Minimum internal diameter for sanitary sewers: What minimum diameter is typically adopted for small gravity sanitary sewers to avoid blockage and to facilitate maintenance?

Introduction / Context:
Minimum sizes are standardized so that sewers are less likely to clog, are maintainable with standard equipment, and can pass typical base flows without excessive deposition. Knowledge of the typical minimum diameter is a core design guideline.

Given Data / Assumptions:

• Gravity sanitary sewer serving residential/commercial premises.
• Conventional design standards and self-cleansing velocity criteria.

Concept / Approach:
Industry practice commonly adopts 150 mm (15 cm) as the minimum for lateral and small branch gravity sewers. Smaller diameters increase risk of clogging and complicate maintenance. The 150 mm size allows reasonable velocities under normal flows and compatibility with cleaning tools.

Step-by-Step Solution:
Identify the code/practice minimum: 150 mm.Compare against options and choose 15 cm.

Verification / Alternative check:
Public works standards frequently state 150 mm as the minimum internal diameter for new gravity sewers; larger sizes are chosen as flows warrant.

Why Other Options Are Wrong:
10–12.5 cm: Too small for maintenance and self-cleansing velocities.20–30 cm: Possible but not the minimum; used when capacity demands it.

Common Pitfalls:

• Confusing house plumbing pipe sizes with public sewer minimums.
• Ignoring that storm or combined sewers may have different practical minima.

Final Answer:
15 cm