A number of studies have been performed and reported
the occurrence of contaminants in surface water, groundwater and drinking water
(Boleda et al., 2011; Kolpin et al., 2002; Sodré et al., 2010). The study indicated the presence of contaminant in the range of ?g.L-1
to ng.L-1 levels. A study conducted by Mompelat et. al., 2009,
identified 90 Pharmaceutical and Personal Care Products (PPCPs) in treated and
tap water(Mompelat et al., 2009). Similarly, 25 PPCPs were detected in drinking water
by Vulliet et. al., 2011(Vulliet and Cren-Olivé, 2011). Carmona et al. 2014 found a significant amount of
ibuprofen, naproxen, propylparaben and butylparaben in tap and mineral waters(Carmona et al., 2014). The presence of PPCPs in drinking water in trace
amount raises an alarm in relation to possible adverse effect on living being
and also disturbing natural resources (Dai et al., 2015).

The different AOPs methods such as ozonation, UV, photocatalysis
(TiO2/UV,  ZnO/UV)  and UV/H2O2, O3/OH-,
O3/UV, Fenton reaction (Fe(II)/H2O2), and
UV-A/Fe+2/H2O2 have been studied for the
treatment of drinking water (Kim and Tanaka, 2009; Klavarioti et al., 2009). The choice of AOPs depends upon the contaminants to
be removed from water. These methods not only control odour and taste but also
act as disinfectant (Gerrity et al., 2010). PPCPs are known as emerging contaminants, their
adverse effects have been proven, not satisfactorily removed by traditional
treatment methods but AOPs could be proven efficient in removing them from
contaminated water. PPCPs can be degraded by UV/H2O2
process and degradation efficiency depends on the properties of the water
matrix, such as turbidity, alkalinity, etc (Postigo and Richardson, 2014; Wols and Hofman-Caris, 2012).  For the removal of atrazine,
herbicide from the surface and drinking water, various AOPs such as ozonation,
O3/H2O2 and O3/UV were tested and
found that O3 with H2O2 and UV are efficient
system (Beltrán et al., 2000; Saquib et al., 2010).

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Water treatment plant equipped with O3/H2O2
or UV/H2O2 are efficient in removal of PPCPs such as
carbamazepine, fluoxetine, naproxen, gemfibrozil, diclofenac and ibuprofen (Borikar et al., 2015). Lin et. al., 2016 showed the efficiency of AOPs in
removing the compounds such as caffeine, indomethacin, and sulfamethoxazole
from the water(Lin et al., 2016). The performance of different AOPs in efficiency to
remove contaminants from water has presented highest average ranking for O3/H2O2
system compared to other process (Lin et al., 2016). A study carried out by Ahmed et al. (2017) found
ozonation to be efficient method for the degradation of endocrine disruptor
compounds and pesticides(Ahmed et al., 2017). The two oxidant systems, UV/chlorine and UV/H2O2
were investigated for their capacity to PPCPs removal from water and outcome of
the study showed that UV/chlorine oxidant system is more efficient (Yang et al., 2016).   

Ozonation behaviour on the removal of broad
range of water contaminants is difficult to predict due to difference in
pollutant structure and water matrices (Lee et al., 2013).Tay et. al., 2010 investigated the reaction
kinetics and degradation mechanism of parabens including methylparaben,
ethylparaben, propylparaben and butylparaben during ozonation. They also
characterize degradation by-products and found hydroxylation was the major
reaction of degradation of the parabens(Tay et al., 2010). Liu J. et al. 2015 studied removal of
antibiotics in an industrial-scale water treatment plant equipped with AOPs(Liu et al., 2016). Table 1 represents the efficiency of AOPs in
removal of pharmaceutical contaminants from water.Evolution
of toxicity of transformation productsAOPs are very commonly used for treatment of drinking
water and majorly cause loss of biological activity of the parent compounds
without generation of any toxicity. For example, conventional ozonation and
AOPs can inactivate estrogenic compounds, antiviral compounds, antibiotics and
herbicides (Dodd
et al., 2009; Lee et al., 2008; Mestankova et
al., 2011; Mestankova et al., 2012). However, oxidation of parent compound can also lead to
a transformation product with similar biological activity. The ability of an AOP
to structurally transform a chemical leads to the potential contaminant with
new chemical toxicity. Further, there are various reported incidence where an
increase in toxicity (e.g. mutagenic activity) has been detected upon AOPs
treatment of some of the molecules from EPA’s contaminant compound list (CCL3).
For example, AOPs treatment of nitrobenzene quinolone, methamidophos,
N-nitrosopyrolidine and N-nitroso-di-n-propylamine lead to evolution of
mutagenicity. Oxidation of quinolone by •OH lead to generation of estrogenic
activity in treated water. Mutagenic N-nitrosodimethylamine (NDMA) is formed
during ozonation of harmless dimethylsulfamide (a metabolite of fungicide
tolylfluanide). NDMA is also formed upon oxidation of bromide- and
dimethylsulfamide-containing waters and ozonation of dimethylamino containing
functional groups (Gunten et al., 2010; Schmidt and
Brauch, 2008; Yang et al., 2002).

