Environmental Engineering Reference
In-Depth Information
activated carbon. When more organic matter is present along with the emerging contami-
nants, the removal eficiency of emerging contaminants decreases owing to the competi-
tion for adsorption sites.
25.3.5 Chlorination
Chlorination is a conventional oxidation process in which the organic matter, ammonia,
and pathogens get removed. This process is inexpensive compared with AOPs. However,
many studies have reported the formation of organochlorine compounds such as trihalo-
methanes and haloacetic acids, which are carcinogenic in nature. These compounds are
formed as the by-products of chlorination.
25.4 Advanced Oxidation Processes
AOPs are a good alternative to existing water treatment methods. Different AOPs include
photochemical and nonphotochemical processes such as Fenton, photo-Fenton, H
2
O
2
/ U V,
O
3
, O
3
/H
2
O
2
, O
3
/ U V, O
3
/H
2
O
2
/UV, and photocatalysis. In these methods, highly reactive
oxygen species such as hydroxyl radicals, hydrogen peroxide, and superoxide are produced.
The main oxidizing species is the hydroxyl radical. It has a very high oxidizing potential.
The reaction rate is very high and since these radicals are nonselective in nature, they
degrade the organic pollutants as well as microorganisms present in water. AOPs are ind-
ing increased usage in water and wastewater treatment because of the nonselectivity of the
oxidative species generated and the reduced time requirement for degradation. AOPs are
successfully used for the treatment of emerging contaminants. Commonly used oxidizers
in AOPs are hydrogen peroxide and ozone. They are capable of degrading organics in water.
The intermediates formed during the reactions can be either more or less toxic than the par-
ent compound. To reduce toxicity, these oxidizers are used in conjunction with UV radiation.
AOPs can be classiied into photochemical and nonphotochemical processes depending on
the way hydroxyl radical is generated. In case of a photochemical process, a light source,
usually UV, is used along with catalysts or oxidizers. Different AOPs are described below.
Relative oxidizing powers of different oxidizing species are given in Table 25.1.
TABLE 25.1
Relative Oxidation Power of Some Oxidizing Species
S. No.
Oxidizing Species
Relative Oxidation Power
1
Chlorine
1.00
2
Hypochlorous acid
1.10
3
Permanganate
1.24
4
Hydrogen peroxide
1.31
5
Ozone
1.52
6
Atomic oxygen
1.78
7
Hydroxyl radical
2.05
8
Positively charged hole on
titanium dioxide,
TiO
2
+
2.35
Source:
Carey, J.H.,
Water Pollut. Res. J. Can
., 27, 1, 1992.
