Slow sand filtration (biological filtration) differs from rapid sand filtration primarily because:
- A Slow sand filters use a biologically active schmutzdecke layer and require no coagulation ✓
- B Slow sand filters require prior addition of alum as coagulant
- C Slow sand filters remove only turbidity but not bacteria
- D Rapid sand filters work without prior sedimentation
Correct answer: A. Slow sand filters use a biologically active schmutzdecke layer and require no coagulation
Explanation
Slow sand filters rely on a surface biological layer called schmutzdecke (dirty skin) composed of algae, bacteria, and protozoa that trap and digest impurities, including bacteria (up to 99% removal without chemicals). They filter at 0.1–0.4 m/hr. Rapid sand filters work at 4–5 m/hr but require prior coagulation (alum) and sedimentation, removing turbidity mechanically without the biological action.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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