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67
above settling tanks is dried on the land, and is afterwards
ploughed in; the land, after a considerable interval, is
cropped with cabbages, etc. A portion of the "low level"
sewage is pumped up from the low level outfall and mixed
with the " high level" sewage at the high level outfall.
About four years ago the Council completed the first instalment
of a system of septic tanks and bacterial percolating
filters for dealing with a portion of the "low level" sewage.
The result proved so satisfactory that the Council decided to
extend the installation, and further septic tanks and bacterial
percolating filters were put down.
The sewage, after passing from the primary screen, falls
into a detritus tank, where the gross solids are deposited.
From the detritus tanks the sewage flows into "Dortmund"
tanks, from the bottom of which the sludge is drawn off at
frequent intervals. Firom the "Dortmund" tank it flows
into the septic tanks, the flow from these tanks being varied
as required by the conditions arising from time to time. The
scum on the septic tanks is held back by scum boards, and
forms a complete covering over the whole of the tanks. The
septic liquid is then taken through equalising channels into
the various filters. The sewage is distributed on the circular
filters by revolving sprays, and on the rectangular beds by
fixed distributors designed by the Council's Engineer.
Measures have also been taken by means of special
hydraulic tanks to intercept the solids in suspension both between
the septic tanks and filters, and between the primary
and secondary filters.
Six of the percolating filters are circular, 60 feet in
diameter, and 5 feet deep. Two of the percolating filters
are below ground level, rectangular, 5 feet deep, with open
jointed herring-bone pipes for taking off the effluent and for
ventilation.