A Review on Advanced Oxidation Processes for Effective Water Treatment

Advanced oxidation processes (AOPs) such as fenton, ozonation, sonolysis, photocatalysis, UV photolysis, and wet air oxidation are one amongst the most suitable techniques for water and wastewater treatment. These, AOPs have also been chosen for the complete degradation of various categories of emerging pollutants that could not be managed byany conventional technologies.The mineralization is achieved by chemical reactions between the various reacting species generated and the pollutants. The present article provides avivid view of the mechanistic features of various AOPs and its possible synergisation for process enhancement to achieve better treatment efficiency. current World Environment Journal Website: www.cwejournal.org ISSN: 0973-4929, Vol. 12, No. (3) 2017, Pg. 470-490 cONTAcT Saravanan Pichiah saravananpichiah@iitism.ac.in Department of Environmental Science and Engineering, Indian Institute of Technology (ISM), Dhanbad 826004, Jharkhand, India. © 2017 The Author(s). Published by Enviro Research Publishers This is an Open Access article licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (https://creativecommons.org/licenses/by-nc-sa/4.0/ ), which permits unrestricted NonCommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. To link to this article: http://dx.doi.org/10.12944/CWE.12.3.02 Article history Received: 24 November 2017 Accepted:16 December 2017


introduction
The availability of the earth's fresh water resources has drastically depleted or contaminated due to improper water management, leading to thevulnerable situation.On the other hand the demand for safe potable water is increasing day-byday due to the exponential growth of population and inability of the traditional treatment 1 .There are many traditional and contemporary purification techniques available or practiced for delivering safe water as per the local standards and these spans from basic filtration, adsorption to most advanced techniques; membrane separation and advanced oxidation processes (AOPs).Amongst the contemporary, the latter one is recognized as highly effective in delivery of safe drinking water free of organics, inorganics, and microbes [2][3] .Owing to strong reactive oxygen species generation ability of AOPs it was considered for treating different types of water and wastewaters containing thevarious classification of pollutants including endocrine disrupting chemicals (EDCs), persistent organic pollutants (POPs), total organic carbon (TOC) and micropollutants [4][5] .This AOPs is a broad classification wherein consists of various techniques for the generation of reactive oxygen species and is shown in Figure 1.In general, the realistic aim of any water purification techniques is to render water that is free of toxic matters (organic, inorganic and biological).In this regard, AOPs are characterized as best water treatment/purification processes that involve generation of hydroxyl radical (*Oh) in sufficient quantity to affect water purification at standard temperature and pressure 6 .The significant advantage of AOPs over all existing chemical and biological processes is that they are totally "environmental-friendly" as they neither transfer pollutants from one phase to the other (as in chemical precipitation and adsorption) nor produce massive amounts of hazardous sludge [7][8][9][10] .The first AOP based water purification/treatment in full scale was proposed in early 1980s and followed by considerable achievements have been reported 11,12 .Thus, the present review will emphasize on the various aspects of AOPs for efficient water management.

Fig. 1: classifications of Advanced Oxidation Process
The enhanced degradation of various categories of pollutants by different AOPs has drawn attention from various research communities.The AOP allows the in-situ generation of various reactive oxygen species (ROS) via different process such as sonolysis, ozonation, UV, Fenton processes, etc.These ROS are subsequently utilized towards the degradation of the various categories of pollutants.The insights on the process parameters and degradation of different AOPs have been conferred.Thus, the present article consolidates the significant works that was reported by the various researchers for efficient water management.

