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14
sifting the material composing. the lower three feet of the bed. The bed was then filled once a
day until the 30th May, when tidal fillings were again commenced.
Between the 8th May and 31st December, 1899, the bed was filled 361 times, and deait with
a total of 170,191,973 gallons, or with an average of 471,446 gallons per filling. This gives an
average capacity of 27.6 per cent., or 1.5 per cent. lower than 11th March. The actual fall in
capacity between the middle of May and the end of December, 1899, was 7.7 per cent. Since the
end of the year 1899 the capacity has gradually diminished until it reached, on 3rd March, 1900,
21.8 per cent., and 20.1 per cent. on 21st April.
The purification effected during the period extending from the 8th May to 31st December,
1899, averaged 82 per cent., as measured by the removal of the dissolved oxidisable matters from the
chemical effluent. The results of these examinations are given in Table 7 pp. 25 to 34.
On February 20th a hole was dug to the bottom of the bed after it had been draining for
five or six hours, and it was found that liquid remained in the bed filling it to the surface of the
old 3-feet bed. The water could be baled out, but the hole filled itself again by the liquid oozing
very slowly through its sides from the surrounding parts of the bed. In other words the lower half
of the bed remained permanently full of liquid. This was probably due to the consolidation of the
unsifted fine pan breeze in the lower part of the bed by the weight of the upper layer of three feet;
this upper part being coarse-sifted coke apparently remained open and unconsolidated. The
liquid contents of the lower three feet of the bed were accordingly only forced through the drainpipes
during the process of emptying by the weight of liquid in the upper half of the bed, and as
soon as the contents of the upper three feet reached the lower three feet of the bed they remained
there while the bed was resting in air until they were forced out after the next filling. It appeared
therefore that only the upper three feet of the bed was aerated by direct contact with the air when
the effluent flowed out, and that the lower three feet of the bed must have obtained its oxygen from
the liquid which had been in contact with the upper half of the bed. This condition of things
probably explains the low nitrification which has always been found in effluents from this bed, as
compared with those derived from other experimental beds in use on the works. It explains also
the low liquid capacity of this bed. In all probability the liquid capacity was nearly double that
which was shown by measuring the volume of the effluent discharged. The capacity measured was
indeed that of the upper half of the bed only, the lower half remaining entirely filled with liquid.
III—REPORT AS TO FURTHER EXPERIMENTS ON THE BACTERIAL
TREATMENT OF CRUDE SEWAGE AT THE SOUTHERN OUTFALL
(CROSSNESS).
1.—INTERMITTENT TREATMENT OF CRUDE SEWAGE BY SINGLE CONTACT IN
A COKE-BED 13 FEET IN DEPTH.
A brick-lined tank, 22 feet 6 inches long by 10 feet 8 inches wide, and having a superficial
area equal to 2/363of an acre, had already been filled with coke to a depth of 6 feet for the experiments
which were described in the previous Report on the Crossness Experiments. The coke fragments
were about the size of walnuts. This 6-foot bed had been acting as a secondary bed for about eight
months, and had treated the effluent from a coke-bed fed with crude sewage for that period.
On February 27th, 1899, this bed was first filled with crude sewage, and its depth was increased
by adding new coke fragments of similar size to the old, until on March 8th the bed was 12 feet deep.
On April 11th another foot of coke was added giving the bed a total depth of 13 feet. This increase
in depth was made in order to ascertain whether an increase in the depth of the bed in any way
detracted from its efficiency. It will be evident that the capacity of a bed per unit of superficial area
is increased in proportion to this increase in its depth.
In charging this bed crude sewage was pumped by a special engine and pump direct from the
suction culvert of the main pumping engines of the Outfall, and was distributed from a trough over
the surface of the bed. The liquid sank readily into the body of the coke-bed.
As soon as the bed was filled and the liquid appeared at the surface of the bed, the pumping
was stopped. The sewage was then allowed to remain in contact with the coke for three hours, after
which it was allowed to flow away slowly from the bottom of the bed.
The filling took from 20 to 25 minutes, and the draining away of the effluent from an hour to
an hour and a half.
From February 27th to March 25th inclusive, the bed was filled once daily with sewage, and
after March 27th it was generally filled twice daily.
The bed was charged once only on Saturdays and remained empty on Sundays and on holidays.
The other rests were due either to accidents occurring to the driving belt or to the pump, or to the
necessity arising for making alterations in the arrangements or for making capacity determinations of
the bed.
The coke which was used in this bed occupied, while it was new, 49 per cent. of the whole
volume of the bed, and the interstitial air or liquid occupied 51 per cent. This liquid space was
reduced by June 8th to about 34 per cent., and by October 10th to about 28 per cent. This loss in
capacity was equal to about 63 gallons decrease of liquid capacity per week, and threatened speedily
to put an end to the work of the bed. In order to prevent further loss of liquid capacity the coarser
suspended matter of the crude sewage was allowed to subside before the sewage flowed upon the bed.
With this object a large wooden tank was placed upon the top of the coke-bed, and the crude
sewage was pumped into this and was allowed to overflow upon the bed.