Municipal Separate Storm Sewer Systems (MS4)
A Municipal Separate Storm Sewer System (MS4) is a system designed for collecting or conveying stormwater (i.e. roads with drainage systems, curbs, gutters, ditches, man-made channels, or storm drains) that is owned or operated by a public entity but not part of a publicly owned treated water works facility.
Operators of MS4s include:
- Municipalities and local sewer districts
- State and Federal DOTs
- Public universities, public hospitals, military bases, correctional facilities
A MS4 Permit allows MS4s to discharge stormwater directly to water bodies without being treated. A MS4 Permit must be obtained by the operator through the Environmental Protection Agency's (EPA) National Pollutant Discharge Elimination System (NPDES) program.
ITD MS4 Permits
In some cases ITD is listed as a co-permittee with local municipalities that share or are connected to the same MS4 conveyance system. The following are MS4 Permits currently held by ITD:
Annual MS4 Permit Reports
Annual reporting is required for MS4 permit holders. Contact Brad Wolfinger
to request any supporting documentation.
Medium and Large MS4s:
Medium and Large MS4s Permit designations are based on populations served by a system. Population served by combined sewer systems (systems that convey sanitary wastewaters and stormwater through a single pipe to a publicly owned treatment water works facility) are not included in the population for the MS4 permitting process.
- A Medium MS4 is a system that is located in an area that serves a population of 100,000 to 249,999 people.
- A Large MS4 is a system that is located in an area that serves a population of 250,000 people or greater.
- NPDES Permit Requirements for Medium and Large MS4s
Only a select subset of small MS4s, referred to as regulated small MS4s, require an NPDES stormwater permit.
- Small MS4s are any MS4 that is not a Medium or Large MS4 covered by Phase I of the NPDES Stormwater Program
- Regulated Small MS4s are all small MS4s located in "urbanized areas" (UA) as defined by the Bureau of the Census, and those small MS4s located outside of a UA that are designated by NPDES permitting authorities.
- NPDES Permit Requirements for Small MS4s