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72
Outfalls.
The difficulties experienced in treating sewage by sedimentation during the
unprecedented drought periods of 1933 and 1934 were again encountered during
the shorter drought period of 1935. A lengthy investigation was carried out on the
use of various chemicals and methods of their application to accelerate the sedimentation
of the suspended solids present in sewage.
Consideration of cost ruled out the use of some agents, and, of the others subjected
to test, it was found that the best results were obtained by the use of ferric
salts in solution, accompanied by not too violent agitation. Failure to obtain ferric
salts in the quantities required (when added to the dry weather flow of London
sewage a dose of only 1 grain per gallon requires the use of 18 tons per day) led to a
method of preparation being worked out in the laboratory. This was followed by
trials on a semi-manufacturing scale, and finally a full scale plant was designed and
erected at the Southern outfall. The successful working of this plant was followed
by the erection of an improved and larger plant at the Northern outfall.
The activated sludge plant at the Northern outfall has continued to treat,
with satisfactory results, ten million gallons per day of sedimented sewage. In
connection with this plant, two types of sedimentation tank are used, and one type
has consistently given inferior results to the other type.
The inlets and outlets of the less satisfactory tanks have been radically altered,
with the result that all the tanks now work equally well. The construction of five
activated units, similar to the one now in use, is proceeding, and it is hoped that two
units will be at work in the early summer of 1936.
The experimental work at the Southern outfall on the digestion of sewage sludge
in a covered concrete tank of 25,000 gallons working capacity was concluded in the
early part of the year. During the three years in which this work was conducted
the effects of variations in working temperature, detention period, stirring, etc.,
have been studied. From the lessons learnt, a plant of 1,800 tons working capacity
has been designed, and is now being constructed at the Northern outfall.
The dry matter in individual cargoes of sewage sludge sent to sea from the
Northern outfall varied from 4 per cent. to 14 per cent., the average being 8.6 per
cent.—practically the same as last year. In the case of the cargoes from the Southern
outfall, the variations were from 3 per cent. to 12 per cent., with an average of 6.9 per
cent., also similar to that of last year. The dry solids in the greater number of
cargoes from both outfalls did not diverge much from the average.
The condition of the river, which had been affected by the prolonged dry
weather of 1933 and 1934, improved steadily after October when the increase in
the rainfall made itself felt. The steps taken by the Council, described above,
also contributed materially to the improved conditions. The new works of the
Middlesex County Council at Mogden and the new units at the Northern outfall as
they come into being will do much to restore the balance between oxygen absorption
and re-absorption at the critical times of low flow of fresh water and high
temperature.
Vehicular
tunnels.
The systematic investigations, commenced in 1928, as to the condition of
the air in Blackwall and Rotherhithe tunnels, were continued during 1935. As
revealed by the periodical traffic census taken by the valuer, there has been a remarkable
increase in the amount of motor traffic passing through each of the tunnels
in the past few years, the figures for 1935 being more than double those for 1928.
The results of the air examinations made in 1935 indicate that in spite of this increase
the ventilating plant at Rotherhithe tunnel has, with a few exceptions, maintained
the atmospheric conditions in a satisfactory state. Even when the carbon monoxide
limit figure of 20 parts per 100,000 was exceeded, the excess was not large.