Slow sand filter is differentiated from rapid sand filter by -

Comparison of Rapid and Slow sand filters Rapid sand filter Slow sand filter Space Occupies very little space Occupies large area Rate of filtration 200 m.g.a.d 2-3 m.g.a.d Effective size of sand Chemical coagulation and sedimentation Plain sedimentation Preliminary treatment By back washing By scraping sand bed Washing Required Not required Frequent washing Essentially physical Both physical and mechanical Mechanism of action Essentially physical Both physical and mechanical Operation Highly skilled Less skilled Loss of head allowed 6-8 feet 4 feet Removal of turbidity Good Good Removal of colour Good Good Removal of bacteria 98-99 % 99.9-99.99% Suitability For big cities For small towns Ref: Park 25th edition Pgno : 751