How well-mixed is well mixed? Hydrodynamic-biokinetic model integration in an aerated tank of a full-scale water resource recovery facility

Abstract

Current water resource recovery facility (WRRF) models only consider local concentration variations caused by inadequate mixing to a very limited extent, which often leads to a need for (rigorous) calibration. The main objective of this study is to visualize local impacts of mixing by developing an integrated hydrodynamic-biokinetic model for an aeration compartment of a full-scale WRRF. Such a model is able to predict local variations in concentrations and thus allows judging their importance at a process level. In order to achieve this, full-scale hydrodynamics have been simulated using computational fluid dynamics (CFD) through a detailed description of the gas and liquid phases and validated experimentally. In a second step, full ASM1 biokinetic model was integrated with the CFD model to account for the impact of mixing at the process level. The integrated model was subsequently used to evaluate effects of changing influent and aeration flows on process performance. Regions of poor mixing resulting in non-uniform substrate distributions were observed even in areas commonly assumed to be well-mixed. The concept of concentration distribution plots was introduced to quantify and clearly present spatial variations in local process concentrations. Moreover, the results of the CFD-biokinetic model were concisely compared with a conventional tanks-in-series (TIS) approach. It was found that TIS model needs calibration and a single parameter set does not suffice to describe the system under both dry and wet weather conditions. Finally, it was concluded that local mixing conditions have significant consequences in terms of optimal sensor location, control system design and process evaluation.

A compartment model for risk-based monitoring of lactic acid bacteria cultivations

“…Indeed, the compartment model could complement the process design phase at an early stage; in particular, it allows exploring and testing different scenarios with a short simulation time. Using a CFD simulation instead requires much longer simulation times and more computational resources [41]. However, we believe that a detailed analysis of the best candidates would be needed subsequently with a high resolution CFD simulation to substantiate the results.…”

Compartmental Modelling in chemical engineering: A critical review

“…For the study of wastewater treatment plants again, Rehman [89], Rehman et al [90,91] developed and compared different modelling ways. In this study, a full reactive CFD-biokinetics model is created to simulate the reagents and products concentration profiles in the system.…”

“…For reactive systems, the concentration of the species can be chosen as compartments definition criteria [31,90]. The criterion must be the concentration of the reagent or the product of the fastest reaction influencing the chemical reaction of interest [91,92]. The choice of concentration as the definition parameter can be a problem in some complex cases because the initial CFD simulation must include a complete or a simplified kinetic model.…”

“…For twenty years, the number of industrial applications using compartmental model approaches continues to increase (e.g. bioreactors [84], water treatment [91], crystallization [80], CO 2 valorisation [116], gas turbine modelling [31]...). The purpose of this work is to make an inventory of the different compartmental modelling approaches developed in the literature so far and to analyze the different models in order to propose a definition for compartmental modelling in chemical engineering.…”

Computational fluid dynamics-based design of anoxic bioreactor zone in wastewater treatment plant

“…The flow velocity in the outer channel of an orbal oxidation bioreactor in a full-scale wastewater treatment plant in China was simulated and monitored under actual operational conditions [14]. Rehman et al [15], presented a numerical study to visualize the local impacts of flow mixing by proposing an integrated hydrodynamic-biokinetic model for an orbal bioreactor in a full-scale WWTP.…”