Advancing Low-Energy Biological Nutrient Removal through Low Dissolved Oxygen Operation: Is Biological Selection the Key?

Introduction In the activated sludge process (ASP), dissolved oxygen (DO) is an important factor for aeration cost, nitrification efficiency, nutrient removal, and N2O emission. Even though, the term 'low DO' has been used in a somewhat subjective manner and can vary depending on the context and specific requirements, the overall objective of this study is to pave the way for full-scale implementation of low DO BNR through scientific investigation to systematically answer questions on the impact of DO concentrations on nitrification, denitrification, BPR, microbial ecology, and greenhouse gas emissions. This abstract presents results developed as part of the WRF No. 5083 titled 'Advancing Low-Energy Biological Nitrogen and Phosphorus Removal.' Methodology To quantify the impact and benefits of low DO operation in the capacity and performance of existing ASP, several participating facilities that were operating at low DO and high DO conditions were studied as part of this project. Long-term operational data from the facilities was assessed and compared with the baseline information collected during the literature review. Mixed liquor samples from the participating utilities were collected and assessed through lab-scale studies. Low dissolved oxygen control and batch test procedures were developed as part of the project and used to investigate the impact of low DO on biokinetic rates. This information provided a unique comparison and tabulation of important biokinetic parameters to better understand the implications of low DO BNR on plant capacity and performance. In addition, this provides guidelines for the selection of proper important biokinetic coefficients for process modeling. Results and Discussion Bench-scale, and full-scale experiments were conducted to investigate the impact of DO concentrations on nitrification, denitrification, BPR, microbial ecology, and N2O emissions with the goal to advance low DO BNR. The following biokinetic insights were collected as part of this study. Nitrification: Specific nitrification rates for ASP operating with long-term exposure to low DO conditions were approximately 60-80% of the nitrification rates at high DO as presented in Figure 1. This study concluded that long-term low DO operation results in a shift in the nitrifier community structure, with comammox, and in some case ammonia oxidizing archaea, being selected over ammonia oxidizing bacteria. Experimental results confirm that these communities selected under stable low DO operation have a higher oxygen affinity than AOB and Nitrobacter-related NOB. TIN Removal: The maximum TIN removal rates were found at DO concentrations ranging from 0.2 to 0.5 mg/L. At this DO range, the nitrification rate equals the denitrification rate, which would therefore lead to complete SND in non-carbon limiting conditions as depicted in Figure 2. At DO concentrations of less than 0.2 mg/L, the TINRR is inhibited by nitrification and at DO concentrations higher than 0.5 mg/L, TINRR is limited by denitrification. This observation may change from plant to plant since there are multiple parameters that could affect the results including floc size and porosity, mixing intensity, and microbial population. Biological Phosphorus Removal: Biological phosphorus removal is not impacted by low DO operation. Based on the experimental results from 12 full-scale plants, the low DO phosphorus uptake rates measured in this study were higher compared to the P uptake rates for high DO/ conventional systems. Phosphorus uptake rates slows down significantly when the DO is 0.1 mg/L suggesting that the KDO for PAO is lower around 0.1 mg/L. N2O Emissions: In this work, pilot and bench-scale testing demonstrated that long-term exposure of biomass to low DO does not result in an increase in total N2O emissions (Fig. 3a). Even though this study did not investigate the specific N2O production mechanisms and pathways associated with low DO BNR, the work suggests that the N2O emissions are lower than conventional high DO systems since the relative abundance of AOB and Nitrobacter-related NOB are relatively low (Fig. 3b) limiting the N2O produced via NH2OH oxidation pathway and nitrifier denitrification by AOB. In the low DO BNR systems, Nitrospira-related NOB and comammox were dominant nitrifiers suggesting they do not have the same reduction pathways of NO2 and NO to N2O and therefore produce less N2O comparable to AOB. Conclusion DO is an important parameter influencing aeration cost, nitrification efficiency, nitrogen and phosphorus removal efficiency, treatment plant capacity and N2O emission. As presented in this study, there are several WRRF that utilize low DO operation in the ASP achieving effluent water quality with respect to N and P and require less energy than conventional ASP. The DO setpoints in such plants vary depending on loading conditions, aeration controls, and operator's preferences. Therefore, selecting a single DO setpoint to represent low DO BNR is difficult. Based on the experimental results obtained during this study, low DO can be defined as the bulk liquid DO concentration or range in the aerobic zones where simultaneous N and P removal reactions occur, often ranging from 0.2 mg/L to 0.5 mg/L. Results from this study demonstrate that long-term exposure to biomass at these DO concentrations allow for biological selection of microorganisms with very high oxygen affinities.