Table 7.10Guideline values for verification of microbial qualitya (see also Table 5.2)

OrganismsGuideline value
All water directly intended for drinking
E. coli or thermotolerant coliform bacteriab,cMust not be detectable in any 100 ml sample
Treated water entering the distribution system
E. coli or thermotolerant coliform bacteriabMust not be detectable in any 100 ml sample
Treated water in the distribution system
E. coli or thermotolerant coliform bacteriabMust not be detectable in any 100 ml sample
a

Immediate investigative action must be taken if E. coli are detected.

b

Although E. coli is the more precise indicator of faecal pollution, the count of thermotolerant coliform bacteria is an acceptable alternative. If necessary, proper confirmatory tests must be carried out. Total coliform bacteria are not acceptable as an indicator of the sanitary quality of water supplies, particularly in tropical areas, where many bacteria of no sanitary significance occur in almost all untreated supplies.

c

It is recognized that in the great majority of rural water supplies, especially in developing countries, faecal contamination is widespread. Especially under these conditions, medium-term targets for the progressive improvement of water supplies should be set.

From: 7, Microbial aspects

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Guidelines for drinking-water quality: Fourth edition incorporating the first and second addenda [Internet].
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