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Recently, forward osmosis (FO) has attracted growing attention in many potential applications such as power generation, desalination, wastewater treatment and food processing. However, there are still several critical challenges, including concentration polarization, membrane fouling, reverse solute diffusion and the need for new membrane development and draw solute design in FO. These challenges are also the current research focus on FO. This paper aims to review the recent developments in FO, focusing on the opportunities and challenges. It begins with discussing the advantages of the FO process over pressure-driven membrane processes. These potential advantages lie in FO's low energy consumption, low fouling propensity, reduced or easy cleaning, low costs, high salt rejection and high water flux. Next, the recent applications of FO, as the outcomes of the above advantages, are described. The key part of this review is a detailed discussion of five critical challenges faced by FO and their relationships. Finally, the future of FO is viewed. This review provides a clear outline for FO-concerned researchers on the recent developments in FO. Refereed/Peer-reviewed

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). Refereed/Peer-reviewed