View report page

4
Although the extreme variation in numbers was wide, a number of samples gave results fairly
similar. Thus experiments 6, 7, 9, 10, 12 and 14 yielded the following numbers—263, 290, 400, 470,
120 and 450, of which the mean is 332.
Crossness crude sewage.
2,800, highest number, March 2nd, 1898. Experiment 3.
180, lowest number, April 20th, 1898. Experiment 8.
852, average of 5 cultures of 5 samples. Experiments 3, 4, 8, 11 and 15.
Excluding the results obtained in experiment 3, experiments 4, 8, 11 and 15 yielded the
following results—500, 180, 400 and 380, of which the mean is 365.
To what extent the spores are present in the sewage when it is first discharged into the sewers
is not known, therefore it cannot be said that there is either an increase or a decrease in their number
during the passage of the sewage to the outfalls. It is possible that the original spores germinate into
cells in the sewers, but as the struggle for existence between the different species increases, that they
again form spores. Some bacteriologists hold that bacteria proceed to sporulation chiefly or solely when
the cells have attained their highest development. Others assert that bacteria change from a transitory
"growth form" to a "persistent spore form" in order to avoid extinction when the conditions for
their growth are unfavourable.
Future work will show whether the conditions prevailing in the biological filters at the outfall
works lead to an increase or a decrease in the number of spores. As spores are peculiarly resistent to
external influences their presence in large numbers in a liquid is to be regarded with some suspicion.
Fortunately, however, the great majority of spores of aerobic organisms in sewage belongs to species
which are quite harmless.
Taking the figures 3,899,259 and 3,526,667 as representing the total number of bacteria, and
332 and 365 as representing the number of spores in one c.c. of average samples of Barking and
Crossness sewage respectively, the ratios of spores to bacteria are 1 to 11,744, and 1 to 9,662
respectively. It may be worthy of note that the actual number of spores and the number relatively
to the total number of bacteria present in one gramme of moist soil are much greater than these.
3. Liquefying bacteria.
The number of liquefying bacteria present in 1 c.c. of the different samples varied as follows—
Barking crude sewage.
900,000 highest number, May 4th, 1898. Experiment 14.
220,000 lowest number, April 15th, 1898. Experiment 6.
430,750 average of 9 cultures of 4 samples. Experiments 6, 9, 13 and 14.
Crossness crude sewage.
400,000 May 4th, 1898. Experiment 15, was the only record of the exact number.
The number of liquefying bacteria found in the sewage was always very great. It was a
matter of some difficulty to determine the exact number, since many of the micro-organisms liquefy
the gelatine so rapidly as to "flood" a plate before it is ready for counting. It is probable, therefore,
that the above estimations of the number of bacteria present which cause liquefication of gelatine are
too low.
The importance of collecting information as to the number of liquefying bacteria which are
present in sewage is considerable, since it has a distinct bearing on the ability of sewage to cause the
liquefaction of its own solid or suspended matter. That this liquefaction may be and probably is
largely due to putrefactive bacteria is not necessarily a cause for alarm, because the several stages
of putrefaction are steps leading in the direction of ultimate purification. It must not be concluded,
however, that because a micro-organism liquefies gelatine it will also liquefy all the various and
complex kinds of organic matter existing in sewage. But it is safe to assert that a liquid rich in
bacteria of varied species, many of which are capable of producing liquefaction of gelatine, is likely
also to be rich in ability to dissolve solid or suspended organic matter.
It is probable that not only the bacteria generally, but the liquefying bacteria in particular,
multiply greatly during the passage of the sewage along the sewers to the outfall.
The ratio between the number of liquefying bacteria and the total number of bacteria in
Barking and Crossness sewage is 1 to 9 and 1 to 8.8 respectively.
Before proceeding to discuss the species of micro-organisms present in London crude sewage,
it is to be noted that all the above figures refer to aerobic bacteria.
4. Species of micro-organisms present in London crude sewage.
As the work in this direction is still in progress and demands careful and prolonged study, the
following description must be looked upon as being only a record of the chief results which have been
obtained up to the time of writing this report.
The limits of this report prevent any but a passing allusion to the work of others. The subject,
however, has been carefully studied by such competent observers, among many others, as Mr. Jordan* ;
Sir Henry Roscoe and Mr. Lund†; Mr. J. Parry Laws and Dr. Andrewes‡. That the valuable results
which have been obtained are not more complete, is to be attributed to the fact that the precise
study of the bacteria in sewage is certainly the work of many years. To a considerable extent the
present enquiry covers new ground.
* Reports of the State Board of Health of Massachusetts (1888-90, Part II.).
† Transactions of the Royal Society, 1892.
‡ Micro-organisms of sewage, 1894, London County Council. Apart from its intrinsic merit as a most
valuable contribution to scientific literature, this report is of especial interest as it deals with the bacteriology of
London crude sewage.