Which statement best describes design expectations for sewer pipes used in municipal sewerage systems?

Introduction / Context:
Sewer design aims to transport wastewater reliably while minimizing maintenance. Key considerations include hydraulic performance, self-cleansing velocity, and durability against abrasion by grit.

Given Data / Assumptions:

• Gravity flow dominates municipal sewers except at pumping stations.
• Self-cleansing velocity threshold typically ~0.6–0.75 m/s for domestic sewers (depends on grit).
• Abrasive wear is a real issue due to sand, grit, and scouring during storms.

Concept / Approach:
Good design checks both hydraulics and materials. Hydraulics ensures adequate slopes and velocities; materials selection ensures the pipe withstands chemical and mechanical attack (including abrasion).

Step-by-Step Solution:
Step 1: Confirm gravity conveyance: most sewers are laid to grade to avoid energy costs.Step 2: Provide slope such that v_min ≥ v_self-cleansing at dry weather flow to reduce deposition.Step 3: Select materials/linings that resist abrasion (e.g., vitrified clay, ductile iron with liners, HDPE).Step 4: The option combining all three requirements is correct.

Verification / Alternative check:
Standard design manuals and codes (municipal practice) list self-cleansing velocity checks and material abrasion resistance as essential criteria, confirming the combined statement.

Why Other Options Are Wrong:

• (a) alone omits self-cleansing and abrasion resistance.
• (b) alone ignores gravity layout and durability.
• (c) alone ignores hydraulics.
• (e) still omits abrasion resistance.

Common Pitfalls:
Undersloping pipes to save depth can cause silting; ignoring grit leads to rapid wear and increased O&M costs.

Final Answer:
All of the above