In the last decade, a number of methods have been developed for the in-situ characterization of fouling layers formed in confined geometry. Laser sheet at grazing incidence and ultrasonic time domain reflectometry are relevant methods to measure fouling layer thickness. However, these two methods have never been used simultaneously during the same filtration run. The objective of this study was to compare values measured by both methods. After validation of the thicknesses given by each method on an especially designed calibrated gauge, measurements were made simultaneously by both methods on porous fouling layers formed on two membranes with different permeabilities. The results show that, in the case of a compact fouling layer, the thicknesses given by the two methods are the same. However, for more porous layers, such as concentration polarization layers, thicknesses differ, although the growth kinetics is identical. Thus, laser sheet at grazing incidence and ultrasonic time domain reflectometry are two complementary methods to determine fouling layer thickness and/or its growth kinetics according to operating conditions.

In this work, we propose a real-time analysis of the growth of a granular material by a high frequency ultrasonic method (around 5MHz) based on normal incidence reflectometry. This method is non-destructive. It can be applied to the general case of cloudy solutions in optically opaque sedimentation cells. Results were obtained on glass balls used as a model granular material. Glass balls were sedimenting in water. Measurement of the deposit thickness (varying from several dozen of micrometers to several millimeters) leads to morphological and dynamic properties such as the volume solid fraction and the deposit growing rate. The strong influence of the particle size distribution (median diameter, broadness of the size distribution) on the dynamic properties of the particle packing was determined quantitatively. Our results are in good agreement with the classical model of sedimentation for an isolated sphere (Stokes law).