Regulation mechanism of optimal bubble diameter for ozone wastewater treatment.

Efforts have been made to enhance ozone utilization efficiency and reduce energy costs in ozone-aerated wastewater treatment. Microbubbles, characterized by their large interfacial area and efficient gas-liquid mass transfer, are extensively used to boost ozone utilization. However, generating microbubbles demands significant energy, and their actual efficiency requires careful evaluation due to the absence of universal strategies for designing microbubbles with optimal performance. This study established an integrated mathematical model for ozone oxidation in wastewater treatment, based on comprehensive ozone reaction kinetics, microbubble mass transfer control theory, and mass conservation equations for gas-liquid phase components. Simulation results showed a pollutant removal rate with a prediction error of less than 20 %, confirming the model's reliability. Further analysis revealed that although the initial mass transfer rate of 0.5 mm bubbles is lower than that of 0.1 mm bubbles, the total mass transfer quantity is reduced by only 8.7 %. This suggests that an optimal bubble diameter can balance mass transfer efficiency and energy consumption. Based on this finding, we integrated ozone production and bubble generation energy consumption to develop a regulation mechanism for optimizing bubble diameter, minimizing total energy consumption while meeting target removal rates. Results indicate that the optimal bubble diameter is closely related to water depth: as depth increases, the optimal bubble diameter also increases. At a depth of 2.1 meters, the optimized bubble diameter reduces energy consumption by 33.5 % and 16.2 % compared to millimeter-sized and 100-micron bubbles, respectively. Sensitivity analysis shows that total energy consumption is more sensitive to changes in specific ozone energy consumption, while variations in bubble generation energy remain relatively stable. These results underscore the feasibility of using the proposed model to guide energy-efficient bubble size selection.