ANS:D - 0.75 m/sec

The self-cleaning velocity recommended for Indian conditions to prevent settling down of sewage both at the bottom and on the sides of a large sewer is typically around 0.75 m/sec. This velocity ensures that the flow of sewage is adequate to keep the solids in suspension and prevent them from settling at the bottom or adhering to the sides of the sewer pipe. By maintaining this velocity, sedimentation and deposition of solids are minimized, reducing the risk of blockages and maintenance issues in the sewer system. Therefore, a self-cleaning velocity of approximately 0.75 m/sec is often recommended to ensure effective transportation of sewage and to prevent sedimentation and buildup within large sewer systems in Indian conditions.