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With regard to the Outfall Works, it was Bazalgette's original
intention that when the sewage reached the Outfalls at Barking and
Crossness, it should be stored in reservoirs on the flood tide and
discharged into the river during the ebb tide.
For a time these measures sufficed, but it became evident by 1880
that further measures were necessary to prevent undue pollution of
the Thames.
Sedimentation channels were therefore constructed at the Northern
Works and the reservoirs at the Southern Outfall Works turned into
sedimentation channels. After chemicals had been added to the
sewage, it was allowed to flow slowly through the channels in which
a considerable portion of the solids in suspension fell to the bottom
and the sewage thus partially purified passed on to the river. Subsequently
the use of chemicals was discontinued. The whole of the
sewage of London is still partially purified in this manner at both
Outfalls. The solids which are thus abstracted, or sludge as it is termed,
is further settled in sludge settling tanks and then pumped to the
sludge tanks which store the sludge whilst awaiting the ships which
take it to the Black Deep well out beyond the Nore Lightship in the,
Thames Estuary. These four ships, the Henry Ward, J. H. Hunter,
John Perring and the Edward Cruse have a capacity of about 1,500
tons and normally make the round trip in under twelve hours. That
is they can make one trip per tide.
Owing to various causes, the quality of the river water has
deteriorated in recent years and the Council has entered upon a
programme of extensive works as its contribution to the improvement
of the river. So far activities have been concentrated at the Northern
Outfall Works where the need is more pressing. At these Works the
whole of the sewage of London, North of the Thames, including that
of the out county Boroughs which is admitted, is treated. Five Outfall
sewers each 9 feet by 9 feet, discharge to the Works an average daily
flow of approximately 200,000,000 gallons. The sewage is settled in
thirteen sedimentation channels and some of the effluent discharges
direct to the Thames. Some 60,000,000 gallons per day are given
further treatment in the Activated Sludge Plant where sewage effluent
and activated sludge are mixed in these six sets of aeration channels.
The mixed liquor gravitates to these Final Settling Tanks where the
activated sludge settles out and the purified effluent discharges to the
Thames. A Sludge Digestion Plant utilising two compartments of the
old reservoirs to treat 1,200 tons of sludge per day and producing
600,000 cubic feet of gas per day has recently been completed.
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