Operational criterion for sewer velocities: Which statement best describes the lower limit that should not be violated during design to avoid deposition?

Introduction / Context:
Self-cleansing velocity is the minimum flow velocity needed to prevent sedimentation of solids in gravity sewers. Maintaining velocities below this threshold leads to deposition, odor, and reduced capacity, while extremely high velocities risk abrasion and structural damage.

Given Data / Assumptions:

• Gravity sewers carrying domestic wastewater with some grit.
• Typical target minimum velocities around 0.6–0.75 m/s at half-full (varies by code and context).
• Upper velocities limited by scouring/erosion constraints.

Concept / Approach:

Design aims to maintain V ≥ V_sc (self-cleansing). Excessively high velocities are also undesirable, but the key “must not violate” lower bound is the self-cleansing limit to avoid chronic deposition and maintenance issues.

Step-by-Step Solution:

Verification / Alternative check:

Sewer design manuals specify minimum slopes or minimum velocities consistent with self-cleansing criteria derived from settling and shear considerations.

Why Other Options Are Wrong:

(a) reverses the concept; (c) and (d) provide unrealistic absolute numbers; (e) is overly prescriptive and not universally correct across sizes/flows.

Common Pitfalls:

Designing only for peak flow velocity and ignoring low-flow conditions; neglecting solids characteristics and grit load.

Final Answer:

Velocity should not be less than the self-cleansing velocity