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), CH(4), 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 approximately 0.6 V to approximately 1.5 V for the highest H(2) harvest rate (4.3 m(3) H(2)/m(3).d). CH(4) generation consistently occurred in the continuous single-chamber MEC, and its electron fraction 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.