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The "original capacity" here shown was taken about 7 days after the experiments had
been started; it is probably below the total capacity, since it was taken while the beds were being
emptied, and the drainings of the beds were therefore not included. No doubt a more trustworthy
method of estimating capacity consists in filling the bed after it has drained for a definite time,
which should be the same in all processes of taking capacities. It might be well that a standard
"drainage period" should be recognised generally, since the wetness of the bed-material must
affect the so-called capacity considerably. Thus a bed which had been draining for one hour only
would require less liquid to fill it than one which had been draining for ten hours. In the case
of the one-acre bed at Barking it is known that the effluent will continue to drain away for
several days.
2.—PRIMARY AND SECONDARY TREATMENT IN BEDS OF COKE 10 FEET IN
DEPTH.
For the purpose of these experiments iron super-structures 4 feet in height were bolted to
the existing tanks, and the joints were caulked and made watertight. Thus the depth of the
tanks was increased to 10 feet. The tanks were then filled to a depth of 9 feet 9 inches with coke,
and were worked in two series as follows—
Series A.—
(1.) A primary coarse bed. (A.)
(2.) A secondary coarse bed. (A 1.)
Series B.—
(1.) A primary coarse bed. (B.)
(2.) A secondary fine bed. (B 1.)
The three coarse beds were composed of coke of such a size that it would pass a 2-inch
mesh and be rejected by a ½-inch mesh.
The fine bed was composed of coke of a size which would pass a ½-inch and be rejected
by a 1/16-inch mesh. In this instance the coke which had been in use in the previous experiments,
and was thus in good bacterial condition, was mixed with two-fifths of its bulk of new material
in forming the deeper bed. The arrangement of the tanks allowed the effluent from the primary
beds to drain completely into their respective secondary beds.
The sewage supplied to the beds was obtained from the same source as during the previous
experiments, and was thus of the strongest nature, and contained sand and other suspended
mineral and vegetable matters which should not be allowed to reach the bed, and which a
short period of sedimentation would, in a very great measure, remove It speaks well for the
beds that they have been able to cope so satisfactorily with the heavy work which was thrown
upon them.
In series B the depth of the primary coarse bed was reduced to 9 feet 6 inches, in order that
its liquid contents should not overflow when it passed into the secondary fine bed, whose capacity
was somewhat smaller, owing to its being made of finer material.
The procedure was precisely similar to that adopted in the previous experiment. The two
primary beds were filled simultaneously, were allowed to rest full for two hours; they were then
drained into the secondary beds, and the effluent was allowed to remain for a further period of two
hours in contact with the coke. The crude sewage and each of the effluents were separately
sampled and subjected to examination.
Percentage Capacities of the Beds—The beds were charged by means of a fixed syphon
made of one-inch pipe from an overhead water-tank, which was kept constantly full." The
volume delivered by this syphon per minute was ascertained, and the capacity was then
determined bv the length of time required for fillineg each bed.

The progressive decrease in the capacity of the beds shows that they were choking. This
cannot be wondered at, considering that the very worst of the sewage was being dealt with, and
that sand and other solid matter was being admitted to the beds. Latterly, trouble had been
experienced in filling the primary beds, owing to the accumulated mass of undissolved matters
on the upper part of the beds. The beds, however, were doing very satisfactory purifying work,
and were removing from 40 to 50 per cent. of the dissolved putrescible matters.
It was noticed that after seven months' work the coarse bed B had sunk to the extent of
6 inches. On 20th February, 1900, 9 inches of new coke, similar to that originally used, was
placed on the top of this bed, and the depth of the bed was thus brought up to 9 feet 9 inches, as in
the case of the other three tanks, the capacity of the bed having fallen sufficiently to prevent its
contents from overflowing the secondary fine bed.