Slow Sand Filters (SSF) differ from Rapid Sand Filters mainly because SSF relies on:
Choose the correct answer
High pressure filtration
Biological action (Schmutzdecke layer) at slow rate 0.1–0.4 m/h
Chemical dosing during filtration
Carbon adsorption
Correct Answer
B. Biological action (Schmutzdecke layer) at slow rate 0.1–0.4 m/h
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SSF relies on biological action: a biologically active layer (Schmutzdecke) of algae, diatoms, bacteria on top of sand removes pathogens by predation, adsorption, and biological oxidation. Rate = 0.1–0.4 m/h. No coagulant needed. Cleaned by scraping (not backwashing). SSF is best for turbidity < 50 NTU.
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