The correct answer is: A. More than the self-cleansing velocity.
The self-cleansing velocity is the minimum velocity of flow in a sewer that is required to prevent the deposition of solids. If the velocity of flow is less than the self-cleansing velocity, solids will deposit on the sewer walls and create a blockage. This can lead to flooding and other problems.
The self-cleansing velocity is affected by a number of factors, including the diameter of the sewer, the slope of the sewer, and the type of solids present in the wastewater. The self-cleansing velocity is typically in the range of 0.7 to 1.5 m/s.
If the velocity of flow in a sewer is greater than the self-cleansing velocity, the solids will be scoured from the sewer walls and carried away in the wastewater. This can lead to erosion of the sewer walls and other problems.
Therefore, it is important to ensure that the velocity of flow in a sewer is not less than the self-cleansing velocity. This can be done by designing the sewer with the appropriate diameter and slope, and by using appropriate methods to remove solids from the
wastewater.Options B, C,
and D are incorrect because they do not represent the minimum velocity of flow required to prevent the deposition of solids in a sewer.