In Situ Image Analysis for the Investigation of Microbubble Size, Rise Velocity and Microbubble-Microparticle Interaction During Flotation
: (Alternative Format Thesis)

  • Bert Swart

Student thesis: Doctoral ThesisPhD


Dissolved air flotation is a method of water clarification widely utilised in the removal of suspended solids within water and wastewater treatment. Understanding the complex effect that bubble characteristics, particle characteristics and flow patterns have on particle removal efficiency is necessary for the effective design of future dissolved air flotation processes and can aid in the development of updated mathematical models of the dissolved air flotation process. The presence of microplastic particles in the environment and the fate of these particles within water and wastewater treatment is of much recent concern, with research necessary regarding methods of microplastic removal from water. The main aim of this thesis was the development of unified experimental and analytical methods which can be used to: (1) determine the characteristics of generated microbubbles, (2) quantify the effect that process parameters such as particle shape, size, density, surface charge and hydrophobicity have on flotation performance, and (3) determine flow patterns during flotation. The application of microbubble flotation to the removal of different microplastic particle types, sizes, densities, and shapes from water was also investigated.

A method of in situ image analysis followed by semi-automated MATLAB image analysis was developed and used to characterise microbubbles of diameter 20 – 150 µm, generated via a regenerative turbine pump at different operating conditions. The method utilised a side-stream viewing slit with a backlighting and camera system. Microbubble size distribution and rise velocity was determined in water at different temperatures, surfactant type and surfactant concentration. The developed method allowed for fast, simple, and accurate determination of microbubble size with continuous sampling and observation.

A batch flotation method was developed to determine flotation performance, utilizing in situ image analysis of microplastic particle size distribution and concentration before and after flotation. The flotation of spherical polyethylene and non-spherical polyethylene, polypropylene, polyvinyl chloride, and polymethyl methacrylate microplastic of different sizes and densities was investigated in water with different concentrations of surfactant and salt. The study provided important quantification of the effect of particle size, shape, density, surface charge and hydrophobicity on the flotation of microplastic particles and was discussed in relation to existing literature regarding the interaction of particles and bubbles.

The research culminated in the design and development of a lab scale dissolved air flotation rig that allowed for the in situ determination of microbubble and microplastic particle size and spatial distribution throughout the flotation tank. Simultaneous tracking of particle motion also allowed for the determination of flow patterns within the tank. Flow patterns were seen to vary with changing flowrate and microplastic particle concentration was seen to reduce within the separation zone of the flotation tank with the addition of salt, illustrating the ability to observe varying flotation performance under different conditions. The study showed that simultaneous investigation of flotation performance and flow patterns within the flotation tank is possible with the described experimental equipment and methodology.

In summary the research project achieved important objectives towards the aim of developing unified experimental methods for the direct in situ measurement of key flotation parameters. The experimental and analytical methods developed can be used in future work to quantify the complex effects of bubble and particle characteristics on flotation performance as well as the effect of flow patterns on the fate of bubble-particle aggregates.
Date of Award13 Sept 2023
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorJannis Wenk (Supervisor) & John Chew (Supervisor)

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