3.1.      Toxicity
development during degradation of different class of individual compounds after
AOPs treatment

Different
class of toxicant usually present in drinking water such as halogenated
compounds, olefines, nitro compounds, ethers, alcohols and phenols,
nitrogen-containing compounds, organophosphorous compounds, amides, and
N-nitroso compounds have been assessed to know the development of toxicity
during degradation of different class of individual compounds after AOPs
treatment of drinking water (Mestankova et al., 2016).
Majority of treatment cases resulted in a loss of biological activity upon
oxidation of the parent compounds without generation of any form of toxicity.
However, an increase in mutagenic activity was detected by oxidative
transformation of the some CCL3 parent compounds

N-Nitroso
compounds are one of the common classes of contaminants of drinking water.
N-nitroso compounds include some of the major carcinogens like nitrosamines (Lijinsky, 1970).
Nitrosamines are byproducts of various natural as well as manufacturing
processes. It also occurs in water as a byproduct of disinfection water
treatment which has been a recent cause of concern (Richardson et al., 2012).
The extent of formation is proportional to disinfection process applied and the
composition of the treated water.  NDMA
is one of the most important compounds of this category. A few precursors which
form NDMA during oxidation have also been identified (Mitch et al., 2003).
The two processes of chloramination and chlorination are important source of
NDMA. Further, ozonation is also reported as responsible for formation of
N-nitrosamines or their precursors (Krasner et al., 2013).

Evolution
of mutagenic activity upon photolysis, ozonlysis and OH oxidation of N-nitrosodimethylamine,
N-nitrosodiethylamine, N-nitrosodi-n-propylamine, N-nitrosopyrrolidine and
N-nitrosodiphenylamine was evaluated. Results revealed increased mutagenicity
upon OH radical treatement (TAMix strain) in case of N-nitroso-di-n-propylamine
and N-nitrosopyrrolidine and upon photolysis (TA98 strain) of
N-nitrosodiphenylamine. The mutagenicity formation of N-nitrosopyrrolidine and
N-nitrosodiphenylamine was reported in absence of S9. There could be a possible
degradation pathway which leads to formation of direct mutagens during
oxidative treatments of waters. Oxidation of Nitroso compounds can also be
similar to its metabolism. In any case, there is a strong possibility of
generation of mutagenicity upon advanced oxidation process based treatment of
nitroso compounds (Mestankova et al., 2014).

Chlorination process apply to water purification
system may generate disinfection by products (DBPs) which are the issue of
concern because of their potential to lead in the process of carcinogenesis,
teratogenesis and mutagenesis. The main precursor of DBPs are dissolved organic
nitrogen which on chlorination and chloramination may produce nitrogenous
disinfection by-products (N-DBPs) (Dotson et al., 2009; Pehlivanoglu-Mantas and Sedlak,
2008). N-DBPs are more toxic than carbonaceous
disinfection products (Plewa et al., 2007; Wagner et al., 2012). A number of reports suggested that some
N-nitrosamines have been recognized as emerging non-halogenated N-DBPs in
drinking water (Choi and Valentine, 2002; Krasner et al., 2013; Richardson et al., 2007). The removal of dissolved organic matter from
drinking water can be accomplished by widely used O3/biological
activated carbon technique which was found to be effective in removal of
precursors of trihalomethanes and haloacetonitriles (Krasner et al., 2013; Tan et al., 2017)  and
nitrosamines precursors (Gerrity et al., 2015; Zhao et al., 2008). The concentration of N-nitrosodiethylamine
(NDEA) in drinking water is varying due to difference in drinking water
quality. The water samples of public water system were analyzed from US and
China country found NDEA concentration beyond 5ng/L in 2% and 15% samples
respectively (Bei et al., 2016). Removal of the precursors of NDEA during
drinking water treatment process and its toxicity effect on human health and
adult zebrafish were studied with the outcome that advanced water treatment
process showed much higher removal efficiency to NDEA than the conventional
process (Zheng et al., 2017).