Self-cleansing velocity: For a sewer containing 1 mm sand particles (specific gravity 2.65) and 5 mm organic particles (specific gravity 1.2), what is the minimum velocity typically required to move the solids without deposition?

Introduction / Context:

Sewer design targets a self-cleansing velocity that prevents the settlement of typical solids—heavy mineral grit (e.g., 1 mm sand, SG ≈ 2.65) and lighter organic particles (e.g., 5 mm, SG ≈ 1.2). Achieving a minimum velocity reduces maintenance and odors.

Given Data / Assumptions:

• Sand particles: 1 mm, SG 2.65 (heavier, settle faster).
• Organic particles: 5 mm, SG 1.2 (larger but near-neutral density).
• Objective: select a practical minimum velocity for self-cleansing.

Concept / Approach:

Empirical practice and sediment transport considerations suggest minimum velocities around 0.45–0.6 m/s for sanitary sewers to move common grit under normal flows. The heavier sand dictates the controlling criterion; hence values near the upper end of common minima are used.

Step-by-Step Solution:

Verification / Alternative check:

Design handbooks cite 0.45 m/s (about 1.5 ft/s) as a commonly accepted minimum for sanitary systems carrying typical grit loads, supporting the chosen value over lower velocities.

Why Other Options Are Wrong:

• 0.30–0.40 m/s: Often insufficient for 1 mm mineral grit, risking deposition.
• 0.50 m/s: Acceptable and conservative, but when choosing a minimum from listed options, 0.45 m/s reflects the standard benchmark.

Common Pitfalls:

• Ignoring that grit, not organics, typically governs the required minimum velocity.

Final Answer:

0.45 m/s