Page 170 - Industrial Wastewater Treatment, Recycling and Reuse

144 Industrial Wastewater Treatment, Recycling, and Reuse

3.2.1 Acoustic Cavitation
In the case of acoustic cavitation, also termed sonication (US), cavitation is
produced using high-frequency sound waves, usually ultrasound, with fre-
quencies in the range of 16 kHz to 2 MHz (Lorimer and Mason, 1987;
Suslick, 1990). Alternate compression and rarefaction cycles of the sound
waves result in various phases of cavitation such as generation of the bub-
ble/cavity, growth phase, and finally the collapse. During the compression
cycle, the average distance between the molecules decreases, while during
rarefaction the distances increase. If a sufficiently large negative pressure is
applied to the liquid, such that the average distance between the molecules
exceeds the critical molecular distance necessary to hold the liquid intact, the
liquid will break down and voids or cavities will be created. These cavitation
bubbles may grow in size until the maximum negative pressure has been
reached. In the following compression cycle of the sound wave, these cav-
ities will compress, i.e., decrease in volume, and some of them may implo-
sively collapse. Because these events occur over extremely small time
intervals (micro- to nano-seconds), other transport processes are absent,
and the final collapse stage is adiabatic in nature, thus producing very high
local temperatures (up to 10,000 K) and pressures (up to 2000 atm).
Since 1990, there has been an increasing interest in the use of ultrasound
to destroy organic contaminants present in wastewater (Fındık and Gu ¨ndu ¨z,
2007; Francony and Petrier, 1996; Hamadaoui and Naffrechoux, 2008;
Merouani et al., 2010; Nagata et al., 2000; Shrestha et al., 2009; Wang
et al., 2006; Weavers et al., 1998). Many researchers have reported that ultra-
sonic irradiation processes were capable of degrading various recalcitrant
organic compounds, such as phenolic compounds, chloroaromatic com-
pounds, aqueous carbon tetrachloride, pesticides, herbicides, benzene based
compounds, polycyclic aromatic hydrocarbons, and organic dyes. The effi-
ciency of the acoustic cavitation reactor depends on the intensity of the cav-
ity collapse, which in turn depends on the total number of cavitational
events occurring inside the reactor and the final cavity collapse pressure. This
again depends on several operational parameters such as frequency of ultra-
sound, irradiating surface, intensity of sound waves, calorimetric efficiency
of ultrasonic equipment (power dissipated into the system per unit power
supplied), physicochemical properties of the liquid medium, and the pres-
ence of air and solid particles. For the best results, these parameters need
to be optimized.