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Table 1

Advantages and disadvantages of commonly used MNB generation methods (Xiong et al. 2016)

MethodPrincipleAdvantageDisadvantage
Dissolving–releasing gas Dissolving gas with pressure and releasing gas while reducing pressure Dissolve the gas in water at high pressure and form a supersaturation state, then enter the gas to reduce pressure and form a large number of fine bubbles. The size and strength of the bubbles depend on the release conditions, including surface water tension. At high pressure, the gas is highly soluble, so the number of bubbles is large and more bubbles rise stably. The efficiency is low since the whole process is continuous. 
Dissolving gas with pressure and releasing gas with impeller Fine bubbles are produced directly by the gas dispersed by the impeller, or in combination with the pressurized gas, performing three processes of mixing gas and water, releasing gas and gas dissolving. Simple principle, efficiently produced in the combination of gas–water mixtures, more dissolution and release of the gas into the pump. The process is complex, usually forms large bubbles (>50 μm). 
Dispersing gas method High-speed rotational flow The gas–water mixture flows into the circulating hollow, forming a pressurized gas. The gas cuts into ultra-fine bubbles and rotates as the water passes. Generally high quality and efficient. The flow path is difficult to design and produce. 
Flow-path section change The flow cross-section slowly decreases and then quickly increases; the water collapses violently and vortices are formed. Repeating the process leads to stronger turbulence and finer bubbles. A large flow path makes it easy to repair and less likely to block up. Difficult to adjust oxygen content during streaming significant changes in conditions. 
Fine porous materials The pressure gas forms fine bubbles through the strength of small porous materials. The quality of the bubbles mainly depends on the porous material. The method is simple. Stuck frequently. 
MethodPrincipleAdvantageDisadvantage
Dissolving–releasing gas Dissolving gas with pressure and releasing gas while reducing pressure Dissolve the gas in water at high pressure and form a supersaturation state, then enter the gas to reduce pressure and form a large number of fine bubbles. The size and strength of the bubbles depend on the release conditions, including surface water tension. At high pressure, the gas is highly soluble, so the number of bubbles is large and more bubbles rise stably. The efficiency is low since the whole process is continuous. 
Dissolving gas with pressure and releasing gas with impeller Fine bubbles are produced directly by the gas dispersed by the impeller, or in combination with the pressurized gas, performing three processes of mixing gas and water, releasing gas and gas dissolving. Simple principle, efficiently produced in the combination of gas–water mixtures, more dissolution and release of the gas into the pump. The process is complex, usually forms large bubbles (>50 μm). 
Dispersing gas method High-speed rotational flow The gas–water mixture flows into the circulating hollow, forming a pressurized gas. The gas cuts into ultra-fine bubbles and rotates as the water passes. Generally high quality and efficient. The flow path is difficult to design and produce. 
Flow-path section change The flow cross-section slowly decreases and then quickly increases; the water collapses violently and vortices are formed. Repeating the process leads to stronger turbulence and finer bubbles. A large flow path makes it easy to repair and less likely to block up. Difficult to adjust oxygen content during streaming significant changes in conditions. 
Fine porous materials The pressure gas forms fine bubbles through the strength of small porous materials. The quality of the bubbles mainly depends on the porous material. The method is simple. Stuck frequently. 
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