NPDES Permit Limits: A Primer

The origins of the National Pollutant Discharge Elimination System (NPDES) program can be traced to the 100-year history of fires on Ohio’s Cuyahoga River. After the 13th fire in 1969, the Clean Water Act (CWA) was enacted to protect, restore and maintain water quality. It is the channel through which we have been improving our nation’s waterways ever since. Through Section 402 of the CWA, the NPDES regulations establish the permits program that controls the discharge of pollutants in water.

Today, the Cuyahoga River supports aquatic life and human recreation—thanks in large part to the nearby facilities and entities that obtained, understood and implemented NPDES permits.

Overview of Water Quality Standards

The NPDES program targets the elimination of pollutants in water discharge—both stormwater and wastewater—through establishing permit limits and monitoring practices.

For those new to these programs, some frequently asked questions include: What is the basis for permit limits? Why are some pollutants measured and some are not? How is the allowed discharge concentration established?

Answers to these questions are found by better understanding the designated beneficial use of receiving waters, knowing the common pollutants associated with specific activities and applying the best available technology to remove pollutants from water.

Designated Beneficial Use

Section 401 of the CWA establishes standards to protect the physical, chemical and biological integrity of “Waters of the United States.” Every receiving water has an optimal condition. Some have clear water with rocky bottoms that support trout and cold-water fish populations. Others are muddy, warm and slow-moving, supporting sediment transport to a river delta and providing an ecological niche for catfish.

Therefore, water quality standards are determined by aquatic life use and the use by humans, which may include agriculture, drinking water, industrial supply, fishing and contact recreation. These determinations are made at the state level, based on pollutant criteria relevant to the ecological health of each state’s waterways.

Each use has a set of “pollutants of concern” with associated concentration thresholds, which must be met for the stream to be used safely for its designated beneficial use. We meet these standards by establishing effluent permit limits.

Impairment Status

When a stream’s water quality has a pollutant load for one or more parameters that is higher than the concentration that supports the stream’s designated beneficial use, we define that stream as “impaired.” This can affect permit limits in some significant ways.

Let’s examine this through a fictional ice cream manufacturing facility, “Yummy! Ice Cream Corporation.” Yummy! wants to double production and add a whole new line at its existing facility—which, in turn, triggers a major amendment of its wastewater discharge permit.

Yummy! discharges to “Sweetwater Stream,” designated for aquatic life and recreation. Sweetwater Stream is “impaired” due to degradation of fish and wildlife populations. The “State Environmental Agency” (SEA) has established a Total Maximum Daily Load (TMDL) for Sweetwater Stream and established a waste load allocation for Chemical Oxygen Demand (COD) in authorized discharges.

During permitting, Yummy! identified the presence of COD concentrations in their proposed wastewater discharge. The SEA modeled the impact of the increased COD load to Sweetwater Stream. Then it established a Water Quality-Based Effluent Limitation (WQBEL) for Yummy!’s discharge of COD to Sweetwater Stream. The WQBEL would maintain or restore the physical, chemical and biological health of the fish and wildlife population. This may require Yummy! to utilize a pre-treatment system prior to the discharge to Sweetwater Stream to achieve the reduced COD limits allowed by the NPDES permit.

Exceptional Water Quality

What if Sweetwater Stream had been designated as “exceptional”—a water that exceeds the quality necessary for the propagation of fish, shellfish, wildlife and recreation? Exceptional waters can include drinking water supplies, designated protected waters and others that exceed water quality standards—either on a pollutant basis or a water-body basis.  

To illustrate, imagine the fictional “Sour Gas Company” proposes to operate a compressor station near “Crystal Lake,” the public drinking water supply reservoir for a nearby town. Discharges from the compressor station may include washwater from the cleaning of aprons and trays beneath transfer points, as well as hydrostatic test water discharges from SEA-regulated pipeline cleaning.

In this example, Sour Gas applies for an NPDES permit to discharge to Crystal Lake. Drinking water supplies are one example of waters that are designated as “exceptional quality” and must not be degraded; therefore, Sour Gas may not receive authorization to discharge wastewater into Crystal Lake.

If pollutant loading causes degradation of the water quality standard, the permit will be denied by the SEA. If modeling performed by the SEA of the proposed discharge is acceptable, permit limits will be established to protect the “exceptional” designation of Crystal Lake. (The limits will be protective of the lake, a water-body bases, rather than representative of the company’s discharge.)

If the SEA modeling determines the discharge may cause degradation, Sour Gas may have to route its water to a different watershed, or to discharge to the local, publicly owned treatment works, rather than discharge to Crystal Lake.

Groundwater Protection Zones

While not all groundwater is maintained by an active surface water connection, some aquifers can be affected by impacts at the surface—notably in areas with fissured limestone or sand layers. When this is the case, NPDES permits may contain limits that protect groundwater quality from surface water infiltration.

Consider the fictional “Rocky Cement Factory.” Rocky Cement proposes to build a new kiln over the “Deep Springs Aquifer” recharge zone. NPDES authorizes discharge to surface water; however, when a sensitive groundwater infiltration zone occurs in a wastewater discharge area, overlapping regulations are likely to establish additional effluent limitations and even prohibit certain activities—like underground storage of hazardous substances.

Water pollution abatement in areas of recharge will consider stormwater during and after construction, as well as wastewater disposal through additional NPDES permit constraints. For example, to protect the sensitive recharge zone, Rocky Cement may be required to install management practices to remove at least 80% of the total suspended solids in its runoff and discharges.

Keeping the Water Clean

Thanks in part to lessons learned from the Cuyahoga River, NPDES permitting continues to provide the framework for thoughtful pollutant management and, ultimately, has resulted in cleaner water throughout the U.S. The pollutants of concern, monitoring frequency and permit limits associated with the quality of water discharging from a facility are driven by protecting the physical, chemical and biological integrity of surface waters. Understanding discharge impacts, along with using the best available technology, will result in NPDES permits that continue to help improve our water quality and keep your facility in compliance.

Article originally published in the Spring 2023 issue of Currents. Subscribe today to stay current on environmental insights and regulatory updates that impact your projects.