In the last edition of our book on "Sewage Disposal," in speaking of
precipitation we said that "the purification of sewage by chemicals
has been the subject of misapprehension, owing to the extravagant
advantages which have been claimed for the system by its advocates."
This is even more true now than it was two years ago, inasmuch as in
the recent scheme for dealing with the sewage of the Thames Valley
chemical treatment _per se_ was relied on to produce from the sewage
of a future population of 350,000 an effluent at all times fit to be
discharged at one point into the river Thames above London; but the
Parliamentary Committee rejected it. One part of the report of this
Committee deserves attention, when speaking of sewage treatment by
chemicals. It is as follows: "Your committee believe that in these
cases the process of filtering the chemically purified effluent through
earth ought, if possible, to be adopted, which was not provided for
in the scheme under their consideration." This opinion is exactly
in accordance with our experience, and is that which we have held
throughout. It is at the root of the whole matter, because efforts
are made by those interested in chemical processes to attain as high
a standard of purity as possible with the attendant heavy expense of
chemicals. Experience shows that it is impossible at all times and
seasons to be sure of a constant and uniformly high standard of purity,
and that chemical works should be supplemented by a filtration area,
however small. The addition of this, however, enables a lower standard
of effluent from the precipitation tanks to be admissible, and this can
be attained with very simple and inexpensive chemicals.
In the course of our practice we have had to advise as to the majority
of the processes, and to design the works for their being carried into
operation. We have found that the cost of such works complete varies
from 0.091 to 0.166 pound per head of the population, and that the
average cost of the works at several towns which we have been connected
with is 0.123 pound per head. This figure may be conveniently followed
by that of the cost of treatment, which we find varies from 0.036 to
0.110 pound per head per annum, and an average of several places gives
0.06 pound per head per annum. The above figures apply only to places
where the very highest standard was sought to be attained, but our more
recent experience leads us to modify the arrangement of the works and
the cost of treatment, so as to rely on filtration of the effluent as
an important factor. We estimate that under these conditions the cost
of the works complete would be about 0.075 pound per head, and the
cost of treatment 0.04 pound per head per annum. The disposal of the
sludge has always been a difficulty in these works, but this is now
overcome in two ways: either by digging it into the ground, as is done
at Birmingham now, or by pressing it into cakes in filter presses.
It is found at Birmingham that one ton of sludge with 90 per cent.
of moisture is produced from 1,000 people. There the lime process is
used. We have found that about one ton to 2,000 people is produced
where a salt of alumina or iron is used with the lime. At Birmingham
the sludge is dug into the land adjoining the works, and it is found
that one square yard of land will take one ton of sludge with 90 per
cent. of moisture once in three years, which results in three yards of
land being required to be provided for each ton of sludge. This system
of digging in sludge is successfully carried out as regards freedom
from nuisance. Where land is not available to dig in the sludge, it is
necessary to make it portable for removal and disposal away from where
it is produced. This is best effected by filter presses. Appliances are
made for this purpose, by which the sludge is pressed to a consistency
of about 50 per cent. of moisture. The cost of effecting this is about
0.007 pound per head per annum. It is found in practice that where the
sludge is produced by straining the solids from sewage before passing
it on to land for purification, it requires a little lime to enable
the press to work well. About two barrow loads of lime for each ton of
pressed sludge suffices.
It has been thought that the cost of precipitation would be covered,
and even a profit gained, by the sale of the sludge. This hope,
however, is not nearer realization now than it was in the time, now
gone past, when chemical processes were relied on to turn sewage from
a profitless into a profitable commodity. There is, consequently,
less justification now than there was at that time for adopting a
precipitation system for sewage disposal. It is entirely a question
of carefully considering the engineering and financial points
involved, regardless of the sanguine representations of interested
or enthusiastic advocates of any particular system. As the estimated
manurial value of the sludge which is precipitated from sewage by the
addition of chemicals does not seem to be capable of realization,
we think that probably the reason may be found in the fact that the
chemicals arrest that process of decomposition which is essential to
the conversion of the organic matters into nitrates for vegetation to
utilize.
This explanation will be understood in the light of what we have
already described in regard to "nitrification." If this view is
correct, it would follow that the more completely and permanently
the sludge is deodorized by the chemicals, the less capable is it of
passing through the necessary stages of decomposition by which its
manurial value can be realized. As mistakes are constantly being made
in regard to the weights of sludge with varying degrees of moisture,
the following table may be useful:
Tons. Per cent. Tons. Per cent.
100 of sludge with 90 of moisture = 50 with 80
100 " " " 33.3 " 70
100 " " " 25 " 60
100 " " " 20 " 50
100 " " " 16.6 " 40
100 " " " 14.3 " 30
100 " " " 12.5 " 20
100 " " " 11.76 " 15