When Less is More (GHGs): Comparing The Carbon Impact of Common Nitrogen Treatment Processes Using ASM2d Models Demonstrates Surprising Tradeoff For Low DO Processes

Introduction
As public agencies make commitments to reducing their climate footprints, the importance of rigorous carbon accounting in wastewater treatment grows. Often, these efforts have focused on plant energy balance, primarily focused on reducing aeration energy use or achieving energy neutrality. While aeration is indeed the largest energy demand at wastewater treatment plants, a plant's greatest climate impact may instead stem from the direct release of greenhouse gasses (GHGs) (Vasilaki et al. 2019). Specifically, methane and nitrous oxide (N2O), two gasses produced during biological treatment, have much higher impact per unit of mass than carbon dioxide released during electrical generation. According to the IPCC, the global warming potential of one unit of methane (on a 100-year timescale) is 21 times higher than CO2, equal to 21 CO2 equivalents (CO2e), while N2O is 310 times more potent than CO2 (IPCC 1996). Off-gas testing for these two GHGs can be expensive and technically challenging, but process models can be used to estimate their production. The present study uses process models to estimate and compare the GHG impact (in CO2e) of various wastewater treatment processes so that utilities can make informed and sustainable choices when selecting process upgrades or retrofits.
Materials and Methods
To compare the overall climate impact of common wastewater treatment technologies, a predictive model for GHG emission was developed and paired with the established IWA ASM2d process model. The GHG model normalized nitrous oxide & methane release, along with energy and chemical (alum, polymer, and methanol) demand, into CO2-equivalent GHG emissions units as defined by the IPCC. This model considered both direct emission of gasses and emission of GHG dissolved in effluent. Model inputs included (1) the dissolved nitrogen gas mass balance, (2) the relationship between dissolved oxygen and nitrous oxide production in biological nitrogen removal (BNR), and (3) the electricity and (4) chemical usage trends. This model was applied using commercial simulator software (MassFlow, UnU Inc.), first to validate the model, and subsequently to compare simulated GHG emissions from 7 representative BNR processes. Simulated conditions were based on a flow of 20,000m3/d and influent characteristics as per table 1. Simulated processes included the Modified Ludzack-Ettinger (MLE) and Anaerobic/Anoxic/Oxic (A2O) conventional activated sludge processes, a membrane bioreactor (MBR) process, integrated fixed film activated sludge (IFAS), biological aerated filter (BAF), aerobic granular sludge (AGS) and Partial Nitritation/Annamox (AMX) processes. GHG production was calculated per unit of total nitrogen removed, which assumed complete conversion of influent ammonium into nitrogen gas. Sidestream nitrogen sources (i.e. centrate from sludge dewatering) were also considered in the process model. Results and Discussion The model-predicted GHG generation per unit of total nitrogen removal ranged from 15.2-15.5 CO2e/kgTNremoved for the MBR, IFAS and ANAMMOX processes, and 11.2 CO2e/kgTNremoved for the BAF process (Fig. 1). In all cases, total GHG impact was dominated by N2O emission (69.2%), while the contribution of methane (7.2%) and CO2 embodied in electrical energy for aeration (22.1%) was minor (Fig 2). Carbon embodied in process chemicals such as coagulants, flocculant polymers and methane (required for denitrification) were nearly negligible at only 1.5% of total GHG impact. Simulated CH4 emission was similar for most processes apart from BAF and anammox (Fig 3). The Anammox process was the largest N2O emitter at 59.8 gN2O-N/kgNHx-Nremoved, and BAF was lowest at 18.4 gN2O-N/kgNHx-Nremoved (Fig 4). Total GHG generation ranged from 18.4-59.8 gN2O-N/kgNHx-Nremoved in all processes. Excluding Anammox, all other processes' emissions matched previously reported ranges of 22.6-48.6 gN2O-N/kgNHx-Nremoved (Vieiral et al. 2019). N2O emissions from BNR processes are affected by pH, water temperature, stripping factors, solids retention time, and dissolved oxygen (DO), but the effect of reactor DO is proportionally large. Previous ASM2d modelling work by Massara et al. (2016) found that emission of N2O peaked at DO concentrations between 1.0-1.5 mg/L, while a later paper by the same author found even higher N2O emissions at even lower DO conditions (0.8-1.1 mg/L). In this study, the BAF process, which was simulated based on installation data from the Proteus biofilter (Tomorrow Water, Anaheim, CA) demonstrated the lowest N2O generation, and subsequently the lowest total GHG impact. This result was driven primarily by high dissolved oxygen concentrations in this process (5-6 mg/L). The complete conference paper will also include empirical data validating the modelled N2O removal in fullscale Proteus biofilters. This study found that while the single-stage Anammox process uses significantly less energy and chemical carbon than other processes, it nevertheless demonstrated the highest GHG impact of all processes modeled, primarily as a result of N2O production in low-DO, high-nitrite conditions within the granule/biofilm. The complete conference paper will compare these results with both modelled and empirical GHG emissions from a two-stage Anammox process, where the partial-nitritation reaction occurs at high-DO conditions, to test whether N2O emission in Anammox systems can be minimized by adjusting the process configuration. This change in process configuration has the potential to shift AMX from the most GHG-intensive option, to one of the least intensive. MBR demonstrated a similarly high GHG impact to Anammox in this model, though this was driven primarily by CO2 emissions embodied in the high electrical energy inputs required to aerate MBRs running at high MLSS, which leads to a degradation in alpha factor.
Overall, this study demonstrates that holistic comparisons of the climate impacts of various treatment processes require integration of (1) existing process models, (2) emissions factors for process inputs (electrical energy and process chemicals), and (3) models of direct GHG emissions (N2O and CH4). Results from this integrated approach suggest that nitrous oxide emissions dominate total GHG impact for most common BNR treatment processes, while electrical energy use remains important in some cases (such as MBR). Results of the GHG emission model were sensitive to changes in process DO, influent nitrogen loading and an empirical N2O emission factor. Since this factor was selected based on off-gas testing results published in previous studies, continued collection of empirical N2O production data from full-scale installations is needed to continue calibrating and improving the underlying GHG emissions models based on real-world data.