Rev Chem Eng 2015; 31(3): 263–302 Archina Buthiyappan, Abdul Raman Abdul Aziz* and Wan Mohd Ashri Wan Daud Degradation performance and cost implication of UV-integrated advanced oxidation processes for wastewater treatments Abstract: Advanced oxidation processes (AOPs) are commonly used for treating recalcitrant wastewater with varying degree of efficiency, depending on several operating parameters. In this review, a comparative study among selected AOPs integrated with ultraviolet (UV) (UV/Fenton, UV/H2O2, UV/O3, UV/TiO2, UV/persulfate, UV/H2O2/O3, and UV/TiO2/H2O2) was conducted. The cost implication, changes in kinetics, changes in reaction rates, and effects of various parameters such as type of contaminants, pH, catalyst loading concentration of oxidants, and type of UV light are explained and concluded in this paper. From this review, it is concluded that UV-integrated AOPs are efficient for wastewater treatment. However, a few aspects must be considered including process scale-up, kinetics of combined processes, reactor configuration, modeling of a system, and optimization of operating parameters to enhance the process efficiency. Keywords: cost evaluation; energy calculation; hydrogen peroxide; integrated AOPs; recalcitrant wastewater. DOI 10.1515/revce-2014-0039 Received September 11, 2014; accepted January 29, 2015; previously published online May 9, 2015 Abbreviations BOD COD CR DOC Fe+2 Fe+3 biological oxygen demand chemical oxygen demand color removal dissolved organic compounds ferrous ion ferric ion *Corresponding author: Abdul Raman Abdul Aziz, Faculty of Engineering, Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia, e-mail: azizraman@um.edu.my Archina Buthiyappan and Wan Mohd Ashri Wan Daud: Faculty of Engineering, Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia H2O2 HO· TOC hydrogen peroxide hydroxyl radical total organic carbon 1 Introduction Advanced oxidation processes (AOPs) depend on the generation of free radicals such as hydroxyl radical (OH), hydroperoxyl radical (HO2·), and superoxide radical (O2·-) (Ayoub et al. 2010, Glaze et al. 1987). Hydroxyl radical (OH·) plays a central role among these free radicals for degradation of a variety of recalcitrant pollutants in AOPs for wastewater treatment (Guittonneau et al. 1990, Wang and Xu 2012). The importance of hydroxyl radical in wastewater treatment has been recognized by researchers in the last decades due to its non-selective behavior which is capable of oxidizing a wide range of hazardous compounds to carbon dioxide and water or lower-chain compounds which can then be treated biologically (Guittonneau et al. 1990, Schulte et al. 1995, Arslan et al. 1999, Trabelsi-Souissi et al. 2011). Furthermore, hydroxyl radical is a powerful oxidant with oxidation potential of E0 = 2.73 V and shows faster rates of oxidation as compared to other conventional oxidants (Zhang et al. 2005, Rosenfeldt et al. 2006). The most widely discussed AOPs for water and wastewater treatment are ultraviolet (UV) (Lester et al. 2008, 2012, Avisar et al. 2010), H2O2/UV (Bledzka et al. 2012), ozone/UV (Bustos et al. 2010), ozone/H2O2 (Jung et al. 2012), ozone/H2O2/UV (Shu 2006), photocatalytical oxidation (Riga et al. 2007), Fenton, and photo-Fenton reaction (Bianco et al. 2011, Duran et al. 2011, Hasan et al. 2012a,b, Patel et al. 2013). A number of studies have been conducted using combinations of O3, H2O2 and UV, ultrasound (US), solar light, and catalysts such as TiO2, ZnO, Fe2O3, SnO2, ZnS, and CdS (Domínguez et al. 2005, Riga et al. 2007, Lucas et al. 2010). It has been found that energy-dissipating components such as US, UV, solar light, and microwave in combination with oxidants and catalysts offer better treatment efficiency compared to Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 264 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT single systems. AOPs combined with energy-dissipating components have revealed some advantages such as increase in the rate of generation of hydroxyl radicals, less fouling of solid catalyst, higher degradation and mineralization efficiency, complete mineralization, lower reaction time, better reaction rate, and elimination of mass transfer limitation. In the literature, many AOP combined technologies were explored extensively for a variety of pollutants. Ultrasound in combination with AOPs is one of the most reviewed combination systems. Recently, Eren (2012) has reviewed the use of ultrasound with biochemical, electrochemical, ozonation, photolysis, photocatalysis, and Fenton processes for the degradation of textile dyes and dye bath. Besides that, Mahamuni and Adewuyi (2010) conducted a critical review on the cost estimation of AOPs involving ultrasound for wastewater treatment. It is noted that Yang et al. (2013) have critically reviewed the application of UV-based AOPs for the treatment of organic micropollutants in waste and wastewater. This research was focused mainly on direct UV photolysis, UV/H2O2, and UV/TiO2 and their efficiency in degrading micropollutants. Apart from that, there are a number of reviews reported recently on AOPs such as heterogeneous photocatalytic oxidation (Gaya and Abdullah 2008) and Fenton oxidation (Garrido-Ramírez et al. 2010, Babuponnusami and Muthukumar 2013). It is understood that a comparative review performance of different types of UV-integrated AOPs varying the pollutants is not reported in the literature. Therefore, this present work is focused on reviewing the degradation efficiency of different combinations of UV with AOPS and their cost implication for recalcitrant wastewater treatment. Among other energy-dissipating components, UV integrated with AOPs has received great attention among researchers working on wastewater treatment. UV oxidation is a destruction process that mineralizes or oxidizes a variety of organic contaminants found in wastewater by addition of oxidants and catalysts and irradiation with UV light. In UV-AOPs system, the contaminants are destructed by direct oxidation, UV photolysis, and synergistic action of UV with oxidants and catalysts. As reported in the literature, UV is attaining wide attention for drinking water treatment, microbial disinfection, and degradation of organic compounds through direct or indirect photolysis (Alkan et al. 2007, Bin and Sobera-Madej 2012, Lester et al. 2012). The efficiency of UV light absorption has been proven to be improvised by the addition of oxidants or photosensitizing agents (Banat et al. 2005, Jung et al. 2012, Liu et al. 2013). The available literature shows that radiation has been useful for environmental applications such as drinking water treatment, wastewater treatment, decolorization of dyes (Chang et al. 2010), oxidation of organic compounds, pesticides removal (Chelme-Ayala et al. 2010), leachate treatment (Hu et al. 2011), pharmaceutical compounds degradation, and palm oil refinery effluents degradation (Leong and Bashah 2012). On a separate note, there is growing research focus on application of UV for the degradation of recalcitrant organic pollutants (Azimi et al. 2012, Matafonova and Batoev 2012, Zoschke et al. 2014). This paper aims to review the current status of these effective and still emerging UV-integrated AOPs for wastewater treatment. In this present work, processes such as UV/H2O2, UV/O3, photocatalysis, photo-Fenton, UV/Persulfate, and hybrid methods such as UV/TiO2/ H2O2, UV/TiO2/O3, and other combinations are discussed in detail. These processes have been given special attention since they have potential to degrade refractory compounds, toxic chemicals, pesticides, and other emerging pollutants under ambient conditions. Since AOPs are about generating sufficient hydroxyl radicals to oxidize the chemical present in the wastewater, it is very crucial to find a way to accelerate its production for developing more efficient treatment systems. The free radicals are effectively produced if the treatment is supplemented with other energy-dissipating components besides UV (such as solar and US), combination of more than one oxidants and catalyst. This work highlighted the mechanism of the different processes, operating parameters that contribute to the efficient production of radicals, and the possible combination of different oxidants, catalysts, and energy-dissipating components such as microwave and US for better technical feasibility of treatment system in detail. Besides, the technical and economic feasibility of a process is important to consider as these are the most important aspects of any treatment system. However, very few studies in the literature have addressed both economic and technical feasibility of the AOPs. Recently, an attempt has been made by Mahamuni and Adewuyi (2010) to estimate the cost of ultrasound combined with AOPs for wastewater treatment. Their study showed that the cost of the treatment system is considerably affected by the type of pollutants, design of reactor, and the consumption of chemicals. To date, there is no review reported on the cost estimation of UV-based AOPs, and in this paper, special attention was given to the cost calculation of UV AOPs. The possible components that need to be included when calculating the capital, maintenance, and operational costs based on the