The bacteriological examination of water supplies.

Date:: 1939

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

14/68 page 6

$any marked deviation from the normal findings must be followed forthwith by suitable measures for the protection of the consumers. . : Routine control for big water undertakings should involve daily testing, but neither a detailed bacteriological examination nor a full chemical analysis should normally be necessary, though these, of course, are demanded when a new source of supply is to be introduced or when suspicion arises in the course of the ordinary routine. in water, as in milk control, it is far more important to examine numerous samples by a simple test than occasional samples by a more complicated test or series of tests. It is not possible to prescribe schedules setting out the fre- quency with which bacteriological tests should be made on all the various water supplies in use in this country. Each water undertaker should take expert advice as to the minimum neces- sary in order that effective watch can be maintained on the condition of the water from each of the sources from which water is drawn and on that of the water as it reaches consumers. The nature of the supply itself, its known lability to accidental admixture with water of doubtful purity, such as that from fissured strata, from floods, from fractured sewers or contami- nated subsoil, and its exposure to unavoidable risks, such as those incurred in repairs and alterations of wells, reservoirs, mains, etc., should be the overriding considerations in deter- mining how often tests should be made and how thorough they should be. In general, it will not be too onerous for the larger undertakings to have daily routine tests. For smaller under- takings, where this would impose too great a financial burden but where frequent tests are nevertheless required, a compromise may be necessary as advised by the expert. In many such supplies the water will in any case require treatment and the protection of the consumers will depend as much on the care with which this is applied and continuously controlled as on the frequency of bacteriological tests. In the case of small supplies such as those used by a single, or a few households, routine examination is obviously out of the question; but in such cases measures of complete protection of the source from risks of contamination are usually practicable, and a few careful bacteriological and chemical analyses will then give assurance that this has been achieved for an in- definitely long period. TYPE OF EXAMINATION. Various tests are available for controlling the bacteriological purity of waters, such as the plate count at 22° C. and 37° C., the presumptive coliform count, the differential coliform test, and the tests for faecal streptococci and for Clostridium welch.$

The bacteriological examination of water supplies.

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