The Anaerobic Digestion Model No. 1, implemented in the GPS-X computational simulator, was calibrated to simulate the steady state anaerobic co-digestion of municipal wastewater sludge and restaurant grease trap waste (GTW). Substrate characterizations combined with anaerobic respirometric analysis were applied for model calibration. Initial biomass concentrations and distributions were estimated using methane production rate curves together with effluent values from full-scale anaerobic digesters. Two separate datasets obtained from steady state mesophilic lab-scale experiments were used to calibrate and validate the model. The modified model was able to predict reasonably well the steady-state results of biogas production, CH4 and CO2 contents, pH, alkalinity, COD and VSS observed within the evaluated GTW loading. Finally, the calibrated model was validated using pilot-scale co-digestion results obtained during a previous study. The model output was in good agreement with measured values of methane production, pH, ammonia, alkalinity and COD. (C) 2014 Elsevier B.V. All rights reserved.