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8
The Single Coke-bed.
One of the tanks (A) was filled to a depth of four feet with coke of uniform size, each fragment
being about as large as a walnut. The coke was drawn from the retorts of the gasworks at the
Crossness Outfall, and was found by long soaking in water to absorb 15 per cent. of its weight of
water. The coke-bed had a sewage capacity of 3,000 gallons, which is equal to 50 per cent. of the volume
occupied by the coke and air space ; the volume of the coke amounts to 960 cubic feet.
This coke-bed is best described as " The Single Coke-bed," since it has been used for the
purpose of subjecting the sewage to a single process only.
The Double Coke-bed.
The other two tanks (C, B) have been filled to a depth of six feet with precisely similar coke,
tanks is 4,500 gallons. As compared with the single coke-bed, the capacity of the bed is therefore
increased in proportion to the increased volume of coke which it contains.
Tank C is at a higher level than tank B, and the two coke-beds have been, therefore, easily
rendered available for a process of double treatment. The raw sewage has been pumped into tank C,
and the effluent from this tank has been allowed to flow by gravitation into tank B and there to
undergo a second treatment.
In order to avoid confusion the two tanks, C and B, have been called "The Double Coke-bed,"
and for purposes of distinction tank C has been called the "Primary," and tank B the "Secondary
Coke-Bed."
It should be stated that the effluent from the Primary Coke-bed was passed through a small
laboratory coke-vessel pending the preparation of the Secondary Coke-bed. This laboratory coke-vessel
consisted of a glass bottle of 2.75 gallons capacity, which was filled with small coke, and which
had a sewage capacity of 1.25 gallons, or about 45 per cent. of that of the coke which it contained.
2.—method of working and present condition of the coke-beds.
After each coke-bed has been filled from above to the level of the upper surface of the coke, a
process which occupies about seven minutes for the four-foot bed, the sewage remains in contact
with the coke for three hours. The liquid is then allowed to slowly flow by gravitation from the
bottom of the coke-bed, the process of emptying requiring an hour for the four-foot bed, and the cokebed
is then allowed to stand empty for about eight hours in order that the surface of the coke fragments
may become aerated. In the case of the secondary coke-bed the aeration process lasted only
seven hours.
The coke-bed requires to be daily filled and emptied and aerated for about four weeks before it is
"matured" and begins to exert its full purifying effect upon the sewage. During this interval the
coke is doubtless becoming sown with bacteria, whioh are the active purifying agents, and which are
present in large numbers in the raw sewage. This process of "maturing" the coke-bed has usually been
carried on during its construction, by constantly treating the coke with sewage while it is being introduced
into the tank.
In the earlier experiments with the "single coke-bed," when it was being filled twice a day, and
before it was fully "matured," the coke-bed evidently became overworked, and accordingly returned a foul
effluent, and the coke itself became foul. A fortnight's rest in an empty condition restored its activity,
and it has not again given any unsatisfactory results whilst being filled with sewage once a day, and
latterly when it has again been filled twice a day.
The coke-beds are not filled on Sundays. They have also rested for four consecutive days, including
Sunday, during the Whitsun recess, and on July 30th and August 1st, 5th, 6th and 7th, the latter rest
being partly due to the necessity of repairing the sewage pump. The coke-beds rested on five other
days. With these exceptions the coke-beds have been continuously at work since the date when they
were started.
The surface of each working coke-bed is broken up to a depth of several inches twice a week
by being raked over; this keeps the surface open, and there is no appearance of its becoming
clogged.
At longer intervals a hole extending from the surface of the coke-bed to the bottom of the tank
has been dug in the coke. The whole of the coke has on every occasion been found to be perfectly
sweet when it has been thus exposed, and possesses only a slight earthy odour. The surface of the
upper portion, to a depth of three or four inches, is not quite bright, but it emits no foul smell.
The use of comparatively large pieces of coke in constructing the coke-bed enables the bed
to hold a charge of sewage which is considerably greater than that which would fill a similar bed
constructed of the smaller coke which was formerly employed. A single daily filling of the coke-bed
deals, therefore, with almost as large an amount of sewage as the double daily filling of an ordinary
bed. The larger size of the coke will doubtless also admit of the coke-bed being considerably
deepened without losing the possibility of fully aiirating the coke. It may be confidently anticipated
that the larger coke and greater depth of the bed will thus enable the necessarily large amount
of sewage to be dealt with.
Since the single coke-bed settled down to a steady rate of working, the number of daily fillings
has been increased to two, and the depth of one of the coke-beds has been increased from six to
thirteen feet in order to ascertain the possibility of working with deeper coke-beds.
3.—history of each coke-bed.
The Single Coke-bed.
The single coke-bed was filled with coke to a depth of of 4 ft. It was first charged with sewage
on April 22nd, and from that date until June 23rd it was charged with crude sewage twice daily,