Periodic Reporting for period 1 - InnoDAF (Innovative model-based design and operational optimization of Dissolved Air Flotation)
Reporting period: 2019-09-01 to 2021-08-31
Water scarcity is being recognized as a global threat to human activity and water reuse strategies deserve special attention. Traditional wastewater treatment technologies deals with diluted wastes with diffuse emissions of methane and nutrients and are not deemed sustainable. The time has come to start redesigning sewage treatment focusing on maximizing the reuse in line with the cradle-to-cradle concept. This project is paving the way for the exploitation of a promising up-concentration approach, Dissolved Air Flotation (DAF), through a holistic modelling and experimental campaign to characterise and optimise processes and parameters. DAF, being explored as an emerging separation technology for up-concentration, is however vastly still a black box. Fundamental and applied research (spanning TRL 3-6) about DAF to optimize its performance is, therefore, urgently needed. Nevertheless, a fundamental understanding of complex systems can be achieved by using complex, yet powerful, mathematical modeling frameworks. In the case of DAF, this boils down to the interplay between three phases (solids, liquid and gas) in a three-dimensional space by means of partial differential equations. CFD (computational fluid dynamics) is specifically designed for this purpose and shows how velocity within the DAF tank changes as a function of design and operational variables. The reliability and accuracy of CFD simulation, however, are constrained due to the lack of physical understanding on the mechanism of the drag force, a dominating momentum exchange mechanism between phases. As for DAF, it could be even more challenging due to the difficulty for the mesh of given size (prefer coarse considering the computational load limit) to capture the flow behavior at both micro-scale (bubble) and macro-scale (reactor) level. Besides, the average bubble and floc size were usually assumed due to the complexity of describing the particle size distribution dynamics, which may not sufficiently capture the flow behavior, the key to DAF. InnoDAF aims to propose a multi-scale hypothesis of the three-phase interactions in DAF based on mechanistic models obeying principles such as mass, energy and momentum conservation. The model is completed by considering the bubble breakage/coalescence, and bubble-solid attachment/detachment. This complete model is used to optimize DAF to advance its TRL level significantly from both operational and system design perspectives.
A 3-D three phase CFD model was developed, and parameter sensitivity analysis, mesh independence were conducted. The CFD simulations successfully predicted the form and magnitude of the stratified flow occurring in a DAF tank. It captured the trends compared to the experimental measurements as the air content increases along with height. To simulate the bubble breakup and coalescence process in a bubble-water system, one of the promising approaches is to use population balance models (PBMs), which describe the variation in a given population property over space and time in a velocity field. CFD-PBM results showed that the eddy capture is the dominating mechanism for the bubble coalescence in the most volume of the CZ. Simultaneously, the velocity gradient mechanism and turbulent induced mechanism also play a key role in bubble coalescence in the regions with drastic flow transition and in the nozzle downstream. No rheological property was considered in the model, as the solid content was too low to have an impact on the viscosity. CFD-PBM simulations reveal that increasing the recirculation rate is better than increasing the volume fraction in the smaller recirculated flow to achieve higher bubble number density and smaller bubble size if the same amount of gas flow rate is injected. The baffle with proper transverse corrugate can equalize and reduce the total coalescence rate. By establishing a synergy between fluid dynamics and bubble size to achieve the best CZ efficiency, the generic CFD-PBM approach developed in this study has laid a solid basis for the optimal design and operation of DAF systems. CFD-PBM model was further completed by considering the kinetics of bubble-particle attachment/detachment for the DAF systems. CFD-PBM-Kinetics model considered the probabilities of the collision, attachment, and stability of bubble-particle. Processes involving particle surface chemistry and hydrodynamics, such as DAF, flocculation, flotation can be modelled with this multi-scale CFD-PBM-Kinetics model. The extended Derjaguin−Landau−Verwey−Overbeek (XDLVO) theory is helpful to explain the particle bubble interaction and was included in the probability calculation. We found that the attachment probability decreases as the particle−bubble velocities increase both for the approach with and without the inclusion of XDLVO.
Our project has addressed a number of theoretical and modelling issues in population balance of microbubbles in DAF, bubble-particle attachment/detachment kinetics and microbubble size control. A generic PBM considering all major mechanisms leading to bubble collision in a turbulent flow was coupled with CFD for the DAF. With this generic CFD-PBM model, various DAF systems with different operational conditions can be modelled and new configuration design can be reasonably proposed and tested. The CFD simulations successfully predicted the form and magnitude of the stratified flow occurring in the DAF. It also captured the trends compared to the experimental measurements as the air content increases along with height. In addition, the CFD-PBM simulations were found to fit the experimental data quite well. However, the CFD-PBM model developed could not include the bubble-particle attachment/detachment phenomenon, CFD-PBM model needs to be further completed by the inclusion of kinetics model for describing the bubble-particle attachment/detachment. CFD-PBM model was further completed by considering the kinetics of bubble-particle attachment/detachment for the DAF systems. CFD-PBM-Kinetics model considered the probabilities of the collision, attachment, and stability of bubble-particle. Processes involving particle surface chemistry and hydrodynamics, such as DAF, flocculation, flotation can be modelled with this multi-scale CFD-PBM-Kinetics model.
The European research area benefited from this mobility as the ER has unique research experience, key to the success of the proposed project. Experience with working abroad of the ER is highly evaluated by future employers as this is regarded as open-minded and taking the initiative. The ER is successfully offered a job as the senior CFD expert by the company of AM-Team, which is a partner of this project. We seek to further test the robustness of the model-based findings of the Marie Curie fellowship in a real world context by filing a patent, to refine them in light of this testing, and to provide more training materials based upon them for environmental professionals. The publication of Min Yang’s first research article in the research domain of PBM, has demonstrably led to an enhanced profile among the CFD and PBM communities.
The European research area benefited from this mobility as the ER has unique research experience, key to the success of the proposed project. Experience with working abroad of the ER is highly evaluated by future employers as this is regarded as open-minded and taking the initiative. The ER is successfully offered a job as the senior CFD expert by the company of AM-Team, which is a partner of this project. We seek to further test the robustness of the model-based findings of the Marie Curie fellowship in a real world context by filing a patent, to refine them in light of this testing, and to provide more training materials based upon them for environmental professionals. The publication of Min Yang’s first research article in the research domain of PBM, has demonstrably led to an enhanced profile among the CFD and PBM communities.