Flux behavior and membrane fouling in pressure-assisted forward osmosis

This study investigated pressure-assisted osmosis (PAFO), which uses the external pressure together with osmotic gradient across a membrane, to improve flux through forward osmosis (FO) membranes. Experiments were performed in a laboratory-scale PAFO system, which allows the application of external pressure up to 13 bar on the feed solution side. Deionized water (D.I. water) and synthetic seawater (35,000 mg/L NaCl) were used as feed solutions, and MgCl2 was used as a draw solution. Humic acid was used as a model foulant to examine the characteristics of membrane fouling. A theoretical model based on osmotic transport theory incorporating internal/external concentration polarization, and mass balance equations were used to analyze the performance of FO and PAFO system. Results indicated that the addition of external pressure to the osmotic pressure allowed higher flux in PAFO than FO. Nevertheless, the flux in PAFO was less than the sum of the flux in RO and that in FO, which attributed to the increase in the internal concentration polarization by the external pressure. Under the test conditions in this study, fouling by sodium alginate and humic acid was negligible in FO, RO, and PAFO.