This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Water, Subject name: Social & Preventive Medicine
All are true for Rapid sand filters except: 
Select and explain. 
A. No preliminary storage of raw water is required, B. Operation requires highly skilled persons, C. Frequent washing is not required, D. Can be gravity type or pressure type
C: Frequent washing is not required
Answer: C: Frequent washing is not required 
Explanation: 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