A New Microbubble Method for Dissolved Air Flotation

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Dissolved air flotation (DAF) is a water treatment process for the separation of particles from water and wastewater (Figure 1). In a DAF facility air is dissolved in the water under high pressure and then released into a basin at ambient pressure (Agarwal, Ng and Liu, 2011). The pressure difference cause air bubble nucleation and growth. Bubble coalescence would take place when they grow to a certain size that allows them to touch each other. Suspended solids in the water would subsequently attach to the bubble-water interface and rise to the top due to buoyancy, thereby separating solids from the liquid. DAF was first recognized as a method of separating particles such as mineral ore in the early 1900s. A US patent was filed in 1905 for a process using pressurised aeration followed by pressure release (Edzwald, 1995). Since then DAF has become established in many fields and is widely used in the water industry. Multiple flotation techniques already exist: dissolved air (pressure) flotation, electro-flotation, dispersed induced air flotation, nozzle flotation, column flotation, centrifugal flotation, jet flotation, cavitation air flotation. The distinguishing feature of DAF is the employment of small scale bubbles, typically 30-100 micrometres in diameter, where extremely small particles have to be separated (Rubio, Souza and Smith, 2002). The most common method for microbubble production uses compression of an air stream to dissolve air into a liquid which in turn becomes supersaturated. This liquid is then depressurised via flow through a nozzle system producing microbubbles via cavitation (Zimmerman et al., 2008). Current research ((Edzwald, 2010) shows that the energy required to generate bubbles via conventional methods (e.g. compressed air) contributes significantly to the operating cost. The hydrodynamics of air-liquid-solid flows especially in high capacity DAF is also far from optimised. The overall aim of this project is to investigate alternative energy efficient microbubble production methods and assess their suitability to high capacity DAF facility. Experimental and computational tools will be used to elucidate the hydrodynamics of the multiphase (gas-liquid-solid) system. Aims:1. Enhancing the energy efficiency and separation performance of DAF by using improved microbubble generation methods.2. Determine efficiency of new method compared to old method (i.e. cost, energy etc.).3. Look at the scale up and implementation of new method in existing treatment facilities.Specific objectives:Computational modelling- Use computational fluid dynamics (CFD) modelling to elucidate the hydrodynamics of existing and improved DAF systems- Couple CFD and MATLAB to solve the reaction and mass transfer characteristics - Use CFD modelling to assess and select the most appropriate operating conditions (e.g. gas and liquid flow rates) for solid separations - USE CFD results to inter- and extrapolate the separation performanceExperimental Work- Characterisation of bubbles, visualisation of flow paths, gas transfers etc.- Lab scale DAF experiments to verify the computational work.- Investigate the removal of different types of contaminants/pathogens.