Sonolysis
It involves efficient utilization of ultrasonic sound with a frequency range of 20 Khz -10 Mhz 13,14 .It does not utilize any hazardous chemicals asmediator's and hence regarded aseco friendly process.The organic molecules are degraded as a result of explosions of the cavitation bubbles formed as a result of ultrasonic/hydrodynamics.The generated cavitation bubbles tend to fluctuate in their size until get collapsed at their resonance size, leads to dissipation of the stored energy causing the explosions 13 .The formation of these cavitation bubbles highly dependents on the ultrasound frequency 15 .In general, lower frequencies leads to formation of lesser concentration of active bubbles due to presence of high water vapor content in the collapsing bubbles 15 .however, high frequency leads to collision among the active bubbles which generates smaller number of reactive species leading to lesser efficiency 15 .The schematic of working principle of Sonolysis is depicted in figure 2.
The explosion of these bubbles leads to degradation of the pollutants via pyrolysis at extreme pressure and temperature (500-10,000 atm& 3,000-5000 K respectively) 16,17 subsequently leading to the generation of *Oh by the dissociation of water molecules.Thus formed *Oh further reacts with the pollutants leading to their simplest form 18,19 .The degradation at the bubble-liquid interface is dominated by the *Oh.Also, the migration of free radicals from the bubble-liquid interface into the bulk liquid leads to secondary reactions in it.It is facile sludge free process with no additional generation of secondary pollutants, promoting it as one of the most preferred techniques over photolysis, photo-Fenton and Fenton process, etc. 14 .Moreover, it has inherent ability to treat cloudy water and leads to the efficient degradation of volatile and sparingly soluble organic matters causing high turbidity 19 .Nevertheless, its lower viability due to higher energy consumption and lesser mineralization efficiency limitations directs towards intensifying with various other AOPs like photocatalysis (sono-photocatalysis), photofenton (sono-fenton), ozonolysis (sono-ozonation), sonophotolysis, etc.

Fig. 2: Schematics of working principle of Sonolysis (a) Bubble interior (b) liquidbubble interface (c) Bulk solution
The mineralization efficiency can also be enhanced by varying the operational parameters such as initial substrate concentration, ph, catalyst loading and ultrasonic power 20 .Among the intensified,sonophotolysis involves the synergistic effects of UV irradiation and ultrasound waves towards the mineralization of the pollutants in the absence of catalysts.Whereas, sono-photocatalysis involves the use of UV irradiation and ultrasound waves in the presence of a semiconductor photocatalyst.The synergistic effect of UV/ultrasonic waves leads to enhanced generation of the reactive radicals leading to improved mineralization efficiency.The enhanced generation was attributed to the formation of electron-hole pair as a result of excitation in the semiconductor photocatalyst 21 .Additionally, presence of photocatalyst improves the bubble cavitation phenomenon resulting in enhanced migration of the reactive species towards the liquid bulk region 21 .Thus, intensification of the above process leads to increased concentration of the free radicals as a result of ultrasound on the peroxide species 22 .In case of sono-ozonolysis, the mass transfer of the *Oh and ozone are enhanced leading to in-situ generation of h 2 O 2 /hO* (each O 3 molecules degrades generating 2*Oh) 22 resulting effective mineralization of the pollutants.The reactive species are generated as a consequence of cavitation phenomenon in water and thermolysis of ozone.The mineralization can be further enhanced either by increasing the pressure or adding peroxide 23 .The combination helps in achieving efficiency levels > 80 % within a ph range of 5.5 -6.5 24 .