available research papers are also included. We believe that this study will help the researcher who is working on Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT the cost evaluation of the UV-AOPs and also to transfer the laboratory-scale technique to a larger-scale system. This paper also could help in giving an idea to researchers on the possible ways of hydroxyl radical production for maximizing the efficiency and minimizing the cost. 2 U V-based AOPs for treatment of contaminated water UV-based AOPs consist of a variety of methods that show non-selective oxidation of organic compounds. The main reactive species is the hydroxyl radical, which degrades many refractory organic pollutants with high reaction rates. The industrial effluents contain high chemical and biological oxygen demand; aliphatic and aromatic hydrocarbons; suspended solids; many different ­macromolecules such as humic acids, sediments, and chlorinated organic and inorganic salts; high concentrations of bactericides, emulsifiers, and detergents; and large amount of ammonia, nitrogen, and heavy metals that are unable to be fully degraded or oxidized by a single UV system. Therefore, the combination of UV radiation with other systems is attracting researchers for efficient removal of recalcitrant organic compounds (Garcia et al. 2007, Lester et al. 2011). This section discusses the most widely applied UV-based AOPs in sequence: UV/Fenton, UV/H2O2, UV/TiO2, UV/ozone, UV-persulfate, and various combinations of UV-AOPs. 2.1 UV/Fenton Fenton oxidation has been successfully utilized in wastewater treatment for degradation of various hazardous compounds in the last two decades (Babuponnusami and Muthukumar 2013). Fenton involves oxidation of ferrous ion to ferric ion and decomposition of hydrogen peroxide to hydroxyl radical as shown in Eqs. (1)–(5) (Zhang and Pagilla 2010). Fe 2+ + H 2O2 → Fe 3+ + OH - + OH ⋅ (1) Fe 3+ + H 2O2 → Fe 2+ +⋅O2 H + H + (2) Fe 2+ + OH ⋅→ Fe 3+ + OH - (3) Fe 2+ +⋅O 2 H → Fe 3+ +⋅HO2 - (4) Fe 3+ +⋅O 2 H → Fe 2+ + O2 + H + (5) Ferric ion can be reduced to ferrous ion in the presence of excess amount of hydrogen peroxide [Eq. (2)]. 265 Apart from hydroxyl radical, Fenton reaction also produces ­hydroperoxy radicals (·O2H) which may also be helpful in the oxidation of organic pollutants. Lately, a number of studies have been conducted using Fenton reaction accelerated by UV irradiation. UV-assisted Fenton process produces more hydroxyl radical compared to conventional Fenton process and is capable of increasing the degradation rate. Equation (6) represents the photochemi­cal regeneration of Fe2+ and additional production of hydroxyl radical by photo-reduction of aqua-Fe3+ complex (Giroto et al. 2008, De la Cruz et al. 2013). Fe(OH) 2+ + hv→ Fe 2+ + OH ⋅ (6) The literature shows that combination of the Fenton reaction with UV radiation results in better degradation of organic contaminants compared to the typical Fenton reaction. The investigated organic contaminants include polymers, pesticides, reactive dyes, EDTA, landfill leachate, sulfonylurea herbicide, oil refinery wastewater, penicillin, ibuprofen, 2-chlorophenol, livestock wastewater, acetaminophen, malathion pesticide, polyphenols, tea-manufacturing wastewater, palm oil refinery effluent, simulated industrial wastewater, phthalic anhydride, and naval derusting wastewater (Kusic et al. 2006a,b, Giroto et al. 2008, Zarora et al. 2010, Zhang and Pagilla 2010). Photo-Fenton process can be carried out at room temperature and atmospheric pressure (Lu et al. 2012). In addition, iron salt is non-toxic and can be easily separated in the form of sludge, and H2O2 is an environmentally friendly compound (Liu et al. 2007). In many studies, photo-Fenton has been reported to be more efficient for degradation and decolorization of organic contaminants compared to Fenton treatment (Qiu and Huang 2010). Besides, Yeber and Cid (2013) recently reported that UV/Fe system was efficient for removal of fish oil from fishmeal mill wastewater. The UV/Fe system successfully achieved good organic matter mineralization while improving effluent biodegradability and disinfection in their study. The reduction of chemical oxygen demand (COD) from 5,562 mgO2/l to 1,218 mgO2/l was observed in their study. Table 1 summarizes the effectiveness and experimental conditions for removal of various pollutants from various types of wastewater utilizing photo-Fenton treatment. Based on the comprehensive studies of photoFenton for wastewater treatment, factors that may affect degradation efficiency are outlined and discussed in detail in the following section. In addition, Figure 1 was also constructed to outline the main operating parameters and its effects on different type of UV-AOPs. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM UV-C lamp (30 W) Low-pressure mercury vapor lamp (20 W, 254 nm) Azo Dye C.I. Acid Red 14 Mordant Red 73 azo dye Reactive Dye Low-pressure mercury UV lamp (30 W, 254 nm) UV light (15 W UVA lamp (UVP BL-15 365 nm) Reactive Black B C.I. Acid Yellow 23 UV-A irradiation (150 W, 360 nm) Reactive Black 39 A Heraeus UV immersed lamp TN 15/35 with a nominal output of 15 W Mercury lamp (5 W, UV-C, 254 nm) Malathion pesticide C.I. Reactive Yellow 84 Low-pressure mercury vapor lamp of 12 W (254 nm) Pesticides Low-pressure mercury lamp (20 W, 254 nm) Low-pressure mercury lamp (125 W, UV-C 254 nm) Low-pressure mercury vapor lamp of 12 W (254 nm) Pesticide C.I. Acid Blue 9 UV-lamps Pollutant 60 30 60 100 180 60 180 45 180 400 120 UV-irradiation time (min) Table 1: Summary of studies on the removal of pollutant by (UV/H2O2/Fe). Elmorsi et al. (2010) COD: 85 CR: 99 [H2O2]: 20 mm [Fe2+]: 0.1 mm [Dye]: 40 mg/l pH value: 3 UV light intensity: 45.3 W/m2 Modirshahla et al. (2007) Neamtu et al. (2003) Daneshvar and Khataee (2006) CR: 100 CR: 98 COD: 81 TOC: 50 COD: 90 CR: 100 Huang et al. (2008) Vujevic et al. (2010) Arslan-Alaton et al. (2009) CR: 100 COD: 84 TOC: 53 TOC: 98 CR: 100 TOC: 73.8 Zhang and Pagilla (2010) Malathion removal: > 70 Qiu and Huang (2010) Abdessalem et al. (2010) TOC: 93 CR: 98 Abdessalem et al. (2010) TOC: 90 [Fe2+]: 0.1–1 mm [H2O2]: 1–100 mm pH: 3 [Fe3+]: 1 mm [H2O2]: 50 mm R = [H2O2]/[Fe3+]: 50 and 100 Malathion:H2O2: 1:100 Molar ratio H2O2:Fe(II): 40:1 pH: 2.5 [Fe3+]: 1.5 mm [H2O2]: 35 mm [H2O2]: 647.06 mm UV irradiation: 45 W [Fe2+]: 0.72 mm pH: 3 [Pollutant]: 50 mg/l [Fe2+]: 0.2 mm [H2O2]: 10 mm [Fe2+]: 0.396 mm [Pollutant]: 5 × 10-2 mm [H2O2]: 2.5 mm pH: 3 Temp.: 25°C. [H2O2]/[Fe2+]: 2.0–3.5 [H2O2]/[Dye]: 7.0 [Dye]: 100 mg/l [Fe2+]: 0.5 mm [H2O2]: 2.5 mm Molar ratio of the Fenton reagent: 1:5 pH: 3 Temp.: 23°C Molar ratio H2O2/Fe2+: 20:1 pH: 3 References Treatment efficiency (%) Experimental conditions 266 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 80 160 Low-pressure mercury lamp (8 W, UV-C 254 nm) Medium-pressure mercury vapor lamp (30 W, UV-C, 254 nm) UV-A irradiation (366 nm) High-pressure mercury lamp (125 W) High-intensity 254 nm UV grid lamp Reactive red 241 (RR241) EDTA Chlorimuron-ethyl (sulfonylurea herbicide) Oil refinery wastewater UV-lamp with nominal power of 125 W (wavelength 254 nm) Xe lamp (290–400 nm wavelength range) Medium-pressure Hg lamp of 150 W (wavelength 254 nm) Near-UV (black light) fluorescent lamps (15 W, 352 nm) Penicillin G Ibuprofen 2-Chlorophenol Polyphenols in oolong tea manufacturing wastewater Landfill leachate A Heraeus UV immersed lamp TN 15/35 with a nominal output of 15 W) C.I. Reactive Red 120 100 45 120 60 240 60 240 30 40 Medium-pressure mercury (15 W, wavelength 254 nm) Everdirect supra turquoise blue FBL-textile dye UV-irradiation time (min) UV-lamps Pollutant (Table 1: Continued) [Dye]: 50 mg/l [Fe2+]: 0.5 mm, [H2O2]: 2 ml/l [H2O2]: 5.88×10-2 mm [Fe2+]: 30 mg l Temp.: 25°C pH: 4.0 [EDTA] = 5 mm [H2O2]: 100 mm pH: 3.0 [H2O2]: 68.4 mm [Fe3+]: 0.33 mm [Fe2+]: 0.14 mm [H2O2]: 11.77 mm Molar ratio of the Fenton reagent: pH: 3 Molar ratio H2O2/Fe2+: 20 [H2O2]: 20 mm [Fe2+]: 1 mm Mixing: 120 rpm pH: 3.5 [H2O2]: 0.32 mm [Fe2+]: 1.2 mm pH: 3 pH: 2.5–4.0 [H2O2]: 22 mm [Fe2+]: 0.45 mm [H2O2]: 14.71 mm [Fe2+]: 7.19×10-2 mm Stock solutions of H2O2 0.95 mm, FeSO4.7H2O: 0.36 mm H2O2:Fe2+: 1.63–15.25 pH: 3 H2O2: 5.2 Fe2+: 3.6 Agitation speed: 20 rpm Molar ratio H2O2/Fe2+: 20:1 pH: 3 Experimental conditions TOC: 96 COD: 100 COD: 95–97 COD: 60 COD: < 81 TOC: 95.6 CE removal: 94.6 COD: 50 TOC: > 80 CR: 98 COD: 85 TOC: 73 CR: 100 TOC: 18.6 COD: 56.8 TOC: 95 Treatment efficiency (%) Sabaikai et al. (2014) Kavitha and Palanivelu (2003) Mendez-Arriaga et al. (2010) Saghafinia et al. (2011) Tony et al. (2012) Gozzi et al. (2012) Ghiselli et al. (2004) Hu et al. (2011) Patel et al. (2013) Neamtu et al. (2003) Liu et al. (2007) References A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT 267 Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM UV-lamps UV lamps (40 W, wavelength 254 nm) UV-C lamps (280–200 nm; 55 W) and UV-A lamps (400–320 nm; 70 W) UV-lamp with wavelenghth 254 nm UV-C emitting irradiation with a maximum at 254 nm UV-A emitting irradiation with a maximum at 365 nm Low-pressure mercury vapor lamp (12 W, 254 nm) Low-pressure mercury UV lamp (UV-A, 365 nm) Medium-pressure UV lamp (450 W) Pollutant Livestock wastewater Acetaminophen Palm oil