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20
SANITARY CIRCUMSTANCES OF THE AREA.
WATER.
Apart from the small portion of the district served by the
Rickmansworth and Uxbridge Valley Water Company, the area
is served by the Colne Valley Water Company. The water
originating in the gathering grounds in the Chilterns is pumped
at a number of stations from wells about 200 feet deep in the chalk.
At the Bushey pumping station in its passage to the softening
tanks it is injected with ammonia and chlorine. The softening
tanks are first charged with saturated lime water. As the chlorinated
water enters the tanks it is dosed with ammonium sulphate
to inhibit objectionable tastes. To assist the precipitation of the
calcium carbonate about half a grain per gallon of sulphate of
alumina is added to the hard waters entering the tanks. During
the entire period of sedimentation of some seven to ten hours the
chloramine is in contact with the water. The water gravitates to
the pumps through a floating arm pipe, the mouth of which draws
at a depth of two to three feet from the surface. As the softened
chlorinated water passes to the supply pumps, it is dechlorinated
with the requisite amount of sulphur dioxide. This treatment
effects reduction in the total hardness from twenty-three to between
ten to twelve grains per gallon.
At the Eastbury Pumping Station the softening is carried out
by the Base Exchange process, use being made of the synthetic
zoolite " doucil." A proportion of the water is softened to zero
and is then bulked with an approximately equal amount of
unsoftened water, the total hardness of the resultant mixture being
about ten. This soft water is then chlorinated with chloramine
and passes through a contact tank with baffles to ensure adequate
mixing and the necessary period of contact to effect sterilisation.
On passing from the tank to the pumps it is dechlorinated by sulphur
dioxide.

Of the 554 samples of softened supply water which was
examined 96-2 per cent, showed the absence of B.Coli. in 100 c.c.