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It will be noted that in these comparative experiments the total number of bacteria in the crude
sewage usually exceeded those found in the effluents from the coke-beds. On three separate occasions,
however, the effluent from the 4-foot coke-bed contained more organisms than the crude sewage, namely,
expts. 2, 4, and 10, as compared with expts. 1, 3, and 9. In round numbers the averages were over six
million, over four million, and over four million in the crude sewage, the effluent from the 4-foot bed,
and the effluent from the 6-foot bed respectively. The percentage reduction of bacteria was on an average
27-7 in the case of the 4-foot bed effluent and 32.4 in the case of the 6-foot coke-bed effluent.
Although the percentage reduction in the total number of bacteria is fairly satisfactory, it must be
conceded that the average number of microbes left in the effluent is very large, namely, 4,437,500 in the
case of the 4-foot coke-bed, and 4,150,000 in the case of the 6-foot coke-bed.
It is probable that the conditions prevailing in the coke-beds may have been satisfactory as regards
the solution of suspended organic matter and even as regards the partial destruction of dissolved organic
matter, and yet have fallen short of leading to active nitrification and total purification. In the
Massachussets experiments, when the reduction in the amount of organic matter was satisfactory and
nitrification was in active progress, the total number of bacteria suffered a marked diminution.
In brief, it may be, that the coke-beds were highly efficient from the point of view of solution of
suspended organic matter and partial destruction of dissolved and offensive substances, and as a preliminary
measure tending in the direction of complete disintegration and ultimate purification of the
crude sewage, but that they were unable to bring about the complete resolution of the organic matter; if
this were the case, sufficient pabulum may have been left in the effluent to allow of the continued growth
and multiplication of the sewage bacteria.
Indeed, so long as organic matter in an assimilable form remains in the effluent, multiplication of
existing micro-organisms may be expected to take place until the self injurious products of these bacteria,
or a lack of nutritive material, or some other conditions adverse to microbial life lead to their destruction
or to a cessation of their powers of multiplication.
The presence of bacteria in enormous numbers in an effluent does not perhaps necessarily imply that
the effluent is of a degraded character and highly putrescible ; it may only mean that the liquid has passed
through a previous stage of putrefaction, preparatory to its purification, in which case the danger so far as
nuisance is concerned may be regarded as potential and not actual. It is as certain on the one hand that
the addition of an infinite number of bacteria to a pure liquid, would be followed by a decrease in their
number owing to the lack of nutritive material, as it is on the other hand that the removal of all the
bacteria from a foul liquid would be a useless precaution, since Nature has always at her command a host
of micro-organisms ready to attack the effete matter of the vegetable and animal kingdom, and effect directly
or indirectly its purification.
Nevertheless, a liquid swarming with living bacteria is usually a liquid still undergoing putrefaction,
and is likely also to contain germs of a harmful nature.
It may be worthy of note that out of the sixteen samples of Crossness and the nine samples of
Barking crude sewage examined during a period extending from February to August, 1898, on no occasion
was it found that the total number of bacteria was other than very large. It is evident then that the
various substances, such as waste matters from manufactories, which are discharged in large quantities
into the London sewers, and yet may be regarded as foreign to sewage, do not inhibit the growth of the
sewage bacteria to any marked extent. It is important to note this fact, because if the bacterial
purification of sewage should be adopted on a large scale, disastrous results might arise if the sewage
was at one time rich in microbial life and at another almost sterile owing to the presence of foreign
substances inimical to bacteria.
In conclusion, it must be borne in mind that the coke-beds at Crossness were purposely constructed
so as not to lead to mere mechanical filtration of the suspended matter, much less of the bacteria in the
crude sewage.
In estimating the total number* of bacteria gelatine plates were used. From 0.11 to 1.0 c.c. of crude sewage
or effluent diluted with 10,000 times its volume of sterile water (i.e., 0.00001 to 0.0001 c.c. of the original fluid) was
added to 10 c.c. of iterile gelatine contained in a test tube. Alter the gelatine had been melted, it was poured
into a Petri's capsule, and after solidification had taken place the plate was inverted, incubated at 20 C.,
and the colonies subsequently counted at as late a date as the liquefaction of the gelatine and the crowding
of the colonies allowed of.

These results are shown in graphic form in Diagram 4.
* See B 3, page 2—First Report.