Abstract Since wastewater treatment has been a subject of interest, non-Newtonian flows, such as sludge flows, have been widely studied in the literature, most of the time from a rheological point of view. It is well known that the hydrodynamic behaviour of sludge flows is of prime importance to optimise process parameters of wastewater plants on one hand, as well as those of excess sludge retreating processes on the other hand. The present study is devoted to a synthesis of articles dealing with the rheological characterisation of biological wastewater treatment plant sludges (activated or concentrated ones) and some other concentrated suspensions such as microorganism ones. Attention is notably given to the rheological methods used and to rheological equations and behaviours observed in the different articles. Correlation proposed by some authors linking rheological properties to physico-chemical parameters of suspensions, or to some operating parameters of a process involving the use of sewage sludges are also given in this literature review.