Optimization of TFC-PES hollow fiber membranes for reverse osmosis (RO) and osmotically assisted reverse osmosis (OARO) applications

Abstract

The osmotically assisted reverse osmosis (OARO) has been recently proposed to increase the water recovery of the reverse osmosis (RO) process, in which RO has reached its limitations. Strong membranes are essential for both OARO and RO processes. Here, we have developed high mechanical strength thin-film composite (TFC) hollow fiber membranes for both RO and OARO processes. The newly developed TFC hollow fiber membranes consist of a polyamide layer synthesized via interfacial polymerization on the inner surface of PES hollow fiber substrates that have been optimized by controlling the bore and dope fluid flow rates, fiber dimension and morphology. The TFC-PES hollow fiber membranes have a pure water permeability (PWP) of around 2.5–3 L/(m2 h bar) (LMH/bar) and a NaCl rejection of around 97.5–98% for brackish water desalination at 20 bar. The water permeability drops from 2.15 to 0.06 LMH/bar when the NaCl concentration increases from 0.035 mol/L (2000 ppm) to 1.2 mol/L at 30 bar for OARO due to the concentrative and dilutive concentration polarization (ECP and ICP) that decreases the effective driving force. The structural parameter (S) and burst pressure (PB) of the newly developed membranes increase from 550 to 800 μm and from 47 to 104 bar, respectively, with an increase in the dope to bore flow rate ratio (D/B ratio) because of the thicker substrate wall and reduction in porosity. The optimal TFC-PES hollow fiber membrane for OARO has a burst pressure of 95 bar, structural parameter of 795 μm and water permeability of 0.09 LMH/bar when using 1.2 mol/L NaCl for OARO. To the best of our knowledge, this inner-selective TFC-PES hollow fiber membrane has the highest burst pressure to date, it has impressive high mechanical strength and good RO and OARO performance.