Electricity production from xylose in fed-batch and continuous-flow microbial fuel cells

A medium-scale (0.77 l) air-cathode, brush-anode microbial fuel cell (MFC) operated in fed-batch mode using xylose (20 mM) generated a maximum power density of 13 ± 1 W/m3 (673 ± 43 mW/m2). Xylose was rapidly removed (83.5%) within 8 h of a 60-h cycle, with 42.1% of electrons in intermediates (8.5 ± 0.2 mM acetate, 5.9 ± 0.01 mM ethanol, 4.3 ± 0.1 mM formate, and 1.3 ± 0.03 mM propionate), 9.1% captured as electricity, 16.1% in the remaining xylose, and 32.7% lost to cell storage, biomass, and other processes. The final Coulombic efficiency was 50%. At a higher initial xylose concentration (54 mM), xylose was again rapidly removed (86.9% within 24 h of a 116-h cycle), intermediates increased in concentration (18.4 ± 0.4 mM acetate, 7.8 ± 0.4 mM ethanol and 2.1 ± 0.2 mM propionate), but power was lower (5.2 ± 0.4 W/m3). Power was increased by operating the reactor in continuous flow mode at a hydraulic retention time of 20 h (20 ± 1 W/m3), with 66 ± 1% chemical oxygen demand removal. These results demonstrate that electricity generation is sustained over a cycle primarily by stored substrate and intermediates formed by fermentation and that the intermediates produced vary with xylose loading.