A New Microbubble Method for Dissolved Air Flotation

溶气浮选（Dissolved air flotation，简称DAF）是一种用于从水和废水中分离颗粒的水处理工艺（图1）。在一个水处理设施中，空气在高压下溶解在水中，然后在环境压力下释放到一个水池中（Agarwal、Ng和Liu，2011年）。压力差导致气泡成核和生长。当气泡生长到一定的尺寸，使它们能够相互接触时，就会发生气泡聚结。水中的悬浮固体随后会附着在气泡-水界面上，并由于浮力而上升到顶部，从而将固体与液体分离。在1900年代初，浮选法首次被认为是一种分离颗粒（如矿石）的方法。1905年申请了一项美国专利，该专利涉及一种使用加压曝气然后压力释放的工艺（Edzwald，1995年）。从那时起，它已经在许多领域建立起来，并广泛应用于水行业。已经存在多种浮选技术：溶解空气（压力）浮选、电浮选、分散诱导空气浮选、喷嘴浮选、柱浮选、离心浮选、喷射浮选、空化空气浮选。分离的显著特点是使用直径通常为30-100微米的小尺度气泡，其中必须分离极小的颗粒（鲁比奥、Souza和Smith，2002年）。最常见的微泡生产方法是压缩空气流，将空气溶解成液体，液体又变得过饱和。然后，该液体通过流经喷嘴系统减压，该喷嘴系统通过空化产生微泡（齐默尔曼等人，2008年）。目前的研究（Edzwald，2010年）表明，通过传统方法（例如压缩空气）产生气泡所需的能量对运营成本有很大影响。空气-液体-固体流动的流体动力学，特别是在高容量的流化床中，也远未优化。本项目的总体目标是研究替代的节能微泡生产方法，并评估其对高容量微泡设备的适用性。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.采用改进的微泡产生方法，提高了微泡分离器的能量效率和分离性能.确定新方法与旧方法相比的效率（即成本、能源等）。3.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生物处理系统的流体动力学--结合CFD和MATLAB来解决反应和传质特性--使用CFD建模来评估和选择固体分离的最合适的操作条件（例如气体和液体流速）--使用CFD结果来内推和外推分离性能实验工作--气泡的表征，流路的可视化，气体传输等--实验室规模的模拟实验，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.