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It is to be noted that up to the twenty-fourth day after the original inoculation the presence
of St. pyogenes aureus was easily demonstrated. But a negative result was obtained in the case of
culture (9)—thirty-fifth day. Nevertheless, in culture (10) a single colony was discovered, i.e., on the
thirty-eighth day subsequent to inoculation. Later, the microbe was no longer discoverable in
cultures made from the sewage.
From this experiment it would seem that St. pyogenes aureus is capable of retaining its
vitality in Crossness crude sewage for a considerable time.
In conclusion, and speaking in general terms of all the experiments, it may be stated that
the cholera bacillus (pathogenic), B. prodigiosus (non-pathogenic), and St. Pyogenes aureus
(pathogenic) are capable of retaining their vitality in Crossness crude sewage in competition with
the very numerous bacteria normally present in the liquid for a considerable time. But it must
be insisted upon that the conditions prevailing in laboratory test-tube experiments are widely
different from those in operation in biological coke-beds. Nevertheless, and looking at the
subject from the point of the epidemiologist, these experiments are far from reassuring, and,
indeed, would seem to indicate that the antagonism of the saprophytic bacteria normally present
in sewage would not suffice for the destruction of pathogenic species either in the sewers or, later,
in the bacterial beds. Since the bacterial beds are purposely constructed so as not to mechanically
hold back the suspended matters, and as it is to be feared from these experiments that the biological
processes at work in the coke-beds would not rapidly destroy the life, although they might inhibit
the multiplication of pathogenic germs accidentally introduced into the crude sewage, the effluents
from the beds ought to be regarded as no more safe in their possible relationship to disease than
the raw sewage itself. It must be remembered, however, that the number of the pathogenic
germs added to the sewage in these experiments was vastly greater than could conceivably take
place under natural conditions, and that, notwithstanding the enormous number introduced, there
was definite indication of a somewhat rapid decrease in their numbers.
For records relating to the passage through the bacterial beds of pathogenic germs
habitually or occasionally present in crude sewage reference must be made to other sections of this
Report and also to previous Reports.
2.—THE INOCULATION OF THE 13-FOOT COKE-BED AT CROSSNESS WITH A
SPECIAL SEWAGE MICROBE.
Most observers who have had any practical experience of the so-called biological treatment
of sewage are agreed as to the necessity of treating new bacterial beds with small but gradually
increasing doses of raw sewage until they have become thoroughly matured. Further, it is
generally believed that a mature bed is one which has become by a natural process of selection
charged with the special bacteria concerned in the work of purification. By varying the conditions
in a number of ways changes, favourable or the reverse, may readily be induced. For example,
by overtaxing the coke-beds a change commonly spoken of as "sickening" may set in when the
chemical results will no longer be satisfactory. In such a case it may be conjectured that the wrong
kind of bacteria have gained the ascendancy. Conversely, by greatly diminishing the amount of
sewage to be "treated," a particularly good effluent may often be obtained. Here presumably the
special bacteria are able to multiply abundantly and to exercise their specific qualities to the best
advantage. Exactly which kind of bacteria are directly beneficial and which act as intruders is
not yet clearly known, although the broad fact that putrefactive and nitrifying germs are necessary
has long been established. Nor is it properly understood under what conditions the special
bacteria should be placed in order to allow them to exercise their beneficial qualities to the best
advantage. Bacterial processes so widely different as those which aim at encouraging the growth
of aerobic micro-organisms and those which are essentially or largely anaerobic in character, have
been put in operation by different workers. And each observer claims for his own process
peculiar advantages. But it will be gathered from what has been said that probably
all the bacterial processes in practical operation at the present time aim at allowing
certain bacteria or groups of bacteria to gain the ascendancy by a natural jorocess of selection.
Although this is certainly a rational method of "treating" sewage it is to be thought of that
the future of the biological treatment of sewage may possibly lie in the direction of a real or
apparent interference with nature's methods. In the absence of absolute knowledge as to the
exact conditions under which sewage should be placed so as either in one or a succession of stages
to foster the growth of the bacteria directly concerned in the work of purification and to. inhibit the
growth of unnecessary or harmful germs it is conceivable that the addition (continuously or intermittently)
of pure cultures in large amount of selected microbes might exercise a beneficial effect.
Which kinds of bacteria should be added, and under what conditions, cannot safely be affirmed in
the present state of our knowledge. The experiments might be conducted so as to aid or abet
nature rather than with the object of interfering with or upsetting the natural order of things, and
they might occupy several stages. Further, they might be carried out in connection with bacterial
beds already for some time in operation or new beds might be sown with special bacteria previous
and preparatory to the application of sewage. It is conceivable that they might be divided into
two stages—the first aerobic or semi-anaerobic, the bacteria used belonging to the class of putrefactive
aerobes and facultative anaerobes, and the second stage purely aerobic, the micro-organisms
involved in the process being those of nitrification, or, preliminary to the above, and as an initial
stage anaerobic bacteria might be employed. Lastly, the experiments might be conducted from the
epidemiological point of view rather than with the sole idea of obtaining an effluent chemically
sound. For as it is known that certain micro-organisms are antagonistic to others, the special
bacteria selected might be saprophytes, the products of whose growth act as bactericidal agents to
pathogenic germs.
Of course it will be argued that nature's own methods are the best, and that any interferance
with the natural course of events is not to be recommended, and that it would be wise to first
determine more accurately the precise functions of the different bacteria concerned in the work of