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Municipal Separate Storm Sewer Systems (MS4)
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A MS4 is a system designed for collecting or conveying storm water (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. Anything that is put into a MS4 goes to the river without being treated.
Operators of MS4s include:
- Municipalities and local sewer districts
- State and Federal DOTs
- Public universities, public hospitals, military bases, correctional facilities
A NPDES permit must be obtained by the operator of a MS4.
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Medium and Large MS4s:
Medium and Large MS4s focus on the size of the population in the area where the system is located. The population served by combined sewer systems (systems that convey sanitary wastewaters and storm water through a single pipe to a publicly owned treatment water works facility) 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
NPDES Permit Requirements for Medium and Large MS4s
Small MS4s:
Only a select subset of small MS4s, referred to as regulated small MS4s, require an NPDES storm water permit.
- Small MS4s are any MS4 that is not a Medium or Large MS4 covered by Phase I of the NPDES Storm Water 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
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