In this study it is demonstrated that a correct description of the mixing behavior of an anaerobic digester is necessary to accurately model and predict the reactor performance, especially at a larger scale operation. A lab-scale (3.781) and a pilot-scale (120l) anaerobic digester were operated at both mixing and non-mixing conditions. At lab-scale no significant difference in performance was found between these conditions. In each case about 0.25 lCH(4)/I-reactor/d of biogas was produced. The model predictions were in close agreement with measured values. At pilot-scale, however, the influence of mixing became important. A reduction of about 10% of methane production occurred during unmixed conditions. Furthermore also a build-up of volatile fatty acids (up to 3 gCOD/l) prevailed. This behavior could not be predicted by using a CSTR approach. In contrast, it was accurately predicted with the proposed model, which includes a more rigorous mixing next to common reaction kinetics. Prediction of the daily biogas production assuming a perfectly mixed reactor revealed an overestimation of the biogas production by 10%. It is therefore recommended to use more accurate mixing models in order to predict biogas production from anaerobic digesters. (C) 2014 Elsevier B.V. All rights reserved.