For a separate sewerage system carrying only sanitary sewage under gravity, which cross-sectional shape is generally the most hydraulically efficient and economical to construct and maintain?

Introduction / Context:
Choosing a sewer cross-section affects construction cost, self-cleansing ability, and maintenance. In separate systems (without storm flow), the shape must perform well over a broad range of partial-flow depths.

Given Data / Assumptions:

• Separate sewerage system (no storm runoff).
• Gravity flow, steady uniform design condition.
• Standard construction methods and materials.

Concept / Approach:
Circular sections are structurally efficient (uniform ring compression under earth loads), economical to fabricate, and hydraulically satisfactory. They provide reasonable velocities at partial depths and are easy to clean and inspect. Egg-shaped sections are advantageous for combined systems with large flow variation; however, for separate sanitary sewers, circular pipes dominate.

Step-by-Step Solution:
Assess hydraulic efficiency: circular sections have high hydraulic radius for a given area, especially near half-full.Consider structural and construction aspects: circular pipes are easy to produce and strong in ring compression.Select the widely adopted option: circular.

Verification / Alternative check:
Most municipal standards specify circular PVC, vitrified clay, concrete, or ductile iron pipes for sanitary sewers; noncircular shapes appear mainly in large combined sewers or special cases.

Why Other Options Are Wrong:

• Rectangular/parabolic/trapezoidal: Rare for buried sanitary sewers due to structural and maintenance drawbacks.
• New-egg: Useful for combined flow variation, not necessary for separate sanitary flows.

Common Pitfalls:
Assuming egg-shaped is always superior; in separate systems it adds cost/complexity with limited benefit.

Final Answer:
Circular