New insights in dynamic modeling of a secondary settler—II. Dynamical analysis J.Ph. Chancelier ∗ M. Cohen de Lara ∗ C. Joannis † F. Pacard ∗ ∗ Cergrene. ENPC, La Courtine, 93167 Noisy le Grand C´ edex, France. fax: 33-1-43 05 70 78 † LCPC-Division Eau, BP 19, 44340 Bouguenais, France. fax: 33-40 84 59 98 Abstract A dynamic model of the settling process in the secondary settler of a wastewater treatment plant is given by a nonlinear scalar conservation law for the sludge concen- tration under the form of a partial differential equation (PDE). A numerical algorithm is given which also includes a mathematical model of the aeration tank. Theoretical and numerical simulations are then compared with real data. The evolution of the shock corresponding to the rising of a sludge blanket is described by an ordinary differ- ential equation (ODE). As a consequence, regulation strategies of the rising of a sludge blanket in case of important water admission to the plant are proposed. We end briefly with two possible extensions. A model with two classes of particles in interaction is introduced to take into account the particle size change, as well as a model giving the distribution of residence times to take into account its effect on the velocity. Key words. clarification, conservation numerical scheme, control, entropic solution, hyperbolic equation, mathematical modeling, secondary settler, thickening, waste water treatment plant. 1