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
No preliminary storage of raw water is required
Operation requires highly skilled persons
Frequent washing is not required
Can be gravity type or pressure type
Water