Rejection of nine haloacetic acids and coupled reverse draw solute permeation in forward osmosis

The rejection of haloacetic acids (HAAs) by forward osmosis (FO) and the coupled reverse draw solute permeation were experimentally determined and mathematically modeled by using the solution-diffusion model for both the AL-FW (active layer facing the feed water) and the AL-DS (active layer facing the draw solution) orientations. The rejection ratio for each HAA increased with the increase of draw solute concentration for the AL-FW orientation. In contrast, the HAA rejection ratio could reach its maximum under a medium osmotic pressure difference for the AL-DS orientation. The rejection ratios for all HAAs were higher than 94.6% for the AL-FW orientation and ranged from 73.8% to 89.1% for the AL-DS orientation under a draw solute concentration of 1 mol/L NaCl. The reverse draw solute flux for the AL-FW orientation was lower than that for the AL-DS orientation. The model-predicted HAA rejection results matched well with the experimental rejection ratios for the AL-FW orientation. However, the model over-estimated the rejection ratios for the AL-DS mode, likely due to the adoption of inaccurate mass transfer coefficient for internal concentration polarization. Regarding the reverse draw solute permeation, a general agreement between the model prediction and experimental data was observed for both orientations.