In a water treatment plant, chlorination produces a free residual chlorine (FRC) of 0.5 mg/L at the farthest consumer point. As per WHO and Indian practice guidelines, this concentration is considered adequate for ensuring microbiological safety because it indicates:

• A Adequate contact time has been achieved, as the Ct product exceeds 15
• B The minimum required FRC of 0.2 mg/L at consumer end is met, providing disinfection assurance ✓
• C Breakpoint chlorination has been exceeded, eliminating combined chlorine
• D The chlorine demand of the water is zero, so no further dosing is needed

Explanation

WHO (2017) and Indian practice require a minimum free residual chlorine of 0.2 mg/L (and ideally 0.5 mg/L) at the consumer end as assurance that disinfection is adequate and recontamination during distribution is prevented. Option A wrongly conflates residual concentration with the Ct concept (concentration × time), which is a design calculation, not a field verification. Option C confuses breakpoint with residual maintenance. Option D is incorrect because zero chlorine demand is virtually never the case in piped distribution.

Reference: Guidelines for Drinking-water Quality, WHO / Park's Textbook of Preventive and Social Medicine, 4th / 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.