refinery effluent Simulated industrial wastewater Simulated industrial wastewater Phthalic anhydride in aqueous medium Simulated dyehouse wastewater Naval derusting wastewater (Table 1: Continued) 60 60 120 60 60 30 120 80 UV-irradiation time (min) UV Light Intensity: 5 mW/cm2 Temp.: 20°C pH: 5 [H2O2]: 0.1 mm [Fe2+]: 0.01 m H2O2 flow rate: 50 ml/h, [Fe2+]: 2 ppm pH: 2.5 Temp.: 40°C), H2O2 mass: 2.125 g H2O2:FeSO4·7H2O: 15:1 Agitation speed: 250 rpm pH value: 3.88 [Fe2+]: 5.01 mm [H2O2]: 30 mm pH: 1.9 [Fe3+]: 8.0 mm [H2O2]: 30 mm pH: 3 R = [H2O2]/[Fe3+]: 40 [Fe2+]: 0.1 mm Temp.: 29±3°C. [Fe2+]: 0.97 mm [H2O2]: 30 mm [Fe2+]: 7.19×10-2 mm [citric acid]: 0.3 mol/l [H2O2]: 2400 mm Experimental conditions COD: 93 TOC: 61 TOC: 98.7 Kim et al. (2010) Grcic et al. (2011) Trabelsi-Souissi et al. (2011) Dopar et al. (2011) Dopar et al. (2011) kobs = 12.94 × 10-2 m-1 s-1 kobs = 9.71 × 10-2 m-1 s-1 Leong and Bashah (2012) Duran et al. (2011) Park et al. (2006) References COD: > 75 COD: 83 BOD: 94 TOC: 71 COD: 70–79 CR: 70–85 Treatment efficiency (%) 268 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT pHi [Oxidant]i [Catalyst]i • UV/H2O2-mostly alkaline • UV/OzoneAcidic (Direct oxidation, Ozone), Alkaline (Indirect oxidation,OH radical) • Fenton based oxidation - pH3-5 • TiO2/UVAlkaline • Strongly dependeny on amount of H2O2 • Excess amount causes scavenging effect and decrease the efficiency • Excessive amout formed iron complex • Reduce the efficiency by scavenging the reaction • Light penetration is reduced by sludge formation 269 UV light Intensity [Pollutant]i • Concentrated pollutant need more dosage of oxidant as well as catalyst. • Tubidity of pollutant, decrrease the amount of UV light that pass through the system • Proper choice of UV source • Turbidity effects the light penetration • Optimize the design of photoeractor for efficient distribution of light Figure 1: Operating parameters and their effects on treatment efficiency of UV-AOPs. 2.1.1 Factors affecting wastewater treatment using photo-Fenton process 2.1.1.1 Operating pH The hydroxyl radical generation by photo-Fenton processes is strongly dependent on the initial pH of the solution since pH value has a significant effect on the oxidation potential of OH radicals. This is due to the inverse relation between the oxidation potential and pH value (Eo = 2.8 V and E14 = 1.95 V) (Lide 2004). At acidic conditions, efficiency of photo-Fenton process is improved regardless of types of pollutants to be degraded (Philippopoulos and Poulopoulos 2003). This is because acidic conditions favor the formation of OH radical, whereas H2O2 is decomposed into O2 at higher pH values, and it loses its oxidation ability. However, extreme acidic conditions are also not suitable for efficient OH radical generation. Hydrogen peroxide gets solvated and becomes more stable at pH below 2, reducing its reactivity with ferrous ion to generate hydroxyl radical (Duran et al. 2011). Axonium ion (H3O2)+ is formed at higher concentration of H+ making hydrogen peroxide more stable and preventing it from reacting with ferrous ion (Feng et al. 2006). Therefore, pH value needs to be optimized for efficient production of OH radical to improve the overall degradation efficiency. Besides OH radical, availability of ferrous iron in reaction medium is also influenced by pH value. Different forms of iron species in relation to pH values are summarized in Table 2. It is clear from the table that at very low pH, iron is present in the form of ferric iron complex which acts slowly with hydrogen peroxide to form OH radical (Lucas and Peres 2006). Most of the ferrous iron precipitates as Fe (OH)3 and forms amorphous oxyhydroxides (Fe2O3·nH2O) at higher pH (Ghiselli et al. 2004). Presence of iron precipitates does not only decrease the light absorption but also increase the cost of post-treatment process. Most of the studies concerning Fenton and photo-­ Fenton processes have suggested pH = 3 as the optimum pH value. For instance, Lucas and Peres (2006) studied the decolorization of azo dye Reactive Black 5 by photo-Fenton oxidation within pH range of 1–3. The color removal efficiencies of 32.6%, 61%, and 98.6% were obtained at pH 1, 2, and 3, respectively. The result showed that maximum color removal was obtained at pH 3. Lower degradation at pH 1 and 2 was caused by the hydrogen scavenging effect due to excessive H+ as mentioned earlier. In contrast, Park et al. (2006) have discovered pH = 5 as the optimum pH value for degradation of livestock wastewater through photo-Fenton process. The authors commented that hydrogen peroxide was the most stable in the range of pH 3–4, but the decomposition rate increased rapidly at pH 5 and decreased above pH = 5. Therefore, the optimum pH was fixed at 5 for this study. In addition, Dopar et al. (2011) highlighted the effect of UV type on pH for treatment of simulated industrial wastewater. It was observed that optimum pH value varied with UV type used. The highest degradation rate was obtained at pH of 3.88 ([Fe2+] = 5.01 mm and [H2O2] = 30 mm) for UV-C irradiation, while pH of 1.9 ([Fe2+] = 8.39 mm and [H2O2] = 30 mm) was Table 2: Iron species generated in photo-Fenton as a function to pH value (Neamtu et al. 2003). Fe3+ species Fe(H2O)63+ Fe(OH)(H2O)52+ Fe(OH)2(H2O)4+ pH range 1–2 2–3 3–4 Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 270 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT observed as optimum pH for UV-A light. From the literature, photo-Fenton also shows efficient results in acidic conditions like Fenton process, but it has to be optimized for each type of UV system, as evident from results obtained by Dopar et al. (2011). 2.1.1.2 Ferrous ion concentration The amount of ferrous ions is one of the main factors influencing the process due to its action as a catalyst to decompose hydrogen peroxide to generate OH radicals. According to previous studies, the minimum ferrous ion concentration required for the reaction is between 3 and 15 mg/l. As reported by Vujevic et al. (2010) and Tony et al. (2012), degradation rate increased with an increase in concentration of ferrous ion till a certain concentration, and it became inefficient above that value. Increasing ferrous ion above the optimal value has adverse impacts on reaction, where it may act as hydroxyl radical scavenger according to Eq. 7 (Vujevic et al. 2010). Excessive Fe2+ can result in a great self-consumption of free radicals, which can inhibit the oxidation reaction as presented in reaction Eqs. (1) and (2). Besides, enormous increase of ferrous ion leads to formation of brown turbidity, which hindered the absorption of UV light and made the process efficiency drop (Herrera-Melián et al. 2000). Furthermore, the sludge formed requires post-treatment, and this is not economically viable (Tony et al. 2012). So the concentration of ferrous ion should be as low as possible to minimize the formation of sludge. This is because sludge removal requires post-treatment, and this is not economically viable (Tony et al. 2012). Fe 2+ + OH ⋅→ HO- + Fe 3+ (7) Vujevic et al. (2010) conducted a study on decolorization and mineralization of reactive dye by UV/Fenton process. The effects of Fe2+ concentration on dye degradation efficiency were investigated within the range from 0.05 to 1.0 mm in this study. The results indicated that mineralization increased from 35% to 73.8% with an increase in the initial Fe2+ concentration from 0.05 to 0.5 mm. Therefore, it can be concluded from the results that degradation rate decreased when iron concentration is above 0.5mm. The degradation of terephthalic acid (TPA), isophthalic acid (IPA), and benzoic acid (BA) from terephthalic acid wastewater by AOPs was evaluated by Thiruvenkatachari et al. (2006). The authors have reported that enhancement in the removal efficiency was observed for all three target organic species in the photo-Fenton process compared to UV/H2O2 system due to more hydroxyl radical in UV-assisted Fenton. Besides, it was found that the removal efficiency increased with increase in Fe concentration. The rate of organic destruction is faster in the early stage of the reaction than in the later stage. It is because ferrous ion catalyses H2O2 quickly in the first stage of reaction to form hydroxyl radical, so more degradation of organic compounds occurs in the early stage of the reaction. In this study, selectivity of hydroxyl radicals for certain specific organic materials was observed. Complete removal of BA was observed within a short period of reaction time when only about 60% of TPA and IPA were degraded. The amount of ferrous ions is one of the main parameters that influence the efficiency of the photo-Fenton process. On the other hand, ferrous ion concentration depends on the type of organic contaminants present in the wastewater and therefore should be optimized for each particular treatment system. 