Municipal Separate Storm Sewer System (MS4)

About 10% of the City's sewer collection system is called a Municipal Separate Storm Sewer System (MS4). In an MS4 system, a set of pipes are designated only for rainwater runoff into rivers and streams nearby. The State of Delaware oversees how we manage this MS4 system through a special permit, along with New Castle County and the Delaware Department of Transportation. Meanwhile, another set of pipes in the sewer system is only for carrying sanitary sewage to the wastewater treatment plant.

The Delaware Department of Natural Resources and Environmental Control (DNREC) issued the National Pollutant Discharge Elimination System (NPDES) permit DE 0051071/State Permit WPCC 3063A/96 on May 7, 2013 (effective date). This permit allows the Principal Permittees (New Castle County and the Delaware Department of Transportation or DelDOT) and the Co-permittees (towns of Bellefonte, Elsmere, and Newport and the cities of Delaware City, New Castle, and Wilmington) to discharge stormwater from their municipal separate storm sewer systems (MS4).

As part of the permit requirements, the principal permittees must collaborate with all co-permittees, or the City of Wilmington may independently develop and submit to DNREC a Stormwater Pollution Prevent and Management Plan (SWPP&MP). This plan is designed to control the quality of stormwater discharged from the MS4 and must meet all permit conditions. It must include measurable goals and describe the necessary controls to prevent the discharge of any materials other than stormwater into the MS4.

Municipal Separate Storm Sewer System (MS4) Infographic

The SWPP&MP should detail measures aimed at reducing pollutant discharge from the MS4 and provide a schedule for implementation. The City has chosen to develop and implement its own SWPP&MP separate from the principal permittees. You can access the City's SWPP&MP through the links provided below: