MABR Biofilm Technology Reduces N2O Emissions: Results from Eight Full-scale Mainstream & Sidestream Applications

Background Nitrous oxide (N2O) is produced during biological nitrogen removal at wastewater treatment plants (WWTPs) (Kemmou, L. and Amanatidou, E., 2023). N2O emissions can contribute up to 75% of greenhouse gas (GHG) emissions from a WWTP (Daelman MR et al., 2013), therefore it is critical for facilities to mitigate N2O emissions to minimize their GHG footprints. Membrane aerated biofilm reactor (MABR) technologies have demonstrated greenhouse gas (GHG) reduction due to their high aeration efficiencies and have also shown great potential for N2O mitigations. However, studies on N2O emissions from MABR technologies were mainly carried out by modelling or using lab-scale systems (He and Daigger, 2023; Bunse et al., 2024). Measurement of N2O emissions from full-scale MABRs is rare (Uri-Carreño et al., 2024; Long, Z. et al., 2023). Although literatures indicated that MABR technologies can reduce N2O emissions, the extent of reduction may depend on many factors. In this paper, N2O emissions from eight full-scale MABR systems are reported with affecting factors examined. The main objectives include, (1) to establish the benchmark(s) of N2O emissions from MABR technologies; and (2) to improve the understanding of how MABR technologies can mitigate the N2O emissions. Methodologies Figure 1 shows the N2O monitoring techniques and the approach to determine the N2O emission factors. Table 1 provides a simplified description for the full-scale MABR plants in this study. Results and discussion 1. N2O emission benchmark for MABR unit process Table 2 lists all available N2O EFs from the study, with the data focusing on MABR unit process. Table 1 indicates that MABR exhaust is the main source of N2O in MABR unit process, with the mean N2O EF of 0.55%±0.65% in MABR exhaust. The total N2O EF from MABR unit process was slightly higher, with the mean N2O EF of 0.61%±0.90%. Comparing with the N2O benchmark from IPCC 2019 (EF of 1.6%), MABR unit process reduced N2O emission by approximately 2/3 to remove the same amount of NH4+-N. In addition, MABR exhaust can be easily collected and the final disposal of N2O in the exhaust would be much more cost effective due to the low volume of the exhaust. Following factors might have contributed to the low N2O EF from MABR tanks. ◠Counter diffusion in MABR biofilm: N2O generated by AOB nitritation and de-nitritation in MABR biofilm will diffuse either across the membrane into the gas phase or across the outer biofilm into the bulk liquid. The outer biofilm might serve as a mass transfer barrier and even a N2O sink, resulting in low N2O transfer into the bulk liquid. ◠Denitrification in the biofilm and/or the mixed liquor: the bulk liquid in MABR tanks is normally anoxic or anaerobic. In addition, carbon source in the feed is often available for the biofilm and suspended biomass if present. The heterotrophic denitrification by the biofilm and/or the suspended biomass would further minimize the N2O concentration in the bulk (Conthe, M. et al., 2019). ◠Intermittent mixing in MABR tanks: the intermittent mixing would minimize the N2O emission into the air. 2. N2O emission affecting factors in MABR unit process Table 3 lists the main operational conditions and biofilm characteristics of MABR unit process at these plants, which were grouped into mainstream and sidestream applications. Among all these conditions, bulk NO2--N concentration is identified as the key factor affecting N2O emission for MABR technologies. This seems reasonable because nitrite affects the activities of all bacteria involved in N2O production (Kemmou, L. and Amanatidou, E., 2023). Figure 2a below show that a linear relationship (R2=0.7682) between the N2O EF and the bulk NO2-N concentration, which varied from 0 to around 120mgN/L. HigherNO2-N concentration results in higher N2O emissions probably because high NO2-N concentration might promote AOB denitritation and inhibit heterotrophs denitritation. Since bulk NO2--N concentration is relatively low in most MABR applications, a plot of the N2O EF at the bulk NO2-N concentration less than 4mgN/L is carried out (Figure 2b). Figure 2b shows that the impact of bulk NO2-N concentration become weaker when it becomes lower than 4mgN/L. 3. Impact of MABR unit process on downstream unit processes MABR unit process might have a positive impact on the N2O emission in the downstream aerated tanks due to process intensification by the MABR biofilm. The monitoring campaign at Site 1, which compares the N2O emissions between two identical lanes (Site 1a and 1b), indicated that the lane with MABR retrofitting (Site 1a) had much lower N2O EF. Further studies should be carried out to confirm and understand the impact of MABR unit process on the whole plant. Conclusions Data from eight full-scale MABR plants demonstrates, 1. MABR technologies reduce N2O emissions by approximately 2/3 compared with IPCC 2019 benchmark. 2. MABR exhaust is the main source of N2O emissions in MABR unit process, making it easy for gas-phase disposal to further reduce N2O emissions. 3. Bulk NO2-N concentration is the key affecting factor for N2O emissions in MABR technologies. However, once it becomes low, the impacts become weaker. 4. Counter diffusion in MABR biofilm plays a critical role in N2O mitigations.