Water Environment Federation (WEF)

Forward (Direct) Osmosis: A Novel and Prospective Process for Brine Control

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Courtesy of Courtesy of Water Environment Federation (WEF)

This paper reports the study of a novel process - Forward (Direct) Osmosis (FO) that potentially can be used for brine control. The driving force in the described FO process is provided by a draw solution comprising highly concentrated ammonium bicarbonate or fructose solution. Using a commercially available FO membrane, experiments conducted using a crossflow, flatsheet membrane cell yielded water fluxes ranging from 0.3 to 6.5 gal⋅ft-2⋅d-1 for a wide range of draw and feed solution concentrations. It was found, however, that the experimental water fluxes were lower than expected due to internal and external concentration polarizations (CP). SEM images of the FO membrane showed that it has a loose fabric porous support layer, which caused internal CP to a large extent. Explanations were given for the transport phenomena in the FO membrane. The dense selective layer of a common cellulose acetate RO membrane was split from the supporting layer and tested. The result confirmed our conclusion that an ideal FO membrane should be one with a thin dense selective layer without loose fabric support layer. This study also suggests that applying the FO process to concentrate brine to simplify its handling is feasible when using an ideal FO membrane and a suitable draw solution, which require further research.

Forward (direct) osmosis (FO) is a novel and prospective water treatment process that emerges as a result of water scarcity and energy crisis. In the FO process, a solution of considerably high concentration (draw solution) is utilized to generate a hydrostatic osmotic pressure gradient across a semi-permeable membrane to extract freshwater from feed solution (such as seawater or brine), which is on the other side of the membrane. Since this process capitalizes the phenomenon of natural osmosis, little energy and thus, little cost is required as compared to traditional technologies such as RO process or distillation.

Previous work on FO through semi-permeable membranes, while limited, still exposed two significant drawbacks. Experiment results showed that RO membranes were not ideal for FO as water flux was found to be low (McCutcheon et al., 2005; Ng et al., 2006). Finding a suitable draw solute is also crucial for the FO process. To date, the FO process still needs to be better understood. The objective of this report is to investigate the relationships among the draw solution concentration, feed water salinity, membrane structure and the permeate flux behavior in the FO process. The draw solution concentration was varied over a wide range (up to 6M), processing feed waters with salinities as high as 2M of sodium chloride. Based on these experiment results, explanations were proposed for permeate water flux behavior in the FO process.

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