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241
A limit to the rapidity of filtration has been generally adopted.
It is represented by the passage of about 540 gallons through each
square yard of the upper surface area of the filter in '24 hours, or
gallons through each square foot of surface per hour. Water
passed through well-constructed filters at a rate not exceeding this,
becomes under ordinary conditions bright and clear. 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 1890, was as follows:—
West Middlesex, 1.24; Grand Junction, 2.03; Chelsea, 1.75.
The requirements of the Act in this respect, therefore, were fully
carried out. The filtering area per million gallons of average daily
supply, possessed by the several Companies, is:—Grand Junction,
0.63 acre; Chelsea, 0.68; and West Middlesex, 0.88.
The filtering basins are lined with brickwork on concrete,
and contain layers of sand, and of fine and 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 the filtering media ranges from eight feet in the
case of the Chelsea Company, to two feet nine inches in that of
the Grand Junction Company. The thickness of the upper layer
of sand, which is the most effective of the layers, ranges from
two feet to four feet six inches.
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, 6in.; fine gravel, 9in.;
coarse gravel, 9in.; boulders, 1ft. Total thickness. 5½ft Another and
more modern kind of filter used by this Company, is made up of sand,
2ft., gravel, 9in. Total thickness, 2ft. 9in.
Chelsea.—Thames sand, 4ft. Gin.; shells, &c., 3in.; gravel, 3f. 3in. Total
thickness, 8ft.
Sand, it may be added, varies in quality, the Harwich sand,
which is remarkably fine and uniform in grain, being the best.
The sand becomes clogged in course of time with deposited mud