The bacteriological examination of water supplies.

Date:
1939
Catalogue details

Licence: Public Domain Mark

Credit: The bacteriological examination of water supplies. Source: Wellcome Collection.

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    led to a desire to have the water examined. It is of great 1m- portance, in those cases where the water to be sampled is sub- mitted to preliminary sterilisation by means of chloramine or chlorine and may therefore contain traces of these reagents at the sampling point, that such traces should be immediately destroyed; otherwise erroneous results will be obtained due to the continued action of the sterilisant during the period of transit of the sample to the laboratories. A sample of the corre- sponding raw water before chlorination should be taken at the same time as the chlorinated sample. Detailed instruc- tions, setting out fully the precautions to be taken and the information that should be supplied, are given in Appendix A. SECTION III. ; THE RATIONALE OF THE BACTERIOLOGICAL ANALYSIS OF WATER. The direct search for the presence of specific pathogenic bacteria in water is unlikely to prove of value for routine control purposes. There are several reasons for this. If a given supply receives a single contamination, for example from a typhoid carrier, it will probably be about a fortnight before a case of typhoid develops and another week or even longer before it is diagnosed and reported to the Medical Officer of Health. Since typhoid bacilli die out fairly rapidly in water it is improbable that the bacteriologist, after this lapse of time, will succeed in demonstrating their presence. In addition, the organisms usually gain access to the water in only very small numbers, and the technical difficulties of isolating small num- bers of typhoid bacilli from water contaminated with excretal material are very great. If, of course, the contamination is repeated or continuous, then the chance of finding the bacillus is rather greater, but in practice suspicion is almost invariably thrown on the water as the result of epidemiological rather than of bacteriological inquiry. The earlier workers, realising the difficulty of demonstrating directly the presence of pathogenic organisms, adopted an in- direct approach to the solution of the problem. It was sug- gested that the water should be examined for evidence of excretal or sewage pollution, the assumption being that if pollution of this type did occur, the water must be regarded as potentially dangerous. Experience has shown that this reasoning was sound in practice. During the last century methods were evolved for the detection of organic matter in water by chemical means, and further methods were elaborated, depending on the estimation of free and albuminoid ammonia, chlorine, nitrates and nitrites, for distinguishing between organic matter of animal and vegetable origin. In this way considerable progress was made, but in