Effects of feed and draw solution temperature and transmembrane temperature difference on the rejection of trace organic contaminants by forward osmosis

Abstract

The effects of feed and draw solution temperature and transmembrane temperature difference on the rejection of 12 trace organic contaminants (TrOCs) by two forward osmosis (FO) membranes were investigated. The membrane structure parameter (S) and the reverse salt (NaCl) flux selectivity (RSFS) were constant over the temperature range of 20–40°C, suggesting that within this range, the solution temperature did not significantly influence the membrane polymeric structure. Draw solution properties, including diffusivity, viscosity, and osmotic pressure, played an important role in water and reverse salt (NaCl) flux behaviour and TrOC rejection. Pure water and salt (NaCl) permeability coefficients of the two forward osmosis membranes increased as both the feed and draw solution temperatures increased from 20 to 40°C due to an increase in solute diffusivity and a decrease in water viscosity. Rejection of charged TrOCs was higher than that of neutral TrOCs and their rejection was insensitive to temperature variation. On the other hand, rejection of neutral TrOCs decreased significantly when the feed and draw solution temperatures were 40 and 20°C, respectively, due to the increase in their diffusivity at an elevated temperature. By contrast, rejection of neutral TrOCs increased when the feed and draw solution temperatures were 20 and 40°C, respectively. The reverse salt (NaCl) flux increased due to an increase in the draw solute diffusivity. In addition, at a higher draw solution temperature, the dilution effect induced by higher water flux and the hindrance effect enhanced by a higher reverse salt (NaCl) flux led to the increase in the rejection of neutral TrOCs.