorily to the layman interested 
in its adoption. 

In this process, sometimes called the direct-oxidation process, 
all grit is removed and the sewage is passed through fine screens 
before entering the electrolytic tank. In the electrolytic tank 
the sewage passes in thin sheets between electrodes and an 
electric current is discharged through it. A recent develop- 
ment has been the addition of lime to the sewage at some point 
in its passage through the electrolytic tank. From the elec- 
trolytic tank the sewage flows to a sedimentation tank, where 
sludge is accumulated, and from which the liquid effluent is 
finally disposed of. 

It is claimed that the action of the electricity electrolyzes 
the sewage, releasing chlorine, which acts as a powerful disin- 
fectant. The constituents of the sewage are oxidized so that 
the dissolved oxygen, nitrates, and relative stability are increased 
and the sludge is rendered non-putrescible. It is said that the 
addition of lime increases the efficiency of sedimentation and 
enhances the effect of the electric current. The results obtained 
by tests at Easton, Pa., are shown in Table 101. It will be 
observed from this table that the combination of lime and 
electricity does not have a more beneficial effect than either one 
of them alone. The amount of sludge produced by the com- 
bination is about the same as by chemical precipitation alone, 
but the character of the sludge produced with electricity is less 
putrescible. The cost of the treatment as estimated at Elm- 
hurst is shown in Table 102. 

As a result of the tests at Decatur, comparing lime alone 
with lime and electricity together, Dr. Ed. Bartow stated : 

The purification by treatment with lime alone was 
greater than that obtained in several of the individual 
samples treated with lime and electricity. 

, l Reference 17. 2 Reference 19. 3 Reference 21. 



DISINFECTION OF SEWAGE 



489 



TABLE 101 

COMPARATIVE RESULTS OBTAINED FROM THE TREATMENT OP SEWAGE BY 
LIME ALONE, ELECTRICITY ALONE, AND LIME AND ELECTRICITY COMBINED 

(Creighton and Franklin, Journal of the Franklin Institute, August, 1919) 





Lime and 








Electricity 


Lime Alone 


Electricity Alone 




Change, 
Parts 


Change, 
Per 


Change, 
Parts 


Change, 
Per 


Change, 
Parts 


Change, 
Per 




per 
Million 


Cent 


per 
Million 


Cent 


per 
Million 


Cent 


Chlorine 


+1.2 


+ 1.9 


+ 12.3 


+ 18.2 


+ 1.6 


+2.2 


Nitrites 


+0.014 


+58.3 


-.005 


-10.0 


-0.01 


-20.0 


Nitrates 


+0.13 


+23.6 


+ .005 


+0.8 


-0.15 


-20.0 


Ammonia 


-3.3 


-18.3 


+0.2 


+1.3 


+0.9 


+6.6 


Albuminoid am- 














monia 


-3.6 


-12.1 


-0.4 


-1.7 


-0.5 


-2.3 


Oxygen demand. . . 


-13.0 


-20.5 


-7.7 


-8.9 


-6.5 


-10.0 


Dissolved oxygen . . 


+1.78 


+40.9 


-0.93 


-19.1 


+ 1.61 


+40.1 


Total bacteria at 














37 (Thousands) 


-343 


-92.7 


-373 


-82.4 


-165 


-37.8 


Total bacteria at 














20 (Thousands) 


-688 


-92.2 


-1074 


-90.1 


-635 


-70.0 


B. Coli (Thou- 














sands) 


-77.9 


-99.85 


-96.3 


-92 3 


-45 


-81.8 


Oxygen absorbed 














in 5 days 


-3.40 


-81.6 


-1.03 


-21. 


+ 1.24 


+31 





DISINFECTION 

277. Disinfection of Sewage. Sewage is disinfected in order 
to protect public water supplies, shell fish, and bathing beaches; 
to prevent the spread of disease; to keep down odors, and to 
delay putrefaction. Disinfection is the treatment of sewage 
by which the number of bacteria is greatly reduced. Steriliza- 
tion is the destruction of all bacterial life, including spores. 
Ordinarily even the most destructive agents do not accomplish 
complete sterilization. Chlorine and its compounds are practi- 
cally the only substances used for the disinfection of sewage. 
The liine used in chemical precipitation, the acid used in the Miles 



490 ACIDIFICATION, ELECTROLYSIS, DISINFECTION 



Acid Process, the aeration in the activated sludge process, all 
serve to disinfect sewage, but are not used primarily for that 
purpose. Copper sulphate has been used as an algaecide but 
never on a large scale as a bactericide. 1 Heat has been suggested, 
but its high cost has prevented its practical application to the 
disinfection of sewage. 

TABLE 102 

COST OF ELECTROLYTIC TREATMENT, ELMHURST, LONG ISLAND, AND 
EASTON, PENNSYLVANIA 







Three 




One Million Gallon 


Million 






Gallon 


Item 


unit at 


unit at 


unit at 




Easton, 


Elmhurst, 


Elmhurst, 




Dollars 


Dollars 


Dollars 


Hydrated lime: 








Elmhurst, 1300 pounds at $7.90 ton. 1 
Easton, 3720 pounds at $6.75 ton. j 


12.56 


5.14 


15.42 


Electric power electrolysis: 








Elmhurst, 85 kw-h. at 4 cents j 
Easton, 185 . 5 kw-h. at 2 . 26 cents j 


4.19 


3.40 


9.60 


Electric power, light and agitation: 








Elmhurst, 60 kw-h. at 4 cents | 
Easton, 6 . 25 kw-h. at 8 . 05 cents J 


0.50 


2.40 


7.20 


Heating 


1.25 






Labor and supervision 


15.00 


12 50 


15 00 


Maintenance, repairs and supplies 


1 50 


1 00 


3 00 


Sludge pressing and removal 




5 11 


15 33 


Total 








35.00 


29 55 


65 55 


Cost per million gallons < 


35.00 


29.55 


21.86 





The action which takes place on the addition to sewage of 
chlorine or its compounds is not well understood. The idea that 
the bacteria are burned up with " nascent " or freshly born 
oxygen, has been exploded. 2 Likewise the idea that the toxic 
properties of chlorine have no effect has not been borne out by 

1 Reference 24. 
. 2 Inorganic Chemistry, by Alexander Smith. 



DISINFECTION OF SEWAGE 491 

experiments. It has been demonstrated, particularly by tests 
on strong tannery wastes, that the action of chlorine gas is more 
effective than the application of the same amount of chlorine 
in the form of hypochlorite. All that we are certain of at present 
is that the greater the amount of chlorine added under the same 
conditions, the .greater the bactericidal effect. 

