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52
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."
Methods.—For a description of the various methods employed the Report itself must be
consulted (pages 22, 23, 24, 25, 27). Here it is permissible only to give the amounts of crude
sewage or effluents to be added to the nutrient media in order to obtain the best results with the
different processes.
Total number of bacteria—From 1/100,000 to 1/10,000 c.c. Gelatine plate cultures, incubated at
20° C *
Spores of aerobic bacteria—From 1/10 to 1 c.c. Gelatine plate cultures. Cultures previously
heated to 80° C. lor 10 minutes.
(Incubated at 20° C.)
Liquefying bacteria—From 1/100,000 to 1/10,000 c.c. Surface gelatine plate cultures.
bated at 20° C.)
B. coli—From 1/100,000 to 1/10,000 c.c. Surface phenol (0.05 per cent.) gelatine
plate cultures. (Incubated at 20° C.)f
Spores of B. enteritidis sporogenes— 1/10,000; 1/1,000; 1/100; 1/10 c.c. Milk cultures, heated to 80° C. for 10
minutes, and cultivated anarëobically.
(Incubated 37° C.)‡
The Biological Treatment of Sewage (Section I., pages 18-20, Second Report).
In speaking of the discoveries which have paved the way towards a scientific knowledge of
the true nature of putrefactive processes, it was said that—
"One of the most important discoveries of recent times was that made by Schlœsing 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."
In discussing the aim and object of the biological treatment of sewage it was pointed out
that—
"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."
Further, that—
"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 treatment 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 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."
Two of the most remarkable phenomena to be observed in the study of biological coke-beds are.
(1) the almost complete removal of the suspended matters, and (2) the production of effluents
which, although they usually still contain a quite appreciable amount of oxidisable organic
matter, are, as a rule, non-putrescible so far, Indeed, as this may be judged of by storing them in
(a) completely filled and stoppered bottles and (b) partially filled and stoppered bottles.
It appears quite certain that the non-putrescible character of many effluents from bacterial
coke-beds is not due to the absence of putrefactive bacteria. This is, of course, very evident from
the numerous records which have been given of the bacterial composition of the Crossness effluents.
In particular, the following paragraph (page 27, Second Report) is worth quoting—
"In experiments 9 and 10 (col. 5, Table I.) it was sought to discover the smallest amount
of crude sewage and of effluent which in broth cultures at 20° C. would produce growth, indol
reaction and offensive smell. No growth occurred in either case, when as little as 0,0000001 c.c.
was inoculated into the broth; but when 0.000001 c.c. was used, growth occurred both in the case
of the crude sewage and of the effluent, and the cultures had an offensive smell and gave indol
reaction."
Probably the explanation lies, in part at all events, in a change in the character of the
organic matter still remaining in the effluents. Possibly also the liquid acquires some quality
inhibiting the putrefactive bacteria from exerting their putrescent effect.
Some stress was laid upon the fact that in certain 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 matters being all that is urgently called for, as, for example, where the
* In illustration of such cultures see Plate 1. fig. No. 2, First Report.
+ „ „ Plate 1, fig. No. 1, First Report.
‡ „ „ Plate 1, fig. No. 14, First Report.