This study investigates the effect of using ultrasound on water flux through a forward osmosis membrane when applied for seawater desalination. A synthetically prepared solution simulating seawater with scaling substances and organic foulants was used. The parameters considered include membrane cross-flow velocity, flow configuration (co-current versus counter-current), direction of ultrasound waves relative to the membrane side (active layer versus support layer), and type of draw solution (NaCl versus MgCl2). The study revealed that applying a continuous ultrasound frequency of 40 kHz was effective in enhancing water flux, especially when the ultrasound source faces the membrane active layer, irrespective of the used draw solution. The highest water flux enhancement (70.8% with NaCl draw solution and 61.9% with MgCl2 draw solution) occurred at low cross-flow velocity and with the ultrasound waves facing the membrane active layer. It was also observed that the use of ultrasound generally caused an adverse effect on the water flux when the ultrasound source faces the membrane support layer. Moreover, applying the ultrasound at the membrane support layer increased the reverse solute flux. For all tested cases, higher water flux enhancement was observed with NaCl as a draw solution compared to the cases when MgCl2 was used as a draw solution.

This review considers the forward osmosis (FO) membrane process as one of the feasible solutions for water desalination. Different aspects related to the FO process are reviewed with an emphasis on ultrasound assisted FO membrane processes. The different types of membranes used in FO are also reviewed and discussed; thus, their configuration, structure and applications are considered. Coupling ultrasound with FO enhances water flux through the membrane under certain conditions. In addition, this review addresses questions related to implementation of an ultrasound/FO system for seawater desalination, such as the impact on fouling, flow configuration, and location of fouling. Finally, the mechanisms for the impact of ultrasound on FO membranes are discussed and future research directions are suggested.

The effect of ultrasound on water flux through forward osmosis membrane for seawater desalination was investigated using the factorial design approach. Sodium chloride (NaCl) was used to simulate the dissolved solids content. In every test, the initial draw solution (DS) concentration was fixed at 4.5 M for NaCl and 2 M for MgCl2. Parameters considered in the investigation included membrane crossflow velocity (0.25 and 1.0 cm/s), flow configuration (co-current versus counter-current), direction of ultrasound waves relative to the membrane side (active layer versus support layer), and type of draw solution (NaCl versus MgCl2). A two-level factorial design was considered in the analysis of the results obtained from the experimental work. Based on the factorial design analysis, crossflow velocity and use of ultrasound have a positive effect on water flux enhancement for both draw solutions. However, the velocity effect on water flux enhancement was more pronounced than that of the use of ultrasound. The effect of flow configuration was statistically insignificant for both draw solutions. The interaction effect between crossflow velocity and ultrasound was statistically significant for both draw solutions. However, the interaction between crossflow velocity and flow configuration was only significant for the case of MgCl2. The three-way interaction was insignificant for both draw solutions. The developed factorial model equations were used to predict other flux data in ultrasound-assisted FO systems and showed adequate representation of these data at relatively similar conditions after adjustment of the model for the baseline conditions of the evaluated cases.