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The table shows that the number of spores of bacteria in the crude sewage (25 samples), the
effluent from the 4-foot coke-bed (14 samples), the effluent from the G-foot primary coke-bed (8
samples), and the effluent from the G-foot secondary coke-bed (3 samples), averaged 340, 252, 256,
and 320 per c.c. respectively. The percentage reduction in the effluents as compared with the
crude sewage being 25, 24, and 5. The average number of spores in the 14 samples of crude
sewage corresponding to the 4-foot coke-bed effluents was 322; in the 8 samples corresponding to
the 6-foot primary coke-bed effluents, 310; and in the 3 samples corresponding to the 6-foot
secondary coke-bed effluent the average was 503. Calculated from these figures the percentage
reduction in the number of spores of bacteria was 21 as regards the 4-foot coke-bed effluents, 17 as
regards the 6-foot primary coke-bed effluents, and 36 in respect of the 6-foot secondary coke-bed
effluents.
Usually a rise or fall above or below the mean in the number of spores of bacteria in the
raw sewage was associated with a similar increase or decrease in the number in the corresponding
effluents. Thus as regards the crude sewage and 4-foot coke-bed effluents there was in this
respect a relation between the two in 11 out of 14 samples; and the raw sewage and 6-foot primary
coke-bed effluents showed a similar correspondence in six out of eight comparative samples. The
same relationship was noticed with reference to the 6-foot secondary coke-bed effluents, but here
only three samples were examined.
So far as may be judged from a rise or fall above or below the mean the figures show no
distinct parallelism between the total number of bacteria and the number of spores of bacteria in
the case either of the crude sewage or of the effluents from the 6-foot primary coke-bed. But as
regards the 4-foot coke-bed effluents there is evidence of a distinct relation.
It will be understood that in the above remarks, when mention is made of a relationship, it
does not imply a parallelism as regards percentage deviation from the mean.
In the Second Report it was shewn that the percentage reduction in the number of bacteria
and the number of spores of bacteria in the 4-foot coke-bed effluent as compared with the crude
sewage was 27'7 and 38 respectively. Here and when dealing with a larger number of records,
i.e., including those given in Addenda A and C, as well as those contained in the body of the
Report, the figures are slightly different, being in the first case 32 per cent, and in the second
25 per cent.
It is difficult to gauge the exact significance of the number of spores of aerobic bacteria
and the number relative to the total number of bacteria. 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.
AH hough the actual number of spores in sewage* is large, the number relative to the total
number of bacteria is very small. Thus in the 26 samples above recorded for every microbe
present in the spore-form there were present over 21,000 in the vegetative-form. And this result
is not in all probability to be traced solely to the comparative infrequency of bacteria in sewage
capable of forming spores, but also to the active and continued multiplication of the microorganisms
in the presence of an abundance of suitable pabulum. In the case of effluents from
bacterial beds it is conceivable that a smaller reduction in the number of spores of bacteria as
compared with the reduction in the total number of bacteria, or an actual increase in the number
of spores in the effluent as compared with the crude sewage might be a good rather than a bad sign
as indicating that the organic pabulum was becoming exhausted and the conditions for bacterial
life so unfavourable as to lead the microbes to form snores so as to escape extinction.
In mv reports to the Local Government Board on the bacteriological examination of soils I
have shewn that in surface soils not only is the number of spores very large, but also that they are
very numerous in relation to the total number of bacteria. Thus in soils it is common to find the
ratio between the number of spores and number of bacteria—1:2;1:8;1:4;1:5;1:6;1:7;
1: 8; 1: 9 and 1: 10. Sometimes the ratio may be 1: 20 ; 1: 30; 1: 40, or lower, but here the