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186
that portion of the sewage which was untreated by precipitation.
At the Southern outfall at Crossnesss, then uncompleted,
2,228 millions of gallons of sewage were only partially treated,
with 407 tons of lime, and 113 tons of sulphate of iron, which
yielded 46,873 tons of sludge. The untreated sewage discharged
into the river at this outfall during the summer, was
deodorized by means of 923 tons of manganate of soda, and
532 tons of sulphuric acid. The cost for lime at both outfalls,
during 1891, was about £8,146, and that for sulphate of iron,
£3,184. The cost of manganate of soda and sulphuric acid,
used at the outfalls and storm overflows, was £27,525; the great
bulk of which, the Committee stated, would be saved when, as
now, the precipitating operations are in full working order.
To sum up: 32,400 millions of gallons of sewage were treated
at the two outfalls, and 582,873 tons of sludge were produced;
whilst 8,923 tons of lime, 1676 tons of sulphate of iron, 1423
tons of manganate of soda, and 879 tons of sulphuric acid were
used, the cost of these chemicals being £38,855.
In previous Annual Reports I summarised the objections
raised by Sir Henry Roscoe to the system of sewage-disposal
now in operation, in reports made by him, after elaborate investigations
carried out for the late Metropolitan Board of
Works. In common with other eminent authorities, he favours
the view that the sewage, crude or after precipitation, must
ultimately be taken to the land. At Paris it is stated that 5,000
gallons, on an average, are applied daily to the acre. At this
rate, 30,000 acres would be required for irrigation purposes,
were all the sewage of London, averaging 150 million gallons
daily, disposed of in this manner. A larger proportion of a
partially purified effluent, presumably, might be used, thus
reducing the amount of land required. The land, however,
has yet to be found, and in any case a large outlay would be
required on main and branch sewers, &c. Projects for conveying
the sewage to the land have been before the public for
thirty years and more, but nothing has been done, and probably