A three-phase Eulerian–Lagrangian model to simulate mixing and oxygen transfer in activated sludge treatment

Abstract

We introduce a novel modelling tool for the activated sludge process (ASP) based on large-eddy simulation and multiphase Eulerian–Lagrangian coupling. Aeration-driven sludge activation is a key part of wastewater treatment and represents the vast majority of its energy consumption. Our model allows interaction among the liquid (wastewater), solid (sludge) and gas (air bubbles) phases, to provide insight on the fluid dynamics taking place during ASP. The model uses an Eulerian–Lagrangian point-particle algorithm that respects the discrete nature of both sludge flocs and air bubbles. Four-way coupling is implemented, where the interaction between solid particles is handled by a soft-sphere collision model. The analysis was focused on quantifying the Oxygen transfer from gas to liquid to solid, i.e., the conditions for aerobic bacteria activation. Such transfer is complex due to the dispersed nature of the gas and solid phases and the turbulent mixing occurring in the tank. Unlike box-modelling approaches, our three-dimensional model describes the evolution in space and time of the concentrations of these species and the Oxygen exchange, without a priori assumptions on the nature of the mixture. The model was validated versus experimental data using the interphase exchange of Oxygen as the key parameter, exhibiting in all cases an excellent agreement with measurements that qualitatively improves Eulerian–Eulerian approaches in five different tests. Subsequently our model was used to simulate a realistic scenario within the aeration basin of a wastewater plant and explore its results across a wide parameter range (aerator distribution, dissolved Oxygen levels, air flow rate, sludge size, bubble size). This allow us the explore the time evolution of the activation process and therefore test its performance versus the air flow rate injected (hence, energy). Our results indicate that the initial dissolved Oxygen levels within the basin (related to weather conditions and aeration frequency) are critical for sludge activation, with initial anoxic conditions being very taxing. For a given flow rate, bubble screens (i.e, more aerators) provide significantly better performance. Finally, we compare model estimations of bacterial Oxygen uptake with field data obtained from real-life ASP in wastewater plants, finding a good agreement. We therefore present to the community a reliable and extendable model that solves the fluid mechanics and the basic eco-hydraulics of the three-phase system encountered in wastewater plants, with minimal empirical inputs. This is a valuable and precise tool to test the operations and design of ASP and similar processes.