Nowadays, one of the major challenges in the wastewater sector is the successful design and reliable operation of treatment processes, which guarantee high treatment efficiencies to comply with effluent quality criteria, while keeping the investment and operating cost as low as possible. Although conceptual design and process control of activated sludge plants are key to ensuring these goals, they are still based on general empirical guidelines and operators' experience, dominated often by rule of thumb. This review paper discusses the rationale behind the use of Computational Fluid Dynamics (CFD) to model aeration, facilitating enhancement of treatment efficiency and reduction of energy input. Several single- and multiphase approaches commonly used in CFD studies of aeration tank operation, are comprehensively described, whilst the shortcomings of the modelling assumptions imposed to evaluate mixing and mass transfer in AS tanks are identified and discussed. Examples and methods of coupling of CFD data with biokinetics, accounting for the actual flow field and its impact on the oxygen mass transfer and yield of the biological processes occurring in the aeration tanks, are also critically discussed. Finally, modelling issues, which remain unaddressed, (e.g. coupling of the AS tank with secondary clarifier and the use of population balance models to simulate bubbly flow or flocculation of the activated sludge), are also identified and discussed.

A 2D model of a confined impinging jets mixer having the same geometry of the mixing chamber of a Reaction Injection Moulding, RIM, machine is introduced for the flow field simulation in a Computational Fluid Dynamics, CFD, code. From the CFD simulations the flow field structures and dynamics are clearly established. In addition, the numerical parameters affecting the 2D model simulations are studied, setting for each parameter a validity range. The 2D model is validated and used in the study of some operational parameters: the Reynolds number, the Froude number and the momentum ratio between the opposed jets. The validation of the CFD simulations is also made by comparison with experimental results. The limitations of the 2D model, for simulating the actual 3D flow field, are assessed; from the 2D/3D comparison, it is clearly shown that the introduced model can predict the main flow field features.

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.

Gas–liquid mass transfer in wastewater treatment processes has received considerable attention over the last decades from both academia and industry. Indeed, improvements in modelling gas–liquid mass transfer can bring huge benefits in terms of reaction rates, plant energy expenditure, acid–base equilibria and greenhouse gas emissions. Despite these efforts, there is still no universally valid correlation between the design and operating parameters of a wastewater treatment plant and the gas–liquid mass transfer coefficients. That is why the current practice for oxygen mass transfer modelling is to apply overly simplified models, which come with multiple assumptions that are not valid for most applications. To deal with these complexities, correction factors were introduced over time. The most uncertain of them is the α-factor. To build fundamental gas–liquid mass transfer knowledge more advanced modelling paradigms have been applied more recently. Yet these come with a high level of complexity making them impractical for rapid process design and optimisation in an industrial setting. However, the knowledge gained from these more advanced models can help in improving the way the α-factor and thus gas–liquid mass transfer coefficient should be applied. That is why the presented work aims at clarifying the current state-of-the-art in gas–liquid mass transfer modelling of oxygen and other gases, but also to direct academic research efforts towards the needs of the industrial practitioners.

Citation: Karpinska AM and Bridgeman J (2018) CFD as a tool to optimize aeration tank design and operation. Journal of Environmental Engineering 144(2).Copyright statement: ©2017 In a novel development on previous computational fluid dynamics (CFD) studies, the work 9 reported here employed an Eulerian two-fluid model with the shear stress transport (SST) 10 − turbulence closure model and bubble interaction models to simulate aeration tank 11 performance at full scale and to identify process performance issues resulting from design 12 parameters and operating conditions. The current operating scenario was found to produce a 13 fully developed spiral flow. Reduction of the air flow rates to the average and minimum 14 design values led to a deterioration of the mixing conditions and formation of extended 15 unaerated fluid regions. The influence of bubble-induced mixing on the reactor performance 16 was further assessed via simulations of the residence time distribution (RTD) of the fluid. 17Internal flow recirculation ensured long contact times between the phases; however, hindered 18 axial mixing and the presence of dead zones were also identified. Finally, two optimization 19 schemes based on modified design and operating scenarios were evaluated. The adjustment 20 of the air flow distribution between the control zones led to improved mixing and a 20 % 21 improvement to the mass transfer coefficient. Upgrading the diffuser grid was found to be an 22 expensive and ineffective solution, leading to worsening of the mixing conditions and 23 yielding the lowest mass transfer coefficient compared to the other optimization schemes 24 studied. 25