Water Supply Vol 3 No 5 pp 117124 © IWA Publishing 2003
Formation and prevention of hardly removable particle layers in inside-out capillary membranes operating in dead-end mode
IWW Rheinisch-Westfälisches Institut für Wasserforschung, Moritzstr. 26, D-45476 Mülheim an der Ruhr, Germany (E-mail: email@example.com)
Although numerous ultra- and microfiltration dead-end plants with capillary membranes are already operative, some phenomena are still unexplained. Therefore, the fundamental processes taking place inside a capillary membrane were observed. Initially, the flow field depending on axial and radial position inside the capillary was determined. Then, particle transport and deposition were theoretically studied by determination of particle trajectories considering influences of particle concentration and walleffects on hydrodynamics, DLVO-forces, buoyancy, gravitation, diffusion and interparticular forces. Following these calculations, incoming particles with a diameter smaller than the so-called "limiting diameter", which depends on operational and geometrical boundary conditions, due to depositions are widely and evenly distributed. Larger particles do not deposit until they are at a certain distance from the water inlet. The larger the particle size, the longer the distance. If the particle is larger than the so-called "corkforming diameter" then the particles are transported to the dead-end of the capillary which may cause a clogging of the capillary. This "corkforming diameter" depends on operational as well on geometrical boundary conditions. These theoretical predictions are confirmed by experimental results from investigations with spherical latex and non-spherical walnut particles. To avoid this clogging, the deposition of the particles should be evenly distributed, which means that the “corkforming diameter” should be as large as possible. That goal could be achieved by operating the membrane plant with short and/or wide capillaries. However, a small permeate flux, a small membrane resistance and/or a small membrane surface potential succeed as well. Another possibility could be to operate the capillaries with a very small cross flow.
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