Biogas plants, increasing in number, produce a stream of fermentation residue with high organic content, providing an energy source which is by now mostly unused. We tested this biomass as a potential feedstock for catalytic gasification in supercritical water (T >= 374 degrees C, p >= 22 MPa) for methane production using a batch reactor system. The coke formation tendency during the heat-up phase was evaluated as well as the cleavage of biomass-bound sulfur with respect to its removal from the process as a salt. We found that sulfur is not sufficiently released from the biomass during heating up to a temperature of 410 degrees C. Addition of alkali salts improved the liquefaction of fermentation residues with a low content of minerals, probably by buffering the pH. We found a deactivation of the carbon-supported ruthenium catalyst at low catalyst-to-biomass loadings, which we attribute to sulfur poisoning and fouling in accordance with the composition of the fermentation residue. A temperature of 400 degrees C was found to maximize the methane yield. A residence time dependent biomass to catalyst ratio of 0.45 g g (1)h (1) was found to result in nearly full conversion with the Ru/C catalyst. A Ru/ZrO2 catalyst, tested under similar conditions, was less active. (C) 2012 Elsevier Ltd. All rights reserved.