Ozonation
Ozone iscategorized as strong oxidant and a powerful disinfectant 25 after fluorine.Theoretically ozone has been reported to oxidize both inorganic and organic pollutants but practically ozone is highly selective towards pollutant 25 .hence ozoneis considered as an electrophile with high selectivity in its reaction 26 .Ozonation for water treatment is highly applicable for degrading microbes 26 , decolorization 27,28 , micropollutants removal 29,30 , non-protonated amines 26 and taste and odor removal 26,31 .This technique is commercially implemented in European countries like Switzerland, Germany and etc as an oxidizer and in wastewater treatment plants 32 .Ozone generator, cooling system for ozone generator, pre-treatment unit for dehydrating the air added into the Ozonator and post-treatment reactor for removing excess ozone in the off-gas are the unitsrequired for implementation of ozonation 33 .The schematic of a typical ozone reactor is shown in figure 3.  4.This ozone decomposition process can be accelerated by escalating the ph or hydrogen peroxide concentration 26 .It is well known that Ozone is the predominant disinfectant in water while Oh and O 3 both act as oxidants in the oxidation process in water with O 3 being highly selective and *Oh is being highly reactive 26 .hence ozonation can simultaneously be used for disinfection and oxidation.In the inception ozone concentration in water decreases rapidly and then follows firstorder kinetics while in the second phase of ozone decrease through oxidation following second-order kinetics 26,35,36 . 25he half-life of dissolved ozone varies from seconds to hours depending upon the water 26,28,37 ph, alkalinity, natural organic matter content 26,28 .Among the three the natural organic matter can react with ozone or scavenge *Oh i.e. directly or indirectly affecting ozone stability 26 .It has been reported that ozone is potent to react with substances directly or indirectly with *Oh even at neutral ph as hydrogen peroxide, superoxide ions 25 .Ozone, *Oh or the blend of the two and subsumed throughout the ozonation of organic and inorganic compounds.Ozone oxidation is enhanced by electron donating groups like -Ch 3 , O -, -OCh 3 26,32 and is reduced by electron withdrawing groups like -Cl, -NO 2

26
. Sulfidic groups and compounds containing amino groups and double bond also exhibit high ozone reactivity while the reactivity with the saturated compounds is low being accumulated during the ozonation process 26 .Low reactivity is also observed in the reaction of ozone with methyl and ethyl ether.Two electron oxidation liberating oxygen atom from ozone is followed in ozonation for oxidizing inorganic compounds 26,38,39 .The oxidation potential decreases with protonation of species due to decrease in nucleophilicity 26 .hence the Ozonation can be effectively applied to varieties of pollutants as shown in the Table 1.influence of Process conditions higher dosage of ozone at elevated ph with low bromide concentration increases the abatement efficiency of micro-pollutants 29 .While low alkalinity and low dissolved oxygen concentrations decrease the oxidation capacity of the system 26 .For active compounds like diclofenac, sulfamethoxazole, carbamazepine, trimethoprim, hydrochlorothiazide, phenazone, tramadol, metoprolol etc.with ozone and *Oh are independent of ph change without variation in degradation efficiency; howeverelevated ph readily converts O 3 resulting in the abatement of ozone-resistant 29 .

limitation of Ozonation
Low oxidation by ozone is reported for chlorinated benzenes, geosmin, and methylisoborneol (MIB), trihalomethanes (ThMs) etc 25 .Cryptosporidium paravumoo cyst are a resistant microbe against disinfection requiring higher amount ozone exposure and hence formsundesirable by-products in drinking water 26,47 .Ammonia removal by ozone is slow as it possesses weak oxidation potential 26 .It has been seen that the oxidation products produced by ozonation can have unknown toxic matters however these are in low concentration as compared to parental compounds having negligible antimicrobial and estrogenic activities.The toxic matter includes formaldehyde, ketones, phenols, nitromethanes and carcinogens like bromates, N-nitrodimethylamine (NDMA) 32,48 .Bromate a potential carcinogen formation occurs when ozone and *Oh react with bromide in theliquid phase..Studies showed that h 2 O 2 application in ozonation enhanced the organic substance and Trihalomethanes (ThM) precursor oxidation.Its addition increases the ozone transfer rate 51,[52][53][54][55][56] , follows a single electron transfer forming hO 2 -which initiates the ozone decomposition cycle forming *Oh.The h 2 O 2 incorporation in ozonation especiallyincreases the color removing efficiency but treating drinking water, the oxidation of pesticides, aromatic compounds, and chlorinated solvents mainly applies O 3 /h 2 O 2 50,61 .Benefits of O 3 /h 2 O 2 usage include shorter reaction time, allowing higher application of ozone doses and low agglomeration of ozone at the reactor outlet.Process conditions like reaction time increment after ozone addition, intensifying ph and applying hydrogen peroxide can enhance the oxidation property of ozone.however, increase in reaction time and ph is not economically viable however h 2 O 2 being a low-cost reagentmost commonly applied in drinking water treatment for achieving higher efficiency.Ozone decomposition by h 2 O 2 incorporation is initiated by *Oh and superoxide formation 26 .In case of treating surfacewaters using conventional ozonation and O 3 /h 2 O 2 much difference was not observed in the transformation of ozone to hO*.Butin case of groundwater, containing parachlorobenzoic acid (pCBA) the oxidation of it by conventional ozone is 20 % and was increased to 50 % by incorporating h 2 O 2 along with O 3

