Impact de la loi de comportement rhéologique sur la sédimentation en batch de boues activées: approche expérimentale et numérique

Abstract

Rheology appears as a prominent field of study for the characterization of the behavior of activated sludge. This work aims at testing different models linking the viscosity of a suspension of activated sludge to its concentration using numerical simulations of batch settling. Sludge samples obtained from a wastewater treatment plant have first been used to perform standard tests for the characterization of activated sludge, along with rheological measurements aiming at obtaining realistic viscosity values and the rheogram of the sludge. It appears the latter can be well described by a Bingham model. Then, a three-dimensional model coupling equations of fluid dynamics, sludge settling velocity and rheology has been used to simulate batch settling in a cylindrical column. Three different relationships originating from literature linking sludge viscosity and concentration have been investigated: they differ from one another mainly by a more or less steep increase of the viscosity with respect to the concentration. The vertical profiles and the contours of concentration obtained with each model are compared. The influence of a change of rheological model is particularly obvious at the level of the concentration distribution within the sedimentation domain, the irregularity of the concentration contours becoming more important as the fluidity of the suspension increases. A simulation using one of the previous models modified to generate even greater values of viscosity tends to confirm this observation. © Société Hydrotechnique de France, 2013.