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26
Sanitary Circumstances.
pumped during heavy rainfalls; one square yard of roof surface
will discharge double the rainfall that would drain from an equal
area of roadway.
In 1910, the whole of the pumping was done by steam-driven
machinery, and there were then seven pumping engines installed,
varying in capacity from three to fifteen million gallons per day,
but the Board considered that the time had arrived to increase the
pumping power, in view of the rapid increase of population in the
united districts.
Further additions of steam engines would have entailed
additional boilers and buildings; for, although the whole machinery
is not in operation every day, still, all of it must be kept in a state
of readiness for use, and a steam plant would have required boilers
to be kept under steam, with their fires banked, ready for work on
short notice.
Various schemes were considered, but eventually it was
decided that the new reserve plant, which was to be installed only
to pump large volumes of rainfall and sewage, should be driven by
gas; and a large installation of pumping machinery has been
recently erected, capable of pumping twenty million gallons per
day, operated by gas from the mains of the local Gas Company ;
and the station is now equipped with pumping capacity to lift
thirty million gallons by steam, and twenty million gallons by gas.
The convenience of having two systems was demonstrated during
the past coal strike. When the strike began, the Board's reserve
stock of coal was not large ; so during the whole month of March
the working of the station was altered so that all the sewage
pumping was done by gas-driven machinery.
SLUDGE DISPOSAL.
I have already mentioned how the sewage is treated with
chemicals, settled in tanks, filtered, and then discharged on the
ebb tide into the river. The volume thus dealt with is from three