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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.
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 2j 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 1889, was as follows:— West Middlesex, 1.27; Grand Junction,
1.93; 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, 0.66 acre; Chelsea, 0.71; and
West Middlesex, 0'91.
The filtering basins are lined with brickwork or 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 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, 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, 5^ft.