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National Pollutant Discharge Elimination System (NPDES)

Peak Flows at Sewage Treatment Plants

Many sewage treatment processes may be used for complying with Clean Water Act (CWA) requirements. Most municipalities use a series of unit processes to treat wastewater prior to discharge including the following:

  • preliminary treatment or screening to remove large solids,
  • primary clarification (or preliminary sedimentation) to remove floating and settleable solids,
  • biological treatment (also referred to as secondary treatment) to remove biodegradable organic pollutants and suspended solids, and
  • disinfection to deactivate pathogens.  

Some facilities also provide more advanced treatment which is designed to reduce constituents, such as nitrogen and phosphorus, that are not removed in any significant quantity by traditional biological treatment processes. Some municipalities currently experience high peak influent flows during periods of wet weather that exceed the treatment capacity of existing biological or advanced treatment units.

Under these peak flow conditions, in order to prevent damage to the wastewater treatment plant and maintain future effective operations, some plant operators divert a portion of the flow around biological or advanced treatment units. The diverted flow is then either recombined with flows from the biological treatment units or discharged directly into waterways.

Experts Forum on the Public Health Impacts of Wet Weather Blending

EPA is interested in better understanding the implications of blended discharge effluent from sewage treatment plants, such as wastewater treatment or water resource recovery facilities. On June 19-20, 2014, EPA assembled a group of public health experts to discuss the public health implications of blended effluent discharges from publicly owned treatment works (POTWs) into waterways.

These public health experts were enlisted to ensure that EPA has up-to-date information on the pollutant discharges that may be associated with the different engineering options available to address wet weather blending at POTWs in order to consider the potential public health implications of these different options. The experts forum did not include discussion of the application of the agency’s bypass regulation at 40 CFR 122.41(m) going forward (the bypass regulation prohibits the intentional diversion of waste streams from any portion of a treatment facility except where neccessary for essential maintenance to assure efficient operation). Rather, the forum was solely concerned with the potential public health impacts of blended discharges from POTWs.

  • Draft Literature Review – A draft literature review prepared for the public health experts on blending practices and the discharge of pollutants for different blending scenarios.
  • Introductory Presentation – The engineering experts introductory presentation of technical issues.
  • Forum Summary – The summary identifies major themes expressed by the forum experts but is not a transcript. This document does not represent the views of EPA.

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