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(a) Mineral matter, principally sand, grit and mineral road detritus which are
not affected by the septic action, and which, when working on a large scale, would to a
great extent be separated, by previous sedimentation, from the other constituent of the
deposited solids.
(b) Organic matter, comprising, in addition to faecal matter, husks of cereals, chips
of wood, paper, etc., which latter are only acted upon by the bacteria in the septic tank
with extreme slowness.
A much more accurate idea of the action which occurred in the septic tank is obtainable,
therefore, when the reduction of the solid matter is calculated as a percentage of the total
solid organic matter deposited in the tank. By reference to Table X. it will be seen that this
amounted to 71.4 per cent.
With respect to the figures given in Table X., the following particulars may be useful—
(a) The figures representing the quantity of crude sewage supplied to the tank are
obtained by calculation from the quantities indicated in the meter which was fixed to
the inlet pipe.
(b) The sludge left in the tank at the end of the first period (column 2) was not
removed before the second period commenced, but was left in the tank to mix with the
sludge which was deposited during the second period. It is noteworthy that the sludge
finally obtained contained less liquid than the former quantity since it had been more
fully drained.
(c) The averages of the amounts of suspended matter in the crude sewage and in
the settled sewage are the averages of the daily estimations made during the time over
which the experiments extended. The average amount of solid matter in the sludge
is the result of an estimation which was made after the sludge had been well mixed
in the tank in order to obtain an average sample.
(d) The proportions of the mineral and organic constituents of the suspended matter
in the sewage have been calculated on the basis of—
41.6 per cent. of mineral matter and 58.4 per cent. of organic matter in the
suspended solids in the crude sewage,
40.9 per cent. of mineral matter and 59.1 per cent. of organic matter in the
settled sewage.
These percentages were obtained from the averages of the daily estimations which
were made on the crude and on the settled sewage during the three weeks' experiment
from December 24th, 1899, to January 13th, 1900.
These components of the suspended solids were not estimated during the experiments
lasting from November 1st, 1900, to October 5th, 1901.
(e) The figures under the heading "Calculated quantity of sludge left in the Tank"
are the differences between the amounts of suspended matter found in the crude sewage
and in the settled sewage.
(f) The calculated quantity of sludge left in the tank is the quantity which would
have been found in the settling tank if no septic action had taken place. The difference
between this quantity and that actually found, is the quantity of solid matter which
was liquefied by the action of the bacteria in the settling tank.
(g) The figures representing the proportions of the mineral and of the organic
constituents of the solids in the sludge were obtained by direct estimation made on
a portion of the sludge itself.
VI.—As to the Action of the Coke-beds.
Monthly and general averages of the various chemical estimations made on the crude
sewage, on the settled sewage, and on the coke-bed effluent are set forth in Tables VIII. and IX.
From these tables it will be seen that the average amount of purification effected on the
crude sewage throughout the whole of the experiments, by the combined action of the settling
tank and of the coke-beds, was 58.7 per cent. when measured by the amount of total putrescible
matter (both suspended and dissolved) removed, and 55.7 per cent. when measured by the
removal of the dissolved putrescible matter only.
The figures representing the estimations made of the effluents from the two coke-beds
will be found to agree very well with each other except in the case of the estimations of nitric
nitrogen. The divergence under this heading, however, is not so great as to necessitate any
inquiry as to the cause of the difference, since the difference is well within the ordinary variations
to which coke-beds are liable in this respect.
VII.—As to the Putrescibility of the Crude Sewage, of the Settled Sen-age of the Chemical Effluent and of
the Coke-bed Effluent.
During this series of experiments the putrescibility of the crude sewage, of the settled
sewage, of the effluent from the chemically treated and sedimented sewage, and of the
effluent from the coke-beds was tested at the Southern Outfall Works. The observations and
estimations made in this connection are set forth in Table XI.
The methods adopted in making these tests depended on—
(a) The estimation of the amount of oxygen absorbed from permanganate in three
minutes at 80° Fahr. by the putrescible matter in the various liquids at the time of
the collection of the samples, and the repetition of the estimation after they had been
kept for a period of seven days in an incubator at 80° Fahr.
(b) The examination of the liquids, after seven days' incubation at 80° Fahr., as
to visible signs of putrescence, as to the presence of sulphuretted hydrogen, and as to
their offensive nature.