View report page

20
This experiment showed that rapidly settled sewage could be dealt with in coke-beds
without reducing their capacity, and that this treatment yielded a satisfactory effluent. The
period over which this experiment extended was too short to admit of septic action being
set up in the settling channels; but it seems reasonable to anticipate that if the rapid
sedimentation were accompanied by septic action, a still greater amount of purification would
be effected.
3. ADDITIONAL PARTICULARS OF THE EXPERIMENTS ON THE BACTERIAL OR
NATURAL PROCESS OF SEWAGE PURIFICATION AT THE SOUTHERN OUTFALL
WORKS (CROSSNESS).
(a) Treatment of Settled Sewage by Continuous Feed through a Distributor to a four
foot Coke-bed. (Series II.)
From November 20th, 1899, to May 26th, 1900, experiments were carried out with the
continuous feeding of a coke-bed with settled sewage supplied through a distributor. For this
purpose the 4-foot single bed in tank B was used, and various methods of continuously feeding
the bed with settled sewage were adopted.
A decided decrease in the amount of purification of the settled raw sewage was apparent
when a quantity of sewage equal to that dealt with on the intermittent principle was passed
continuously through the bed. This decrease was probably due to—
(a) The overcharging of the bed with sewage.
(b) The reduction of the bacterial action owing to the chilling of the sewage by
contact with the cold air during the winter time.
It appears desirable that the temperature of sewage should not fall below 53° Fahr. if the
bacteria are to rapidly carry on their purifying action. This temperature is always exceeded
when the sewage is supplied to the bed on the intermittent system.
The continuous feeding of the bed further appeared at times to have the effect of washing
the useful deposit from the surfaces of the coke fragments.
The various methods of continuous feeding adopted during these experiments did not
effect a satisfactory purification, and they did not deal with the sewage as rapidly as it was dealt
with by the intermittent treatment.
(b) Treatment of Crude Sewage by Continuous Passage through a Settling Tank and by
Subsequent Intermittent Passage through Coke-Beds. (Series III.)
These experiments were commenced on November 1st, 1900, and lasted until October 5th,
1901. They were undertaken with the view of supplementing the results obtained from the rough
experiments (Series II.) on the septic treatment of sewage, which had lasted from May 3rd to
July 28th, 1900.
I.—Particulars as to the Construction of the Tanks and of the Coke-beds.
Tank C, which had been previously used as a 6-foot primary coke-bed, was emptied of the
coke and was used as a settling and septic tank for the crude sewage before it underwent treatment
in the coke-beds. Tank B, which had been used as a 4-foot single coke-bed, was increased in
depth to 6 feet with coke fragments which passed through a 2-inch mesh and which were
rejected by a 1-inch mesh. This bed was called the 6-foot single bed No. 1. Tank A was filled
to a depth of 6 feet with coke fragments similar in size to those forming bed No. 1. This bed
was called the 6-foot single bed No. 2. It had previously contained 13 feet of coke, and the
6-foot bed was simply formed by removing sufficient coke to leave a bed of 6 feet in depth.
Both beds were underdrained with loosely jointed earthenware agricultural drain-pipes.
The capacity of the septic tank was 9,000 gallons, but the actual quantity of liquid in the
tank varied somewhat during the working.
II.—Method of working the Tank and Beds.
For the purpose of measuring the quantity of sewage which was supplied to the settling
and septic tank, a meter was fixed to the delivery pipe which conveyed the crude sewage to the
septic tank.
From the settling tank an outlet-pipe conveyed the settled sewage to either No. 1 or
No. 2 coke-bed, or when the coke-beds were not being filled, to the main effluent discharge
culvert.
At first the settled sewage was supplied to the beds by allowing it to overflow from wooden
troughs, but this method was afterwards replaced by one in which the liquid fell from the
distributing pipes into perforated wooden trays placed upon the surface of the beds.
The filling of each bed occupied about half an hour ; the bed remained full for two hours
and occupied one hour in emptying. The resting period varied with the number of fillings per
24 hours.
The crude sewage samples for analysis were taken at the inlet to the tank, and the settled
sewage samples were taken at the outlet.
The arrangement of the tanks is shown in the diagram on p. 19. It should, however, be
borne in mind that the bottom of tank C was on the same level as the top of tanks A and B.
III.—An to the Number of Times the Coke-beds were filled.
The coke-beds were filled four times a day until the middle of January, 1901 (see table
VII., p. 36). From January 14th to 21st, 1901, No. 1 bed was supplied with settled sewage on