Dead-end ultrafiltration has proved itself as a technique for reclamation of backwash water of sand filters and as a pretreatment step before spiral wound reverse osmosis. A direct result of dead-end filtration is a flux decrease in time caused by the accumulation of material in the membrane pores and on the membrane surface. Different cleaning techniques are used to remove this accumulated material. Recently a new technique has been introduced, the AirFlush®. This technique makes use of air to create higher turbulence as compared to a water flush. At Delft University of Technology in co-operation with X-flow and Norit Membrane Technology research has been started into the fundamentals of the combined air and water flush. As in many industrial processes, an equal division of water and air over an installation is very important. To check the distribution of water and air over the cross-sectional area of tubular and capillary membrane modules two different test installations have been built. The results from the experiments show that for tubular membrane modules the water and air distribution over the cross-sectional area of the module is not always equally divided. Improvements have to be obtained by a better air distribution system. For capillary membrane modules the distribution of water and air over the cross-sectional area is more equally divided. The results from the experiments are discussed taking into account the theory of two-phase flow. It is shown that from the theory of two-phase flow the good distribution for the capillary membrane module can be explained by the large frictional pressure drop compared to the hydrostatic pressure drop.

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