SND: Why You Should Believe

INNOVATION AND IMPACT Over the past two years, NEW Water, the brand of the Green Bay Metropolitan Sewerage District, completed full-scale demonstration testing of low dissolved oxygen (DO) setpoints to investigate the dynamics of simultaneous nitrification and denitrification (SND). The testing completed at NEW Water gave insight into: -SND dynamics quantified both in the field and via bench scale testing related to COD availability, DO setpoint impact, and COD type -Confirmation that SND does occur in both the field and bench scale testing -Identification of controlling parameters for SND, including COD:TN, floc size and DO setpoint -Identification of emissions risks, and the importance of SRT and aeration control to minimize emissions With the results from NEW Water, we can start to have a better prediction of the level of SND that will occur in our biological nutrient removal (BNR) designs. INTRODUCTION/BACKGROUND NEW Water is collaborating with Black & Veatch and EnviroMix (Xylem) to pilot a transition from their existing anaerobic-oxic process to an anaerobic-anoxic-oxic (A2O) process at low DO concentrations. The full-scale A2O test basin has an average primary effluent (PE) flow of 7 mgd and includes the addition of EnviroMix mixing technology and aeration and process control strategy (Figure 1). The demonstration at NEW Water has been operational for two years. Three different low DO aeration control strategies have been compared, as shown in Table 1, including static low DO setpoints of 0.3 mg/L and 0.5 mg/L, and FlexZone operation, an aeration and mixing control strategy to dynamically transition DO setpoints. METHODOLOGY Sensors: Two YSI VARiON ion-selective electrode (ISE) sensors measuring ammonia (NH4) and nitrate (NO3) were installed within the demonstration basin to support continuous, in-situ nitrification and denitrification rate calculations. One sensor was located at the end of the anoxic zone, while the second was installed in the middle of subzone 2 (SZ2), approximately one-third of the way through the low DO aeration zone. DO was monitored in each aerated zone using optical sensors, and biomass concentration was estimated using a total suspended solids (TSS) sensor. In-situ Rate Calculations: Probe data was used to calculate in-situ nitrification and denitrification rates occurring in SZ1 and SZ2 under low DO conditions. The in-situ rates were calculated based on NH4 and NO3 values from both sensors on a one-minute frequency and utilizing a TSS probe and assumed MLVSS/MLSS ratio. Bench-Top Rate Testing: The impact of DO setpoint on nitrification rate was measured using benchtop nitrification rate testing. A DO controller was used to maintain a constant DO setpoint during three-hour rate tests ranging between 0.1 mg/L and 4 mg/L. Similarly, the denitrification rate was measured on the bench top with a range of DO setpoints, carbon-to-nitrogen ratios (C/N), and carbon sources, including endogenous decay products (no external carbon), acetate, and a synthetic fermentate mixture. RESULTS Full Scale SND Simultaneous nitrification-denitrification has been observed under conditions where bulk oxygen is limiting, reduced oxygen diffusion into flocs creates a thin aerobic layer over an anoxic core, and organisms with greater oxygen affinity are favored. This has resulted in reported reductions in total effluent nitrogen in the literature (Rieger et al. 2012; Regmi et al. 2022). Here, SND was observed across low DO control strategies. Mass balance calculations indicated SND increased with decreasing DO setpoints (Figure 2). The calculated nitrification and denitrification rates grouped by test phase are shown in Figure 3, with the correlation of nitrification rate and denitrification rate show for different DO bins. Figure 4 shows how the PE COD:TKN related to denitrification rate. Dissolved nitrous oxide (N2O) was also measured throughout testing to understand emission dynamics (Figure 5). Key observations from full-scale SND measurements include: -SND was consistently quantified using overall mass balance calculations in the aerobic zones during low DO testing. -At low DO conditions (<0.5 mg/L), the denitrification rate was consistently between 2 and 2.5 mgN/gVSS-hr despite the nitrification rate ranging from 2 to 6 mgN/gVSS-hr, indicating that denitrification was likely limited by carbon availability. -At higher DO setpoints (>0.5 mg/L), the