Page 146 - Industrial Wastewater Treatment, Recycling and Reuse

120 Industrial Wastewater Treatment, Recycling, and Reuse

2.4.2.1 Electro-oxidation
Electro-oxidation is a relatively recent development along similar lines as
electro-coagulation where anodic oxidation on the anode surface is employed
in place of direct chemical/photochemical oxidation. An alternate variation
of this process involves only the generation of an oxidizing species using the
electrochemical route followed by direct oxidation by the generated oxidiz-
ing agents. Thus, the basic mechanism is the same while the location and the
form of the oxidation reaction are slightly different. Similarly to the conven-
tional oxidation process, common oxidizing agents are the hydroxyl radical,
hydrogen peroxide, chlorine, and ozone. The design of the electrochemical
cell, selection of electrodes, operational parameters, and cost of power are
critical parameters for the application of electro-oxidation in wastewater
treatment. Typically, the available electro-oxidation processes are believed
to be effective in degrading refractory pollutants that are difficult to degrade
otherwise. These mainly include toxic compounds and ammoniacal nitrogen.
However, information on practical application in this area is scarce and
requires substantial research and developmental work for industrial wastewa-
ter treatment, especially with respect to the nature of effluents/pollutants and
their concentrations.

2.5 CAVITATION

Cavitation is a phenomenon of formation, growth, and collapse of micro
bubbles (cavities) within a liquid. Cavities are generated by decreasing local
pressure (either because of desorption of dissolved gases or evaporation of
liquid). The cavities, when traveling in a region of higher pressure, implode
(collapse) under certain conditions, resulting in very high pressure and tem-
perature near the location of collapse. Realization of such very high pressure
(>1000 atm) and very high temperature (>5000 K) is usually harnessed for a
variety of applications ranging from disinfection of water to the carrying out
of reactions (Pandit and Joshi, 1993; Suslick et al., 1997; Didenko et al.,
1999; Gogate and Pandit, 2004, 2005; Ranade et al., 2008, 2013).
Cavitation is also used to treat industrial effluents. Collapse of cavities
generates hydroxyl ions, which are useful in realizing many benefits in efflu-
ent treatment including the following:
• Reduction in COD
• Reduction in color
• Reduction in ammoniacal nitrogen
• Increase in BOD to COD ratio (and therefore enhancing the effective-
ness of bio-digesters).