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234
If the sewage exceeds a certain proportion, as compared with
the river water, the whole of the river is deprived of its
dissolved oxygen, and the " healthy" growths are thus
practically killed.
Hence the addition of deodorants has for its object the
destruction by chemical oxidation of the putrescent material.
Deodorization, however, is only the first step in the process
of purification, and for the growth of the necessary " healthy "
organisms, the deodorized sewage must be brought into contact
with free dissolved oxygen, which, under proper conditions,
is obtained from the large volume of unpolluted water. But
should the river water be insufficient in quantity, or have
lost its oxygen by previous pollution, the water, plus sewage,
cannot support the life of the " healthy" organisms, and no
further addition of an oxidizing chemical can ever restore
the river to its normal condition. " The only natural remedy
then remaining is the slow absorption of atmospheric oxygen
at the surface of the water."
So deodorization (or " chemical oxidation ") of the sewage
is only a means to an end, that end being the presence of a sufficient
quantity of free dissolved oxygen. Hence it remains to consider
whether this same end cannot be brought about by different
means, as, for example, aeration of the sewage before entering
the river.
This point being reserved, the action of the two deodorants,
viz., the bleaching powder and manganate of soda acidified with
sulphuric acid, is discussed in detail. The river was very foul
at the beginning of the experiment with the bleaching powder,
which was used to the extent of 3 grains per gallon of sewage,
with the effect that on the whole the putrescent odour of the
sewage was effectually destroyed throughout the entire period,
July 2nd to August 26th. It was concluded, therefore, that
bleaching powder is a good deodorant; but a " general impression
" formed was, that it is " not possible to express any
favourable opinion as to the action of the bleaching powder
upon the general condition of the river.'' Hence the necessity