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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 147

also gives the advantage of better energy dissipation due to a larger irradiating
area, is another new development with a promising future for medium- to
large-scale applications, but more work is required in terms of testing this
equipment for operation at high frequency and high power dissipation.

3.2.1.2 Optimization of Operating Parameters for Acoustic Cavitation
3.2.1.2.1 Effect of Frequency
Higher frequencies of sound waves are suited for effective destruction of
pollutants using acoustic cavitation up to an optimum value (Francony
and Petrier, 1996; Hua and Hoffmann, 1997; Hung and Hoffmann,
1999; Petrier et al., 1996). A higher frequency leads to higher cavitational
collapse intensity; hence, the total quantum of collapse pressure increases,
leading to higher cavitational activity. But at a very high frequency, the cav-
ity generation may reduce due to the short time of the rarefaction phase
(higher frequency) and may require higher driving pressures. This can result
in the immediate collapse of cavities without attaining maximum size (smal-
ler life time), thereby causing reduced degradation efficiency (pollutant mol-
ecules experience cavitational conditions over a shorter time due to the
shorter life span of the cavitational bubble). The optimum frequency found
in the acoustic cavitation reactor depends on the type of pollutant to be trea-
ted and the concentration along with the reactor configuration (Weavers
et al., 1998). Also, continuous operation with high frequencies at a larger
scale of operation (and hence at higher power dissipation levels) leads to
an erosion of the transducer surface due to the sudden and immediate implo-
sion of the cavities present inside the reactor. Moreover, the power required
for the onset of cavitation increases with an increase in the frequency of irra-
diation; hence the process may become uneconomical at much higher fre-
quencies of irradiation. Instead of using a single high frequency transducer,
reactors having multiple low frequency transducers located at different and
strategic locations are found to be more energy efficient due to their ability
to generate a similar cavitational intensity as in the high-frequency acoustic
reactor. Sivakumar et al. (2002) have described a design where six trans-
ducers in total have been attached on the opposite faces of a rectangular
cross-section irradiating 25 and 40 kHz either individually or simulta-
neously. They have reported that significantly higher collapse pressure pulse
is generated at the end of the cavitational event for the multiple-frequency
operation as compared to the single-frequency operation. This results in
higher transformational yields. Thus, dual- or triple-frequency reactors