We have studied, for the first time, the fouling behavior of pressure retarded osmosis (PRO) hollow fiber membranes under low, moderate and high hydraulic pressures. The thin film composite (TFC) polyethersulfone (PES) membrane has a high water permeability and good mechanical strength. Membrane fouling by gypsum (CaSO4 center dot 2H(2)O) sealants, sodium alginate, and the combined foulants was examined under various pressures up to an ultrahigh hydraulic pressure of 18 bar In the combined fouling experiments, the membranes were conditioned by one of foulants followed by the other. Flux decline results suggested that such conditioning could increase the rate of combined fouling because of the change in membrane surface chemistry. Specially, the co-existence of gypsum crystals and alginate under 0 bar led to the synergistic combined fouling and resulted in a greater flux decline than the sum of individual fouling. Interestingly, such gypsum-alginate synergistic fouling was not observed under high pressure PRO tests because the increased reverse salt flux inhibited the formation of gypsum crystals. Therefore, alginate fouling could be the dominant fouling mechanism for both (1) alginate conditioning and then sealants fouling, and (2) sealants conditioning and then alginate fouling PRO processes under 8 bar and 18 bar. Since the reverse salt flux increases from 5.6 +/- 1.1 g/m(2) h at 0 bar to 74.3 +/- 9.7 g/m(2) h at 8 bar and finally to 150.5 +/- 2.5 g/m(2) h under 18 bar, the reverse salt ions lead to substantial declines of normalized flux under 8 bar and 18 bar because the reverse sodium ions not only reduce the effective driving force across the PRO membrane but also induce a significant cake enhanced sodium concentration polarization layer and facilitate alginate gelation near the membrane surface. Therefore, the removal of alginate type foulants from the feed water stream may become essential for the success of PRO processes under high pressures. (C) 2015 Elsevier B.V. All rights reserved.