The filterability of the activated sludge is an important characteristic that offers valuable information about the filtration process in membrane bioreactors (MBR). MBR operators rely on permeability values to monitor the process. However, results obtained in this and other studies have demonstrated that permeability alone is a weak parameter, as it provides no specific information about which factors are determining it. Aiming to elucidate the causes affecting the filterability of the activated sludge, some research groups have quantified and correlated the filterability with different activated sludge parameters, mostly for municipal MBRs. This paper however, offers a different approach focusing on an industrial scenario. A representative sample of eleven full-scale industrial MBRs has been selected in centre-Europe to conduct a study, in order to quantify both, the impact of parameters and constituents of industrial activated sludge on filterability. Temperature, membrane configuration, hydraulic retention time, solids retention time, and the soluble organic fraction did not show any impact on filterability whereas the mixed liquor suspended solids and the food to microorganism ratio demonstrated a slight influence. Among all parameters analyzed, the colloidal fraction played the most important role in the fouling process indicating that for industrial activated sludge, the membrane acts as a biomass barrier merely, since a high percentage (>75%) of the soluble organic fraction is small enough to pass the membrane (0.03 mu m). Finally, filterability measurements suggest that on the whole, an optimization in the operation of industrial MBRs is plausible. (C) 2011 Elsevier B.V. All rights reserved.