A New Microbubble Method for Dissolved Air Flotation

溶气浮选（DAF）是一种从水和废水中分离颗粒的水处理工艺（图1）。在DAF设施中，空气在高压下溶解在水中，然后在环境压力下释放到盆地中（Agarwal， Ng和Liu， 2011）。压力差导致气泡成核和长大。当它们长到一定的大小，使它们能够相互接触时，就会发生气泡合并。悬浮在水中的固体随后会附着在气泡-水界面上，并由于浮力上升到顶部，从而将固体从液体中分离出来。20世纪初，DAF首次被认为是一种分离矿物矿石等颗粒的方法。1905年，一项美国专利被提交给使用加压曝气的过程，随后是压力释放（Edzwald, 1995）。从那时起，DAF已经在许多领域建立起来，并广泛应用于水工业。多种浮选技术已经存在：溶气（压力）浮选、电浮选、分散诱导气浮选、喷嘴浮选、柱浮选、离心浮选、射流浮选、空化气浮选。DAF的显著特点是采用小尺度气泡，通常直径为30-100微米，其中必须分离极小的颗粒（Rubio， Souza和Smith， 2002）。生产微泡最常见的方法是压缩气流，将空气溶解成液体，使其过饱和。然后，这种液体通过喷嘴系统通过空化产生微气泡进行减压（Zimmerman et al., 2008）。目前的研究（Edzwald, 2010）表明，通过传统方法（如压缩空气）产生气泡所需的能量对运行成本有很大影响。特别是在大容量DAF中，空气-液-固流动的流体动力学也远未优化。该项目的总体目标是研究替代节能微泡生产方法，并评估其对高容量DAF设施的适用性。实验和计算工具将用于阐明多相（气-液-固）系统的流体动力学。目的:1。采用改进的微泡生成方法提高DAF的能源效率和分离性能。确定新方法与旧方法相比的效率（即成本，能源等）。看看新方法在现有处理设施中的规模扩大和实施情况。具体目标：计算建模-使用计算流体动力学（CFD）建模来阐明现有和改进的DAF系统的流体动力学-耦合CFD和MATLAB来解决反应和传质特性-使用CFD建模来评估和选择最合适的操作条件（例如气体和液体流速）用于固体分离-使用CFD结果来相互分析和推断分离性能。流动路径的可视化，气体传输等-实验室规模的DAF实验，以验证计算工作。-调查不同类型污染物/病原体的清除情况。

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.