Anaerobic methane oxidation: High-rate performance of a continuous bioreactor using nitrate and nitrite as electron acceptors

The anaerobic oxidation of methane coupled to denitrification (N-AOM) is an innovative biotechnology able to simultaneously remove dissolved methane and NO3-/NO2-, minimizing the environmental impact of anaerobic effluents discharge. Most N-AOM studies have been implemented in batch reactors. However, studies in continuous systems are required to assess the potential of this biotechnology for full-scale applications. In this work, a continuous N-AOM reactor was implemented with a hydraulic retention time as short as 17 h. Process performance was evaluated using NO3- alone and in a mixture with NO2-. Methane elimination capacity (EC) values from 20 to 39 g m -3h- 1 were reached when NO3- was supplied as the only electron acceptor. When the mixture of NO3- and NO2- was fed, EC values from 48 to 61 g m -3h- 1 were recorded, which constitute the best N-AOM performance so far reported in the literature. The amount of nitrogen directed to N2O formation was always below 0.55% of the total nitrogen removed. Data from the reactor and further batch tests evidenced that the pH value was a critical factor influencing methane oxidation, denitrification, and N2O emission. Microbial charac-terization using 16S rRNA sequencing and metagenomics revealed that the archaeon Methanoperedens nitro-reducens and the bacterium Methylomirabilis oxyfera were not present in the reactor. Instead, bacteria belonging to Proteobacteria and Bacteroidetes phyla, and archaea from the Euryarchaeota and Halobacterota phyla were the most abundant microorganisms. Based on the metagenomic analysis, the potential metabolic mechanisms involved in the N-AOM process were proposed.

Automated summary

The anaerobic oxidation of methane coupled to denitrification (N-AOM) is a promising biotechnology for reducing the environmental impact of anaerobic effluents. In this study, a continuous N-AOM reactor was used to evaluate its performance, achieving methane elimination capacity values of 20-39 g m -3h- 1 with NO3- as the electron acceptor, and 48-61 g m -3h- 1 with a mixture of NO3- and NO2-. The pH value was found to be a critical factor influencing the process. Metagenomic analysis revealed potential metabolic mechanisms involved in N-AOM.