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The maximum possible rate of treatment by each coke-bed is possibly not yet reached. This
is an important point to be determined, since the practicability of applying this method to the
treatment of the whole of the London sewage depends largely upon the superficial area which will
be required for the laying down of the coke-beds. The above daily rate of treatment will
naturally be augmented as the depth of the coke-bed is increased, and if the satisfactory working of
the 13-foot bed is maintained, it will treat a volume of raw sewage equal to at least three-and-a-half
million gallons per day.
(f) Secondary treatment.
It has been stated that the purification effected by a single treatment of the raw sewage in the
coke-beds amounts to a complete removal of the suspended matters, and to a further removal of an
average of at least 51.3 per cent. of the dissolved putrescible oxidisable matter. The primary sixfoot
coke-bed actually removed on the average 49.9 per cent. of dissolved impurity, and a second
process has effected thus far an additional purification of about 19.3 per cent., giving a total average
purification of the clarified raw sewage amounting to about 69.2 per cent.
(g) Effect of the effluent on fish.
Fish die at once when they are placed in the present effluent produced by chemical precipitation,
probably because there is a serious deficiency of dissolved oxygen in the impure liquid, and therefore
their respiration cannot be maintained. Not only goldfish, but roach, dace and perch have lived for
months in the first effluent from the coke-beds, and they apparently would live and thrive in this
liquid for an indefinite period.
(h) Bacteriological character of the effluent.
The results obtained by the bacteriological examination of the effluent by Dr. Houston (see pp. 17
to 41) thus far seem to indicate that the coke treatment does not by any means remove the bacteria from
the crude sewage, and indeed does not materially reduce the number. It shows that the presence of many
of the bacteria in the effluent is possibly unobjectionable, and is probably necessary for the purpose of completing
the purification of the effluent when it has flowed into the river; but it further shows that
some of the bacteria whose presence might be looked upon as undesirable in drinking water pass
through the coke-beds.
4.—General Conclusions.
The above considerations show that neither on chemical, not possibly on bacteriological grounds
can any serious objection be raised to the introduction of the effluent from the coke-beds into a portion
of the river Thames which is cut off by locks from the Intakes of the Water Companies, and the water
from which is not employed for drinking purposes, and cannot be used for drinking on account of its
"brackish" nature. The effluent certainly will not cause any deposit upon the river-bed, and will
even tend to render the turbid water of the lower river more clear and transparent. At the same
time, the liquid discharged from the outfall into the river will be sweet and entirely free from smell.
Further, it will carry into the river the bacteria necessary for completing its own purification in contact
with the aerated river water, and under no conditions can it therefore become foul after it has mingled
with the stream. The effluent will in no way interfere with fish-life in the stream.
5.—Advantages of bacterial over chemical treatment.
As compared with the present process of chemical precipitation and sedimentation, the bacterial
process presents the following advantages—
(a) It requires no chemicals.
(b) It produces no offensive sludge, but only a deposit of sand or vegetable tissue which
is free from odour.
(c) It removes the whole of the suspended matter, instead of only about 80.0 per cent.
thereof.
(d) It effects the removal of 51.3 per cent. of the dissolved oxidisable and putrescible
matter, as compared with the removal of 17 per cent, only, effected by the present chemical
treatment.
(e) Further, the resultant liquid is entirely free from objectionable smell, and does not
become foul when it is kept; it further maintains the life of fish.
SECTION II.
CONSTRUCTION AND WORKING OF THE COKE-BEDS.
1.—Construction and details of the coke-beds.
For the purposes of the present experiments three brick-lined tanks with brickwork bottoms
were specially provided with suitable outlets and with means of charging them with raw sewage by
means of a pump and distributing troughs. A series of parallel loose-jointed stoneware drain pipes
were laid on the bottom of the tanks, and were connected with suitable outlets; this arrangement
served to draw off the effluent.
Two of these tanks (A, B) are precisely similar in dimensions, being 22 feet 6 inches long,
10 feet 8 inches wide, and 12 feet deep, and the superficial area of each is 2/363 rd of an acre. The
third tank (C) is of less regular shape, but of the same area as (A) and (B), and 6 feet in depth.