The IWA Anaerobic Digestion Model No. 1 (ADM1) was used to simulate the full-scale anaerobic sludge digester of Ankara Central Wastewater Treatment Plant. The digester is a completely mixed, once through continuous flow type which is being fed with a primary and secondary sludge mixture. The variability and unpredicted nature of the primary sludge composition was expected to challenge the model. A one-year dynamic data set from the digester was used for model calibration and validation in the study. The standard ADM1 model was corrected in the study with regard to nitrogen and carbon mass balances. A good correlation between the measured and simulated data was obtained for biogas yield, total volatile fatty acids and pH after heuristically calibrating stoichiometric and kinetic parameters of the ADM1 model. The simulation is sensitive to influent composition.