An experimental study on the effect of viscosity on micro-bubble size distribution and rise velocity in a bubble column

Dissolved air flotation (DAF) is a proven solid-liquid separation technology that is being used in water and wastewater treatment as an alternative process to conventional sedimentation operation. Due to its smaller footprint and ability to cater for higher liquid loading rates, it is ideal in many urban water treatment plants where space is limited and is usually designed for larger capacities. DAF systems generate microscale air bubbles to lift suspended particles in influent solution to the top of a rectangular tank and remove them by scrapers. Due to the recent focus on different applications of micro and nanoscale air bubbles across many industrial operations, researchers and engineers are trying to explore the application of DAF in other industries such as mineral ore processing, chemical industries, and sludge thickening operation in wastewater treatment plants (WWTPs). However, these applications have yielded low treatment efficiencies and less predictable performances, which demands more studies on the use of DAF in the high solid concentration of solid-liquid separation applications. While higher solid concentrations alter several physicochemical parameters in influent, an increase in slurry solution viscosity is a major concern. As a result, this thesis sought to identify the effect of viscosity on the microbubble (MB) size distribution and rise velocities which are identified as among the main factors affecting DAF performance. A laboratory-scale micro bubble generation system attached with a bubble column was designed and built to investigate the effect of viscosity on system dynamics. Shadow imaging technology and particle image velocimetry were utilised to measure bubble size distribution and bubble rise velocities respectively for different viscosities. Solutions were prepared by mixing commercially available Xanthan Gum powder in different concentrations. The results of these experiments identified interesting variations of bubble sizes and rise velocities concerning viscosity (ranging from 1 mPas to 67.6 mPas). An increase in viscosity reduced bubble sizes and narrowed size distribution (from 60 - 200 μm to 30 – 70 μm) while reducing mean bubble rise velocities from 57 mm/s to 9 mm/s. The results of these experimental studies were critically analysed, and it was identified that reduction of bubble coalescence in high viscous solutions resulted in smaller bubble sizes, while increased drag forces slow down the rise velocity of bubbles. Moreover, this study provides essential baseline information for future studies when trying to improve DAF efficiency in high solid content applications during solidliquid separation operations