Understanding Nitrous Oxide (N2O) Impacts on Carbon Footprint When Considering the Full Footprint of Nutrient Removal

Introduction Water resource recovery facilities (WRRFs) have adopted low dissolved oxygen (DO) operation to reduce operating costs of activated sludge processes. However, the relationship between DO levels and greenhouse gas (GHG) emissions is complex and has attracted considerable interest. This modelling study investigated the impact of low DO conditions focusing on nitrous oxide (N2O) emissions, energy consumption (expressed as carbon dioxide equivalent, CO2-eq), and the beneficial nutrient removal, which includes reduced external carbon and chemical inputs. Although low DO operation in Biological Nutrient Removal (BNR) processes may increase the risk of N2O production, the net environmental impact may be surprisingly positive. This presentation will shed light on the carbon footprint (CF) and trade-offs between N2O emissions, energy consumption, and system performance of low DO operation at WRRFs. Materials and methods: A process model was developed within SUMO v.22 simulation platform (Dynamita, France). The process model (Figure 1) was based on an existing WRRF in Texas (USA) and was calibrated and validated utilizing special sampling and historical influent as well as operational data (RAS, MLSS and WAS) obtained from the facility. Dynamic simulations were conducted over a 720-day period using historical data for low DO (0.5 mg/L) and conventional DO (2 mg/L) with two different influent characteristics (carbon rich, high C, and carbon-deficient, low C). The model outputs from the four scenarios were used to compute indirect GHG emissions stemming from electricity and chemicals. The aeration energy necessary to maintain the designated DO levels in the reactors was determined based on airflows derived from the model. Using a conversion factor of 25 scfm/hP, the total blower power requirement was estimated. This blower power was then converted to kWh and multiplied by the daily CO2-eq of consumed electricity. The chemical phosphorus removal process involved the use of alum, while methanol addition was presumed to reduce nitrates to a maximum concentration of 8 mg/L. Daily CO2-eq values for alum and methanol were employed to estimate carbon emissions from chemical consumption, contributing to the calculation of total indirect GHG emissions in a BNR system. Results and Discussion: The results presented in Figure 2 indicate that the cumulative indirect GHG emission rates were consistently lower for low DO operation across all four scenarios, regardless of the influent carbon content. The average emissions for low DO operation were 58% lower than high DO operation (red dashed lines in Figure 2). Figures 3 and 4 illustrate the total GHG emissions from non-N2O sources and reveal that high DO scenarios had a 28% higher average aeration energy, which posed denitrification challenges. To address these challenges, high DO operations required post-anoxic carbon dosing, which increased the CF. Conversely, low DO operations exhibited lower aeration energy and minimal chemical addition, with Case C requiring 98% less carbon dosing than Case A and D requiring 88% less than Case B. Based on blower energy use, external carbon, and chemical coagulant addition operating a BNR system at low DO setpoints results in a significant reduction in CF for the known emission sources. The key question the industry is facing is how much N2O is emitted from low DO setpoints, and if this N2O emission offsets the CF reduction from energy, carbon, and chemical addition. While there has been research investigating the impact of low DO operation on N2O production, the rate of N2O production for low DO system is far from an established metric. For low DO systems to have a net reduction in CF, the resulting N2O emissions would need to be less than the carbon reduction from the known parameters (energy use, chemical addition, carbon addition). If the CO2 reduction is converted to kg N2O, and related to kg N2O production per influent N, an estimation of how much N could be emitted as N2O can be made while maintaining a net CF benefit. As shown in Figure 5 an increase in the N converted to N2O of 0.4 to 0.8% would need to occur before low DO would result in a net increase in CF. Using IPCC guidelines, a typical activated sludge system emits 0.7% of the influent nitrogen as N2O. Therefore, a low DO system would only result in a net increase in CF if the N2O emissions doubled compared to a conventional DO system. Conclusions: Understanding N2O emissions is a major driver for WRRFs. While European and Australian facilities have advanced the understanding on the dynamics associated with N2O production in activated sludge, the impacts of low DO operation on the net CF of BNR facilities needs to be better understood. An important consideration when evaluating the net CF is not just the N2O emissions and energy impacts, but the potential reduction in carbon and chemical addition by moving to lower DO setpoints. The simulation work completed highlighted key aspects of N2O emissions:

*N2O emissions in a low DO system would need to double the N2O emissions from a conventional DO system to offset the carbon emissions savings from energy, carbon addition, and chemical addition savings from low DO operation

*The lower the influent COD, the larger the net carbon emission benefit of operating at low DO concentrations in a BNR basin