Significance of Biological Hydrogen Oxidation in a Continuous Single-Chamber Microbial Electrolysis Cell

A single-chamber microbial electrolysis cell (MEC) that used a high density of nonmetal-catalyst carbon fibers as the anode achieved high volumetric current densities from 1470 +/- 60 to 1630 +/- 50 A/m(3) for a hydraulic retention time of 1.6-6.5 h. The high current density was driven by a large anode surface area and corresponded to a volumetric chemical oxygen demand (COD)-removal rate of 27-49 kg COD/m(3).d. Observed H-2 harvesting rates were from 2.6 +/- 0.10 to 4.3 +/- 0.46 m(3) H-2/m(3).d, but the H-2 production rates computed from the current densities were 16.3-18.2 m(3) H-2/m(3).d. Tracking all significant electron sinks (residual acetate, H-2, CH4, biomass, and soluble microbial products (SMP)) in the single-chamber MEC showed that H-2 reoxidation by anode-respiring bacteria recycled H-2 between the cathode and the anode, and this caused the large discrepancy in H-2 production and harvest rates. H-2 recycle accounted for 62-76% of observed current density, and this made the observed Coulombic efficiency 190-310% at steady state. Consequently, the cathodic conversion efficiency was only 16-24%. The current density added by H-2 recycle also increased the applied voltage from similar to 0.6 V to similar to 1.5 V for the highest H-2 harvest rate (4.3 m(3) H-2/m(3).d). CH4 generation consistently occurred in the continuous single-chamber MEC, and its electron traction of consumed acetate was 7-25%. Because of methane formation and biomass/SMP accumulation, the overall H-2 recovery was moderate at 1.8-2.0 mol of H-2/mol of acetate in the MEC. Thus, this study illustrates that a single-chamber MEC with a high anode surface area can generate high volumetric rates for COD removal and H-2 generation, but H-2 recycle and methanogenesis present significant challenges for practical application.