On which of the following parameters are the slow sand filters better than rapid sand filters?

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

C_u ⇒ 1.8 to 3.

Gravel -30-75 cm thick

D₁₀ = 0.35 mm - 0.55 mm

C_u = 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%)