Production of electricity from samples obtained during anaerobic digestion of grass silage was examined using single-chamber air-cathode mediator-less microbial fuel cells (MFCs). The samples were obtained from anaerobic reactors at start-up conditions after 3 and 10 days of operation under psychrophilic (15 degrees C) and mesophilic (37 degrees C) temperatures. Electricity was directly produced from all samples at a concentration of 1500 mg COD L-1. Power density obtained from the samples, as a sole carbon source, ranged from 56 +/- 3 m(-3) to 31 +/- 1 W m(-3) for the mesophilic and psychrophilic samples, respectively. Coulombic efficiencies ranged from 18 +/- % to 12 +/- 1% for the same samples. The relationship between the maximum voltage output and initial COD concentration appeared to follow saturation kinetics at the external resistance of 217 Omega. Chemical oxygen demand (COD) removal was over 90% and total phenolics removal was in the range of 30-75% for all samples tested, with a standard amount of 60 mg L-1 total phenolics removed for every sample. Our results indicate that generating electricity from solution samples of anaerobic reactors utilizing grass silage is possible, opening the possibility for combination of anaerobic digestion with MFC technology for energy generation. (C) 2010 Elsevier Ltd. All rights reserved.