In this report, tests on the impact of the sludge properties on the oxygen transfer at low and high solids contents are presented. Additional to the oxygen transfer tests, the activated sludge was intensively analysed to examine the changes of the -factor in relation to the sludge properties (rheology, EPS, CST, etc.). The a-factor did strongly decrease in all sludge types at increasing MLSS or increasing viscosity, respectively. In the second test stage, the impact of the aeration system was examined in detail. For these tests, the same sludge from a membrane bioreactor was used throughout. Apart from the impact of the power density in the reactor and the specific air throughput, the main focus was on the economic efficiency of the examined systems in cases of high MLSS. It became apparent that up to solids contents of 18 g/l the fine-bubble aeration is the most economically efficient method.