In order to control and scale up activated sludge's processes, such as aeration tanks in wastewater treatment plants, it is important to get precise rheological measurements of sludge flows in a sufficiently large range of solid concentration. Indeed, the development of new processes such as Membrane Bioreactors enhances the range of biomass concentrations encountered in wastewater treatment plants. As a consequence, in this paper, we are interested by measuring flows of sludge with solid contents varying from about 2.5 to 57 g/L (activated to concentrated sludge). Considering the consistency of these sludges, among available measuring geometries, concentric cylinders (CC) and double concentric cylinders (DCC) are tested. This leads to the conclusion that, in our case, only the CC system can be used to perform the experiments. Flow and dynamic measurements are then carried out using the CC geometry. The analysis of several rheological parameters is presented for various total suspended solid (TSS) values (27 g/L to about 57 g/L). Finally, empirical equations are proposed to model the influence of the TSS concentration on rheological parameters such as the yield stress, the flow index and also the consistency index and the energy of cohesion of the sludge material. (c) 2006 Elsevier Ltd. All rights reserved.