Suction-lift sludge removal and non-Newtonian flow behaviour in circular secondary clarifiers: Numerical modelling and measurements

We present a computational fluid dynamics (CFD) model that predicts the sedimentation of activated sludge in a full-scale flat-bottom circular secondary clarifier that is equipped with a suction-lift sludge removal system. The axisymmetric single-phase model is developed using the general-purpose CFD solver Fluent. A convection-dispersion equation, which is extended to incorporate the sedimentation of activated sludge in the field of gravity, governs the mass transfer in the clarifier. Sludge removal by suction-lift in the near-bottom region of the clarifier is simulated using negative source terms on all field equations. The standard k-ε turbulence model is used to compute the turbulent motion, and our CFD model accounts for buoyancy flow and non-Newtonian flow behaviour of the activated sludge in the clarifier. The rheological flow behaviour was measured for varying sludge concentrations and temperatures. These measurements show that at low to moderate shear rates typical of secondary clarifiers, the relationship between shear stress and shear rate follows the Casson law for pseudo-plastic flow behaviour. The rheology study was carried out together with measurements of the settling velocity during on-site measurements. The well-known double-exponential law is used to describe the dependence of the settling velocity on the concentration. Light scattering was applied to measure the local sludge distribution in the clarifier. The computation of the local sludge distribution in the clarifier by the CFD model compares well with concentration profile measurements for two different treatment plant loadings. Our CFD computations show further that the sludge viscosity dominates the flow and the sedimentation of activated sludge within the sludge blanket.