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water is to be derived would be purchased outright, certain
villages would be abolished, drains would be dug out and
cleansed, burial grounds would be cleared of their contents, and
the whole area would be devoted to its one purpose of gathering
water for the supply of London. We assert, then, that there is
no comparison between the two alternatives, and that if London
wants to have pure water its engineers should go at once to a
pure source rather than adopt mechanical means, however ingenious,
for clarifying water which has once been sewage.
It seems to be maintained by some that so great is the power
of filtration, that it really does not much matter where water
comes from, and that notwithstanding all we know as to the
material which flows into the Thames and Lea, their water has
only to be filtered to be rendered wholesome. The wisdom
of trusting everything to a process which is so rough and ready,
and so liable to break down in its various details, must be
seriously doubted.
We have lately been told that after the filtration to which
London water is subjected it is sometimes absolutely sterile; but
that is not the point. The real criterion by which filtration has
to be judged is not its power of occasionally producing a pure
effluent, but of always keeping back such micro-organisms as
may be present in the water. As a matter of fact, a certain
percentage only appears to be kept back, and so long as the
occurrence of floods in the river is found to influence the
character of water in the pipes, so long must we be allowed to
distrust filtration as a reliable means of producing a pure water
from a doubtful source, although it is of course admitted that
when no pure source is available it is the best that can be done.
The assertion made lately that the bacillus of typhoid fever
has never been discovered in the water supplied by the London
companies has been accepted by some as showing that, at least
so far as typhoid is concerned, London water is innocuous. In
relation to this, it may be well to recall to mind the