The short-term fouling behavior of forward osmosis (FO) membrane in an osmotic membrane bioreactor (OMBR) was investigated, using NaCI or MgCl2 as the draw solutions. The effect of membrane orientation, mixed liquor suspended solids (MLSS) concentration and draw solution (DS) osmotic pressure on water flux and membrane fouling behaviors was examined, along with the effects of simulated elevated salinity on sludge properties and on membrane fouling. Water flux and membrane fouling were not significantly affected by both MISS concentration (4.91-12.60 g/L) and osmotic pressure (3.0-15.0 MPa), but were severely affected by elevated salinity, due to changes in activated sludge properties, in particular the increase in extracellular polymeric substances (EPS) and sludge hydrophobicity. MgCl2 as the DS showed more significant influence on activated sludge properties and membrane fouling than NaCI but gave rise to lower salt accumulation. Analyses of the membrane foulants showed that small sludge floc/particles and EPS (in particular, proteins) were enriched in the fouling layer. UPLC-MS/MS analyses of the proteins showed that hydrophobic proteins were the main cause of membrane fouling. (C) 2013 Elsevier B.V. All rights reserved.