Pumping of dewatered sludge: Slipping or flowing behavior?

• Measured rheological behaviors of dewatered sludge are device-dependent.
• A shear localization appears during rheological measurements.
• Usual rheological models no longer works with dewatered sludge.
• Pipe flow of dewatered sludge has to be modeled considering a slipping behavior.

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 From rheological analysis, we first clearly established that current rheological models are no longer valid for this purpose.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.