Magnetite-mediated electrically connected community for shortening startup of methane-dependent denitrification in a membrane biofilm reactor

Denitrifying anaerobic methane oxidation (DAMO) not only mitigates methane emission from wastewater treatment, but compensates for the deficiency of current nitrogen removal process. However, the doubling time of DAMO microbes is usually more than two weeks, resulting in a long-period startup of a methane-dependent denitrification reactor especially with anaerobic sludge as inoculum. The study presented here found that, in the presence of magnetite, the nitrate removal rate reached 17.6 +/- 0.4 mgNO3- -N/L/d after the 120-day experiments, which was approximately 2.4 folds higher than that in the absence of magnetite (7.3 +/- 0.1 mgNO3- N/L/d). The nitrate removal rate in the presence of magnetite reached 7-8 mgNO3- -N/L/d at day 80-85, which shortened the startup periods of approximately 35-40 days. Microbial morphology based on scanning electron microscope and transmission electron microscope showed that, the surface of cells was deposited by magnetite and they might be connected via magnetite. Further investigations via cyclic voltammetry and electrochemical impedance spectroscopy showed that magnetite mediated an electrically connected community with a higher capacitance and lower resistance than that in the absence of magnetite. Microbial community analysis showed that, in the presence of magnetite, the potential DAMO bacteria (Methylacidimicrobium and Methylotetracoccus species), methanogens (Methanothrix species) as well as nitrate-reducing bacteria (Bellilinea and Dechloromonas species) were enriched. They might participate in direct interspecies electron transfer (DIET) via magnetite substituting for c-type cytochromes as the electrical connection components, since methane oxidation in the presence of magnetite was insensitive to the external addition of potential intermediates, such as hydrogen.

Automated summary

The presence of magnetite can enhance nitrate removal rate, shorten the startup period of DAMO reactor, and promote the formation of an electrically connected microbial community. The microbial community in the presence of magnetite is enriched with potential DAMO bacteria, methanogens, and nitrate-reducing bacteria, which may participate in direct interspecies electron transfer via magnetite as the electrical connection component.