The correct answer is D. All the above.
The circular section of sewers provides maximum hydraulic mean depth because it has the smallest perimeter for a given area. This means that the water flowing through a circular sewer experiences less friction than it would in a sewer of another shape. As a result, circular sewers can carry more water with less energy loss.
Circular sewers are provided for separate sewerage systems because they are the most efficient shape for carrying wastewater. Wastewater is a mixture of water and solids, and the solids can settle out and form deposits in the sewer if the sewer is not designed properly. Circular sewers are less likely to form deposits than sewers of other shapes, and they are also easier to clean if deposits do form.
Circular sewers work efficiently if the sections run at least half full because this minimizes the amount of energy required to move the wastewater through the sewer. When the sewer is running less than half full, the water flows in a turbulent manner, which creates more energy loss. When the sewer is running at least half full, the water flows in a laminar manner, which creates less energy loss.
In conclusion, the circular section of sewers provides maximum hydraulic mean depth, circular sewers are provided for separate sewerage systems, and circular sewers work efficiently if the sections run at least half full.