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plant with interchanging septa devised by Colonel Butler and Mr. Tabor will produce
a better final effluent than the two-tier plant previously experimented with. The
modification was introduced to avoid the use of long two-tier channels, where the
effect of a prolonged travel of the mixture of activated sludge and sewage might
be unfavourable or of the still longer single channels such as are used at Sheffield.
The new inter-communicating septum, in addition to making possible the
extended application of the two-tier channel, introduced other conditions which,
it was seen on theoretical considerations, were likely to improve the results of
treatment. The small scale plant consisting of three two-tier treatment channels
with appropriate sedimentation and re-activation tanks which has been in use in
the research carried out at the Southern Outfall for many years has been adapted
for a comparative trial of the new type of double channel, one channel having been
fitted with a flow-transposing septum of the new type, and one of the other channels
worked in the usual way, each receiving the same amount of uniform mixture
of sedimented sewage and activated sludge so that all experiments have been
strictly comparable.
Both new and old types of channels have been subjected in the course of the
experiments to widely varying conditions, always varied similarly in both systems.
The results in both cases have been satisfactory up to the maximum flow for which
the plant was constructed, yielding inoffensive, stable effluents such as might at all
times be discharged with safety into the river at the outfalls. The main object of the
series of experiments was, however, to study the comparative efficiency of the two
plants. From the beginning it was observed that more rapid and better mixing of
activated sludge and effluent was effected in the new design than in the unmodified
two-tier channel. As intimate mixture and rapid and frequent change of liquid at
the liquid and air interface were the two chief aims, it was realised that this rapid
admixture and the eddying which was also observed were favourable circumstances
which should lead to improved results.
Under all the varied conditions of treatment which were tested, whether a
reduction of the time of treatment or variations in the proportion of free "humus"
(activated sludge) or the supplementary use under all these conditions of additional
air introduced into the lower channel by blowing, there was uniformly a superiority
of results in the new type of channel. When the supplemental air supply was used
this amounted to from 20 per cent. to 50 per cent. in the quality of the effluent and
from 8 per cent. to 18 per cent. under the less favourable condition of absence of
air blowing.
How far this improvement of the effluent can be taken as an indication of the
possibility of treating a larger volume in any given unit of plant in the same time
has not yet been fully determined, nor could any figures so obtained be applied for
determining the ultimate capacity of the large plant to be erected.
Outfalls.
At the outfall laboratories, systematic daily examinations of the sewage reaching
the outfalls and the resulting effluent and sludge, as well as of the water of the river
Thames at both high and low tides have been continued. The systematic analyses
of samples in connection with the working of the outfalls have also been made at
these laboratories in addition to the research work undertaken in connection with
the biological treatment of sewage. The condition of the water of the river Thames
during June rendered it necessary that resort should be had to the use of chemical
precipitants in the treatment of the sewage at both outfalls. This treatment was
commenced on the 26th July and ceased on the 16th October, when the reduction of
temperature and increased flow of the river had improved its condition.
Slndge
digestion.
In addition to experiments with the modified two-tier channels for treatment
of sewage with activated sludge, some work has been done on digestion of ordinary
sewage sludge such as is sent to sea. It was found that the behaviour, on digestion,
of sludge from the Council's works was similar to that observed at other places in this