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76
On arrival at the works the sewage first passes through automatic
screens where rags are removed and thence to special tanks where the
mineral grit is removed. Flow in excess of three times the dry weather flow
rate is diverted to storm water tanks where it is stored until it can be returned
for full treatment. Flows up to 50,000,000 gallons daily pass forward
for primary settlement in eight circular tanks, 105 ft. dia., where the suspended
solid matter is removed as a thick sludge.
The liquid from the primary settlement tanks is passed into twelve
aeration channels, each 25ft. wide and 420 ft. long into which compressed
air is introduced, together with biologically active sludge. The subsequent
biological action results in a highly purified liquor. The final effluent is
separated from the activated sludge in sixteen conical bottomed tanks 67ft,
dia., and discharged through a culvert to the River Wandle some two miles
away.
The sludge removed from the primary settlement tanks is pumped to the
existing heated sludge digestion plant. Here it is rendered innocuous in a
fermentation process which yields approximately 300,000. cu. ft. of methane
gas daily. After digestion, the sludge is pumped onto land, allowed to dry
and then ploughed into the top-soil.
The gas collected from the digestion process is used as fuel in six
engine-generator sets, totally 4,500 b.h.p. which supply all the power
required for the treatment works.
Further stages of treatment are to be introduced, so as to improve the
effluent quality above the normal requirements, due to the poor dilution
ratio of effluent to river water. Construction of these units will commence
during 1970/71 and these are expected to cost a further £600,000.
The new works will become vested in the Greater London Council in
April, 1970.