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In these comparative experiments the number of spores of bacteria in the crude sewage usually
exceeded those found in the effluents from the coke-beds, but on three separate occasions the number in the
effluent from the 4-foot coke-bed was greater than in the crude sewage—namely, experiments 6,10, and 16
as compared with experiments 5, 9, and 15. The averages were 407, 252, and 390 in the crude sewage, in
the effluent from the 4-foot bed and in the effluent from the 6-foot bed respectively. The percentage
reduction of spores of bacteria was on an average 38 in the case of the 4-foot coke-bed effluent, and 4 in the case
of the 6-foot coke-bed effluent. In the latter case, however, the average is based on two experiments only.
In the case of the 4-ft. coke-bed effluent, the percentage reduction in the number of spores was slightly
greater than the percentage reduction of the total number of bacteria. Yet the number of spores
remaining in the effluent, namely, 252 on an average, was large. Spores of bacteria are peculiarly
resistant to unfavourable physical conditions; fortunately, however, the majority, at all events, of the spores
of aerobic micro-organisms found in sewage belong to species which are believed to be harmless.
Taking the figures 6,140,000, 4,437,500 and 4,150,000 as representing the total number of bacteria,
and 407, 252 and 390 as representing the number of spores in one cubic centimeter of an average sample of
Crossness crude sewage, of effluent from 4-ft. bed, and of effluent from 6-ft. bed respectively, the ratios of
spores to bacteria are as 1 to 15,086, 1 to 17,609, and 1 to 10,641 respectively. It appears, then, that
there was a slight diminution in the number of spores, relative to the total number of bacteria, as a result
of the treatment of the sewage in the 4-ft. coke-bed The opposite is the case as regards the 6-ft. bed,
but here only two samples of effluent were examined.
Comparing diagrams 3 and 4 as regards crude sewage, no direct parallelism appears to exist between
the total number of germs and the number of spores, although to some extent a rise or fall in the total
number of bacteria was associated with a rise or fall in the number of spores. When, however, a similar
comparison is made in the case of the 4-ft. coke-bed effluent, it will be noted that a parallelism does exist
between the total number and the number of spores of bacteria.
In estimating the number of spores of bacteria, *the following plan was adopted:—To 10 c.c. of sterile
gelatine in a test tube was added 1 c.c. of diluted sewage or effluent (1:10), and the mixture heated to 80° C. for
ten minutes and then poured into a Petri's capsule. After the gelatine had become quite solid the plate was
inverted and incubated at 20° C.

These results are shown in graphic form in Diagram 5.
Although the number of liquefying bacteria in the crude sewage largely exceeded those in the
effluent from the 6-ft. coke-bed, the difference between the liquefying microbes in the 4-ft. coke-bed
effluent and in the crude sewage was not great, and in four out of the eight comparative experiments the
number was greatest in the former—namely, in experiments 2, 4, 10, and 24, as compared with experiments
1, 3, 9, and 23.
The averages were 860,000, 762,500, and 250,000 in the crude sewage, in the effluent from the 4-ft.
bed, and in the effluent from the 6-ft. bed.
The percentage reduction was on an average 11.3 in the case of the 4-foot coke-bed effluent and
70'9 in the case of the 6-foot coke-bed effluent. But as regards the 6-foot coke-bed effluent, the average
is based on the result of the examination of two samples only.
It is to be noted that in the case of the 4-foot coke-bed effluent the reduction in the liquefying
bacteria, as compared with the reduction in the total number of microbes and in the spores of microorganisms,
was very small.
The ratio between the number of liquefying bacteria and the total number of bacteria in Crossness
crude sewage, in the effluent from the 4-foot bed and the effluent from the 6-foot bed, is 1 to 7.1, 1 to 5.8,
and 1 to 16.6 respectively.
As regards the 4-foot bed, it would seem as if the biological treatment of the sewage gave rise
to a slight increase in the number of liquefying bacteria relative to the total number of germs. The
opposite holds good in the case of the 6-foot bed, but here only two samples were examined.
Comparing diagrams 3 and 5 as regards the crude sewage it will be seen that a rise or fall in the
total number of bacteria was to some extent associated with a rise or fall in the number of liquefying
bacteria. But when in the case of the 4-foot coke-bed effluent a similar comparison is made, it will be
noted that the parallelism between the two was more distinct.
Comparing diagrams 3, 4, and 5, it follows from what has been already said as regards the 4-ft. cokebed
effluent, that if the total number of bacteria and the number of spores, and the total number of bacteria
and the number of liquefying organisms, are related in each case, then there must be some relation between
*See B 4, page 2—First Report.