Another class of contaminants is
halogenated compounds. Trihalomethanes and haloacetic acids are most common halogenated compounds which occur as
byproducts of drinking water treatment and have gained significant amount of
research efforts. After these, haloacetaldehydes is the third most common
contaminant group in this (Krasner et al., 2006).  In terms
of drinking water toxicity, it is reported that haloacetadehyde has much higher
magnitutde of toxicity than Trihalomethanes and haloacetic acids causing effect
even at lower concentrations. Since the identification of chloroform as the
first kind of transformation product in drinking water, many such toxic
halogenated transformation products have been identified. There are total nine
species of bromo and chloro substituted haloacetaldehyde including chloro-,
bromo-, dichloro-, bromochloro-, dibromo-, trichloro-(exists as chloral
hydrate), bromodichloro-, dibromochloro-, and tribromo derivatives of
acetaldehyde. However, trihalomethanes are now regulated by majority of nations
around the world. There are very little existing regulations and guidelines for
haloacetaldehyde species like chloral hydrate. It has been reported that
haloacetaldehyde formation potentials (HAFP) were highly increased in case of
ozonolysis given before or after conventional treatment. Further, the
brominated HAs are much greater cause of health concern than the chlorinated
derivatives. Therefore, monitoring of halogenated products other than
trihalomethanes, haloacetic acid after advanced treatment acquires much greater
importance. Considering chloralhydrate to be only haloacetaldehyde as
transformation products (and ignoring other haloacetaldehyde derivatives) underestimates
the overall health risk of halogenated compounds. Further examination of
different haloacetaldehyde species, identification of their precursors, and
factors responsible for their formation needs to be conducted (Mao et al., 2016).

 

Ozonation method applied to water purification may
lead to the formation of bromate (BrO3-) species which is classify as a
potential human carcinogen (Kurokawa et al., 1990). The permissible value for BrO3- in drinking water
is 10 ?g BrO3-L-1 as set by different agencies (Directive, 2003; Edition, 2011; EPA, 2006). HOBr/OBr- are the key intermediate in the
formation of BrO3- (Von Gunten and Hoigné, 1994). The efficiency of conventional ozonation and AOP
(O3/H2O2) method for the treatment of surface
water spiked with micropollutants along with formation of oxidation by product
bromate has been investigated and outcome was that the addition of H2O2,
could affect the formation of BrO3-. Chlorothiazide  and tramadol N-oxide  are the transformation product of
hydrochlorothiazide and tramadol respectively was assessed by kinetic  modeling (Bourgin et al., 2017). Perfluorocaboxylic acids (PFCA) are used in many commercial
applications such as stain repellants, non-stick cookware and fire-fighting
foams because of having certain properties like chemical stability,
hydrophilic-lipophilic tendencies and thermally labile. On the other hand due
to these properties there is a concern that it can persistent in the
environment (water, air & soil) and having the adverse effects on wildlife and
humans. Fluorotelomer alcohols are PFCA precursors which oxidize in the
atmosphere to fluorotelomer carboxylic acids and fluorotelomer unsaturated
carboxylic acids as indicated by several studies (Scott et al., 2006; Sinclair et al.,
2006; Zhao et al., 2013). AOP (O3 & UV) treatment of water showed that
fluorotelomer compounds can be readily oxidize to PFCA which raises concern for
the use of these methods for the drinking water purification (Anumol et al., 2016).

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PPCPs are persistent organic pollutants. These
pollutants have many inputs routes which includes runoff in agricultural
(animal husbandry) activities, leaching into groundwater via landfills etc (D??az-Cruz et al., 2003). These products have both hydrophobic and
hydrophilic nature. Therefore, some of these can accumulate in plant and other
organisms, be in water or in sediment and soil (Meredith-Williams et al., 2012)). Furthermore, PPCPs can have adverse effect on
health and even be carcinogenic in nature (Birnbaum and Fenton, 2003). A number of PPCPs present in waste water are
recalcitrant compounds which cannot be completely degraded by waste water
treatment and such degradation product may be source of increased toxicity (Vieno and Sillanpää, 2014; Ziylan and
Ince, 2011)

Evaluation of toxicity of transformation products
of Atrazine (herbicide), caffeine (psychotropic), diclofenac (inflammatory), carbamazepine
(antiepileptic) and triclosan (antimicrobial) reported enhanced toxicity after
treatment with UV/H2O2 due to transformation products (Rozas et al., 2016). Mestankova et al (2016) have
reported that mutagenicity of quinolone increased (in TA98 strain) upon
oxidation by OH radical (–S9) as well as upon ozone oxidation (both –S9 and
+S9) (Mestankova et al., 2016). Other than toxicity, some transformation
products may retain biological activity of parent compound or can even have
higher potency of the same activity. For example, Kusari et al (2009) has
reported that photolysis of difloxacin in water lead to its degradation to
sarafloxacin which also possessed activity against both Gram-positive and
Gram-negative bacteria(Kusari et al., 2009).

