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MODERN PAPER-MAKING

Surface water from springs and small streams or 'bums' may often be
satisfactorily freed from suspended matter by passing it through a series of
tanks filled with clean coke or sand. The water is made to flow upwards
through the cone in the first tank and down through the coke in the second
tank, and so on until it is sufficiently cleaned. The coke must, of course, be
renewed periodically.

Another method is to fill the tanks first with drain-pipes and then with a

HARD WATER INLET

[Paterson Engineering Co. Ltd.
FIG. 151.—TYPICAL VERTICAL WATER SOFTENER, LIME-SODA TYPE
layer of coarse stones, then smaller stones, followed by a layer of gravel, and
finally a layer of clean sand on top. In this way the water, passing through
the various layers, deposits the suspended vegetable matter and slime, and
finally appears at the top in a clean and clear state. The sand, etc., has to be
removed frequently and washed in a washing machine, and is then put back
in place ready to be used again.
These methods are both, of course, entirely mechanical, and no change
takes place in the actual composition of the water. When a Bell filter is- used
the action may be partly chemical as well, as the water may come in contact
with alum and so be softened and actually freed from colour by chemical action.