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was dealing with a small portion of the present effluent before it entered the river.
This bed was arranged only to treat the chemical effluent; it was not of satisfactory
construction, nor was it filled with suitable coke material. Further, it was impossible,
owing to its low level, to empty it at all states of the tide, and accordingly it could
not be filled as often as was necessary, in order to obtain the knowledge which was
required. At the desire of the Committee, however, this bed has been maintained
in action for over eight years, and it is now still giving admirable results in purifying
the present sewage effluent.
But with the sanction of the Committee I have had in constant operation,
and under most careful observation, a series of smaller coke-beds. These beds have
received a special supply of raw sewage which came direct from the main sewer,
and was entirely unmixed with chemicals. The process of working these beds has
been subject to constant improvements and extensions, which have been suggested
by experience and observation. I will only here describe the final method of experimental
treatment which has been adopted.
The raw sewage, which has been screened from its coarser matters only,
is pumped continuously into a settling tank, the rate of supply being so adjusted to
the capacity of the tank that the sewage remains about six hours in the tank before
it flows away through an elbow pipe from beneath the surface of the liquid into
the coke-beds.
In this tank practically the whole of the suspended or floating particles of the
sewage settle as sludge, and are allowed to remain undisturbed. It is found that a
considerable proportion of this sludge disappears by bacterial action. During the first
period of six months, before the bacterial action was fully started, 25.6 per cent. of the
sludge disappeared, and during a subsequent similar period more than 50 per cent. of
the sludge was removed by bacterial action. The portion which disappeared was the
most putrescible portion of the sludge, which is at present carried to sea in the Council's
sludge boats. Its disappearance would reduce the freight which these boats are
required to remove. But it is probable that a considerable further reduction in the
"sludge" cargoes might be effected by properly devised detritus settling tanks in
which road sand would subside, and from which it might be removed to the marshes
without giving offence. This reduction in the amount of sludge, which is secured
by bacterial action alone, would be of the greatest importance.
The sewage as it flows from the settling tank is less pure than the effluent
which is at present discharged into the river at Crossness, and is quite unsuitable
for direct discharge into the stream. But by the subsequent coke-bed treatment
it is rendered sufficiently pure to support the life of fish, and to insure it against
undergoing even in summer time any offensive change.
In the experimental treatment, the settled sewage is therefore allowed to
flow into a coke-bed, which is a tank filled with suitable coke fragments to a depth
of six feet. As soon as the bed is filled to the surface of the coke, the supply is
stopped, and the sewage liquid is allowed to remain in contact with the coke for
two hours. It is then drained off from the bottom of the bed, and constitutes the
"bacterial effluent." After the liquid has flowed away, the coke-bed remains empty
for two hours, and is then refilled with a fresh supply of the settled sewage, which
is treated in precisely the same way as the preceding charge. The coke-bed
receives and purifies four charges of settled sewage in the twenty-four hours.
Conclusions arrived at by the Experimental Treatment.
The points which have been established by the experimental work are the
following:—
1. That by suitable continuous undisturbed sedimentation the raw
sewage is deprived of matter which would choke the coke-beds, and the
sludge which settles out is reduced in amount by bacterial action to a very
considerable extent. This reduction might undoubtedly be increased by
the preliminary removal of road detritus.
2. That the coke-beds, after they have developed their full purifying
power by use, have an average sewage capacity of about 30 per cent, of
the whole space which has been filled with coke.
3. That the sewage capacity of the coke-bed, when the bed is fed with
settled sewage, fluctuates slightly, but undergoes no permanent reduction.
The bed does not choke, and its purifying power undergoes steady improvement
for some time.
4. That coke of suitable quality does not disintegrate during use.