Towards a detailed understanding of oxygen transfer in wastewater treatment: the effect of bubble size distribution

Approximately 50 to 80% of the energy cost in biological wastewater treatment is accounted for by the aeration system that provides air to the bioreactor hosting the microorganisms of the activated sludge. These microorganisms require air to convert organic carbon and inorganic nitrogen into waste gasses. In order to reduce the costs associated with wastewater treatment it is therefore crucial to optimise the operation and design of aeration systems. To achieve this, the mechanisms driving the aeration processes need to be understood at a fundamental level. The objective of this doctoral research is to improve the general understanding of oxygen transfer by taking into account the effect of bubble size. In a first part, it is illustrated how computational fluid dynamics can be used to predict the spatial distribution of dissolved oxygen in an aerated river stretch subjected to different scenarios. Next an experimental study was conducted to investigate the effect of activated sludge process conditions, more specifically liquid viscosity and air flow rate, on the spatial dynamics of the bubble size distribution (BSD). A high-speed camera together with appropriate image analysis tools were used to obtain the size distribution of bubbles generated in a cylindrical bubble column under different process conditions and at different heights. These measurement data were subsequently used for the development of a population balance model that predicts the BSD dynamics. Finally, a new modelling approach was suggested that takes into account the BSD to estimate the spatial distribution of the volumetric oxygen transfer coefficient or KLa. Based on the results presented in this doctoral dissertation, it should be concluded that more advanced monitoring and modelling approaches, which take into account the spatial distribution of bubble size, are valuable tools to more accurately estimate aeration efficiency