Nitrous oxide emissions from two full-scale membrane-aerated biofilm reactors

The upcoming change of legislation in some European countries where wastewater treatment facilities will start to be taxed based on direct greenhouse gas (GHG) emissions will force water utilities to take a closer look at nitrous oxide (N₂O) production. In this study, we report for the first time N₂O emissions from two full-scale size membrane aerated biofilm reactors (MABR) (R1, R2) from two different manufacturers treating municipal wastewater. N₂O was monitored continuously for 12 months in both the MABR exhaust gas and liquid phase. Multivariate analysis was used to assess process performance. Results show that emission factors (EF_N2O) for both R1 and R2 (0.88 ± 1.28 and 0.82 ± 0.86 %) were very similar to each other and below the standard value from the Intergovernmental Panel on Climate Change (IPCC) 2019 (1.6 %). More specifically, N₂O was predominantly emitted in the MABR exhaust gas (NTR_exh) and was strongly correlated to the ammonia/um load (NH_x,load). Nevertheless, the implemented Oxidation Reduction Potential (ORP) control strategy increased the bulk contribution (NTR_bulk), impacting the overall EF_N2O. A thorough analysis of dynamic data reveals that the changes in the external aeration (EA)/loading rate patterns suggested by ORP control substantially impacted N₂O mass transfer and biological production processes. It also suggests that NTR_exh is mainly caused by ammonia-oxidizing organisms (AOO) activity, while ordinary heterotrophic organisms (OHO) are responsible for NTR_bulk. Different methods for calculating EF_N2O were compared, and results showed EF_N2O would range from 0.6 to 5.5 depending on the assumptions made. Based on existing literature, a strong correlation between EF_N2O and nitrogen loading rate (R² = 0.73) was found for different technologies. Overall, an average EF_N2O of 0.86 % N₂O-N per N load was found with a nitrogen loading rate >200 g N m⁻³ d⁻¹, which supports the hypothesis that MABR technology can achieve intensified biological nutrient removal without increasing N₂O emissions.