NPDES three

The level of treatment required of each discharger is determined by the category of discharge and the condition of the receiving water body. Minimum limits established for a category of discharges regardless of their location are called “technology-based.” Limits driven by the water quality standards and current conditions of the receiving waterbody are said to be “water quality-based.” In the Clean Water Act’s two-part system, each permit must contain limits for each pollutant based on the more stringent of the two types of controls in that particular situation. It is not unusual for a single permit to contain some limits of each type.

“Technology-based” permit limits

Clean Water Act regulations establish minimum pollutant control limits for numerous categories of industrial discharges, for sewage discharges and for a growing number of other types of discharges. Regardless of any site-specific considerations, all dischargers must meet at least the minimum limits that apply to all others in the same category. These limits are said to be “technology-based.” In each category, they represent levels of technology and pollution control performance that the EPA expects all dischargers in that category to achieve.

A “technology-based effluent limit” (TBEL) is simply a minimum level of performance that the EPA or a state permitting agency has decided must be achieved by dischargers in a given category, regardless of the nature or size of the “receiving water.” Decisions about how to best achieve that level of performance are typically left to permitees. Permits may prescribe specific point source control practices or technologies, or best management practices for non point sources, but they seldom do.

Many people understandably (but incorrectly) assume that technology-based limits represent the current state-of-the-art in pollution control technology. The Clean Water Act called for the technology-based limits to be based on the performance of the “best available technology economically achievable” (CWA, Section 301(b)(2)). This has been described as representing the average performance of the best performers. However, over thirty years after the passage of the Act, individual dischargers often are able to achieve a higher level of performance than what is specified by national technology-based limits. While EPA is continuing to develop technology-based limits for the categories of discharges that don’t yet have them, the agency has generally not been updating technology-based limits as technology advances.

Water quality-based permit limits

In many cases, technology-based limits are not enough to protect a receiving water body. If the discharge is large or highly concentrated, or if the receiving water is small or ecologically sensitive, the discharge can easily overwhelm the water body unless the discharger does more than simply meet the minimum technology-based limits.

The authors of the Clean Water Act understood that technology-based limits often would not be enough. Rather than discard the pre-1972 “receiving water quality-based” approach, they improved it by requiring that it be used as a backstop to the technology-based approach. This crucial backstop system requires each discharger to use as much additional treatment as necessary to meet water quality standards for the receiving water itself.

Water quality-based effluent limits (WQBELs) are often required when an applicant requests a permit for a relatively large-volume discharge to a relatively small stream. They may be necessary when the receiving water is naturally limited in its capacity to absorb pollution, or when a water body already carrying a heavy pollution burden cannot absorb any more without violating water quality standards. As pollution pressures in a watershed increase, permit limits are more frequently driven by water quality standards. Unlike the minimum technology-based effluent limits that have an economic feasibility component, water quality-based limits are set to assure that water quality standards (uses, criteria and antidegradation) are not violated, regardless of the economic consequences.

Monitoring compliance with these limits

Most of the monitoring of permitted discharges is done by permittees themselves. State and federal officials make spot checks (often in response to public concerns or questions), but the vast majority of the information about discharges and permit compliance is compiled and submitted by permittees.

Individual permittees submit monitoring reports to the permitting agency. The Discharge Monitoring Reports (DMRs) are typically due monthly. A failure to report properly and on time is a permit violation. The permitting agency compiles DMR information and must make it available to EPA and the public. When violations are evident, enforcement actions are in order. General permits usually do not have similar monthly reporting requirements, but they may have other requirements such as annual reporting.