Low DO & ABAC Strategies: Navigating Nitrogen Management Pathways for Seasonal Operations in Coastal Small- to Medium-Size Maine Utilities

Relevance Optimizing nitrogen removal-and phosphorus when required-is critical for meeting tightening regulatory standards, protecting ecosystems, managing operational costs, and ensuring long-term sustainability (Doyle et al., 2025; Jimenez et al., 2024; Leong et al., 2023). Both research and field implementation show that low dissolved oxygen (DO) operation, especially when combined with advanced control strategies like ammonia-based aeration control (ABAC), STAR, and Ammonia versus Nitrate (AVN) control, can significantly enhance biological nutrient removal while reducing aeration energy use (Jimenez et al., 2024; Li et al., 2022). However, full-scale implementation requires careful system design, reliable monitoring, and sufficient operational resources. Currently, of the 15,000+ publicly owned treatment works (POTWs) in the US, only about 40% employ advanced treatment processes, and fewer than 10% are trialing advanced control strategies such as ABAC, STAR, or AVN (US EPA, 2022). Most small, decentralized, and seasonal plants lack a clear understanding of the benefits and potential challenges of low DO strategies, despite their strong potential to deliver energy savings and effluent quality improvements over traditional biological nutrient removal (BNR) processes. Coastal wastewater utilities in Maine, face the dual challenge of increasingly stringent TN regulations and highly variable influent loads due to seasonality, septic flows, tourism, and industrial discharges-all within limited capital budgets. Recognizing these risks, the York Sewer District (YSD) is proactively preparing for compliance and resilience by evaluating targeted low-capex innovations such as low DO strategies, ABAC, STAR, AVN through process simulation. By thoroughly assessing current operational constraints and implementing advanced control strategies, YSD aims to quantify both energy and nutrient compliance benefits, providing a data-driven and scalable pathway toward regulatory readiness for similar smaller utilities. Methodology YSD adopted a simulation-led, multi-scale approach to evaluate low DO and ABAC for nitrogen management: 1.Baseline Assessment: Influent and effluent characterization (flow, COD, BOD, TKN/TN, diurnal patterns) at low, medium, and high-end influent ranges typical of small plants. Aeration system performance and energy use were benchmarked to establish a baseline. 2.Predictive Process Modeling and Scenario Analysis: A calibrated dynamic model simulated plant performance under multiple nitrogen control strategies-conventional DO control, ABAC, and time-variable low DO operations with feedback-factoring in the effects of extreme loading and seasonal variation. 3.DO Setpoint Optimization: A clear operating concept was implemented: front-end aeration zones were maintained at higher DO (1.0–1.5 mg/L) to support BOD removal and nitrification capacity, while back-end zones operated at lower DO (0.3–0.8 mg/L) to enable simultaneous nitrification-denitrification (SND) and minimize air input. 4.Performance Evaluation: Pre- and post-implementation assessments compared energy consumption, effluent quality, and process stability. Cost-benefit analysis will determine the value of low DO/ABAC strategies versus conventional upgrades, with a focus on costs. 5.Monitoring, Risk Management, and Operator Engagement: Pre- and post-implementation assessments compared energy consumption, effluent quality, and process stability. Cost-benefit analysis will determine the value of low DO/ABAC strategies versus conventional upgrades, with a focus on sustainability. Results Modeling predicts that low DO operation (0.8–1.2 mg/L during off-peak periods) combined with ABAC can provide a 15–25% reduction in aeration energy use compared to baseline operations, while supporting SND pathways. Ongoing work, including continued modeling and early operational results, show improved nitrogen removal stability under highly variable influent conditions, associated with extended anoxic volumes and optimized control. ABAC and similar advanced controls are demonstrating potential to reduce over-aeration and keep effluent ammonia within regulatory limits. Cost-benefit analyses, currently underway, will clarify the payback period for investments in advanced control systems (SCADA upgrades and online sensors), aiming to confirm a robust, low-capital pathway for performance improvement. YSD's ongoing efforts focus on continually optimizing controls, monitoring, and operator training to ensure reliable, year-round treatment. This initiative demonstrates the value of evaluating low DO operations and advanced control strategies for small and medium-sized POTWs. Through structured implementation, real-time monitoring, and flexible operations, YSD is charting a model for other utilities to achieve regulatory compliance and adapt to future nutrient management challenges with minimal capital investment.