Assertion–Reason on sewer operation: Assertion (A): Minimum self-cleansing velocity should occur in a sewer at least once per day. Reason (R): If initial deposition is not flushed, it obstructs flow and promotes further deposition, eventually blocking the sewer.

Introduction / Context:
Self-cleansing velocity is the minimum flow velocity needed to keep solids in suspension or move them along without deposition. Designing and operating sewers to achieve this velocity periodically is essential for long-term reliability.

Given Data / Assumptions:

• Normal domestic sewers carrying settleable solids and grit.
• Daily diurnal peaks provide higher velocities than nighttime base flows.
• Standard thresholds (e.g., around 0.6–0.75 m/s for sanitary sewage) apply depending on grit content and pipe roughness.

Concept / Approach:

If solids deposit and remain, they reduce the effective flow area and increase roughness, thereby causing additional energy loss and further deposition. This positive feedback can culminate in blockages and sanitary overflows. Ensuring at least once-daily attainment of the self-cleansing velocity flushes sediments out.

Step-by-Step Solution:

Verification / Alternative check:

Operational records show that lines failing to hit cleansing velocities develop slime, grit banks, and odors, necessitating jetting or flushing trucks.

Why Other Options Are Wrong:

(b) denies the causal link; (c) and (d) contradict tested practice; (e) conflicts with field evidence.

Common Pitfalls:

Ignoring peak attenuation in large trunks; choosing grades from tables without checking resulting velocities for actual design flows.

Final Answer:

Both A and R are true and R is the correct explanation of A