Selection of inorganic-based draw solutions for forward osmosis applications

In this investigation, a protocol for the selection of optimal draw solutions for forward osmosis (FO) applications was developed and the protocol was used to determine the most appropriate draw solutions for specific FO applications using a currently available FO membrane. The protocol includes a desktop screening process and laboratory and modeling analyses. The desktop screening process resulted in 14 draw solutions suitable for FO applications. The 14 draw solutions were then tested in the laboratory to evaluate water flux and reverse salt diffusion through the FO membrane. Internal concentration polarization was found to lower both water flux and reverse salt diffusion by reducing the draw solution concentration at the interface between the support and dense layers of the membrane. Draw solution reconcentration was evaluated using reverse osmosis (RO) system design software. Analysis of experimental data and model results, combined with consideration of the costs associated with the FO and RO processes showed that a small group of seven draw solutions appeared to be the most suitable. The different characteristics of these draw solutions highlighted the importance of considering the specific FO application and membrane types being used prior to selecting the most appropriate draw solution.

Research highlights▶ A protocol for the selection of optimal draw solutions for forward osmosis (FO) applications was developed. ▶ Experimental data and model results showed that a small group of seven draw solutions appeared to be the most suitable. ▶ It was proven that internal concentration polarization is strongly dependent on the diffusion coefficient of the draw solution ▶ The large draw solution matrix, in terms of both constituents and concentrations, made it possible to quantitatively compare the effect of internal concentration polarization on water flux and reverse salt diffusion. ▶ The protocol developed can be used for draw solution selection as new FO membranes and draw solutions are developed and applied to new FO applications.