GGR Consulting, LLC
NPDES Stormwater and Environmental Consulting and Training 


Municipal Seperate Storm Sewer Systems (MS4s):

Polluted stormwater runoff can flow through a Municipal Separate Storm Sewer System (MS4), which can then discharge into a nearby waterbody. To help prevent harmful pollutants from being washed or dumped into an MS4, the MS4 operator (i.e. County, City, Special District, etc.) that owns and operates the MS4 must obtain a NPDES (National Pollutant Discharge Elimination System) permit and develop a stormwater management program.

  • Phase I, issued in 1990, requires medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges.
  • Phase II, issued in 1999, requires regulated small MS4s in urbanized areas, as well as small MS4s outside the urbanized areas that are designated by the permitting authority (US EPA of an authorized State), to obtain NPDES permit coverage for their stormwater discharges.

Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general or generic permit. Each regulated MS4 is required to develop and implement a stormwater management program (SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges.  

An MS4 is a conveyance or system of conveyances that is:

  • Owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.;
  • Designed or used to collect or convey only stormwater (including storm drains, pipes, ditches, etc.);
  • Not a combined sewer; and
  • Not part of a Publicly Owned Treatment Works (sewage treatment plant).

The MS4 Program contains elements called minimum control measures that, when implemented, should result in a significant reduction in pollutants discharged into receiving waters. The minimum measures include:

  • Public Education/Outreach: Describes BMPs (Best Management Practices) that will provide educational outreach regarding the harmful impacts of polluted stormwater runoff to the general public.
  • Public Participation/Involvement: Describes BMPs that involve the public in developing, implementing, and reviewing MS4 management programs to help reduce stormwater pollution.
  • Illicit Discharge Detection and Elimination (IDDE): Describes BMPs for identifying and eliminating illicit discharges and spills to storm drain/MS4 systems.  
  • Construction Site Runoff Control: Describes enforcable BMPs for MS4s and construction site operators to address stormwater runoff from active construction sites.
  • Post-Construction Runoff Control: Describes BMPs for MS4s, developers, and property owners to address stormwater runoff (stormwater management systems) after construction activities have ended.
  • Pollution Prevention/Good Housekeeping: Implement a program to reduce pollutant runoff from municipal operations and properly train staff in different methods of pollution prevention.

Additional Requirements May Include:

  • Developing a Stormwater Managment Plan 
  • Developing Standard Operating Procedures
  • Water quality monitoring
  • Develop and submit Annual Reports
  • Many permits have re-opener clauses for TMDLs (Total Maximum Daily Loads) 








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