Page 145 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 119

AOPs have been shown to be effective for the destruction of many
refractory pollutants (Hoffmann et al., 1995). Using an oxidation process
in wastewater treatment is based on the attack of a highly reactive and oxi-
dizing agent such as a hydroxyl radical on organic/inorganic pollutant spe-
cies, resulting in the destruction of the pollutants through oxidation
reactions. The process has two inherent components, the first being the gen-
eration of an oxidizing agent, for example, hydroxyl radicals, and the second
being the oxidation reaction. A number of oxidizing agents can be
employed: the most conventional are oxygen, chlorine, hydrogen peroxide,
and hypochlorite. The hydroxyl radical is a comparatively strong oxidant
that can destroy many pollutants otherwise difficult to destroy. Different
methodologies have been developed for the generation of suitable oxidizing
species which are then allowed to react (Gogate and Pandit, 2004; Kim et al.,
2004; Metcalf & Eddy, Inc. 2003). In wastewater treatment, a number of
AOPs that are commonly used include Fenton processes, photo-Fenton
processes, ozone oxidation processes, oxidation using UV or ozone and
UV, oxidations using a peroxide such as hydrogen peroxide+UV or
ozone + hydrogen peroxide + UV, and photo-oxidations using TiO 2 . These
and other such varied combinations are being tried for mineralization of pol-
lutants and for wastewaters containing phenolics. AOPs have been found to
be highly effective using such combinations. Ozone is hazardous, needs to
be generated in situ, and the off gases need strict monitoring. H 2 O 2 , on the
other hand, is convenient to use and is available as a solution with good shelf
life. Wet air oxidation is suitable for wastewaters containing organics in the
concentration range that is rather low for incinerating but high or toxic for
biological treatment. Wet air oxidation is conventionally carried out in the
liquid phase. The process requires severe temperature and pressure condi-

tions (125–300 C and 0.5–20 MPa) and oxidizes organics and oxidizable
inorganic pollutants using a gaseous source of oxygen (e.g., air). A number
of catalysts, usually metal based, have also been reported for wet air oxidation
processes to reduce the severity of process conditions such as temperature or
pressures and to enhance reaction. However, compared to most other pro-
cesses, the wet air oxidation process requires high capital and operating costs
and may have difficulty in degrading refractory pollutants. The process finds
useful application in the pulp and paper industry. Heterogeneous photoca-
talysis is an emerging field that is believed to overcome many of the draw-
backs of the traditional water treatment methods and provides an alternative
for mineralization of pollutant species with reduced cost and without use of
hazardous chemicals.