Page 196 - Industrial Wastewater Treatment, Recycling and Reuse

170 Industrial Wastewater Treatment, Recycling, and Reuse

cavity collapse (Weavers et al., 1998). By the use of a cavitation process
(either US or HC), ozone decomposes thermolytically in a cavitation bubble
3
and yields molecular O 2 and atomic oxygen O( P), which reacts with water
•
molecule to form OH radicals. Another advantage of the combined process
of cavitation and ozone is the increased mass transfer of ozone from the gas
phase to the bulk solution to react with a substrate because of the turbulence
created as a result of high velocity water jet formation during cavity collapse
and/or its oscillation. Hence, in this process, organic pollutants can be
possibly degraded by the direct attack of ozone in aqueous solution and
•
by the attack of OH radicals generated as a result of the decomposition
of ozone under the cavitation. The following reactions are known to occur
during the combined process of cavitation and ozone.

3

O 3 + ÞÞÞ ! O 2 +O P (3.34)
• •
H 2 O+ ÞÞÞ! OH + H (3.35)
3
•
O P +H 2 O ! 2 OH (3.36)
• •
OH + OH ! H 2 O 2 (3.37)
•
OH + pollutant molecule ! CO 2 ,H 2 O, etc: (3.38)
O 3 + pollutant molecule ! CO 2 ,H 2 O, etc: (3.39)

•
The combined process of cavitation and ozone produces two OH rad-
icals for every O 3 molecule and hence can significantly enhance the degra-
dation efficiency of the combined process. In this combined process, ozone
loading depends on the type of pollutant molecules to be treated and the
initial concentration of the pollutant molecules. It has been reported that
a dosage of O 3 exceeding the optimum value could result in the release
of unreacted O 3 from the system (He et al., 2007). Hence, optimization
of the O 3 dosage is necessary to minimize the energy consumption and
the amount of O 3 in the exhaust gas. Figure 3.7 shows the schematic of a
laboratory-scale setup used for the combined process of cavitation and ozon-
ation. In the case of the combined process of US and O 3 , a gas sparger is
needed for the uniform distribution of ozone gas inside the US reactor.
However, the poor energy efficiency of the US reactor makes these pro-
cesses uneconomical on a larger scale. On the other hand, HC can be used
more effectively on a larger scale in combination with O 3 . The ozone gas
can be injected directly at the throat or vena contracta of the cavitating
device, thereby exposing O 3 directly to the cavitating conditions. This also