Microbial fuel cells
Electricity production from microbial fuel cells has been accomplished using acetate and butyrate in domestic wastewater.
Hydrogen can be produced from wastewater rich in carbohydrates by biological fermentation but much of the energy remains in the form of soluble products such as acetate and butyrate. One way to recover this lost energy is to use dual-chamber microbial fuel cells (MFC) that feed on the organic acids and produce electricity. The anaerobic and aerobic chambers are separated by a proton exchange membrane that facilitates proton transfer.
This technique has been modified by US researchers who have constructed a membrane-free single-chamber MFC that permits increased mass transfer to the cathode in a smaller, cheaper design. Bruce Logan and colleagues from Pennsylvania State University used the cell to generate electricity from domestic wastewater dosed with acetate and butyrate.
Using a carbon paper anode and a platinum-containing carbon paper cathode, power was produced principally by the direct transfer of electrons to the anode via bacterial biofilms growing on the electrode, as indicated by cyclic voltammetry. The maximum power densities and current densities were 661mW/m2 and 2.2A/m2, respectively, for acetate, and 349mW/m2 and 0.77A/m2 for butyrate, indicating the superior performance of acetate.
Power levels in both cases were more than 50 per cent greater than those obtained in dual-chamber MFCs. But overall energy recoveries were only three to seven per cent for acetate and two to five per cent for butyrate, indicating that processes other than electricity generation were responsible for electron and energy losses. Logan proposes that the loss routes might include diffusion of oxygen through the cathode into the anode chamber, the onset of methanogenesis, and the presence of alternative electron acceptors in solution.
H Liu et al, Environ. Sci. Technol., 2004 (DOI: 10.1021/es048927c)