Hints from the Health Department. Leaflet from the archive of the Society of Medical Officers of Health. Credit: Wellcome Collection, London
[Report of the Medical Officer of Health for London County Council]
During the period of rest from July 12th to l6th, 1901, septic action occurred in the
settling tanks, and large masses of sludge were lifted tot the surface by the gas evolved during
the anaerobic action. The sludge which had been deposited in the tanks was very much disturbed
by this movement, as well as by the alteration of the level of the liquid in the tanks
while the settled sewage was being syphoned off for the purpose of filling the coke-beds. Further,
on account of the small size of the tanks some of the sludge, which had been lifted by the
gas bubbles, was drawn through the syphon pipes and thus gained access to the coke-beds.
On January 28th, 1901, the settling tank A was thoroughly cleaned out and a fresh start
Throughout the period of experiment the amount of suspended matter or sludge entering
the tank in the crude sewage had been carefully determined, and it was known how much
should be present as a sediment on the bottom of the tank. But when the quantity of this sludge
was estimated, during the clearing out of the settling tank, it was found that the amount was
considerably less than that which had entered the tank. No less than 20.2 per cent. of this total
solid matter was found to have been liquefied by septic action between January 28th and August
6th, 1901, in settling tank A; this is equal to 37.4 per cent. of the solid organic matter in the
sewage supplied to the tank. The reduction of the quantity of solid matter in tank 13 was
only (3 per cent. of the total solids, or 12.8 per cent. of the solid organic matter in the sewage
supplied to the tank between February 9th and August 6th, 1901.
The great difference between the amounts of septic action which had occurred in the two
tanks is doubtless due to the fact that tank A, in which the septic action appears to have been the
more pronounced, had not been disturbed during three weeks from June 5th to 28th, on account of
the period of rest accorded to the coke-bed which was worked in connection with the tank A;
whereas tank B was practically in continuous use. As has been already stated the disturbance of the
sludge during working was considerable, and undoubtedly interfered with the septic action. There
seems also little doubt that the low temperature of these tanks, due to their free exposure on all
sides to the air, is inimical to bacterial change. Similar experiments carried out at the
Southern Outfall, with settling tanks constructed in masonry and sunk in the ground, gave much
more uniform and satisfactory results, and this appears to be due to their maintaining a higher
and more uniform temperature, as well as to their feed being so arranged as to avoid disturbance
of the sedimented sludge.
The amount of sludge produced weekly in each of the two subsidence tanks averaged about
9 gallons, and it contained 89.5 per cent. of moisture. The average quantity of sewage which
passed through each tank was 1,620 gallons weekly in the case of tank A, and 1,412 gallons in the
case of tank B.
V.—As to the action of the coke-beds.
The results of these experiments, as far as the action of the coke-beds is concerned, may be
considered very satisfactory. Bearing in mind the small size of the settling tanks and the small
amount of septic action which took place in them, the chemical condition of the final effluent
and the small loss of capacity of the coke-beds compare very favourably with the results of other
experiments and prove the great advantage of preliminary sedimentation.
The average percentage amount of purification effected by the combined action of the
settling tank and of the coarse bed of Series A, as measured by the relative quantities of oxygen
absorbed from permanganate by the dissolved putrescible matter in the crude sewage and in the
final coke-bed effluent, was 51.8; the highest average purification effected by the double ragstone-beds
with two fillings per day amounted to 51'6, but the double coke-beds of this series
of experiments yielded the best results with one filling per day, when an average of 64'9 per
cent. of purification was effected. It will thus be seen that the purification effected by the
settling tank and coarse coke-bed of Series A was equal to that effected by the double ragstonebeds.
The following table sets forth the figures representing such results.
|Average percentage purification effected throughout the experiments, by the beds on the liquid supplied to them.|
|Series I. experiment— Coarse ragstone-bed||20.6|
|Series II. experiment— Primary coarse bed A||46.3|
|Secondary coarse bed A 1||31.9|
|Primary coarse bed B||47.0|
|Secondary fine bed B 1||52.1|
|Series III. experiment— Coarse coke-bed A||38.8|
|Fine coke-bed B||51.2|
From these results it will be seen that the fine coke-bed in the last series of experiments
dealing with settled sewage, effected a purification practically equal in amount to the best results
obtained from any single bed in previous experiments. But it will be remembered that in the
instances where the amount of purification effected by a single bed in previous experiments was