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Water: Combined Sewer Overflows

Combined Sewer Overflows (CSO) Home



Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe.  Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body.  During periods of heavy 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, or other water bodies.

These overflows, called combined sewer overflows (CSOs), contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris.  They are a major water pollution concern for the approximately 772 cities in the U.S. that have combined sewer systems.

CSOs may be thought of as a type of "urban wet weather" discharge.  This means that, like sanitary sewer overflows (SSOs) and stormwater discharges, they are discharges from a municipality's wastewater conveyance infrastructure that are caused by precipitation events such as rainfall or heavy snowmelt.

EPA's CSO Control Policy, published April 19, 1994, 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.  EPA's Report to Congress on implementation of the CSO Control Policy assesses the progress made by EPA, states, and municipalities in implementing and enforcing the CSO Control Policy.


Experts Forum on Public Health Impacts of Wet Weather Blending - EPA held a forum of public health experts on June 19-20, 2014 to discuss the public health implications of discharges of ‘blended’ effluent from publicly owned treatment works (POTWs) into waterways.

Post Construction Compliance Monitoring Guidance (PDF) (252 pp, 9.01MB, About PDF) - This document presents  guidance on how to conduct effective post construction compliance monitoring, as provided in the 1994 Combined Sewer Overflow (CSO) Control Policy (59 Fed. Reg. 18688), which established a national approach under the National Pollutant Discharge Elimination System (NPDES) permit program for controlling discharges into the nation’s waters from combined sewer systems (CSSs). The CSO Control Policy defines expectations for regulated communities, state water quality standards (WQS) authorities, and NPDES authorities. One of these expectations is that regulated communities should develop Long Term CSO Control plans (LTCP). The ninth element of a Long Term CSO Control Plan is the development of a post construction compliance monitoring program adequate to verify compliance with water quality-based requirements and ascertain the effectiveness of CSO controls. EPA expects, however, that all CSO communities, regardless of whether they have an LTCP, will conduct post construction compliance monitoring.

A Screening Assessment of the Potential Impacts of Climate Change on CSO Mitigation in the Great Lakes and New England Regions (Final Report) (PDF) (50 pp, 339K, About PDF) - This report is a screening-level assessment of the potential implications climate change has had on CSO mitigation in the Great Lakes and New England Regions.

Report to Congress: Combined Sewer Overflows to the Lake Michigan Basin (PDF) (88 pp, 1.51MB, About PDF) - This EPA Report to Congress provides an assessment of the occurrences of combined sewer overflows from Publicly Owned Treatment Works (POTWs) into the Lake Michigan basin.  The Report also provides information about the enforcement of existing regulations concerning discharges and the future steps the EPA plans to take to minimize such overflows.

The Long-Term Control Plan-EZ (LTCP-EZ) Template: A Planning Tool for CSO Control in Small Communities - Planning tool for small communities that must develop an LTCP to address water pollution problems related to CSOs.  This tool provides a framework for the organization and completion of an LTCP. It includes step-by-step instructions and a detailed template.

Using Green Infrastructure     

Financial Capability Assessment Framework (8 pp, 412K, About PDF)

Proposed Peak Wet Weather Discharge Policy

Report to Congress: Impacts and Control of CSOs and SSOs (August 26, 2004)

This page contains information about CSOs, EPA's CSO Control Policy, and current CSO-related program activities.

  • CSO Control Policy - Information on the CSO Policy and a link to the Policy itself.
  • CSO Demographics -  Where cities with combined sewer systems are located.
  • Principal Guidance Documents -  A description of EPA's key guidance documents on CSO control, and links to the documents.
  • Principal Memoranda -  Links to key policy memoranda released since 1994 on CSO program issues.
  • Wet Weather - EPA program activities for other "wet weather" pollution sources such as stormwater and sanitary sewer overflows.

For information on the CSO national program, please contact Mohammed Billah at billah.mohammed@epa.gov.

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