View report page

156
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
attention to the further question of the "collection and disposal
of the sludge" resulting from the precipitation process, and by
means of a 30 inch press dealt with 1,787 tons of sludge, which
yielded 523 tons of "cake." It has been ascertained that this
cake cannot be burned without nuisance, except at an expense
that would be prohibitory. It is evident, therefore, that if the
wet sludge is not removed from the works, by farmers and others,
for manurial purposes, the alternative must be, either to raise up
low lying land with the cake—87 acres to a depth of three feet
annually—or to take the sludge or cake out to sea. With a view
of ascertaining the expense of the latter course, the Board