How do filtration rates compare?

SSF 0.1 to 0.4 m/h; RSF 3 to 6 m/h

SSF 3 to 6 m/h; RSF 0.1 to 0.4 m/h

How is each filter cleaned?

SSF by scraping the top layer; RSF by upward-flow backwashing

SSF by backwashing; RSF by scraping

What pre-treatment does each need?

SSF only plain sedimentation; RSF coagulation, flocculation and sedimentation

SSF needs coagulation; RSF needs none

Neither needs any pre-treatment

Which statement on effluent quality is correct?

SSF removes 98 to 99 % bacteria; RSF removes about 80 to 90 %

SSF removes 80 to 90 %; RSF removes 98 to 99 %

SSF removes none, RSF removes 99 %

2070 Chaitra · 8 marks

Slow sand filter versus rapid sand filter

Short answer

Slow sand filters use low rate (0.1–0.4 m/h), fine sand (effective size 0.2–0.3 mm), biological schmutzdecke action, scraping cleaning, and no pre-coagulation. Rapid sand filters use higher rate (3–6 m/h), coarser sand (0.45–0.7 mm), adsorption, require coagulation-flocculation, and are backwashed.

Differentiate between slow sand filter and rapid sand filter.

Point	Slow sand filter	Rapid sand filter
Rate of filtration	0.1 to 0.4 m/h (100 to 400 l/h/m²)	3 to 6 m/h (3000 to 6000 l/h/m²)
Effective size of sand	0.2 to 0.3 mm	0.45 to 0.7 mm
Uniformity coefficient	2 to 3	1.3 to 1.7
Depth of sand	0.9 to 1.2 m	0.6 to 0.75 m
Area required	Large	Small
Pre-treatment	Plain sedimentation only (no coagulant)	Coagulation, flocculation and sedimentation needed
Principal action	Straining and biological (schmutzdecke)	Straining and adsorption within the bed
Cleaning	Scraping the top 1 to 3 cm every 1 to 3 months	Backwashing by upward flow every 24 to 72 h
Loss of head	Up to 1.0 to 1.2 m	Up to 2.5 to 3.0 m
Quality of effluent	Bacteria removal 98 to 99 %	80 to 90 % bacteria removal
Operation	Simple, unskilled labour	Skilled operation, more complex
Construction cost	Higher (large area)	Lower per unit, but equipment cost
Running cost	Low	High (chemicals, power for washing)
Suitable for	Small towns and rural areas, low turbidity	Large towns and cities, turbid water
Wash water	Not needed	2 to 4 % of filtered water

✓ Answer checked by Claude Opus 5.5 (high effort)

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