In the present work, non-Newtonian properties of activated sludge are investigated using a bench test device specially developed for the purpose of this work. This set-up consists in a bioreactor equipped with a helical ribbon impeller (HRI configuration), and a recirculation loop of the sludge (pipe flow (PF) configuration). The HRI configuration is equipped with a motor and a torque-meter allowing in situ determination of sludge rheogram under stirred reactor conditions. On the PF configuration, differential pressure and flow transducers are implemented, allowing in situ determination of sludge rheogram under piping conditions. Both configurations are then compared. The yield stress, flow and consistency indexes of sludge suspensions (Herschel–Bulkley rheological law coefficients, $\tau_y$, $K$ and $n$) are determined over a large range of total suspended solid (TSS) concentrations, (12–38 g L$^{−1}$) in the laminar flow regime. The analysis of experimental data shows that measurements performed with the PF configuration have to be corrected in order to take into account a wall slip phenomenon. A correction model is proposed that shows that slip mechanism is mostly due to wall static depletion at low solid concentrations (12–17 g L$^{−1}$), then turns to shear induced migration effect at higher solid concentrations (26–38 g L$^{−1}$).