ANS:C - both (a) and (b)

The efficient operation of a sewer system requires careful consideration of flow velocities to ensure optimal performance and prevent issues such as sedimentation, debris buildup, and sewer blockages. Let's evaluate each statement: a) Minimum velocity of 0.45 m/sec is maintained at its minimum flow:

• Maintaining a minimum velocity ensures that sediment and solids are carried along by the flow, preventing their deposition and accumulation within the sewer pipes. A velocity of 0.45 m/sec is often considered the minimum to prevent sedimentation.

b) Maximum velocity of 0.90 m/sec is maintained at its maximum flow:

• A maximum velocity limit is also crucial to prevent excessive erosion of the sewer pipe walls and to avoid the risk of hydraulic jumps or turbulent flow conditions. A velocity of 0.90 m/sec is typically considered a safe upper limit to prevent these issues.

Considering both statements:

• It is essential to maintain a minimum velocity to prevent sedimentation and ensure proper conveyance, especially during low-flow periods.
• Similarly, a maximum velocity limit is necessary to prevent erosion and maintain the structural integrity of the sewer system during peak flow conditions.

Therefore, the efficient working of a sewer system requires both the maintenance of a minimum velocity at low flow (a) and the limitation of velocity at peak flow (b). So, the correct answer is both (a) and (b).