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equal to that of the fine coke-bed B of the last experiment, the coke-beds were dealing with
sewage which had received previous treatment in a primary coke-bed, whereas in the last experiment
the coke-bed had been dealing with sewage which had been only imperfectly settled and
which had been subjected to but very slight septic action.
In the case of the coarse coke-bed the amount of purification effected was greater in the
last series of experiments than that effected by the secondary coarse-bed A 1 in the previous
series of experiments. The result obtained by the coarse coke-bed A is hardly comparable with
the results obtained by the primary coarse beds of Series I. and II., as the sewage supplied to
them had not been treated in any way, whereas in Series III. the sewage had been previously
subjected to sedimentation.
It will therefore be seen that the coke-beds used in the experiments of Series III. effected
practically as much purification on a sewage which had been previously settled as the secondary
coke-beds of Series I. and II. effected on a sewage which had been previously treated in primary
coke-beds. But the advantage of the method of previous settlement and treatment in a single
coke-bed, over that of primary and secondary treatment in coke-beds without previous settlement,
is that in the former process the primary coke-beds are rapidly thrown out of action by loss
of capacity by choking, while in the latter process the choking is avoided. The results of the experiments
at the Council's Outfall Works and at other centres indicate that when the settlement of the
sewage is conducted on a large scale, and is accompanied by energetic septic action, the cokebeds
do not lose capacity at all by the accumulation in them of sewage sludge, but only by the
growth of purifying bacteria upon the coke surfaces. This growth is necessary and desirable if it
is kept, as it can be kept, within reasonable limits.
From November 7th, 1900, to January 27th, 1901, the samples of sewage supplied to the
two settling tanks were mixed together before being analysed, as were also the samples of the
settled sewage flowing from the two settling tanks to the coke-beds; on and after January 28th
these samples were all separately analysed.