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34
The only advantage that can possibly accrue from the use
of the alum are the following:—(i.) The effluent and sludge
from pure lime treatment are strongly alkaline (too strong in fact
to render either of much value to vegetation) and the alum tends
to neutralise in a slight measure this alkalinity. (2.) With a pure
lime effluent there is a tendency for a secondary decomposition
to set in, with the result that it gives off offensive smells after
standing—alum tends to check this decomposition and consequent
evolution of smells. (3.) If too much lime is employed, some
of the offensive matter that was originally in suspension gets
dissolved, held in solution and not precipitated—and a less
pure effluent results.
The next obvious question is whether something may not
be substituted for the alum which will give a heavier precipitate
which will settle more rapidly, which will at least equal the
alum in its clarifying and purifying effects, and which will tend
to check secondary decomposition in the effluent at least as
effectually as the alum.
My experiments with carbferalum, which is a mixture of
alum, proto-sulphate of iron, and carbon, furnish results but
little better than those from an equivalent amount of lime; but
the protosulphate of iron is employed along with lime on many
farms and also at the sewage outfall works for the Metropolis,
and I have carefully gone into the relative powers and merits of
this iron salt and alum.
I find from many experiments that the protosulphate
possesses all the advantages (as a precipitant) of the alum, with
none of its disadvantages, and that the results of the employment
of 2½ grains to the gallon of this re-agent are in the main
as efficient as 5 grains of alum, and that the iron excels the alum
in its power of keeping the sludge and effluent sweet. It is
necessary that the lime should be added first, so as to ensure
marked alkalinity of the sewage.