Fouling behavior of forward osmosis (FO) was systematically investigated under different conditions. According to experimental results, minimal membrane fouling was observed in the FO mode with the least organic load. Severe membrane fouling was observed with the increase of Ca2+ concentration or pH when organics were feed solution (FS). Temperature of solution played a remarkable role in membrane performance. Flux reduced to 80.02% and 81.11% of initial water flux when the temperature of both FS and draw solution (DS) was set at 23 degrees C and 35 degrees C, respectively. However, flux decreased to 62.41% by only elevating the temperature of DS to 35 degrees C. In contrast, flux decreased to 56.22% by only elevating the temperature of FS to 35 degrees C. Moreover, membrane was easily fouled in the pressure retarded osmosis (PRO) mode than that in the FO mode. Flux declined to 32.66% in the PRO mode, compared with that of 81.11% in the FO mode. In terms of the types of DS, more flux decline was occurred when CaCl2 was used as DS, compared with that in NaCl DS system. It was attributed to the reverse permeation of Ca2+ and rapid change of osmotic pressure as the decrease of CaCl2 concentration. The fouling index revealed that membrane orientation and solution temperature were the most important factors in fouling behavior of FO. (C) 2016 Elsevier B.V. All rights reserved.