On the Number of Grid Layers Required for Accurate Simulation of a Convection-Diffusion Model for Secondary Settling Tanks

Abstract

One-dimensional models that can adequately describe the flow and settling process in secondary settling tanks (SSTs) are an important tool with respect to control and optimisation of activated sludge wastewater treatment systems. In this study, the focus is on the one-dimensional convection-diffusion model proposed by Hamilton et al. (1992). Like most one-dimensional sedimentation models, it is based on a mass balance equation, which is discretized by dividing the settler into a number of horizontal layers with equal height. The advantage that convection-diffusion models have over most other one-dimensional models is that they generate mesh independent concentration profiles. Nevertheless, a minimal number of layers nmin is required to ensure convergence of the concentration profiles with respect to the number of layers. In this paper, the required number of layers is determined for a wide range of loading and operational conditions, and the sensitivity of n min with respect to the model parameters is assessed. The required number of layers is shown to depend strongly on the operational status of the settling tank and on the value of the diffusion coefficient