Continuous anaerobic oxidation of methane: Impact of semi-continuous liquid operation and nitrate load on N2O production and microbial community

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.

Automated summary

The study demonstrates the feasibility of enriching a microbial community capable of anaerobic oxidation of methane coupled to nitrate reduction using activated sludge. It shows that methane and nitrate removal rates are dependent on the applied nitrate loads, with no N2O emissions detected under certain conditions of dissolved O2 and pH control.