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168
The quantities of sewage, etc., dealt with during 1928 were as follows:—
Sewage treated— Million gallons.
Northern outfall 61,505.6
„ „ (daily average) 168.0
Southern outfall 37,675.5
,, „ (daily average) 102.9
Sludge sent to sea— Tons.
Northern outfall 1,853,220
„ „ (daily average) 5,063
Southern outfall 931,398
„ „ (daily average) 2,545
The sludge vessels made 1,949 trips and travelled altogether 215,360 nautical
miles.
Sewage
treatment.
The constantly increasing flow of sewage to the main drainage outfalls necessitated
consideration of the question of the possibility of dealing with the sewage
by the methods at present in operation, namely, precipitation of the solids by
natural means in channels at the outfalls, the discharge of the resulting effluent
into the River Thames at the outfalls and the sending of the solid parts of the sewage
to sea in sludge ships. When the outfalls were completed by the Metropolitan
Board of Works about the year 1875, the average daily quantity of sewage flowing
to the outfalls was 139 million gallons. New methods of sewage treatment were
adopted between the years 1890 and 1893 ; the average daily flow having, at the
latter date, risen to 186 million gallons. At the present time the average daily
flow is 270 million gallons. A report made in the year 1891 by Sir
Benjamin Baker and the then chief engineer of the Council (the late
Sir Alexander Binnie) put at 300 million gallons a day the maximum
quantity which should be concentrated at the present outfalls. Experience
has proved that the calls which are being made upon the London main
drainage system and on the capacity of the river as a final place of disposal, have
reached a point when further steps should be taken. Alternative methods are
(1) the transference of the outfalls to a point nearer to, or on, the sea coast, and (2)
purification of sewage at the outfalls. The cost of the first alternative has not
been estimated, but there is no doubt that vast capital expenditure would be entailed.
Nor can the certainty of opposition from areas adjacent to that selected for any
new outfall be disregarded.
Improved methods of sewage treatment have been installed in several provincial
towns, and some of these were inspected by the Main Drainage Committee in the
year 1920. As the result of this visit experimental plant was installed at the Southern
outfall in connection with the biological treatment of sewage on the activated sludge
basis, using various types of apparatus. At an early stage the experiments established
the fact that London sewage responded to treatment with activated sludge.
Later experiments were directed towards the treatment of the effluent discharged
into the river from the settling or sedimentation channels and it has been found
that effluent can be more expeditiously dealt with than crude sewage. The tankshave
been altered from time to time and the quantities of liquid which can be dealt
with have been largely increased. The existing experimental plant reached an
ultimate capacity of about 150,000 gallons of sedimented effluent a day.
In view of the success attained with the comparatively small experimental plant
the Council was advised that the work of purifying the effluent should be commenced
at the Northern outfall on a practical scale, with a plant capable of treating from.
5 to 10 million gallons a day.
Before committing itself to any definite action, the Council felt that, in a matter
of such magnitude and of so technical a nature, the evidence should as far as
possible, be conclusive. The advice of Professor W. E. Adeney, D.Sc., F.R.C.Sc.I.,.