What is a MS4?

A Municipal Separate Storm Sewer System (MS4) is a storm water conveyance or system of conveyances that is regulated under the National Pollution Discharge Elimination System (NPDES).   The operator of an MS4 is required to obtain a permit to discharge storm water from the MS4 into local surface waters.  The objective of this permitting mechanism is to protect surface waters from polluted storm water runoff.

More Information about the MS4

A MS4 (Municipal Separate Storm Sewer Systems) is a conveyance or system of conveyances that is:

  • Owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.
  • Designed or used to collect or convey stormwater (including storm drains, pipes, ditches, etc.)
  • Not a combined sewer
  • Not part of a Publicly Owned Treatment Works (sewage treatment plant).
  • The term MS4 does not solely refer to municipally-owned storm sewer systems.


Membership in INAFSM connects you to a network of floodplain and stormwater professionals, many of whom have responsibilities for MS4 programs.  The INAFSM Stormwater Committee has work groups focused on certain requirements for the MS4 program.  Visit these pages under the Stormwater Committee tab (link if possible) to find useful resources for MS4 implementation.  Work groups offer opportunities throughout the year for training and peer discussion.  The INAFSM annual conference in September includes several sessions about MS4 requirements and programs.