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Water: Basics

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) (28 pp, 222K, About PDF) provides more information on Water Quality-based permit limits.

Technology-Based Limits

The intent of technology-based effluent limits in NPDES permits is to require a minimum level of treatment of pollutants 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) (37 pp, 166K, About PDF) - provides overview of the process for establishing technology-based effluent limits.

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