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51
Next dealing with the ten samples in which B. coli was distinctly below the average,
namely, those in which the number was 300,000 or less per c.c. In three of these samples the
number of spores of B. enteritidis was over 1,000, five times the number was an average one, and
in the remaining two the number was below the average.
Secondly, dealing with the effluents from the 4-foot coke-bed, a similar although perhaps
less well marked want of correspondence between the number of B. coli and B. enteritidis is
noticeable.
The records as regards the other coke-bed effluents are not very numerous and do not lend
themselves readily to comparisons of the above sort.
In general summary of these results it may be said that—
(1) Although the total number of bacteria, the number of spores of aerobic microbes,
the number of liquefying micro-organisms, the number of B. coli. and the number of
spores of B. enteritidis sporogenes was, on an average, less in the effluents from the cokebeds
than in the raw sewage, the reduction was not well marked, and frequently the
number was actually greater in the effluents than in the corresponding samples of crude
sewage.
(2) The 4-foot coke-bed yielded on the whole better results than the 6-foot primary
and proved nearly as efficient as the 6-foot secondary coke-bed.
(3) There was no very definite evidence, so far as may be judged from these results,
of any special selective* process being effected by the bacterial treatment of the sewage
in the coke-beds, since the bacterial composition of the effluents was very much the same
as that of the raw sewage.
(4) The number of microbes, both in the crude sewage and effluents, was always more
than one million, but usually less than ten million per c.c.; the number of B. coli and
spores of B. enteritidis sporogenes was usually more than 100,000, and at least 100, but
less than 1,000, respectively, per c.c. More than one-tenth of the bacteria liquefied
gelatine, and only about one in 20,000 was present in the spore form.
(5) As the bacterial composition of the effluents differed only slightly from the crude
sewage, so far as this could be judged of by the records which have been enumerated, it
cannot, in the absence of direct proof to the contrary, be safely considered that the
effluents from the coke-beds are less dangerous than the raw sewage in their possible
relation to disease.
It must, however, be remembered that in the present case there are practical points which
first of all demand consideration. In this connexion it will not be out of place to quote from
page 21 of the Second Report.
Thus in dealing with the general results obtained at Crossness it was said—"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
from the dissolved oxidisable matter removed by the treatment, was on an average 50 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 bv 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 considerable
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, 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
* That, however, some selective process is at work in the coke-beds need hardly be doubted. For
example, raw sewage contains little or no oxidised nitrogen, whereas the effluents from bacterial beds contain
nitrites, and nitrates often in considerable amount. This alteration in the chemical state of the liquid is brought
about by nitrifying bacteria, and it is highly probable that these particular microbes multiply and accumulate
in the coke-beds, and that the effluents from mature coke-beds contain more nitrifying organisms than the crude
sewage. Some experiments of a tentative character which I have carried out seem to support this hypothesis.