Consideration of sludge rheology is very important for a proper solids transport and to obtain the necessary settler capacity. Although a true yield stress does not exist, large apparent viscosities at low shear rates inhibit an efficient sludge transport. Research indicates that the dimensionless ratio of gravity to viscous forces is an important operational parameter towards settler capacity. To improve solids transport scraper mechanisms are utilized that introduce extra shear to (i) overcome the pseudo-yield stress and (ii) liquefy the sludge. INTRODUCTION Efficient transport of thickened sludge in secondary settling tanks largely depends on the low-shear rheological properties of the suspension as low shear stresses prevail in the solids blanket. Thickened sludge characterized as Bingham or Herschel-Bulkley fluids show a yield stress below which no solids move. Increased yield stresses thus prevent solids transport along the bottom floor (i.e. settler capacity); they impact the solids blanket height and velocity field in the settling tank (Lakehal et al., 1999). The primary goal of this study is to experimentally investigate whether a yield stress exists for sludge as Barnes & Walters (1985) described how this stress only results from the sensor’s insensitivity. The objectives were to set up an alternative rheological model that accurately describes low-shear rheology; to determine its impact on the internal sludge transport in secondary settlers by computational fluid dynamics simulations; and to identify the mechanisms of the scraper as sludge removal device. METHODS Comprehensive rheology sampling was conducted with a sensitive rotational stresscontrolled Bohlin CVO rheometer. Sludge at different solids concentrations and from different Belgian municipal wastewater treatment plants were investigated in terms of their rheological behavior. The extracted rheological model was subsequently imported in the computational fluid dynamics (CFD) software FLUENT (Fluent Inc., UK) enabling the computation of the solids concentration and flow velocity fields. The CFD model was validated for a circular secondary settler operating with a zeolite dosage to improve the sludge settling properties (De Clercq, 2003). RESULTS The rheological experiments clearly indicated the non-existence of a yield stress; large apparent viscosities prevail at shear rates below 1 s though (FIGURE 1). An alternative Herschel-Bulkley rheological model has therefore been proposed, i.e. ( ) n γ m 0 γ K e 1 τ τ & & + − = − where τ and γ& are the shear stress and shear rate respectively. Further, τ0, K and n are the yield stress, fluid consistency index and flow behavior index respectively. The material parameter m has been introduced by Papanastasiou (1987) to control the growth of stress as the shear rate increases. Its original introduction to avoid numerical instabilities in CFD code now obtains a physical interpretation as well. Whereas the material parameter and flow behavior index appeared to be constants, the dependencies of yield stress and fluid consistency index on the solids concentration were described by a power and quadratic model respectively.