Abstract
The occurrence of a variety of organic pollutants has complicated wastewater treatment; thus, the search for sustainable and effective treatment technology has drawn significant attention. In recent years, bulk nanobubbles, which have extraordinary properties differing from those of microbubbles, including high stability and long residence times in water, large specific surface areas, high gas transfer efficiency and interface potential, and the capability to generate free radicals, have shown attractive technological advantages and promising application prospects for wastewater treatment. In this review, the basic characteristics of bulk nanobubbles are summarized in detail, and recent findings related to their implementation pathways and mechanisms in organic wastewater treatment are systematically discussed, which includes improving the air flotation process, increasing water aeration to promote aerobic biological technologies including biological activated carbon, activated sludge, and membrane bioreactors, and generating active free radicals that oxidise organic compounds. Finally, the current technological difficulties of bulk nanobubbles are analysed, and future focus areas for research on bulk nanobubble technology are also proposed.
Highlights
Nanobubbles are recognised as spherical packages with diameters of less than 1 μm that exist at the solid–liquid interface or dispersed in the liquid medium (Alheshibri et al 2016)
It can be found that large amounts of previous studies about bulk nanobubble-based flotation method focus on the removal of metal ions, little researches pay special attention to the organic matter, the flotation process is the common step of water treatment in plants, which deserves the concentration in-depth
The aerobic biological methods are extensively used for water treatment, but there is rare review highlighting the improvement of organics biodegradation efficacy via combination of bulk nanobubbles and aerobic biological technologies, clarifying the mechanism to promote more profound study and application
Summary
Bulk nanobubbles can enhance the flotation efficiency to remove organics like grease. Oxygen mass transfer rate is accelerated for microbials biodegradation processes. The collapse of bulk nanobubbles forms free radicals which promote oxidation process
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