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13
In order to secure even distribution of the sewage, and to prevent the sediment of sludge
on the bottom of the tank from being disturbed, the crude sewage flowed upon a board 8 inches
square, fixed about 6 or 7 inches below the open end of the feed pipe.
A 1-inch iron pipe was used in each tank as a syphon for conveying the settled sewage
from the settling tanks to the coke-beds in the lower tanks. One end of each of these pipes
was continued to a depth of 5 feet below the top of the settling tank, the other end of each
syption pipe discharged the sewage upon the first of a series of three consecutive wooden platforms;
the last platform of each series was perforated and was placed about 2 feet above the
surface of the coke in the lower tanks. The object of these platforms was to secure aeration of
the settled sewage.
The total capacity of each of the subsidence tanks was 1,000 gallons, but the depth at
which the tanks were worked was only 9.5 feet and gave a working capacity of 950 gallons.
The two lower tanks were used for the coke-beds. The bed which was worked in connection
with settling tank A was known as the "Coarse Coke-bed A." It was composed of the
coke which had been used in connection with the previous series of experiments, and the size
of the fragments was such that they would pass through a 2-inch mesh and be rejected by a
½-inch mesh.
The other tank was known as "Fine Coke-bed B." The coke of which it was composed
had also been used before and was of such a size as would pass a ½-inch mesh and be rejected
by a 1/16 -inch mesh. Both of the tanks were filled with coke to a depth of 6 feet. The beds
were underdrained with perforated 2-inch iron pipes arranged as a "fourway," with three open
ends, while the fourth end was connected with the outlet tap., Large pieces of coke were
arranged around the pipes in order to prevent choking.
II.—Method of working the tanks and beds.
When the two settling tanks were first started they were filled with roughly screened
crude sewage to within 6 inches of the top and this was allowed to settle for 24 hours; sufficient
of the sedimented sewage to fill the coke-beds was then syphoned off to the beds. The sewage
from tank A was always supplied to coke-bed A, while that from tank B always passed on to bed B.
After the beds had been filled from the tanks, the latter were immediately filled up with fresh
sewage, which was allowed to settle until it was time for the coke-beds to receive a fresh charge.
This method of procedure was maintained throughout the experiments of Series III., except
as regards the number of consecutive hours during which the sewage remained undisturbed in
the settling tanks. After the coke-beds had been filled, they remained undisturbed for two
hours; they were then emptied, and remained with air filling the interspaces until the time
arrived for re-charging them with sewage. This rest period necessarily varied with the number
of charges which the beds received during the twenty-four hours. The number of fillings per
day ranged between one and three. The beds were filled at 4 a.m., 2 p.m. and 10 p.m., when
they were receiving three fillings per day.
III.—Particulars as to the number of times the coke-beds were filled.
By reference to Table III., p. 32, it will be seen that the coarse coke-bed A was filled with
settled sewage 287 times between November 8th, 1900, and August 6th, 1901, and that the
fine coke-bed B was filled 313 times during the same period. Both of these coke-beds always
rested on Sundays.
On several days during January, February and March, 1901, it was impossible to fill
the coke-beds on account of the tanks having become frost-bound. This was due to their elevated
position giving free exposure on all sides to the air.
Both beds rested from April 24th to May 5th, 1901. This was thought to be advisable
because their purifying powers had apparently deteriorated. By reference to Table III. it will
be seen that immediately before this period, the beds had been receiving three fillings per day
for nine days ; this was apparently too much work for the beds to deal with satisfactorily.
It must be borne in mind that these beds were dealing with sewage which had undergone very
imperfect sedimentation, on account of the small size of the settling tanks and the imperfect
manner of feeding and discharging them; and it was further shown, later on, that the sewage
had undergone very slight anaerobic or septic action in these tariks. The sewage supplied to the
beds was therefore not in a suitable condition for undergoing rapid purification in the coke-beds.
The coarse coke-bed A rested also from June 6th to 27th, 1901, because it showed a
decrease in the efficiency, which was apparently due to overwork.
The other days on which the beds rested were either public holidays or times at which
the boilers of the engines were being cleaned, or days on which the pumps were being repaired.
IV.—Relative to the action of the settling tanks.
The settling tanks were primarily arranged for the purpose of intercepting the grosser
suspended matter from the sewage which passed through them. Such matters consist principally
of sand and road detritus and of substances chemically known as "cellulose," which include
water-logged wooden refuse and the husks of cereals. Since sand and mineral road detritus are
not acted upon by bacteria at all, and cellulose is only very slowly broken up by aerobic bacterial
action, it follows that these substances should be intercepted, since, if they are allowed to enter
the coke-beds, the capacity of the beds becomes very rapidly reduced.
It was anticipated that bacterial change would occur in the sediment, and the tanks were
carefully watched in order to detect any signs of so-called "septic" or anaerobic bacterial action
which might take place in them. On December 13th, 1900, an examination of the deposit in the
settling tanks was made and indicated that some of the sediment or sludge had disappeared
from the bottom of the tanks.