Halton Region's Operations-Focused Nitrous Oxide Monitoring and Mitigation

Halton Region in Ontario, Canada, collects and treats wastewater from over 600,000 people. The Region's WRRF operations staff have learned that N2O emissions can be an indicator of stress to the biology causing treatment inefficiencies. By adjusting process conditions to remove stressors, staff hope to both optimize treatment and prevent N2O emissions. The Region initiated a two-year project to pursue 'low hanging fruit' N2O mitigation strategies through operational optimization they believe are achievable based on current global experience. The objectives include: (1) Measuring WRRF N2O emission factors; (2) Identifying the operating parameters influencing N2O emissions and identifying low-cost operational optimization mitigation techniques; (3) Verifying the effectiveness of operational changes to reduce N2O emissions.

The Region installed online dissolved N2O sensors at two of its largest facilities. Figure 1 illustrates the locations of dissolved oxygen and N2O sensors. Once N2O emissions are measured within different areas of the plant, actions can be taken to reduce these emissions.

Table 1 summarizes the measured N2O emission factors from the first eight months of monitoring. Interestingly, the emission factors differ dramatically between Halton's two WRRFs and differ from the typical default which reaffirmed their decision to measure N2O.

Operations staff used a standard procedure and field data collection sheet at both plants to conduct quarterly bioreactor nitrogen profiling. The nitrogen concentration results for Mid-Halton WWTP in

#Figure 2 show the highest nitrification rates were at positions 4 and 5 which corresponded to the highest measured N2O concentrations.

#N2O concentrations varied continuously on an hourly, daily, and seasonal basis (Figure 3). High data sampling frequency from the N2O sensors, to capture short-term variation in emissions, presents data collection, handling and manipulation challenges. In addition, five different sources of data from each facility must be compiled together: 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 data which is maintained manually by staff, the plant's digital operator logbook, and the N2O sensor maintenance log.

#Operations staff have observed the solids retention time can significantly influence N2O production. Figure 4 illustrates a period in March 2024 when the mixed liquor concentration at Mid-Halton was being increased in tank #12 in preparation to bring tank #11 back online which had been out of service for several months for repairs. During the initial rise from March 9 to 14 the N2O concentration decreased by about 50%. Then Tank #11 was brought online March 14 which divided Tank #12's mass inventory and the ammonia load was spread across two tanks resulting in lower mixed liquor concentrations and a further 50% decrease in N2O concentration. By the end of March, the N2O concentration was about 85% lower than at the beginning of the month, which clearly demonstrated how operational changes can influence N2O generation.

At Skyway, the Region dewaters biosolids using a centrifuge about 6 hours per day for 5 or 6 days per week. The centrate is stored and then returned to the headworks the same day typically over a four-hour period from 4 to 8pm. This slug load of ammonia reaches the aeration tanks in the late evening around midnight when influent loads are low. The impact of this time-varying ammonia loading is observable in the N2O concentrations of Figure 5. Each day when there is dewatering there is a corresponding spike in N2O generation around midnight, while on the weekend when there is no dewatering there is no midnight N2O spike.

Many of the Region's WRRFs experience seasonal alkalinity deficiency during warmer drier months of the year which results in low effluent pH. The Region observed an inverse relationship between N2O and pH at both plants: higher N2O is seen when effluent pH is lower (Figure 6). This relationship is stronger at Skyway which does not control pH in secondary treatment, while the relationship is not as strong at Mid-Halton where magnesium hydroxide is added as alkalinity supplement (although the dosing is limited to manage operating costs).

Dissolved oxygen (DO) that is too low or too high is the most common N2O risk and optimizing aeration is the most common mitigation strategy. Due to difficulty with aeration turndown and control, both Skyway and Mid-Halton have quite high DO concentrations, routinely >4 mg/L, whereas a DO of 1-2.2 mg/L would be recommended by the Cobalt Water Global N2ORisk AI/ML Decision Support System (Figure 7) being used on this project. The Region observed that changes in N2O concentrations are often aligned with DO changes (Figure 8).

Halton Region's operations team has found value from N2O measurement. The need to collect many data sources on regular basis and review changes in plant operation has been valuable for understanding how its facilities are performing. Operations staff have been keenly interested in the new data and have taken initiative to mitigate N2O emissions, for example at Skyway they are working to manage dewatering centrate returns over a longer duration. The project team is selecting several operational changes that will be field tested during 2025 to assess low-cost N2O mitigation.