Which statement about sanitary sewers is most universally correct for design and public health protection?

Introduction / Context:
Sewer design balances hydraulics, public health, and economics. This question focuses on which guidance is broadly applicable across most projects.

Given Data / Assumptions:

• We consider normal gravity sewers (not force mains).
• Statements include size adequacy, flow condition, and burial depth.

Concept / Approach:
Adequate sizing to pass peak sanitary flow (plus infiltration/inflow) prevents surcharge and overflows. Gravity sanitary sewers are typically designed to run part full at design peak, not full. Burial depth depends on local topography, frost depth, and service connections—not a universal fixed depth.

Step-by-Step Solution:
Assess (a): Adequate sizing is a universal requirement to avoid overflows and backups.Assess (b): Normal operation aims for part-full flow with free surface; full flow indicates potential surcharge.Assess (c): Depth varies project-to-project; 2–3 m is not a universal rule.Hence only (a) is consistently correct.

Verification / Alternative check:
Standard manuals specify minimum slopes and capacities to maintain self-cleansing velocities while avoiding surcharge—implying part-full, not full, operation.

Why Other Options Are Wrong:

• (b) Incorrect: design for full flow is atypical for gravity sewers.
• (c) Incorrect: burial depth is site-specific; sometimes shallower or deeper.
• (d) Incorrect: because (b) and (c) are not universally true.
• (e) Incorrect: pressurized operation is for force mains, not gravity sewers.

Common Pitfalls:
Assuming that “full” signals good capacity. In sewers, steady part-full flow with a free surface is desired to allow ventilation and minimize surcharge risk.

Final Answer:
Sewers must be of adequate size to avoid overflow under peak conditions.