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III
CHAPTER XXVI.
MAIN DRAINAGE.
The district drained by the Council's main drainage system has an area of nearly
149 square miles with a population (in 1921) of 5,333,387. This includes an area of
nearly 32 square miles, with a population of 850,138, outside London.
Drainage
area and
statistics.
The following statement shows the quantities of sewage, etc., dealt with during
1922:—
Sewage treated— Million gallons.
Northern outfall 55,137.2
„ „ (daily average) 151.1
Southern outfall 32,417.4
„ „ (daily average) 88.8
Sludge sent to sea— Tons.
Northern outfall 1,391,000
„ „ (daily average) 3,811
Southern outfall 824,000
„ „ (daily average) 2,258
The sludge vessels made 2,215 trips and travelled altogether 244,372 miles.
The Council's by-laws prescribe the methods for the drainage of premises, and
subject to these, the metropolitan borough councils are charged with the control
of house drainage. Disputants have a right of appeal to the Appeals Committee of
the Council (see p. 108). The metropolitan borough councils provide local sewers
for house drainage and surface water, the plans of these sewers being subject to the
approval of the Council. From the point at which local sewers discharge into main
sewers the Council becomes entirely responsible. The main sewers, many of which
are on the lines of streams formerly discharging into the Thames, now connect with
intercepting sewers which run roughly parallel to the Thames. In turn the intercepting
sewers connect with the outfall sewers which convey the sewage to the outfalls,
where, after the extraction of solid matters, the effluent is run off into the river
and the solids are sent to sea in specially designed sludge vessels. For the disposal
of rain-water, storm-relief sewers have been constructed which discharge into the
Thames by the shortest practicable route. Many main sewers also have storm
water outlets to the river. Sewage and storm water flow principally by gravitation,
but pumping stations are necessary at certain places.
London
sewage
system.
The severe drought during 1921 had caused the tidal portion of the Thames
to show signs of pollution owing to the fact that the flow of upland water had been
so small and the consequent downward displacement due to the flow of fresh water
had been reduced to a minimum. The rainfall during the earlier months of 1922
was not sufficient to restore normal conditions and, in order to avoid the possibility
of nuisance, arrangements were made in July for the use of chemical re-agents
to be resumed. About this time, however, more rain fell and the consequent improvement
in the condition of the lower river made the use of chemicals unnecessary.
The improvement has been maintained, but in the absence of exceptional rainfall
time must elapse before conditions can become normal.
Sewage
treatment.
Experiments in connection with the biological treatment of sewage on the
activated sludge basis have continued during the year and useful knowledge has bee
gained, but the conditions affecting London sewage appear to be more difficult
than those at other places where this system is at work. New tanks have been
equipped with different types of appliances for agitating the sewage.
Further repairs to the pier at the northern outfall, which have proceeded
continuously since 1913, were authorised during the year at a cost not exceeding
£2,000. The repair of the roadwavs at the outfall has been undertaken at a cost
of £1,000.
Northern
outfall.