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32
While conducting the long series of experiments embracing
nearly 100 analyses, I soon found that it was useless comparing the
results of work upon sewage taken on different days. I found
that even though the sewage of one day be to physical tests
the same (even the specific gravity being identical) with
that of another day, the effect of equal amounts of chemical
on the two samples may vary. It is extremely difficult to
account for the discrepancies which I encountered under
this head. Obviously the nature of the original refuse matter
which finds its way into the sewers cannot vary much
in Finchley from day to day, and the only explanation I have to
offer is that the sewage was collected at different hours, and it is
conceivable that the sewage may vary in nature, at different
hours, throughout the same day.
In every case I worked with fresh sewage pumped up into
cans, so the difference in the precipatability of the sewage was
not due to stages in staleness and decomposition of such sewage.
I performed experiments to this end, and found that more
matter can be chemically precipitated from fresh sewage than
from stale. A sample of fresh sewage, for instance, containing
180 parts per 100,000 of total solids yielded a supernatant fluid
showing 86 parts per 100,000 in 6 hours from the addition of the
precipitants, whereas the same sewage after 24 hours gave 93
parts, and after 48 hours, 94. The probable explanation is that
the organic matter quickly decomposes and thereby breaks up into
much finer particles, which may escape being carried down by the
chemical precipitant, and probably the gases which are given off in
increasing quantities as fermentation and putrefaction advances,
by their constant bubbling up through the sewage, keep lifting up
the lighter particles, and thus prevent them from settling completely.
These facts, then, point to one conclusion only, and serve to
drive home two important truths, i.e., (a) There is a distinct
advantage—so far as the purity of the effluent is concerned—in
treating the sewage as fresh as possible; (b) There is a disadvantage
in allowing sewage to remain in the tanks much longer
than 6 hours.