Biological sludge was treated in a membrane bioreactor with rotating ceramic membranes. The filtration resistance due to the formation and removal of cake on membranes was studied as a function of the operational conditions, i.e., pressure and rotation speed. A pressure-step procedure was used with a relaxation step between the pressure steps. It was demonstrated that the cake compressibility was crucial for a close fit of the mass balance model to the experimental data. From modeling, limiting flux, initial specific cake resistance, and cake compressibility could be estimated. Limiting flux increased with membrane rotation speed, while parameters describing the pressure dependency of the filter cake resistance confirmed the compressible nature of the sludge cake. Furthermore, cake compressibility was found to decrease with increasing membrane rotation speed. (C) 2012 Elsevier B.V. All rights reserved.