A slow sand filter is being installed for rural water purification. The purification mechanism in the biological layer (schmutzdecke) primarily involves:
- A Predation and biological metabolism by microorganisms ✓
- B Chemical adsorption onto sand grain surfaces
- C Electrostatic attraction of colloidal particles
- D Mechanical straining of particles larger than pore size
Correct answer: A. Predation and biological metabolism by microorganisms
Explanation
The gelatinous biological layer (schmutzdecke or hypogeal layer) that forms on the surface of slow sand filters contains microorganisms that ingest and metabolize organic matter and pathogens. This biological action is the defining mechanism distinguishing slow sand filtration from rapid sand filtration, which relies mainly on physical processes. Straining plays a minor role but is not the primary mechanism.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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