Grease trap waste (GTW) presents a challenge to wastewater treatment processes due to its slow biodegradation kinetics, high oxygen demand, and risks of pipeline blockage. The objective of this work was to evaluate the feasibility of GTW as an organic-rich co-substrate to improve biomethane production in the anaerobic digestion of municipal waste sludge (MWS) from sewage treatment, one of the most abundant feed materials to municipal anaerobic digesters. Waste characterization confirmed the high organic content of GTW at 138 gVS/L, which was 626% higher than that of MWS (19 gVS/L). The methane potential of GTW approximated 145 L(methane)/L(GTW). which was more than 15 times higher than that of MWS (8.9 L(Methane)/L(MWS)). When GTW was added as a co-substrate in addition to MWS, the high methane potential and organic content of GTW resulted in significant improvement in methane production during the anaerobic codigestion of MWS, e.g. a 65% increase at the GTW loading of 5.5 gVS/L, representing a less than 4% (vol/vol) addition of GTW. Thus, the operational feasibility of anaerobic co-digestion using GTW as the co-substrate is enhanced by the insignificant volumetric GTW loading required for significant improvements in methane production. Process inhibition and reduction in biogas production, however, occurred with higher GTW loadings, suggesting the importance of proper GTW loading rates for the implementation of anaerobic codigestion processes effective in improving biomethanation of municipal waste sludge. (C) 2010 Elsevier Ltd. All rights reserved.