Aeration of wastewater with a ceramic membrane nanobubble generator: Experimental validation and modelling of its oxygen transfer

Aeration is important for wastewater treatment. A significant amount of energy is used for the aeration process, sometimes up to 80 % of the total energy used at the wastewater treatment plant. Small sub-micrometer gas bubbles (“nanobubbles”) have been suggested as an alternative method for full or supplementary aeration to ensure an efficient process. This study investigates the aeration efficiency of a membrane-based nanobubble generator. The aim is to understand the role of nanobubbles in enhancing the aeration transfer efficiency, and to investigate whether nanobubbles can act as an oxygen reservoir, releasing oxygen into the liquid phase when the concentration falls below saturation. Data confirmed that the nanobubble generator enhances aeration efficiency compared with conventional methods. The gas content within the generated nanobubbles was examined, revealing negligible gas content within the nanobubbles themselves. These results suggest that the observed increase in the standard oxygen transfer efficiency (SOTE) was primarily attributable to the turbulence of liquid flow at the membrane surface and to the rapid transfer of oxygen from the formed bubbles to the liquid, rather than to significant oxygen storage in the nanobubbles themselves. The SOTE of clean water increased with the liquid flow rate and was 20 % at a flow rate of 10 L/min and 54 % at 20 L/min. Aeration happened quickly and the nanobubbles did not release a significant amount of oxygen after the generation process. • Nanobubble generation enhances SOTE from 20 % to 54 %. • The enhanced oxygen transfer is not due to an oxygen reservoir within the nanobubbles. • The nanobubble had an average size of 300 nm and a zeta potential between −10 and − 20 mV. • Higher liquid crossflow velocity increase the rate of oxygen transfer to the liquid phase. • Microbubbles were minimized at high flow rates.