MS4 is an acronym for Municipal Separate Storm Sewer Systems
Polluted stormwater runoff is commonly transported through municipal separate storm sewer systems (MS4s), and then often discharged, untreated, into local water bodies.
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 stormwater (e.g., storm drains, pipes, ditches),
not a combined sewer, and
not part of a sewage treatment plant, or publicly owned treatment works (POTW).
To prevent harmful pollutants from being washed or dumped into MS4s, certain operators are required to obtain NPDES (National Pollutant Discharge Elimination System) permits and develop stormwater management programs (SWMPs). The SWMP describes the stormwater control practices that will be implemented consistent with permit requirements to minimize the discharge of pollutants from the sewer system.
Quick links to relevant information on stormwater discharges from municipal sources: