Effect of Hydraulic Retention Time on Nitrate Removal Through Methane Oxidation Coupled with Denitrification in Membrane Biofilm Reactor After Air Ingress

Greenhouse gas generated from wastewater treatment plants has attracted much attention as it has the potential to be recovered and used as an energy source. In this study, a membrane biofilm reactor was designed to simultaneously enhance nitrate removal and reduce methane (CH4) emissions during methane oxidation coupled with the denitrification process. The enrichment of CH4-driven denitrification microbes with a relatively short hydraulic retention time (HRT) and its effects on the stable operation of the reactor were studied within 250 d. With an increasing HRT from 8 to 20 h, a removal rate of up to approximately 0.51 mg/L·h−1 was achieved, which also kept the effluent NO2−-N below 0.5 mg/L. Microbial community analysis showed that the diversity and uniformity of microorganism communities decreased with the addition of CH4 as a carbon source, and the microbial structure changed significantly. Compared with that of seed sludge at the phylum level, the relative abundance of Proteobacteria increased significantly, Alphaproteobacteria and Sphingobacteriia continued to become enriched, and the abundance of Methylocystis increased significantly. Neither denitrifying anaerobic methane oxidation (DAMO) archaea nor bacteria were found in the sequencing analysis. Methylocystis was the dominant CH4 oxidizing bacteria, in synergy with the co-occurrence of autotrophic and heterotrophic denitrifying bacteria, which likely join up in nitrogen removal. Unlike the systems described in most methane-driven denitrification studies, our system achieved nitrate removal without detectable DAMO microbes.