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Water: Combined Sewer Overflows

Wet Weather Discharges

Proposed Policy
Peak Wet Weather Discharges from Municipal Sewage Treatment Facilities

EPA proposed for public comment a new policy for addressing very high or "peak" flow events at municipal wastewater treatment plants that are a result of significant storm events.  The policy follows the joint recommendations of the Natural Resources Defense Council (NRDC) and the National Association of Clean Water Agencies (NACWA).  The proposed policy describes limited circumstances when certain management techniques may be used by the operator of a municipal wastewater treatment facility to address very high flows that result from storm events.  The policy also indicates how the management of peak flows must be documented in National Pollutant Discharge Elimination System (NPDES) permits.

Aging sewer line infrastructure in many communities allows rain and snow melt to enter sanitary sewer systems.  During significant storm events, these high volumes can overwhelm certain parts of the wastewater treatment process and may cause damage or failure of the system.  Operators of wastewater treatment plants must manage these high flows to both ensure the continued operation of the treatment process and to prevent backups and overflows of raw wastewater in basements or on city streets.  The proposed policy encourages municipalities to make investments in ongoing maintenance and capital improvements to improve their system's long-term performance.

The policy outlines the limited circumstances when these management techniques can be used and how they must be documented in NPDES permits.  The policy also stipulates that all NPDES permit limits must be met at all times.  The policy encourages further public participation via the National Pollution Discharge Elimination System (NPDES) permit process, and provides for public notification when certain management techniques are used.

The proposed Peak Wet Weather policy is substantially different from the November 2003 proposed "blending" policy.  It requires that discharges must still meet all the requirements of NPDES permits and that operators demonstrate that all feasible measures are used to minimize wet weather problems.  It also prohibits the use of these peak flow management techniques in systems where high peak flows are due to poor system maintenance or a lack of investment in upgrades to improve treatment capacity.  The policy is designed to provide greater national consistency while still incorporating flexibility to recognize site-specific issues.

EPA encouraged interested parties to read the proposed policy and supporting materials and to provide written comments.  Comments have been received or postmarked on or before January 23, 2006.


"Wet weather discharges" refers collectively to point source discharges that result from precipitation events, such as rainfall and snowmelt. Wet weather discharges include storm water runoff, combined sewer overflows (CSOs), and wet weather sanitary sewer overflows (SSOs). Storm water runoff accumulates pollutants such as oil and grease, chemicals, nutrients, metals, and bacteria as it travels across land. CSOs and wet weather SSOs contain a mixture of raw sewage, industrial wastewater and storm water, and have resulted in beach closings, shellfish bed closings, and aesthetic problems.

Under the NPDES permit program, there are three program areas that address each of the wet weather discharges described above. (NPDES requirements from runoff from concentrated animal feeding operations (CAFOs) are described elsewhere) These programs share a range of cross-cutting issues and affect a similar group of stakeholders. EPA believes that wet weather discharges should be addressed in a coordinated and comprehensive fashion to reduce the threat to water quality, reduce redundant pollution control costs, and provide State and local governments with greater flexibility to solve wet weather discharge problems. To identify and address cross-cutting issues and promote coordination, EPA established the Urban Wet Weather Flows Federal Advisory Committee in 1995.


The links presented below provide more information related to the three NPDES program areas that address wet weather discharges, the Urban Wet Weather Flows Federal Advisory Committee, and wet weather flows research.

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