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treatment of sewage is conducted under control; secondly, that the process always gradually secures
the destruction of the pabulum on which bacteria feed, and hence leads to their death; thirdly, that
the balance of evidence points to the probability that some, at all events, of the pathogenic organisms
are crowded out in the struggle for existence in a nutritive medium containing a mixed bacterial
flora,* their vitality being weakened or destroyed by the enzymes of the saprophytic species; †
fourthly, that while it is true that bacteria produce poisonous substances in their growth, it also is true
that their chemical poisons are toxic in proportion to the dose, and, moreover, are highly unstable and
readily break down into their elementary and innocuous constituents; and, lastly, that in some cases
it may not be necessary to attempt the complete purification of the sewage, the solution of the
suspended matters and partial destruction of the putrescible matter in solution being all that is
urgently called for, as, for example, where the effluent is of relatively small bulk and is turned into a
stream the water of which is not used for domestic purposes (as is the case in the lower Thames) or
else when the effluent is to be subsequently treated by land irrigation.
It has been said that the processes of putrefaction and nitrification are dependent on the vital
activity of bacteria, and that the destruction of the organic matters in sewage can be effected by the
life processes of living germs. But it will be asked under what conditions the bacteriological
purification of sewage can be best carried out, and whether there is any process known which in point
of economy, practicability and freedom from obvious danger compares with the methods ordinarily in
use—namely, chemical treatment and land irrigation. These are questions which cannot be fully
answered in the present state of our knowledge.
As regards the conditions essential to the successful biological treatment of sewage, it may be
said at the outset that the presence in the sewage of any substance inimical to microbial life must, if
possible, be guarded against. Thus, it is well known that acidity inhibits the growth of bacteria, and
that slight alkalinity is a favourable condition. Again, many of the waste products of manufacturing
processes are poisonous to bacteria, and in the preliminary treatment of the sewage with certain
chemicals it may also be rendered unfit for bacterial life. But whether the sewage is best placed
primarily under aerobic or under anaerobic conditions is a question still open to discussion. All,
however, are agreed that the final treatment must be aerobic.
As regards a comparison between the bacteriological treatment of sewage and its treatment by
chemical precipitation or by land irrigation, it may safely be said that even the most powerful
advocates of treatment with chemicals or treatment on land are dissatisfied with the general results that
have been obtained in the past, and it is a significant fact that the believers in the possibility of the
so-called self-purification of sewage are rapidly increasing in numbers. Moreover, the records, which
are daily accumulating, of the practical trials of bacterial coke-beds in different parts of the country tend in
the direction of demonstrating the possibility of purifying sewage biologically, although the conditions
under which such treatment is best carried out are not yet fully known.
One point which has been rather lost sight of is that there is probably no bacterial process in
practical operation at the present time which is not eminently successful in that it places the sewage
in a most favourable state for its final purification by land irrigation, or by other methods. The mere
solution of the great mass of the suspended matters by bacterial agencies, which is, perhaps, common
to all the different processes at present under trial, is a sufficient vindication of the enormous advantage
to be gained by the biological treatment of sewage. And, even when an effluent is excessively foul, it
is, perhaps, not wholly justifiable to condemn it in unmeasured terms, because such foulness may possibly
be a step, although not of the most direct character, leading towards ultimate purification.
The scope of this report permits only of a brief allusion to some of the methods which are in
operation for dealing bacteriologically with raw sewage and partially purified effluents. No attempt,
however, will be made to describe their respective merits.
To the State Board of Health of Massachusetts belongs the honour of having carried out a
laborious series of experiments upon the purification of sewage at a time when the whole subject was
enshrouded in mystery. The publication of the valuable results of this investigation at once aroused
the interest of sanitarians in this country.
In 1892 Mr. Scott-Moncrieff introduced his cultivation bacteria-bed. He founded his system on
the following well-recognised truths—
(1) That bacteria under favourable conditions are capable of indefinite multiplication.
(2) That there exist in sewage, bacteria which are capable of peptonizing solid organic
matter, or in other words, of preparing it, by a process comparable to that of digestion, for
its final disintegration.
(3) That in nature the purification of the refuse of the organic world is effected by the
life history of these or similar micro-organisms.
In this system the crude sewage passes into the bottom of a bed filled with flint, coke and
gravel, and fitted with a false bottom. The liquid portion rises through this bed and the suspended
matters are held back at the bottom and undergo solution by the action of bacteria. In order to still
further improve the quality of the effluent, by oxidation and by the action of micro-organisms, the
effluent from this primary bed is afterwards passed down nitrifying channels. Since 1892
Mr. Moncrieff has modified his original plans in a number of important respects.
*In this connection reference may be made to certain interesting experiments carried out by Lawes and
Andrewes, and published in their joint report to the London County Council on Micro-organisms in Sewage, 1894.
† It must be distinctly understood that I do not imply that such organisms as the typhoid bacillus or the
cholera vibrio would necessarily lose their vitality, or even suffer a diminution in virulence under the conditions
prevailing in a biological filter. In the absence of actual experiments with the particular sewage in question, I am
not prepared to say more than that I believe that if these germs did gain access to the sewage they would suffer
a diminution in numbers primarily in the sewers, and secondarily in the coke-beds.—A. C. H.