What is a municipal separate storm sewer system (MS4)? What is a municipal separate storm sewer system (MS4)?

St. Croix River and Mississippi River confluence

A municipal separate storm sewer system (MS4) is a means of transportation, individually or in a system, (e.g. roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, storm drains, etc.) that are:

• owned or operated by a public entity (e.g. cities, townships, counties, military bases, hospitals, prison complexes, highway departments, universities, etc.) with jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes. This includes special districts under State law (sewer, flood control, or drainage districts, etc.), an authorized Indian tribal organization, or a designated and approved management agency under section 208 of the Clean Water Act;

• designed or used for collecting or transporting stormwater;

• not a combined sewer; and

• not part of a publicly owned treatment works.