The bacteriological examination of water supplies.

Date:
1934
Catalogue details

Licence: Public Domain Mark

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

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    THE BACTERIOLOGICAL EXAMINATION OF WATER SUPPLIES. PRELIMINARY NOTE. This Memorandum has been prepared for circulation with the following objects :— | (1) To provide a description of technique the adoption of which will ensure sufficient uniformity in the practice of bacteriological examination of water to permit of comparison of the results obtained in different laboratories. (2) To explain to Sanitary Inspectors and others the pre- cautions necessary in obtaining and transmitting samples to the laboratory. (3) To assist in assessing the results of bacteriological examination in terms of hygienic quality. I. INTRODUCTION. The bacterial flora of different waters varies so much that it is impossible to lay down absolute standards to which all safe waters must conform. Each supply must be judged on the sum of knowledge obtained by topographical and laboratory examination in the light of accumulated hygienic experience of water supplies in general. Even from the same supply, a sample of water may, on a particular arbitrary laboratory standard, give one day a result indicating potential danger. Later, perhaps the next day, a similar examination may give a result apparently suggestive of safety; yet from the epidemiological point of view the supply has probably not altered at all. On the other hand, frequent and regularly repeated examinations, which reveal the normal range of variation of the flora, often enable a standard to be established for any one water so that any marked variation from such a norm would at once be regarded with suspicion. The main object of the bacteriological examination of water is to find whether excretal pollution is present, but a single exam- ination, however favourable the result, does not justify the con- clusion that the water is suitable for drinking purposes. Not until several examinations with satisfactory results have been made at different times of the year, and especially after heavy rainfall, is it legitimate to conclude that the water is free from dangerous excretal pollution. Reasons may exist for suspecting that a water supply may be liable to pollution in’ circumstances that only occur at long intervals. For example, a prolonged summer drought, with an increased seasonal demand for water, (often augmented by an increased summer population), may involve the drawing of underground supplies from a much wider area than normal, with consequent risk of polluted infiltration reaching the water-bearing strata. Especially when such a drought is followed by heavy
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