Question

In comparing slow sand filters (SSF) and rapid sand filters (RSF), which of the following is an incorrect statement
1. SSF require larger area for its installation
2. Cost of operation of RSF is lower
3. Depreciation of SSF is lower
4. SSF cannot meet variations in the demand
5. SSF requires cleaning at a frequency of one month

Answer 1

Correct Answer - Option 2 : Cost of operation of RSF is lower

Difference between SSF and RSF:

	Slow sand filter	Rapid sand filter
Pre-treatment	Effluents from plain sedimentation coagulation, not req.	Coagulation, flocculation, and sedimentation is the 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 30m × 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 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%)
Cost of operation	Lower than RSF	Higher than SSF
Depreciation	Lower than RSF	Higher than SSF