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DIVISION I-CHEMICAL.
SECTION I.
GENERAL OBJECTS AND CONCLUSIONS.
1.—Introductory.
After duly considering the different methods of sewage treatment which have been at present
proposed (see pages 19, 20), it appeared to me that the intermittent treatment in bacteria coke-beds was
the one most likely to be suitable to the conditions at the Council's Northern and Southern Outfalls.
Much information has already been laid before the Main Drainage Committee on this method, but
chiefly with respect to the results obtained by treating the effluent from the chemical precipitation of
London sewage. I considered that further information and experience was desirable before the
Committee could be advised to embark on the scheme of treating the whole or even a considerable
portion of the raw sewage by this process, which is usually called that of " intermittent filtration
through coke-beds." This treatment undoubtedly depends upon the action of bacteria, and the term
" filtration," which implies mechanical removal of suspended impurity should be abandoned.
On the 15th of February, 1898, the Council approved, on the recommendation of the Main
Drainage Committee, my proposal that further experiments should be made on this process of
intermittent bacterial treatment in coke-beds, and the chief engineer, Sir Alexander Binnie, willingly
accorded his cooperation. Dr. Houston was retained for the purpose of conducting bacteriological
investigations during the first year of the experimental work, and a grant was voted by the Council for the
purpose of laying down coke-beds on a small scale at Crossness and for maintaining their working.
Thanks to the cooperation of Mr. E. J. Beal, the superintendent, and of Mr. J. W. H. Biggs, the
assistant chemist at Crossness, these experimental coke-beds have been constructed and worked to
my entire satisfaction.
In the present report the details of construction and the method of working the coke-beds are
explained, and the general results arrived at, after about ten months experience of the filtration, are
stated.
It appeared to me to be necessary that the crude sewage at the Northern Outfall should be
subjected to similar experimental treatment, and a further sum was accordingly voted by the
Main Drainage Committee on 21st July, 1898, to equip small experimental coke-beds at the Northern
Outfall. The full particulars of these coke-beds and of the results obtained by them, will be included
in a special report in due course.
The Committee has already received, in a Report which they ordered to be printed and
circulated on the 16th June, 1898, the results obtained by Dr. Houston from a bacteriological
examination of the crude sewage as it arrives at Crossness and at Barking. In the present Report
the results are stated, which were obtained by Dr. Houston from a bacteriological examination of the
Crossness sewage after its passage through the coke-beds (pages 17 to 41).
It should be explained that the experiments on intermittent treatment in the coke-beds have
been carried out by filling the coke-beds with sewage until the coke is just submerged, then allowing
the sewage to remain in contact with the coke for several hours, and finally draining the liquid away
completely. The coke is then allowed to remain for some time in contact with the air, which fills the
interstices of the coke-bed when the liquid has flowed away. This series of processes is repeated at
regular intervals.
Fuller details as to the construction and working of the coke-beds will be found in Section II.
(pages 7 to 10) of this Report.
2.—Objects of the so-called "filtration" experiments.
The process of "filtration" of raw sewage, or of its bacterial treatment in coke-beds, is already
in use for purifying the sewage of several towns containing a population less numerous than that of
London. But in order to render this process adaptable to the treatment of London sewage it
appeared to be necessary to use coke-beds which were deeper and which could be more rapidly filled
and emptied than those usually employed elsewhere. And further, since London sewage is of a
special character, consisting not only of domestic drainage but of manufacturing refuse of the most
varied description, it seemed advisable to ascertain by a lengthened experience whether its purification
was possible, and whether it could be regularly maintained by the process of bacterial treatment.
With these objects in view, coke of fairly large size was employed, and the coke was introduced into
tanks of considerable depth so as to enable the layer of coke to be progressively increased in
thickness during the progress of the experiments.
The main objects of the experiments were to ascertain—
(a) The effect of using the coke in fragments about the size of a walnut.
(b) The effect of increasing the depth of the layer of coke beyond the usual limit.
(c) The extent to which the raw sewage underwent purification by the treatment.
(d) The practicability of maintaining the constant passage of raw sewage through the
same coke-bed, without deterioration occurring, either in the bed or in the effluent.