2.4 Industrial MSGP and Municipal MS4 Programs

Multi-Sector General Permit (MSGP) and Municipal (MS4) Permits

In addition to the CGP, CESSWI inspectors must understand the two other pillars of the NPDES stormwater program: industrial facilities (MSGP) and municipal storm sewer systems (MS4).

Multi-Sector General Permit (MSGP) for Industrial Activities

The EPA Multi-Sector General Permit (MSGP) regulates stormwater discharges associated with 11 statutory categories of industrial activity (40 CFR § 122.26(b)(14)(i)–(xi)), organized into industrial sectors (Sector A through Sector AD)—ranging from timber products (Sector A), metal mining (Sector G), and landfills (Sector L) to automobile salvage yards (Sector M), scrap recycling (Sector N), concrete/asphalt batch plants (Sector D/E), and air or ground transportation maintenance facilities (Sector S/P). Note that construction activity itself was originally Category (x) under the Phase I industrial rule before being given its own dedicated Construction General Permit.

Key elements of an industrial MSGP program include:

• Industrial SWPPP & Control Measures: Identifying potential pollutant sources (loading docks, outdoor material storage, fueling areas, waste dumpsters, dust collection vents) and implementing good housekeeping, spill prevention, and structural controls.
• Routine Facility Inspections & Quarterly Visual Assessments: Inspecting industrial areas at least quarterly and collecting a grab sample of stormwater discharge once each quarter to visually assess water quality characteristics (color, odor, clarity, floating solids, settled solids, suspended solids, foam, and oil sheen).
• Benchmark Monitoring & Numeric ELGs: Analyzing stormwater samples for sector-specific parameters (such as total suspended solids, chemical oxygen demand, copper, zinc, or aluminum). Benchmark values are action-level reference concentrations: exceeding a benchmark is not a direct permit violation by itself, but triggers mandatory Additional Implementation Measures (AIM) / corrective action tiers to reduce pollutant concentrations. By contrast, certain high-risk industrial sectors are also subject to binding numeric Effluent Limitation Guidelines (ELGs) (such as runoff from coal piles, cement kiln dust, or airfield deicing), where exceeding the numeric limit is an enforceable permit violation.
• No Exposure Certification (NEC): An eligible industrial facility meeting 40 CFR 122.26(g)’s shelter and no-exposure conditions, including the regulation’s limited exceptions may submit an NEC every 5 years to be conditionally exempt from MSGP coverage.

Municipal Separate Storm Sewer System (MS4) Permits

A Municipal Separate Storm Sewer System (MS4) is a publicly owned conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, and storm drains) designed or used for collecting or conveying stormwater that is not a combined sewer and not part of a Publicly Owned Treatment Works (POTW).

• Phase I MS4s (1990): Cover Medium MS4s (serving incorporated populations of 100,000 to 249,999) and Large MS4s (serving populations of 250,000 or more). Phase I MS4s are typically regulated under individual NPDES permits requiring wet-weather chemical monitoring, industrial facility inspections, and comprehensive watershed management.
• Phase II MS4s (1999): Cover Small MS4s located within Census urban areas with at least 50,000 people (or areas separately designated by the permitting authority), as well as non-traditional MS4s (state Departments of Transportation [DOTs], public universities, military bases, penitentiaries, and public hospital campuses). Phase II MS4s are typically covered under a General Permit and must develop, implement, and enforce a Stormwater Management Program (SWMP) designed to reduce pollutant discharges from the MS4 to the Maximum Extent Practicable (MEP) through Six Minimum Control Measures (MCMs):
  1. MCM 1 — Public Education and Outreach: Distributing educational materials and performing outreach to inform citizens, businesses, and contractors about the impacts of polluted stormwater runoff.
  2. MCM 2 — Public Involvement and Participation: Providing opportunities for citizens to participate in SWMP development and implementation (public hearings, citizen advisory panels, storm drain marking, stream cleanups).
  3. MCM 3 — Illicit Discharge Detection and Elimination (IDDE): Developing a comprehensive storm sewer system map showing all outfalls and receiving waters, enacting an ordinance prohibiting non-stormwater discharges into the MS4, and conducting dry-weather outfall screening to detect and eliminate illegal sanitary sewer cross-connections, floor drain tie-ins, and unlawful dumping.
  4. MCM 4 — Construction Site Stormwater Runoff Control: Enacting an enforceable local ordinance requiring erosion and sediment controls and waste management on construction sites disturbing 1 acre or more (or less than 1 acre within a common plan), reviewing site plans, receiving public complaints, and conducting municipal site inspections and local enforcement.
  5. MCM 5 — Post-Construction Stormwater Management in New Development and Redevelopment: Enacting an ordinance requiring permanent structural and non-structural stormwater BMPs (such as bioretention, permeable pavement, extended detention basins, and green infrastructure) on projects disturbing $\ge 1$ acre, coupled with recorded long-term Operation and Maintenance (O&M) agreements and post-construction inspections.
  6. MCM 6 — Pollution Prevention / Good Housekeeping for Municipal Operations: Implementing an O&M program and employee training to prevent stormwater pollution from municipal streets, parking lots, fleet maintenance garages, salt/deicer storage domes, parks, and storm drain catch basin cleaning.

Current industrial permit status

EPA’s 2021 MSGP is administratively continued after February 28, 2026 for previously covered facilities/operators. New facilities and new operators cannot obtain coverage by submitting an NOI under that expired permit; EPA’s conditional No Action Assurance is enforcement discretion, not permit coverage. No-exposure certifications remain available to eligible facilities. Inspectors must distinguish existing coverage, exclusions, and unpermitted new discharges before applying routine industrial inspection requirements.