A B S T R AC T Since aeration is the largest cost factor in membrane bioreactor (MBR) operation it is clear that the biggest leap towards energy and operational costs savings can be achieved by improving the use of air. Many basics of the complex two-phase fl ow in membrane modules and in the overall MBR tank as well as their interactions, however, are still poorly understood. This work focuses both on fundamental studies on shear stress exerted by rising bubbles and on optimising the

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... of tank and module accordingly in order to obtain an improved deposition control at minimum energy input. For both, parameter studies were carried out by numerical simulations which were validated with experimental measurements. The optimum bubble size/channel width combination depended on the superimposed liquid velocity. The relationship between the liquid circulation velocity and the aeration intensity was measured for different reactor and module geometries. A modifi cation of the Chisti model for airlift loop reactors was also performed which can be used as a design rule for tank and module geometry or aeration rate. At the same gas fl ow rate, a 30-50% increase in liquid circulation velocity was achieved by a simple modifi cation of the sparger and the entry zone to the riser section.