MS4
Stormwater Discharges From Municipal Separate Storm Sewer Systems (MS4s)
Under the 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.
The definition of medium and large MS4s focuses on the size of the population in the area where the system is located. The population served by combined sewer systems does not figure into this total.
A medium MS4 is a system that is located in an area with a population between 100,000 - 249,999.
A large MS4 is a system that is located in an area with a population of 250,000 or more.
Other MS4s, including those under the 100,000 threshold, are also large and medium MS4s. These systems were designated by NPDES permitting authorities due to interrelationships to other medium and large MS4s.
Small MS4s are defined as any MS4 that is not a medium or large MS4 covered by Phase I of the NPDES Stormwater Program. Only a select subset of small MS4s, referred to as regulated small MS4s, require an NPDES stormwater permit.
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 through the links provided below:
- NPDES Permit Requirements for MEDIUM and LARGE MS4s
- NPDES Permit Requirements for Regulated SMALL MS4s
EPA Stormwater Educational and useful Materials:
- http://cfpub1.epa.gov/npdes/stormwatermonth.cfm
- http://cfpub1.epa.gov/npdes/stormwater/menuofbmps/index.cfm
Case Study: Orange County Municipal Activities Program
