A public health engineer compares two water filtration systems for a town supply. In the slow sand filter, which mechanism is PRIMARILY responsible for removal of bacteria from raw water?

• A Mechanical straining by the fine sand grains alone
• B Sedimentation within the deep gravel support layer
• C Coagulation of organisms by alum dosed before filtration
• D Biological action of the schmutzdecke layer on the sand surface ✓

Explanation

The slow sand filter removes bacteria chiefly through biological purification. A gelatinous film called schmutzdecke, rich in algae, diatoms and protozoa, forms on the sand surface within a few days and oxidizes organic matter while retaining microorganisms. Mechanical straining plays only a minor part because bacteria are far smaller than the pore size of sand. Alum coagulation belongs to rapid sand filtration pretreatment, and gravel provides support, not purification.

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.