Question

Self-cleansing velocity is:

• A. The minimum velocity of flow required to maintain a certain amount of solids in the flow
• B. The maximum velocity of flow required to maintain a certain amount of solids in the flow
• C. Such flow velocity as would be sufficient to flush out any deposited solids in the sewer
• D. Such flow velocity as would be sufficient to ensure that sewage does not remain in the sewer

Hint:

Self-cleansing velocity in sewers is designed to prevent sedimentation, not to maximize flow speed.

Answer 1

The Correct Option is C

Sewers carry a mix of liquid and solid waste, and if the flow is too slow, solids settle out and accumulate at the bottom of the pipe, eventually causing blockages and odour problems.
Self-cleansing velocity is the minimum flow speed engineers design for specifically to prevent that: it's fast enough that any solids which do settle get picked back up and swept along, rather than sitting and building up over time.
It's not about capping velocity at a maximum, and it's a narrower, more specific idea than just "sewage shouldn't sit still", it's tied directly to moving solid deposits.
So the correct answer is such flow velocity as would be sufficient to flush out any deposited solids in the sewer.

Answer 2

Another way to pin this down is to ask what goes wrong if a sewer is designed below this velocity, and check which definition matches that failure mode:

1. Maintaining a certain amount of solids in flow (as a minimum): Close to the idea, but "maintaining solids in the flow" undersells the actual engineering goal, which is specifically about recovering and moving on solids that have already dropped out of suspension, not just keeping the ones currently suspended.
2. Maintaining a certain amount of solids in flow (as a maximum): If velocity were capped at a maximum instead, there would be nothing stopping it from running too slow and letting solids settle, which is the opposite of what self-cleansing design tries to prevent.
3. Flushing out deposited solids: If a sewer runs below this velocity, solid particles settle at the invert and stay there, silting up the pipe over time; designing for a velocity that flushes out any such deposits directly prevents this failure mode.
4. Ensuring sewage does not remain in the sewer: Falling short of this describes stagnant sewage sitting in the pipe generally, a detention-time issue, which is a different failure mode from solids settling and accumulating.

The specific failure this velocity is engineered to prevent is solids depositing and staying put, which matches one definition precisely.

Therefore, the correct answer is Such flow velocity as would be sufficient to flush out any deposited solids in the sewer.