NPDES Permit Limits Additional Resources

• Whole Effluent Toxicity (WET) Guidance
• Watershed- based Permitting
• Nutrient Permitting
• Per- and Polyfluoroalkyl Substances (PFAS)

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WET Guidance

• Methodology

  • Clarifications Regarding Toxicity Reduction and Identification Evaluations in the National Pollutant Discharge Elimination System Program -- The document supplements existing EPA guidance and policies on WET and TREs/TIEs but does not change current policy or legal authority.
    • Clarification Document
    • Transmittal Memo
  • Clarifications Regarding Whole Effluent Toxicity Test Methods Recently Published at 40 CFR Part 136 and Guidance on Implementation of Whole Effluent Toxicity in Permits -- Supplemental memorandum to "Clarifications Regarding Flexibility in 40 CFR Part 136 WET Test Methods," guidance provided in the Technical Support Document for Water Quality-Based Toxics Control, WET Control Policy .
  • Errata for the Effluent and Receiving Water Toxicity Testing Manuals
  • Methods for Aquatic Toxicity Identification Evaluations: Phase I Toxicity Characterization Procedures Second Edition -- Aid in conducting aquatic organism Toxicity Identification Evaluations. Part one of a three part set.
  • Methods for Aquatic Toxicity Identification Evaluations: Phase II Toxicity Identification Procedures for Samples Exhibiting Acute and Chronic Toxicity -- Technical guidance on how to identify the cause of whole effluent toxicity. Part two of a three part set.
  • Methods for Aquatic Toxicity Identification Evaluations: Phase III Toxicity Confirmation Procedures for Samples Exhibiting Acute and Chronic Toxicity - Technical guidance on how to identify the cause of whole effluent toxicity. Part three of a three part set.
  • Toxicity Identification Evaluation: Characterization of Chronically Toxic Effluents, Phase I -- Methods to characterize the chemical/physical nature of the constituents in effluents which cause their chronic toxicity.

• Programmatic

  • Whole Effluent Toxicity (WET) NPDES Spreadsheet -- This spreadsheet can be used to analyze valid acute or chronic WET test data using statistical approaches. The results generated by the spreadsheet can be used by NPDES permit writers for reasonable potential (RP) determinations in accordance with EPA’s TSD (see pages 53-57, Chapter 5) and for NPDES WET compliance determinations (see TSD pages 112-113, Chapter 6). NPDES permittees and WET testing laboratories may also find the spreadsheet helpful when analyzing valid WET test data.
    • Whole Effluent Toxicity (WET) NPDES Spreadsheet
    • Disclaimer: Please note that neither the EPA nor any of its employees assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information disclosed. Furthermore, the NPDES WET Spreadsheet is supplied “as-is” without guarantee or warranty, expressed or implied, including without limitation, any warranty of merchantability or fitness for a specific purpose.
    • Factsheet on the Whole Effluent Toxicity (WET) Analysis Spreadsheet
  • Clarifications Regarding Flexibility in 40 CFR Part 136 Whole Effluent Toxicity (WET)
  • Clarifications Regarding Toxicity Reduction and Identification Evaluations in the National Pollutant Discharge Elimination System Program (03/27/2001) -- Provides additional clarification on conducting toxicity reduction evaluations and toxicity identification evaluations with respect to WET testing. The document supplements existing EPA guidance and policies on WET and TREs/TIEs, but does not change current policy or legal authority.
    • Clarification Document
    • Transmittal Memo
  • Draft National Whole Effluent Toxicity (WET) Implementation Guidance under the NPDES Program (12/28/2004) -- This draft document gathers together previous EPA guidance, policy, and other information and organizes it in one comprehensive document. The document also addresses a number of important issues that have been raised by states, regulated industry, and interested stakeholders on the implementation of WET limits in NPDES permits.
    • Transmittal Memo (PDF)
    • Draft Guidance (PDF)
  • EPA Whole Effluent Toxicity (WET) Control Policy (07/01/1994) -- Policy for the development of effluent limitations in NPDES permits to control WET for the protection of aquatic life
  • Generalized Methodology for Conducting Industrial Toxicity Reduction Evaluations (TREs) (April 1989) -- This document presents a generalized methodology for designing and conducting a TRE and 10 supporting case studies which illustrate various approaches that have been used in the performance of TREs to date.
  • Memorandum: Certification of "Accuracy" of Information Submissions of Test Results Measuring Whole Effluent Toxicity (03/03/2000) -- Clarifies that EPA intends that a certification of "accuracy" occurs in information submissions of testing data for WET, but as the layperson uses the term, not "accurate" as the term is used to describe the quantifiable performance of a measurement system.
  • Method Guidance and Recommendations for Whole Effluent Toxicity (WET) Testing (07/01/2000) -- This document provides guidance and recommendations on the conduct of the approved WET test methods and interpretation of WET test results reported under the NPDES program. This guidance partially fulfills the obligations of a legal settlement agreement that resolves a judicial challenge to the WET final rule. The document provides guidance on the following issues: nominal error rate adjustments, confidence intervals, concentration-response relationships, dilution series, and dilution waters.
  • National Policy Regarding Whole Effluent Toxicity (WET) Enforcement (08/14/1995) -- Memorandum clarifying national enforcement policy related to single exceedances of WET limits and response when toxicity reduction evaluations are inconclusive.
  • National Pollutant Discharge Elimination System Test of Significant Toxicity Implementation Document (06/01/2010) -- The transmittal memo and implementation document provides the basis for implementing the Test of Significant Toxicity (TST) approach under NPDES for permitting authorities (states and regions) and persons interested in analyzing WET test data using the traditional hypothesis testing approach as part of the NPDES Program.
    • Transmittal Memo
    • Implementation Document
  • National Pollutant Discharge Elimination System Test of Significant Toxicity Technical Document (06/01/2010) -- This document provides the technical basis for the TST approach under NPDES for permitting authorities (states and regions) and persons interested in analyzing valid WET test data using the traditional hypothesis testing approach as part of the NPDES Program.
  • Toxicity Reduction Evaluation Guidance for Municipal Wastewater Treatment Plants (August 1999) -- Provides a general framework for conducting TREs at publicly owned treatment works (POTWs) and describes the available methods and procedures. 
  • Toxicity Reduction Evaluation Protocol for Municipal Wastewater Treatment Plants (April 1989) -- Provides a systematic framework for conducting a toxicity reduction evaluation (tre) and a description of the available methods and procedures.
  • Understanding and Accounting for Method Variability in Whole Effluent Toxicity Applications Under the NPDES Program (06/01/2010) -- This document was developed to address questions raised by stakeholders on the issue of analytical variability that might be associated with WET testing and how it relates to the NPDES permit program.
  • Whole Effluent Toxicity (WET) Control Policy (07/01/1994) -- This policy is intended to promote nationwide compliance with statutory and regulatory requirements for the control of WET.

