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134
CHAPTER XXIV.
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 sludge vessels made 2,187 trips and travelled altogether 241,868 nautical
miles.
London
sewage
system.
The Council's by-laws prescribe how premises shall be drained, and subject
to these, the metropolitan borough councils control house drainage. Disputants
have a right of appeal to the Appeals Committee of the Council. The
metropolitan borough councils provide local sewers for house drainage and surface
water, the plans of these 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.
Sewage
treatment.
The experiments in connection with the biological treatment of sewage on the
activated sludge basis have been continued during the year, tanks being equipped
with different appliances for agitating the sewage, namely (i.) the diffusion of air
through the sewage by means of porous tiles fitted in the bottom of the tank; (ii.)
the agitation of the sewage by paddle wheels which constantly expose fresh layers
of the sewage to the air; and (iii.) the spraying of the sewage into the air through
a nozzle. The experiments have established the fact that London sewage responds
to this treatment and that, owing to its regular character, it is more easily treated
in this way than the sewage of some large towns where trade refuse forms a larger
part of the sewage.
Later two of the tanks were used for experiments in the treatment of the
effluent discharged into the river from the settling or sedimentation channels.
Generally speaking it is found that effluent can be more expeditiously dealt with
than crude sewage and that the results given in one tank vary from those in the
other. In order to secure the best conditions the tanks are altered from time to
time. Experiments on these lines are being continued. New methods of biological
treatment of sewage are being adopted in other places and careful attention is being
paid to the results. Sewage disposal is a comparatively costly matter and very