Interrelated Resistance Mechanisms in Fouling Layers on Reverse Osmosis Membranes for Water Treatment
Final Report Abstract
Cake layer formation by colloidal particles (colloidal fouling) and the related increase in the hydraulic resistance is, besides biofouling, one of the main causes of energetic performance loss in membrane based desalination processes. Although this loss is economically highly relevant, physicochemical mechanisms behind the cake layer related increase of hydraulic resistance are still poorly understood. Relevant interactions between simultaneously occurring membrane phenomena like concentration polarization (CP), surface charge screening, and coupled fluxes through membranes are insufficiently investigated. One important reason for that circumstance is the lack of differentiated experimental methods by means of which the measured overall hydraulic resistance (or its change) could be explicitly attributed to a specific type of resistance. The derivation of such relationships is highly complex since the formation of a cake layer, with its associated hydraulic resistance RF, also influences the resistance of the concentration polarization layer RCP (altering to RCP,F) and of the membrane RM (altering to RM,F) by a variety of potentially unknown coupling mechanisms. Therefore, the measured fouling related increase of the overall hydraulic resistance OHR is not necessarily equivalent to RF but may also be due to associated variations of RCP,F and/or RM,F. Within the present study innovative experimental and numerical methods have been applied to achieve a differentiated quantitative assessment of the cake related resistance mechanisms in dense membrane applications (reverse osmosis (RO) and nanofiltration (NF)). A fluid domain is developed to reproduce hydrodynamic flow conditions of spiral wound modules operated with industrial facilities parameter. Continuum mechanical calculations show small dead zones at the exterior boundaries that are excluded for further simulations. The major part of the fluid domain is characterized as laminar flow. The fluid domain is cut into subdomains of different size to fit the requirements of the various numerical approaches. Euler-Euler and Euler-Lagrange approach are applied to investigate dispersed multiphase flow with particles in micrometer scale. A coupled fluid-granular simulation is established using unresolved CFD-DEM method, resolving time and dimensions in nanoscale. Solid phase and fluid phase are interacting by exchanging mass, heat and momentum. Cross-flow (CF) experiments show that CP is significantly increased in presence of a colloidal cake. This mechanism, commonly referred to as cake enhanced concentration polarization (CECP), is usually associated with a significant increase in OHR. However, for nanofiltration (NF) membranes the contradictory effect that CECP results in OHR smaller than the sum of the individually measured values of RM, RF and RCP is shown. As it was not possible to differentiate between the single resistances and to assess their individual contribution in order to acquire generalizable quantitative information on the parameters that specifically determine the extent of RF in CF mode an innovative dead-end (DE) filtration method is proposed. By using this method it can be determined how RF reacts to changes in the feed water composition and the associated change in CP. By fixing the extent of CP to a maximum steady-state value the osmotic equilibrium across the membrane (and the trans-membrane osmotic pressure respectively) is preserved as long as flux and feed water composition are maintained. Accordingly, when all filtration parameters besides the presence of a cake layer are maintained, any change in OHR can be directly attributed to a change in RF. DE experiments show that the only explanation for the determined reduction of resistance in case of simultaneous measurement in CF can be a significant reduction of the membrane resistance RM in presence of a cake layer and CP. It is postulated that this so far unknown effect is caused by an extended transport of ions into the membrane pores by a coupled convective salt transport causing a broadening of the pores by electrostatic or osmotic effects. This effect can thus only be seen with less dense membranes for which the salt transport is not only driven by diffusion but also by water flux as for NF and in parts RO membranes.
Publications
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Backwash of dead-end capillary membranes: numerical simulation of multiphase flow and the effects of different operation conditions, Membranes, 3 249-265 2013
Mansour H., Keller A., Gimbel R., Kowalczyk W.
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Eine neue Versuchsanlage für die systemunabhängige Analyse und Quantifizierung von Deckschichtwiderständen auf Umkehrosmosemembranen, 10th Aachen Conference Water and Membranes, Aachen, Germany, 29. ‐ 30.10.2013
Keller, M., Gimbel, R.
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Investigation of Inlet Boundary Conditions on Capillary Membrane with Porous Wall During Dead-End Backwash, Journal of Membrane Science & Technology, 2015, 5:138
Mansour H., Kowalczyk W.
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Measuring hydraulic layer resistance and correlated effects in colloidal fouling of salt-retaining membranes, Water Science and Technology: Water Supply
M. Keller, S. Panglisch, R. Gimbel
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Relevance of hydraulic layer resistance and correlated effects in colloidal fouling of salt‐retaining membranes, IWA Specialized Conference ‐ Advances in particle science and separation: Meeting tomorrow's challenges, Oslo, Norway, 22. ‐ 24.06.2016
Keller, M., Panglisch, S., Gimbel, R.
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Zur Bedeutung hydraulisch bedingter Deckschichtwiderstände beim Fouling salzrückhaltender Membranen unter besonderer Berücksichtigung von Kopplungseffekten, Dissertation, Faculty of Engineering, Department of Mechanical and Process Engineering, University of Duisburg-Essen, 2016
Keller, M.
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Analysis of Fluid-Dynamical and Multiphase Flow Aspects of Capillary Membrane Backwashing, Dissertation, Faculty of Engineering, Department of Mechanical and Process Engineering, University of Duisburg-Essen, 2017
Mansour, H.
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Eine neue Messtechnik zur Differenzierung der Einflüsse von Wassermatrixeffekten auf den Deckschichtwiderstand bei Entsalzungsmembranen, DGMT Tagung "Neue Entwicklungen in der Membrantechnik", Kassel, Germany, 08. - 09.02.2017
Keller, M., Panglisch, S.
