Electroactive microorganisms in bulk solution contribute significantly to methane production in bioelectrochemical anaerobic reactor

[Display omitted] •Polarized bioelectrode enriches electroactive microorganisms (EAM) in bulk solution.•EAM in bulk solution promote interspecies electron transfer for methane production.•EAM contribute to greater methane production in bulk solution than on bioelectrode.•Methane production strongly depends on EAM concentration in bulk solution.•Bioelectrochemical methane production represents a 3-dimensional space technology. The role of anaerobic microorganisms suspended in the bulk solution on methane production was investigated in a bioelectrochemical anaerobic reactor with the electrode polarized at 0.5 V. The electron transfer from substrate to methane and hydrogen were 25% and 7.5%, respectively, in the absence of the anaerobic microorganisms in the bulk solution. As the anaerobic microorganisms increased to 4400 mg/L, the electrons transferred to methane increased to 83.3% but decreased to 0.3% in hydrogen. The electroactive microorganisms (EAM), including exoelectrogens and electrotrophs, enriched in the bulk solution that confirmed by the redox peaks in the cyclic voltammogram was proportional to the anaerobic microorganism. The methane yield based on COD removal was dependent on the anaerobic microorganisms in the bulk solution rather than on the bioelectrode surface. The EAM suspended in the bulk solution are enriched by the polarized electrode, and significantly improve methane production in bioelectrochemical anaerobic reactor.