A slow sand filter is functioning normally in a rural piped water scheme. What percentage reduction in bacterial load does a properly working slow sand filter achieve?
- A 60 to 70 percent
- B 80 to 90 percent
- C 98 to 99 percent ✓
- D 100 percent, producing sterile water
Correct answer: C. 98 to 99 percent
Explanation
A mature slow sand filter with a developed schmutzdecke removes 98 to 99 percent of bacteria, along with most cysts and ova, making it highly effective where raw water has low turbidity. Option D is wrong because filtered water still requires disinfection before supply; filtration alone does not guarantee sterility. Rapid sand filters remove fewer bacteria (about 80 to 90 percent) and depend on prior coagulation and subsequent chlorination.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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