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Water: Sanitary Sewer Overflows

Sanitary Sewer Overflows and Peak Flows


Properly designed, operated, and maintained sanitary sewer systems are meant to collect and transport all of the sewage that flows into them to a publicly owned treatment works (POTW). However, occasional unintentional discharges of raw sewage from municipal sanitary sewers occur in almost every system. These types of discharges are called sanitary sewer overflows (SSOs). SSOs have a variety of causes, including but not limited to blockages, line breaks, sewer defects that allow storm water and groundwater to overload the system, lapses in sewer system operation and maintenance, power failures, inadequate sewer design and vandalism. EPA estimates that there are at least 23,000 - 75,000 SSOs per year (not including sewage backups into buildings). The untreated sewage from these overflows can contaminate our waters, causing serious water quality problems. It can also back-up into basements, causing property damage and threatening public health.

Additionally, aging sewer line infrastructure in many communities allows rain and snow melt to enter sanitary sewer systems.  During significant wet weather events it is possible for influent flows to exceed the treatment capacity of existing secondary treatment units. Known as "peak flows," these wet weather flows are sometimes diverted around secondary treatment units and then either recombined with flows from the secondary treatment units or discharged directly into waterways from the treatment plant in order to prevent any damage to the treatment facility. 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.


Compendium of Performance Information for Facilities that Blend During Wet Weather – EPA is developing a compendium of performance data from facilities that blend, including those that provide auxiliary treatment and those that do not.

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

Public Listening Sessions Held on SSO Rulemaking and Peak Flows - Find information about the latest SSO and Peak Flow policy discussions.

Proposed Peak Wet Weather Discharge Policy - This policy was proposed and has not been finalized.

Report to Congress: Impacts and Control of CSOs and SSOs

Featured Case Studies, Fact Sheets, and Other Information - Contains a series of case studies that document innovative techniques for addressing overflows of sanitary sewers.

SSO Toolbox - Provides educational materials and other useful links for municipalities looking to EPA for assistance in understanding and adopting parts of the Draft SSO Proposed Rule.

SSO Federal Advisory Subcommittee - Contains information on the activities of the SSO Subcommittee, which was formed in 1995 to provide a forum for stakeholders to identify and evaluate issues associated with the development of an SSO Rule.

Enforcement and Compliance Information - Provides information regarding the Clean Water Act’s enforcement mechanisms in wet weather discharge scenarios.

FAQs - Contains Frequently Asked Questions related to SSOs.  This is the best place to start when looking for general information on EPA's efforts to control SSOs.

Wet Weather Discharges - Contains information on EPA's program activities for "wet weather" pollution sources, which include stormwater runoff, combined sewer overflows (CSOs), and wet weather sanitary sewer overflows (SSOs)

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