Production of Electricity from Acetate or Butyrate Using a Single-Chamber Microbial Fuel Cell HONG LIU, † SHAOAN CHENG, † AND BRUCE E. LOGAN* ,†,‡ Department of Civil and Environmental Engineering, and The Penn State Hydrogen Energy (H2E) Center, The Pennsylvania State University, University Park, Pennsylvania 16802 Hydrogen can be recovered by fermentation of organic material rich in carbohydrates, but much of the organic matter remains in the form of acetate and butyrate. An alternative to methane production from this organic matter is the direct generation of electricity in a microbial fuel cell (MFC). Electricity generation using a single-chambered MFC was examined using acetate or butyrate. Power generated with acetate (800 mg/L) (506 mW/m 2 or 12.7 mW/ L) was up to 66% higher than that fed with butyrate (1000 mg/L) (305 mW/m 2 or 7.6 mW/L), demonstrating that acetate is a preferred aqueous substrate for electricity generation in MFCs. Power output as a function of substrate concentration was well described by saturation kinetics, although maximum power densities varied with the circuit load. Maximum power densities and half-saturation constants were P max ) 661 mW/m 2 and K s ) 141 mg/L for acetate (218 Ω) and P max ) 349 mW/m 2 and K s ) 93 mg/L for butyrate (1000 Ω). Similar open circuit potentials were obtained in using acetate (798 mV) or butyrate (795 mV). Current densities measured for stable power output were higher for acetate (2.2 A/m 2 ) than those measured in MFCs using butyrate (0.77 A/m 2 ). Cyclic voltammograms suggested that the main mechanism of power production in these batch tests was by direct transfer of electrons to the electrode by bacteria growing on the electrode and not by bacteria-produced mediators. Coulombic efficiencies and overall energy recovery were 10-31 and 3-7% for acetate and 8-15 and 2-5% for butyrate, indicating substantial electron and energy losses to processes other than electricity generation. These results demonstrate that electricity generation is possible from soluble fermentation end products such as acetate and butyrate, but energy recoveries should be increased to improve the overall process performance. Introduction Harvesting products from wastewater in order to make the process more economical and sustainable is the next frontier in wastewater treatment (1, 2). Hydrogen production from wastewater by biological fermentation has drawn much attention as a method of producing a valuable product during treatment of wastewaters containing high concentrations of carbohydrates (3-7). One mole of glucose can theoretically be converted into 12 mol of hydrogen, but the maximum yield via known fermentation routes is only 4 mol of hydrogen when acetate is the sole byproduct. While the maximum efficiency of hydrogen production is therefore 33%, typically only 15% of the energy is recovered as hydrogen (2, 8) with the remainder of the organic matter present as fatty acids and alcohols. To improve the economics of hydrogen production from wastewater, additional processes are needed to recovery the remaining energy. One approach is to link hydrogen pro- duction with methane production by using a two-stage process (2). Although two-stage anaerobic treatments have been used to make methane, it has not yet been proven outside of the laboratory that hydrogen can be recovered at high concentrations from the first stage using actual waste- waters. A second approach is to use phototrophic bacteria to recover additional hydrogen from the byproducts of hydrogen fermentation (9, 10). Although solar energy is free, the availability of sufficient land area and the instability of sufficient solar energy at the plant would make such a process difficult for wastewater treatment applications. A third approach is to recover the remaining energy directly as electricity in a microbial fuel cell (MFC). While electricity production has been shown in MFCs using glucose or acetate, much remains to be done in order to use this technology for wastewater treatment. Bacteria present in wastewater, anaerobic reactor sludges, and marine sediments have been shown to produce electricity in a MFC (11-14). Bacteria that have been identified to be capable of making electricity in fuel cells, most of which are metal-reducing bacteria, include Geobacter sulfurreducens (15, 16), Geobacter metallireducens (13, 16), Shewanella putrefaciens (17, 18), Clostridium butyricum (19), Rhodoferax ferrireducens (20), and Aeromonas hydrophila (15). It has also been recently shown that electricity generation in an MFC resulted in large part from the production of mediators, or electron shuttles, by a microbial community consisting of primarily three bacteria: Alcaligenes faecalis, Enterococcus faecium, and Pseudomonas aeruginosa (12). Many MFCs contain two chambers (16-18, 20). One chamber contains electrochemically active bacteria growing under anaerobic conditions that grow as a biofilm attached to the anode. The other chamber is kept aerobic by sparging water with air and contains the cathode. The two chambers are typically separated by a proton exchange membrane (PEM), which allows the transfer of protons from the anode to cathode chamber and that helps to physically block oxygen diffusion into the anode chamber. Recently, single-chamber MFCs have been developed that use a cathode exposed directly to air instead of air-sparged water (11, 21, 22). There are several advantages of using a single-chamber MFC versus a two-chambered system: increased mass transfer to the cathode; decreased operating costs, because it is not neces- sary to sparge the water; an overall decrease in reactor volume; and a simplified design. Power output can further be increased in a single-chamber MFC by removing the PEM (11). Although there is increased oxygen diffusion into the anode chamber in the absence of the PEM, the formation of an aerobic biofilm on the cathode inner surface (facing the anode) removes any oxygen that diffuses into the chamber, preventing the loss of anaerobic conditions in the anode chamber. The lack of a PEM also substantially decreases the cost of the materials needed to make a MFC. The primary fermentation end products during biohy- drogen production are acetic and butyric acids. Thus, to link * Corresponding author phone: 814-863-7908; fax: 814-863-7304; e-mail: blogan@psu.edu. † Department of Civil and Environmental Engineering. ‡ The Penn State Hydrogen Energy (H2E) Center. Environ. Sci. Technol. 2005, 39, 658-662 658 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 2, 2005 10.1021/es048927c CCC: $30.25  2005 American Chemical Society Published on Web 12/03/2004