MS4 stands for Municipal Separate Storm Sewer System.

• A. Municipal Sanitary Sewer System
• B. Main Storm Sewer System
• C. Municipal Separate Storm Sewer System
• D. Multi-Stage Sewer System

Answer: (C)

MS4 stands for Municipal Separate Storm Sewer System, a term used to describe the network that collects and conveys urban stormwater runoff. The emphasis is on “separate” from the sanitary sewer system that carries domestic and industrial wastewater to treatment plants. This separation helps prevent stormwater from mixing with wastewater, reducing treatment needs and the potential for pollution during rain events.

In practice, a municipal storm sewer system includes pipes, outfalls, and drainage features that channel runoff from streets, parking lots, and other impervious surfaces directly to nearby water bodies. The designation signals regulatory focus on controlling pollutants in urban runoff through permits and best management practices.

The other options don’t fit because they describe systems that aren’t the standard term for this concept: a sanitary sewer system handles wastewater, not just stormwater; a “Main” Storm Sewer System isn’t the recognized designation; and a “Multi-Stage Sewer System” isn’t a standard term in environmental regulation.