26
. Compounds like geosmin and 2-methylisobrneol (MIB) produced by algae are also difficult to oxidize by ozone as they contain saturated ring system.however, they are well suited to be treated by O 3 /h 2 O 2 having high oxidation rate constant 26 .The Bromate formation is low in O 3 / h 2 O 2 process than the conventional ozonation 51 .
O 3 /catalyst homogeneous and heterogeneous catalyst addition in ozonation process also plays a key role in enhancing the oxidation reaction 57 .Metal oxides like Fe 2 O 3 , Al 2 O 3 -Me, TiO 2 -Me, MnO 2 , Ru/CeO 2 and metal ions like Fe 2+ , Fe 3+ , Mn 2+ , etc. have been used as catalysts 57 .Successful removal of chemical oxygen demand, organochlorides, and total organic carbon had been achieved by combining iron and manganese 57,58 .Combinations like O 3 /TiO 2 , Ru/CeO 2 /O 3 33 , Al 2 O 3 /O 3 showed better efficacy in removing total organic matters 57 .Granular activated carbon is also considered as a catalyst for thedestruction of bio-refractory compounds 57,60 .

uv bases AOP
The UV treatment is generally applied as a tertiary for killing the microbes and degradation of those aquatic organic compounds which can absorb UV light.On the absorption of UV light, the electrons in the pollutant excite from the ground state to the excited state, (equation.Low pressure, medium and high-pressure lamp.The former is monochromatic in nature and emit light of wavelength 253.7 nm.Medium and high pressure lamps emit awide range of wavelengths in the UV region and penetrate deeper because of their higher intensity and take less time for the completion of the treatment.The limitation with the application of them is that they are energy intensive 62 .Lamps like Pulse radiation lamps and Excimer lamps were also utilized for generation of UV irradiation 63 .The main component of UV treatment system includes lamps, ballast and a reactor.Mercury arc lamps are generally used as the UV lamps.Ballast is a support device, which mainly performs two functions: first, they provide appropriate voltage for the reliable starting of the lamp; secondly, they maintain a continuous current flow to the lamp, to prevent the lamps from short-circuiting.Finally, the Reactor, they are of two types: contact and non-contact reactor.The contact reactors are the one in which the UV lamps remain in submerged condition.The lamps are generally enclosed within quartz sleeves in order to prevent them from damages.Non-contact reactors are the one in which a transparent material is placed between the water sample and the lamps.The lamps can be positioned either in perpendicular or parallel to the wastewater flow direction 62,64 .The major strength of this process is eco-friendly and shorter treatment time.It also does not require the use chemicals; therefore, no residual products are left after the treatment 62 .This technique has been applied for the degradation of numerous organic compounds, including EDCs (Endocrine disrupting chemicals) and various industrial solvents.It is an effective method for degrading NDMA (N-nitrosodimethylamine) a well-known potent carcinogen and mostly found in wastewater in trace amounts [65][66][67]  The *Oh formed, then either completely mineralizes the organic substances to form CO 2 or h 2 O or form some easily degradable intermediate substances.
Being an unstable and reactive molecule, O 3 gas is generated in-situ in the experiment.Thus,the generated O 3 is then sparged into the reactor in which UV lamps are installed 75,76 .These UV lamps are enclosed within Quartz sleeves in the contact reactors, which ensurebetter transmission of UV.In the non-contact type UV lamp and wastewater are separated by a transparent separator.Schematics of a UV reactor configuration are shown in figure 5.

