Thin-film composite (TFC) hollow fiber membrane with double-polyamide active layers for internal concentration polarization and fouling mitigation in osmotic processes

Internal concentration polarization (ICP) and severe irreversible fouling occurring within the porous and tortuous substrates of the forward osmosis (FO) and pressure retarded osmosis (PRO) membranes have significantly hidden their applications for water purification and osmotic power generation. This study experimentally demonstrates that designing a double-skin structure in FO and PRO membranes can effectively control ICP and their fouling propensity. Thin-film composite (TFC) hollow fiber membranes consist of an inner polyamide selective skin and an outer polyamide sealing layer were successfully fabricated by double interfacial polymerizations on a tailored polyethersulfone (PES) fiber substrate (termed as dTFC-PES). Due to the outstanding rejection of the outer polyamide sealing layer, the penetration of inorganic salts and foulants into the substrate is sufficiently blocked. As a result, not only ICP and fouling inside the membrane are effectively minimized, but also sustainable FO and PRO performances are achieved. By using real wastewater contains multiple inorganic salts and organic foulants as the feed, the dTFC-PES membrane shows a quite low flux decline of 29% in FO operations under the PRO mode at an ultrahigh feed recovery of 80%. Under PRO tests for power generation, the membrane power density slightly drops to 90.8% of the initial value after a 12-h test at ΔP=15 bar. In addition, since foulants are primarily accumulated on the surface of the polyamide sealing layer, physically flushing the fouled membrane surface by either freshwater or commercial cleaner Genesol 704 can efficiently restore the water flux back to its initial level with a recovery rate of 87% or 98%, respectively. This study may offer useful insights and meaningful strategies for the development of effective antifouling FO and PRO membranes.