Under the National Pollutant Discharge Elimination System (NPDES) storm water program, operators of large, medium and regulated small municipal separate storm sewer systems (MS4s) require authorization to discharge pollutants under an NPDES permit.
Medium and large MS4 operators are required to submit comprehensive permit applications and are issued individual permits. Regulated small MS4 operators have the option of choosing to be covered by an individual permit, a general permit, or a modification of an existing Phase I MS4's individual permit. | ![](https://webarchive.library.unt.edu/eot2008/20081109225438im_/http://yosemite.epa.gov/icons/ecblank.gif) | ![](https://webarchive.library.unt.edu/eot2008/20081109225438im_/http://yosemite.epa.gov/R10/WATER.NSF/34090d07b77d50bd88256b79006529e8/234bb9d73c9eff2a88256ad1006be803/Body/0.596!OpenElement&FieldElemFormat=jpg) |