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247
several local companies, in 1887, was as follows :—West Middlesex,
1.33; Grand Junction, 1.80; Chelsea, 1.75. 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. 3in.; Barnes sand, 1ft.; gravel
screened to different sizes, and arranged in layers, 2ft. 3in. Total
thickness, 5½ft.
Grand Junction.—Harwich sand, 2ft. 6in.; Hoggin, Gin.; fine gravel, 9in.;
coarse gravel, 9in.; boulders, 1ft. Total thickness, 5£ft. 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.
Sand is the efficient agent in filtration as practised, and
this it may be added varies in quality. In the Report for 1886,
the "West Middlesex" Company, was referred to as prominent
in "respect of the uniformity in the quality of its water," this
Company using “to an appreciable extent sand from
Harwich which is remarkably fine and uniform in grain."
The filtering basins are lined with brickwork or concrete,
and contain layers of sand, and fine end coarse gravel arranged
in descending order. The water passes vertically downwards,
and is collected by pipe and brick drains laid on the floors of
the basins, and led by conduits to the well of the pumping-engine.
The thickness of this filtering media ranges from eight feet in
the case of the Chelsea Company to three feet in that of the
Grand Junction Company. The thickness of the upper
layer of sand, which is the most effective of the layers, varies
from two feet to fcur feet six inches. The sand becomes
clogged in course of time, and then the upper layer is scraped
off and removed. Renewals of sand should take place periodically
as the layer becomes reduced below an assigned minimum.
But filtration avails little if the water is allowed, as
frequently happens, to be fouled in the cisterns; whether
through the uncleanly condition of those receptacles or by the