Full-Scale Testing of Intentional Operational Changes to Influence Nitrous Oxide Emissions

Halton Region collects and treats wastewater from over 600,000 people in Ontario, Canada. In early 2024, the Region installed online dissolved nitrous oxide (N₂O) sensors at their two largest wastewater treatment plants (WWTPs), Skyway and Mid-Halton, and initiated a two-year project to measure and mitigate N₂O. N2O is a potent greenhouse gas (GHG) and the Region's largest emission from wastewater treatment. The objectives of the project include: (1) Measure N2O emission factors for Mid-Halton and Skyway; (2) Identify operating parameters influencing N2O, and (3) identifying low-cost operational optimization N2O mitigation techniques. Results of objectives 1 and 2 were presented at WEFTEC 2025. This presentation focuses on the full-scale mitigation testing (Objective 3) conducted at the Mid-Halton WWTP. Skyway WWTP is testing optimizations not directly related to N2O and therefore not reported here. Table 1 summarizes the operational optimization strategies tested at full-scale at Mid-Halton WWTP with potential to reduce N2O emissions. These strategies were developed and refined with the plant operations staff, as they considered current operational practices and limitations that exist both in operations (i.e. maintaining effluent quality) and physical equipment and control systems. The testing was constrained by the requirement to work within the plant's existing systems; there were no changes to SCADA programming and no capital improvements. As of writing, three of four tests are complete and the fourth will finish imminently (Feb. 2026). Tables 2, 3 and 4 summarize the process changes for each test and the schedule. Some tests included additional field sampling and data collection such as bioreactor dissolved oxygen (DO) profiling. The Region's Technical Services supported operations staff with customized field data collection sheets and standard procedures. Other than some additional grab sampling, the testing was constrained to using the existing data sources and frequency. For example, there was no online ammonia sensor available, only the typical composite sampling for ammonia available 2-3 times per week. Data and information sources included SCADA data for online instrumentation and flowmeters (including N₂O sensors), laboratory data from the Region's laboratory information management system, a plant operations spreadsheet with daily summary operating parameters maintained by plant staff, digital operator logbook, and N₂O sensor maintenance log. An important activity was weekly check-in meetings between plant operations, the Region's Technical Services, and consultant. These meetings provided immediate information about what was happening at the plant, allow fast decisions on adjusting test plans, and give plant staff an open forum for discussing how to run the plant and what changes would be acceptable within their experiences. Increasing the RAS rate from 45% to 75% during test periods 3 and 10 (Figures 1 to 3) saw slight N₂O reductions, however in test period 5 N₂O increased despite the higher RAS likely due to high DO risk. Running the secondary swing zones under anoxic conditions resulted in higher N₂O emissions, not lower (Figures 4 and 5). Although operations staff were able to step feed sufficient carbon to achieve low DO, there was little denitrification and higher nitrite leading to higher N2O. There were also limitations in airflow control – the law of unintended consequences found it was difficult to predict ahead of time all the follow-on effects of operational changes. Optimizing aeration is a common approach in the literature for mitigating N2O. The difficulty at Mid-Halton, however, was the constraints on both the physical airflow distribution (only one airflow controller for the whole train and limits on how the manual dropleg valves could be adjusted) and limits on how SCADA controls could be tweaked. Testing included using one DO probe for control (instead of normal averaging two probes) and adjustments to DO setpoints. Lowering the DO did not provide significant N₂O reductions (Figures 6 and 7). Because airflow cannot be controlled independently at the front and back of the tank, DO remained high in the back, leading to elevated N₂O. Slight reductions were observed when DO setpoints were lowered to 1.6–1.8 mg/L (from the normal 2.0–2.5 mg/L); however, the improvement was marginal, N₂O remained elevated due to high DO risk, and further lowering the setpoint was not recommended operationally. The higher MLSS / higher SRT testing will be finished imminently. Preliminary results (Figure 8) suggest that other factors, such as high DO, overshadowed SRT-driven changes in N₂O. Results of all tests will be fully described in the full paper. Halton Region's N2O project has demonstrated the need for considering site-specific operational practices and limitations. These results indicate it can be challenging to determine the exact cause of N₂O reductions during full-scale operations, as N₂O is highly influenced by multiple parameters. Halton Region's experience demonstrates, however, that to improve things sometimes we need to figure out how to make them worse. Understanding the conditions when N2O emissions have been higher has been a useful exercise. The Region plans to consider the outcomes of the intentional mitigation tests and how their typical operations may be modified for a more sustainable and optimized treatment process.