Chlorine is applied either in the form of a bleaching powder 
or a gas. In ordinary commercial bleach (calcium hypochlorite) 
the available chlorine is about 35 to 40 per cent by weight. In 
order to add one part per million of available chlorine to sewage 
it is necessary to add about 25 pounds of bleaching powder or 
85 pounds of liquid chlorine per million gallons of sewage. This 
can be computed as follows: 

The molecular weight of calcium hypochlorite is 
127.0. This reacts to produce two atoms of available 
chlorine with a molecular weight of 70.9. If the bleach- 
ing powder were pure the available chlorine would there- 
fore represent 70.9 -f- 127, or 56 per cent of its weight. 
Then to obtain one pound of chlorine it would be neces- 
sary to have 1.79 pounds of pure bleaching powder. 
Since 1,000,000 gallons of water weigh approximately 
8,300,000 pounds, in order to apply one part per million of 
chlorine to 1,000,000 gallons of sewage it is necessary to 
apply 1.79X8.3 or 14.9 pounds of pure bleaching powder. 
Commercial bleaching powder is only about 60 per cent 
calcium hypochlorite. It is therefore necessary to add 
14.9 -j- 0.60 or about 25 pounds of commercial bleach. 

Since liquid chlorine is very nearly pure, approxi- 
mately 85 pounds of it applied to 1,000,000 gallons of 
sewage are equivalent to a dose of one part per million. 

Commercial bleaching powder is a dry white powder which 
absorbs moisture slowly, and which loses its strength rapidly 
when exposed to the air. It is packed in air-tight sheet iron 
containers, which should be opened under water, or emptied 
into water immediately on being opened. The strength of the 
solution should be from \ to 1 per cent. The rate of the appli- 
cation of the solution to the sewage may be controlled by auto- 
matic feed devices, or by hand-controlled devices. 

Commercial liquid chlorine is sold in heavy cast steel con- 
tainers, which hold 100 to 140 pounds of liquid chlorine under a 
pressure of 54 pounds per square inch at zero degrees C. or 121 
pounds per square inch at 20 degrees. 



492 ACIDIFICATION, ELECTROLYSIS, DISINFECTION 

The amount of chlorine used is dependent on the character 
of the sewage to be treated, the stage of decomposition of the 
organic matter, the desired degree of disinfection, the period of 
contact, and the temperature. The amount of chlorine is 
expressed in parts per million of available chlorine, regardless of 
the form in which the chlorine is applied. In general about 15 
to 20 parts per million of available chlorine with 30 minutes' 
contact at a temperature of about 15 C. will effect an apparent 
removal of 99 per cent of the bacteria from the raw sewage. 
The effect is only apparent because many of the bacteria encased 
in the solid matter of the sewage escape the effect cf the chlo- 
rine, or detection in the bacterial analysis. Stronger and older 
sewages, higher temperatures, and shorter periods of contact 
will demand more chlorine to produce the same results. A 
septic effluent will require more chlorine than a raw sewage 
because of the greater oxygen demand by the septic sewage. 
The results of experiments on disinfection made at different 
testing stations have shown such wide variations in the amount 
of chlorine necessary, as to demonstrate the necessity for inde- 
pendent studies of any particular sewage which is to be chlori- 
nated. For instance, at Milwaukee approximately 13 p.p.m. 
of available chlorine applied to an Imhoff tank effluent effected 
a 99 per cent removal of bacteria, whereas the same result was 
obtained at Lawrence, Mass., on crude sewage with only 6.6 
p.p.m. and at Marion, Ohio, only 9 per cent removal of bacteria 
was obtained by the addition of 4,815 p.p.m. to crude sewage. 
The Ohio and Massachusetts reports show irrational variations 
among themselves. For instance, 6.2 p.p.m. applied to a septic 
effluent effected 88 per cent removal whereas in another case 
7.6 p.p.m. effected only 36 per cent removal. At Lawrence in 
one case it took 8.6 p.p.m. to remove 99 per cent from a sand 
filter effluent, but only 6.3 p.p.m. to effect the same result in the 
effluent from a septic tank. The most consistent results are 
those found at Milwaukee which show a steadily increasing 
percentage removal with increasing amounts of chlorine. 

Some time after sewage has received its dose of chlorine the 
number of bacteria may be greater than in the raw* sewage. 
Such bacteria are called after-growths. Certain forms of bac- 
teria, particularly the pathogenic or body temperature types, 
are most susceptible to disinfecting agents. These are killed 



REFERENCES 493 

off and leave the sewage in a condition more favorable to the 
growth of more resistant fonns of bacteria. As the latter are 
non-pathogenic and are generally aerobic their presence is 
usually more beneficial than detrimental, as they hasten the action 
of self-purification. 

REFERENCES 

The following abbreviations will be used: E.G. for Engineering and 
Contracting, E.N. for Engineering News, E.R. for Engineering Record, 
E.N.R. for Engineering News-Record, M.J. for Municipal Journal, p. for 
page, and V. for volume. 

No. 

1. Grease and Fertilizer Base for Boston Sewage, by Weston, E.N. V. 75, 

1916, p. 913 and Journal American Public Health Association, April, 
1916. 

2. Getting Grease and Fertilizer from City Sewage, by Allen. E.N. V. 75, 

1916, p. 1005. 

3. New Haven Tests Five Processes of Sewage Treatment. E.N.R. V. 79, 

1917, p. 829. 

4. Recovery of Grease and Fertilizer from Sewage Comes to the Front. 

E.N.R. V. 80, 1916, p. 319. 

5. Miles Acid Process may Require Aeration of Effluent, by Mohlman. 

E.N.R. V. 81, 1918, p. 235. 

6. Promising Results with Miles Acid Process in New Haven Tests. 

E.N.R. V. 81, 1918, p. 1034. 

7. Baltimore Experiments on Grease from Sewage. E.N. V. 75, 1916, 

p. 1155. 

8. Report on Industrial Wastes from the Stock Yards and Packingtown 

in Chicago to the Trustees of the Sanitary District of Chicago, 1914, 
pp. 187-195. 

9. The Separation of Grease from Sewage, by Daniels and Rosenfeld. 

Cornell Civil Engineer. V. 24, p. 13. 

10. The Separation of Grease from Sewage Sludge with Special Reference 

to Plants and Methods Employed at Bradford and Oldham, England, 
by Allen. E.G. V. 40, 1913, p. 611. 

11. Acid Treatment of Sewage, by Dorr and Weston. Journal Boston 

Society of Civil Engineers, April, 1919. E.G. V. 51, 1919, p. 
510. M. J. V. 46, 1919, p. 365. 

12. The Miles Acid Process for Sewage Disposal. Metallurgical and Chemical 

Engineering, V. 18, p. 591. 

