In membrane desalination, the phenomenon of concentration polarization is the fundamental base of a variety of other effects influencing membrane performance, which is extensively discussed in literature, e.g. scaling and fouling. The experimental research on directly measuring concentration polarization is mainly based on micro-electrodes or optical methods while the latter has the advantage of being an insitu technique without disturbing the measured phenomenon. Optical methods like Shadowgraphy, Schlieren or (Digital) Holographic Interferometry have been employed in different variations measuring the second-, first- or zeroth-order derivative of the refraction index in the investigated solution. The refraction index, however, depends not only on the concentration but also on the temperature, which is assumed to be constant throughout most published research. As the influence of temperature differences of only few K on the refraction index is in the same order of magnitude as concentration differences of few g/kg, especially in reverse osmosis and membrane distillation, it should be an aim to be able to resolve both concentration and temperature polarization simultaneously. There are optical methods, which are capable of simultaneous investigation of heat and mass transfer boundary layers, such as two-wavelengths Holographic Interferometry. The experimental requirements on accuracy and alignment of equipment are even higher in comparison to single-wavelength Holographic Interferometry. In this work, three alternative approaches are presented and compared in view of measurement accuracy and applicability of the proposed algorithms for quantitative evaluation in post-processing. These are two-wavelengths Schlieren, single-wavelength Digital Holographic Interferometry and combined single-wavelength Digital Holographic Interferometry with LaserSchlieren. The present study is focused on concentration and temperature polarization in reverse osmosis and membrane distillation; however, the findings about optical methods are not limited to these desalination technologies.     «

In membrane desalination, the phenomenon of concentration polarization is the fundamental base of a variety of other effects influencing membrane performance, which is extensively discussed in literature, e.g. scaling and fouling. The experimental research on directly measuring concentration polarization is mainly based on micro-electrodes or optical methods while the latter has the advantage of being an insitu technique without disturbing the measured phenomenon. Optical methods like Shadow...     »