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Microbubbles (MBs) and nanobubbles (NBs) refer to bubbles of micrometer to nanometer sizes. Due to several unique attributes, including long-term stability, negative surface charge, and the ability to generate reactive oxygen species, MB and NB technology has garnered significant attention in water and wastewater treatment and ecological restoration. Recently, several studies have shown the beneficial effects of MBs and NBs in membrane defouling, pathogen deactivation, environmental remediation, etc. However, there is limited knowledge of the physical and biochemical interactions between MBs and NBs and microbial communities; the mechanistic roles of MBs and NBs on micropollutants removal during aerobic wastewater treatment and anaerobic digestion; and the engineering limitations on versatility and scalability of MB and NB technology, among others. This review fills this gap by providing a systematic discussion on the fundamentals of MBs and NBs, including their size and concentration, physicochemical properties, and generation methods. The latest advances on MB and NB applications to water and wastewater treatment and ecological restoration are then critically discussed. The review thus identifies the challenges of implementing MB and NB technology and concludes with future research directions for a broader understanding of MB and NB technology.

No abstract

Nitrogen, a critically important nutrient that boosts yields in agriculture and food production, is currently overused to meet the rising demand for food. Surplus nitrogen ends up in the environment in excess of the capacity of natural nitrogen cycle, thereby leading to serious environmental pollutions, such as eutrophication of water bodies and emission of nitrous oxide (a highly potent greenhouse gas) to atmosphere. Aquaponics− bioponics is an emerging soilless technology for nitrogen recovery that links organic vegetable production to aquaculture effluent remediation (aquaponics) or organic waste recycling (bioponics). This Review presents the concept of aquaponics−bioponics for nitrogen recovery. Nitrogen transformations and nitrogen mass distributions in aquaponic−bioponic systems are critically discussed, along with the nutrient availability of several organic composts that can be integrated with the systems, and the microbial communities involved. This discussion is followed by a dynamic nitrogen modeling for managing nitrogen from different wastes in aquaponics−bioponics. Various emerging engineering technologies that could improve aquaponics−bioponics are presented, including aeration with microbubbles and/or nanobubbles, cocultivation with algae, process automation with Internet of Things, and integration with indoor vertical farming (plant factory with artificial light). Overall, the Review lays out the state-of-art in aquaponics−bioponics and highlights potential approaches for developing highly efficient nitrogen recovery technologies from diverse organic waste streams.