In a slow sand filter, the layer chiefly responsible for biological purification of water is:

• A The gravel support layer at the base
• B The schmutzdecke (vital layer) on the sand surface ✓
• C The coarse sand layer in the middle
• D The supernatant water column above the sand

Explanation

The schmutzdecke is a thin biological layer of algae, diatoms, bacteria and protozoa that forms on the sand surface within days of commissioning. It acts by straining, adsorption and predation, removing up to 99 percent of bacteria and oxidising organic matter. The sand and gravel layers provide physical support and filtration, but the vital layer performs the biological purification; if it is disturbed, filtered water quality deteriorates sharply until it re-forms.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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