National Pollution Discharge Elimination System 101

For Part I, the permit writer identifies the water quality standards applicable to the water body receiving the discharge. These are found in the surface water quality standards established by the primary enforcement agency (e.g. state, territory, or authorized Indian tribes).

For Part II, water quality data on the discharge and receiving water is collected and analyzed. Historical data can be used if collected using appropriate sampling procedures and analytical methods, as evaluated by the primary enforcement agency.

NPDES and primacy agency permits do not have to include effluent limits for all parameters for which there is a water quality standard in a receiving water. All permits will include TBELs, which for municipal wastewater treatment plants includes the secondary treatment standards mentioned above and found at 40 CFR 133.102, at a minimum.

Under Part III, to evaluate the need for WQBELs, regulatory agencies conduct a reasonable potential (RP) analysis. The RP analysis is an evaluation of historical effluent quality data that determines mean concentrations and the statistical variability of those values. If the mean exceeds the water body water quality standard or if the coefficient of variability of the individual values exceeds a given number, reasonable potential exists for the effluent to exceed water quality standards and an effluent limit will be established in the permit for that parameter.

For water bodies that are not meeting their water quality standards, Total Maximum Daily Loads (TMDL) must be determined. A TMDL is the maximum amount of a pollutant that a water body or segment can receive and still meet the water quality standard. The results of the TMDL calculation is used to determine the Waste Load Allocation (WLA), or effluent limit, for each discharger to a water body or segment for the pollutant in question. The states, territories, and tribes prioritize the order for water bodies to conduct TMDL studies using various criteria.

The Part IV calculation is permit-specific and takes a number of factors into consideration, including receiving water body volume or frequency, duration, and magnitude of flow.  A fact sheet issued with the permit discusses how each effluent discharge limit is established. Dischargers are encouraged to review the fact sheet in addition to the permit.

Details on establishing WQBELs can be found in Chapter 6 of the Permit Writers Manual (https://www.epa.gov/sites/default/files/2015-09/documents/pwm_chapt_06.pdf).

It is important to note that the process for drafting and finalizing NPDES (and State issued) permits requires multiple opportunities for internal and public comment. Dischargers are encouraged to carefully review all permit documents and provide comments, as necessary, to the permit issuing agency during the public comment period. Traditionally, there has been little to no opportunity to voice concerns once the public comment period ends.

This article and the previous one emphasize the importance of dischargers knowing their state surface water quality standards and how they are developed. They should also know their own effluent data and how it is used to establish permit limits. The level of the permit limits can have substantial technological and cost implications for the permit holder.

More information can be found at https://www.epa.gov/npdes.