Physical Aging and Thixotropy in Sludge Rheology

Abstract

Abstract Sewage sludge presents a dual rheological behaviour with an abrupt change between the two regimes. Using a new technique of reconstruction of the velocity profile, the behaviour can be modelled by a unique equation including liquid and solid components but also a structural parameter. It is also rigorously demonstrated that the only one rheological behaviour in steady state in the liquid regime is a truncated power-law which can be defined only for a shear rate and a shear stress higher than a critical value, γ̇c and τc. Moreover, the critical shear rate and shear stress increase with the solid content and depend on the fractal dimension of flocs which implies that thixotropic effects are all the more important as the sludge is thick and fresh.