Size Distributions of Micro-Nano Bubbles (Mnbs) Generated by a Regenerative Turbine Microbubble Generator Using Ozone: Significance and Potential Importance of the Nanobubble Fractions

The generation of microbubbles usually results in a distribution of bubbles containing both micro- and nanobubbles. Whilst the enhancement achieved by utilising microbubble generators is well reported, the specific size of the bubbles is rarely include and there is a genuine paucity of data concerning the relative significance of the nano and the micro bubble size distributions. Accordingly, the current work has measured the size distributions generated by a regenerative turbine microbubble generator using ozone to establish the size ranges generated and assess the relative significance of the nanobubble fraction. The size distributions were measured using two different technique that are suitable for the proposed size ranges. In the case of the nano bubble fraction, the size distribution was measured using nanoparticle tracking analysis (NTA) and the microbubble size distribution was measured using focus-beam reflectance measurement. Sauter mean diameters of 217 nm and 37 µm were reported for each fraction and half of the microbubbles were less than 5000 nm in size. Comparison of the size and number concentrations of both fractions revealed the majority of the gas was contained within the micro bubble fraction and hence this controlled the overall mass transfer performance of the system. Further, the nanobubbles were observed to be stable for 18 hours with little change in size or number indicating no net transfer and hence could pose a risk to downstream exposure of ozone. However, the risk of residual ozone was insignificant ozone residual could reach a maximum of 3.2 µg L -1 .