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138
Teported, as the result of experiment, that while the treatment
by lime and iron" does exert a distinctly purifying effect upon
the liquid part of the sewage," the effluent, nevertheless, "retains
a sufficiently unpleasant odour to prohibit its being discharged
into the river, during warm weather, at all states of the tide; "but
that "the addition of manganate of soda and sulphuric acid, or
other suitable oxidising agent," to the effluent, "would so far
deodorise and purify that effluent as to allow of its being afterwards
discharged into the river throughout the year at all states
of the tide." This supplementary treatment, however, would be
unnecessary during great part of the year, when the river water
and the sewage are cool, and the latter much diluted by rainfall.
When necessary, the addition of manganate of soda, in some proportion,
ranging between 0.5 and 1.5 grains of the crude commercial
manganate to one gallon of the effluent, together with sulphuric
acid corresponding to about one-third of the manganate used,
suffices to deodorise and purify the effluent to such an extent as
to render its discharge into the river unobjectionable at all states
of the tide. The chemists recommended that the treatment by
lime and iron be carried on throughout the year, and that when
the effluent is to any appreciable extent offensive it be treated
with the manganate. So successful has this plan proved, and so
greatly have the Board succeeded in reducing the cost of the
manganate (viz., from £40 to £11 per ton), that, in the view of
the Board, which, however, is by no means universally accepted,
the necessity for costly land filtration of the effluent, as recommended
by the Royal Commission on Metropolitan sewage
discharge, to ensure the removal of all odour and the possibility
of "secondary fermentation," no longer exists. In order to prevent
nuisance within the metropolis, arising from the discharge
of offensive gases from sewer ventilators, the Board are now
applying, at numerous stations, manganate of soda and sulphuric
acid to the sewage as it flows through the main sewers. It is
expected that this plan will ensure the arrival of the sewage at
the outfalls in a deodorised condition, and thus assist in the production
of a satisfactory effluent. The Committee devoted much