An innovative osmotic membrane bioreactor (OMBR) is currently under development, based on forward osmosis (FO) driven by an osmotic pressure difference. For the OMBR to be both technically and economically viable, the performance of the FO membranes has to be sufficiently high, while membrane fouling and draw solution leakage has to be low. The effect of temperature, membrane type, membrane orientation, type and concentration of draw solution was investigated to optimize FO membrane performance using deionised water. Membrane fouling and draw solution leakage was investigated in this work by laboratory scale FO experiments using an activated sludge solution from a membrane bioreactor (MBR). The best FO performance with an activated sludge solution was found with a FO-type membrane J = 6.21/m(2) h at 20 +/- 2 degrees C with 0.5 M NaCl corresponding to pi = 24 bar). Draw solutions from salts consisting of monovalent ions (NaCl and NaNO3) performed better than salts consisting of bivalent ions (MgSO4 and ZnSO4), while the flux of the FO membrane were non-linearly related to the concentration of the draw solution. Internal concentration polarization did hamper the FO performance as a result of the thickness and structure of the porous substructure of the investigated membranes. Both reversible and irreversible membrane fouling were not found during the FO experiments using activated sludge solutions. Furthermore no substantial draw solution leakage was found for the FO membranes at various draw solution concentrations for different draw solutions. (C) 2008 Elsevier B.V. All rights reserved.