A New Microbubble Method for Dissolved Air Flotation

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 performance

Experimental 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.

We will deliver the Centre's impact aims in depth and breadth through the following objectives:
1) Ensuring that skilled recruits are available to industry to enhance the global competitiveness of UK plc thereby filling an industry-identified skills gap in appropriately trained water informatics professionals - Beneficiary: Industry;
2) Maximising the recruitment opportunities for graduates, by providing them with the professional and development skills needed to succeed - Beneficiary: Students;
3) Promote the work of the CDT to the widest possible audience so that the true value of the investment in the centre is realized - Beneficiary: Communities (both public and academic);
4) Create and develop the next generation of academics - Beneficiary: Academia / Students.
Economic and Societal Impact: Water professionals are faced with increasingly complex problems of ensuring sustainable use of water resources, given a rapidly expanding demand for energy and food from a growing population, and the dynamic nature of our world. Simultaneously we see an explosion in new data and in computational power, which allows us to build more and more complex models of our environment. Organisations such as Toshiba and IBM expect the Centre to support them in developing a 'real business opportunity' in Smart Utility systems. Partners such as the Environment Agency and MET Office feel that WISE will give them access to essential skills in long term planning and climate impact assessment. HR Wallingford and Wessex Water see the opportunity to maintain and enhance their global advantage in technology and catchment management expertise. The impact on the industrial sectors relevant to this Centre will be guided and supported by our Advisory Board. To facilitate wider impact we will also work through regional and national groups, networks, and Learned Societies.
We will undertake the following activities in support of our pathways to impact:
1) Bi-Annual WISE Mini-Conference: One day events to engage current Partners and additional end-users including the student cohort and established research projects.
2) Annual 'Hackathon': A sector specific one day event will be an opportunity for the students to focus on a real industry problem and provide solutions.
3) Short Film: To facilitate outreach, we will produce a short film to promote the awareness of the centre topic and the research of its students.
4) Case Studies: We will jointly develop a number of case studies for our website to showcase research and allow industry to understand how it can benefit from engagement with the Centre.
5) Third Party Events and Activities: Our student cohort and supervisors will work with existing and new networks to develop new relationships.
6) Public Engagement: The Centre will benefit from RCUK funded "Public Engagement with Research Catalyst" projects based at Exeter, Bath and Bristol. We will also engage with the British Science Association.
Impact on Knowledge Creation: The training approach has been designed to facilitate the transfer and dissemination of knowledge. From Year 2 onwards students will work in other institutions and/or with our industry partners for 3-6 months. We have agreement from our overseas and industrial partners to host placements. In terms of the wider academic and industrial sectors, students will be expected to attend and present at leading national and international conferences, and at our bi-annual mini-conferences.
Broader Impact on Postgraduate Students: The Centre has worked with partners to develop an environment that will provide training across a wide range of interdisciplinary topics. Bespoke skills-based workshops, novel approaches and strong relationships with partners are key features of this environment. Specifically our students will undertake modules within the University of Exeter Business School, which will give them the opportunity to explore challenges facing leaders in industry around the globe.