The evolution of the concentration polarization layer during crossflow reverse osmosis in a slit channel has been studied. Digital Holographic Interferometry allows visualizing the polarization layer as an interference fringe pattern. An especial module with four windows has been designed to see the development of the polarization layer along the membrane channel. Several experiments with a constant transmembrane pressure of 6 bar, three different feed concentrations (3, 6 and 9 kg/m 3 ) and three different Re (13, 38 and 111) had been carried out. The process has been modelled simulating the experimental conditions. The computed results were found to be consistent with the experimental ones. All the experiments show a continuous increase of the polarization layer along the channel, regardless of the crossflow velocity, except near the outlet of the cell due to an edge effect. This increase in the polarization layer is greater at lower Re, although it does not influence very much the permeate flux. In contrast, what substantially affects the permeate flux, much more than Re number, is the feed concentration due to its osmotic pressure.

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Digital Holographic Interferometry (DHI) has been used to visualize the polarization concentration layer during crossflow RO. This technique is based on the fact that changes in the concentration of the solution produce changes in its refractive index.Therefore, the concentration profile formed due to the polarization phenomenon can be

An optical method (real-time holographic interferometry) has been used to visualize concentration changes in the vicinity of the membrane surface during the dead-end reverse osmosis of salt solutions. This interferometric technique is based on the fact that changes in refractive index, which are associated with changes in concentration, can be visualized as interference fringes. Reverse osmosis experiments with NaCl and Na 2 SO 4 solutions with feed concentration in the range of 1-7 kg/m 3 at a constant pressure of 600 kPa have been conducted. Interferograms obtained under different experimental conditions, as well as permeate flux and membrane rejection, are presented. Concentration profiles in the concentration polarization layer have been determined from these interferograms and compared with those calculated using the mixed convection-diffusion and osmotic pressure theory or Fick's second law of diffusion, depending on whether the profiles correspond to the development or the disappearance of the layer. The reasonable agreement obtained between experimental and calculated results seems to support the validity of the mathematical models proposed in the range of the experimental conditions studied.

Real-time holographic interferometry, previously used to measure concentration profiles in the polarized layer during membrane processes, has been applied to visualize the buoyancy effects on dead-end reverse osmosis of salts by rotating the cell 90° and 180° from its original position (0°). Sets of experiments have been carried out, each one with the membrane in a different gravitational orientation, using NaCl and Na2SO4 with a feed concentration ranging from 1 to 7 kg/m3 at a constant pressure of 600 kPa. The interferometric fringe patterns obtained in each membrane position were very different. At the 0° position, the evolution of the polarization layer was observed by means of several interferometric fringes parallel to the membrane surface. At 180° position, the interferometric fringe pattern obtained pointed out the existence of a natural convection or buoyancy flow in the vicinity of the membrane surface which prevented the growth of the polarization layer. Finally, with the membrane vertically placed, an intermediate situation occurred. The combination of the diffusive movement of the solute (horizontal) and the natural convection currents (vertical) caused by the density gradient established in the module produced deformations on the interference fringes. The main consequence of the buoyancy effect is a great enhancement of the membrane performance and, therefore, a higher effectiveness of the reverse osmosis process.