A tool is developed for the validation of membrane fouling: the Membrane Fouling Simulator (MFS). The MFS uses the same materials as spiral wound reverse osmosis and nanofiltration membranes, has similar dimensions and hydrodynamic behaviour and is equipped with a sight glass. Using the MFS, fouling can be monitored by: (1) operational parameters like pressure drop, (2) non-destructive (visual and microscopic) observations using the window and (3) analysis of coupons sampled from the membrane sheet in the MFS. The major advantages of the MFS are: (i) representativeness of spiral wound membranes and (ii) the small size requiring small amounts of water and chemicals, reducing research costs and enhancing the possibility to test several MFS units in parallel. The MFS can be applied for early warning, characterization of the fouling potential of feed water, comparison of different pre-treatment schemes and for evaluation of fouling control applying different chemicals. The MFS can be used in model studies to develop integrated membrane systems less susceptible to fouling. The MFS is a suited tool for testing newly developed membranes. The first MFS tests showed that the results are representative for membranes used in practice under the same operating conditions. A comparison study of the MFS and spiral wound membrane modules showed the same pressure drop development in time and the same fouling accumulation. (c) 2006 Elsevier B.V. All rights reserved.