Biofilm increases N₂O production in a sidestream partial nitritation system under low dissolved oxygen conditions

Biofilm-based process such as integrated fixed-film activated sludge (IFAS) appears to be a promising alternative to upgrade and retrofit the conventional activated sludge (CAS) technology for sustainable wastewater treatment. However, limited studies have attempted to evaluate the effects of biofilm carriers on the nitrous oxide (N₂O) emissions, which contributes ∼50 % of total greenhouse gas emissions at wastewater treatment plants. In this study, comparisons were conducted between a CAS reactor versus a IFAS reactor, both performing partial nitritation (PN). It was experimentally demonstrated that the biofilm carriers significantly increased N₂O production by 3.04 times in the sidestream PN system under low dissolved oxygen (DO) conditions (e.g., ˂0.5 mg O₂/L) via stimulating the hydroxylamine oxidation and heterotrophic denitrification pathways, which was primarily attributed to the introduced anoxic microenvironments and altered microbial community distributions. When the biofilm carriers were removed from the IFAS reactor, the N₂O emission factor decreased from (1.64±0.04)% to (0.63±0.06)%, and the sludge microbial community evolved towards that of the CAS reactor. In addition, enhancing aeration for higher DO levels would narrow the gaps between two reactors and N₂O production in the IFAS reactor was even 16 % lower at higher DO levels (e.g., 1-1.5 mg O₂/L). This work reveals that biofilm presence stimulates N₂O emissions from the sidestream nitritation process under low DO concentrations, thus informing the development of IFAS technology for efficient pollutant removal with a minimized carbon footprint.