The Vernier scale (Wentworth grading) is used to classify drinking water quality based on turbidity measured in Nephelometric Turbidity Units (NTU). The maximum permissible turbidity for treated piped water supply in India according to BIS IS:10500 is:

• A 1 NTU
• B 10 NTU
• C 25 NTU
• D 5 NTU ✓

Explanation

BIS IS:10500:2012 specifies the maximum permissible turbidity of treated drinking water as 5 NTU in the absence of an alternative source, with a desirable limit of 1 NTU. WHO Guidelines for Drinking-water Quality (4th edition) recommend a turbidity of <0.1 NTU entering the distribution system for effective disinfection. High turbidity protects micro-organisms from disinfectants (interferes with UV and chlorination) and is aesthetically unacceptable. Turbidity is measured using a nephelometer.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.