Choice of sewerage system: In regions where light rains occur frequently and are fairly uniform through the year, which sewerage system is generally preferred?

Introduction / Context:
Selection of a sewerage system (separate, combined, or partially combined) depends on rainfall pattern, topography, environmental regulations, and economics. The pattern and intensity of stormwater strongly influence the hydraulic and treatment implications of the chosen system.

Given Data / Assumptions:

• Rainfall is light but frequent and fairly uniform through the year.
• Urban setting where storm runoff volumes are moderate.
• Historical textbook perspective on system choice (recognizing modern CSO controls).

Concept / Approach:

Where rainfall is frequent but light, the combined system historically offered simplicity—one network for both sanitary sewage and storm runoff—keeping velocities adequate and avoiding a dry-weather underutilized storm system. Modern practices may favor separate systems to avoid combined sewer overflows (CSOs), but the classical exam answer associates uniform, light rainfall with the combined system choice due to economy and adequate self-cleansing flows.

Step-by-Step Solution:

Verification / Alternative check:

Where heavy, sporadic storms occur, separate systems reduce peak loads to WWTPs; where storms are light and frequent, combined systems maintain flow in one conduit but must manage CSO impacts.

Why Other Options Are Wrong:

Separate system is favored for intense or infrequent storms; partially combined is a compromise but not the classical recommendation for the stated rainfall regime; “none” is incorrect.

Common Pitfalls:

Ignoring environmental constraints; overlooking CSO consent decrees; applying a one-size-fits-all rule without local analysis.

Final Answer:

Combined system