Guest Column | March 7, 2024

Mixing Zones: Balancing Compliance, Environmental Protection, And Cost Efficiency

By Karoline Qasem

0324-Fehr Graham

Wastewater dischargers can be both environmentally and fiscally responsible by mastering the mix of objectives.

Imagine pouring a tablespoon of chocolate syrup into a tablespoon of milk. The taste is overwhelmingly chocolatey. In a glass of milk, that tablespoon of chocolate syrup makes the drink near perfect. But what happens when you pour that same tablespoon of chocolate syrup into a gallon of milk? You might not even notice it.

That analogy gives insight into the concept behind environmental mixing zones when it comes to waste entering public waters. While the concentration at the discharge point may be too high for a tiny stream, that same amount is diluted and safer in the mighty Mississippi River.

For wastewater facilities nationwide, understanding the dilution process at the point of discharge is key to securing flexibility in environmental permit compliance.

Using Mixing Zones

From an environmental standpoint, a mixing zone refers to an area in a river or lake where wastewater is discharged. As the pollutants move away from the discharge location, they mix with fresh water and become diluted — ideally, this dilution reduces pollutant concentrations to levels that adhere to safety standards, protecting fish, plants, and humans who use the water for recreation.

The idea behind a mixing zone is to offer flexibility to wastewater treatment facilities as they try to meet U.S. EPA guidelines. Wastewater treatment facilities are designed to clean up wastewater before releasing it — in some cases, for longer periods depending on the complexity of the treatment. The goal of the mixing zone is to provide a balance between economic feasibility and environmental protection by the time wastewater reaches the main waterway. Mixing zones allow wastewater facilities to study their impacts on local waterways and determine how much their water needs to be cleaned before it’s safe to discharge. Perhaps it isn’t at a safe level the moment it reaches the river, but if it doesn’t overwhelm the water, it gets to a safe water quality standard as it reaches a permissible distance.

Consider the chocolate milk analogy. That tablespoon of chocolate is concentrated when mixed with the tablespoon of milk, but considerably less intense when mixed with a gallon. If the level of pollutants isn’t too dense for fish to pass through safely, treatment facilities can avoid excess cleaning processes — and higher costs — by proving that discharged pollutants reach an environmentally safe level at a certain distance.

Obtaining Flexibility With Mixing Zones

When a facility applies for a National Pollutant Discharge Elimination System (NPDES) permit, it needs to prove it can meet the stated limit of pollutants discharged into the local body of water. But what the permit writer might not tell you is that, with proper testing and planning, a facility can be approved for a higher pollutant level if it can prove that these pollutants will dilute to safe levels within the confines’ mixing zone.

How large is that zone? It depends on several factors, including the nature of the water body, effluent water characteristics, pollutant properties, the design of the discharge port, etc. It might take a long time for pollutants to dilute in a small stream, while that would happen almost immediately in a large lake.

Any time a mixing zone is an option, it creates the possibility for a community to have substantial savings on unnecessary water cleaning.

Regulatory compliance for mixing zones is complex, focusing not only on cost-effectiveness but also on stringent environmental protection standards. In some cases, having a proper mixing zone might not be enough. In sensitive ecological areas, a facility may be required to conduct a mussel survey to determine if mussels are present beneath the discharge area. This requirement is to assess the impact on these sensitive species, reflecting the larger ecological considerations of these zones. If that’s the case, there is no flexibility with mixing zones, and the permit may not be granted. Fehr Graham’s certified diver conducts mussel surveys, which is a requirement in these situations.

It’s also important to note that not all waterways in all locations qualify for mixing zone options. Regulations differ and are specific to location. Consider a wastewater site slightly upstream from a common swimming nook. A mixing zone likely won’t be an option in that scenario.

Permit renewal requirements also vary. In Illinois, for example, permit holders were asked recently to re-evaluate their mixing zone years to ensure the plan put in place still works. A drought or an influx of pollutants could change the safety levels for a facility.

Calling On Mixing Zone Experts

The complexity of these guidelines, combined with understanding the minimum data requirements for mixing zone studies, suggest that technical professionals should be engaged.

Any time a mixing zone is an option, it creates the possibility for a community to have substantial savings on unnecessary water cleaning.

Experts use sophisticated near-field models such as the Cornell Mixing Zone Expert System to determine the dilution levels in the water as it travels downstream. That helps identify the best way to meet water quality standards, either through implementing best practices or installing diffusers at the outfall point where wastewater enters the river.

Experts can also help write a mixing zone plan for the state environmental regulatory agencies that details not only the data needs but also outline proposed methodologies, data collection strategies, and modeling approach.

The first step in using the flexibility that mixing zones offer is one that many facilities miss — realizing that these options exist. Now that you have moved beyond that barrier, you’re free to bring in experts who can walk you through the steps of testing and planning that will help you obtain an NPDES permit.

The result? A stable ecosystem and substantial cost savings. And that’s a win-win.

About The Author

Karoline Qasem, PhD, PE, is a powerhouse in water resources engineering. At Fehr Graham, she specializes in watershed, water quality, hydrodynamic modeling, regulatory permits, nutrient criteria development, watershed planning, and stormwater management. Her groundbreaking research, particularly at the interface of environmental engineering and ecology, has revolutionized our understanding of urban streams. Reach her at