denitrification rate was strongly correlated to the nitrification rate, with the denitrification rate typically 30% of the nitrification rate. The maximum denitrification rate occurred at high nitrification rates (>6 mgN/gVSS-hr). At this high nitrification rate, the denitrification rate matched those observed at low DO setpoints (approximately 2 mgN/gVSS-hr). This indicates that nitrate availability is a critical aspect of SND at slightly higher DO setpoints. This may be associated with diffusion into flocs or possibly denitrification kinetics. -Although the lower COD:TKN data set was limited, a significant decrease in denitrification rate in the low DO zones was observed when COD:TKN was less than 6 in the PE. Denitrification rates averaged a more similar number at higher COD:TKN. -Emissions were dependent on an ecology that is stabilized for low DO operation. Benchtop Scale Testing Nitrification and denitrification kinetics were further explored with benchtop testing. Nitrification followed a similar pattern to previously reported conditions (Sabba et al. 2024, Jimenez et al. 2024) (Figure 6). Benchtop denitrification rates were also measured at low DO conditions. With no external carbon added, no denitrification was observed in the lab. The greatest denitrification rate was observed with a complex carbon source at the highest C/N ratio. Additionally, when the DO setpoint was above 0.1 mg/L, denitrification was reduced significantly (Figure 7). Key observations from bench-scale SND measurements include: -Nitrification would likely not be limiting until the DO reached 0.1 mg/L. At this DO, the nitrification rate (1.2 mgN/gVSS-hr) is less than the average denitrification rate under low DO conditions -The denitrification rate at 0.1 mg/L is significantly higher than at a 0.25 mg/L DO setpoint and is highly dependent on the availability of carbon. -Interestingly, the denitrification rate in the full-scale (2 to 2.5 mgN/gNSS-hr) at <0.5 mg/L DO is similar to the denitrification rate in benchtop experiments at 0.1 mg/L DO. This may be an indicator that the DO profiles within the aerobic zone are critical, as small pockets of lower DO (~0.1 mg/L) may be a large driver for SND rates in full-scale aeration basins. The benchtop results also highlight the criticality of floc size, as a larger floc may experience more volume at less than 0.1 mg/L and drive higher SND rates. CONCLUSIONS AND SIGNIFICANCE SND was confirmed to be occurring with full-scale data based on lab measurements and online sensors. In addition, bench scale experimentation examined dynamics related to DO concentration, COD availability, and COD type on nitrification and denitrification rates at low DO concentrations (<0.5 mg/L). The results provide insights into why SND has been variable at different sites, and why the occurrence of SND was a hotly debated topic at WEF RBITT 2025. This project provides the following key insights: -Carbon matters. Higher PE COD:TKN ratios resulted in higher rates of denitrification in the low DO zones. When a facility has a COD:TKN ratio of less than 6, it may result in decreased rates of SND. -Denitrification is highly sensitive to DO concentration. In lab tests, the denitrification rate almost disappeared at a DO of 0.25 mg/L, but was similar to the observed rate at a DO of 0.1 mg/L. This relatively small change in DO likely lead to large variations in observed SND rates, and site-specific mixing and airflow distribution likely contribute to variations in observed SND in aeration basins. Maintaining a DO of less than 0.5 mg/L also appears to be a critical control factor. -Nitrification rate influences denitrification rate under low DO conditions. When the DO is greater than 0.5 mg/L, the denitrification rate is correlated to the nitrification rate. This means that SRT and HRT will have a large impact on the SND rates in the system. -Floc size will be a critical factor. This relates to the DO and nitrification rate influences on SND rates. The smaller the floc, the more sensitive SND will be to DO concentration as the micro-gradients in the floc will play a critical role in SND. -SRT and controls matter for SND and emissions. More stable SRTs and allowing for biological adaptation were critical for both SND rates as well as N2O production rates. SND designs should consider the design SRT, control equipment, and control loops carefully to maximize the level of SND occurring in the system.