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The fall was made to correspond with the gradient used in the construction of sewers of this
size, and the end A was conveniently placed for filling with sewage, which could be emptied when required
by opening the outlet C.
At B there was a 9-inch opening (provided with a tight fitting cover) through which the necessary
apparatus could be placed in position; the lower end of the sewer was carried vertically upwards for 8
feet, and at the point D a small hole was drilled to admit a ⅜-inch steam pipe. By injecting high
pressure steam through this pipe a current of air was drawn through the sewer, the velocity of which
could be regulated with ease; when the steam-cock was fully open it attained a maximum of 15 feet per
second.
The following is the mode of experiment adopted—the sewer was first filled with sewage and
allowed to remain fall for about a day, then emptied by opening the outlet C; after varying intervals a
current of air of definite velocity was drawn through the sewer by means of the steam injector, and a.
simultaneous determination made of the number of micro-organisms in the air surrounding the sewer,,
and in the sewer itself.
In the later experiments the sewer was allowed to remain empty for a lengthened period, comparative
determinations being made from time to time to ascertain whether the walls had dried sufficiently to give
off any of the micro-organisms clinging to them. The velocity of the air-current used in the above
experiments was 5 feet and 15 feet respectively, the latter being far in excess of any current that would
normally obtain in a sewer.
The carbonic acid in the air within the sewer was not determined, inasmuch as it would have
been practically identical with the carbonic acid in the air surrounding the sewer. On reference to
Table II. it will be seen that in most cases the number of micro-organisms found in the air surrounding
the sewer exceeds those found in the air within; in the only case in which the number of microorganisms
found within the sewer exceeds in a marked degree the organisms found without, the increase
is entirely due to moulds, and not to bacteria. It is really remarkable to find that no organisms are
given off from the walls of a sewer which has been empty and open to the air at both ends for such a
lengthened period as twelve days. The sewage with which the sewer had been kept full for several
periods of 24 hours would contain no less than three to four million organisms per cubic centimetre,
and immense numbers of these must of necessity have been clinging to the walls of the sewer. From
this it is evident that the conclusions arrived at from my previous experiments in the King's Scholars'
Pond sewer hold good for smaller sewers.
3.—Experiments on stagnant sewage.
Tabic III. In this series of experiments the same 9-inch sewer was used, and at about a foot from
the point B a dam was placed across the sewer occupying two-thirds of its sectional area (diagram 2).
Sewage was then pumped in to the level of the dam and allowed to remain stagnant for several
weeks. For the first sixteen days both ends of the sewer were kept open, then after the addition of a
considerable quantity of solid sludge the ends were closed. From time to time comparative determinations
were made similar to those described in the first series of experiments.
It will be seen on examining Table III. that the maximum percentage of carbonic acid only
reached 15.27 vols, per 10,000. This is probably explained by the fact that the air space in this experimental
sewer being very small the air was more frequently changed.
The number of micro-organisms in the sewer air in experiments 2 and 4 considerably exceeds
the number of micro-organisms in the air without the sewer at the same time; this increase, in my
opinion, loses its significance because it is due to an augmentation of the moulds and not of the bacteria;
it is, however, difficult to offer any explanation of the cause as the determinations within and without
the sewer were always made simultaneously.
Fulham-road sewer.
Table IV. An experiment was made on September 6th in this sewer, belonging to the Fulham
Vestry, at a point close to the Town Hall. It may be within the recollection of the Committee that owing
to many complaints as to the condition of the sewers in this locality, Mr. Dibdin made several determinations
of the amount of carbonic acid and ammonia in two of the Fulham Vestry sewers, and also in the
Council's main sewer into which they discharged their contents. The results he obtained revealed an
unusual state of things, the carbonic acid in the Fulham-road sewer reaching, on August 16th, the
abnormally high percentage of 93.10 vols, per 10,000, the highest hitherto recorded as far as I am aware.
The sewer was only some 400 yards long, its upper end being a dead end, and its lower end discharging its
contents into the Council's main sewer. It contained a large amount of heavy deposit and was otherwise
in a most filthy condition. It was practically without ventilation, as nearly all the ventilators had
been closed for some time past owing to the number of complaints about the bad smell. In spite of all
these apparently unfavourable conditions it will be noticed, on reference to Table IV., that the number
of micro-organisms is particularly low, only averaging a little over one per litre of air. A comparative
determination of the micro-organisms in the air above the sewer was not made, as no convenient place
could be found free from the dust caused by the heavy traffic in the vicinity.
4. The question of identity of the various micro-organisms isolated from sewer air and fresh air.
A satisfactory answer to this question is of the utmost importance in deciding whether sewer air
is dangerous or not. There are two possible sources of the micro-organisms in sewer air. They may,
in the first place, be derived from the sewage and sewer walls, or secondly, from fresh air gaining access
to the sewer by ventilation. Were their source the sewage or walls of the sewer, their number would
increase with the length of sewer traversed by the air, or with the length of time during which the air
was in contact with the sources of contamination.
It has, however, been abundantly proved that sewer air almost invariably contains a smaller
number of micro-organisms than fresh air in the same vicinity and at the same time, and, moreover,
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