In this study, organic fouling behavior of forward osmosis (FO) membrane operated in FO mode (i.e., the dense layer facing the feed solution) and pressure retarded osmosis (PRO) mode (i.e., the porous layer facing the feed solution) was investigated. Organic foulants prepared from the reverse osmosis reject of a water reclamation plant were used at low concentration. The results on the effect of membrane orientation on fouling revealed that in the PRO mode, the FO membrane fouled more easily than when operated in the FO mode, and the fouling became more severe with increasing organic concentration in the feed solution. With TOC of 50 ppm and lesser, even in the presence of 5 mM Ca2+, minimal membrane fouling was observed in the FO mode, corroborating membrane resilience. In the PRO mode, severe membrane fouling was observed when Ca2+ was present together with 30 and 50 ppm of TOC in the feed solution. The fouling layer resistance in the absence of Ca2+ was found to be an order of magnitude lower than the membrane inherent resistance. However, in the presence of 5 mM Ca2+, the fouling layer resistance was higher, but in the same order of magnitude as the membrane inherent resistance. (C) 2012 Elsevier B.V. All rights reserved.