13. Miles Acid Treatment of Sewage, by Winslow and Mohlman. Journal 

American Society Municipal Improvements, Oct., 1918. M. J. V. 45, 

1918, pp. 280, 297, and 321. 

14. New Electrolytic Sewage Treatment. M.J. V. 37, 1914, p. 556. 



494 ACIDIFICATION, ELECTROLYSIS DISINFECTION 

No. 

15. Electrolytic Sewage Treatment. M.J. V. 47, 1919, p. 131. 

16. Electrolytic Treatment of Sewage at Durant, Oklahoma, by Benham. 

E.N. V. 76, 1916, p. 547. Municipal Engineering, V. 49, 1916, 
p. 141. 

17. Electrolytic Treatment of Sewage at Elmhurst, Long Island, by Travis. 

Report to the President of the Borough of Queens, Aug. 31, 1914. 
E.R. V. 70, 1914, pp. 292, 315, and 429. M.J. V. 39, p. 551. Muni- 
cipal Engineering, V. 47, p. 281. 

18. Tests of the Electrolysis of Sewage at Toronto, by Nevitt. E.N. V. 71, 

1914, p. 1076. 

19. Electrolytic Treatment of Sewage Little Better than Lime Alone, by 

Bartow. E.R. V. 74, 1916, p. 596. 

20. Electrolytic Sewage Treatment Not Yet an Established Process. 

E.N.R. V. 83, 1919, p. 541. 

21. Tests of Electrolytic Sewage Treatment Process at Easton, Pa. Journal 

of the Franklin Institute, Aug., 1919. E.N.R. V. 83, 1919, p. 569. 

22. The Disinfection of Sewage. U. S. Geological Survey, Water Supply 

Paper, No. 229. 

23. Sewage Disinfection in Actual Practice, by Orchard. E.R. V. 70, 1914, 

p. 164. 

24. Water "and Sewage Purification in Ohio. Report of the Ohio State Board 

of Health, 1908, pp. 738-762. 

25. Water Purification, by Ellms. Published in 1917 by McGraw-Hill 

Book Co. 

26. Electrolytic Sewage Treatment, A Half Century of Invention and 

Promotion. E.N.R. V. 86, 1921, p. 25. 



CHAPTER XX 
SLUDGE 

278. Methods of Disposal. Sludge is the deposited suspended 
matter which accumulates as the result of the sedimentation 
of sewage. The methods for the disposal of sludge as discussed 
herein will include the disposal of scum. Scum is a floating 
mass of sewage solids buoyed up in part by entrained gas or 
grease, forming a greasy mat which remains on the surface of the 
sewage. 1 The sludges formed by different methods of sewage 
treatment are described in the chapter devoted to the particular 
method. The disposal of sludge is a problem common to all 
methods of sewage treatment involving the use of sedimentation 
tanks. 

Sludge is disposed of by: dilution, burial, lagooning, burning, 
filling land, and as a fertilizer or fertilizer base. Certain methods 
of disposal, such as burning or as a fertilizer, demand that the 
sludge be dried preparatory to disposal. Sludge is dried on dry- 
ing beds, in a centrifuge, in a press, in a hot-air dryer, or by 
acid precipitation. 

279. Lagooning. This is a method of sludge disposal in 
which fresh sludge is run on to previously prepared beds to a 
depth of 12 to 18 inches or more, and allowed to stand without 
further attention. The preparation of the lagoons requires 
leveling the ground, building of embankments, and, if the 
ground is not porous, the placing of underdrains laid in sand 
or gravel. At Reading, Pa., 2 approximately one acre was 
required for 1,700 cubic yards of wet sludge. The results of 
lagooning at Philadelphia are given in Table 103. 2 

1 American Public Health Association definition. 
* Sewage Sludge by Allen. 
495 



496 



SLUDGE 



TABLE 103 

RESULTS OF DRYING SLUDGE IN LAGOONS AT PHILADELPHIA 
("Sewage Sludge" by Allen) 



Treatment 


Days 


Depth, 
Inches 


Per Cent, 
Moisture 


Rainfall, 
Inches 


Cubic Yards 
per Acre 


Screened 





12.20 


82 8 





1600 


Screened 


26 


7 67 


57 


o 


1000 


Screened 


49 


3 50 


51 6 


43 


470 


Screened 





13.50 


90 1 


o 


1800 


Screened 


62 


7 00 


61 


3 14 


950 


Crude 





12 00 


88 7 





1600 


Crude 


59 


4.70 


62 8 


2 59 


640 















During the period of standing in the lagoon the moisture 
drains out and evaporates and the organic matter putrefies, 
giving off gases and foul odors. In the course of three to six months, 
biological action ceases and the sludge has become humified and 
reduced to about 75 per cent moisture. In the utilization of 
this method of disposal the lagoons must be removed from 
settled districts and should occupy land of little value for other 
purposes. The odors created at the lagoons may be intense 
and offensive. The land so used is rendered unfit for other pur- 
poses for many years. 

The digestion of sludge in special tanks is a form of lagooning 
in which an attempt is made to maintain septic action as a result 
of which a portion of the sludge is gasified or liquefied, leaving 
less to be cared for by some of the other methods of treatment 
or disposal. The results obtained by digestion tanks has not 
been entirely satisfactory. A partial drying and consolidation 
of the sludge may be effected, however, by the process of decanta- 
tion, in which the supernatant liquid is run off, followed by further 
sedimentation, rendering the final product more compact. 

280. Dilution. In the disposal of sludge by dilution, as in 
the disposal of sewage by dilution, there must be sufficient 
oxygen available in the diluting water to prevent putrefaction, 
and a swift current to prevent sedimentation. Such conditions 
exist in localities along the sea coast, and in communities 



DILUTION 497 

situated near rivers, when the rivers are in flood. In some sea- 
coast towns, for example at London and Glasgow, the sludge is 
taken out to sea in boats, and dumped. Since it is not necessary 
to discharge sludge continuously, it can be stored to advantage 
in the digestion chamber of a tank, until the conditions in the 
body of diluting water are suitable to receive it. 

The amount of diluting water to receive sewage sludge has not 
been sufficiently well determined to draw reliable general con- 
clusions. A dilution of 1,500 to 2,000 volumes may be considered 
sufficiently safe to avoid a nuisance provided there is a sufficient 
velocity to prevent sedimentation. Johnson's Report on Sew- 
age Purification at Columbus, Ohio (1905), states that a dilution 
of 1 to 800 is sufficient to avoid a nuisance. The character 
of the sludge has a marked effect on the proper ratio of dilution, 
the sludge from septic and sedimentation tanks requiring a 
greater dilution than that from Imhoff tanks. 

