Emissions, Energy, and Performance: Low Energy BNR at Full Scale

BACKGROUND:
NEW Water will upgrade their aeration basins at their Green Bay Facility over the next decade. In preparation, NEW Water is collaborating with Black and Veatch and EnviroMix to pilot a transition from an anaerobic-oxic (AO) process to an anaerobic-anoxic-oxic (A2O) process at low dissolved oxygen (DO). This includes the addition of EnviroMix's mixing technology and aeration control strategy. The setup includes two full-scale parallel aeration basins, designated as test and control, each with its own secondary clarifier and effluent for independent evaluation (Figure 1).

OBJECTIVES:
While many facilities have demonstrated the ability to achieve nutrient removal at low DO setpoints (Water Research Foundation Project 5083 on this topic is ongoing), a side-by-side demonstration to explore not only energy and nutrients, but to understand the impacts of low DO and control strategies on nitrous oxide (N2) emissions and process stability has not previously been completed. This demonstration focuses on three objectives:
- Validate low DO energy savings and nutrient removal
- Understand control requirements and flexibility improvements
- Assess N2 production from low DO systems
APPROACH:
Over seven months, the test basin DO was systematically reduced from an initial 3 mg/L to 0.3 mg/L, followed by four months of FlexZone operation. The control basin operated at a constant DO setpoint of 3 mg/L throughout the demonstration. Table 1 details the progression of DO setpoints in the test basin's first three zones (SZ1, SZ2, SZ3). The fixed AE at the end of the basin operated at a DO between 1.5-3.0 mg/L DO during all test phases. FlexZone operation utilizes a novel aeration and mixing control strategy developed by EnviroMix to transition environments based on changing influent loading. Individual subzones can operate as aerated, unaerated/mixed, or with dynamic DO setpoints. This results in variable DO ranging from 0.2 to 0.8 mg/L.
RESULTS:
Energy savings and effluent performance: Throughout the demonstration, the difference between airflow required per load in the test basin compared to the control basin has increased. During low DO (<0.5 mg/L) and FlexZone operation, a 22-30% reduction in airflow was observed (Figure 2). Figure 3a shows higher phosphate concentrations in the test basin anaerobic zone compared to the control during low DO and FlexZone operation, suggesting more active EBPR, characterized by phosphate release during sCOD uptake. Additionally, effluent phosphorus was more stable in the test basin than the control basin (Figure 3b). A reduction of effluent nitrate in the test basin was observed as DO setpoint decreased (Figure 4). This decrease is attributed to the IMLR and to SND in the low DO zones. Figure 5 shows the calculated mass of denitrification occurring in the anaerobic and anoxic zones compared to the mass due to SND in the low DO zones. NOx removal was calculated using grab samples from each zone. Figure 5 shows an increasing trend of SND within the low DO operation period. The mass of NOx removed by SND is correlated with increasing primary effluent COD load (r2 = 0.72) and specifically, increasing sCOD load (Figure 6). To investigate the fate of dynamic COD fractionation, SND operation, and IMLR ratio, various simulations are in progress (using the newest Sumo version, v24).

Growth pressures: The impact of DO swings on biological growth pressures were assessed. Figure 7 shows preliminary modeling work assessing the impact of DO control on OHO and PAO growth, where instantaneous growth rate swings in a wider range with DO variability. This may be an indication of biological instability. This emphasizes the importance of having a well-tested and well-design low DO controls strategy with DO setpoint ranges in each zone, sensitive and flexible aeration valve control, and proper placement of sensors. The low DO operation has been going through constant testing and optimization in the past four months and improvements have been observed in process stability and effluent quality.

N2 production from low DO systems: N2 assessments in the test basin (two sensors: one in SZ1, one in SZ2) highlighted that emissions initially increased when DO levels were lowered to 0.5 mg/L but subsequently stabilized and returned to levels similar to high DO operation after the system adapted to the new conditions (Figure 8). This indicates that reduced N2 emissions at low DO can be achieved with stabilization over time. A probe failure in August temporarily halted data collection, with monitoring to resume in winter 2025 to assess the impact of low temperatures. Liquid temperature ranged from 16-26&deg;C and SRT from 7-32d.To understand the key factors impacting N2, multiple correlations were performed (data not shown). It was evident that aeration intensity plays a major role in N2 off-gassing, highlighting the importance of stable aeration control in reducing N2 emissions (Figure 9).
CONCLUSIONS:
This study is among the few to use full-scale, side-by-side testing to evaluate low DO impacts and control strategies. Key findings include energy and nutrient removal improvements from low DO operation, such as enhanced EBPR, nitrogen removal, SND, and aeration savings. The study also underscores the necessity of maintaining stable growth conditions in DO control to achieve nutrient removal objectives and mitigation.