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the requirements of Section 4 of the Metropolis Water Act of
1852 are complied with—viz., that "every Company shall
effectually filter all water supplied by them within the metropolis
before the same shall pass into the pipes for distribution." The
effectual filtration of river water depends, according to Sir F.
Bolton, upon:—
1. A sufficient area of properly constructed iilter beds, constantly cleaned,
and fresh sanded from time to time as the original thickness is reduced
;
2. The rate of filtration being controlled and limited to a certain speed ;
3. The water delivered into the filter beds having been previously stored
in subsiding reservoirs, and the capacity of these reservoirs being such
as to avoid the necessity for the intake of turbid and muddy water
during the time of extraordinary and heavy floods, which tend to foul
and choke the filters.
"The rate of filtration should not exceed 540
gallons per square yard of filter bed each 24 hours, or 2½ gallons
per square foot per hour. Filtration ought to be effectual at this
rate, which, for all practical purposes, may be considered as a
standard. Effectual filtration is greatly facilitated by previous
subsidence." The average rate of filtration, per square foot of
filtering area, in gallons per hour, by the several local companies
in 1886, was as follows:—West Middlesex, 1½; Grand Junction,
15/6; and Chelsea, 1¾. The requirements of the Act in this
respect, therefore, are fully carried out.
The filter beds are formed as follows:—
West Middlesex.— Harwich sand, 2ft. Sin.; Barnes sand, lft.; gravel
screened to different sizes, and arranged in layers, 2ft. 3in. Total
thickness, 5½ft.
Grand Junction.—Harwich sand, 2ft. 6in.; Hoggin, 6in.; fine gravel, 9in.;
coarse gravel, 9in.; boulders, lft. Total thickness, 5½ft. Another and
more modern kind of filter is used by this Company made up of sand, 2ft.,
fine gravel, 9in., pipe drains, 3in. Total, 3ft.
Chelsea.—Thames sand, 4ft. 6in.; shells, &c., 3in.; gravel, 3ft. 3in. Total
thickness, 8ft.
The filtering basins are lined with brickwork or concrete,
and contain layers of sand and firm and coarse gravel arranged in
descending order. The water passes vertically downwards, and