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What is an MS4?

A municipal separate storm sewer system (MS4) is a conveyance or system of conveyances (roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, storm drains, etc.) that is also:
  • Owned or operated by a public entity (which can include cities, townships, counties, military bases, hospitals, prison complexes, highway departments, universities, etc.) having jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes, including special districts under State law such as a sewer district, flood control district or drainage districts, or similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under section 208 of the Clean Water Act that discharges to waters of the United States;
  • designed or used for collecting or conveying stormwater;
  • which is not a combined sewer; and
  • which is not part of a publicly owned treatment works.

Municipal Separate Storm Sewer Systems (MS4s)

The MS4 program is responsible for the regulation of polluted stormwater runoff that is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), from which it is often discharged untreated into local waterbodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a National Pollutant Discharge Elimination System (NPDES) permit and develop a stormwater management program. The MS4 systems may be:
 
Large MS4 250,000+ Population
  • Louisville and Jefferson County MSD
  • Lexington-Fayette County Urban Government

Medium MS4 100,000+ Population
  • None in Kentucky 

Small MS4 >10,000 Population or Population Density of 1,000/ square mile
  • 100 Phase II covered under 44 permit
The Phase II Permit, Fact Sheet, and the Division’s guidance for preparing a Phase II Stormwater Quality Management Plan can be found in the forms library via the link below:


The stormwater management plan consists of six minimum controls. The MS4 permittee must adopt ordinances that prohibit illicit discharges of non-stormwater into the MS4; prevent erosion and provide sediment control for construction sites and provide stormwater control measures for long term stormwater management. The MS4 permittees must submit an annual report detailing their compliance with the permit.  The U.S. EPA page linked below has more information on MS4s and links to documents for developing a program and other resources for permitting, funding, and training opportunities:


For more information contact: 

Environmental Scientist V
502-782-7044