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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 149

intensity because of the formation of more cavities over a small area (number
density of cavities), which start coalescing with each other and result in larger
bubbles collapsing in the gas-liquid compressible dispersion, leading to a
lower pressure pulse at the time of collapse (Fındık and Gu ¨ndu ¨z, 2007;
Gogate et al., 2003; Hamadaoui and Naffrechoux, 2008). On the other
hand, if the intensity is changed by changing the transmittance area of ultra-
sonic equipment, at lower intensities, the same power dissipation is taking
place over a larger area, resulting in uniform energy dissipation and a larger
active area of cavitation and higher cavitational yields.

3.2.1.2.4 Effect of Physico-chemical Properties of Liquid
The physico-chemical properties of the liquid medium, such as vapor pres-
sure, surface tension, solvent viscosity, and presence of impurities/gases, also
crucially affect the performance of the sonochemical reactors by altering the
cavity dynamics. The cavity formation (inception) and the number of cav-
ities being generated, the initial size of the nuclei/cavity, and the maximum
size reached by the cavities before collapse depend mainly on these liquid
phase physico-chemical properties. Threshold power requirement for cav-
itation inception can be defined as the minimum power required for the
onset of the cavitation process, i.e., the formation of the cavities, and this
should be as low as possible so that the energy effectively available for the
growth of the cavities is larger (the total supplied energy is utilized for gen-
eration of cavitational nuclei and for the growth of cavities followed by sub-
sequent collapse), leading to a higher collapse pressure pulse (Gogate and
Pandit, 2000a). The total quantum of pressure/temperature pulse generated
as a result of cavitation is the product of the pulse generated by the collapse of
a single cavity (lower initial cavity size results in higher collapse pressure/
temperature pulse) multiplied by the number of cavities generated in the
reactor deciding the overall transformational energy delivery. It is hence
advisable to have a large number of cavitational events occurring in the reac-
tor with lower initial size of the cavitating nuclei. Higher surface tension,
lower viscosity, and lower vapor pressure favor cavitation. Therefore, the
liquid phase physico-chemical properties should be adjusted in such a
way so as to lower the cavitation inception threshold, resulting in easy gen-
eration of cavities and at the same time increasing the number of cavities
generated with lower initial size, allowing them to grow and collapse more
violently. This will result in higher energy delivery to the system and a
higher extent of transformational efficiency.