281. Burial. Sludge can be disposed of by burial in trenches 
about 24 inches deep with at least 12 inches of earth cover, 
without causing a nuisance. The ground used for this purpose 
should be well drained. This method of disposal is generally 
used as a makeshift and has not been practiced extensively 
because of the large amount of land required. Insufficient infor- 
mation is available to generalize on the amount of land required 
or the time before the land can be used for further sludge burial, 
or for other purposes. Indications are that the sludge may 
remain moist and malodorous for years and that the land may be 
rendered permanently unfit for further sludge burial. Under 
some conditions the land may be used again for the same or 
other purposes. For example, Kinnicutt, Winslow and Pratt * 
state that 500 tons of wet sludge can be applied per acre and : 

The same land, it is claimed, can be used again after 
a period of a year and a half to two years, if in two months 
or so after covering the sludge with earth, the ground is 
broken up, planted, and, when the crop is removed, again 
plowed and allowed to remain fallow for about a year. 

282. Drying. Before sludge can be disposed of to fill land, 
by burning, or for use as a fertilizer filler it must be dried to a 
suitable degree of moisture. The removal of moisture from the 

1 Sewage Disposal by Kinnicutt, Winslow and Pratt. 



498 SLUDGE 

sludge decreases its volume and changes its characteristics so 
that sludge containing 75 per cent moisture has lost all the char- 
acteristics of a liquid. It can be moved with a shovel or fork, 
and can be transported in non-watertight containers. A reduction 
in moisture from 95 to 90 per cent will cut the volume in half. 

The change in volume on the removal of moisture can be 
represented as: 

7(100- P) 
(100- Pi)' 

in which P = the original percentage of moisture; 
PI = the final percentage of moisture; 
7= the original volume; 
7i = the final volume. 

The drying of sludge on coarse sand filter beds is more 
particularly suited to sludge from Imhoff tanks. This sludge 
does not decompose during drying, and is sufficiently light and 
porous in texture to permit of thorough draining. The sludge 
from plain sedimentation or chemical precipitation tanks is 
high in moisture, putrescible, and when placed on a filter bed 
it settles into a heavy, compact, impervious mass which dries 
slowly. In order to avoid this condition the sludge is run on to 
the beds as quickly as possible, to a depth of not more than 6 
to 10 inches. Lime is sometimes added to the sludge at this 
time as it aids drying by assisting in the maintenance of the 
porosity of the sludge, and it is advantageous in keeping down 
odors and insects. 

Sludge filter beds are made up of 12 to 24 inches of coarse 
sand, well-screened cinders, or other gritty material, underlaid 
by 6 inches of coarse gravel and 6 or 8-inch open-joint tile 
underdrains, laid 4 to 10 feet apart on centers, dependent on the 
porosity of the subsoil. The side walls of the filters are made of 
planks or of low earth embankments. The sludge filters at 
Hamilton, Ontario, are shown in Fig. 179. 

The size of the bed is dependent mainly upon the character- 
istics of the sludge. For Imhoff tank sludge which comes from 
the tank with about 85 per cent moisture, the practice is to 
allow about 350 1 square feet of filter surface per 1,000 popu- 
1 Sewage Disposal by Fuller. 



DRYING 



409 



lation contributing sludge. For other types of sludge the area 
varies from 900 to 9,000 square foot per 1,000 population con- 
tributing sludge, and only experiments with the sludge in hand 
can determine the proper allowance. Imhoff recommends 1,080 




^S"C.f.S/udga Pipe. 

Part Plan, 
Movable 

Wooden Trough-, 



l^'Screentdt 



Concrete &&& 




Part Cross Section A-B,' 



FIG. 179. Sludge-drying Beds at Hamilton, Ontario. 
Eng. News, Vol. 73, p. 426. 

square feet per 1,000 population for septic tank sludge, and 
6,480 square feet for sludge from plain sedimentation tanks. 1 
Kinnicutt, Winslow, and Pratt in their book on Sewage Disposal 
state: 

With an average depth of 10 inches per dose of sludge 
of 87 per cent water content, one square foot of covered 
(glass) bed should dry to a spadable condition one cubic 
yard of sludge per year. 



Sewage Sludge by Allen. 



500 SLUDGE 

The sludge is run on the bed in small quantities at periods from 
two weeks to a month apart. In favorable weather Imhoff 
sludge will dry in two weeks or less to approximately 50 to 60 
per cent moisture. It is then suitable for use as a filling material 
on waste land, for burning, or for further drying by heat. Glass 
roofs, similar to those used on green-houses, have been used to 
speed the drying process by preventing the moistening of partly 
dried sludge during rainy weather. In some instances sludge 
has dried to 10 per cent moisture on such beds. Imhoff sludge 
can be removed from the drying beds with a manure or hay 
fork. It has an odor similar to well-fertilized garden soil. It 
is stable, dark brownish-gray in color, is of light coarse material, 
and is granular in texture. 

Sludge presses are suitable for removing moisture from the 
bulky wet sludge obtained from plain sedimentation, chemical 
precipitation, and the activated sludge process. The details 
of a typical sludge press are shown in Fig. 180. The press 
shown is made up of a number of corrugated metal plates about 
30 inches in diameter with a hole in the center about 8 inches 
in diameter. The corrugations run vertically except for a distance 
about 3 inches wide around the outer rim, which is smooth. To 
this smooth portion is fastened, on each side of the plate, an 
annular ring about an inch thick and 2 to 3 inches wide, 
of the same outside diameter as the plate. A circular piece 
of burlap, canvas, or other heavy cloth is fastened to this 
ring, covering the plate completely. A hole is cut in the center 
of the cloth slightly smaller in diameter than the center hole 
in the plate, and the edges of the cloth on opposite sides of 
the plate are sewed together. The plates are then pressed 
tightly together by means of the screw motion at the left end 
of the machine, thus making a water-tight joint at the outer 
rim. Sludge is then forced under pressure into the space 
between the plates, passing through the machine by means of 
the central hole. The pressure ori the sludge may be from 50 
to 100 pounds per square inch. This pressure forces the water 
out of the sludge through the porous cloth from which it escapes 
to the bottom of the press along the corrugations of the sepa- 
rating plate. After a period of 10 to 30 minutes the pressure 
is released, the cells are opened, and the moist sludge cake is 



DRYING 



501 



removed. The liquid pressed from the sludge is highly putres- 
cible and should be returned to the influent of the treatment 
plant. The pressing of wet greasy sludges is facilitated by the 
addition of from 8 to 10 pounds of lime per cubic yard of sludge. 
The cake thus formed is more cohesive and easy to handle. The 
output of the press depends so much on the character of the 
sludge that a definite guarantee of capacity is seldom given by 
the manufacturer. 

