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59
is therefore considerable after periods of heavy rain. All
surface water not discharging into the soil sewers discharges
into the natural watercourses—the Western district draining
into Dollis Brook, or Brent water-shed, and the Eastern portion
of the district draining into the Strawberry vale, or Lea
water-shed.
The district of Finch-ley is drained by two systems of
sewers, the higher portion of the district draining into an outfall
known as the "high level" sewer, and the lower portion
of the district draining into a sewer known as the "low level"
sewer. The outfalls of both sewers are at the sewage works
near Summers Lane.
Sewage Disposal.
The sewage discharged from the "high level" sewer, after
screening, is mixed with lime and proto-sulphate of iron, and
then passes into settling tanks, where the grosser solids are
deposited. The super-natant liquid is drawn off and treated
on rough filters before passing on to the grass land, and
thence into the Strawberry Vale Brook. The sludge from the
settling tanks is dried on the land, and is afterwards ploughed
in ; the land, after a considerable interval, is cropped with
cabbages, etc. At times a portion of the "low level" sewage
has to be pumped up and discharged into the "high level"
outfall.
About six years ago the Council completed the first in
stalment 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 through the automatic screen,
flows into a detritus tank, where the gross solids are deposited.
From the detritus tank the sewage flows into "Dortmund"
tanks, from the bottom of which sludge is drawn
off at frequent intervals. The detritus and "Dortmund"
tanks are in duplicate. From the "Dortmund" tank the