ANS:B - less than the self-cleansing velocity

In sewers, the velocity of flow should not be less than the self-cleansing velocity. Self-cleansing velocity is the minimum velocity required to prevent sedimentation and deposition of solids in the sewer pipe. If the flow velocity is too low, solids can settle out and accumulate, leading to blockages and reduced flow capacity. Therefore, maintaining a velocity above the self-cleansing velocity helps to keep the sewer pipe clear of debris and prevent clogging. Typically, the self-cleansing velocity depends on factors such as the size and slope of the sewer pipe, as well as the characteristics of the wastewater being conveyed. It's often recommended that the flow velocity in sewers should be at least 0.6 m/s (2 ft/s) to ensure self-cleansing. So, the correct option is: less than the self-cleansing velocity.