Organic fouling and reverse solute selectivity in forward osmosis: Role of working temperature and inorganic draw solutions

• We examined the influence of high levels of organic foulants on FO membrane performance.
• We evaluated the influence of temperature on the forward water and reverse salt flux behavior.
• Lower water viscosities played a dominant role in enabling concentrative internal concentration polarization.

The water flux of several draw solutions (DSs, solutes: KCl, NaCl, CaCl2, Na2SO4) and fouling propensity of two different organic foulants (humic acid and alginate) were systematically investigated using forward osmosis (FO) and unpressurized pressure-retarded osmosis. In addition, reverse solute selectivity was evaluated to characterize the water and salt transport mechanisms at different temperatures and in the presence of four different inorganic DS compounds. The influence of solution viscosity has significant implications in FO applications, because the water molecules easily penetrated and diffused throughout the FO membrane active layer (AL) and supporting layer (SL) with increasing temperatures, which is mainly correlated with the lower water viscosities with increasing temperatures. The results indicated that the water flux on average significantly increased from 9.5 to 13.7 and 24.9 LMH when the operating temperature was increased from 5 to 20 and 45 °C, which corresponded to a 44 and 262% increase in the water flux, compared to the FO mode at 5 °C. However, the water flux and viscosity exhibited generally constant trends with respect to the elevating temperature. In addition, elevating temperature increased the reverse solute flux selectivity (RSFS), not only by decreasing the internal concentration polarization (the AL and SL) and the wettability within the effective porosity of the SL, but also via the improvement of water molecule diffusion kinetics rather than solute diffusion. In addition, the RSFS was inversely related to the solute permeability of the different DSs and followed the order Na2SO4 > CaCl2 > NaCl > KCl. These results have significant implications for the prediction of water flux behavior and the selection of DSs at different temperatures in osmotically driven FO processes.