Fig. 5: Schematic representation of a uv reactor consisting of two uv Banks 77
In most of the studies, it was found that the UV/O 3 treatment was successful in TOC and COD reduction besides its pollutant degradation capacity 75,[78][79][80] .In a study performed by hassan et.al., (2017), it was found that the color removal was higher for UV/ In theirstudy, it was found that the UV/O 3 treatment was more effective in COD removal than the O 3 .It was also found that decolourization decreased as the initial concentration of dye was increased 78 .The factors which can affect the treatment includes: ph, initial pollutant concentration, theturbidity of thesolution, amount of O 3 dose, amount of UV dose, types of UV lamps used for the process, reaction time, scavengers present in the water [80][81][82] .Table 4 enlists some selected pollutants removed effectively by UV/O 3 treatment.

Fenton Processes
The Fenton is one of the prominent AOP and the chemistry of reactions related to this includes reactions of peroxides (h 2 O 2 ) with Fe 2+ to generate reactive oxygen species that can degrade the organic as well as inorganic matter inaqueous phase.Fenton chemistry dates back to 1894 when the activation of h 2 O 2 by ferrous salts was reported for oxidizing tartaric acid by henry J. Fenton 89 .In 1934 it was proposed by haber and Weiss 90 that there is a formation of *Oh due to the Fenton reaction.This *Oh has an oxidation potential of 2.73 V. hence it is one of the most active and powerful oxidants, which can be used in the degradation of most organic compounds including emerging 91 .

homogeneous Fenton Reactions
The Fenton process is an easy and economical method to generate highly reactive oxygen species for degrading contaminants.h 2 O 2 is safe and easy to handle, comparatively cheap and easily decomposes into water and oxygen.Similarly, iron is also cheaper and safe to use.This mechanism of decomposition of h 2 O 2 was later revised by Barb et al., [92][93][94] to introduce the chain reactions of the Fenton process.The proposed mechanism which involves the breaking down of h 2 O 2 to produce *Oh /free radical is a sequence of seven reactions in an acidic medium under dark conditions 95 .The reactions are as follows: precipitates to form ferric oxy-hydroxides as the ph increases from its optimum generating an undesirable sludge giving problems in practical applications 97,98 .Additionally, thedrawbacks of application of Fenton process for large-scale wastewater treatment include ph dependency (effective only in the range of ph 2 to 5), generation of iron-based sludge, the necessity of its expensive post-treatment and finally neutralizing the treated water before disposal 99 .Therefore, further research is required on modified Fenton processes such as photo Fenton which can increase the reaction rate by light irradiation.Sometimes the use of chelating agents provides optimum ph for Fenton reactions.Chelating ligands compete favorably with hydroxide ion; hence increase the ph range over which compounds are soluble.The reaction kinetics is as similar to Fenton oxidation at the optimum ph.Various chelators include Fe 3+ -CIP chelate, deferioxamine, cyclodextrin, nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) 100-103 were adopted.however,additionalresearch is required to identify chelating agents that can increase the rate of oxidation, stability, and eco-friendliness.

