Beyond Setpoints: Three Full-Scale Low DO Case Studies

APPLICABILITY Aeration is the dominant energy consumer in biological nutrient removal systems, and conventional dissolved oxygen (DO) setpoints near 2 mg/L are commonly used to provide operational robustness. Interest in lower DO operation has rapidly increased to address several drivers, including: reduced energy costs, stringent nutrient limits, and optimized carbon addition. Although low DO operation (<1.0 mg/L) has demonstrated biological feasibility, full-scale adoption still faces practical implementation challenges. These include aeration system and control limitations, instrumentation performance at low DO, uncertainty related to sludge settleability, and reduced nitrification kinetics during cold-weather operation. Over the past two decades, low DO operation has progressed from theory and small pilot studies to full-scale demonstration and implementation. However, most recent conference presentations and publications continue to focus on theory, modeling, and kinetics, with limited attention to practical design and retrofit considerations for aeration hardware, instrumentation, and controls. As a result, there is little consolidated guidance on how to support low DO operation across different climates and drivers, including energy reduction, carbon efficiency, and more stringent nutrient limits. The objective of this work is to help address this gap through three full-scale case studies illustrating low DO implementation under real operating conditions. RESULTS AND OUTCOMES This presentation evaluates three full-scale facilities operating with low DO conditions. Two facilities are flow-through activated sludge systems- Avon Wastewater Treatment Facility (Avon) and Pomona Water Reclamation Plant (PWRP)- and one is a phased isolation ditch facility operated by Boxelder Sanitation District (BSD). Together, these facilities represent a range of process configurations, climates, aeration systems, instrumentation, and control approaches. Key facility characteristics are summarized in Table 1. At each facility, DO targets were reduced from typical operating DOs > 1.5 mg/L while maintaining stable performance. At PWRP, average DO was progressively lowered to an average of ~0.7 mg/L under highly diurnal loading conditions. Performance at PWRP reflects a highly instrumented system with multiple DO and ammonia probes, high blower turndown, and a responsive airflow control. Typical airflow control loop performance at PWRP demonstrates how instrumentation quality and airflow responsiveness enabled stable low DO operation under highly variable loading (Figure 1) At Avon, DO concentrations were reduced from ~2.0 mg/L to ~0.7 mg/L in 2025 using PID-based DO setpoint control, with short-term increases during the peak ski season to maintain winter nitrification capacity. Avon is also configured with external carbon addition to meet stringent nutrient limits, and operating data will be compared over the course of DO reduction. Physical system flexibility, including two droplegs per zone and a nested diffuser grid, supported improved air distribution at low airflow rates while maintaining adequate mixing (Figure 2). At BSD, operators reduced bulk DO to approximately 0.3 mg/L through time-based aeration sequencing and extended anoxic periods. All three facilities maintained compliance with effluent requirements during low DO operation. At PWRP, aeration energy was reduced by approximately 57% (Figure 3). At Avon, overall facility energy use decreased by approximately 33% compared to pre-upgrade baselines. At BSD, aeration energy was reduced by approximately 25% relative to 2016 baseline conditions. Using existing operating data and practical experience from full-scale facilities, this presentation will compare equipment selection across the three sites, including aeration hardware, blower configurations, airflow control components (valves and actuators), diffuser layouts, and instrumentation. The advantages and limitations of the different equipment and operational control strategies will also be evaluated. Results will be used to summarize how equipment selection, process configuration, and control approach influence stable low DO operation and utility outcomes, including aeration energy, external carbon use, and effluent nutrient performance. Aeration energy use will be quantified for each facility in terms of kWh/MG treated and compared to industry average values. RELEVANCE The presentation is relevant to design engineers planning low DO operations at new and existing facilities, aeration retrofits, or control system upgrades; operators managing diurnal and seasonal variability or exploring alternative operating modes; and utility managers evaluating energy reduction strategies, instrumentation, and reduced reliance on external chemicals. Attendees will gain practical insight into aeration system controllability, instrumentation performance requirements, and defining realistic, adaptive operating envelopes based on full-scale experience.