Measuring In-Situ Nitrification and Denitrification Rates at Low DO

INTRODUCTION/BACKGROUND NEW Water is planning to upgrade the aeration basins at their Green Bay Facility over the next decade. Future permit regulations for total nitrogen (TN) may impact these capital improvement projects. In preparation, NEW Water is collaborating with Black & Veatch and EnviroMix to pilot a transition from an existing anaerobic-oxic process to an anaerobic-anoxic-oxic (A2O) process at low dissolved oxygen (DO) concentrations. The A2O test aeration basin receives an average primary effluent flow of 7 mgd and includes EnviroMix mixing technology and aeration and process control (Figure 1). The demonstration has been operational for two years. In the first year at low DO, simultaneous nitrification-denitrification (SND) was observed via grab samples collected throughout the basin. In the past year, the team has worked on understanding and optimizing SND and developing a novel SND measurement method capturing SND activity in-situ at full-scale using two ammonia and nitrate sensors installed on either side of the low DO zone. This study provides a long-term, longitudinal data set over six months and various low DO control strategies, including static 0.3 mg/L and 0.5 mg/L, and FlexZone control, a strategy to dynamically transition DO setpoints based on influent loading (Table 1). METHODOLOGY Two YSI VARiON ion-selective electrode sensors measuring ammonia (NH4) and nitrate (NO3) were installed to support continuous, in-situ rate calculations. One sensor was located at the end of the anoxic zone, and one was installed in the middle of subzone 2 (SZ2). They recorded NH4 and NO3 concentrations at one-minute intervals, providing high-resolution data. DO was monitored in each aerated zone using optical DO sensors, and biomass concentration was measured using a total suspended solids (TSS) sensor. Sensor data quality was maintained through verification with grab samples collected on a weekly to monthly basis, with matrix adjustments applied as needed. Sensor data was used to calculate in-situ nitrification and denitrification rates in SZ1 and SZ2. The rates were calculated as shown in Figure 2 based on NH4, NO3, and TSS sensor values and a measured MLVSS/MLSS ratio. RESULTS SND has been observed where bulk oxygen is limiting, less oxygen diffusion into flocs creates a thin aerobic layer over an anoxic center, and organisms with greater oxygen affinity are favored (Jimenez et al. 2025). This can result in lower effluent TN outside a designated anoxic zone (Rieger et al. 2012; Regmi et al. 2022). Here, SND was observed in the low DO zone across aeration control strategies. The in-situ nitrification and denitrification rates grouped by test phase are shown in Figure 3. In-situ rates were validated with lab-scale rate tests (results not shown). When the DO setpoint was reduced from 0.5 mg/L to 0.3 mg/L, the nitrification rate stayed the same, suggesting the biomass had fully adapted and/or acclimated to low DO. The nitrification rates are in the upper range of rates observed by Jimenez et al. 2025 across 21 water resource recovery facilities. Low secondary effluent NH4 was maintained throughout the demonstration (data not shown). Additionally, when the DO setpoint was reduced from 0.5 mg/L to 0.3 mg/L the denitrification rate increased. FlexZone operation supported the highest nitrification and denitrification rates, likely because the DO target adjusted according to load. Balancing complete nitrification while achieving denitrification benefits will drive the DO control strategy for full-scale design. The diurnal variation in nitrification and denitrification rates is shown in Figure 4 for a representative weekday (May 9, 2025) with a constant DO of 0.3 mg/L across low DO zones. The peak flow to the basin occurred at 14:00, after which the nitrification rate increased and remained high until the minimum flow occurred at 7:00. At its maximum, the nitrification rate was 32% higher than at its minimum. The denitrification rate did not vary as much, but increased from 0:00 until midday, when the flow was highest. At its maximum, the denitrification rate was 22% higher than at its minimum. Figure 5 shows the relationship between the nitrification and denitrification rate during a stable phase of operation from June 3 to August 4, 2025. The wastewater temperature during this time ranged from 21-24°C. As the nitrification rate increased above 5 mg NH4/g MLVSS-hr, the denitrification rate increased by 17-33%. The nitrate leaving the anoxic zone was consistently less than 0.6 mg/L, and therefore, nearly all nitrate available for denitrification came from nitrification in the low DO zones. CONCLUSIONS AND SIGNIFICANCE Evaluating aeration control strategies to balance nitrification and denitrification at low DO is important for facilities looking to improve TN removal while reducing energy consumption. A novel method of sensor-enabled in-situ rate measurement was advanced, allowing NEW Water to investigate long-term trends in SND. This method can provide a time-saving alternative for SND rate measurement if equipment or personnel are unavailable for lab testing, with the added benefit of live feedback on TN removal. Diurnal variability and a dependence of denitrification rate on the nitrification rate were observed.