Water Quality and Technology-Based Permitting

Effluent limitations serve as the primary mechanism in NPDES permits for controlling discharges of pollutants to receiving waters. When developing effluent limitations for an NPDES permit, a permit writer must consider limits based on both the technology available to control the pollutants (i.e., technology-based effluent limits) and limits that are protective of the water quality standards of the receiving water (i.e., water quality-based effluent limits).

Water Quality-Based Limits

Permit writers must consider the potential impact of every proposed surface water discharge on the quality of the receiving water. A permit writer may find that technology-based effluent limits are not sufficient to ensure that water quality standards, designed to protect the water quality, will be attained in the receiving water. In such cases, the CWA (section 303(b)(1)(c)) and NPDES regulations (40 CFR 122.44(d)) require that the permit writer develop more stringent, water quality-based effluent limits designed to ensure that water quality standards are attained. Chapter 6 of the U.S. EPA NPDES Permit Writer's Manual [PDF Format] provides more information on Water Quality-based permit limits.

• Developing appropriate effluent limits in NPDES permits is a vital component of the water quality standards-to-permits process.
• Watershed Based Permitting takes into consideration the broader context of the watershed in which the discharge is located.
• Where a watershed is listed as impaired, NPDES permits may need to reflect the results of completed Total Maximum Daily Load (TMDL).
• Whole Effluent Toxicity (WET) - Describes the uses of whole effluent toxicity in NPDES permits to protect the receiving water quality from the aggregate toxic effect of a mixture of pollutants in a discharge.
• EPA is developing options to encourage effluent trading for NPDES permittees.
• Metals Translator: Guidance for Calculating a Total Recoverable Permit Limit from a Dissolved Criterion [PDF Format] - A tool for environmental scientists and permit writers to use in calculating the amount of a metal that may be discharged from a facility to a surface water body based on how the metal is distributed or partitioned between water and sediments in the receiving stream.

Technology-Based Limits

The intent of technology-based effluent limits in NPDES permits is to require a minimum level of treatment of ollutants for point source discharges based on available treatment technologies, while allowing the discharger to use any available control technique to meet the limits. For industrial (and other non-municipal) facilities, technology-based effluent limits are derived by

• Using national effluent limitations guidelines and standards established by EPA, and/or
• Using best professional judgement (BPJ) on a case-by-case basis in the absence of national guidelines and standards.

For municipal facilities (publicly owned treatment works or POTWs), technology-based effluent limits are derived from national secondary treatment standards.

Chapter 5 of the U.S. EPA NPDES Permit Writer's Manual [PDF Format] - provides overview of the process for establishing technology-based effluent limits.

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Last updated on April 09, 2007 2:08 PM
URL:http://cfpub.epa.gov/npdes/generalissues/watertechnology.cfm