National Pollutant Discharge Elimination System (NPDES)

NPDES Permit Limits

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).

Effluent limitations guidelines and standards are established by EPA for different non-municipal (i.e., industrial) categories. These guidelines are developed based on the degree of pollutant reduction attainable by an industrial category through the application of pollutant control technologies. 

Technology-based Effluent Limitations (TBELs)

Technology-based effluent limitations (TBELs) in NPDES permits 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.

NPDES Permit Writer's Manual (Chapter 5) -- Read more about TBELs.

Water Quality-based Effluent Limitations (WQBELs)

Section 303(d) of the Clean Water Act (CWA) establishes a process for states to identify waters within their boundaries where implementing technology-based controls is inadequate to achieve water quality standards. States establish a priority ranking of these waters and, for the priority waters, develop total maximum daily loads (TMDLs). A TMDL identifies the amount of a specific pollutant or property of a pollutant, from point, nonpoint, and natural background sources, including a margin of safety, that may be discharged to a water body and still ensure that the water body attains water quality standards. The allocations of pollutant loadings to point sources are called wasteload allocations. 

Effluent limits in NPDES permits must be consistent with the assumptions used to derive the wasteload allocations. Also, in the absence of a TMDL, permitting authorities still must assess the need for effluent limits based on water quality standards and, where necessary, develop appropriate wasteload allocations and effluent limits. This analysis could be done for an entire watershed or separately for each individual discharge.

Permit writers must consider the potential impact of every proposed surface water discharge on the quality of the receiving water. If TBELs are not sufficient to meet the water quality standards in the receiving water, 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 (WQBELs). 

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