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National Pollutant Discharge Elimination System (NPDES)

Combined Sewer Overflow Frequent Questions

Below are questions and answers about CSOs and EPA’s CSO control policy.You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.

  • What are CSOs and why are they important?
    • Combined sewer overflows, or CSOs, are remnants of the country's early infrastructure. In the past, communities built sewer systems to collect both stormwater runoff and sanitary sewage in the same pipe. During dry weather, these "combined sewer systems," or CSSs, transport wastewater directly to the sewage treatment plant. In periods of rainfall or snowmelt, however, the wastewater volume in a CSS can exceed the capacity of the sewer system or treatment plant. For this reason, CSSs are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, lakes or estuaries. CSOs contain not only stormwater, but also untreated human and industrial waste, toxic materials, and debris. This is a major water pollution concern for cities with CSSs. CSOs are among the major sources responsible for beach closings, shellfishing restrictions and other water body impairments.
  • Where are the cities with CSO problems?
    • CSSs serve approximately 860 communities with a total population of about 40 million people. Most communities with CSOs are located in the Northeast and Great Lakes regions, particularly in Illinois, Indiana, Maine, Michigan, New York, Ohio, Pennsylvania, and West Virginia. Although large cities like New York, Philadelphia, and Atlanta have CSSs, most communities with CSO problems have fewer than 10,000 people.
  • How does the CSO Control Policy address CSOs?
    • EPA's CSO Control Policy, published April 19, 1994 (59 FR 18688), is the national framework for control of CSOs. The policy provides guidance on how communities with CSSs can meet Clean Water Act (CWA) goals in as flexible and cost-effective a manner as possible. The policy contains the following fundamental principles to ensure that CSO controls are cost-effective and meet local environmental objectives:
      1. Clear levels of control to meet health and environmental objectives.
      2. Flexibility to consider the site-specific nature of CSOs and find the most cost-effective way to control them.
      3. Phased implementation of CSO controls to accommodate a community's financial capability.
      4. Review and revision of water quality standards during development of CSO control plans to reflect the site-specific wet weather impacts of CSOs.
    • The first milestone under the CSO Control Policy was the January 1, 1997, deadline for implementing minimum technology-based controls (i.e., the "nine minimum controls"). The minimum controls are measures that can reduce the prevalence and impacts of CSOs. They are not expected to require significant engineering studies or major construction. Communities with CSSs are also expected to develop long-term CSO control plans that will ultimately provide for full compliance with the CWA, including attainment of water quality standards.

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