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3
Normal sewer air experiments.
In Table I. we have the results of eight experiments. It will be seen that the carbonic acid of
the sewer air varies from 4.40 as a minimum to 16.04 vols. as a maximum per 10,000 vols. of air, giving
an average for eight estimations of 9.18 vols. per 10,000, the carbonic acid in fresh air varying
during the same period from 2.54 as a minimum to 3.18 as a maximum per 10,000 vols. It is
remarkable that the CO2 of the fresh air should be so small, but to test the accuracy of the analyses
several were made in duplicate.
In five out of the seven determinations given in the same table, the number of micro-organisms
ill sewer air is less than in the fresh air, in the other two cases greater than in the fresh air.
I do not find such a perfect uniformity of results as was found by Drs. Carnelly and Haldane.
This may perhaps be accounted for by the fact that they, working together, were enabled to analyse
the fresh air and sewer air simultaneously, whereas as much as two to three hours frequently elapsed
from the commencement of the analyses of the fresh air to the completion of the estimation of
micro-organisms in the sewer air.
With regard to the statement of the authorities just quoted "that a decrease in the number of
micro-organisms in fresh air was followed by a decrease in micro-organisms in the sewer air," my
experiments are in perfect accord with theirs. This fact is particularly noteworthy also in the series
of deodorising experiments Table III. In the two instances alluded to above, in which the number of
organisms in sewer air exceeded those in the fresh air, the excess is due almost entirely to moulds and
not to bacteria.
Splashing experiments.
Table II. In this series of experiments sewer air only was dealt with. Determinations of the
water, carbonic acid, and micro-organisms of the normal sewer air were made, then splashing was
effected by continuously filling with sewage—shoots placed at about the middle line of the sewer and
at varying distances from the apparatus.
The object was to imitate and estimate, as nearly as possible, the disturbing effect of a house
drain favourably placed in a sewer, and not to obtain violent splashing.
It has been shown by previous experimenters that if the splashing is sufficiently violent to
produce a very fine state of division of the sewage, organisms will be carried some distance, even 50 to
60 yards. Hence the violent splashing, such as is produced by the entrance of a house drain through
the crown of a sewer, should never be permitted. From Table II. it will be seen that sewage running out
of a shoot about the middle of a sewer of the dimensions of the King's Scholars' Pond sewer produces
practically no effect upon the number of micro-organisms in the sewer air.
Deodorising experiments.
Table III. In the third series of experiments the sewage was deodorised for 24 hours previous
to each experiment. For the purposes of deodorising two sheds were erected, one in Piccadilly and
one in Curzon-street higher up the sewer. Work was carried on at both sheds simultaneously.
The deodorants used were manganate of soda and sulphuric acid in experiments I., II. and III.,
manganate of soda alone in IV., sulphurous acid in v. and VI., and carbolic acid in VII. and VIII.
The method of deodorising with manganate of soda and sulphuric acid, manganate of soda alone,
and sulphurous acid alone, was identical to that employed for some time past in deodorising the
sewage during the summer months.
In deodorising with carbolic acid the strength of the solution used was 0.5 per cent. and 500
gallons were delivered per hour from each shed.
Observations as to the amount of water, carbonic acid, and micro-organisms in the fresh air and
in the sewer air were also made during the employment of deodorants.
It will be seen in Table III, that the number of organisms in fresh air was very low during the
course of these experiments, which were carried out in January, February, and early March of this year.
With the decrease in the number of organisms in the fresh air, we find a concomitant decrease in the
number of micro-organisms in the sewer air. As was anticipated, the use of deodorants does not affect
the relation between the number of organisms in fresh air and sewer air respectively, but although the
quantity of deodorants employed was of course absolutely insufficient for complete disinfection of the
sewage, I thought it advisable as well as interesting to ascertain what effect, if any, the various substances
used exerted upon the organisms in the sewage itself. For this purpose I first of all made determinations
of the number of micro-organisms present in the untreated sewage as it flowed past the staging,
and next in the deodorised sewage. The results, which are the average of three separate determinations
in each case, are given below—
Organisms per C.C.
Untreated sewage 2,618,000
3,179,000
Sewage deodorised with manganate of soda and sulphuric acid 720,000
1,040,000
Sewage deodorised with manganate of soda alone 620,000
Sewage deodorised with sulphurous acid 2,400,000
Sewage deodorised with carbolic acid 1,660,000
1,200,000
I do not put forward these experiments as in any way perfect, for it is manifest that the average
number of micro-organisms deduced from the determinations of the number of micro-organisms on Nov.
20th and Dec. 2nd, would not hold good necessarily for sewage flowing three weeks or more later when
the deodorising experiments were made; nevertheless, I contend that they do show in the case of
manganate of soda and sulphuric acid, manganate of soda alone, and carbolic acid, a strong probability
of these deodorants exercising a certain disinfecting action.