The bacteriological examination of water supplies.

Date:
1939
Catalogue details

Licence: Public Domain Mark

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

    25/68 (page 17)
    I 17 a water should fall into Class 1 of our grading. Even making allowance for sampling and other errors, the appearance of these organisms in quantities of 100 ml., i.e. a fall to Class 2, should at once occasion misgivings as to the adequacy of the chlorination process. Water taken on consumer's premises. The classification laid down here for piped water supplies refers essentially to the quality of the water entering the distri- bution service and not to that of the water taken on the con- sumer’s premises. Whatever the quality of the water entering the distribution service may be, some slight deterioration is liable to occur during its passage to the consumer. Organic matter may be sucked in to a variable extent through defective water mains, and further contamination may occur in service reservoirs and cisterns or from washers on the service taps. Coliform organisms of the non-faecal type may gain limited access from these sources, but their appearance in any con- siderable numbers should be regarded with grave suspicion. The appearance of faecal coli among them suggests at once that undesirable material is gaining access to the water. It is diffi- cult to lay down standards for water of this type, but if careful comparison between the water before and after , distribution shows that any considerable deterioration has occurred, par- ticularly in relation to Bact. coli I, then measures should be taken to ascertain the source of the pollution. Small rural supplies. As pointed out above, the classification we have suggested refers essentially to waters distributed to populations of more than minimal size. In rural districts, where a piped water supply may at present be economically unobtainable, and where reliance has to be placed largely on private supplies, it may be difficult to reach the standard of Class 3 or 4. In such in- stances everything possible should be done to prevent the access of pollution to the water. By relatively simple measures, such as the removal of obvious sources of contamination from the catchment area, and by attention to the coping, brick lining, and covering of the well, it should usually be possible to reduce the coliform count for a shallow well water to a level between ro and 25 per 100 ml. Persistent failure to do this, especially if the water frequently gives a presumptive coliform count of 50 or over per 100 ml., should lead as a rule to condemnation of the supply, though there are circumstances, such as where the pollution appears to be mainly of non-human origin, when an exception may be made to this rule. Once more we must insist on the importance of adequate bacteriologist’s analysis may be, a water can never be regarded