In a horizontal flow slow sand filter operating normally, the approximate percentage reduction in bacterial count achieved is:

• A 60 to 70 percent
• B 75 to 80 percent
• C 85 to 90 percent
• D 98 to 99 percent ✓

Explanation

A slow sand filter removes 98 to 99 percent of bacteria through the schmutzdecke layer and biological action within the upper sand bed. This far exceeds the 80 to 90 percent bacterial removal of a rapid sand filter, which relies mainly on physical straining and flocculation. The biological layer is the reason the slow filter also oxidises organic matter and ammonia.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.