A water sample requires chlorination before distribution. The applied chlorine dose is 0.5 mg/L and after 30 minutes of contact the residual free chlorine is 0.2 mg/L. What is the chlorine demand of this water?

• A 0.3 mg/L ✓
• B 0.7 mg/L
• C 0.2 mg/L
• D 0.5 mg/L

Explanation

Chlorine demand equals the difference between the amount of chlorine applied and the amount of residual chlorine remaining at the end of a specified contact period, usually 30 minutes. Here, 0.5 mg/L applied minus 0.2 mg/L residual gives 0.3 mg/L consumed by organic matter, ammonia and reducing agents in the water. Students who add the two figures arrive at 0.7 mg/L, which is the distractor built on reversing the subtraction.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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