Page 178 - Industrial Wastewater Treatment, Recycling and Reuse

152 Industrial Wastewater Treatment, Recycling, and Reuse

dissolved gases or some impurities in the liquid medium acting as cavitational
nuclei. Yan and Thorpe (1990) have studied the effect of the geometry of
cavitating devices (orifice plates) on the inception of cavitation. They
observed that for a given size orifice, the cavitation inception number
remains constant within an experimental variation narrow bound for
a specified liquid. The cavitation inception number does not change with
the liquid velocity, is a constant for a given orifice size, and is found to
increase with an increase in the orifice size and dimension. Moholkar and
Pandit (1997) have discussed these observations in terms of the variation
in the turbulent fluctuating velocity magnitude with the orifice dimensions
specifically.
The major advantages of HC are the following:
• It is one of the cheapest and most energy-efficient methods of generating
cavitation.
• The equipment used for generating cavitation is simple.
• Maintenance of such reactors is negligible.
• The scale-up of these types of reactor is relatively easy.
• Independent of the wastewater composition, wastewater having a high
chemical oxygen demand (COD) can be treated more effectively for
COD reduction or pretreatment.
• It can be used at multiple locations in the existing treatment process,
i.e., before and after the biological treatment process. It can also serve
multiple applications such as complete oxidation of refractory pollut-
ants; breakdown of complex molecules into smaller biodigestible
molecules that can be further degraded by conventional processes,
hence increasing the efficiency of conventional processes; and also
for disinfection, thus reducing the quantum of chemicals used for
disinfection.
These advantages make HC a useful technique that can be utilized success-
fully at an industrial scale. Researchers have now started to look at HC as a
future technology for wastewater treatment. In last few years focus has been
shifted from ultrasound based reactors toward the study of HC for the deg-
radation of organic pollutants as a potential technique to be used on a larger
scale. There are few reports available on the applications of HC to degrade
pollutants (Braeutigam et al., 2009; Bremner et al., 2008; Chakinala et al.,
2009; Padoley et al., 2012; Patil and Gogate, 2012; Pradhan and Gogate,
2010; Saharan et al., 2012; Sivakumar and Pandit, 2002; Wang and Zhang,
2009; Wang et al., 2011a,b).