Channelizing the osmotic energy of proximate sea bittern for concentration of seawater by forward osmosis under realistic conditions to conserve land requirement for solar sea salt production

The present investigation relates to partial dewatering of seawater or seawater-like feed solutions (π = 20–26 bar) by Forward Osmosis (FO) using concentrated brine/virgin sea bittern (π = 549–715 bar) available in proximity. An Aquaporin Inside™ hollow fibre FO membrane module (0.6 m2 active area) was used for the study. Initial studies with deionized water as feed and sea bittern as draw revealed that the reverse solute flux was 4560–6066 mg m−2 h−1, with salt rejection efficiency of 99.45–99.87%. Studies with ca. 36,000 mg L−1 NaCl feed and 24 °Bé [°Bé= 145(1–1/ρ), where ρ = specific gravity] concentrated sea brine draw resulted in 26.41%, 13.29% and 9.15% dewatering of feed (v/v) in continuous single pass (CSP) mode, with average flux (Jw) of 6.17, 6.48 and 6.73 L m−2 h−1, for feed to draw (F/D) ratios of 10:1, 20:1 and 30:1 v/v, respectively. Outlet feed concentrations estimated from density data were in good agreement with the values computed considering pure water permeation. FO experiments were subsequently undertaken with 2.84 °Bé natural seawater as feed and 29 °Bé virgin sea bittern as draw. The feed and draw flow rates were 45 and 1.5 L h−1, respectively, in the CSP mode in accordance with their relative abundance in a salt works. The extent of dewatering was 11% (v/v) and average Jw was 8.3 L m−2 h−1. When the same study was repeated in recirculation (RC) mode maintaining feed, draw and F/D ratio constant, the extent of dewatering was enhanced to 16.14% over 30 min of operation. A plot of Jw versus ∆π was also obtained, which exhibited deviation from linearity at large driving force. This was ascribed to dilutive internal concentration polarization and its effect was quantified. CSP mode was advantageous in terms of higher throughput, reduced contamination of feed with draw constituents, and lower circulation cost, while RC mode gave higher extent of dewatering over extended periods of operation. 10.17–19.22% of the pent-up osmotic energy in the draw amount taken was utilized for dewatering, all experiments considered. Partial dewatering of inlet stream by outlet stream through FO would help conserve 9–11% of land requirement for solar salt production from 3.5 to 2.8 °Bé seawater. It would also enable safer discharge of sea bittern owing to its ca. fourfold dilution.