Photo -Fenton Process
Irradiating Fenton reaction solutions with ultraviolet and visible light increases its reaction rate and the efficiency towards waste degradation 99 .The increase in efficiency is correlated to the photochemistry of Fe 3+ complexes [Fe 3+ (Oh) -] + and [Fe 3+ (RCO 2 ) -] 2+ to dissociate into Fe 2+ .The photochemistry of Fe 3+ gives an advantage to Fenton processes because the reduced Fe 2+ reacts with h 2 O 2 forming *Oh as per reaction 9 104 .The photo-Fenton reaction reaches its optimum around ph 2.8 and asthe ph increases above this the Fe 3+ precipitates as oxyhydroxides and as ph decreases below the optimum, the concentration of Fe(Oh) 2+ will decline 105 .The Photolysis of hydrogen peroxide takes place under UV light irradiation as shown (equation 18): Although the quantum yield by photolysis of hydrogen peroxide shown in reaction 10 is good, due to the weak absorption of light irradiation by h 2 O 2 , its activity in photo Fenton reactions is not significant.Similar to Fenton, photo-Fenton has the drawbacks of high cost for sludge processing and the requirement of narrow ph range, limits its application in waste/wastewater treatment 97,106 .
heterogenous Photocatalysts heterogeneous photocatalysis is different compared to other treatment methods involving oxidation and reduction simultaneously, with the use of light irradiation and a light absorbing photocatalyst.
Various compounds can utilize light irradiation to catalytically undergo photolysis to produce redox reactions.These compounds usually have a band structure with empty conduction and a filled valence band 89 .Once light irradiation is incident on a semiconductor and if the energy of photons exceeds the energy gap of the semiconductor, electrons are excited from valence band to the conduction band leaving behind the holes 90,91 .The activated electrons in the valence band and holes in the conduction reactswith water generating*Oh, superoxide (*O2 -) and peroxide radicals (*OOh).These radicals further degrade various pollutants to obtain products such as CO 2 and h 2 O [92][93][94] .The mechanism of photocatalysis and degradation of contaminants is illustrated in figure 6.
The other classification of photocatalyst also experimented for its applicability.Owing to the limitation of utilizing the visible light the conventional photocatalyst materials were hybridized with suitable cocatalyst.Table 4 shows the various hybridized heterogeneous photocatalysts utilized for waste degradation.Fenton Reactions using heterogeneous catalysts In conventional Fenton process removal of dissolved iron from the treated water is one of the biggest challenges which prompted the use of heterogeneous catalysts along with Fenton process.The investigation has been carried out on the use of supported iron catalysts to decrease sludge formation and to increase the ph range over which the Fenton reaction is effective.In heterogeneous catalysts supported Fenton process reactant molecules get adsorbed on the active sites present on the surface of the catalyst.The products get desorbed after the reaction takes place 112 .The use of goethite (a-FeOOh), hematite (a-Fe 2 O 3 ), FeS 2 /SiO 2 , Ilmenite (FeTiO 3 ) and Titanomagnetite (Fe 3 TiO 4 ) as heterogeneous Fenton catalysts has been studied by various researchers [113][114][115][116] .The advantage of an ideal heterogeneous catalyst is its ability to separate from water and function in wide range of ph.Various pollutants degraded using Fenton/heterogeneous catalysts are tabulated in table 5.

Other AOPs
The above discussed processes are the most prominence among the AOPs with practical applicability.Apart there are few like wet air oxidation (WAO), super critical wet oxidation process (SWOP) process and electron beam radiation.Off this WAO utilizes the molecular oxygen or air as oxidizer in high pressure and temperature environment.
This extreme condition allows the generation of free radicals that decomposes the waste 123 .Thus temperature and pressure are the controlling factor for WAO process.Most of the organic acids excluding acetic and propionic acid 124 are converted to CO 2 at high temperature.Figure 7 depicts the simple functionalities of wet oxidation.

Fig. 7: Flow operational diagram of WAO process
huge operational expenditure and investment is required to maintain a WAO system as it handles an extreme reaction environment.
Figure 8 illustrates Reactor design adopted for WAO system.
Fig. 8: Reactor design for SWOP system SWO Pemploys supercritical condition refers to the extreme temperature and pressurized condition, where water changes its polarity and became a nonpolar solvent to get miscible with the organic part.
In supercritical condition, water can be mixed with the oxygen creating a homogeneous mixture which is perfect medium for organic degradation.When organic compounds and oxygen are dissolved in water above the critical point they are immediately brought into intimate molecular contact in a single homogeneous phase.With no interface transport limitations at sufficiently high temperatures, the kinetics is fast and the oxidation reaction proceeds rapidly to completion.This supercritical water oxidation (SCOW) is also referred as hydrothermal oxidation (hTO).This process uses hydrogen peroxide in a homogeneous mixture to oxidize the toxic effluents above the critical point of water.If the organic material is in solid phase it requires the heterogeneous catalytic SCOW.

