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It might reasonably be inferred that in the coke-beds there was a relative increase in the liquefying
bacteria, and a relative decrease in the number of spores, as a result of the biological treatment of the
sewage.
It must be admitted that the above results are not satisfactory from the bacteriological point
of view, particularly when it is remembered that an effluent ought to be judged, not only by the
percentage amount of purification effected, but also by the actual state it is in. Yet it is to be
considered that these results assume a different complexion when viewed side by side with the
chemical data.
It has been shown in Division I. of this report that the percentage purification, as judged by the
dissolved oxidisable matter removed by the treatment, was on an average 51.3 per cent., and that the
suspended matter was entirely removed. It has been stated that the results thus obtained surpass
considerably those yielded by chemical treatment and appear to justify the claims put forward by the
supporters of the biological treatment of sewage, especially since, so far as can be seen, no nuisance
or danger arises as a result of the treatment.
In the body of the report a number of reasons are given, showing that it is unwise in the
present state of our knowledge to recklessly condemn an effluent on bacteriological ground alone,
without full knowledge of all the requirements of the case. In the attempt to treat sewage on
biological lines it is to be noted that the solution of the suspended matter and even the partial
destruction of putrescible matters by microbial agencies afford sufficient ground for justifying the
process, at all events as a preliminary measure. Whether this preliminary treatment is to be
supplemented by further treatment, either by passage through other coke-beds, or by land irrigation or
by any other method, is a matter largely dependent on circumstances.
In the present case there are practical points which first of all demand consideration, and
although it may be most desirable to obtain an effluent chemically pure and bacteriologically above
suspicion of danger it is to be thought of that an effluent not altogether satisfactory in one or other, or
even in both, of these respects may yet fulfil all necessary requirements without passing out of the
range of practicability. In certain cases it may be imperative to obtain an effluent bacteriologically
sound, but it does not follow that a similar result is urgently called for in other cases, as, for example,
where an effluent is turned into a watercourse which is not used for drinking purposes, and which
already may contain practically all the bacteria that are found in sewage.
It might reasonably be argued that where an effluent is turned into a river already grossly
polluted and below the lowest level of "intake" for waterworks purposes, that the chemical state of
such effluent was (from the practical point of view, at all events) of possibly even greater importance
than the bacteriological. Some such state of things pertains in the case of London sewage and the
river Thames. Here the initial consideration is to avoid fouling the river with putrescible matters to
such an extent as to constitute a grave public nuisance. From this point of view it is evident that an
effluent rich in putrescible matter is not permissible, but it is not as certain that an effluent rich in
bacteria is equally to be condemned on practical grounds.
In conclusion, it must not be considered from what has been said that any attempt is made to
minimise the importance of the bacteriological state of the effluent, or to disguise the true significance
of the results that have been obtained. No doubt the question will eventually have to be faced—
Are the advantages gained by chemical purification sufficiently great to outweigh the possible danger arising
from the discharge of an effluent bacteriologically unsound into the river Thames ?
That the water itself of the Thames is in an unsatisfactory state from the bacteriological point
of view may be deduced from the results shown in Table 2. This table also shows that the effluents
from the Barking and Crossness precipitation-tanks are no better, if indeed they are not worse, than
average samples of the raw sewage.
III.—SUMMARY OF RESULTS* SHOWN IN TABLE I.
The results are those derived from the bacteriological examination of Crossness crude sewage, and
of the effluents respectively from the 4-foot coke-bed, from the 6-foot coke-bed, and from the laboratory
coke-vessel (see pages 28 to 31).

These results are shown in graphic form in Diagram 3.
*I desire to record my thanks to Dr. Klein for advice and help of the greatest value.—A. O. H.