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33
I experimented, therefore upon fresh average samples of
the Finchley sewage; and the experiments were conducted in
many series upon the same sewage taken at the same time from
the sewer; the figures of the analyses were all submitted to the
Sanitary Committee.
I first took many measured quantities of the fresh sewage
and treated these with known and varying amounts of lime, and
my best results were obtained when 9 grains to the gallon of
lime were added to what one may classify as an average sample of
sewage—that is to say less than 9 grains gave less satisfactory
results, but more than 9 grains did not affect any appreciable improvement
in the effluent.
Similar experiments were performed with alum, with the
result that 12 grains were found the most satisfactory amount
to use.
Next the 2 substances were combined in all proportions
within the limits of 10 grains of one to 1 grain of the other, and
after verifying results by a series of control experiments, I found
that 7 grains of lime and 5 of alum gave the best results with
average fresh sewage of the district.
I found that 7 grains to the gallon of lime, and 5 of alum,
separate about 25 per cent. of the total solids in solution in
the sewage as well as almost the whole of the solids in suspensoin.
Alum acts by yielding a flocculent precipitate, and
I find that this settles more slowly than the lime precipitate,
that it is more easily disturbed after settlement, and
that alum tends to form a certain amount of floating scum;
these are distinct drawbacks to its use. Next I found that so
far as clarification of the sewage was concerned, better results
are got by 10 grains to the gallon of lime, than by 5 grains of
lime and 5 of alum; i.e., there is about equal clarification, but
this is more quickly effected. Lime is only about half the
price of an equal weight of alum, and the question naturally
arises as to why the alum should be employed at all, and why
its place should not be taken by an equal amount of lime.