Fig. 9: Schematics of ScOW
It is most efficient method in case of environmental remediation as complete removal of oxidisable material can be achieved by this process.Till date inorganic substances like ammonia or cyanides can also be converted to CO 2 , h 2 O, and N 2 .SCOW consists of four steps, starting from pressurizing the reagent, reaction, salt separation and depressurization.Wastewater and oxidant are mixed and fed in the SCOW reactor.The oxidants are normally heated before reaction and this reaction condition allows the exothermic reaction between the waste and oxidant.The heat energy obtained in the oxidation reaction helps to activate the reagent optimally to oxidizable condition.After the treatment the salt precipitation is visible inside the reactor, because of reduced salt solubility.Though the process requires high energy, this demand can be recovered by utilization of this hot stream for preheating or energy production.A number of works that study in depth the organic reactions in SCW, focusing on the influence of the properties of water and in the kinetics modelling, has been developed over the last few years.The table 6 consists of the list of some organizations that have incorporated SCOW plant for water treatment.Figure 9 shows the schematics of a SWOP process for water/wastewater treatment while the table 6 lists the organization that incorporated the same.conclusion The review explicitly discussed on the robustness of various AOPs that has been extensively used forwater management.Their treatment efficiency depended on various process parameters that were clearly reviewed.however, limitations such as high operational costs, high energy consumption and lower generation of the reactive species initiated the need for process intensifications.The strength of the intensified system clarified their role on treating various pollutants.Though various AOPs for water treatment are discussed, the heterogeneous Photocatalysis is most widely preferred over the rest of the processes for real-time applications.Over all thereview presented the clear scenario of the AOPs for water and wastewater treatment with a benefitto the community.

Fig. 3 :
Fig. 3: A typical reactor assembly for ozonation.(A=liquid circulation pump; B=Ozone liquid analyzer; c=Ozone gas analyzer) 34 Aquatic Phase Reaction of Ozone In water, ozone is wobbly and hence undertakes reaction with the elements of water components 25 .The ozone decomposition mechanism in water entangle complex sequence of atoms and single electron transport with intermediate *Oh formation and further involves the formation of Oh -, hO 2 , O 2 -, O 3 -, hO 3 , Oh, O 2 and hO 4 entities as shown in figure 4.This ozone decomposition process can be accelerated by escalating the ph or hydrogen peroxide concentration 26 .It is well known that

O 3 in comparison to individual treatment with O 3 80 .
Tehrani et.al., (2010) studied the removal of Reactive Blue 19 dye through the UV/O 3 and O 3 treatment.

h 2 O 2
Ozone combined with hydrogen peroxide has emerged as a new dimension in water treatment which can oxidize both inorganic and organic substances more effectively than standalone process.The first studies for wastewater treatment by using O 3 /h 2 O 2 were performed by simultaneous by Nakayama et.al., and hango et.al., While, Brunet et.al., and Duguet et.al., performed for treating drinking water 1) from where the electrons are transferred to an oxygen molecule which converts both O 2 and the pollutant molecule into a radical (equation.2).The radical being a highly reactive species oxidizes other molecules to acquire a stable form.

Table 2 : various categories of pollutants treated by uv photolysis category Specific Pollutant Ref.
68akai et al., (2012)found 222 nm Kr-Excimer -UV lamps to be a better option for NDMA degradation in the place of low pressure (LP) and filtered medium pressure lamps (FMP)68.Sanches et.al., 2010 utilized low-uv/O 3 Process Treatment by combining UV and O 3 is a wellestablished advanced oxidation technology.This technique is more advantageous than the individual UV and O 3 technologiessince it combines the advantages of both.The ultraviolet light coming in contact with the ozone (O 3 ) breaks it to form *Oh, through the following reaction steps 75 :