Getting Control of Low DO with ABAC

ABSTRACT: This study highlights the necessity of controls and process optimization when transitioning from conventional BNR to low energy BNR (low DO) to achieve nutrient removal objectives to allow for biological adaptation to new low DO settings. Key start-up considerations for the ABAC low DO transition include allowing time for ecological adaptation and control system fine-tuning. Although modeling validated the design, the WRRF experienced start-up challenges. Early operation showed promising EBPR and densification, but further on-site testing is needed to optimize P uptake and denitrification. RELEVENCE: The paper will focus on the lessons learned related to the process control of the ABAC Low DO process. It will give insight into other utilities implementing similar intensification projects through challenges and lessons learned. BACKGROUND: The 25 MGD Boulder WRRF recently converted the 4-stage bardenpho process to AO SND process using ABAC (Figure 1). This intensification project was one of the first of its kind to transition to low DO using ABAC with hydrocyclones and tight nitrogen and P limits (Table 1) in a cold-weather environment. The intensified low DO process was selected (1) to obtain P removal without the need of additional tankage and (2) to reduce energy consumption with no internal mixed liquor pumping and lower airflows. During the design, ABAC was selected over constant low DO since modeling showed improved performance with respect to response to ammonia fluctuations using ABAC. METHODOLOGY: This paper describes the challenges with converting the WRRF to low DO operation. Low DO conversion takes time and is especially challenging with daily maximum ammonia and nitrate limits and ongoing construction and maintenance activities. The timeline of the key activities during the upgrade is shown in Table 2. RESULTS: Figure 2 shows the process control and instrumentation for this project. Constant Low DO: To acclimate the MLSS to low DO, the aeration control in Z4-8 was changed to constant low DO in May 2025 and was operated in this mode for about three months. The effluent concentrations were examined to evaluate the SND and EBPR performance of the low DO process. Average effluent ammonia, nitrate, and TP were 0.3, 12.7, and 1.9 mg/L, respectively, during the low DO operation, as presented in Figure 3. The effluent quality is compared with before and after the upgrades and with model predictions for the low DO process. The constant low DO process achieved lower effluent TP and ammonia, but higher nitrate compared with before the upgrades. This shows the adaptation of nitrifiers to the low DO environment and occurrence of EBPR. The model underpredicted effluent TP and nitrate. DO profile for the low DO and ABAC process is presented in Figure 4. ABAC: Upgrade to the ABAC process was conducted in September 2025. The average effluent nitrate and TP concentrations with ABAC were 13.4 and 1.5 mg/L, respectively (Figure 3), exceeding the treatment targets. Table 3 shows the model results for ABAC using default kinetics (ammonia probe in Zone 7 and Zone 8). Process modeling was also conducted using adjusted kinetics for KDO AOB and NOB based on onsite rate testing (Figure 5). Compared with model predictions, the model underpredicted effluent TP and nitrate. The Z9 with supplemental carbon proved to be instrumental in meeting the daily max NO3 limit; however, it also removed SND process volume. In addition, dosing acetic acid in this zone resulted in a secondary release of P, and may interfere with the feast/famine cycle necessary for selecting granules capable of EBPR. Discussion SND: Comparing monthly average effluent NO3 (Figure 3), the NO3 was lower (< 10 mg-N/L) during constant low DO operations compared to the ABAC operation (12-14 mg-N/L). The higher NO3 values indicate that the ABAC set points are not optimal for SND. Testing of different max DO set points in a stepwise manner is underway. Fine tuning of the DO set points is expected to improve denitrification. EBPR: Figure 6 illustrates the correlation between the P uptake and P release mass rates observed. P uptake varied linearly with P release and was 105% of the P release which is lower than the 1.2-1.5:1 typical ratio. This confirms the activity of PAOs but there is opportunity for improvement for P uptake. Figure 7 compares the P release and uptake before and after the upgrade. The constant low DO and ABAC process showed P release and uptake, with higher P uptake for the constant low DO process compared with ABAC. Process optimization for EBPR is required. Process Improvement: Process improvement measures taken for optimization of the new process are presented in Table 4. Biological activity testing is underway (Table 5). Future optimization efforts will inform near-term operational adjustments for improved performance, calibrate the existing plant model, and provide the City with a stronger technical basis for future optimization of bio-P and SND. SUMMARY: While extensive process modeling validated the design, the WRRF faced challenges with start-up and adaptation to the low DO process using ABAC. Start-up and a few months of operation showed that the Low DO SND process achieved EBPR but there is substantial opportunity for improvement. The process successfully achieved densification with biological and physical selection, but onsite testing is critical for understanding P uptake and denitrification.