Clean water is becoming a scarce resource. As a consequence reclaimed wastewater can be an attractive alternative to replace existing water resources for non-potable reuse options. In order to achieve this goal advanced treatment technologies are required. An example of such technology is the membrane bioreactor (MBR) process. In the MBR process the biomass (activated sludge) that biologically degrades the pollutants in wastewater is separated from the purified water (effluent) by an advanced membrane filtration step, instead of the conventional sedimentation step. The superior effluent quality that can be achieved with MBR technology offers possibilities for reuse options such as irrigation and industrial process water. Application of the MBR process is however restrained by the relatively high operation and maintenance costs, related to the problem of membrane fouling (accumulation of substances on the membrane and the accompanying decrease of the system performance). In order to make MBR technology a feasible alternative more knowledge about the fouling process is required. In the research described in this thesis a method used to characterise the filterability of activated sludge is discussed and applied at full-scale MBR plants in the Netherlands. In addition the factors the influence the filterability are analysed. This is important, because good filterability forms the starting-point for an efficient MBR filtration process.