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19
Plan of tanks used in connection with the bacterial or natural treatment of sewage.
The figures against the inside of the lines representing the sides of the tank C indicate the
depth of sludge referred to on p. 18 (Series II.).
The figures against the outside of the lines of the plan indicate the dimensions of the
tanks.
The pipe with the valves was used in connection with the experiments of Series III.
As a result of the whole series of experiments, it was proved that the crude sewage of
London could not be satisfactorily dealt with in coke-beds unless it was subjected to a preliminary
settling process; since, although the coke-beds deliver a satisfactorily purified effluent, they choke
up very rapidly if they are constantly charged with crude, unsettled sewage.
Full details of the working of this bed were published in the Third Report on the "Bacterial
Treatment of Crude Sewage" on pp. 14-16, and the details of the analytical results up to
the end of 1899 appear on pp. 35-37. The averages of the analyses of the crude sewage and
of the coke-bed effluent, given in this Report in table 6, p. 35, complete the publication of information
respecting this series of experiments.
In the Second Report tanks A and B were stated to be 12 feet deep; this measurement
was, however, only approximate, and in no way affected the experiments referred to in that
Report. On making exact measurements of the depth of the tank for the purpose of this experiment,
it was found that the depth was 13 feet. This measurement includes a coping which was
not considered in the previous measurement.
(c) Experiments on the Rapid Sedimentation of Crude Sewage on a Large Scale and
the Subsequent Treatment of a Portion of the Settled Sewage in a Small Coke-bed.
During three weeks, between December 24th, 1899, and January 13th, 1900, the coke-bed
experiments, then in progress at the Southern Outfall "Works at Crossness, were stopped in order
to carry out a large scale experiment on the rapid sedimentation of the whole of the crude
sewage received from South London and the subsequent treatment of a small portion of the sedimented
sewage in a coke-bed. The sewage received at this Outfall underwent sedimentation in
the settling channels, but was not treated with chemicals during the three weeks. It was
allowed to flow more rapidly than usual through the channels; the rate of flow averaging 7.4
feet per minute, whilst the usual rate of flow of the chemically-treated sewage is 1.5 foot per
minute.
Four channels, each of 6,000,000 gallons capacity, were used, and two of 3,000,000
gallons capacity each. Their length was 581 feet each.
A portion of this rapidly sedimented sewage was afterwards treated in a small coke-bed
contained in a tank of 280 gallons capacity. The bed was filled three times a day. Its capacity
remained constant during the experiments, and the bed suffered in no way from choking.
The amount of purification effected on the crude sewage by the sedimentation and by
the bed was as follows—
By the rapid sedimentation in the settling channel—
Purification from total putrescible matter, 16.1 per cent.
Purification from dissolved putrescible matter, 0.4 per cent.
By the bacterial action of the coke-bed on the settled sewage—
Purification from dissolved putrescible matter, 52.9 per cent.
By the combined action of the settling channel and of the coke-bed—
Purification from dissolved putrescible matter, 58.7 per cent.