Adopted minimum diameter for gravity sewers: In municipal practice, what is the minimum sewer diameter generally adopted for new gravity sanitary sewers?

Introduction / Context:
Minimum diameter rules help ensure operability, maintenance access, and reduced blockage risk. This question reaffirms the standard adopted minimum size used at the start of system planning and design.

Given Data / Assumptions:

• Gravity sanitary sewer context.
• Typical municipal standards and best practices.

Concept / Approach:
A 150 mm minimum is widely accepted for public sanitary sewers. It balances hydraulic capacity, cleaning access, and constructability, while smaller sizes can hamper maintenance and increase clog risks.

Step-by-Step Solution:
Recall practice minimum: 150 mm.Select 15 cm among options.

Verification / Alternative check:
Design manuals and local authority specifications commonly list 150 mm as the minimum diameter for gravity sewers; larger diameters are used for higher contributing populations or combined flows.

Why Other Options Are Wrong:
10–12.5 cm: Below accepted minimum; escalates blockage risk.20–25 cm: Valid sizes but not the minimum.

Common Pitfalls:

• Assuming house drain sizes apply to public sewers.
• Overlooking that pressure sewers or special systems may differ.

Final Answer:
15 cm