Effects of membrane orientation on process performance in forward osmosis applications

Forward osmosis (FO) has attracted growing attention for its great promise in desalination, wastewater treatment, liquid food processing and power generation. However, there is no clear agreement on the selection of membrane orientation in these applications. This study investigates the effects of membrane orientation on FO performance in saline water desalination without fouling, and with inorganic or organic fouling. The results show that the feed solution component and the concentration degree could influence the selection of membrane orientation. When severe membrane fouling or scaling occurs, the isoflux point occurs relatively early and FO mode (active layer facing the feed) provides a more stable and higher water flux than that in the alternative membrane orientation, i.e. pressure retarded osmosis (PRO) mode (support layer facing the feed). Additionally, lower fouling but higher cleaning efficiency is observed in FO mode compared with PRO mode. Therefore, in the applications of treating feed solutions with higher fouling/scaling tendencies (e.g. wastewater treatment) or treating higher salinity water (e.g. seawater desalination), FO mode is more favourable. While PRO mode is preferred when using the solutions with lower fouling/scaling tendencies as the feed (e.g. brackish water desalination), or where intensive concentration is unnecessary (e.g. power generation).

► The effects of membrane orientation on FO performance are investigated.
► Selection of the orientation is determined by the occurrence of the isoflux point.
► FO mode provides better performance when severe fouling occurs.
► It provides a guideline for the orientation selection in various FO applications.