Supervisors: John Chew, Jannis Wenk Understanding small scale bubble generation and their applications in wastewater treatment Levke Ortlieb Supervisors: John Chew, Jannis Wenk 22/06/2017

Aeration of activated sludge Motivation Removal of organic waste: aerobic biological treatment Typical bubble size for aeration: 2-5mm Aeration: 45-75% of energy consumption How can aeration efficiency be improved? Aeration of activated sludge Image source: https://www.wateronline.com/doc/thermal-flow-meters-for-airmonitoring-0001

Image source: Garcia-Ochoa, 2010 Transport Mechanism Oxygen has to be transferred from the gas bubble to the cell Rate limiting steps: diffusion through liquid film around bubble and stagnant liquid around solid particles Liquid film Liquid-solid-interface Cell Bulk liquid Gas-liquid interface Gas film Gas bubble Image source: Garcia-Ochoa, 2010

Behaviour of different bubbles in water Small scale bubbles Advantages: Long residence time Large surface area per volume Improved mixing Better mass transfer Behaviour of different bubbles in water Image Source: Khuntia, 2012

Image source: Muroyama, 2013 Research Questions Improved oxygen transfer? Retrofitting to existing plants? Energy and resource savings? Smoother process control? Computational model and experiments Microbubbles Macrobubbles Image source: Muroyama, 2013

Thank you for your attention!

References In case there is a question on why microbubbles – Nanobubbles are not within our consideration because it is not good for mixing (nanobubbles remain stagnant in liquid). Macrobubbles – buoyancy plays a larger role than surface tension, hence it rises rapidly. Microbubbles (in the region of 5-200 microns) – this is a balance between increased surface area, low buoyancy effect vs. surface tension, still induce a reasonable level of mixing