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(e) The amount of sewage which could be treated daily by a superficial unit of the coke-
bed.
(f) The extent to which the effluent underwent further improvement by its passage
through a second similar coke-bed.
(g) The suitability of the effluent for maintaining the life of fish.
(h) The effect of the treatment on the number and nature of the bacteria which were
present in the raw sewage.
3.—General results obtained.
It may be stated that the general results obtained from about 10 months' experience of the
coke-bed treatment point to the following conclusions—
(а) Size of coke.
The use of ordinary gas coke, in pieces about the size of walnuts, seems to be attended with the
following advantages, as compared with the use of smaller coke. The larger coke enables the bed to
hold a larger volume of sewage. The beds now in use had an original capacity for sewage which was
nearly equal to the volume of the coke which they contained, in place of only 20 or 30 per cent. of
that volume, as is shown by beds containing smaller coke. The use of the larger coke also allows the
bed to be more rapidly filled and emptied, and to be more completely emptied and aerated.
(б) Depth of coke.
Coke-beds similar in character, but differing in depth, have been found to give practically
identical purifying effects. At present experience has been obtained only with a four-foot and with a
six-foot bed; but I am arranging to extend these depths considerably.
[June 21st, 1899—A bed 13 feet in depth has now been working satisfactorily for over nine
weeks, and has given a purification approximately equal to that effected by the four-foot bed.]
(c) Chemical purification effected by a single treatment.
The sewage had been roughly screened before reaching the coke-beds, and it was free from
larger matter usually described as "filth," and from coarse sand and heavy mineral road detritus, but
it contained all the suspended solid matter usually termed " sludge."
During part of the period reported on, the coke-beds received only one charge of this raw
sewage daily; but latterly all the coke-beds have received two charges daily of raw Crossness sewage.
The coke-beds have removed the whole of the suspended matter or "sludge "frotn the crude
sewage; and they have yielded an effluent which occasionally shows a slight turbidity, apparently due
in ordinary flow, mainly to the presence of bacteria, but which is increased in storm flow by fine
clay and mud.
Not only has the suspended matter been removed, but the removal of the dissolved oxidisable
and putrescible matters of the raw sewage has been secured to the average extent of 51.3 per cent. by
the single process, the four-foot coke-bed giving 52.7 and the primary six-foot coke-bed 49'9
per cent. The effluent thus produced remains free from objectionable odour when it is kept
in open or in closed vessels, provided the bacteria present in it are not removed or killed by special
subsequent treatment. This effluent could, therefore, produce no offensive smell when it is introduced
into the river.
(d) Permanency of coke-beds.
Since the coke-beds have become regular in their action, neither the effluent nor the coke itself
has become foul. The coke which has been in use for upwards of 12 months has shown no general
tendency to break up, but the surface of each piece of coke has become partially covered with soft
matter which consists mainly of fine particles of coke with some fine sand, woody and vegetable
tissue, cotton and woollen fibres and diatoms. Below this thin film the coke is perfectly hard and no
sand has penetrated two millimetres below the surface. The sand when examined by polarised light
was found not to be granite sand derived from road detritus. The ash in the coke has been reduced
in amount by about 25 per cent. during its exposure to sewage in the coke-bed.
The capacity of the four-foot coke-bed, has during the period in review been reduced from
50 to 33 per cent, of the whole volume of the bed, and this reduction of capacity appears to be mainly
due to fragments of straw and chaff, apparently derived from horse-dung, and to woody-fibre derived
from the wear of wood pavements. The treatment of raw sewage by this coke-bed is being continued
in order to ascertain whether the decrease in the capacity continues. It has been ascertained that
the original capacity is not restored in any degree by prolonged aeration, which proves that the deposit
on the coke surface was not organic matter of animal origin; but it has been found that the vegetable
tissue, which appears to be the main cause of the decrease in capacity, can be in great measure
separated from the raw sewage by a brief period of sedimentation before the sewage is allowed to flow
into the coke-bed.
(e) Amount of sewage which can be treated daily by a superficial unit of the coke-bed.
The volume of sewage which can be passed through the coke-bed per unit of superficial area
has not yet attained its maximum, since the depth of the coke-bed is being further increased. Itoriginally
amounted to 555,000 gallons per acre per day for the four-foot coke-bed, and to 832,500 gallons
per acre per day for the six-foot coke-bed. This represents in each case one filling per day;
but, as has been already stated, two fillings have been made successfully for six months, and this
corresponds to 1,665,000 gallons per acre per day for the six-foot coke-bed. These amounts are
reduced after ten months' working of the coke-beds to 370,000 gallons per acre for a single filling
of the four-foot coke-bed, and to 673,400 for the six-foot coke-bed.