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53
The two principal intercepting sewers, namely, those of
the Mutton Brook Valley and the Dollis Valley, were nearly
completed before the close of 1885, and three quarters of a
mile were also constructed of the tunnel sewer, which takes
the sewage of the western and southern sides of the district.
The year 1889 saw the completion of the scheme.
Sewage reaches the farm in Summers Lane by either the
low or high level sewer. The sewage from the low level sewer
passes through a screen, and is then pumped to join the high
level sewage. The mixed sewage is screened, and, after the
addition of lime and sulphate of iron, passes into one of three
precipitation tanks (each having a capacity of 226,260
gallons). The tank effluent is next treated in a double set
of rough contact beds, in each of which it remains standing
about five hours. The effluent is finally run over several
acres of meadow before entering the brook at the bottom of
the valley.
The sludge is drawn off after a precipitation tank has
been used one week, and is pumped on to the land at the
highest level of the farm. It is there left to drain and
evaporate, and is then ploughed in, the land being used for
crops of cabbages, etc.
The dry weather flow of sewage is estimated at about
700,000 gallons per diem, in the proportion of about four of
low level sewage to three of high level sewage. The storm
flow may be any volume up to ten times the above. In times
of storm a portion of the sewage is treated in the ordinary
way, and the remainder by broad irrigation. The subsoil
of the farm is stiff clay, and there is only a shallow surface
soil over most of the area, so that considerably more land has
to be utilised than would be required with more suitable
soil.
The total area of the farm comprises close upon 80 acres
belonging to the Council, and 33 acres rented from the Burial
Board.