Reducing energy costs in wastewater treatment through low DO and suboxic methods approaches

As energy costs rise and environmental regulations tighten, wastewater utilities are exploring new strategies to improve efficiency without compromising treatment performance. One promising approach is operating at lower dissolved oxygen (DO) levels during biological nutrient removal (BNR).

In a recent article published by the California Water Environment Association (CWEA), Carollo’s Michelle Young and Natalie Beach, in collaboration with the Los Angeles County Sanitation Districts (LACSD), share how a pioneering retrofit at the Pomona Water Reclamation Plant (WRP) is showing just how effective low DO and suboxic strategies can be.

Why Lower DO Matters in Wastewater Treatment

Traditional activated sludge systems often maintain DO levels above 2 mg/L to convert ammonia to nitrate. While effective, this approach is energy-intensive, accounting for up to 60% of a plant’s total energy use. Reducing DO targets can significantly cut costs, but making this transition requires the right tools, systems, and strategies to maintain performance.

With funding from the U.S. Department of Energy, Carollo and LACSD partnered to pilot low DO operations at Pomona WRP, a 15 MGD facility already facing the need to upgrade aging, inefficient blowers. The project became an opportunity to transform how the plant delivers nitrogen removal while slashing its energy demand.

Pomona’s Low DO Transformation: Achieving Major Energy Savings

To support low DO operations, Pomona WRP replaced outdated centrifugal blowers with high-efficiency turbo blowers, installed precision air control valves, and upgraded process monitoring equipment. The facility also integrated a machine learning–based control system, DO/Nmaster™, that uses real-time ammonia and DO measurements to dynamically adjust aeration.

By gradually lowering DO setpoints and allowing time for process adaptation, the facility was able to maintain nitrogen removal performance, even under high-influent ammonia loads. “This excellent performance was achieved at the same sludge retention times as before,” said Michelle, “demonstrating that process capacity was not compromised.”

The results speak for themselves. Pomona reduced aeration energy consumption from 1,300 to 550 kWh/MG of influent, putting the facility well below industry averages. High-efficiency blowers contributed a 40% savings, while reduced DO setpoints added another 30% in energy reduction.

The study also revealed operational benefits beyond cost savings. More precise ammonia control could reduce the need for supplemental ammonia during disinfection, opening new doors for process optimization.

Building a Path Toward More Efficient Treatment

Pomona WRP’s experience demonstrates how thoughtful design, modern control systems, and collaborative planning can enable utilities to successfully transition to low DO operations. “This is the first application where state-of-the-art high-efficiency blowers and controls were combined with a focused design to reach low DO operations while maintaining or improving nitrogen and phosphorus removal,” said Beach.

As utilities face mounting energy challenges, these findings provide a scalable blueprint for reducing operational costs while meeting performance goals.

Read the full article in CWEA to learn more about this cutting-edge project and its implications for the future of wastewater treatment.