Continuous anaerobic oxidation of methane: Impact of semi-continuous liquid operation and nitrate load on N_2O production and microbial community

摘要

This work proves the feasibility of employing regular secondary activated sludge for the enrichment of a microbial community able to perform the anaerobic oxidation of methane coupled to nitrate reduction (N-AOM). After 96 days of activated sludge enrichment, a clear N-AOM activity was observed in the resulting microbial community. The methane removal potential of the enriched N-AOM culture was then studied in a stirred tank reactor (STR) operated in continuous mode for methane supply and semi-continuous mode for the liquid phase. The effect of applying nitrate loads of similar to 22, 44, 66, and 88 g NO3- m(-3) h(-1) on (i) STR methane and nitrate removal performance, (ii) N2O emission, and (iii) microbial composition was investigated. Methane elimination capacities from 21 +/- 13.3 to 55 +/- 12 g CH4 m(-3) h(-1) were recorded, coupled to nitrate removal rates ranging from 6 +/- 3.2 to 43 +/- 14.9 g NO3- m(-3) h(-1). N2O production was not detected under the three nitrate loading rates applied for the assessment of potential N2O emission in the continuous N-AOM process (i.e. similar to 22-66 g NO-3 m(-3) h(-1)). The lack of N2O emissions during the process was attributed to the N2O reducing capacity of the bacterial taxa identified and the rigorous control of dissolved O-2 and pH implemented (dissolved O-2 values = 0.07 g m(-3) and pH of 7.6 +/- 0.4). Microbial characterization showed that the N-AOM process was performed in absence of putative N-AOM archaea and bacteria (ANME-2d, M. oxyfera). Instead, microbial activity was driven by methane-oxidizing bacteria and denitrifying bacteria (Bacteroidetes, alpha-, and gamma-proteobacteria). (C) 2021 Elsevier Ltd. All rights reserved.