Effect of Different Draw Solutions on Concentration Polarization in a Forward Osmosis Process: Theoretical Modeling and Experimental Validation

Abstract

Forward osmosis (FO) is a promising membrane technology to promote water sustainability. However, the concentration polarization (CP) effects, including external (ECP) and internal concentration polarization (ICP), are critical issues reducing FO efficiency. In this study, a solution diffusion model was built to investigate the effects of different draw solutions (NaCl, KCl, Na2SO4, and MgCl2) on the intrinsic CP effects, and results were experimentally validated in a laboratory-scale FO system. The modeling results showed a significant loss of a draw solution’s theoretical osmotic pressure (OP) (up to 50-90%) in the CP effects, especially ICP (up to 40-72%). Among the various draw solutes, the solute with fewer ions dissociated (NaCl and KCl), a higher diffusion coefficient, and a low viscosity allowed solutes to diffuse more freely and maintain a higher effective OP with much lower CP. The ECP effects could be mitigated by increasing the flow rate, which improves mixing and reduces the boundary layer. The CP effects, ICP effect particularly, became more prominent when increasing the draw solution concentration. The in-depth analysis of the relationship among ICP, ECP, and water flux confirmed that the increase in draw solution concentration resulted in a significant loss of OP because of the CP effects. Apart from the membrane properties, it is vital to control the CP effects by selecting a draw solution with a greater diffusion coefficient to maintain a higher effective OP.