A model of reactive settling of activated sludge: Comparison with experimental data

Abstract

A non-negligible part of the biological reactions in the activated sludge process for treatment of wastewater takes place in secondary settling tanks (SST) that follow biological reactors. It is therefore of interest to develop models of so-called reactive settling that describe the spatial variability of reaction rates caused by the variation of local concentration of biomass due to hindered settling and compression. A reactive-settling model of an SST is described by a system of nonlinear partial differential equations and a numerical scheme is introduced for the simulation of hindered settling of flocculated particles, compression at high concentrations, dispersion of the flocculated particles in the suspension, dispersion of the dissolved substrates in the fluid, and the mixing that occurs near the feed inlet. The model is fitted to experiments from a pilot plant where the SST has a varying cross-sectional area. For the reactions, the Activated Sludge Model No. 1 (ASM1) is used with temperature-adjusted standard coefficients. The constitutive functions for hindered settling and compression are calibrated to a series of conventional batch settling experiments after the initial induction period of turbulence and reflocculation has been transformed away. The overall conclusion is thus that a careful treatment of batch-settling data to calibrate the sedimentation-compression model is sufficient to ensure good predictability of the reactive sedimentation process in a pilot SST, while the optional inclusion of hydrodynamic dispersion (modelled with two parameters) or a term modelling the mixing of the suspension near the feed inlet at most marginally improve predictability.