2.1.1.3 Hydrogen peroxide concentration The concentration of hydrogen peroxide contributes to the degradation efficiency of the targeted pollutants in photoFenton process. Hydrogen peroxide can generate powerful hydroxyl radical by acting as an electron acceptor as in Eq. (8) (Alkan et al. 2007, Elmorsi et al. 2010). The absorbance of UV light by hydrogen peroxide can generate two hydroxyl radicals as shown in Eq. (9) H 2O2 +e- →OH ⋅+OH - (8) H 2O2 +hv→OH ⋅+OH ⋅ (9) Based on the observed result, degradation efficiency increased with the amount of hydrogen peroxide. However, the concentration of hydrogen peroxide should be selected carefully to avoid excessive hydrogen peroxide as it can decrease the degradation efficiency or does not improve the degradation. This might be due to auto-decomposition of hydrogen peroxide to oxygen and water together with recombination of hydroxyl radical, which results in scavenging effect as shown in (10) and (11). 2H 2O2 → 2H 2O+O2 (10) H 2O2 + OH ⋅→ H 2O+HO2 ⋅ (11) HO2 ⋅+OH ⋅→ H 2O + O2 (12) OH ⋅+OH ⋅→ H 2O2 (13) Besides, dosage of hydrogen peroxide highly depends on the type of organic contaminants, so it is very important to optimize the value based on the treatment. Papic et al. (2009) evaluated the degradation of three different Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT reactive dyes using homogeneous photo-Fenton process. The optimum H2O2 concentration was found to be 20 mm for C.I. Reactive Yellow 3 (RY3), 2.5 mm for C.I. Reactive Blue 2 (RB2), and 50 mm for C.I. Reactive Violet 2 (RV2), which is in accordance with the optimum molar ratio of Fe2+/H2O2. Duran et al. (2011) evaluated the photo-Fenton mineralization of synthetic municipal wastewater effluent containing acetaminophen in a pilot plant. The results showed that the degradation rate continuously increased with the peroxide flow rate, as more ·HO radicals were being generated. Further analysis was conducted to understand the effects of hydrogen peroxide by analyzing the remaining and consumed H2O2 in the solution. The increase in H2O2 flow rates was not proportional to the increase in degradation rate. This might be due to formation of oxonium ion (H3O2+) and intermediate product through H2O2reaction with organic pollutants. H2O2 contributes to hydroxyl radical scavenging capacity at higher H2O2 dosages. Therefore, optimal H2O2 should be determined to achieve the most efficient degradation. 2.1.1.4 Initial concentration of pollutant Initial pollutant concentration plays a significant role in efficiency of photo-Fenton process. It has been observed that degradation potential of UV decreases with increase in pollutant concentration (Zhang and Pagilla 2010). This is because light penetration decreases with increase in turbidity. It does not only result in decrease in photo-degradation rates of targeted contaminant but also significantly reduces the photo-reduction of ferric ions (reaction 6) and H2O2 to OH radicals (reaction 9). Tokumura et al. (2013) evaluated efficiency of photoFenton process for simultaneous color wastewater treatment. The initial Orange II concentration varied from 22 to 78 ppm to determine the effects of initial Orange II concentration. All the other operating parameters were kept constant with the initial hydrogen peroxide concentration of 297 ppm, initial total iron ion concentration of 10 ppm, a solution pH of 2.5, and a total UV-A light intensity of 94.7 W/m2. It was reported that k decreased with increasing initial Orange II concentration from 22 ppm (k = 0.445 min-1) to 78 ppm (k = 0.334 min-1). From the above studies, it can be concluded that photo-Fenton process becomes inefficient at higher concentration of pollutant. In this situation, more hydroxyl radicals are required for efficient degradation which can only be possible through utilizing high concentrations of iron salt and hydrogen peroxide. 271 2.1.1.5 Operating temperature Based on the literature review, there is a limited number of studies that have highlighted effects of temperature on degradation efficiency. The generation of OH· and HO2· can be promoted by increasing reaction temperature, but the generation of OH radical is not favorable at high temperature. It can be supported by the study conducted by Zhan et al. (2013). This study evaluated the effects of solution temperature ranging from 30°C to 70°C on the oxidation of mercury. The result illustrated that oxidation efficiency increased when the solution temperature increased from 30°C to 40°C, but there was a sharp decline following further increase of the solution temperature to 70°C. Torrades and García-Montaño (2014) also investigated the optimization of parameters for degradation of real dye wastewater by photo-Fenton reactions. The degradation studies of Tartrazine were conducted at different temperatures in the range of 298–333 K under optimum experimental conditions with pH = 3, [H2O2] = 73.5 mm, and [Fe(II)] = 1.79 mm. The rate constant increased with temperature till the temperature reached 323 K. The rate constant decreased above this temperature. This was a result of decomposition of hydrogen peroxide into water and oxygen at higher temperature. Thus, based on the previous studies, increasing temperature can be considered a way of intensification of the Fenton process. But the temperature and amount of H2O2 should be adjusted effectively to achieve a high mineralization. More studies need to be conducted to see the effects of higher temperature on the amount of ferrous ion, mechanism of reaction, and intermediate generation. 2.2 UV/H2O2 Photolysis of hydrogen peroxide (H2O2) using UV light remains the most often investigated advanced oxidation method for water and wastewater treatment. UV/H2O2 is still a promising technique because no sludge is produced at the end of reaction with easier handling and shorter reaction time to degrade contaminants compared to other AOPs (Aleboyeh et al. 2005, Autin et al. 2012). Besides, UV/H2O2 system is the most appropriate AOP technology for removing toxic organics since hydrogen peroxide is cheaper and requires fewer safety precautions. Oxidation in UV/H2O2 system may occur via one of three general pathways such as hydrogen abstraction, electron transfer, and radical addition. The reaction between UV light and hydrogen peroxide generates powerful hydroxyl radical (·OH) as shown in reaction (14) Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 272 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT (Xu et al. 2011, Zuorro and Lavecchia 2013). Radiation with a wavelength < 400 nm is suitable to photolyze H2O2 molecule into hydroxyl radicals with a yield of two ·OH radicals per quantum of radiation absorbed [Eq. (14)]. The other reactions also occur depending on the process condition [Eqs. (15)–(17)]. The photolysis reaction of H2O2 depends on the conditions of the system including temperature, initial concentration of H2O2, pollutant, and pH (Malik 2004, Banat et al. 2005, Shu et al. 2006b, Yonar et al. 2006, Li et al. 2011a). H 2O2 + hv→ 2⋅OH (14) H 2O2 + HO⋅→ H 2O+HO2 ⋅ (15) HO⋅+ HO2 - → HO2 ⋅+OH - (16) H 2O2 + HO2 ⋅→ HO⋅+ H 2O2 + O2 (17) According to previous studies, degradation rate of UV/H2O2 strongly depends on pH (Shu and Chang 2005a, Muruganandham and Swaminathan 2006a). At high pH value, hydrogen peroxide deprotonates and generates H2O2/HO2- equilibrium (Aleboyeh et al. 2005). As shown in reaction (18), non-dissociated molecule of H2O2 reacts with HO2- species and is broken down to oxygen and water instead of hydroxyl radical (Schrank et al. 2007). Decomposition of H2O2 decreases the amount of hydroxyl radical available and eventually decreases degradation rate. Besides, hydrogen peroxide shows a very high self-decomposition rate at higher pH (Shu and Chang 2005b, Schrank et al. 2007). As a conclusion, raising pH results in reducing dye degradation rate since H2O2 dissociates into water and oxygen rather than hydroxyl radicals in alkaline condition. So it is necessary to determine the optimum value of pH for each treatment system investigated. HO2 - + H 2O2 → H 2O + O2 + OH - (18) Rauf et al. (2008) also studied the photolytic decolorization of Rose Bengal by UV/H2O2 and observed that pH played an important role in degradation. The study showed that the maximum dye decolorization was 90% at pH 6 for dye concentration of 0.005 mm and H2O2 of 0.042 mm. The rate of dye decolorization was low in highly acidic solutions. The maximum dye decolorization was achieved at pH 6.6, and the decoloration reduced at alkaline pH. The authors attributed the enhanced dye decolorization in less acidic environment to formation of OH radical when peroxide anions were irradiated by UV light (15). The result was also supported by the study conducted by Jiraroj et al. (2006). The authors observed a rapid degradation of PbEDTA in acidic solutions, while lead precipitation was achieved at pH higher than 6. Slower degradation was observed at high pH conditions because a small number of H2O2 is depronotaed to yield HO2- anion. The generated anion acts as a hydroxyl radical scavenger since this HO2- anion absorbs UV and generates ·OH with a higher molar absorption coefficient compared to that of H2O2. Besides, the reaction between ·OH and HO2- is faster than that of H2O2 and HO2-. Chelme-Ayala et al. (2010) also investigated the degradation of pesticides in natural water by UV irradiated H2O2 system. At an initial H2O2 concentration of 8.8 × 10-4 m, the degradation of pesticides at different pH was investigated. The authors observed that pH had little effects on degradation efficiency of pesticides. In addition to the above