• Case Studies

  • Effluent and Ambient Toxicity Testing and Instream Community Response on the Ottawa River, Lima, Ohio -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system
  • Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact on Five Mile Creek, Birmingham, Alabama -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
  • Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Back River, Baltimore Harbor, Maryland -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
  • Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Kanawha River, Charleston, West Virginia -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
  • Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Naugatuck River, Waterbury, Connecticut -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
  • Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Ohio River, Near Wheeling, West Virginia -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
  • Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Scippo Creek, Circleville, Ohio -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.
  • Validity of Effluent and Ambient Toxicity Tests for Predicting Biological Impact, Skeleton Creek, Enic, Oklahoma -- Study report evaluating relationship of whole effluent toxicity to biological impacts in a freshwater system.

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Watershed-based Permitting

• Guidance

  • Watershed-Based National Pollutant Discharge Elimination System (NPDES) Permitting Implementation Guidance --  Describes EPA's recommended steps and ideas for watershed-based permitting implementation under the NPDES permit program. This approach, aimed at achieving new efficiencies and environmental results, provides a process for considering all stressors within a hydrologically defined drainage basin or other geographic area, rather than addressing individual pollutant sources on a discharge-by-discharge basis.
  • Watershed-Based National Pollutant Discharge Elimination System (NPDES) Permitting Technical Guidance  -- A follow up to the 2003 Watershed-based NPDES Permitting Implementation Guidance, it provides more detail concerning a number of permit development and issuance questions not addressed previously. It focuses on helping NPDES authorities develop and issue NPDES permits that fit into an overall watershed planning and management approach with input from watershed stakeholders. 

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Nutrient Permitting

EPA Resources

• Compilation of Cost Data Associated with the Impacts and Control of Nutrient Pollution -- A compilation of data collected from a range of sources including published, peer-reviewed journals, government-funded research and reports, academic studies and other data sources. This report is a collection of research data from 2000 through 2012 including references to the literature cited. 
• Nutrient Recycling Challenge -- EPA is partnering with pork and dairy producers, the U.S. Department of Agriculture, and environmental and scientific experts to find affordable technologies that recycle nutrients from livestock manure and create valuable products.
• Watershed-based Permitting -- Watershed-based permitting is a process that produces NPDES permits that are issued to point sources on a geographic or watershed basis. Since nutrient pollution often has far-field, cumulative impacts on a waterbody, stakeholders may wish to pursue a watershed-based permitting approach to NPDES permits.
• Water Quality Trading -- Water quality trading under the Clean Water Act is an option for compliance with a water quality based effluent limitation (WQBEL) in a NPDES permit.
• Discharge Monitoring Report (DMR) Pollutant Loading Tool -- The DMR Pollutant Loading Tool helps determine which facilities are discharging, what pollutants they are discharging and how much, and where they are discharging. The tool calculates pollutant loadings from permit and DMR data from EPA's Integrated Compliance Information System for the National Pollutant Discharge Elimination System (ICIS-NPDES). Data are available from the year 2007 to the present.
• National Study on Nutrient Control and Water Treatment Technologies -- EPA is considering a national study on nutrient removal at municipal wastewater treatment plants, also called water resource recovery facilities (WRRFs). This study would be a multi-year effort and fill crucial and fundamental information gaps that exist for nationwide nutrient contribution from WRRFs, and the nutrient control measures and practices available for these facilities. In the fall of 2016, EPA plans to collect basic information from all facilities nationwide to identify the secondary treatment facilities that should be surveyed.

Non-EPA Resources

• Nutrient Modeling Toolkit -- Developed by the Water Environment Research Federation (WERF), the Nutrient Modeling Toolkit is used to quantitatively link nutrient loads to water quality and ecological response indicators on a site-specific basis.

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Per- and Polyfluoroalkyl Substances (PFAS)

• Memorandum: Recommendations from the PFAS NPDES Regional Coordinators Committee Interim Strategy for Per- and Polyfluoroalkyl Substances in Federally Issued National Pollutant Discharge Elimination System Permits (pdf) (254.72 KB, November 22, 2020)  -- This memorandum transmits recommendations developed by the PFAS NPDES Regional Coordinators Committee, a workgroup comprised of EPA Headquarters and Regional contacts.