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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 155

3.2.2.2 Optimum Operating Conditions
3.2.2.2.1 Effect of Operating Pressure and Cavitation Number
The operating inlet pressures to the cavitating device and cavitation number
are the two important parameters that affect the cavitational intensity
generated in the reactor. The number of cavities being generated and the
pressure/temperature pulse generated due to cavity collapse depend very
much on the inlet pressure and the cavitation number. The operating inlet
pressure and cavitation number also depend on the type of effluent to be
treated because the physico-chemical properties (surface tension, density,
etc.) of the effluent affect the cavity generation rate and its subsequent
dynamic. A lower cavitation number or higher operating inlet pressure to
the cavitating device is more useful for oxidizing organic pollutants because
the number of cavitational events, and thus the final collapse pressure (which
is equal to the number of cavities generated multiplied by the collapse
pressure of single cavity, as explained before) also increases, resulting in more
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HO radical generation. Decreasing cavitation number to a very low value
may also not be effective due to conditions of super-cavitation and condition
of optimum cavitation number exists. The optimum cavitation number was
found to be in the range of 0.15–0.4 depending on the type of effluent to be
treated and the geometry of the cavitating device used (Saharan et al., 2013;
Senthilkumar et al., 2000; Sivakumar and Pandit, 2002). A cavitation
number below the optimum number results in choked cavitation, with
an outcome of reduced cavitational intensity. This should be avoided to
obtain the maximum effect (Saharan et al., 2011).

3.2.2.2.2 Effect of Geometry of a Cavitating Device
The optimum cavitational yield of HC is dependent on several operating
parameters: number of cavitational events occurring inside a cavitating
reactor, residence time of cavity in the low-pressure zone (maximum size
reached by the cavity before its collapse), and the rate of pressure recovery
downstream of the throat (Bashir et al., 2011). These parameters depend
on thegeometryof thecavitating deviceandtheflow conditionsoftheliquid,
i.e., the scale of turbulence and the rate of pressure recovery. All these param-
eters need to be optimized considering interactive effect to get the enhanced
cavitational yieldfrom theHC because considering only one parameterin the
designofacavitatingdevicewouldnotresultinthepossibleoptimizationofall
cavitational conditions for the desired effects. This is because none of these
parameters is independent. Thus, the cavitational condition can be altered
bychangingtheratiooftheperimeterofcavitatingholestothecross-sectional