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 (NO) production. In this study, we report for the first time NO emissions from two full-scale size membrane aerated biofilm reactors (MABR) (R1, R2) from two different manufacturers treating municipal wastewater. NO 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) 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, NO was predominantly emitted in the MABR exhaust gas (NTR) and was strongly correlated to the ammonia/um load (NH). Nevertheless, the implemented Oxidation Reduction Potential (ORP) control strategy increased the bulk contribution (NTR), impacting the overall EF. A thorough analysis of dynamic data reveals that the changes in the external aeration (EA)/loading rate patterns suggested by ORP control substantially impacted NO mass transfer and biological production processes. It also suggests that NTR is mainly caused by ammonia-oxidizing organisms (AOO) activity, while ordinary heterotrophic organisms (OHO) are responsible for NTR. Different methods for calculating EF were compared, and results showed EF would range from 0.6 to 5.5 depending on the assumptions made. Based on existing literature, a strong correlation between EF and nitrogen loading rate (R = 0.73) was found for different technologies. Overall, an average EF of 0.86 % NO-N per N load was found with a nitrogen loading rate >200 g N m d, which supports the hypothesis that MABR technology can achieve intensified biological nutrient removal without increasing NO emissions.