mentioned investigation, Schrank et al. (2007) showed the important role of pH for efficient degradation and necessity of pH optimization for each UV/H2O2 system as it varies with pollutant types. Initial concentration of H2O2 may either enhance or inhibit the photoreaction rate of a UV/H2O2 system. A study conducted by AlHamedi et al. (2009) discovered that the rate of dye decoloration was directly proportional to H2O2 concentration till a certain concentration. Above that value, the increase in dye decoloration was not linear. This is because the solution undergoes self-quenching of ·OH radicals with excess amount of H2O2 at higher concentration and produces HO2· radicals as shown in Eq. (19). H 2O2 +⋅OH → H 2O + HO2 ⋅ (19) In addition, Elmorsi et al. (2010) also studied the effects of initial concentration of H2O2 (2.5 × 10-5, 2.5 × 10-3, 2.5 × 10-1, and 5 × 10-1 m) on photo-degradation efficiency of an H2O2/ UV system using fixed concentration of Mordant red 73 azo dye solution (5 × 10-2 mm) at pH 3.0 and 25±2°C. The highest degradation rate of the dye was observed with H2O2 concentration of 2.5 × 10-3 m (rate constant of 0.0863 min-1). Further increase of concentration of H2O2 to 5 × 10-1 m inhibited the reaction, as confirmed by a decrease in the rate constant to 0.0103 min-1. Therefore, the optimum concentration of H2O2 in the reaction course must be reached to maximize the degradation rate. Muruganandham and Swaminathan (2006a) also investigated the effect of addition of H2O2 (5–25 mmol/l) on decolorization of Reactive Yellow 14. It was found that addition of 5–20 mmol/l of H2O2 increased the decolorization from 40.1 to 60.1%. However, a further increase of H2O2 to 25 mmol/l decreased the decolorization from 60.1 to 59.2% within 60 min. Enhanced decolorization was observed by adding H2O2 up to 20 mmol/l due to increase of hydroxyl radical. However, at high H2O2 concentration, hydroxyl radicals act as a quencher and causes decrease in treatment efficiency. In addition, Lester et al. (2010) also studied the photodegradation of the antibiotic sulphamethoxazole (SMX) in water using Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT a medium-pressure UV lamp combined with H2O2. They observed, at higher concentration of hydrogen peroxide (50–150 mg/l), that the efficiency of pollutant degradation did not show any significant improvement in comparison with 50 mg/l of H2O2. The authors concluded that concentrated H2O2 may decrease the availability of light for direct photolysis of pollutant, and scavenging of the hydroxyl radicals by H2O2 also causes the efficiency to drop at higher concentrated H2O2. These observations are also in agreement with the result of other studies (Malik 2004, Shu and Chang 2005a, Riga et al. 2007, Li et al. 2008). UV radiation is another prominent parameter which has significant effects on degradation of pollutants in UV/ H2O2 system besides initial concentration of H2O2 and pH (Muruganandham and Swaminathan 2006a). Haji et al. (2011) investigated degradation of methyl orange (MO) dye using AOP by exposing MO aqueous solution to UV irradiation after addition of hydrogen peroxide (H2O2). AOP process was carried out by studying the influence of dye concentration, H2O2 dosage, UV irradiation power, and the area of the solution exposed to the UV source. The effect of irradiation power was investigated by changing the power from 0 to 30 mW. The result showed that linear increase in dye degradation rate was observed with increase in UV irradiation power. Increase in UV radiation power caused a rise in the formation of hydroxyl radical from hydrogen peroxide and increased the degradation rate. On top of that, the type of UV lamp also affects pollutant degradation in UV-based processes. Linden et al. (2007) studied the effectiveness of UV/H2O2 processes using low and medium-pressure lamps to degrade endocrine-disrupting chemicals (EDCs). The authors observed that the emission spectrum of a medium-pressure mercury UV lamp covered most of the major absorbance range of contaminants investigated. Therefore, the authors concluded that a medium-pressure mercury UV lamp was more effective than a low-pressure mercury lamp. In this study, 17-alpha-estradiol (E2) and 17-beta-ethinyl-estradiol, suspected EDCs, were degraded between 80 and 99.3% at H2O2 concentration of 15 ppm and a UV dose of 1000 mJ/cm2 using either low- or medium-pressure lamps. On the other hand, Rosario-Ortiz et al. (2010) used a custom-made low-pressure (LP) UV collimated beam system in UV/H2O2 process to degrade six pharmaceuticals (meprobamate, dilantin, carbamazepine, primidone, atenolol, and trimethoprim). They found that the removal of the contaminants correlated with the reduction in UV absorbance. Higher dose of UV radiation was needed to promote the generation of hydroxyl radical to overcome the scavenging effects and increase the oxidation efficiency of pollutants. They also observed that the choice 273 of UV lamp depends on the type of pollutant and the ­system’s condition. In addition, a number of researchers have also investigated the efficiency of UV/H2O2 in pilot-scale systems. De la Cruz et al. (2013) investigated the potential use of UV alone, UV/H2O2, and UV/Fenton to treat micro-pollutants in a continuous effluent from a municipal wastewater treatment plant (MWTP) at pilot scale. They observed degradation > 80% with UV irradiation and H2O2 within very short reaction times. The degradation threshold was also achieved within the retention time of 10–67 s, and the maximum amount of H2O2 needed was only 50 mg/l. Ghafoori et al. (2014) proposed a methodology for the photo-reactor scale-up, which included all reaction mechanisms for binary degradation of aqueous polyvinyl alcohol (PVA) by UV/H2O2 process. They determined the intrinsic kinetic parameters using the experimental data obtained in a laboratory-scale batch recirculating system. The performance of a pilot-scale continuous-flow photoreactor was predicted using the kinetic parameters influencing the photochemical reaction of PVA in a batch-scale reactor without any parameter adjustment. Such kinetic parameters were used because they are independent of the photoreactor geometry and operation. Therefore, they can be used to predict the performance of the pilot-scale photoreactor with different geometry and mode of operation. The authors highlighted three important conditions to use this developed method including a valid kinetic scheme, detailed mechanistic kinetic model to identify the intrinsic reaction kinetic expression, and an accurate mathematical model. In this study, computational fluid dynamics (CFD) was used to simulate the results of the proposed model. A good agreement between the CFD simulation result and the experimental data was observed, and thus the reliability of this scaled-up methodology was confirmed. A pilot-scale plug-flow photo-reactor (UV/H2O2) was used to treat azo dye acid orange 10 by Shu and Chang (2005c). The key parameters in this experiment, including flow rate, hydrogen peroxide concentration, UV input power, pH, and initial dye concentrations, were investigated. Ultimately, the obtained result was compared with the ones from a batch reactor, and it was observed that the degradation rate of dye in a pilot-scale plug-flow photoreactor (UV/H2O2) was 233 times higher than that of batch reactor with the same UV light source. Besides, the time needed to achieve 99.9% of color removal of 20 mg/l acid orange 10 dye was only 26.9 min, which was much shorter than that of batch reactor which required 186.4 min to obtain < 1.0% of residual color. Ninety percent decolorization was reached at the flow rate of 1.63 l min-1 with lower Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 274 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT hydrogen peroxide concentration of 0.233 mm. The result showed that light penetration was the most important factor for enhancement of degradation rate in UV/H2O2 and any UV-based processes. The pilot-scale system showed a better removal efficiency compared to batch-scale reactor. It is due to the larger gap that the batch reactor resulted in ineffective decolorization. It is therefore suggested that it is important to reduce the gap between the reactor wall and UV lamp, especially in large-scale systems for better degradation efficiency. This is because there is less light penetration when the gap is larger, causing less hydroxyl radical to be produced and decreasing the efficiency of the process. More applications of UV/H2O2 for wastewater treatment is presented in Table 3. As discussed earlier, the effectiveness of UV/H2O2 process strongly depends on factors such as UV radiation, retention time, initial pH of the solution, initial concentration of hydrogen peroxide, initial concentration of pollutant, and type of pollutant. It is noted from the literature that all parameters have significant impacts on the efficiency of the UV/ H2O2 process in batch- or large-scale system. UV/H2O2 has been successfully used to degrade different types of pollutants including synthetic dyes, landfill leachate, pharmaceutical, emerging pollutants [diethyl phthalate (DEP), chlorophenols, and clofibric acid], and domestic wastewater. The results showed