The bacteriological examination of water supplies.

Date:
1939
Catalogue details

Licence: Public Domain Mark

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

    10/68 (page 2)
    years, will provide the necessary normal standard for any par- ticular water, lapses from which must at once arouse suspicion. For example, in a deep well supply in which it is known that the normal bacterial content is low, a small increase would be of great significance and require most careful consideration. Similar alteration in an upland surface or river supply, especially if shown to be seasonal and paralleled by the experience of previous years, is of less significance. When a supply is drawn from multiple sources each contributory source must be similarly examined. The intervals which may be permitted between successive bacteriological examinations depend on the character of the supply. An upland surface supply collected from moorland, where there is little possibility of human contamination, may hardly need examination apart from that applied to the actual distributing mains. A water derived from wells sunk in fissured strata, such as chalk or limestone, especially where the area overlying is becoming built up and where increased pumping, drawing water from greater distances underground, may be required to cope with a larger demand, needs very frequent, perhaps daily, examination to ensure the detection of sudden pollution. Such conditions as prolonged drought followed by heavy rains should call at once for a series of bacteriological examinations at short intervals whether the supply is from a deep well, a spring or a river. It should always be borne in mind that the most which bacteriological tests can prove is that at the time of examination bacteria indicating excremental pollution did or did not grow under laboratory conditions from a sample of the water. A satisfactory result apart from, other data does not exclude the possibility of dangerous pollution at some future time. The results of the laboratory examination should therefore be reported with the caution that the considera- tions mentioned above impose and should be _ interpreted similarly. SECTION II. TAKING OF SAMPLES. The importance of good sampling cannot be over-estimated. Upon it depend the deductions which are ultimately drawn from the results of the examination. Every care must therefore be used to prevent any risk of contamination of the water in taking the sample. Since the laboratory examination of a sample should always be considered in conjunction with the topographical circum- stances of the supply, it is important in sending a sample of water for bacteriological examination to send with it very full details of the nature and source of the supply and methods of sterilisation or other purification and of the reasons that have