Continuous anaerobic oxidation of methane: Impact of semi-continuous liquid operation and nitrate load on N2O production and microbial community
2021
Valenzuela, Edgardo I. | Ortiz-Zúñiga, María F. | Carrillo Reyes, Julián | Moreno-Andrade, Iván | Quijano, Guillermo
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 ∼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. ∼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 O₂ and pH implemented (dissolved O₂ 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, α-, and γ-proteobacteria).
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