(176c) Development of Thin-Film Composite FO Hollow Fiber Membranes and Their Potential Applications

Forward osmosis (FO) process has gained much attention recently because of its potential to either reduce energy consumption (in seawater desalination) or produce energy (pressure retarded osmosis). However, the viability of FO technology for large-scale implementation has been impeded by two main technical challenges: (1) the identification of a suitable draw solution which is able to generate a high osmotic pressure and is readily regenerated; and (2) the lack of appropriate membranes for FO application that can produce a reasonably high flux. This paper describes the development of novel hollow fiber FO membranes with a focus on the effect of porous substrate on the performance of resultant FO membranes. We also evaluate their potential applications. Thin film composite FO hollow fibers are fabricated by a two-step preparation ? a phase inversion for a hollow fiber substrate followed by an interfacial polymerization for a RO-like skin layer on either the outer or inner surface of the porous substrate. The water permeability A and NaCl salt permeability B of the FO hollow fiber membranes are determined by the RO-like selective layer, while the structure of the porous substrate affects the degree of internal concentration polarization (ICP) occurred inside it. The ICP is always a big concern in the FO process as it causes a significant flux decline. Under conditions of highly concentrated feed and draw solutions, the water flux is only determined by the membrane structural parameter, and the osmotic pressures of the feed and draw solutions. Thus, the key membrane ?characteristic' or the structural parameter, needs to be minimized to alleviate concentration polarization issue. Various polyethersulfone (PES) hollow fiber substrates with different structures are designed and fabricated. The substrates with similar UF-like inner skin layer but different sponge structures formed beneath the outer layer can be obtained by controlling the phase inversion occurred at the outside of nascent hollow fiber while maintaining the same bore fluid conditions. The ultra-thin polyamide-based RO-like selective layer is made on the UF-like inner surface of the substrate using the same condition of interfacial polymerization. As such, the selective layers of the FO membranes are anticipated to be similar. Therefore, the effect of substrates on the FO membrane can be studied independently from the influence of the inner selective layer. In addition, double-skinned substrates with an UF-like inner skin and an NF-like outer skin are designed and fabricated by the phase inversion plus plasma post-treatment. This structure is expected to overcome another potential problem encountered in the FO process - membrane fouling when the porous substrate faces the feed stream in the FO process. The performance of the FO membranes with/without plasma treatment are evaluated and compared.