In water purification, slow sand filtration removes pathogens primarily through which mechanism?

• A Mechanical straining of particles through fine sand pores
• B Chemical coagulation with alum followed by sedimentation
• C Chlorination of filtered water removing residual organisms
• D Biological activity of the Schmutzdecke (zoogloeal layer) on the sand surface ✓

Explanation

Slow sand filtration achieves pathogen removal (including Giardia, Cryptosporidium, bacteria, viruses) primarily through biological degradation in the Schmutzdecke — a gelatinous mat of microorganisms, algae, and organic matter on the sand surface. This biological layer develops over weeks and is the key purification mechanism; it must not be disturbed during cleaning. Rapid sand filtration, by contrast, relies mainly on mechanical straining after coagulation-flocculation.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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