In-situ micro-rheology of a foulant layer at a membrane surface

Abstract Fouling remains an operational limitation in membrane-based water purification. Despite extensive research on fouling, little is known about the mechanical properties of foulant deposits, their response to operating conditions and how these relate to fouling reversibility and resistance to permeation. Here, we demonstrate in-situ measurements of the micro-scale rheology of a foulant layer deposited at a membrane surface. A custom-made membrane cell was employed, fabricated with an optical window to enable high-resolution, real-time observation with a confocal microscope. Permeate flux and applied pressure were accurately maintained using a software-based controller. A particle-tracking technique was employed to assess the mechanical properties at different depths within the foulant layer. Through the analysis of particle trajectory statistics, the mechanical properties of the alginate layer are obtained in terms of the frequency-dependent storage and loss modulus. Importantly, this method enables the simultaneous measurement of foulant layer micro-mechanics and its macroscopic manifestation — the hydraulic resistance during filtration. Results demonstrate the elastic-gel characteristics of the foulant layer, and the variations in its storage and loss moduli upon flux increase, indicating micro-scale ‘stiffening’ of the material, as it undergoes compaction. In addition, the observed increase in the complex viscosity coincides with an increase in the hydraulic resistance. The foulant properties also exhibit clear depth variations, where the foulant’s apparent ‘stiffness’, as manifested by its complex viscosity, increases at closer proximity to the membrane surface. The presented approach provides fundamental insight and opens new possibilities for studying foulant mechanical properties, in-situ, which can lead to more effective cleaning and back-pulsing strategies for foulant removal, e.g., by selecting the most appropriate frequency and intensity.