In biological Wastewater Treatment Plants operated with the activated sludge process, the resulting suspension is a very complex non-Newtonian material. A better understanding of sludge flows could lead to improvements in the process itself. For the purpose of sludge characterisation, rheology is a well-known relevant investigation tool. In this study, Theological measurements have been carried out on sewage sludge with different total suspended solid (TSS) contents ranging from 4 to 43 g/L. Three measuring geometries, a concentric cylinders (CC) system, a double concentric cylinders (DCC2) system and a helical ribbon impeller (HRI) have first been compared in their ability to accurately characterise the rheological properties of sludge suspensions. The Herschel-Bulkley model has then been chosen to represent the flow curves. The evolution of the model parameters drawn as a function of TSS content for CC and HRI systems showed that, with both geometries, viscoplastic (VP) and shear-thinning (ST) behaviours increased with increasing TSS. Finally, the role of exocellular polymers in terms of adsorbed exopolysaccharides (EPS) concentration has also been investigated. As the sludge ages, the results showed that a decrease in EPS concentration induced a decrease in both VP and ST properties. (C) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.