8) Pergamon PH: 50273-1223(98)00453-3 War. Sci Vol. 38. No.3 , pp. 95-102.1998. IAWQ C 1998Publishedby Elsevier ScienceUd. Printedin Great BrilailL All rights reserved 0273-1223/98 S19'(]() + o·(]() A GENERALIZED SETTLING APPROACH IN THE NUMERICAL MODELING OF SEDIMENTATION TANKS G. Mazzolani, F. Pirozzi and G. d'Antonoi Departmentof Hydraulic and Environmental Engineering 'Girolamo Ippolito', University of Naples 'Federico /I', via Claudio 21, 80125 Naples, Italy ABSTRACT Numerical models for the prediction of turbulent flow field and suspended solid distribution in sedimentation tanks are characterized by refined modeling of hydrodynamics, but apparently weak modeling of settling properties of suspensions. It is known that sedimentation tanks typically treat highly heterodisperse suspensions, whose concentrations range from relatively high to low values. However, settling is modeled either by considering one or more particle classes of different settling velocity. without accounting for hindered settling conditions. or by treating the suspension u monodisperse, even in regions of low concentration. A new generalized settling model is proposed to account for both discrete settling conditions in low concentration regions of the tanks and hindered settling conditions in high concentration regions. Settling velocities of heterodisperse suspensions are then determined UI function of particle velocities in isolation and their total concentration. The settling model is used in the framework of I transport model for the simulation of hydrodynamics and solid distribution in • rectangular sedimentation tank. Results show that solid distribution is mainly affected by particle interactions in the inlet region and by settling properties of individual particles in the outlet region. Comparison of the proposed settling model with other settling models suggests that a generalized approach of the modeling of settling properties of suspensions is I primary concern to obtain reliable predictions of the removal rate. @ 1998 Published by Elsevier Science Ltd. All rights reserved KEYWORDS Density currents; particletransport; sedimentation tanks;settlingmodels; settlingvelocity. INTRODUCTION Sedimentation tanks are widely used in water and wastewater treatment plants to remove the settleable fraction of suspended solids from water. However, thereis a widegap between designpractice of theseunits and hydrodynamic research. Design has reliedtraditionally on parameters, such as detention time, hydraulic and solid loadings, which are based on idealized one-dimensional representations of the actual hYdrodynamics and solid transport. Tracer studies (El-Baroudi and Fuller, 1973) have shown that flow pattern in sedimentation tanks may presentlarge recirculation regions and the resulting mean detention time is smaller than the theoretical one. Furthermore, measurements of velocity fields in several sedimentation tanks (Larsen, 1977) have shown that the flow field is roughly two-dimensional and it may be governed by densimetric forces, whichgenerate a bottom density current undertheclarified suspension. Themain features of the density-induced flowfieldin sedimentation tanks may not be modified significantly by individual settling properties of settleable particles, which, conversely, may strongly affect solid distribution and removal rate (DeVantier and Larock, 1987; Lyn et 01., 1992). The settling velocity of a particle contained in a heterodisperse suspension, as typically found in sedimentation tanks, is known to be dependent on both its individual characteristics (size, density, shape, porosity, etc.) and interactions with other particles. In the case of low concentration, particle interactions are negligible and thus each particle 95