Short- and long-term performance of the thin-film composite forward osmosis (TFC-FO) hollow fiber membranes for oily wastewater purification

Fouling behavior of thin-film composite (TFC) membranes synthesized on sulfonated polyphenylenesulfone (sPPSU) hollow fiber substrates was investigated for separating oil-water emulsions under forward osmosis (FO). The water and salt reverse fluxes of the TFC-FO hollow fiber membranes were found to be stable for a period of 28 days when deionized (DI) water was used as feed. A series of fouling experiments were carried out, and it was observed that water flux decreased faster with increasing oil concentration of the feed solution. In addition, the rate of flux decline was rapid in the initial few hours, because of the cake-enhanced concentration polarization (CECP). Furthermore, we observed that the ratio of salt reverse flux to water flux (J_s/J_w) was much lower, compared to that of using DI water as feed. This may indicate that the oil fouling plugs some defects in the selective layer and the oil fouling layer hinders the salt transport, because of the CECP mechanism. Further investigations revealed that the FO fouling could be efficiently washed off using a solution containing 1 g/L NaOH and 0.3 g/L sodium dodecyl sulfate (SDS), and the water flux could be effectively recovered, to a large extent, within 5 days when separating a 500 ppm oil-water emulsion. Overall, the newly developed FO membranes can recover ∼80% of the water at a high average water flux of 10.4 LMH using 1 M NaCl as draw solution from a 500 ppm oil-water emulsion containing a low salt concentration of 0.5 g/L. This, in turn, demonstrates the potential of the FO membranes for oily wastewater reclamation.