Micro and nanobubbles in water and wastewater treatment: A state-of-the-art review

Air-water micronanobubbles MNBs can be produced by four different methods and be used to improve (1) oxygen transfer in microbiological processes with high consumption rate, (2) algae flotation, (3) decomposition of organic compounds, and (4) fouling mitigation in membranes. The macrobubbles and slugs are currently implemented in the industry, and MNBs are investigated in the labs. The current review maps ways to explore the full potential of MNB flows through basic definitions, state-of-the-art analytical techniques, bubble generation methods, and implementation of engineering principles in water and wastewater treatment processes. The future of air-water flows is in the intentional integration of MNBs in industrial processes alongside macrobubbles and air slugs. The most significant promise held by MNBs is in a conscious minimization of the role water treatment chemicals play in water treatment processes. A deliberate generation of OH· radicals for inactivation of pathogens and oxidation of micropollutants may start the movement away from chemical inactivation towards more sustainable water resources. Mitigation of membrane fouling without strong inorganic oxidants will prolong the life of polymer membranes. Better collision and adhesion of MNBs and higher lift of macrobubbles can significantly increase the efficiency of algae flotation, reduce the use of flocculants, and adjust the process for the physical removal of micropollutants. Higher oxygen transfer rates can improve the microbiological treatment and lower the necessity and load of further treatment stages.