Effect of inoculum sources on autotrophic nitrogen removal in anaerobic hollow fiber membrane reactors

Our study aimed to assess the effect of enriched sludge and paddy soil as inocula on autotrophic nitrogen removal using hollow fiber membrane reactors. Mixed sludge from parent reactors and paddy soil were inoculated into two separate upflow anaerobic reactors, UR1 and UR2, respectively. Each reactor was constructed using a plastic cylinder equipped with a U-shaped hollow fiber membrane as a gas diffuser. These reactors were operated for 240 days to compare their nitrogen removal performance under both non-specialized and specialized conditions favoring either anammox, nitrite methane-driven denitrification (MDD), or nitrate MDD processes. UR1 displayed better performance under specialized conditions with higher removal rates of 31.4 mg NH4+-N/L/d, 21.4 mg NO2-N/L/d, and 14.0 mg NO3--N/L/d. Meanwhile, UR2 rapidly adapted to changes and reached a higher nitrogen removal under non-specialized conditions. Furthermore, the dynamics of the microbial communities during the experiment were analyzed in the inocula and biomass after the process. The microbial communities were dominated by bacteria, especially aerobic methanotrophs (e.g., genera Methylocystis and Hyphomicrobium in class Alphaproteobacteria, and genus Methylosarcina in class Gammaproteobacteria). They may directly denitrify or indirectly contribute to denitrification via syntrophic association with heterotrophic denitrifying bacteria (DNB). The higher abundance of DNB (e.g., Gordonia, Ignavibacterium, and Denitratisoma) in UR2 likely contributed to its high adaptation. Archaeal populations were detected in low abundance in both reactors. This study provides evidence for the denitrification capacity of aerobic methanotrophic bacteria, which outnumbered the denitrifying anaerobic methanotrophic bacteria and archaea in hypoxic environments. (C) 2022 The Author(s). Published by Elsevier B.V.