Roles of oxygen in methane oxidation coupled denitrification in membrane biofilm reactors

With a focus on efficient and low-carbon wastewater treatment, methane driven denitrification has garnered significant attention. Membrane biofilm reactors (MBfRs) are commonly employed to enhance methane utilization. However, previous studies have identified the impact of unavoidable oxygen infiltration on reactor performances without specifying the critical oxygen threshold. In this study, two parallel CH₄-MBfRs with different oxygen intakes were established for mutual control. Three different oxygen concentrations, namely 4.5 ± 0.4 mg/L, 1.7 ± 0.2 mg/L, and 0.1 ± 0.1 mg/L, were defined as Rich, Limited, and Poor-oxygen condition, respectively. Results revealed that the maximum denitrification rate in CH₄-MBfRs occurred under Limited-oxygen condition (220 mg N/L/d), followed by Poor-oxygen (108.1 mg N/L/d) and Rich-oxygen (60.0 mg N/L/d) conditions. Notably, methane driven denitrification consistently resulted in short chain fatty acids (SCFAs) production. The maximum SCFAs accumulation of 664.5 mg/L was also observed under Limited-oxygen condition. Microbial community characterization showed that the biofilm was predominated by Pseudomonas, Paracoccus, Lentimicrobium, Arcanobacterium, and Proteiniphilum. Based on the known metabolism characteristics of these microorganisms, it was assumed that these denitrifiers and SCFAs producers contributed jointly to methane driven denitrification, potentially triggered by oxygen. The findings contribute to understanding the role of oxygen in methane oxidation-coupled denitrification.