Rheological characterization of raw and anaerobically digested cow slurry

This study investigates the rheological properties of raw and anaerobically digested cow slurries. Measurements were carried out using a mixer-type rheometer (equipped with a helical ribbon) since this technique is believed to be more appropriate than conventional geometries when dealing with such heterogeneous fluids. Results revealed that slurries are very viscous (e.g. the zero shear rate viscosity of raw slurry at 37 °C is greater than 104 Pa sn), and exhibit pronounced shear-thinning behaviors (flow index ranging between 0.15 and 0.6) and negligible yield stresses. Anaerobic digestion was shown to reduce the slurry apparent viscosity as well as its non-Newtonian character. It has been demonstrated that sieving may lead to a significant decrease of the slurry apparent viscosity, which suggests that literature results are relatively imprecise (since literature studies generally consider sieved samples). Results presented in this paper are important to enable better designs of anaerobic digestion reactors and equipment, especially when a computational fluid dynamics modeling approach is to be used.