Beyond Construction: MS4 and Industrial Stormwater Permits

Municipal separate storm sewer systems (MS4s)

An MS4 is a publicly owned conveyance or system of conveyances (roads with drainage, catch basins, curbs, gutters, ditches, pipes and storm drains) that discharges to waters of the U.S. and isn't part of a combined sewer or a sewage treatment plant. MS4 operators include cities, counties, state DOTs, universities, military bases, hospitals and prisons.

• Phase I (1990) covered medium and large MS4s, generally serving populations of 100,000 or more, under individual permits.
• Phase II (1999) added small MS4s in urbanized areas, usually under general permits.

The six minimum control measures

Small MS4 permits require a stormwater management program built on six minimum control measures (40 CFR 122.34):

1. Public education and outreach on stormwater impacts.
2. Public involvement and participation in developing the program.
3. Illicit discharge detection and elimination (IDDE): mapping the system, finding and stopping non-stormwater discharges such as illegal hookups and dumping, and outfall screening.
4. Construction site stormwater runoff control: an ordinance, plan review, inspections and enforcement for construction sites disturbing one acre or more.
5. Post-construction stormwater management in new development and redevelopment: permanent controls and long-term maintenance. See lesson 11.
6. Pollution prevention and good housekeeping for municipal operations, such as street sweeping, maintenance yards and staff training.

Industrial stormwater

Facilities in listed industrial categories need NPDES coverage for stormwater from industrial activity. EPA issues the Multi-Sector General Permit (MSGP) where it's the permitting authority, and authorized states issue their own industrial general permits. The MSGP groups facilities into sectors identified by letter, such as scrap and waste recycling, vehicle and equipment maintenance at transportation facilities, and many manufacturing types.

Typical industrial permit requirements:

• An industrial SWPPP describing the facility, potential pollutant sources and control measures.
• Control measures such as good housekeeping, minimizing exposure, preventive maintenance, spill prevention and erosion control.
• Routine facility inspections and quarterly visual assessments of stormwater discharges.
• Monitoring for some sectors, such as benchmark and effluent limit sampling, and corrective actions when needed.

Facilities whose industrial materials and activities are not exposed to stormwater may qualify for the no exposure certification exclusion (40 CFR 122.26(g)).

How the programs connect

• Construction site runoff enters the MS4, so the city's construction program regulates your site alongside the construction general permit.
• Industrial facilities under construction or expansion may need construction coverage too.
• Illicit discharge programs catch prohibited construction discharges, such as concrete washout in a storm drain.

Key takeaways

• MS4s are publicly owned storm sewer systems. Phase I covered medium and large systems; Phase II added small urbanized MS4s.
• The six minimum measures: education, involvement, IDDE, construction runoff control, post-construction management and municipal good housekeeping.
• Industrial facilities get coverage under the MSGP or a state industrial permit, with SWPPPs, inspections, visual assessments and monitoring.
• No exposure certification can exclude facilities with no exposure of industrial activity to stormwater.

Last reviewed: October 25, 2026 · Free general education based on the Clean Water Act, EPA NPDES rules and the EPA Construction General Permit. Your governing permit (EPA, state, tribal or local) controls. This is not legal advice, not EPA-approved, and not a California QSP/QSD or other state credential, and it does not issue a certificate. Nothing you enter on this page is saved or sent.