MS4 stands for Municipal Separate Storm Sewer System, which is a network of storm drains, pipes, and ditches owned by a public entity like a city or county that collects and transports stormwater runoff. These systems are called “separate” because they are distinct from sanitary sewers that carry wastewater, and their purpose is to manage rainwater from precipitation and snowmelt, not sewage.
How MS4s work and their purpose
- Collects and discharges runoff: Stormwater flows into catch basins, curbs, gutters, and street drains, which are all part of the MS4.
- Transports to waterways: This runoff is often discharged directly into local rivers, streams, and other water bodies without being treated first.
- Carries pollutants: Because it’s untreated, stormwater runoff can carry pollutants like oil, chemicals, trash, and fertilizers into the environment.
- Regulated by permits: The U.S. EPA regulates MS4s through the National Pollutant Discharge Elimination System (NPDES) permits to help reduce the amount of pollution entering waterways.
- Requires management programs: Permit holders must develop and implement a Storm Water Management Program (SWMP) to address water quality, which includes measures like public education, pollution prevention, and construction site controls.