Professor In S. Kim's research team has identified a flaw in the fluid diffusion theory for non-porous membranes

□ Professor In S. Kim of the School of Earth Sciences and Environmental Engineering at the Gwangju Institute of Science and Technology (GIST, President Seung Hyeon Moon) has proven that the underlying theoretical assumption of solution-diffusion theory * of non-porous membrane ** water treatment processes that hydraulic pressure equals osmotic pressure is wrong.

* The theory that the solvent (water) and the solute (ionic substance) permeate with independent driving force.

** The presence of pores is confirmed, but the separation is small and difficult to quantify.

□ For a long time, research has been actively conducted on the nonporous membrane-based forward osmosis * and reverse osmosis ** processes, and it is known that the nonporous membrane effectively removes ionic substances. However, there has been a lot of experimental evidence that the permeation characteristics cannot be interpreted by the existing fluid diffusion theory with simultaneous water pressure and osmotic pressure, and there has been controversy to verify the reason for this inaccuracy.

* A process of filtering water by osmotic pressure difference by arranging an unprotected membrane between two solutions having different concentrations

** A process of squeezing water through an unprotected membrane by applying a water pressure greater than the osmotic pressure of the solution (eg, seawater)

□ The research team studied the nonporous membranes used in the forward osmosis process. If the theoretical assumption was correct, then the water would not permeate when the same hydraulic and osmotic pressures act in opposite directions. The hypothesis that the salt permeation will be observed in proportion to the salt concentration according to the existing diffusion theory was examined through a dialectical approach.

∘ As a result of the analysis, it was confirmed that the water pressure always had a greater effect than the osmotic pressure. Therefore, it was confirmed that the hydraulic and osmotic pressure in the nonporous membrane are not equal driving forces, and the core assumptions of the conventional fluid diffusion theory are not suitable for analytical purposes.

□ By analyzing the relationship between water pressure and osmotic pressure under the condition that the water movement is not intentionally lowered by the water pressure, the researchers confirmed that osmotic pressure and salt permeation have a linear relationship. This linear relationship allows quantitative measurement of the vulnerability of the non-porous membrane to hydraulic pressure. Analysis of this can confirm the yield hydraulic pressure *. The researchers have successfully quantified the range of proper operating pressure (water pressure) of the nonporous membrane. This is a very significant achievement in terms of field applicability because about 19,000 seawater desalination plants are being constructed and operated worldwide as of 2018.

* After limiting hydraulic pressure, the salt removal ability of the membrane is markably reduced, and this is impossible to explain by fluid diffusion theory.

□ Professor In S. Kim said, "This is of great significance because the core assumption of fluid diffusion theory that hydraulic pressure equals osmotic pressure, which has been long accepted without any doubt, has just been proven to be the main cause of inaccuracy in permeability analysis. The analytical methods proposed in this study are expected to contribute to reliable design, construction, and operation of various non-porous membranes used in water treatment facilities."

□ This research was supported by a grant from Industrial Facilities & Infrastructure Research Program funded by Ministry of Land, Infrastructure, and Transport of the Korean government. The results of the research were published online in the Environmental Science & Technology (ES&T) on February 21, 2018.