Low-DO BNR: A Research Program Looks at How to Do It Cost-Effectively and at Full Scale

The Water Research Foundation and utility partners are collaborating to fund a study of effective, energy-efficient biological nutrient removal methods.

Low-DO BNR: A Research Program Looks at How to Do It Cost-Effectively and at Full Scale

Stephanie Fevig

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Removal of nutrients is a growing challenge for clean-water plants, especially where effluent phosphorus limits are strict.

Plants today are looking toward biological nutrient removal as a cost-effective and sustainable alternative to chemical addition and filtration.

Now The Water Research Foundation has launched a $1 million research project to develop guidelines for designing, implementing and operating low-energy BNR processes at water resource recovery facilities. The aim is to help utilities reduce the environmental footprint and costs of treatment. The work is funded by WRF with $800,000 in-kind contributions from several utility partners.

For clean-water facilities, nutrient management is key to addressing water quality issues in watersheds challenged with rapid urbanization and growing populations. While conventional BNR technologies are effective, they are costly and energy-intensive.

Led by a principal research team of Dr. Jose Jimenez of Brown and Caldwell, Dr. Belinda Sturm of University of Kansas, and Dr. Leon Downing of Black & Veatch, the project aims to advance low-dissolved-oxygen BNR to intensify treatment processes and significantly reduce energy demands and use of chemicals.

The research will use bench-scale, pilot-scale and full-scale testing at several plants throughout the U.S. to cover multiple influent and operational conditions, with the goal to develop a basic understanding of low-DO BNR. Stephanie Fevig, WRF research manager, talked about the project in an interview with Treatment Plant Operator.

How would you describe the trend in effluent permit limits for nutrients?

Fevig: It differs all around the country. Some utilities still don’t have nutrient limits. Some are just starting to see them on the horizon. In Colorado where I live, we’re looking at limits of 15 mg/L total inorganic nitrogen and 1 mg/L for total phosphorus. But around the corner are even stricter limits, down to less than 2 mg/L total nitrogen and 0.1 mg/L total phosphorus.

Is there any support available to help utilities meet those very low limits?

Fevig: In Colorado, for example, the Colorado Department of Public Health and Environment is incentivizing utilities to start making operational changes now to reduce their effluent nutrient discharges. In return they will gain more time to comply with those more stringent limits in the future. While that is unique to Colorado, I believe other state regulators also see that as an opportunity to nudge facilities to try some new operations to prepare for what is to come.

What is driving the trend toward BNR, and low-DO BNR in particular?

Fevig: When we say BNR we’re talking about removal of both nitrogen and phosphorus. We know those nutrients have a role in the formation of harmful algal blooms and cyanotoxins. Depending on the specific waterway or watershed, there might be a discharge permit limit that water resource recovery facilities have to meet. Those facilities are considered point sources, and so they are regulated, while nonpoint sources might not be. This research is important because a lot of the cost burden is on the utilities to meet those low nutrient limits.

What makes conventional BNR costly?

Fevig: The microorganisms using air to break down organic matter and remove nitrogen need big tanks to allow them to grow and thrive. That’s a significant capital investment. And then the electrical energy to run the blowers that provide oxygen for the microorganisms is a significant line item for operations. Utilities have infrastructure in place to meet certain permit limits, and now they’re asked to do more. So they’re looking to get more out of what they have. They’re interested in learning more about low-DO technologies and how to implement them, so they can save on energy and potentially reduce the amount of carbon or chemicals they need.

Where has low-DO BNR been done successfully so far?

Fevig: A great example is St. Petersburg, Florida, which has been testing and operating a low-DO BNR process at full scale for a few years. Two others are Pueblo, Colorado, and the Trinity River Authority in Texas. As part of our research project, we’ll be gathering data from these facilities to help understand the operational adjustments that are making those processes successful.

In essence, how does low-DO BNR operate?

Fevig: Low DO typically refers to levels less than 1 mg/L, although some folks are saying 0.5 mg/L, and success has been demonstrated even as low as 0.2 mg/L. Lowering DO is potentially an easy change for utilities that have the capability to turn down their blowers. The second piece is understanding carbon management. Carbon is important because the organisms that do denitrification want carbon, as so do those that do phosphorus removal. So there’s a need to make sure enough carbon, and the right kind of carbon, is provided to supply both categories of microorganisms.

Are there any pitfalls to attempting low-DO BNR?

Fevig: One concern is the formation of filamentous organisms that cause bulking and result in poor sludge settling. Some utilities that have tried this have found that their clarifiers blow out and effluent TSS goes up. One way our project is addressing that is to understand the role of carbon. We know that carbon can be used up by organisms that do phosphorus removal in an anaerobic zone, but if too much carbon comes out of that zone into a low-DO zone, that can cause the filaments to thrive. So can we select for organisms that can better take up and utilize that carbon for BNR? What are those mechanisms?

What is the role of the pilot scale component of this research?

Fevig: Three utilities are operating pilot plants: the city of Lawrence, Kansas, the Madison (Wisconsin) Metropolitan Sewerage District and the Hampton Roads (Virginia) Sanitation District. Pilot scale testing offers more flexibility with operational changes; obviously you don’t want to make major changes in a full-scale plant. Pilot operations will allow more detailed analysis. There are also demonstration facilities in Rochester, Minnesota; King County, Washington; and Boise, Idaho.

What is the function of the demonstration sites?

Fevig: What we learn from the full-scale plants will be tested at the demonstration sites. They will test new operating parameters to inform facility designs of low-DO systems.

What is the role of the bench scale side of the research?

Fevig: That work is being done at the University of Kansas and Brown and Caldwell’s lab. That will mainly include bioassay testing and evaluating the kinetic parameters of the process and the carbon utilization, mechanisms, conditions that impact BNR. They will also analyze the samples from the various utilities involved.

How will the result of this research be put to practical use at treatment facilities?

Fevig: The research team will provide a Blueprint Guidance Document that synthesizes all the findings. There are four pieces to that. First, it will explain the fundamental mechanisms of low-DO BNR. Second, there will be a decision tree to help utilities compare low-DO BNR to other low-energy BNR technologies, such as membrane aerated biofilm reactors. A third piece will delve into the design, operation and process modeling of low-DO BNR systems. And finally, we will reach out to utilities and others to share all that information. We expect to publish the final work toward the end of 2023.

How would you describe the Water Research Foundation’s role in the project?

Fevig: We have a Project Advisory Committees of volunteers who are experts in the field and can provide external review. They receive quarterly updates from the research team and will provide peer review of the guidance document. When the report is published, we’ll have a webcast to share the findings, and conduct outreach so that utilities can take the knowledge we’ve gained and run with it.   


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