All are true for Rapid sand filters except:
Comparison of Rapid and Slow Sand Filters: Rapid sand filter Slow sand filter aka Biological filter Space Occupies very little space Occupies large areas Rate of filtration 200 m.g.a.d 2-3 m.g.a.d Effective size of sand 0.4-0.7 mm 0.2-0.3 mm Preliminary treatment Chemical coagulation & sedimentation Plain sedimentation Washing By back washing By scraping sand bed Frequent washing Required Not required Mechanism of action Essential physical Both physical & mechanical Operation Highly skilled Less skilled Loss of head allowed 6-8 feet 4 feet Removal of turbidity Good Good Removal of color Good Fair Removal of bacteria 98-99 percent 99.9-99.99 percent Suitability For big cities For small towns