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172
through each square yard of the upper surface area of the filter
in 24 hours, or 2½ 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 1888, 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 filteiing basins are lined with brickwork or concrete,
and contain layers of sand, and 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 pumpingengine.
The thickness of the 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 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 layere, 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 is used by this Company, made up of
sand, 2ft., line 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 Water Examiner's
Report for 1886, the West Middlesex Company was