The simplest form of centrifugal sludge dryer is a machine 
which consists of a perforated metal bowl lined with porous 
cloth in which the sludge is placed. Surrounding this bowl is 




FIG. 180. Filter Press. 

a second water-tight metal bowl so arranged as to intercept the 
water thrown from the sides of the inner bowl as it revolves. 
The peripheral velocity of the inner bowl is about 6,000 feet per 
minute, which makes the effective weight of each particle about 
250 times its normal weight when at rest. Very few data are 
available on the operation of such machines, and their use has 
not been extensive because of the difficulty of starting and 
stopping the machine at each filling, and the difficulty of removing 
the partially dried sludge from the inner basket. The Besco- 
ter-Meer centrifuge, manufactured by the Barth Engineering 
and Sanitation Co., can be operated continuously and the diffi- 
culties of removing the dried sludge from the machine have 



502 



SLUDGE 



been overcome. According to the manufacturers the centri- 
fuge has been operated very successfully in Germany on plain 
septic tank sludge. A removal of 70 per cent of suspended solids 
in the raw sludge and a production of 3,600 pounds of sludge 
per hour, containing 60 to 70 per cent of moisture, can be obtained 
at less than 900 r.p.m. with a consumption of 15 horse-power. 
Extensive tests of the machine were made at Milwaukee 
from October, 1920, to September, 1921, on activated sludge, 





Besco-ter-Meer Sludge Drying Centrifuge at Milwaukee, Wisconsin 
Courtesy, Barth Engineering and Sanitation Co. 

but results of these tests are not as yet available. Indications 
are that the centrifuge has acted as a classifier. The coarser 
particles of sludge have been removed but the finer particles 
have been continuously returned with the liquid to the 
sedimentation tank, ultimately filling this tank with fine 
particles of sludge. An illustration of the unit tested at Mil- 
waukee is shown on this page. 



DRYING 



503 



Experiments on the drying of sludge by acid flotation have 
not progressed sufficiently to allow the installation of a working 
unit. The method, which has been applied principally to 
activated sludge, consists in adding a small amount of sulphuric 
acid to the sludge as it leaves the storage tank. The sludge is 
coagulated by this action, the coagulated material rising to the 
surface as a scum containing about 86 per cent moisture. The 
consistency is such that it can be removed with a shovel. The 
liquid can be withdrawn continuously from below the scum. 

The moisture content of sludge to be used in the manufacture 
of fertilizer must be reduced to 10 per cent or less. None of 




FIG. 181. Direct-Indirect Sludge Dryer. 

Courtesy, the Buckeye Dryer Co. 

the methods of drying described so far can be relied upon for 
such a product and it becomes necessary to use direct or indirect 
heat dryers. There are various types of dryers on the market. 
The details of a Buckeye dryer are shown in Fig. 181. In the 
operation of this machine moist sludge is fed in at the left end 
at the point marked " feed." The hot gases pass from the fire box 
up and around the cylinder which revolves at about eight r.p.m. 
The gases are drawn into the inner cylinder through the open- 
ings marked A which revolve with the two cylinders. The gases 
escape from the inner cylinder through the openings to the 
right and flow towards the left in the outer cylinder, They come 



504 SLUDGE 

in contact with the sludge at this point. The gases then pass 
off through the fan at the left. The sludge is lifted by the small 
longitudinal baffles fastened to the outer cylinder, as the drying 
cylinders revolve. The right end of the cylinder is placed lower 
than the left so that the drying sludge is lifted and dropped 
through the cylinder at the same time that it moves slowly 
toward the right-hand end of the cylinder. These dryers 
require about one pound of fuel for 10 pounds of water evaporated. 
The odors from the dryer can be suppressed by passing the gases 
through a dust chamber and washer. 

A summary of the results from methods of sludge drying 
at Milwaukee 1 follows : 

Excess sludge produced, 12,100 gallons, having 97.5 
per cent moisture, per million gallons of sewage treated. 

Sludge cake produced (by presses), 10,083 pounds 
having 80.3 per cent moisture, per million gallons of 
sewage treated. 

Dried sludge (from heat driers) produced, 2,521 pounds 
having 10 per cent moisture, per million gallons of sewage 
treated. 

Press will produce 3 pounds of cake per square foot of 
filter cloth in four and a half hours, or five operations per 
twenty-four hours. 

Dryers will reduce 6,700 pounds of sludge cake at 
80 per cent moisture to 10 per cent moisture, and will 
evaporate 8 pounds of water per pound of combustible. 

Thickening devices known as Dorr thickeners, patented and 
manufactured by the Dorr Co. and originally intended for 
metallurgical purposes, have been adapted to the thickening 
of sewage sludge. These thickeners are circular sedimentation 
tanks, from 8 to 12 feet deep, more or less, and are made in any 
diameter up to 200 feet or more. An arm, pivoted in the center 
and extending to the circumference, is provided at the bottom 
with a number of baffles or squeegees set at an angle with the 
arm. The arm revolves at from one to fifteen revolutions per 
hour, and the squeegees, in contact with the bottom of the tank, 
scrape the deposited sludge towards a central sump, from which 

1 From Eng. News-Record, Vol. 84, 1920, p. 995. 



DRYING 505 

it is removed by a pump or by gravity, without interrupting 
the operation of the thickener. The sludge so thickened may be 
reduced to 95 or 96 per cent moisture. These devices are ordi- 
narily used only in the activated sludge process in which they 
have been a pronounced success. 



CHAPTER XXI 
AUTOMATIC DOSING DEVICES 

283. Types. Automatic dosing devices are used to apply 
sewage to contact beds, trickling filters, and intermittent sand 
filters. These devices can be separated into two classes; those 
with moving parts and those without moving parts. The latter 
are better known as air-locked dosing devices. Simple devices 
without moving parts are less liable to disorders and are nearer 
" fool-proof " than any device depending on moving parts for 
its operation. 

No one type of moving part device has been used extensively 
in different sewage treatment plants. Designing engineers have 
exercised their ingenuity at different plants, resulting in the 
production of different types. 1 Among the best known forms 
is the apparatus designed by J. W. Alvord for the intermittent 
sand filters at Lake Forest, Illinois. 2 In its operation. , , , 

A float in the dosing chamber lifts an iron ball in one 
of a series of wooden columns, and at a certain height 
the ball rolls through a trough from one column to the 
next, in its passage striking a catch, which opens an air 
valve attached to one of ten bell-siphons in the dosing 
chamber. Each of the siphons discharges on one of the 
ten sand beds, which are thus dosed in rotation. 

Since air-locked dosing devices are in more general use their 
operation will be explained in greater detail. 

284. Operation. The simplest form of these devices is the 
automatic siphon used for flush-tanks, the operation of which 
is described in Art. 61. 

