This study investigated the silica scaling and cleaning behavior in forward osmosis (FO) and how it compared with that in reverse osmosis (RO). The comparison between FO and RO modes shows that, under the hydrodynamic conditions tested, the flux decline rates under silica scaling are very similar in the two modes, but the flux recovery is close to 100% in the FO mode while it is only around 80% in the RU mode. Cellulose acetate (CA) and polyamide (PA) membranes were used to study the effects of membrane materials on silica scaling and cleaning. It is found that the flux decline rates for both membranes are similar, but the flux recovery of the CA membrane is 30-40% higher than that of the PA membrane. AFM force measurements indicate that membrane surface roughness increases the adhesion force between the PA membrane and a silica gel layer, significantly decreasing the cleaning efficiency of the PA membrane. Results from dynamic light scattering and energy-dispersive X-ray spectroscopy indicate that silica scaling is initiated as monosilicic acid deposits on the membrane surface, followed by polymerization/condensation that forms an amorphous silica gel layer at the interface between the membrane and silica particles. (C) 2012 Elsevier B.V. All rights reserved.