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Bacteriological quality of drinking-watera
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Extracted from:
Guidelines for drinking-water
quality, 2nd ed.
Vol. 2 Health criteria and
other supporting information,
1996 (pp. 940-949) and Addendum to Vol. 2 . 1998 (pp. 281-283)
Geneva, World Health Organization.
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| Organisms |
Guideline
value |
|
All water
intended for drinking
|
| E.
coli or thermotolerant coliform bacteriab,c |
Must
not be detectable in any 100-ml sample |
|
Treated
water entering the distribution system
|
| E.
coli or thermotolerant coliform bacteriab |
Must
not be detectable in any 100-ml sample |
| Total
coliform bacteria |
Must
not be detectable in any 100-ml sample |
|
Treated
water in the distribution system
|
| E.
coli or thermotolerant coliform bacteriab |
Must
not be detectable in any 100-ml sample |
| Total
coliform bacteria |
Must
not be detectable in any 100-ml sample. In the case of large supplies,
where sufficient samples are examined, must not be present in 95% of samples
taken throughout any 12-month period |
a Immediate investigative action must be taken if
either E. coli or total coliform bacteria are detected. The minimal
action in the case of total coliform bacteria is repeat sampling; if these
bacteria are detected in the repeat sample, the cause must be determined
by immediate further investigation.
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 indicators of the sanitary quality
of rural 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 in developing countries, faecal contamination is widespread.
Under these conditions, the national surveillance agency should set medium-term
targets for the progressive improvement of water supplies, as recommended
in Volume 3 of Guidelines for drinking-water quality.
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