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18
I—THE BIOLOGICAL TREATMENT OF SEWAGE.
The term biological, as applied to the treatment of sewage by natural processes, although
manifestly convenient, is apt to be misleading. It is meant to denote the endeavour to obtain, under
control, results similar to those which are constantly being produced in nature, and to employ processes
which have been in operation for unknown ages. The term, however, is misleading because it might
give the erroneous impression that sewage does not contain in itself the living organisms which are
necessary to effect its decomposition, and that some new discovery had been made by which sewage
was artificially treated with foreign bacteria, in order to bring about its destruction. This, however,
is not the case.
Sewage already contains all the organisms which are necessary for its decomposition and final
purification. In order to discover a method of biological treatment all that has to be done is to
discover the best and the most practical way of allowing the natural purification by the action of the
bacteria to take place without nuisance or danger.
In brief, the so-called biological treatment of sewage is neither more nor less than the attempt
to imitate nature's own methods of effecting the decomposition and finally the purification of the
effete matter of the animal and vegetable kingdom.
In this connection the testimony of Duclaux may be quoted:—"Whenever and wherever there
is a decomposition of organic matter, whether it be the case of a herb or an oak, of a worm or a
whale, the work is exclusively done by infinitely small organisms. They are the important, almost the
only, agents of universal hygiene; they clear away more quickly than the dogs of Constantinople or
the wild beasts of the desert the remains of all that has had life; they protect the living against the
dead. They do more: if there are still living beings, if, since the hundreds of centuries the world has
been inhabited, life continues, it is to them we owe it."
To describe the steps which have brought our knowledge to such a point that sanitarians now
dare to look forward to a possible solution of the problem of sewage disposal would be to outline the
history of bacteriology. Here it is permissible only to indicate in a few sentences some of the
discoveries which have paved the way towards a scientific knowledge of the true nature of putrefactive
processes.
One of the most important discoveries of recent times was that made by Schloesing and Muntz
in 1877. They proved that nitrification, or the oxidation of ammonia to nitric acid, is due to the vital
activity of bacteria, and thus carried a stage further the important discovery by Schwann and Schultze
(1839) that micro-organisms are the true agents of decomposition. Later, Winogradsky and others
described and isolated, in pure culture, nitrifying organisms.
Pasteur, following up the researches of Cagniard and Schwann, demonstrated, in 1857, the
relation between lactic, acetic, and butyric fermentations and special organisms. Later, he erroneously
assumed that putrefaction was brought about solely by anaerobic bacteria. Since 1857 the science
of bacteriology has advanced by leaps and bounds, and increased knowledge of the nature of putrefactive
processes has, during the last decade, led to the serious attempt to bring about putrefaction,
under control, as a means of disposing of excremental matters.
Putrefaction has been called putrid fermentation, but putrefaction is rather to be looked upon
as a fermentation which is putrid or otherwise, according to the conditions under which it is conducted
and the degree of its completeness.
The products of decomposition by so-called putrefaction are of the most varied kind, and there
is no doubt that putrefaction is brought about, not by a single germ, but by a large number of different
bacteria, some aerobic, others anaerobic. Each one of these germs may produce intermediary products
of widely different character, but all of them, perhaps, tend in the direction of finally resolving highly
complex organic substances into their simplest component parts.
Although the study of fermentation and the proper understanding of the true nature of
putrefactive processes are attended with.great difficulties, the aim and object of the biological treatment
of sewage is sufficiently plain and simple.
In nature the following cycle of transformation takes place:—Dead organic matter decays as
the result of the vital activity of bacteria, and ammonia is liberated. The nitrifying organisms bring
about the oxidation of the ammonia, first to nitrous and then to nitric acid. These acids by reaction
upon the bases, always present, form nitrites and nitrates, and these nourish the living plant. While
the nitrogen is undergoing these changes, the carbon of the organic matter is converted into carbonic
acid, and the hydrogen mainly into water. To some extent also the nitrogen and hydrogen are
liberated in the free gaseous state.
Although organic matter is ultimately resolved into innocuous, inorganic substances by these
processes, the intermediary products of decomposition, as has been already indicated, may be of a highly
complex character. Fortunately, however, these complex substances are for the most part unstable in
character and readily oxidisable, and from the practical point of view they may be regarded therefore
as the earlier stages leading up to the final purification.
Now the organic matters found in sewage are partly in suspension and partly in solution, and
sewage contains in itself the necessary living germs for the destruction of both these forms of organic
matter. The aim and object of the biological ttreatment of sewage is to render soluble by microbial
agencies the solid matters; and to split up by the action of living bacteria both the matter thus
dissolved and the organic compounds which were originally in solution, into their simpler elements.
In the final process of purification, these substances should undergo oxidation induced by the life
"processes of nitrifying organisms, and an effluent should be produced which is free from putrescible
matter and contains only inorganic or mineral substances.
Of course it will be argued that there is great danger in encouraging the life processes of
micro-organisms which are largely derived from the excreta of animals. Such a contention is
certainly justifiable. But in this connection it is to be noted in the first place that the biological