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

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journal contribution
posted on 01.02.2010, 00:00 by Hyung-Sool Lee, Bruce E. Rittmann
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/m3 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/m3·d. Observed H2 harvesting rates were from 2.6 ± 0.10 to 4.3 ± 0.46 m3 H2/m3·d, but the H2 production rates computed from the current densities were 16.3−18.2 m3 H2/m3·d. Tracking all significant electron sinks (residual acetate, H2, CH4, biomass, and soluble microbial products (SMP)) in the single-chamber MEC showed that H2 reoxidation by anode-respiring bacteria recycled H2 between the cathode and the anode, and this caused the large discrepancy in H2 production and harvest rates. H2 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 H2 recycle also increased the applied voltage from ∼0.6 V to ∼1.5 V for the highest H2 harvest rate (4.3 m3 H2/m3·d). CH4 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 H2 recovery was moderate at 1.8−2.0 mol of H2/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 H2 generation, but H2 recycle and methanogenesis present significant challenges for practical application.