Dear Jaedeok Seo

To get an osmotic flow you shuld be have a significant difference between the feed and draw solution (salt concentration).

Regards

Khalid Turki Rashid

Thanks for kind answer. Could I know how much the typical “significant difference” in this field?? Can 0.5 Mol/L be significant??

First of all to generate any water flux, you will need a difference in osmotic pressure across the active layer of the membrane. I assume, that you use a TFC (thin-film composite) membrane consisting of a thin polyamide (active) layer and a porous support. Polarization effects will occur if you run two osmotic solutions (feed & draw): On the feed side the osmolarity of your NaCl solution increase in the boundary layer next to the active layer due to non-ideal mixing and the water transport (external concentration polarization). On the draw side (support) of the membrane the NaCl concentration will decrease for the same reason (internal concentration polarization). The polarization effect in the support is normally stronger in FO applications. Since you are using a RO membrane, which support is denser than the support of FO membrane, internal concentration polarization will be even stronger.

In regard to your second questions, it is not only the difference of osmolarity upon which the flux relies on. It is the concentration range as well. Example: 0.5M NaCl against DI water will aim higher fluxes than 1M NaCl against 0.5M NaCl. Furthermore, membrane properties will also play an important role here. There are publications about how to calculate water fluxes dependent on the osmotic pressure of feed and draw as well as membrane properties (permeability, structural parameter of the support).