Development of Robust and High Performance Hollow Fiber Membranes Used in Pressure Retarded Osmosis Process

Pressure retarded osmosis (PRO) is a process where water flows naturally from a low salinity stream (feed water) at an ambient pressure across a semi-permeable membrane to a pressurized high salinity stream (draw solution) driven by the osmotic pressure difference across the membrane. The increased volume of pressurized solution can be utilized to drive a turbine for power generation. As such, PRO process is considered as a promising approach to harvest salinity gradient energy.

In the PRO process, the key element is the membrane. To make the PRO commercially viable, the membrane has to withstand significant hydrostatic pressures and possess high water flux, as the power density is determined by the trans-membrane hydrostatic pressure and the water flux permeating across the membrane. However, developing robust and high performance membranes suitable for PRO application is a challenging task though many efforts have been reported in recent years. The common problem encountered in thin-film composite (TFC) flat sheet membranes is the membrane deformation under high pressure, which resulted in severe salt leakage through the membrane. Consequently, the power density was depressed significantly.

This presentation will highlight the recent advances in developing novel PRO hollow fiber membranes from the Singapore Membrane Technology Centre. Hollow fiber membranes are self-supported in structure, which do not suffer from the problem caused by the spacer in TFC flat sheet mmebrane. We use the nonsolvent induced phase separation (NIPS) method to fabricate the PES/PEI hollow fiber substrates. Subsequently, a polyamide active layer was synthesized on the inner surface of the as-spun hollow fiber substrate by interfacial polymerization. Eventually, a TFC hollow fiber membrane was composed with an ultra-thin dense polyamide skin layer on the inner surface of a thin and porous support substrate. Our first generation PRO hollow fiber membrane can withstand 9 bar pressure to achieve a power density of 11 W/m².  The newly developed second generation PRO hollow fiber membrane can withstand 15 bar pressure to serve better in the PRO process. In the PRO tests, several pairs of feed water and draw solution were used to evaluate the membrane performance in terms of water flux and power density. Experiments reveal that the second generation PRO hollowfiber membrane can achieve a power density as high as 20 W/m², using synthetic river water (1 mM NaCl) and seawater brine (1 M NaCl) as the feed water and draw solution, respectively, suggesting its great potential for PRO application.