Waste-activated sludge was filtered and the formed filter cake studied. It was shown that average specific filter-cake resistance increases during filtration. A stress-relaxation experiment was performed and the two relaxation times found were of the same order of magnitude as the duration of the filtration stage. It was therefore concluded that the local solid volume fraction of sludge filter cakes changes not only with effective pressure but also with time. In contrast, it is usually assumed in filtration modeling that solid volume fraction is a unique function of effective pressure. A new filtration model was thus developed by adopting the concept of filter-cake creep and applying it in conventional models. It was shown that the model developed, unlike conventional filtration models, satisfactorily describes the experimental data in the case of waste-activated sludge. Furthermore, the model is capable of being scaled with respect to feed concentration and volume.