Municipal Separate Storm Sewer (MS4)

What is MS4?


There are stormwater requirements under the federal Clean Water Act, and these requirements are administered under the Pennsylvania Department of Environmental Protection’s (PA DEP) Municipal Separate Storm Sewer System (MS4) program. The MS4 program requires the Municipalities to incorporate 6 elements into their stormwater management programs, called minimum control measures:

  • Public Outreach and Education
  • Public involvement and participation
  • Illicit discharge detection and elimination
  • Construction site runoff control
  • Post-construction stormwater management in new development and redevelopment
  • Pollution prevention and good housekeeping for municipal operations and maintenance

What is Stormwater?


Storm water is water from precipitation that flows across the ground and pavement when it rains or when snow and ice melt. The water seeps into the ground or drains into storm sewer systems – a system with inlets and pipes that directly connect to local waterways. Storm water becomes a problem when it picks up pollutants such as oil, grease, metals and coolants from vehicles.

Who is in an MS4?


Under the 1987 Clean Water Act Amendments, the U.S. EPA developed storm water regulations to address stormwater that might impact water quality.  These regulations were set up depending upon the US Census population data. The stormwater regulation phase affects about 1,000 municipalities in Pennsylvania.  Those municipalities that are located within and "urbanized area" as defined by the 2010 Census were required to apply for a National Pollutant Discharge Eliminations System (NPDES) permit to discharge stormwater from their municipal separate storm sewer system (MS4). 

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