HYDRODYNAMIC CAVITATION REACTORS                      FOR WATER TREATMENT

With the development of industry and the rapid growth of population, the current water treatment technologies face many challenges. Hydrodynamic cavitation as a green and efficient means of water treatment has attracted much attention. During the hydrodynamic cavitation, enormous energy could be released into the surrounding liquid which causes thermal effects (local hotspots with 4600 K), mechanical effects (pressures of 1500 bar) and chemical effects (hydroxyl radicals). These conditions can degrade bacteria and organic substance in sewage. Moreover, the combination of hydrodynamic cavitation and other water treatment methods can produce a coupling effect.

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When the cavitation bubbles collapse, a large amount of energy is released, which affects the surrounding liquid environment in the form of thermal effect, mechanical effect and chemical effect. The liquid environment generated by hydrodynamic cavitation can be used effectively for water treatment.

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Thermal effect.
When a single cavity collapses, a mass of heat would be released in the local area. As forming an extremely high temperature environment with thousands of kelvins, the hot spot effect appears. Collection of the hot spot effect of all cavitation bubbles forms the thermal effect of hydrodynamic cavitation.
 

Mechanical effect.
The mechanical effects of hydrodynamic cavitation include the strong shock wave generated when cavitation collapses, high-speed microjet and high shear stress. The intensive mechanical effect distorts and breaks the outer wall of microorganisms on the physical plane, making microorganisms inactivated, so as to achieve the purpose of water treatment. 

Chemical effect.
As a bubble collapses, the extreme conditions can break the chemical bonds of the water molecule and form strongly oxidizing hydroxyl radicals -OH and hydrogen peroxide radicals -OH2. Strong oxidizing radicals can effectively degrade organic pollutants in sewage (Vidanage et al. 2020). Meanwhile, the chemical effect of hydrodynamic cavitation is greatly influenced by the environment. The chemical effect of hydrodynamic cavitation can be effectively enhanced and the water treatment quality can be improved (Senthil Kumar et al. 2000), by combining cavitation with non-thermal plasma.