In the operation of sand filters, sprinkling filters, or other 
forms of treatment where there are two or more units to be dosed 

1 A Simple Mechanical Control for Dosing Sewage Beds, by P. Thompson, 
Eng. News-Record, Vol. 84, 1920, p. 1018. 

2 Sewage Disposal by Kinnicutt, Winslow and Pratt. 

506 



OPERATION 



507 



it is desirable that the dosing of the beds be done alternately. 
A simple arrangement for two siphons operating alternately is 
shown in Fig. 182. They operate as follows: with the dosing 
tank empty at the start water will stand at W in siphon No. 2 
and at aa f in siphon No. 1. As the water enters through the 
inlet on the left the tank fills. When the water rises sufficiently, 
air is trapped in the bells, and as the water continues to rise in 
the tank, surfaces a and 6 are depressed an equal amount. When 
b has been depressed to d, a has been depressed to c. Air is 
released from siphon No. 2 through the short leg, and siphon 




Plan. 
.-Bell-. 




: y i/ 


< Dosing 


rti 


f 


~ 1 


J.L.S 




= 


Tank 




i 


= \ 




=. V 




















! - 




[ 














^Sm'ft 




^^ 






Pipe 






Section 








I 


A-A 


k.2 





Fio. 182. Diagram Showing the Operation of Two Alternating Siphons. 

No. 2 goes into operation. Surface c rises in siphon No. 1 as 
the tank empties and when the action of Siphon No. 2 is broken 
by the admission of air when the bottom of the bell is uncovered 
the water in siphon No. 1 has assumed the position of 66' and that 
in No. 2 is at aa'. The conditions of the two siphons are now 
reversed from that at the beginning of the operation and as the 
tank refills siphon No. 1 will go into operation. It is to be 
noted that these siphons are made to alternate by weakening 
the seal of the next one to discharge and by strengthening the 
seal of the one which has just discharged. 



AUTOMATIC DOSING DEVICES 



285. Three Alternating Siphons. This principle can be 
extended to the operation of three alternating siphons as shown 
in Fig. No. 183. These operate as follows: with the dosing 
tank empty at the start and water at aa r in siphons 1 and 2, 
and at bb r in siphon No. 3, the dosing tank will be allowed to 
fill. As the water rises in the tank air is trapped in all the bells 
and surfaces a and 6 are depressed. When surface b has been 
depressed to d, a has been depressed to c. Air is released from 
siphon No. 3 and this siphon goes into action. Surface c rises 





Plan 



c 




y 


,.-1 


Ie/A r v 




E' ir 


r~ii 




r if 


j 






















3 


8 

L 


^ 


^d 



^ 




j 


Section 


j 


A-A. 



FIG. 183. Diagram Showing the Operation of Three Alternating Siphons. 

in siphons 1 and 2 to the position 6, as the dosing tank is emptied. 
At the same time a small amount of water is passed from siphon 
No. 3 to the short leg of siphon No. 1, through the small pipes 
shown, thus filling this leg so that when siphon No. 3 ceases to 
operate the water in siphons 1 and 3 stands at aa' and that in 
No. 2 stands at 66'. Siphon No. 2, having the weaker seal, 
will be the next to operate. During its operation it will fill 
siphon No. 3, leaving No. 1 weak. When No. 1 operates it will 
refill No. 2, leaving No. 3 weak, thus completing a cycle for the 
three siphons. This principle has not been applied to the operation 



FOUR OR MORE ALTERNATING SIPHONS 



50!) 



of more than three alternating siphons and is seldom used on recent 
installations. 

286. Four or More Alternating Siphons. An arrangement 
for the alternation of four or more siphons is illustrated in Fig. 184. 
At the commencement of the cycle it will be assumed that all 
starting wells are filled with water except well No. 1, and that all 
main and all blow-off traps are filled with water. The following 




u 



FIG. 184. Miller Plural Alternating Siphons. 
Courtesy, Pacific Flush Tank Co. 



Section. 



description of the operation of the siphons is taken from the 
catalog of the Pacific Flush Tank Company : 

The liquid in the tank gradually rises and finally 
overflows into the starting well No. 1 and the starting 
bell being filled with air, pressure is developed which is 
transm tted, as shown by the arrows, to the blow-off 
trap connected with siphon No. 2. When the discharge 
line is reached, sufficient head is obtained on the starting 
bell to force the seal in blow-off trap No. 2, thus releasing 
the air confined in siphon No. 2 and bringing it into full 
operation. 



510 AUTOMATIC DOSING DEVICES 

During the time that siphon No. 2 i is operating, 
siphonic action is developed in the draining siphon con- 
nected with starting well No. 2 and as soon as the level 
in the tank is below the top of the well it is drained down 
to a point below the bottom of starting well No. 2. It 
can now be seen that after the first discharge starting well 
No. 2 is empty, whereas the other three are full. . . . There- 
fore when the tank is filled the second time, pressure is 
developed in starting bell No. 2, which forces the seal of 
blow-off trap No 3, thus starting siphon No. 3. ... 

This alternation can be continued for any number of siphons. 
Other arrangements have been devised for the automatic con- 
trol of alternating siphons, but these principles of the air-locked 
devices are fundamental. 

287. Timed Siphons. In the operation of a number of 
contact beds not only must the dosing of the tanks be alternated, 
but some method is needed by which the beds shall be automat- 
ically emptied after the proper period of standing full. To fulfill 
this need the principle of the timed siphon must be employed 
in conjunction with the alternating siphons. Fig. 185 illustrates 
the operation of the Miller timed siphon. Its operation is as 
follows: water is admitted to the contact bed and transmitted 
to the main siphon chamber through the " opening into bed." 
Water flows from the main siphon chamber into the timing 
chamber at a rate determined by the timing valve. The con- 
tact bed is held full during this period. As the tuning chamber 
fills with water air is caught in the starting bell and the pressure 
is increased until the seal in the main blow-off trap is blown and 
the main siphon is put into operation. As the water level in the 
main siphon chamber descends, water flows from the tuning 
chamber into the main siphon through the draining siphon 
and the timing chamber is emptied, ready to commence another 
cycle. 

288. Multiple Alternating and Timed Siphons. 1 The alter- 
nating and timing of a number of beds is more complicated. 
The arrangement necessary for this is shown in Fig. 186. It 
will be assumed at the start that all beds are empty and that all 
feeds are air locked as shown in Section AB except that to bed 
No. 4 into which sewage is running. As bed No. 4 fills, sewage 

1 Design of Siphon by G. H. Bayles, Eng. News-Record, Vol. 84, 1920, 
p. 974. 



MULTIPLE ALTERNATING AND TIMED SIPHONS 511 



is transmitted .through the opening in the wall into the timed 
siphon chamber No. 4. When the level of the water in the bed 
and therefore in this chamber has reached the top of the with- 
draw siphon leading to the compression dome chamber No. 4, 
this latter chamber is quickly filled. The air pressure in starting 
bell No. 4a is transmitted to blow-off trap No. la. The seal 
of this trap is blown, releasing the air lock in feed No. 1 and the 

^-Opening intoBed 



Air Vent is not necessary 
where Siphon discharges 
into an open Carrier, or 
Outlet is not more than 




o 



FIG. 185. Miller Timed Siphon. 

