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25
Sanitary Circumstances.
rivers are running too low to allow much, if any, water being
abstracted for waterworks purposes.
During these periods the filters are being fed from storage
reservoirs, and are really, as it were, borrowing on capital. The
reserves may never be completely exhausted ; but when the river
water again becomes available for waterworks purposes it is likely
to be specially impure in quality, and besides, it is not possible with
depleted reservoirs to purify it rapidly by storage. It follows that
during the storm periods following drought, supplementary processes
of water purification ought to be adopted if most impure water is
to be rendered reasonably pure.
A system in which a river water of reasonably good and
uniform quality is pumped into a nearly full storage reservoir of
adequate capacity at the same rate as water is being abstracted for
filtration purposes is in a state of what may be termed ' quality
equilibrium.' Any falling off in the quantity or the quality of the
river water upsets, in greater or less measure, this equilibrium.
Reservoirs and filters possess, it is true, an immense ' reserve
of purifying ability,' but it would be a most unwise policy to take
too much advantage of this circumstance and abstract river water
independently of its quality, or rush impure river water through
depleted service reservoirs in the hope that the sand filters would
sufficiently remove the impurities. Hence arises the desirability
of considering the feasibility of adopting a new factor in the London
water question, namely, the occasional employment of supplementary
processes of purification.
Subject to these qualifications, I consider that the means
now adopted, or in process of adoption, for the purification of the
Metropolitan water supply are such as to render the water reasonably,
if not absolutely, ' safe ' for drinking purposes."