Ammonia removal through combined methane oxidation and nitrification-denitrification and the interactions among functional microorganisms.

It would be highly beneficial to use the methane produced by anaerobic digestion, which is low cost and accessible, as the carbon source in the removal of nitrogenous contaminants in wastewater. However, there is a knowledge gap regarding coupling systems that entail methane oxidation, nitrification, and denitrification, which restricts their industrial application. In this study, we acclimated a mixed culture to deal with simultaneous nitrification-denitrification coupled to methane oxidation in a laboratory-scale hollow-fiber membrane biofilm reactor, which achieved a steady ammonia removal rate of 38.09 mg N/(L•d). Furthermore, a series of batch experiments were conducted to test methane oxidation coupled to nitrate denitrification (AME-D3), nitrite denitrification (AME-D2), and simultaneous nitrification and denitrification (ME-SND). The molar ratio between methane consumed and nitrate reduced (C/N) equals 10 and 5 mol CH4C mol-1 NO3N in AME-D3 and AME-D2, averagely and respectively. Without methane injection, the removal of nitrates and nitrites was very low, indicating that the coupling of nitrate/nitrite denitrification and methane oxidation was beneficial. The average ammonia removal rates in the 20% O2 and 25% O2 groups were 20.06 and 22.03 mg N/(L•d) in the ME-SND system, respectively. Without methane, the ammonia oxidation rate declined, and large amounts of nitrite accumulated. As traditional ammonia and nitrite oxidation approaches are autotrophic, we proposed the possibility of heterotrophic nitrification-aerobic denitrification (HN-AD). To study the coupling systems, the microbial communities and functional bacteria were analyzed. The results indicated that the system contained a guild of methanotrophs (mainly Methylobacter) and HN-AD bacteria (mainly Chrysobacterium and Comamonas).