Effect of hydraulic pressure and membrane orientation on water flux and reverse solute flux in pressure assisted osmosis

• A novel pressure assisted osmosis (PAO) is investigated.
• Water fluxes in PAO behave differently at FO and PRO modes.
• Reverse solute flux in PAO decreases with increasing hydraulic pressure.
• The important role of membrane channel spacer in PAO is delineated.
• A hybrid FO–PAO–RO system can enhance effectiveness of osmotic dilution.

Forward osmosis (FO) is an emerging technology that has received much global interest due to its potential applications in wastewater reclamation and seawater desalination. One of the major challenges to overcome is the detrimental effects of concentration polarization (CP), which reduce the effective osmotic pressure driving force and thus decrease productivity of the FO process. In this study, pressure assisted osmosis (PAO) was investigated as a method to increase the effective driving force and water flux by combining an osmotic pressure driving force with an additional hydraulic pressure. Experiments were carried out to examine the efficiency of the PAO process using a bench-scale setup specially designed to prevent membrane deformation under the applied hydraulic pressure. Results showed that PAO water flux increased with increasing the applied hydraulic pressure in FO mode (i.e., active layer facing the feed solution). The measured water fluxes were in good agreement with predictions based on a model developed to describe the water flux in PAO operation. However, the PAO water flux was lower than model predictions in PRO mode (i.e., active layer facing the draw solution). This observation is attributed to the spacer ‘shadow effect’ and the resulting reduction in the effective membrane area by the spacers. The results also showed that reverse solute flux decreased with increasing the applied hydraulic pressure in both FO and PRO modes. Although applying hydraulic pressure to FO increases energy consumption, the higher water flux in PAO reduces the number of membrane modules for the FO process. In addition, control of the driving force is easier in PAO than FO, leading to flexibility in system design and operation. Based on these results, a possible combination of FO and RO system with PAO was proposed for allowing higher energy efficiency in seawater desalination.