|M.Sc Student||Epstein Jose Agustin|
|Subject||In-Situ Micro-Rheology of a Foulant Layer|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Guy Ramon|
|Full Thesis text|
Fouling is one of the main drawbacks in membrane processes for water filtration. The performance drop due to fouling can lead to a high rise in the operational costs. Bio-fouling is still regarded as a major challenge in the operation of membrane-based water production in desalinization plants.
This research shows a method to analyze the foulant mechanical properties in-situ, employing a video-based multiple particle tracking micro-rheology technique. Foulants, usually non-Newtonian materials, provide different responses to stresses applied on a water filtration membrane, these operational conditions dependent mechanical properties present many difficulties to be analyzed in-situ. Until now, mainly macro-scale rheometry techniques has been employed to analyze the foulant mechanical properties ex-situ. These analyses are missing to measure the foulant mechanical response while the foulant undergoes intrinsic stresses of the filtration process. We expand the current fouling literature bringing a method to analyze the foulant rheology at different operational conditions.
Micro-rheology enables us to analyze the foulant mechanical properties on the micro-scale, analyzing depth variations with confocal microscopy. The measured frequency-dependent foulant mechanical properties change at different operational conditions, cross-flow, permeate flux, and transmembrane pressure are factors that can change the material properties, with results indicating material behaviors that vary between a viscoelastic solid, gel-like, and viscoelastic liquid. Also, for some conditions a material relaxation can be observed at some alginate depths. We show in this research different elastic and viscous responses for different frequencies, at different operation conditions such as permeate flux and transmembrane pressure.
We show that different cross-flow velocities add an oscillatory non-Brownian motion, the oscillations take place in the direction of the cross-flow. The particle oscillations amplitude is proportional to the foulant stiffness. The observed particles trajectories are analyzed by their micro-scale degree of confinement when the particles are embedded in the foulant. To analyze the particles trajectories confinement quantitatively we compare changes in the mean square displacement (MSD) for a specific foulant layer undergoing different cross-flow velocities. The obtained MSDs show an increase of an apparent anisotropic diffusion when increasing the cross-flow.
In addition, we show the alginate resistance to permeate flux in a dead-end configuration for different permeate fluxes. We can correlate that while we observe a material macro-scale relaxation in the transmembrane pressure vs time curve, we also observe relaxation for some alginate depths in the micro-rheology analysis. Pointing that the observed alginate deposited cake resistance to permeate flux can be an individual contribution of different rheological responses through its material structure.
To summarize, the goal of this thesis is to bring a method that enables the measuring of the fouling mechanical properties in-situ, where we can compare the foulant rheology with its hydraulic resistance at a given operational condition. This insight can help further studies to analyze the impact of the foulant rheology at a given condition on the backwash or cross-flow effectiveness to remove the foulant.