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Combined Sewer Overflows
Frequently Asked Questions

The following is a list of Frequently Asked Questions for Combined Sewer Overflows organized from general questions about the NPDES Program to more technical questions about Combined Sewer Overflows. Click here to search for other program-specific FAQs or to display a list of all NPDES FAQs. Please check back periodically for updates!


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" transport wastewater directly to the sewage treatment plant. In periods of rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, lakes, or estuaries. Combined sewer overflows (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 combined sewer systems. CSOs are among the major sources responsible for beach closings, shellfishing restrictions, and other water body impairments.

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Where are the cities with CSO problems?

Combined sewer systems serve roughly 772 communities with about 40 million people. Most communities with CSOs are located in the Northeast and Great Lakes Regions, particularly in Pennsylvania, Indiana, Ohio, Illinois, Michigan, New York, West Virginia, and Maine. Although large cities like New York, Philadelphia, and Atlanta have combined sewer systems, most communities with CSO problems have fewer than 10,000 people.

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How does the CSO Control Policy address combined sewer overflows?

EPA's CSO Control Policy, published April 19, 1994, at 59 FR 18688, is the national framework for control of CSOs. The Policy provides guidance on how communities with combined sewer systems can meet Clean Water Act goals in as flexible and cost-effective a manner as possible. The Policy contains four 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 the development of CSO control plans to reflect the site-specific wet weather impacts of CSOs.

The first milestone under the CSO Policy was the January 1,1997, deadline for implementing minimum technology-based controls (the "nine minimum controls"). The nine minimum controls are measures that can reduce the prevalence and impacts of CSOs and that are not expected to require significant engineering studies or major construction. Communities with combined sewer systems are also expected to develop long-term CSO control plans that will ultimately provide for full compliance within the Clean Water Act, including attainment of water quality standards.

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Last updated on January 24, 2013 1:31 PM
URL:http://cfpub.epa.gov/npdes/faqs.cfm