Sewer system classification: A rain–sanitary (combined) sewer is constructed to carry which types of flows?

Introduction / Context:

Cities may use separate sewer systems (sanitary and storm sewers) or combined sewers that carry both sanitary sewage and storm runoff in one pipe. The term “rain–sanitary sewer” is a common way to describe a combined sewer designed to handle multiple sources of flow during wet weather.

Given Data / Assumptions:

• The phrase “rain sanitary sewer” refers to a combined sewer.
• Combined sewers convey sanitary sewage, storm runoff, surface drainage, and incidental groundwater inflow.

Concept / Approach:

Combined systems are sized for extreme wet-weather events, so they must carry dry-weather sanitary flow plus wet-weather inflows. These include rainfall-derived inflow (from roof leaders, yard drains), overland surface runoff entering inlets, and some groundwater infiltration/inflow (I/I) through joints and defects.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals explicitly define combined sewers as conveying both sanitary and storm flows, including incidental I/I, validating selection of “All of the above.”

Why Other Options Are Wrong:

• Selecting only one category (sanitary, storm, or groundwater) omits critical design flows and misrepresents combined sewer function.

Common Pitfalls:

• Confusing combined sewers with separate sanitary or storm sewers.
• Overlooking groundwater infiltration as a contributor to wet-weather flow.

Final Answer:

All of the above