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CHAPTER XII.
PROPOSED BACTERIOLOGICAL STANDARDS FOR DRINKING WATERS IN INDIA .
Before dealing with the subject matter of this very important chapter, we wish
it to be clearly understood, that the standards we propose to lay down are tentative
only ; and. we put forward these suggestions thus early, in order that other workers
may test them in different surroundings, and in other parts of the country. Although we
have accumulated a fair amount of information in the Madras Presidency, and, although
we are ourselves satisfied that the lines on which we are working are sound, we are
• fully prepared to admit that objections to our standards may be brought forward, and
that a great deal more experience is necessary before these standards can be considered
to be completely established.
The classification of organisms according to their power of resistance to the action
of sunlight, given in the preceding chapter, will show pretty clearly the line which we
propose to adopt. If this subject is to be thoroughly understood a few points must
be cleared up. In Chapter VIII we have carefully laid. down, that it is not to be
supposed, that hard and fast rules can be formulated when dealing with a subject like
the power of resistance of organisms to the action of sunlight. All we claim to have
established is, that, in any naturally occurring mixture of micro-organisms, a certain
proportion (how great a proportion, we have at present not much idea) of one or more
species will be found to survive the others when the mixture is exposed to prolonged
sunlight. But it is with this resistant residuum that the sanitarian in India is most
frequently called upon to deal. Hence the great importance and utility of this
classification.
In nearly all the water schemes for a large population, and in a fair proportion of
village supplies, surface water, in the shape of lakes, tanks or rivers, provides the supply.
In lakes it has been demonstrated that storage and sunlight are the paramount
factors in the purification of the water. To rivers also the same applies, though in a
less degree. It is generally accepted that sunlight plays a considerable part in the
purification of river waters. Further, most of the rivers in India, for at least 9 months
of the year (if they do not run dry altogether), are extremely shallow, and really do
not depart very widely from the conditions made use of in our experiments, when
emulsions were placed in the sun with a layer of sterile sand at the bottom of dish.
In many of the great rivers in the Madras Presidency, it is not uncommon to find
enormous stretches of water not more than 12 or 15 inches deep. The same also
applies to the Ganges. Consequently, there can be no reasonable deub t that in
a river the susceptible individuals among organisms present will die off with great
rapidity, and that the bacilli, which the analyst finds, can be compared with those
whose powers of resistance hos been studied in the experiments quoted.
No analyst, in any country, would ever venture to give a definite opinion, without
knowing a few elementary fact concerning the source from which a sample has been
taken. Such questions as, whether the samples was drawn from lake, river, well or.
spring—the quantity and date of recent rainfall—the condition, in the case of a river,
whether in heavy flood or nearly dry—are matters of great importance to the bacterio-•
logist who has to pronounce an opinion on the sample. Consequently, in the application
of this method, the question the analyst should always bear in mind is, what chances
has this sample had of being exposed to the action or sunlight? The more exposure it
has had, the more will the arguments involved in this new method apply to the
water ; and the less it has had, the more guarded must be any expression of opinion.
Hence, it is obvious, that, for lakes, tanks, ponds, storage reservoirs, settling tanks,
etc., the method proposed by us is distinctly applicable. The extent to which it
applies to rivers, must depend on the condition of the river itself. The extent to
which it may be applicable to wells, springs, and underground waters generally, is
at present uncertain. This is undoubtedly a somewhat serious limitation to the utility
of the method.