Experimental study of concentration polarization in a crossflow reverse osmosis system using Digital Holographic Interferometry

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 visualized as interference fringes. Experiments with Na2SO4 and NaCl solutions have been carried out. Three variables of the process were studied: crossflow velocity, initial concentration and pressure applied. In each experiment, crossflow velocity was changed every 30 min, in an increasing or decreasing sequence. Few minutes after changing the crossflow velocity the steady-state was reached. Interference fringe patterns of the polarization layer and their corresponding concentration profiles, as well as the permeate flux in different experimental conditions, are presented. The major experimental result is the visualization for the first time in situ and in real-time of the polarization layer in a process of crossflow by a non-invasive method. Experimental results show a close relationship among crossflow velocity, permeate flux and polarization layer. Furthermore, experimental maximum concentration values reached at the membrane surface were compared with values calculated by using the film theory approach and a good agreement was obtained.