Sewerage system is designed for

A sewerage system needs to handle varying amounts of wastewater. It's not enough to design it for just one flow rate.

Maximum Flow: The system must be able to handle the peak flow, which occurs during times of high water usage (like after heavy rain or during certain times of the day). If the system isn't designed for maximum flow, it could overflow, causing pollution and other problems.

Minimum Flow: The system also needs to function properly during times of low flow. If the flow is too low, solids can settle in the pipes, leading to blockages and the production of unwanted gases. Minimum flow is important for maintaining a self-cleansing velocity.

Options A, B, and C are incorrect because they only consider one extreme of flow, which is not sufficient for a well-functioning sewerage system. A good design considers both extremes to ensure reliable and efficient operation.