Courtesy, Pacific Flush Tank Co. 



flow into bed No. 1 is commenced. At the same time the air 
pressure in compression dome No. 4 is transmitted to feed No. 4, 
air locking this feed and stopping the flow into bed No. 4. The 
alternation of the feed into the different beds is continued in this 
manner. 

Bed No. 4 is now standing full and No. 1 is filling. When 
compression dome chamber No. 4 was filled, water started 
flowing through timing siphon valve No. 4 into timing chamber 



512 



AUTOMATIC DOSING DEVICES 



No. 4 at a rate determined by the amount of the opening of the 
timing valve. As this chamber fills compression is transmitted 
to blow-off trap 46 and when sufficiently great this trap is blown 
and timed siphon No. 4 is put into operation. Bed No. 4 is 
emptied by it, and compression dome chamber No. 4 is emptied 

Blow-off TrapNo.la 
Bed No. I 




FIG. 186. Plural Timed and Alternating Siphons for Contact Bed Control. 
Courtesy, Pacific Flush Tank Co. 

through the withdraw siphon at the same time. This com- 
pletes a cycle for the filling and emptying of one bed and the 
method of passing the dose on to another bed has been explained. 
The principle can be extended to the operation of any number 
of beds. 



INDEX 



A. B. C. process of sewage treat- 
ment, 4 

Abandonment of contract, 225 
Access to work, 228, 229 
Accident, contractor's responsibility, 

221, 224 

Acetylene, explosive, 347 
Acid precipitation. See Miles Acid 
Process. 

of sludge, 503 

Acids as disinfectants, 489, 490. 
Activated sludge. Chapter XVIII, 
465-179 

advantages and disadvantages, 
469, 470 

aeration tank, 471, 472 

air diffusion, 475, 477 

air distribution, 473-478 

air quantity, 475, 476 

area of filtros plates, 478 

colloid removal, 358 

composition, 465-469 

cost, 478, 479 

definition, 466 

dewatering, 468, 469, 497-505 

fertilizing value, 469, 470 

historical, 470, 471 

how obtained, 478 

nitrogen content, 468 

patent, 471 

process, 465 

quantity, 469 

reaeration tank, 473 

results, 467, 468, 476 

sedimentation tank, 472 
Advertisement, 214 



Aeration, effect on oxygen dissolved, 
373-375 

of sewage, 371, 376, 465-479 
Aerobes, 363 

Aerobic decomposition, 366, 367 
Aftergrowths, 492 
Aggregates, specifications, 172-174 
Air, see also ventilation, activated 
sludge, compressed air, etc. 

ejectors, 150 

lock dosing apparatus. Chap. XXI, 
506-512 

machinery for activated sludge, 

473, 474 
Algae, 363 
Alkalinity, 358 
Alleys, sewers in, 80 
Alum, 407, 408 
Alvord tank, 427, 429 
Ammonia, 366, 367, 374, 375, 410 

explosives, 297 
Analyses, bacteriological, 364 

chemical, 354, 355 

mechanical of sand, 182 

physical, 352-354 

sewage, 352-364 
Anerobes, 363, 365-367 
Anaerobic, action, 410 

bacteria, 363 

conditions, 367 

decomposition, 365-367 
Ann Arbor, Michigan. Population, 

14 
Annual expense, method of financing, 

157, 158 
Ansonia air ejector, 150, 151 



513 



514 



INDEX 



Antibiosis, definition, 363 
Appurtenances to sewers. Chap. VI, 

99-115 
Arch, analyses, 204-208 

elastic method, 206-208 
vouissoir analysis, 204-206 
brick construction, 312, 313 
centers for brick sewers, 313 
concrete construction, 318-321 
Ardern and Lockett, development of 

activated sludge, 467, 468, 471 
Area of cities, 31 
Asphyxiation in sewer gas, 336 
Assessments, special, 15, 16 
Augers, earth, 21 
Automatic, regulators, 117-121 
siphons, flush tanks, 110 
double alternating, 507 
multiple alternating, 508-512 
timed, 510 
timed and multiple alternating, 

510-512 
triple alternating/ 508 

Bacillus, definition and morphology, 

362, 363 

Backfilling, 328-331 
Backfill, puddling, 330 
weight of, 199, 201 
Backwater curve, 73 
Bacteria, definition and morphology, 

362, 363 

good and bad, 363, 364 
nature of, 362, 363 
nitrifying, 431, 432 
sanitary significance of, 364 
in sewage, 362, 363 
total count, 364 
Bacterial analyses, results in sewage, 

364 

Baffles, scum, 404, 413, 414, 421 
in sedimentation tanks, 404 
in septic tanks, 413, 414 
in Imhoff tanks, 421 
Balls, for cleaning sewers, 338 
Band screen, 384 
Barring, definition, 263 
Bars for screens, 390 



Basins, sedimentation, baffling, 404 

bottoms, 404 

cleaning arrangements, 404 

depth, 401 

economical dimensions, 401-403 

inlets and outlets, 404 

scum boards, 404 

types, 395 
Basket handle sewer section, 67, 

69 

Bathing beaches, pollution, 381 
Bazin's formula, 54 
Bearings, for centrifugal pumps, 131, 
137, 138 

thrust, 138 
Bellmouth, 121, 122 
Bends in pipe, loss of head in, 116 
Berlin, sewage farm, 460, 461 

sewers, date of, 3 
Bids, proposal, 217-219 
Bidder's duties, 215-217 
Bio-chemical oxygen demand, 359 

361 

Biolysis of sewage, 366, 367 
Black and Phelps dilution formulas, 

377-379 
Blasting and explosives, 294-304 

caps, 297, 299, 300 

detonators, 294, 297-300 

firing, 302-304 

fuses and detonators, 297-300 

fuses, delayed action, 291, 300 

fuses, electric, 299, 300 
splicing, 303 

gelatine, 296 

loading holes, 303 

powder, 295 

precautions, 300-302 

priming and loading, 303 

rock, 269 

size of charge, 304, 305 

tunneling, 290, 291 
Bleach, characteristics of for dis- 
infection, 491 
Block sewer, construction, 311-314 

hollow tile as underdrains, 126 
Blocks, vitrified clay, 189, 190 
Boilers, steam, 147-150 



INDEX 



515 



Boilers, efficiencies, 149 

horse-power, 149 
Bond, contractor's, 213, 214, 232 

issues, 14 

Bonds, definition and types, 14-16 
Boring underground, 20 
Bottom, activated sludge aeration 

tank, 472 
Imhoff tanks, 423 
sedimentation tanks, 404 
trickling filter, 451, 452 
Box sheeting, 272 
Branch sewer, denned, 7 
Breast boards, 288 
Brick, arch construction, 312, 313 
and block sewer construction, 311- 

315 

invert construction, 311, 312 
sewer construction, 311-315 
arch centers, 313 
invert, 311-312 
organization, 314, 315 
progress, 314 
row lock bond, 312 
specifications, 188, 189 
sewers, life of, 351 
Bricks for sewers, 316 
British Royal Commission on Sewage 

Disposal, 4 

Broad irrigation. See under Irriga- 
tion. 

