Ultrasound (US) is a longitudinal wave with frequencies above 20 kHz. It belongs to the group of advanced oxidation processes. Ultrasonic devices are mostly based on transducers which are composed of piezoelectric materials. At high frequencies, electrical energy is converted to mechanical energy (vibration, sound). Therefore, at sufficiently high alternating potential, ultrasound will be generated. The ultrasonic wave itself has little effect on removing contaminants from the water. Therefore, the main interactions are associated with ultrasonic cavitation which includes formation, growth and violent collapse of bubbles induced by the pressure fluctuations generated by sound waves in a liquid medium.
The ultrasonic removal of contaminant from water has three basic, practically simultaneous mechanisms. Chemical mechanism is based on the chemical reactions between the generated free OH radicals and the contaminants present in water. Thermodynamical mechanism which, by implosion of gas (air) bubbles, developed in the low pressure phase, results in very high pressure (a few hundred [bars]) and temperature (a few thousand [°C]) thus creating the conditions that cause cellular decomposition of microorganisms. In the same process, free radicals are generated. These radicals cause fast oxidation of organic and inorganic matter, and the degradation of complex compounds in water (solution). They are highly reactive due to their high oxidation-reduction potential (1.8-2.70 V, depending on the pH of the solution/water) and therefore they enhance oxidation reactions more than conventional oxidants. The third mechanism refers to hydromechanic action (it manifests as a sudden multidirectional local movement of the water) which results in strong local sheer stress and by breaking the bond between the molecules which is also caused by the implosion of the air bubbles.
