Nitrous Oxide Emissions from Two Full-Scale Membrane-Aerated Biofilm Reactors

Abstract

Potential changes of legislation in 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 (EFN2O) 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%). N2O was predominantly emitted in the MABR exhaust gas (NTRexh) and was strongly correlated to the ammonia/um load (NHx,load). Nevertheless, the use of diffused aeration in the MABRs increased the bulk contribution (NTRbulk), impacting the overall EFN2O. A thorough analysis of dynamic data reveals that the changes in the external aeration (EA)/loading rate patterns substantially impacted N2O mass transfer and biological production processes.  Different methods for calculating EFN2O were compared, and results showed EFN2O would range from 0.6 to 5.5 depending on the assumptions made. Based on existing literature, a strong correlation between EFN2O and nitrogen loading rate (R2 = 0.73) was found for different technologies. Overall, an average EFN2O of 0.86 % N2O-N per N load was found with a nitrogen loading rate greater than 200 g N m3 d-1, which supports the hypothesis that MABR technology can achieve intensified biological nutrient removal without increasing N2O emissions.

Keywords: Biofilm, IFAS, MABR, N2O, process intensification