that complete decolorization was achieved by using UV/H2O2 with the COD of the sample being successfully reduced from 40 to 90%. Besides, pH 3–5.5 has been found to be suitable for synthetic dye’s degradation, and the value varies with types of dyes. On the other hand, degradation of pharmaceutical, methyl tert-butyl ether (MTBE), and chorophenolcontaining wastewater has been found to be efficient at alkaline conditions. The differences in pollutant removal efficiency in acidic and alkaline solution may be attributed to formation of intermediates or presence of certain species in the reaction solution. The initial concentration of H2O2 also varies with the concentration of pollutants and radiations applied in the system. Furthermore, it is noted from Table 3 that low-pressure lamp has been used most frequently in UV/H2O2 systems for different types of pollutants. Therefore, optimization of all the above mentioned parameters is deemed important to improve the efficiency of a treatment system. Based on the literature, it is clear that UV/H2O2 treatment is one of the promising technologies for wastewater treatment. However, wastewater with high COD and total organic carbon (TOC) value may not be easily treated with this system. There may also be a need for pre-treatment to enhance the degradation as suggested by Li et al. (2008). Besides, UV-H2O2 is relatively expensive compared to other conventional methods. The process should therefore be optimized to become an industrially applicable and costeffective technology for organic pollutant degradation. 2.3 UV/TiO2 For years, the potential of photocatalysis for organic degradation using TiO2 has been extensively studied. Photocatalytic oxidation takes place when titanium (IV) oxide is irradiated by UV light (Autin et al. 2012) with energy higher or equivalent to band gap. As a result, electrons in the valence band excite to the conduction band (LydakisSimantiris et al. 2010). This mechanism results in the formation of a positive hole (H+) in the valence band and an electron (e-) in the conduction band (Shavisi et al. 2014). Species like O2- and OH radicals are produced as a result of O2 and water interaction with the photo-catalyst which helps in degradation of organic pollutants. The principal mechanisms of photocatalytic oxidation are shown in Eqs. (20)–(23). TiO2 + hv-→ H + + e- (20) e- + O2 →O2 - (21) H + + H 2O → HO·+ H + (22) HO + organic →CO2 (23) The degradation mechanism of TiO2 combined with UV treatment that involves indirect photolysis and heterogeneous photocatalysis makes the decomposition of pollutants efficient. UV light with short wavelengths and high energy makes the two reaction pathways possible through OH radical oxidation and electric holes reaction. In the past decade, TiO2 and ZnO have been widely used by numerous authors for degradation of organic matter, and TiO2 has been recognized as a benchmark semiconductor for effective degradation of organic pollutants (Daneshvar et al. 2004, Peternel et al. 2007). TiO2/UV methods have gained considerable acceptance due to their ability to completely degrade a wide range of pollutants. Organic matter that can be successfully treated using TiO2/UV includes synthetic dyes, polyaromatic hydrocarbons (PAHs), volatile halogenated organic solvents, chlorophenols, biphenyls, 2-chloroaniline, 2,4-dichlorophenol, amoxicillin, DEP, lignin wastewater, municipal wastewaters, and pesticides (Tang et al. 1997, Daneshvar et al. 2003, Park et al. 2003, Chang et al. 2004, El Hajjouji et al. 2008, Lin et al. 2012, Mohammadi and Sabbaghi 2014). It should be noted that initial concentration of pollutant, Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM Mercury lamp (45 W, 254.7 nm) Medium-pressure mercury vapor lamp (30 W, UV-C, 254 nm) Monochromatic lamps (254 nm) with nominal power of 15 W Olive mill wastewater Landfill leachate UV lamps (365 nm and 254 nm) UV lamps (365 nm and 254 nm) Low-pressure mercury lamp (254 nm) Low-pressure mercury lamp (15 W, 254 nm) Clofibric acid Diclofenac Humic surface waters MTBE removal 3 120 Low-pressure, mercury vapor sterilization lamp (40 W) Low-pressure mercury lamp (8 W) Low-pressure mercury lamps (254 nm) Low-pressure mercury lamp (254 nm) Low-pressure UV lamp (10 W, 254 nm) Commercial naphthalene sulphonate Melatonin Herbicides simazine Pathogen re-growth in UASB effluent Cyclohexanoic acid 40 60 180 Low-pressure mercury vapor lamps (254 nm) 60 30 5 60 60 70 300 180 60 60 UV-irradiation time (min) 4-Chloronitrobenzene (CNB) Chlorophenols Seawater Low-pressure mercury vapor UV lamp (254 nm, 15 W) Low-pressure mercury lamp (254 nm, 4.9 W) Low-pressure mercury arc lamp (254 nm) Pharmaceutical wastewater Domestic wastewater UV-lamps Pollutant Table 3: Summary of studies on the removal of pollutant by (UV/H2O2). [H2O2]: 1.765 mm Irradiation: 5 mW/cm2 Irradiance: 0.11 mW/cm2 [H2O2]: 5 mm [CHA]: 10 mg/l Molar ratio of H2O2/CHA: 26 [Bacteria]: 106–107 CFU m/l Temp.: 23°C [CPs]: 0.1 mm and 0.4 mm [H2O2]: 5–40 mm pH: 9.5 [Clofibric acid]: 0.0467 mm [H2O2]: 1 mm [Diclofenac]: 0.0314 mm [H2O2]: 1 mm [H2O2]: 3.68 × 10-3 mm & 8.82 × 10-2 mm UV light dose: 68–681 mWs/cm2 [MTBE]: 0.01–0.06 mm Molar ratio H2O2/MTBE: 30 and 100 pH: 6–7 [CNB] = 2.5 × 10-6 mol/l Flow rate: 60 l/h pH: 7.5 [H2O2]: 0.2 mm pH value: 5.9–6.0 [H2O2]: 60 mm [Melatonin]: 20 mg/l [H2O2]: 10 mm pH: 4 [H2O2]: 3.53 mm [Wastewater]: 100 mg/l Temp: 27–37°C [H2O2]: 20 mmol/dm3 [H2O2]: 20 mm Temp.: 25°C pH: 3 [H2O2]: 117.65 mm Temp.: 22°C [H2O2]: 1.471 mm Experimental conditions Yonar et al. (2006) COD: > 95 Alkan et al. (2007) Bacteria reduction: 7 log Yasar et al. (2007) Afzal et al. (2012) k: 2.2 × 10-2±0.003 Li et al. (2011a) Xu et al. (2009) Tureli et al. (2010) Guittonneau et al. (1990) Kinetic constant: maximum value 0.1701 min-1 Pathogens Removal: > 99 COD: 48 TOC: 27 Melatonin Degradation: > 85 CNB removal: 55 Alnaizy and Ibrahim (2009) Kim et al. (2013) TOC: 60 Degradation: 100 Kim et al. (2013) Trapido et al. (1997) Degradation: 100 TOC: 80 Rubio et al. (2013) Disinfection = 99.9 Bin and Sobera-Madej (2012) Hu et al. (2011) COD: 48.2 COD: 7.6 × 10-4 s-1 Drouiche et al. (2004) References COD: 94 Treatment efficiency (%) A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT 275 Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM Low-pressure Hg arc-UV lamp (254 nm) Low-pressure mercury lamp ( 10 W, 254 nm) Low-pressure mercury lamp (6 W, 254 nm) Medium and low-pressure mercury lamp (15 W, 1000 V, 220 V) Low-pressure mercury lamp (254 nm) Amoxicillin Clofibric acid (CA) Chloramphenicol Reactive black 5 Low-pressure mercury arc UV lamp (253.7 nm, 14 W) Low-pressure mercury lamp (15 W) UV lamp (UV power output of 6 W at 254 nm) R-52 Mineralight® lamp (254 nm) Medium-pressure mercury lamp (125 W) Low-pressure mercury arc UV lamp (253.7 nm, 14 W) Acid Blue 74 Rhodamine B Methyl orange Textile wastewater Textile dyeing wastewater Low-pressure mercury arc UV lamps (16 W, 253.7 nm) C.I. Direct Blue 199 C.I. Acid Blue 113 Medium-pressure Hg vapor lamp (254 nm) Textile-dyeing wastewater C.I. Acid Blue 25 UV-lamps Pollutant (Table 3: Continued) 180 180 3 30 7 20 30 10 40 65 50 15 80 UV-irradiation time (min) [H2O2]: 10 mm Temp.: 22°C [Dye]: 10.0 mg/l [H2O2]: 2.94 mm pH value: 4.5 Temp.: 30°C [Antibiotic]: 20 mg/l [H2O2]: 35 mm Irradiation: 600 mW/cm2 [H2O2]: 15.21 mm [Dye]: 38 mg/l [Dye]: 50 mg/l Temp.: 20°C [H2O2]: 45.38 mm pH: 5.7 TOC: 1178 mg/l COD: 3052 mg/l [H2O2]: 5.88 mm UV input power: 560 W [dye]: 20.0 mg/l [H2O2]: 116.32 mm pH: 8.9 UV dosage: 120.70 W/l [H2O2]: 46.53 mm Molar ratio of H2O2/Dye: 1588.05 pH: 3.5–5.5 [H2O2]: 50 mm Molar ratio H2O2/Dye: 1000 [Dye] = 10 μm [H2O2]: 1.67 mm pH: 7 Molar ratio H2O2/MO: 587 [Methyl orange]: 7.80 × 10-5 mol/l [H2O2]: 45.8 mm Temp.: 28°C pH value: 3 [H2O2]: 29.41 mm COD: 5720 mg/l UV power input: 28.0 W/l [H2O2]: 116.35 mm Experimental conditions COD: 92.3 CR: 98.1 CR: 70 COD: 69 CR: 100 Shu et al. (2006a) Schrank et al. (2007) Haji et al. (2011) AlHamedi et al. (2009) Aleboyeh et al. (2005) k: 0.5 min-1 CR: 73 Shu et al. (2005) Shu et al. (2006) 96.6 CR: 90 TOC: 75 Malik (2004) Ghodbane and Hamdaoui (2010) CR: 84 CR: 100 TOC: 18.9 Ince and Gönenç (1997) Zuorro et al. (2014) Kralik et al. (2010) Jung et al. (2012) References COD: 70 TOC: 50 Clofibric acid removal: 99 TOC: 50 Treatment efficiency (%) 276 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT catalyst loading, initial pH of solutions, and light intensity are the important parameters that affect the efficiency of the TiO2/UV system. Majority of the previous studies have reported that degradation efficiency apparently decreases with increasing initial pollutant concentration (Wu 2008). Higher concentration of organic contaminants inhibits the penetration of light causing fewer photons (H+) that reach the surface of the catalyst (Liu et al. 2006). This results in a slower production of hydroxyl radical and reduces the photo-degradation efficiency. Besides, more organic substances are adsorbed on the surface of TiO2, which also contributes to the lower formation of hydroxyl radicals. Wu (2008) investigated the effects of operational parameters on decolorization of C.I. Reactive Red 198 in UV/TiO2based systems. The effects of initial dye concentration