Towards a robust CFD model for aeration tanks for sewage treatment – a lab-scale study

Abstract

Detailed insight into the hydrodynamics of aeration tanks is of crucial importance for improvements in treatment efficiency, optimization of the process design and energy-efficient operation. These factors have triggered increasing interest in the use of Computational Fluid Dynamics (CFD) to evaluate performance of wastewater treatment systems. Whilst factors such as incorrect input assumptions, poor model choice and excessive simplifications have been recognized as potential sources of output errors, there remains a need to identify the most robust strategy to faithfully simulate aeration tank performance. Therefore, the focus of this work was to undertake rigorous transient simulations of the hydrodynamics and oxygen mass transfer in a lab-scale aeration tank in order to work towards the development of robust modeling guidelines for activated sludge systems. Unlike most previous CFD analyses of aeration systems, the work reported here employed the shear stress transport (SST) k − ω turbulence model to account for the turbulent interactions between the phases inducing bubble breakup and/or coalescence, and as a consequence, promoting the formation of bubbles of different sizes and shapes. The results obtained were compared with those arising from an analysis using the standard k − ε (sk − ε) model -and assuming fixed bubble diameter-the most common CFD modeling framework used within the wastewater modeling community. Model validation was achieved using acoustic Doppler velocimetry and particle image velocimetry techniques, and experimentally derived oxygen mass transfer data. Limitations of both turbulence models used and modeling assumptions concerning bubbly flow are discussed. The benefits of the SST k − ω turbulence model are demonstrated, but the need to balance the increased computational expense of this approach compared to the sk − ε model and, indeed, bubble flow modeling are recognized. Thus, this paper presents the first rigorous analysis of turbulence model and bubble flow generation models together for activated sludge system optimization.

CFD as a Tool to Optimize Aeration Tank Design and Operation

“…bubble size, velocity and turbulence in the liquid phase and resulting gas holdup 431 (Karpinska and Bridgeman 2017). Accordingly, higher values of liquid velocities, and air 432 holdup were observed for all operating and design scenarios in zone Z1, resulting in distinctly 433 higher global mass transfer coefficients than in zone Z2, as seen when comparing data 434 summarized in Table 5.…”

“…The multiphase flow in AS tank was simulated with an Eulerian two-fluid model derived 142 from unsteady RANS (URANS) equations and the two-equation SST − turbulence 143 model, following Karpinska and Bridgeman (2017). The governing equations representing 144 conservation of mass and momentum for each phase, described comprehensively in …”

Computational Fluid Dynamic Modelling and Optimisation of Wastewater Treatment Plant Bioreactor Mixer

“…This however is based on assumed hydrodynamics of the bioreactor [11]. Current wastewater treatment design methods make assumptions of the mixing conditions and it is therefore difficult to predict how bioreactor design (i.e., position of inlet, baffles, or membrane structures) affect hydrodynamics, hence overall performance [12].…”

“…In addition, the mixer configuration and influence of this configuration could not be accurately modelled on a prototype designs. However, as demonstrated by Brannock [11], the modelling of hydrodynamics effects of bioreactor configuration in large-scale situations can be undertaken thanks to the development of sophisticated CFD modelling and increased computational power which has contributed to the successful spread of CFD within both academia and industry [12].…”

Development and applications in computational fluid dynamics modeling for secondary settling tanks over the last three decades: A review

“…In the last two decades, CFD has been applied to simulate multiphase flow in wastewater treatment facilities, such as primary settling tanks (PST) (Liu & Garcia, ; Patziger, Günthert, Jardin, Kainz, & Londong, ), aeration tanks (Karpinska & Bridgeman, , , ; Rehman, Maere, Vesvikar, Amerlinck, & Nopens, ), SSTs (Das et al, ; Gao & Stenstrom, ; McCorquodale et al, ; Patziger, Kainz, Hunze, & Józsa, ; Ramalingam et al, ; Weiss, Plósz, Essemiani, & Meinhold, ; Wicklein & Samstag, ; Xanthos, Ramalingam, Lipke, McKenna, & Fillos, ), and sludge digestions (Craig, Nieuwoudt, & Niemand, ; Kariyama, Zhai, & Wu, ). Among these applications, SSTs have been most widely studied.…”