The current design of dewatered sludge pumping devices is based on material flowing properties assuming it is a non-Newtonian highly viscous fluid. From rheological analysis, we first clearly established that current rheological models are no longer valid for this purpose. By using parallel-plates geometry, it was shown that the apparent behavior is dependent on the gap between the plates: results are less representative of sludge intrinsic properties that of the interface interactions between sludge and rotating surfaces. By reproducing dewatered sludge pipe flow at lab-scale, it was highlighted that sludge does not flow but slips along the wall of the pipe. The existence of a thin lubrication layer is suspected and the behavior law to be considered is a slippery law, similar to a Norton-Hoff model. (C) 2016 Elsevier B.V. All rights reserved.