National Pollutant Discharge Elimination System (NPDES)

Peak Flows at Sewage Treatment Plants

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Although a number of sewage treatment processes are used for complying with Clean Water Act (CWA) requirements, most municipalities typically use a series of processes to treat wastewater prior to discharge. The typical series of unit processes includes:

  • 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 increased 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, 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.

  • You will need Adobe Reader to view some of the files on this page. See EPA’s About PDF page to learn more.Forum Announcement (PDF) – EPA announcement on April 18, 2014 invited the public to recommend public health and engineering experts to participate in the forum and submit information related to blending.
  • Agenda – Agenda for the June 19-20, 2014 experts forum.
  • 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.

Compendium of technical information on wet weather treatment technologies

One area identified by the experts’ forum in which additional knowledge is needed concerns the nature of effluent from wastewater treatment facilities while they treat wet weather flows. In response, EPA is developing a compendium of technical information to:

  • provide an overview of the spectrum of design and operational options associated with treatment of wet weather flows, and
  • highlight performance data from wastewater treatment facilities as they treat wet weather flows, including those facilities that blend.
  • Draft Summary of Wet Weather Treatment Performance Data - EPA has identified wet weather performance data describing effluent or flows from the following:
    • facilities that blend primary treated flows with flows from biological treatment units,
    • facilities that blend effluent receiving side-stream treatment with effluent from biological treatment units,
    • primary treatment units operating under wet weather condition, including units where chemically enhanced primary treatment is provided,
    • wet weather treatment units that can be used for side-stream treatment, and
    • secondary treatment units that are modified to increase treatment capacity during wet weather conditions.

EPA is interested in obtaining additional published and unpublished data that describe wet weather performance at wastewater treatment plants. EPA will update the summary of wet weather treatment performance data as it becomes available. In addition, EPA is interested in identifying additional sources of information that describe the potential impacts of wet weather discharges from wastewater treatment plants, including blended discharges. Information related to these areas may be submitted to Kevin Weiss by March 1, 2016.

Sanitary Sewer Overflows (SSOs) and Peak Flow Listening Sessions and Workshop

In 2010, EPA requested and received stakeholder input on whether, and how, to modify the NPDES regulations as they apply to municipal sanitary sewer collection systems and SSOs. The Agency also received input on issues associated with diversions at wastewater treatment plants that are recombined with the flows from biological treatment units prior to discharge.

On July 14-15, 2011, EPA held a workshop on SSOs and Peak Flows.