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Municipal Separate Storm Sewer Systems (MS4)

Under the APDES storm water program, operators of large, medium and regulated small municipal separate storm sewer systems (MS4s) require authorization to discharge pollutants under an APDES 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.

A large MS4 is a system that is located in an area with a population of 250,000 or more.

A medium MS4 is a system that is located in an area with a population between 100,000 - 249,999.

Regulated small MS4s are defined as all small MS4s located in "urbanized areas" (UAs) as defined by the Bureau of the Census, and those small MS4s located outside of a UA that are designated by APDES permitting authority.

In Alaska, the Bureau of the Census recognizes only Anchorage and Fairbanks as urbanized areas.

All operators of regulated municipal separate storm sewer systems (MS4s) are required to:

  • Obtain a APDES permit, and
  • Develop a storm water management program designed to prevent harmful pollutants from being washed by storm water runoff into the MS4 (or from being dumped directly into the MS4), then discharged from the MS4 into local waterbodies.

Storm water management program should meet the standard of "reducing pollutants to the Maximum Extent Practicable (MEP), and include measures to:

  • Identify major outfalls and pollutant loadings
  • Detect and eliminate non-storm water discharges to the system
  • Reduce pollutants in runoff from industrial, commercial, and residential areas
  • Control storm water discharges from new development and redevelopment areas.

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