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

Step 1: Understanding self-cleansing velocity.
Self-cleansing velocity refers to the minimum velocity required in a sewer so that solid particles do not settle and any deposited matter is carried forward by the flowing sewage.
Step 2: Analyzing the purpose of self-cleansing.
The main objective is to prevent deposition of solids, which can lead to blockages, foul odour, and reduced carrying capacity of the sewer.
Step 3: Evaluating the options.
(A) Incorrect, as it does not address removal of deposited solids.
(B) Incorrect, since self-cleansing velocity is not related to maximum velocity.
(C) Correct, because it directly defines the velocity needed to flush out deposited solids.
(D) Incorrect, as this relates more to detention time rather than solid transport.
Step 4: Conclusion.
The correct definition of self-cleansing velocity is (C), which ensures removal of deposited solids in sewers.

Answer 2

Self-cleansing velocity is a design concept used when sizing sewers, aimed at keeping solids from settling and clogging the pipe. Let's check each definition:

1. The minimum velocity required to maintain a certain amount of solids in the flow: This describes keeping solids suspended in transit, which is related, but it's phrased around merely "maintaining" solids in suspension rather than specifically flushing out material that has already settled, which is the actual design purpose of this velocity.
2. The maximum velocity required to maintain a certain amount of solids in the flow: Self-cleansing velocity is a minimum threshold that must be met or exceeded to prevent settling, not a maximum limit, sewers are designed to at least reach this velocity, not to stay under it.
3. Flow velocity sufficient to flush out any deposited solids in the sewer: This directly names the purpose of self-cleansing design, ensuring that any solids that do settle are picked back up and carried along by the flow, preventing long-term deposition and blockage.
4. Flow velocity sufficient to ensure sewage does not remain in the sewer: This describes avoiding stagnant sewage generally, related to detention time and odour control, which is a different design concern from specifically dislodging settled solid particles.

The defining purpose of self-cleansing velocity is specifically about clearing out solids that have deposited, not just general flow adequacy or an upper velocity limit.

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