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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. Sin. Total thickness, 5Jft.
Grand Junction.—Harwich sand, 2ft. Gin.; Hoggin, 6in.; fine gravel, 9in.;
coarse gravel, 9-in.; boulders, 1ft. Total thickness, 5Jft. Another and
more modern kind of filter used by this Company, is made up of sand,
2ft.; gravel, 9in.; drains, 3in. Total thickness, 3ft.
Chelsea.—Thames sand, 4ft. 6in.; shells, &c., 3in.; gravel, 3ft. 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 and
vegetable matter, and then the upper layer is scraped off and removed.
Renewals of sand take place periodically as the layer becomes
reduced below an assigned minimum.
Turbid water is very difficult to filter. The solid impurities in
suspension, consisting chiefly of clay, marl, and chalk, in a very
finely divided state, can be got rid of only by long subsidence previous
to filtration. Turbid water must of necessity be sometimes
admitted, and the filters are then liable to be over-taxed. The fact
that the impurity of river water is so largely increased during periods
of flood, renders it of great importance that there should be sufficient
provision of subsidence and storage reservoirs, to enable the
necessary daily supplies to be delivered during such periods, while
keeping the intakes closed. The position of the several Companies
varies in this respect very considerably. The following statement
shows the number of days' supply, represented by the capacity of
the reservoirs for unfiltered water, belonging to each of the local
Companies:—Chelsea, 14.2; West Middlesex, 7.0; Grand Junction,
3.5. The Grand Junction Company's resource, during floods, of
pumping from the gravel beds adjoining the Thames, is practically an
addition to their provision of storage reservoirs.