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Municipal Separate Storm Sewer System (MS4)
The definition of Municipal Separate Storm Sewer System (MS4) is listed in 40 CFR 122.26(b)(8). In summary, operators of MS4s can include municipalities and local sewer districts, state and federal departments of transportation, universities, hospitals, military bases, and correctional facilities.
The MS4 is a system that is located in an incorporated place or county and categorized by the size of the population it serves:
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Large MS4: a population of 250,000 or more
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Medium MS4: a population between 100,000 - 249,999
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Small MS4: any MS4 that is not regulated under Phase I of the NPDES program as large or medium MS4
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Regulated small MS4: any small MS4 located in an "urbanized area" (UA) or located outside of a UA and brought into the program by the NPDES permitting authority. The interactive map of urbanized areas based on the 2000 US Census can be found in the EPA’s website.
In 2003, the US EPA's NPDES program began to require that municipalities within urbanized areas obtain stormwater discharge permits to better manage publicly owned stormwater collection and discharge systems from town roads and properties. The objective of Phase II MS4 permits is to minimize the impacts to water quality and wetlands from municipally owned stormwater discharge systems. The 5-year permits issued to the operator of MS4 under this program require that the operators identify illicit discharges of contaminants to stormwater, and to implement programs and practices to reduce the discharge of pollutants.
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. Specific NPDES permit requirements for MS4s can be found in the EPA's website through the links provided below:
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Copyright @ January,2005 Prepared by Transportation Environmental Resource Center (TERC)