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191
The small proportion of organic material which the
solid matter invariably contains is, on the other hand,
of the greatest importance in connexion with the use
of the water for drinking purposes. For although the
actual amount of this organic matter is generally quite
insignificant, yet it may be of the most objectionable
character on account of its origin. Thus the
water, both of the Thames and the Lea, receives,
above the points where it is abstracted for the purposes
of the Metropolitan supply, various contributions of
organic matter of animal origin, such as the drainage
from manured land, the effluents from sewage works,
and even raw sewage itself. This animal matter may
at any time be accompanied by zymotic poisons
dangerous to health, and although the chances of such
substances reaching the consumer are greatly reduced,
both by the filtration to which the water is
subjected by the Companies and by the care which is
exercised in excluding the flood-waters from their
reservoirs, yet, in spite of these protective measures,
there is no absolute guarantee that the noxious
ingredients, which may at any time be present, are
wholly removed. Most of the Water Companies are
endeavouring to substitute, as far as possible, water
from subterranean sources for raw river water, whilst
the Board of Conservators have almost completely
stopped the discharge of untreated town sewage into
the Thames.
Table A gives the proportion of organic impurity
present in the waters taken from the Thames, the Lea,
and the deep-wells respectively, from 1868 to 1890.