Application of microbubbles to ozonation for drinking water treatment.

Ozonation is a widely used water treatment process that is used to oxidise contaminants as well as disinfect water. Conventional ozone contactors have a large energy requirement and deep tanks to ensure adequate mass transfer. As a result, the delivery of ozone into water is an energy intensive and expensive process. The use of microbubbles in water treatment is a new technology that has been shown to significantly improve gas-liquid contacting processes. Microbubbles have diameters ranging from 1 – 100 µm, whereas conventional bubbles used in typical ozone contactors have diameters ranging from 2 – 6 mm. Due to their small size, microbubbles have a larger interfacial area and a lower rise velocity than conventional bubbles. Therefore, ozone in the gas phase may be transported more efficiently into the liquid phase. Despite the favourable properties of microbubbles, the mechanism by which microbubbles outperform conventional bubbles is not fully understood, with various conflicting interpretations having been presented in the literature. This work is comprised of several direct-comparison studies of microbubble and conventional bubble ozonation systems under identical conditions. Experiments were normalised for both input and effective ozone dose in order to determine a number of critical performance parameters including: hydroxyl radical production, volumetric mass transfer coefficient, ozone self-decomposition, rate and extent of compound removal and bromate formation. Overall, the observed performance enhancement was attributed to an increase in the volumetric mass transfer coefficient through the combination of an increase in bubble specific interfacial area and a decrease in the mass transfer coefficient. When normalised to effective ozone dose, no enhancement in hydroxyl radical production or increase in bromate formation was observed. In addition, the generation of microbubbles results in a distribution of bubbles containing both micro- and nanobubbles. It was concluded that in order to optimise the overall ozonation process, emphasis should be placed on understanding how to manage the size distribution of the microbubble fraction as the risk of residual ozone from nanobubble survival was deemed insignificant. These findings were then applied to the design of microbubble contactors to determine the economic viability of microbubble generation when applied to ozonation at full scale compared with a conventional bubble ozone contactor.