Header Image

Municipal Separate Storm Sewer Systems

Municipal Separate Storm Sewer Systems (MS4s) are collection systems designed to transport rainwater and snow melt through a series of drains, pipes, ditches and open channels to our streams and rivers. The water that flows into the storm sewer system is called stormwater runoff. Before entering the system, it becomes contaminated by oil and grease from roadways, pesticides from lawns, sediment from construction sites, and carelessly discarded trash, such as cigarette butts, paper wrappers and plastic bottles. In a separate storm sewer system, stormwater runoff is not treated; therefore, all of the pollutants it carries flow directly into our waterways, where they can contaminate drinking water supplies and interfere with the habitat for fish and other aquatic organisms.

The Environmental Protection Agency's Storm Water Phase II Rule establishes an MS4 stormwater management program intended to improve the nation's waterways by reducing the quantity of pollutants that stormwater carries into the system during rainfall. By EPA definition, all MS4 communities are in urbanized areas—areas with a population of 50,000 or more—throughout the United States and are required to obtain a National Pollutant Discharge Elimination System (NPDES) permit through their state environmental agency—in Pennsylvania, the Department of Environmental Protection (DEP)—in order to discharge stormwater runoff into the waterways. These permits require MS4 operators to develop, implement and enforce a comprehensive stormwater management program to reduce the discharge of pollutants, educate the public about stormwater and protect water quality within their watersheds.

The required municipal program must reduce the discharge of pollutants from MS4s to the maximum extent practical, with the goal of protecting water quality and satisfying the requirements of the federal Clean Water Act and the Pennsylvania Clean Streams Law. The program must contain a schedule, best management practices (BMPs) and measurable goals for the Six Minimum Control Measures, and be approved by DEP. More than 1,000 municipalities throughout Pennsylvania must implement a stormwater management program that contains these

Six Minimum Control Measures

  1. Public education and outreach
  2. Public participation and involvement
  3. Illicit discharge detection and elimination
  4. Construction site runoff control
  5. Post-construction stormwater management in new development and redevelopment
  6. Pollution prevention and good housekeeping for municipal operations and maintenance

More information on stormwater permitting regulated by the PA Department of Environmental Protection.

List of all southwestern Pennsylvania MS4 permittees by municipality and the watersheds to which they flow. Map of urbanized areas in Pittsburgh.