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Water: Water Quality Standards Academy

Basic Course: Supplemental Topics (NPDES Permit 9)

NPDES Permit Program

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Effluent Limits: Water Quality-Based Determination

In contrast to making the determination about technology-based effluent limits, when the State/Tribal permit writer moves on to consideration of water quality-based limits, the assessment becomes site specific. That is, potential impacts on the actual receiving waters of the discharge must be assessed in regard to the jurisdiction’s relevant water quality standards.  

Key Point. Water quality-based effluent limits that are established in an NPDES permit apply at the discharge point, generally referred to as the “end of the pipe.” In contrast, water quality standards apply to an entire water body or segment. Thus, as necessary based on an analysis of the impact of pollutants on a receiving water, the permit writer needs to be able to translate the State/Tribe’s water quality standards into water quality-based effluent limits.

The bases for translating water quality standards to effluent limits are the water quality criteria in the State/Tribe’s water quality standards. Water quality criteria are typically expressed using a magnitude, duration, and frequency that differ from the averaging periods for effluent limits.

Water Quality Criteria Effluent Limits
Apply in the receiving water and could include:
  • Magnitude
  • Duration
  • Frequency
Apply at “end of the pipe” and include:
  • Magnitude
  • Averaging period
For example: An acute aquatic life criterion might be expressed as a magnitude of 10 ug/L, for a duration not to exceed 1 hour, at a frequency not to recur more than once in 3 years. For example: 1 mg/L, to be determined over a specified averaging period, such as a daily maximum or a monthly average.

Thus, when establishing effluent limits, the NPDES permit writer needs to follow a procedure that accounts for both the physical and chemical interactions between the effluent and the receiving water. In addition, the procedure must take into account the varying units of expression between the criteria in the water quality standards and the effluent limit to be included in the permit.

For informational purposes only–Not official statements of EPA policy.

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