Rheological behaviour of anaerobic digested sludge: impact of concentration and temperature

Renewable energy is one of the cornerstones of sustainable energy. Biogas from the anaerobic digestion of organic waste materials can provide a clean, easily controlled source of renewable energy, replacing firewood and/or fossil fuels. In order to maintain the requisite constant homogeneous conditions within digesters, operating conditions must be regulated according to the rheological characteristics of the sludge. An accurate estimate of sludge rheological properties is required for the design and efficient operation of sludge pumping and digester mixing. In this paper, we have determined the rheological behaviour of digested sludge at different concentrations and different temperatures, and highlighted common features. At low shear stress, digested sludge behaves as a viscoelastic solid, but shear banding can occur which modifies the apparent behaviour. At very high shear stress, the behaviour fits well with the Bingham model. Finally, we show that the rheological behaviour of digested sludge is qualitatively the same at different solids concentrations and temperatures, and depends only on the yield stress and Bingham viscosity: by normalising the shear stress with the yield stress and the shear rate with the yield stress divided by the Bingham viscosity, a master curve was obtained independent of both temperature and concentration. These two parameters (Yield stress and Bingham Viscosity) increase when the solid concentration increases but decrease when the temperature increases. Furthermore, we show that the rheological behaviour is irreversibly altered by the thermal history. Dissolution of some of the solids may cause a decrease of the yield stress and an increase of the Bingham viscosity. This result suggests that the solid characteristics decreases with temperature and the usual laws used to describe the thermal evolution of the rheological behaviour of fluids are no longer valid with anaerobic digested sludge.