Combined Sewer Overflow Control Policy

Policy Framework

Combined sewer overflows (CSOs) are subject to the National Pollutant Discharge Elimination System (NPDES) permitting program, which was created in 1972 through the Clean Water Act to address water pollution. Through the NPDES program, EPA established a national framework for controlling CSOs, called the CSO Control Policy. Communities with CSOs must comply with the CSO Control Policy and have a permit to discharge.

EPA's Combined Sewer Overflow (CSO) Control Policy (pdf) (1.48 MB) provides guidance on how communities with CSOs can achieve Clean Water Act goals in a flexible, cost-effective manner. The CSO Control Policy also defines expectations for regulated communities, state water quality standards authorities, and NPDES permitting authorities. In December 2000, as part of the Consolidated Appropriations Act for Fiscal Year 2001 (P.L. 106–554), Congress amended the Clean Water Act by adding Section 402(q) (33 U.S.C. (§) 1342 [2020]). This amendment is commonly referred to as the “Wet Weather Water Quality Act of 2000.” It requires that each permit, order, or decree issued pursuant to the Clean Water Act after the date of enactment for a discharge from a municipal combined sewer system shall conform to the 1994 CSO Control Policy.

A Phased Approach

The CSO Control Policy has two phases to help communities manage the complexities and costs of reducing and controlling CSOs.

In Phase I, communities with CSOs must:

• Identify and begin implementing nine minimum controls, which are the minimum technology-based controls to reduce water quality impacts. See Combined Sewer Overflows Guidance for Nine Minimum Controls (May 1995) (pdf) (3.69 MB, 832-B-95-003)
• Develop a long-term control plan to achieve compliance with water quality standards and other CSO Policy and Clean Water Act requirements. See Combined Sewer Overflows Guidance for Long-Term Control Plan (September 1995) (pdf) (13.51 MB, 832-B-95-002)
• Submit the long-term control plan to the NPDES permitting authority.

In Phase II, communities with CSOs must:

• Continue implementing the nine minimum controls.
• Implement the long-term control plan.
• Conduct monitoring to verify water quality standards are being met.