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Sanitaky Circumstances. 27
to four million gallons in the twenty-four hours, but in times of
rainfall it is increased to seven or eight millions, and on some
occasions fourteen millions.
The disposal of the residue left in the tanks is an important
and difficult part of sewage purification.
A large amount of solid matter or sludge is deposited in the
settling tanks; this deposit, containing about 90 per cent, of water,
is cleared out once a week, into a special storage tank.
If allowed to stand for any considerable time, a large
accumulation of this liquid deposit might cause a nuisance; it
cannot be transported in the form in which it comes from the
tanks, so is operated on, in batteries of filter presses, being
forced at a pressure of about 1001b. per square inch between iron
plates, lined with jute cloth. The water passes through the cloth,
which acts as a filter, and the solid matter is retained in between
the plates of the press.
This goes on until a press is filled with the solid matter, all
the water that passes through the cloths draining back to the
sewer to be treated with the incoming sewage.
By this means the sludge is reduced to an easily transportable
pressed cake, containing 50 to 60 per cent, of water, in a form in
which it can be handled or stored, and of some value for agricultural
purposes.
The pressed sludge is in slabs about 2½ inches thick, and
contains valuable manurial properties. In a residential district
like Richmond, it cannot be readily disposed of, and while it may
be useful for agricultural purposes, the expense of handling and
transporting, renders it very difficult to find a market which will
take a constant supply.
At the present moment some 30,000 or 40,000 tons of wet
sludge are produced per annum; this represents about 7,000 tons
of pressed cake—produced day by day in a fairly constant quantity.