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must be considered as a temporary measure sanctioned solely by
conditions of time and of place. That is to say, that considering
the present condition of the outfalls, the arrangements now existing
there, and the conditions arising from drought and high
temperature during the summer months the addition
of some deodorant to the effluent sewage may be advisable
. . The question, however, would assumea different aspect
should these conditions be altered, and then the necessity for
such addition might decrease, or even disappear."
The principles of sewage purification are explained, and
it is assumed " that no amount of chemicals which can, with any
show of reason, be added, is sufficient, or nearly sufficient, to
convert the whole of the foecal matter into harmless forms.
Hence the use of chemicals can only be recommended for the
purpose either of starting a process of purification, or of
simply getting rid of the evil odour." " The purification of
the river must be effected by natural processes." Of
these, by far the most important is the change produced by
living organisms. These " organisms " are classed as " healthy "
and " unhealthy the former requiring free oxygen for their
growth produce a change which has the result of rendering
the organic matter inoffensive: the change due to
organisms which flourish in the absence of free oxygen gives
rise to products which are offensive. Efforts, therefore, should
be made to preserve and assist the life of such organisms as
are capable of effecting the innocuous change, and to prevent
the growth of such as yield offensive products. The oxygen
necessary for the growth of the " healthy " organisms is usually
derived from the air dissolved in all unpolluted running water.
Sewage, however, contains no dissolved oxygen, or at any rate
less than is necessary for the purpose. Hence, in its passage to
the outfall, the sewage has already begun to undergo change
of a putrescent character, owing to the growth of the putrefactive
or " unhealthy" organisms. The products of this
putrefactive change absorb free oxygen, and so when the sewage
is poured into the river, it robs the water of its dissolved oxygen.
Q