(207c) Sedimentation of Activated Sludge in Secondary Clarifiers

We describe on-site measurements and numerical modelling work on the sedimentation of activated sludge in full-scale secondary clarifiers. These solid/liquid separation units represent the final stage in the activated sludge wastewater treatment process. The rheological flow behaviour of the activated sludge was studied experimentally for varying concentrations and temperatures. These measurements show that the relationship between the shear stress and the shear rate follows the Casson law at low shear rates that correspond more closely to the shear rates encountered in secondary clarifiers. At higher shear rates, the Bingham law is more appropriate to describe the non-Newtonian flow behaviour of the activated sludge. The parameters of the double-exponential settling velocity function, which describes the dependence of the settling velocity of the activated sludge flocs on the concentration, were obtained from batch sedimentation experiments. Light scattering was applied to measure concentration profiles in the clarifier. A computational fluid dynamics (CFD) model was developed using the CFD software Fluent. The convection-dispersion equation for turbulent flow, which governs the sludge transport in the clarifier, is extended to incorporate the sedimentation of sludge flocs in the field of gravity. The hydrodynamic model accounts for buoyancy flow and non-Newtonian flow behaviour of the mixed liquor. A one-dimensional sedimentation model, which is based on the convection-dispersion transport equation, was developed within the biological process modelling tool West. Both numerical models are validated using the sludge concentration profiles measured in the clarifier.