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Despite its attractive features for energy saving separation, the performance of forward osmosis (FO) has been restricted by internal concentration polarization and fast fouling propensity that occur in the membrane sublayer. These problems have significantly affected the membrane performance when treating highly contaminated oily wastewater. In this study, a novel double-skinned FO membrane with excellent anti-fouling properties has been developed for emulsified oil-water treatment. The double-skinned FO membrane comprises a fully porous sublayer sandwiched between a highly dense polyamide (PA) layer for salt rejection and a fairly loose dense bottom zwitterionic layer for emulsified oil particle removal. The top dense PA layer was synthesized via interfacial polymerization meanwhile the bottom layer was made up of a zwitterionic polyelectrolyte brush - (poly(3-(N-2-methacryloxyethyl-N,N-dimethyl) ammonatopropanesultone), abbreviated as PMAPS layer. The resultant double-skinned membrane exhibited a high water flux of 13.7 ± 0.3 L/m2.h and reverse salt transport of 1.6 ± 0.2 g/m2.h under FO mode using 2 M NaCl as the draw solution and emulsified oily solution as the feed. The double-skinned membrane outperforms the single-skinned membrane with much lower fouling propensity for emulsified oil-water separation.

Thin film composite (TFC) membranes are state-of-the-art membranes with superior permeability and selectivity and are widely used in various membrane-based processes for desalination, wastewater treatment and other separation applications. These TFC membranes are generally made out of a thin polyamide selective layer that synthesized through interfacial polymerization on the top surface of a microporous substrate. The first commercialized TFC membrane was reported in the 1970s for reverse osmosis (RO) process of seawater desalination. It was later expanded to nanofiltration (NF) process for colour and divalent salts removal in the 1980s. In the early 2000s, the potential use of TFC membrane was explored in the osmoticallydriven process including forward osmosis (FO) process and pressure retarded osmosis (PRO) process. Despite the exceptional performance improvement of TFC membrane was achieved, the existing TFC membranes still suffer from several bottlenecks in terms of fouling resistance, productivity as well as durability upon compaction and chemical attack and have the limited overall separation efficiency. Research in the past has focused mainly on the fabrication of polyamide layer that determines the rejection rate and antifouling resistance of the TFC membrane. This strong research interest on the polyamide layer development can be reflected by the large number of relevant articles published in open literature since 1970s (> 3,000 articles). Nevertheless, over the past 15 years, we have seen growing interest among membrane scientists to study the roles of polymeric substrate and perform in-depth analyses on how the changes in the substrate physicochemical properties could affect polyamide layer structure and thus membrane performance. Recent advancements in the new polymeric materials development and nanomaterials synthesis have opened a lot of opportunities for new generation substrate development. Compared to the pressure-driven membrane processes, the substrate of TFC membranes plays a more significant role in osmotically-driven process as the occurrence of internal concentration polarization (ICP) (within the substrate) to reduce the available driving force for osmosis and may be regarded as an artificial source of inefficiency in FO/PRO process. Considering the importance of TFC membranes for industrial separation process, this review will give a high-quality state-of-the-art account of the subject matter by emphasizing the substrates made by different techniques (e.g., Loeb-Sourirajan phase inversion method, doubleblade casting, electrospinning, and surface modification technique) and various materials (e.g., new polymeric materials, polymer-polymer composite, polymer-inorganic nanocomposite, etc.). More specifically, the article will review the roles of the developed substrates on the chemical and physical properties of polyamide selective layer and further their influences on the TFC membrane performances for both pressure-driven (NF/RO) and osmotically-driven (FO/PRO) processes, aiming to stimulat...

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