Compared with the rapid sand filter, the slow sand filter achieves far greater removal of pathogenic bacteria even without prior coagulation. This superior bacterial removal is chiefly attributable to:
- A The biologically active layer (schmutzdecke) that develops on the sand surface ✓
- B Higher filtration rate compressing particles into the sand bed
- C Addition of alum immediately before filtration
- D Use of finer gravel supporting layers beneath the sand
Correct answer: A. The biologically active layer (schmutzdecke) that develops on the sand surface
Explanation
In the slow sand filter, a gelatinous biological layer called the schmutzdecke or vital layer forms on the sand surface within days. It contains algae, diatoms and bacteria that remove organic matter and up to 98 to 99 percent of bacteria by mechanical strapping, sedimentation and biological action. The rapid sand filter depends mainly on physical straining after chemical coagulation and lacks this vital layer.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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