Industrial scale thin-film composite membrane modules for salinity-gradient energy harvesting through pressure retarded osmosis

The current work highlights development of novel thin-film composite hollow fiber membranes for pressure retarded osmosis applications to harvest salinity gradient energy. The membranes were developed with a specific target of harnessing the salinity gradient energy between wastewater reverse osmosis retentate and seawater reverse osmosis brine via osmotic mixing. The hollow fiber membranes were prepared by coating a polyethersulfone substrate with a thin-film composite polyamide layer via interfacial polymerization, which were assembled into modules of different diameters for lab scale and pilot scale evaluation. As the module size increased from 1-in. to 2-in., and 4-in., the water permeability, tested against a 1000 mg/L sodium chloride solution at 15 bar, decreased from 2.6 L m−2 h−1 bar−1 to 2.0 L m−2 h−1 bar−1, and 1.2 L m−2 h−1 bar−1, respectively. The power density, measured in the lab-scale unit using 1 M sodium chloride draw solution, and DI water feed solution, also decreased from 9.1 W/m2 to 5.3 W/m2 with increasing module size. Pilot scale evaluation of the 4-and 8-in. modules on a 24 m3/day unit resulted in lower power densities of 2.5 W/m2 and 1.5 W/m2 which translated to 0.024 kWh/m3 and 0.05 kWh/m3 of salinity-gradient energy harvested, respectively. [Display omitted] •Novel thin-film composite (TFC) coating developed for polyethersulfone (PES) hollow fiber membranes for pressure retarded osmosis (PRO) applications.•A novel method of pressurizing the reagent stream along the hollow fibers was implemented to ensure homogenous surface coverage of aqueous phase during the interfacial polymerization.•PES hollow fibers and the TFC coating process scaled up from lab scale to industrial scale to demonstrate feasibility of large-scale manufacturing.•In the lab-scale validation of 1-, 2-, and 4-inch modules, the power density remained at ≥ 5 w/m2 indicating the applicability of these membranes in osmotic energy harvesting.•Pilot scale testing of 4- and 8-inch modules was carried out with seawater reverse osmosis (SWRO) retentate as the draw solution and a wastewater reverse osmosis reject as the feed solution.