In water treatment, slow sand filter, when compared to rapid gravity filters, produces

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BPSC AE Paper 6 (Civil) 11 Nov 2022 Official Paper

Answer 1

Explanation:

In water treatment, slow sand filters when compared to rapid sand filters produce Lesser contaminated effluent.

	Slow sand filter	Rapid sand filter
Pre-treatment	Effluents from plain sedimentation coagulation, not req.	Coagulation, flocculation, and sedimentation is a must.
Filter Sand	D10 b/w 0.2 and 0.4 mm Cu ⇒ 1.8 to 3. Gravel -30-75 cm thick	D10 = 0.35 mm - 0.55 mm Cu = 1.2 to 1.8 Gravel-60-90 cm thick
Size of each unit	Large such as 30 m × 60 m. An area from 100-2000 m2. Less flexible on shock loading	Small 5m × 8m. Area < 100m2 More flexible on shock loading
Rate of filtration	100 to 200 litres/hr/m2	3000 to 6000 L/hr/m2
Efficiency	Very efficient in removing bacteria (98-99%) but less efficient in removing colour.	Less efficient in bacteria (80-90%), very efficient in colour removal.
Post-treatment	Disinfection is not must. Almost pure water obtained. Turbidity removal up to 50 ppm	Disinfection is the must. Turbidity up to 35 ppm.
Method of cleaning	Scrapping and removing the top 1.5 to 3 cm thick layer and washing down by hoses.	Backwashing with or without compressed air.
Qty of wash water req.	0.2 to 0.6% of total filtered water is req. in every 1 to 3 months	A large amount of total filtered water is req. in every 1 to 3 days. (1-5%)