Dynamic relationship between membrane fouling and reverse diffusion of inorganic draw solutes in forward osmosis

Both membrane fouling and reverse diffusion limit the application of forward osmosis (FO), and the combined effects further decrease water generation. Herein, the effects of typical inorganic draw solutes (DSs) were investigated on the dynamic relationship between membrane fouling from two organic pollutants and reverse diffusion. The formation of a membrane fouling layer decreased the amount of Ca2+ ions reverse-diffusing from a CaCl2 DS and increased the amount of NH4+ ions reverse-diffusing from a NH4Cl DS. When sodium alginate was used as the feed solute, reverse-diffused ions from the CaCl2/NH4Cl DS resulted in the formation of a threedimensional network structure, further exacerbating membrane fouling. Moreover, greater reverse diffusion of NH4+ from the NH4Cl DS resulted in more membrane fouling in comparison with the CaCl2 DS (i.e., water recovery of 246.4 +/- 4.7 g), resulting in a lower water recovery value of 151.4 +/- 10.6 g. During the filtration of humic acid, the membrane fouling layer caused by the CaCl2 DS led to the formation of a cake filtration layer that was more severe than that formed when the NH4Cl DS was used. This study provides a reliable basis for draw solute optimization to extend the application of FO systems.