The screens are fixed

ANS:C - at an angle 30° to 60° to the direction of flow

If screens were fixed at an angle of 30° to 60° to the direction of flow, it would imply a deliberate design choice that differs from the typical perpendicular arrangement. Such an arrangement might be employed for specific purposes, although it's less common in standard settling tank configurations. Here's how such an arrangement might work:

1. Turbulence Reduction: Angling the screens can help reduce the turbulence caused by the flow of liquid into the settling tank. This reduced turbulence can aid in the settling process by allowing the solid particles to settle more effectively.
2. Increased Surface Area: Angled screens can provide increased surface area compared to perpendicular screens. This increased surface area can enhance the interception and trapping of solid particles, especially if the angle is optimized for the specific flow characteristics of the sewage.
3. Enhanced Solids Capture: The angle of the screens may be adjusted based on the size and density of the solids being captured. A steeper angle might be suitable for larger, denser particles, while a shallower angle might be more effective for finer particles.
4. Flow Control: Angled screens can also help control the flow of liquid within the settling tank, directing it more effectively towards the outlet while allowing solids to settle out.

However, it's important to note that while this arrangement could potentially offer advantages in certain scenarios, it may also introduce complexities in terms of design, operation, and maintenance. Therefore, the choice of screen orientation would depend on the specific requirements and constraints of the settling tank system.