Enhancing efficiency and biological nutrient removal in wastewater treatment: Industry lessons about low DO BNR

As wastewater utilities face tighter nutrient limits and rising energy costs, aeration remains one of the most significant operational challenges. Biological nutrient removal (BNR) is essential for protecting waterways from excess nitrogen and phosphorus, but it is also the most energy-intensive process at most Water Resource Recovery Facilities (WRRFs). Conventional BNR systems typically operate at dissolved oxygen (DO) concentrations of 2 mg/L or higher, driving high energy use and operating costs.

Low dissolved oxygen (low DO) treatment is emerging as a proven alternative. By operating at DO concentrations between approximately 0.2 and 1.0 mg/L, utilities are showing they can significantly reduce aeration energy while maintaining reliable nitrogen and phosphorus removal. Carollo’s Michelle Young and Natalie Beach, along with Tim Drescher (Avon Wastewater Treatment Facility) and Craig Hibbard (Boxelder Sanitation District) authored a recent Rocky Mountain Water Magazine article sharing lessons learned from utilities across the United States that have successfully implemented low DO BNR.

What Is Low DO Biological Nutrient Removal?

Low DO BNR allows microbial communities in activated sludge systems to adapt to reduced oxygen availability. With proper acclimation, some nitrifying microorganisms with a higher affinity for oxygen become dominant, enabling stable nitrification at lower DO concentrations. Operating at reduced DO can also promote simultaneous nitrification-denitrification (SND) within aerated zones, increasing total nitrogen removal. At the same time, polyphosphate-accumulating organisms (PAO) are well-suited to low DO conditions due to their efficient oxygen use, which supports enhanced biological phosphorus removal (EBPR). These conditions reduce aeration energy demand, promote greater use of endogenous carbon, and reduce oxygen carryover into unaerated zones.

Facilities can employ a variety of strategies to operate at low DO. This article focuses on three low DO success stories that explore various process configurations, low and high operating temperatures, and different control strategies.

Avon Wastewater Treatment Facility: Flexible Low DO Operations

The Avon Wastewater Treatment Facility serves tourism-driven mountain communities in Colorado, where flows and loadings vary widely throughout the year. After completing major nutrient upgrades in 2023, including installation of nested aeration grids, the facility transitioned gradually to low DO operation, including during low temperature winter months.

Operators found that low DO does not need to be an all-or-nothing strategy. During peak holiday periods, DO setpoints are temporarily increased to maintain capacity, then lowered once peak loads subside. This flexible approach has allowed Avon to capture energy savings most of the year without compromising reliability.

Boxelder Sanitation District: Long-Term Low DO Success

Boxelder Sanitation District has operated under low DO conditions since 2016, initially motivated by a desire to reduce equipment wear and maintenance costs. By shifting to time-based aeration control and gradually extending anoxic periods, operators reduced bulk DO levels to approximately 0.3 mg/L while maintaining total inorganic nitrogen concentrations of about 1.4 mg N/L and total phosphorus concentrations of approximately 0.3 mg/L.

Over time, the biological system adapted, maintaining strong effluent quality while reducing aeration energy use by approximately 25 percent. The Boxelder experience highlights the importance of patience, data-driven adjustments, and allowing sufficient time for microbial acclimation.

Pomona Water Reclamation Plant: Retrofitting for Low DO Performance

At the Pomona Water Reclamation Plant in Los Angeles County, low DO was achieved through targeted retrofits and advanced controls. Upgrades included high-efficiency turbo blowers, precise airflow control valves, and machine-learning-based ammonia-based aeration control.

DO setpoints were reduced gradually over several months, allowing stable biological adaptation. As a result, average DO levels decreased to below 0.5 mg/L, aeration energy demand was reduced by more than 50 percent, and effluent ammonia remained below 2.5 mg N/L and total phosphorus below 0.3 mg/L, well within permit limits.

A Practical Path Forward for Low DO BNR

As Michelle and Natalie demonstrate, low DO is not an experimental concept but a practical, scalable strategy. With thoughtful planning and collaboration between engineers and operations staff, utilities can reduce energy use, maintain compliance, and advance long-term sustainability goals.

Read the full article in Rocky Mountain Water Magazine to explore these lessons in more detail and learn how low DO biological nutrient removal is reshaping wastewater treatment operations.