Causes of and remedies for membrane fouling in Membrane Bioreactors for wastewater treatment are only poorly understood and described in scientific literature. A Filtration Characterisation Installation and a measurement protocol were developed with the aim of a) unequivocally determination and quantification of the filterability of an activated sludge and b) carrying out short term experiments at labscale to determine foulants and/or fouling propensity determining factors. The installation was tested at three locations in the Netherlands where membrane bioreactors were applied to treat municipal wastewater. The results show that each MBR system creates an activated sludge with very specific filtration characteristics in a wide range. The filterability may well be characterised by the filtration resistance, additional to the clean water resistance, after 20 L of permeate production per square meter membrane area: DR20. Preliminary experiments were carried out to identify fouling substances by testing the theory that polysaccharides in the water phase can be related to fouling behaviour. This relation could not be proved.