Nanobubbles as an emerging technology for water and wastewater remediation: Current outlook and future trends

Nanobubbles comprise an emerging technology with diverse applications, ranging from carbon dioxide (CO2) removal (CDR, e.g., enhanced CO2 dissolution) to agriculture (e.g., increased aerobic water quality) to hydrate-based desalination to environmental remediation. They represent a new frontier in sustainable water and wastewater treatment, as they penetrate biofilms and degrade novel pathogens, microplastics, micropollutants, emerging contaminants (ECs) and persistent organic pollutants (POPs) without generating harmful disinfection by-products (DBPs). This is due to their unique properties and longevity, which catalyze gas transfer and promote reactive oxygen species (ROS) generation, thus reducing dependence on traditional oxidants, such as chlorine. Yet, their effective generation, measurement, high cost and energy demand remain key challenges. Therefore, a selection framework for a suitable nanobubble generation technology is provided here. Furthermore, research has mainly been concentrated in China, while real-world applications are dominant in the United States of America (USA). The nanobubble market size in water treatment will more than double from USD$134 million (2023) to USD$292 million (2033) at an 8.1% compound annual growth rate (CAGR), as a result of increased freshwater demand. In the Global South, where water contamination is widespread, and water scarcity and supply-demand mismatch are on the rise, the use of nanobubbles is limited, necessitating knowledge and technology transfer. Furthermore, future research should focus on digitalization (e.g., artificial intelligence (AI)-driven sensors, digital twins, and internet-of-things (IoT)) and energy recovery and circularity, such as aeration with nanobubbles to boost biogas production, thus supporting the food–energy–environment–water nexus. Finally, the review also critically discusses hybrid treatment processes, such as photocatalysis and ozone-based advanced oxidation processes (AOPs), and the need for improved social license and comprehensive life cycle assessment (LCA) and techno-economic analysis (TEA) studies, which are grossly lagging.