Bucket excavators, 246, 255, 256 
Building material, weight of, 201 
Burkli-Ziegler formula, 47, 425 
Butryn, 366 

Cableway excavators, 246, 250-252 
Cage screen, 384, 385 
Caisson excavation, 285, 286 
Calcium carbide, explosive, 347 
Calumet pumping station, 128, 142 
Cameron septic patent, 411 
Capacity of sewers, diagrams, 57-60 
Capital, private invested in sewers, 17 
Capitalization, method of financing, 

157-160 

Caps, blasting. See blasting. 
Carbohydrate, 366, 367 



Carbon, analysis for, 356 

dioxide, 366, 367 
Carson Trench machine, 250, 251 
Cast-iron pipe, 122, 164, 190, 191 

joints, 164 

quality, 101, 102, 190 
Castings, iron, 101, 102 
Catch-basins, 99, 107-108, 217 

cleaning, 343, 344 

inspection, 337 
Catenary sewer section, 69 
Cellars, depth of, 88 
Cellulose, 367 
Cement. See also Concrete. 

pipe, specifications, manufacture 
and sizes, 171-179 

vs. concrete, 164 
Centrifugal pumps. See pumps, 

centrifugal. 

Centrifuge for sludge drying, 501, 502 
Cesspool, 411, 416, 417 
Champaign, Illinois, septic tank, 415, 

416 

Changes in plan, 222, 223 
Channeling, definition, 263 
Character of surface, 44 
Chemical analyses, 354-362 
Chemical precipitation, 371, 405-409 

chemicals used, 405-407 

preparation of chemicals, 407, 
408 

results, 408, 409 

at Worcester, 408 
Chezy formula, 52, 53 
Chicago. See also Sanitary District 
of Chicago. 

drainage canal, 374, 375 

dilution requirement for sewage, 
380 

early sewers, 3 

method of sewage disposal, 374 

population and density, 29, 30 

trench excavation in, 248 
Chlorine. See also Disinfection. 

disinfectant, 489-493 

in sewage, 358, 374, 375 
Chlorine liquid, application, 491, 492 
Cholera, trans mi ttable disease, 364 



516 



INDEX 



Chromatin, 365 
Chutes for concrete, 187 
Circular sewer section, hydraulic ele- 
ments, 65, 66, 69 

types, 70, 71 
City, growth of area, 31 

growth of population, 24-28 

legal powers, 219 
Clay, life of pipe, 349-351 

manufacture of pipe, 165-167 

specifications for pipe, 168-170 

unglazed for pipe, 165 

vitrified blocks, 167, 189, 190 

vitrified pipe, 165-171 
Cleaning, grit chambers, 398, 400 

sedimentation basins, 404 

sewers, cost, 341 

in N. Y. City, 332 

methods, 337-343 

tools, 338-340 

up after completion of work, 228 
Coccus, 362 

Coefficient of uniformity of sand, 456 
Coffin sewer regulator, 117, 118 
Colloid, nature of, 358 

treatment for, 358 
Color of sewage, 352, 353 
Combined sewer system, 78, 79 
Commercial districts, characteristics 

of and sewage from, 32, 34, 35 
Compensators for pumps, 142 
Compressed air. See also ventilation, 
tunneling, drilling, etc. 

activated sludge, 473-475 

for drilling, 264-268 

in tunnels, 292-294 

transporting concrete, 320, 321 
Concentration, time of flood flow, 

41-i3, 96, 97 
Concrete, aggregates, 172-174 

mixing and placing, 184-188 

pipe, details, 175-179 
manufacture, 171-179 
reinforcement, 177, 178, 209, 
210 

pipe, steam process, 176 
sizes, 175 

pressure against forms, 232, 323 



Concrete, proportioning, 179-183 
qualities, 179, 180 
reinforcement, placing, 178, 326, 

327 

reinforcing steel, quality, 191 
sewer construction, 314-328 

arch, 318-321 

form length, 319 

labor costs, 327, 328 

in open cut, 314-320 

in tunnel, 320, 321 

invert, 315-320 

organization for, 328 

working joints, 319 
sewer costs, 327-329 
strength, 181 
waterproofing, 184 
Conduits, special sections, 67, 70, 71 
Connections to sewers, ordinances, 

344, 345 

record of 92, 238 
Construction of sewers, Chap. XI, 

233-331 
Construction, elements of, 233 

organizations, 315, 328 
Contact bed, 432-437, 506 
advantages and disadvantages, 

432-434 

automatic control, 437, 506 
cleaning, 435 
clogging, 435 
construction, 434-436 
control, 437, 506 
cycle, 436, 437 
depth, 434 
description, 432, 433 
design, 434-436 
dimensions, 434, 435 
loss of capacity, 435 
material, 435, 436 
multiple, 433, 435 
operating conditions, 432-437 
rate, 435 
results, 433, 434 
ripening, 432 
Continuous bucket excavators, 246- 

250 
Contour interval on maps, 79, 80 



INDEX 



517 



Contracts, Chap. X, 211-232 

abandonment of, 225 

assignment, 228 

completion of, 222, 228 

bond, 213, 222 

content, 213, 230, 231 

cost-plus, 212, 213 

disputes, 220 

divisions of, 213 

drawings, 213 

engineer as an arbitrator, 220 

the instrument, 230, 231 

interpretation of, 220, 234, 
235 

lump sum, 212 

nature of, 211, 212 

sample, 230, 231 

time allowed, 222 

types, 212, 213 

unit price, 213 
Contractor, absence of, 222 

bond, 232 

claims against, 228 

duties, 221 

liability, 224 

relations with otKer contractors, 

228, 229 

Contractor's powder, 294 
Control devices, automatic, for sew- 
ers, 117-121 
for niters, 506-512 

inspection of, 336, 337 
Copper sulphate, disinfectant, 490 
Copperas, precipitant, 406-408 
Cor