Visualization by digital holographic interferometry of flux velocity effect in cross-flow reverse osmosis

Abstract

Digital holographic interferometry (DHI) allows interferometric fringe patterns to be obtained, that are indicative of changes in the optical path followed by the light, and are related to changes in the refractive index. In the case of the appearance of the polarization concentration layer during the reverse osmosis process, changes in the concentration distribution, and therefore in the refractive index distribution, can be visualized as an interference fringe pattern. The phenomenon called concentration polarization occurs as filtration proceeds and the solute rejected by the membrane builds up a layer of high concentration near the membrane surface. The formation and development of the concentration polarization layer in the RO process will decrease the effective water filtration driving force and therefore, will decrease the permeate flux. To reduce or control concentration polarization many possible methods have been considered, one of them is the increase of shear at the membrane surface using a greater cross-flow velocity. In this work, DHI has been used to visualize the effect of cross-flow velocity during reverse osmosis of salts. All the experiments were carried out at a constant pressure of 6 bar, using a special module designed with transparent windows to observe the membrane surface. In all experiments a thin film reverse osmosis membrane was used. Two solutes (NaCl and Na2SO4), with different feed concentration (3.5 - 8.5 kg/m3) and different cross-flow velocities (0.002 - 0.017 m/s) (Re = 10 - 77) were used. In every run, the module was filled with the salt solution. The reference state (recorded by a CCD camera) was obtained with the salt solution flowing at the initial cross-flow velocity without pressure. During the reverse osmosis process, different fringe patterns were digitally reconstructed. In all runs, the higher the cross-flow velocity, the lesser the number of interference fringes that appears, showing that the concentration polarization has a minor effect. When higher cross-flow velocities (which reduce the concentration polarization layer) were used, the permeate flux increased.