Long-Term Dissolved-Phase Monitoring of N2O With Gas-Phase Validations During Low DO Operation

[b]Introduction[/b] In 2022, the Los Angeles County Sanitation Districts initiated a project to implement model-predictive aeration control (MPAC) at the Pomona Water Reclamation Plant (PWRP) to support transition to low dissolved oxygen (DO < 1 mg/L) operation. While low-DO operation can reduce aeration energy demand and associated indirect greenhouse gas emissions, it may also increase direct process emissions of nitrous oxide (N₂O). To evaluate this potential tradeoff, continuous dissolved-phase N₂O monitoring was initiated in October 2023 using Unisense sensors. Measurements were validated through seasonal off-gas testing using a photoacoustic spectroscopy (PAS) analyzer connected to a surface emission isolation flux chambers (SEIFC), supported by laboratory analyses of off-gas grab samples. This paper presents (1) monthly N₂O emission factors (EFN₂O%) during the transition to low-DO operation, (2) long-term site-specific EFN₂O% values compared with the Intergovernmental Panel on Climate Change (IPCC) default factor, (3) dissolved- and gas-phase sensor validation results, and (4) operational parameters associated with elevated N₂O emissions. [b]Plant Background[/b] PWRP is a 15-MGD tertiary facility producing ~8 MGD of recycled water (Figure 1). Secondary treatment uses a Modified Ludzack–Ettinger (MLE) process with internal recycle and fine-bubble aeration, and the plant typically operates at an SRT of ~9 days. [b]Methodology[/b] Continuous dissolved-phase N₂O monitoring was conducted in the aerobic zone at the PWRP from October 2023 through October 2025. During this period, aerobic-zone DO levels were gradually reduced from >3 mg/L to <1 mg/L as part of the transition to low DO. Two monitoring locations were selected in aerobic zone Grids 2 and 3 (Figure 2). In Grid 3, the Unisense sensor was installed adjacent to an SEIFC (Figure 3), which was used to collect off-gas emissions during seasonal validation campaigns. Each campaign consisted of at least five days of continuous monitoring. [b]Long-Term Monitoring Results - Key Findings[/b] Transition to Low DO Operation: DO levels in Grid 2 were gradually reduced beginning in October 2023 following implementation of MPAC (Figure 4), while DO in Grid 3 remained relatively constant at ~1 mg/L throughout the study (Figure 5). Low-DO operation was achieved by June 2024. During this transition, the monthly average N₂O emissions factor did not show a notable increase (Figure 6). Long-Term Emissions Factor: Based on available data from October 2023 through November 2024, data collected from the dissolved-phase sensors yielded an average EFN₂O% of 0.10% ± 0.08%. This period represents performance under intended operating conditions. This value is well below the IPCC default emission factor of 1.6% for centralized wastewater facilities with aerobic treatment. Even if a conservative correction factor of 1.48 is applied, based on the highest correlation slope observed during the validation campaigns discussed below, the adjusted EFN₂O % value would be ~0.15%, which remains far below the IPCC default. Impact of Nitrite and SRT: From January 2025 to March 2025, increasing nitrite concentrations were observed in Grid 3 with a corresponding decrease in nitrate concentrations, suggesting impaired nitrification performance (Figure 7). During this period, an increase in the N₂O emissions rates was observed, however the monthly average EFN₂O% remained below the IPCC default value. Nitrite accumulation is widely reported as a key indicator and precursor of elevated N₂O formation in activated sludge systems (Ahn et al., 2010). Between June 2025 and September 2025, the effective SRT was inadvertently reduced from ~9 to ~6 days (Figure 8) when a secondary treatment tank was taken out of service, decreasing the effective process volume while the wasting rate remained unchanged. During this lower-SRT period, increased nitrite and decreased nitrate concentrations were observed in the aerobic tank mixed liquor (Figure 7), indicating impaired nitrification performance. N₂O emissions during this period exceeded the IPCC default emission factor. Emissions during subsequent process recovery continue to be monitored and will be reported in the final paper. [b]Sensor Validation Results [/b] Three validation campaigns were completed in June 2024, February 2025, and June 2025 (Figure 9-11) with a fourth scheduled for February 2026. Across all campaigns, PAS and dissolved-phase Unisense measurements showed strong correlation (R² = 0.87–0.92); however, regression slopes of 1.48, 1.04, and 1.27 indicate that PAS-based emission estimates were consistently higher than those derived from dissolved-phase measurements (Figures 12-14). Off-gas grab samples analyzed by a certified laboratory showed closer agreement with PAS measurements than with dissolved-phase projections. Unisense emission estimates rely on conversion using a mass transfer coefficient (kLa) which uses an empirically fitted proportionality constant derived from Unisense's lab-scale experiments. The near-unity slope observed during the February 2025 campaign (1.04) suggests that, under some operating conditions, the empirical kLa can reasonably approximate effective mass transfer rates in the field. Differences in regression slopes across campaigns likely reflect uncertainties with kLa, sensor calibration, and sensor accuracy near detection limits.