Within the AMEDEUS European project, a work package is dedicated to biological modelling of Membrane Bioreactors (MBR) processes as well as the impact of a pre-treatment by primary sedimentation on sludge production, sludge characteristics and permeate quality. The supposed interest of this pre-treatment is to decrease sludge production and to improve oxygen transfer by decreasing sludge concentration. Two MBR pilots were operated side by side under the same operating conditions, one fed by screened (1 mm screen) raw municipal wastewater and the other by primary settled raw municipal wastewater. This side by side operation allows a true comparison of the performances and model calibration. In average, screened water was observed to contain 30% more solids than settled water. In this study, the sludge age was fixed at 40 days and then modified to 15 days to calibrate the model on a broad range of operating conditions in order to validate and identify the biological activated sludge model (ASM) limits. With a sludge retention time (SRT) of 15 days, sludge production is less important in the pilot fed by settled water because of better bio-accessibility behaviour of settled water. In comparison with ASM1 default parameter values, calibrated parameters are quite different concerning nitrification and denitrification because of an improvement of oxygen transfer induced by lower floc size distribution (K-OA = 0.25-0.3 g O-2/m(3), K-OH = 0.1 g O-2/m(3)). With a SRT of 40 days, model calibration is not validated and the pilot fed with screened water has a sludge production reduction that is less important than the one of the pilot fed by settled water where accumulation of inert organic matter is observed. (C) 2009 Elsevier B.V. All rights reserved.