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the continuous principle; from the latter date until April 15th both beds were thrown out of use,
first on account of lime water from the chemical treatment of the sewage at this Outfall having
gained access to the beds, and afterwards by the choking of the suction pipe of the pump supplying
the settling tank with crude sewage. Owing to a change in the pumping arrangements
which was made in March, it was impossible to give the settling tank a continuous supply of
sewage, and it was also impossible to always fill the beds four times each day.
The other days during the specified period on which the coke-beds were not filled were
either Sundays, when they never were in use; Saturdays, when the height of the sewage in the
main was not sufficient during the morning to work the pump; public holidays and the first week
in August, which latter was fixed as the time for the annual holiday of the coke-bed samplers.
IV.—Ax to the Capacity of the Coke-beds.
The original capacity of the wet coke-beds was not estimated, since in the case of both beds
the coke had been previously in use.
The measurement of October 8th, 1900 (table VII., p. 36), of coke-bed No. 2 was made
when the 13-foot bed of the previous experiments had been reduced to 6 feet. It may, therefore,
be accepted as the capacity of the bed previous to starting this series of experiments, as it had
been draining since July 28th, 1900, and could hardly be expected to appreciably increase in
capacity during the 23 days before the first filling of this series took place.
On examining the other figures representing the capacity at various times, it will be
observed that the capacity of coke-bed No. 2 gradually decreased while the bed was receiving four
fillings per day, but that after it had rested for six weeks its capacity greatly increased; it again
diminished, however, immediately after the resting period, and ultimately remained for about
six months with a fairly constant capacity. This second decrease in capacity, as would be
expected, was slower than before the rest, on account of the reduced number of fillings per day
which the bed received.
The ease with which the capacity of the coke-bed No. 2 was increased by a short rest
indicates that its loss of capacity was mainly due to accumulated organic or putrescible matter in
the bed; this would no doubt be largely avoided by more perfect previous sedimentation of the
sewage and by more energetic septic action, both of which conditions would be fulfilled when
larger settling channels were used.
V.—Relative to the Action of the Settling or Septic Tank..
On January 18th, 1901, the septic tank was roughly covered over, and as a result the
liquid in it became coated with a thick scum, which, on January 28th, was about one inch thick.
This scum continued to grow thicker, and to increase in toughness, especially after the tank
had been closed. The bacterial action appeared to have increased, since gas was evolved in
larger quantity; but the solution of organic matter was not more marked than before the cover
was placed over the tank.
The usual method of working the septic tank was to allow a unit volume of sewage
about six hours to pass through the tank. This was about equal to the average time which a
unit volume of sewage takes to pass through the large settling channels in the chemical and
sedimentation process.
The solution of organic matter continued, but on February 27th, 1901, the pump feeding
the septic tank became choked with sludge, owing to the stoppage of the main pumps at the
east end of the engine house for alterations, and the experimental work had to be stopped. The
pipe in connection with the pump which took the sewage from the main sewers at the Outfall
Works was, therefore, shortened by about 5 feet, in order to prevent choking in the future, and
as a consequence of this shortening of the pipe the end of it was often above the level of the
sewage in the mains. It was, therefore, only when the level of the sewage rose to a certain
height that the feed pipe of the pump was in the liquid, and that the pump could be used for
filling the septic tank. The alteration in the pumping arrangements consequently prevented the
septic tank from receiving in future a continuous feed of crude sewage.
On February 27th, 1901, the scum on the top of the liquid in the septic tank had entirely
disappeared.
On March 28th, 1901, an examination of the septic tank was made, when it was found
that 256 per cent. of the sediment or sludge had been liquefied by bacterial action. During
the whole period that the tank had been in action, 41.2 per cent. of the sludge disappeared.
The great difference in the percentage reduction of sludge in the tank during the period November
1st, 1900, to March 28th, 1901, and the period November 1st, 1900, to October 5th, 1901, is
probably due to—
(а) The efficiency of the septic tank increasing with the length of time during
which it had been in use, since the sludge was not cleared out of the tank at the end of
the first period.
(b) The fact that during the first period the sewage was supplied to the tank almost
uninterruptedly, whereas during the period March 29th, 1901, to October 5th, 1901, the
sewage supply to the septic tank was by no means continuous, owing to alterations in
the pumping arrangements.
The weekly average quantities of crude sewage which passed through the septic tank
were—
From November 1st, 1900, to March 28th, 1901, 131,817 gallons, or 21,969 gallons per day.
From March 29th, 1901, to October 5th, 1901, 89,898 gallons, or 14,983 gallons per day.
By reference to Table X. it will be seen that during the whole period from November
1st, 1901, to October 5th, 1901, the septic action in the settling tank reduced the amount of solid
matter deposited therein by 41'2 per cent. This deposited solid matter consisted of—