A food inspector tests water samples from a town's piped supply. Bacteriological examination shows a Most Probable Number (MPN) of E. coli at 18/100 mL. According to WHO and Indian standards for potable water, this sample is:
- A Unsafe — the coliform count should be zero in treated piped water ✓
- B Safe — the acceptable limit is <50/100 mL
- C Requires repeat testing; single exceedance is not grounds for condemnation
- D Marginally acceptable if turbidity is below 1 NTU
Explanation
WHO and Bureau of Indian Standards (BIS IS:10500) specify that treated piped drinking water must have zero coliforms (including E. coli) per 100 mL. Any detectable coliform in treated, distributed piped water indicates contamination and renders the water microbiologically unsafe. The standard of zero coliforms applies specifically to treated water entering distribution; for water at source (springs, wells), a lower threshold of <10 MPN/100 mL was used in older standards but has been superseded. Turbidity (maximum 1 NTU desirable) is a separate physical parameter and does not modify the bacteriological standard.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.