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7
SANITARY CIRCUMSTANCES OF THE DISTRICT.
Water.
The Borough Water Supply is obtained from the following
sources:—
(a) Deep wells about 300 feet deep which pass through the
London clay 160 feet thick, the Woolwich and Reading
beds 60 feet thick, the Thanet sands 23 feet thick and
penetrate the chalk for about 30 feet. At the depth of
about 300 feet from the surface some two miles of adits
have been cut in various directions to collect the water
from the various isolated fissures which are found in the
chalk.
The water collected from this source has been naturally
softened by passing through the Thanet Sands, and the
hardness is only about 5 (Clark's Scale), i.e., 5 grains
per gallon or 7 parts per 100,000 of salts causing
temporary or permanent hardness.
(b) Petersham Well—Conduits from this well intercept and
collect the water flowing through the gravel beds on top
of the London clay. This water is treated when the
occasion arises on open sand filter beds installed at
Petersham; since 1919, chlorination has been carried
out, and has by now generally proved adequate by itself
for the purification of this water.
Pure Chlorine Gas is used and is administered and
measured by a " Wallace-Tiernan " plant.
It is found with this water that a dose of Chlorine as
small as 1 part in 4,000,000 to 1 part in 5,000 000,
causes a degree of purification adequate for all ???
purposes and superior to that obtained by open sand
filters.
The hardness of this water is about 22° (Clark's Scale),
but as before pumping into the mains it is mixed with
the water obtained from the Deep Well the hardness is
Considerably modified.