The direct photolysis of metronidazole by direct
UVC photolysis is reported to produce byproducts with increased toxicity which
is evident through Allium test. High toxicity potential of photo-transformation
products of diclofenac are reported at concentration equivalent to residual
environmental concentrations of diclofenac degradation products. The mode of
action of toxicity of transformation products of diclofenac remains unclear,
however, more toxic transformation product is less lipophilic than diclofenac
is reported to purely and unspecifically act as a narcotic (Fatta-Kassinos et al., 2011).

3.1.       
Future Directions

AOPs have a significant role in drinking water treatment
not only for removal of parent compound, but also for deactivation of its
biological activity e.g. removal of mutagenic toxicity of cumene hydroperoxide
or the estrogenic activity of MDA. Generation of toxicity through AOPs
transformation product is however, reported in multiple assessments. The formed
products were mostly (but not limited to) either mutagens or had estrogenic
activity. The studies suggest that the possibility of generation of biological
effects as indicated by Ames test and YES assays carried out during advanced
oxidation processes cannot be completely ruled out. Oxidative treatment
processes such as, ozonation, or advanced oxidation lead to generation of a
variety of metabolites from micropollutants (Esplugas et al., 2007) and reaction products from inorganic as well as organic
water matrix (Krasner et al., 2006). Therefore, there is need for collecting important
practical information on AOPs treatment of specific molecules of concern.
However, transformation products identification and evaluation of their
biological effects is not only tedious but expensive also.

Detection and Identification of each of these products
and assessment of their potential impact on human health is neither feasible
nor practical. Moreover, it is impossible to assess the synergistic effect
various permutations and combinations of such reaction products (Martijn et al., 2015). Biological assays offer an efficient and cost effective
approach for the elucidation of transformation products with various biological
activities and modes of action (Escher and Fenner, 2011). Bioassays have unique potential to assess water
quality, independent of specific structural nature of putatively present
pollutants. These assays methodology is flexible and can be modified to a range
of tests from general toxicity to relevant biological effects in a particular
case. For example, activity of antibiotic in water can be assessed by assaying
their action against sensitive bacterial strains (Andrews, 2001). Recombinant yeast assays (RYA) can be used to evaluate
specific biological activities by studying receptor-ligand interaction through
partial reproduction of signaling pathway (Miller, 1997; Noguerol et al.,
2006). Measurement of enzyme activity in such cases can also
serve as important tool for quantification of ligand (pollutant compounds)
concentration (García?Reyero et al., 2001). However, when more than one pollutants have similar
mechanism of action, RYA can only provide information the total biological
activity of given sample.

Implementation of ozone- or UV-based AOPs for the
treatment of drinking water from sources having significant anthropogenic
contamination should be combined with bioassays based assessment of potential
risks (see Figure 2). Bioassays similar to ones mentioned above must be developed
and implemented for monitoring the efficiency of water treatment processes to
remove specific toxic activities (Auriol
et al., 2008; Ávila et al., 2014; Feo et al., 2014). This will help to overcome the scientific limitations
in assessment of toxicity of such transformation products.The occurrence of contaminants in drinking water is a
major health concern. AOPs based technologies for the oxidation of a wide range
of organic compounds have been successfully employed for treatment of these
contaminants. Although AOPs generally
result in loss of biological activity of the parent compounds, toxic potential
of some transformation products cannot be ruled out. Multiple
classes of drinking water contaminants such as halogenated compounds, olefines,
nitro compounds, ethers, alcohols and phenols, nitrogen-containing compounds,
organophosphorous compounds, amides, and N-nitroso compounds may result in toxicity
during AOPs based degradation of individual compounds. However, it is neither practical nor feasible to detect,
identify and evaluate toxicity of each transformation product and to study the
interactions of different classes of such products. Biological assays can serve
as an easy to use technique for evaluation of potential bioactivity of transformation
products of AOPs and thus evaluate the quality of treated drinking water.
Bioassays can be tailored and used as per the specific needs depending upon
type of contaminant and method of AOPs employed for treatment. These methods
are simple, cost effective and can help overcome practical hurdles in ensuring
safety of AOPs treated drinking water. 

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