View report page

230
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 the
late Water Examiner, upon:—
"1. A sufficient area of properly constructed filter 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."
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
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 1890 (and it is much the
same now) was as follows:—West Middlesex, 1.24; Grand Junction,
2.03; Chelsea, 1.75. The requirements of the Act in this respect,
therefore, are fully carried out. The filtering area, per million
gallons of average daily supply, now possessed by the several Companies,
is:—Grand Junction, 0'96 acre; Chelsea, 068; 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