on the rate of dye decolorization were evaluated by changing the initial dye concentration from 10 to 80 ppm at pH 7. The decolorization rate constant declined from 0.2424 to 0.0226 min-1 as dye concentration increased. Liu et al. (2006) also studied the effects of initial dye concentration of C. I. Acid Yellow 17 on mineralization efficiency. The result showed that the removal of dye decreased from 70 to 0.2% as the initial concentration increased from 20 to 150 mg/l. In addition, Tang and Huren (1995) and Chun and Yizhong (1999) found that color removal percentage decreased as the initial concentration increased. The authors attributed it to shielding of the UV light by the dye, which causes the light-triggered catalysts to decrease and reduce the pollutant removal efficiency eventually. A number of studies have reported that pH value affects UV/TiO2 system. Dye degradation depends on the attack by hydroxyl radical, direct oxidation by H+ hole, and direct reduction by e- in the conducting band depending on the properties of the organic pollutant and the pH value (Toor et al. 2006). In an acidic medium, H+ holes react with water molecule producing hydroxyl radical. Hydroxyl radicals are scavenged at higher pH decreasing the oxidation of pollutant by hydroxyl radical (Muruganandham and Swaminathan 2006b). The point of zero charge of TiO2 is at pH 6.8; therefore, below this pH the surface is positively charged and above that it is the opposite. To investigate the effect of pH on TiO2 performance, Cho and Zoh (2007) studied the photocatalytic oxidation of Reactive Red 120. The results showed that acidic solution favored adsorption of negatively charged reactive dye onto the photocatalyst surface and increased the degradation rate. At lower pH, more hydroxyl radicals will be generated to oxidize the dye but at higher pH; TiO2 surface was negatively charged, and the absorption of molecules on the surface of catalyst decreased. Adsorption of TiO2 277 particles onto dye molecules may be less favorable due to the repulsion between negatively charged TiO2 particles and dye molecule. Similar results were reported by Liu et al. (2006). The highest removal of C. I. Acid Yellow 17 (70.6%) was achieved at pH 3, while the lowest (44.3%) was observed at pH 11. The properties of the C.I Acid Yellow 17, which is an anionic dye, make it easier to be absorbed on the surface of the catalyst at lower pH value. Similar results were also reported in other UV/TiO2 reaction systems (Tanaka et al. 2000, Liu et al. 2006). Most of the studies have reported that the efficiency of UV/TiO2 increases with TiO2 dosage until an optimum value after which the degradation efficiency reduces or becomes constant. This is due to excessive TiO2 that causes a shadow effect that interferes with transmission of UV light and hinders formation of electron-hole pairs (Tang and Huren 1995, Thiruvenkatachari et al. 2007). Besides, decrease in the number of surface active sites at higher TiO2 concentration also reduces the degradation efficiency (Toor et al. 2006). Muruganandham and Swaminathan (2006b) evaluated photocatalytic decolorization and degradation of reactive orange 4 by UV/TiO2 process. The effects of TiO2 dosage ranging from 1 to 6 g/l on degradation were investigated. Degradation rate increased from 0.075 to 0.290 mol min-1 as TiO2 dosage increased from 1 to 4 g/l. Further increase of TiO2 dosage did not cause any changes on the degradation rate. The activity of the catalyst became constant above 4 g/l, which may be caused by scattering of light, particle aggregation, and screening effects. Besides, Chang et al. (2004) investigated the effects of TiO2 dosage on degradation of synthetic lignin wastewater. The result showed that the removal efficiency increased with TiO2 dosage until an optimum level of 10 g/l, beyond which the efficiency reduced with further increase of TiO2 dosages due to the shadow effects caused by the excessive amount of TiO2. At optimum dosage of 10 g/l, 50% of decoloration was achieved within 10 min, and by extending the time to 960 min, almost 80% color and dissolved organic compounds (DOC) removal was observed by the researchers. In addition, the literature study conducted by Ramesh et al. (2008) showed that, at a specific light intensity, increase in TiO2 concentration beyond a certain value was found to decrease the efficiency of the treatment. The authors concluded that higher concentration of TiO2 would affect the passage of light through the solution and hence will affect the degree of absorption of light by the catalyst surface. This result was in agreement with the other literature results (Rizzo et al. 2014, Fenoll et al. 2015). Table 4 illustrates the summary of wastewater treatment using UV/TiO2. Various types of organic matter have Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM UV-lamps 9 W lamp (350–400 nm) UV lamp (6 W, 365 nm) UV germicidal lamp (253.7 nm, 18 W) Low-pressure mercury arc bulb (15 W) UV lamp (400 W, 200–550 nm) UV lamp (254 nm and 8 W power) UV tube TLAD Philips 415 W/05 (365 nm) 9 W-lamp (Radium Ralutec, 9 W/78, 350–400 nm). High-pressure mercury lamp (125 W; λ > 253 nm) Medium-pressure (254 nm, 500 W) Medium-pressure mercury lamp (125 W) UV-lamp (40 W) Pollutant Amoxicillin Pesticides Oxidation of organic matter in drinking water Cryptosporidium parvum Petroleum refinery wastewater Methyl tert-butyl ether (MTBE) Olive mill wastewater (OMW) Secondary treated municipal wastewater DEP C.I. Reactive Red 198 Reactive Red 198 Direct Red 23 Table 4: Summary of studies on the removal of pollutant by (UV/TiO2). 100 45 30 180 60 24 H 60 240 45 60 300 90 Temperature: 25°C TiO2 concentration: 0.5 g/l pH: 5 TiO2 suspension: 1.5 g/l pH: 6 Nano-TiO2 film Light intensity: 2–3 mW/cm2 TiO2 suspension: 0.001 g/l pH: 6–8 [TiO2]: 0.1 g/l pH value: 3 Temp.: 318 K TiO2 suspension: 2–3 g/l pH: 2 [MTBE]: 1 mm TiO2 suspension: 1 g/l Filters: 0.45 μm TiO2 suspension: 1 g/l pH: 6 TiO2 suspension: 1 g/l pH: 7 TiO2 suspension: 0.5 g/l [Dye]: 10–80 mg/l pH: 4 Millipore filter membrane: 0.45 μm Temp.: 25°C TiO2 suspension: 0.3 g/l [Dye]: 100 mg/l Temp.: 21–2°C. TiO2 suspension: 4 g/l pH: 2 UV-irradiation Experimental conditions time (min) Decomposition: > 99 Degradation: 99 TOC: 43–46 Sohrabi and Ghavami (2008) Kaur and Singh (2007) Wu (2008) Mansouri and Bousselmi (2012) Lydakis-Simantiris et al. (2010) El Hajjouji et al. (2008) Hu et al. (2008) k: 0.0233 min-1 CR: 57 COD: 22 Coliforms removal: 65 Enterococci: 50 78.6% Saien and Nejati (2007) Ryu et al. (2008) Jin-hui (2012) Chaudhuri et al. (2013) Dimitrakopoulou et al. (2012) References Organic pollutant removal: 90 Log inactivation: 3.3 log10 COD: 25.95 TOC: 8.45 82.14 TOC: 93 Treatment efficiency (%) 278 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM UV-lamps UV lamp (228–420 nm) 40 W-blacklight blue fluorescent lamp (3.6 W, 368 nm) UV lamp (6 W, 352 nm) Medium-pressure mercury lamp (125 W) Blacklight blue fluorescent lamps (40 W, 254 nm) UV-black fluorescent lamps (40 W) UV light (Spectronics, BLE-8T365) was Pollutant Procion yellow H-EXL Reactive Red 120 C.I. Reactive Orange 16 Reactive Red dye 198 (RR dye 198) Reactive Red 120 Direct Yellow 12 C. I. Acid Yellow 17 (Table 4: Continued) 400 150 90 45 120 90 30 [Dye]: 10 mg/l pH: 5 TiO2 suspension: 1 g/l TiO2 suspension: 1.5 g/l pH: 5 [Reactive Red 120]: 100 mg/l TiO2 suspension: 2 g/l pH: 3 TiO2 suspension: 0.3 g/l Temp.: 25°C [Dye]: 100 mg/l pH: 4.6 TiO2 suspension: 2 g/l [Dye]: 50 mg/l pH: 5 TiO2 suspension: 2.0 g/l Millipore filter membrane: 0.45 μm pH: 4.5 pH value: 3 [Dye]: 50 mg/l Temp.: 25°C Flow rate: 0.82 cm/s UV-irradiation Experimental conditions time (min) COD: 73 Liu et al. (2006) Toor et al. (2006) Cho and Zoh (2007) k: 0.0448 min-1 COD: 94 Kaur and Singh (2007) Kartal and Turhan (2012) Park et al. (2003) Barakat (2011) References COD: 95 CR: 61 > 99 Degradation: 100 Treatment efficiency (%) A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT 279 Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 280 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT been successfully treated using TiO2. Based on Table 4, it is found that optimized value of the initial pH of the solution varies with the targeted organic matter. Besides, TiO2/ UV is mostly used for pollutants of low concentration. It might be because UV/TiO2 process is energy intensive and not suitable for high concentrations of pollutants. Retention time of minimum 30 min is required for decolorization or degradation to occur, and it can go up to 400 min based on the targeted pollutants. Therefore, it is necessary to optimize the operating parameter for each process to get maximum removal efficiency. UV/TiO2 systems have also been successfully applied to achieve COD reduction of more than 90% and TOC reduction of about 40%. As a conclusion, the literature shows that heterogeneous photocatalysis is a promising technology for removal of organic pollutants. It is an environmentally friendly, low cost, and sustainable process. Apart from that, the photocatalytic activity and stability of TiO2 are extraordinary. Nevertheless, full-scale TiO2/UV treatment systems have not been successfully developed due to low quantum efficiency, complicated photo-reactor design, inability to reuse titanium dioxide, generation of intermediate products and by-products, and catalyst deactivation. It is necessary to resolve all the above stated limitations to successfully commercialize the system for wastewater treatment. 