muNicipal separate storm sewer system (MS4)

Municipal Separate Storm Sewer System (MS4)

A municipal separate storm sewer system (MS4) is a publicly-owned conveyance or system of conveyances (i.e., ditches, curbs, catch basins, underground pipes, etc.) designed or used for collecting or conveying stormwater and that discharges to surface waters of the state. Examples of MS4 operators include, but are not limited to, municipalities, counties, community development districts, universities, military bases or federal correctional facilities.

Operators of large, medium and regulated small MS4s are required to obtain NPDES permit coverage to discharge to waters of the state.

As implemented by Chapter 62-624, F.A.C., Phase I addresses discharges of stormwater runoff from "medium" and "large" MS4s (i.e., those MS4s located in areas with populations of 100,000 or greater). Under Phase II, the program regulates discharges from certain MS4s not regulated under Phase I, and that meet designation criteria set forth in Chapter 62-624, F.A.C.


Phase I

All Phase I large and medium MS4s were previously designated by EPA, based on the total population within the geo-political boundaries of the municipalities.

A large MS4 is any MS4 that is located in an incorporated place or county with a population of 250,000 or greater.

A medium MS4 is any MS4 that is located in an incorporated place or county with a population between 100,000 and 249,999.

Other MS4s located in areas with populations below 100,000 were also designated into the program as Phase I MS4s by EPA. DEP assumed responsibility for all Phase I MS4 permits when it became the permitting authority in October 2000.


Phase II (City of Daytona Beach)

A Regulated Phase II MS4 is any MS4 not already covered by the Phase I Program as a medium or large MS4. The MS4s is automatically designated if any parts or the entire MS4 is located in an urbanized area boundaries as defined by the U.S. Bureau of the Census (based on the latest decennial census). An urbanized area (UA) is a central place (or places) and the adjacent densely surrounding territory that together has a minimum residential population of 50,000 and a minimum average density of 1,000 people/square mile.

Regulated MS4s that are located outside of the UA may also have potential for being designated by the NPDES Permitting Authority (DEP). For more information on Phase II MS4 designation, see Rule 62-624.800, F.A.C.


Operators of a regulated MS4 are required to:

·         Reduce the discharge of pollutants to the "maximum extent practicable" (MEP)

·         Protect water quality

·         Satisfy the water quality requirement of the Clean Water Act