A community well used by 500 families tests positive for coliform organisms. Shock chlorination is performed. The concentration of free residual chlorine that must be maintained at the consumer's tap after distribution to ensure microbiological safety is:

• A 0.1 mg/L
• B 0.5 mg/L
• C 1.0 mg/L
• D 0.2 mg/L ✓

Explanation

WHO and BIS (IS 10500:2012) specify that free residual chlorine at the consumer's end (tap) must be ≥ 0.2 mg/L to ensure microbial safety during distribution. At the treatment plant exit, a higher residual (0.5 mg/L) is maintained to allow for chlorine demand in distribution pipes. Values below 0.2 mg/L at the consumer tap indicate inadequate protection. Shock chlorination used at point of contamination uses 50 mg/L but this is not the residual maintained in distribution.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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