2.4 UV/Ozone Great attention has been given to water and wastewater treatment utilizing ozone as an oxidant (Sharrer and Summerfelt 2007). Ozone plays a dual role by disinfecting and degrading the target compounds (Summerfelt 2003). Ozone is reported to have successfully degraded many organic contaminants including pesticides, pharmaceuticals, and municipal wastewater (Beltrán et al. 2012, Bin and Sobera-Madej 2012, Cheng et al. 2013, Lester et al. 2013b). The mechanism, kinetics of ozone decompositions, and the chain reaction have been extensively studied (Amat et al. 2005, Sharrer and Summerfelt 2007). In addition, ozone also can be applied directly in its gaseous state and therefore does not increase the volume of wastewater and sludge, and ozone reaction time with pollutant is short. The oxidation of organic contaminants takes place either through direct oxidation by molecular ozone or indirectly by decomposition of ozone (Bustos et al. 2010). Direct oxidation of ozone is favored under acidic condition in the presence of radical scavenger, and alkaline conditions predominate the generation of strong hydroxyl radical (indirect oxidation) (Liu et al. 2004). At alkaline conditions, ozone decomposes to secondary, more reactive, and hence less selective oxidants such as OH·, HO2·, HO3·, and HO4·, are formed (Table 5). Hydroxyl radical is the main oxidant responsible for indirect oxidation in ozone decompositions among others (Tezcanli-Guyer and Ince 2004). Thus, the stability of an ozone solution is highly dependent on the pH. The depletion rate of ozone is reduced in alkaline solutions. This is may be due to the formation of ozonide, O3-, which reacts with H2O2 or OHradicals and reforms ozone. The important factors that affect organic removal in ozone systems are dosage of ozone, composition of water, and reaction-rate constant of ozone with the target contaminant (Huber et al. 2003). Dissolved organic matter and nitrite are the two most important constituents that effect the treatment efficiency. Ozone could absorb the organic matter and improve the particle aggregation by reducing the electrostatic stabilizing effect of organic matters. Ozone is decomposed to OH radical through a radical reaction (refer to Table 5). And it can be promoted by solutes that could transfer hydroxyl radical into superoxide radical ion. At the same time, the action can be also inhibited by compounds such as carbonates that do not promote generation of superoxide radical ion. This could decrease the rate of reaction due to a drop in the free radical to complete the degradation process. Besides that, a radical chain reaction can also be accelerated by the presence of aromatic compounds in the targeted pollutant which may produce additional hydroxyl radical reaction between ozone and aromatic compounds. The Table 5: Mechanism of ozone decompositions at alkaline ­conditions (Tomiyasu et al. 1985). Reaction Rate constants Initial reactions O3+OH- → HO2-+O2 O3+HO2-→ HO2 +O3-· Propagation reactions HO2·→O2-·+H+ O2-·+H+→HO2· O3+O2-·→O3-·+O2 O3-·+H2O→HO·+O2+OHO3-·+HO·→HO2·+O2-· O3+HO·→HO2·+O HO2-+H+→H2O2 H2O2→HO2-+H+ Termination reactions O3+HO·→O3+OHHO·+H2O2→HO2·+H2O HO·+HO2-→HO2·+OH- 40.0 m-1 s-1 2.2 × 106 m-1 s-1 7.5 × 105 m-1 s-1 5.0 × 1010 m-1 s-1 1.6 × 109 m-1 s-1 20–30 m-1 s-1 6.0 × 109 m-1 s-1 3.0 × 109 m-1 s-1 5.0 × 109 m-1 s-1 0.25 m-1 s-1 2.5 × 109 m-1 s-1 2.7 × 109 m-1 s-1 7.5 × 109 m-1 s-1 Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT investigation conducted by Huber et al. (2003) showed that the rate of reaction is depend on the characteristics of the pollutants, which react slowly with ozone and rapidly with hydroxyl radical. The reactivity was found to increase with increases in concentration of organic pollutants and decrease with increases in alkalinity. Lester et al. (2013a) observed that cyclophosphamide (CPD) removal occurred mainly through its reaction with ·OH radicals and decreased with increasing alkalinity and concentration of peptone (more pronounced) and alginic acid (less pronounced). The difference in the rate constant of both model compounds influences the removal efficiency of CPD. Ozone is can easily absorb UV radiation and generate hydrogen peroxide (intermediate), which then decomposes to hydroxyl radical (Tezcanli-Guyer and Ince 2004). And it was found that photolysis of ozone yields more radicals than the UV/H2O2 process (Shu and Chang 2005b). The integration of ozone and UV has been proven to effectively treat many types of pollutants. Molecular ozone reacts slowly and selectively with the organic or inorganic compounds compared to hydroxyl radical. There are a number of obvious differences that could be observed between the ozone and hydroxyl radical with respect to the reaction with organic matters. The rate of ozone consumption is found to decrease rapidly with increasing ozonation. This is because of mass transfer limitations and self-decomposition of ozone at higher concentrations. However, in alkaline conditions, high concentration causes rapid production of highly oxidative free radicals, i.e., HO rapidly reacts with the contaminant, and thus it increases the rate of reaction because of the continuous availability of hydroxyl radical within the system, as clear from Table 5. On the other hand, ozone is selective in nature and attacks conjugated double bonds of the organic compounds, which results in formation of other by-products. Thus, it increases the biodegradability of the wastewater effluent. However, hydroxyl radical follows hydrogen abstraction, electrophilic addition, electron transfer, and radical chain mechanism. In this way, high reaction rates can be achieved, which is one of the advantages of using ozone under alkaline condition. Ozonation appears to be a more efficient technique when it is implemented with UV to treat highly polluted wastewater since it produces additional hydroxyl radical and hydrogen peroxide via photolysis. A number of studies have reported that ozone-assisted UV removes organic contaminants more effectively compared to treatment with ozone alone (Trapido et al. 1997, Vogna et al. 2004, Rosenfeldt et al. 2006). UV photolysis of ozone generates H2O2, which produces hydroxyl radical that reacts with UV 281 radiation as shown in reaction Eqs. (24)–(26) (Gracia et al. 1996, Gong et al. 2008). O3 + hv + H 2O → H 2O2 + O2 (24) H 2O2 + hv-→ 2 HO⋅ (25) 2O3 + H 2O2 → 2 HO⋅+3O2 (26) Ozone/UV has been applied widely in wastewater treatment even though it is known as a most complex system. It can degrade organic matter in different ways: direct ozonation, photolysis reaction, and hydroxyl radiation. Table 6 summarizes the application of O3/UV for treating different types of wastewater together with their experimental conditions and treatment efficiency. Based on Table 6, it is clear that initial pH of the solution, initial concentration of pollutants, flow rate of ozonation, and UV radiation play a crucial role in enhancing the treatment efficiency. Gong et al. (2008) compared the efficiency of O3 and UV/O3 techniques by treating bio-treated municipal wastewater. The result showed that ozonation alone and ozonation with UV radiation were both effective in removing UV-absorbing organic substances from wastewater. However, UV/O3 was found to be much more effective for mineralization by achieving DOC reduction of 90% compared to ozone alone, which achieved only 36% reduction. In addition, Shang et al. (2007) evaluated oxidation of methyl methacrylate (MMA) from semiconductor wastewater by O3 and O3/UV processes. The authors reported that COD and methyl methacrylate removal were 51% and 96%, respectively, in 120 min. The removal efficiency of MMA by the ozone/UV process was higher than that by ozonation alone, which only achieved 24% COD removal. It was because UV radiation could enhance mineralization by decomposing ozone and generating additional hydroxyl radical simultaneously. Irmak et al. (2005) studied the decomposition and degradation of two endocrine disrupters, E2 and bisphenol A (BPA) in aqueous medium by using ozone and O3/UV. In the study, different O3 dosages, lower and upper level, were used for complete oxidation of E2 and BPA. The efficiency was determined based on the initial conversion and complete degradation of the substrate at initial concentration of 0.40 mm. The result indicated that the reaction between BPA and O3 was slower than the reaction between E2 and O3. It was noted that UV coupled with O3 decreased the O3 consumption by 22.5% in converting the same amount of E2, within the limits of the O3 dosages used. The intermediate products formed in the reaction were analyzed by LC-MS and determined to be the oxidation product of E2 via addition of O3/·OH radical to different positions of aromatic ring of E2. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM C-18 ultra aqua column employing UV detection at a wavelength of 210 nm. Low-pressure mercury UV lamp (wavelength 254 nm) UV-C lamp (253.7 nm, 15 W) Low-pressure Hg arc-UV lamp (3 W) