This image shows a person collecting a water sample from an outfall
Image of a person collecting a water sample from an outfall

The Phase II Small MS4 General Permit requires small MS4s (Municipal Separate Storm Sewer Systems) throughout Minnesota to adopt minimum control measures to reduce pollution discharged to receiving waters, including illicit discharge detection and elimination (IDDE). An illicit discharge is defined as any discharge to an MS4 that is not composed entirely of storm water, with exceptions made only for NPDES-permitted industrial sources and activities related to fire suppression. Illicit discharges can occur from diffuse residential properties (e.g., yard waste, pet waste), agricultural sources (e.g., pesticides), private and commercial businesses (e.g., plastics, paint, motor oil), construction sites (e.g., eroded soil), and many other sources. Illicit discharges include but are not limited to deliberate, direct connections from private industry to the storm sewer, accidental discharges related to spills, and damaged utilities. Regardless of the source, all MS4s subject to NPDES General Permit requirements are required to develop, implement, and enforce an IDDE program consisting of the following elements:

  • a storm sewer system map, showing the location of all outfalls and the names and location of all waters of the United States that receive discharges from those outfalls;
  • through an ordinance, or other regulatory mechanism, a prohibition (to the extent allowable under State, Tribal, or local law) on non-stormwater discharges into the MS4, and appropriate enforcement procedures and actions;
  • a plan to detect and address non-stormwater discharges, including illegal dumping, into the MS4;
  • the education of public employees, businesses, and the general public about the hazards associated with illegal discharges and improper disposal of waste; and
  • the determination of appropriate best management practices (BMPs) to meet permit requirements for this minimum control measures.

Although the City of Mankato has had ordinances in place prohibiting illegal dumping and illegal connections to their storm sewer system since 1988, in 2006 the City implemented a detailed IDDE program into their Storm Water Pollution Prevention Program (SWPPP). With coordination from the City Attorney, City Council, and City Manager, the City drafted an Enforcement Response Procedure (ERP) document clearly defining processes for escalation of enforcement to compel compliance. Since inception, the program has detected and eliminated 201 illicit discharges within the City of Mankato and area MS4 systems in Blue Earth County.

A critical component of the IDDE program adopted by the City of Mankato is their illicit discharge public education and outreach program. Internal employees, local contractors, local high-risk businesses, and thousands of residents have been educated via in-person education events, public service announcements (PSAs), and other educational materials. The City credits the success of their IDDE program to working with City Council to adopt a clear and strict ERP for addressing noncompliance, and changing culture and public perception related to illicit discharging and dumping through the success of their education and outreach program, which has become a template education and outreach program across the state.

An important lesson learned cited by City staff is to always verify the cause of the illicit discharge first and include the property owner in the process. In many cases, the property owner may not be aware of the illicit discharge, and involving them in the process can increase cooperation, leading to better long term compliance and elimination of existing illicit discharges.

More information regarding IDDE and developing an IDDE program can be found at the United States Environmental Protection Agency website: For more information regarding the development of the City of Mankato’s IDDE program, contact Rick Baird, Environmental Sustainability Coordinator with the City of Mankato:

Project Location: City of Mankato Urbanized area MS4 systems in Blue Earth County
Completion Date: 2006 to Present (Ongoing)
Organizations Involved: City of Mankato, Blue Earth County
Project Budget Summary: Annual budget of the City of Mankato IDDE program: $2.5 million
Quantitative Outcomes: 201 Illicit Discharges have been discovered and successfully brought into compliance. Thousands of residents, local business owners, and local contractors have been involved in education and outreach programs, leading to fewer complaints year-to-year.

This page was last edited on 8 December 2022, at 15:42.