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 poly-ethersulfone 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 eval-uation 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.

Automated summary

This study focuses on the development of new thin-film composite hollow fiber membranes for harvesting salinity gradient energy in pressure retarded osmosis applications. The membranes are designed to exploit the energy difference between wastewater reverse osmosis retentate and seawater reverse osmosis brine through osmotic mixing. Different-sized hollow fiber membranes were created using interfacial polymerization, and their performance was evaluated at lab scale and pilot scale. As module size increased, the water permeability and power density decreased, resulting in lower energy harvest in the pilot scale evaluation.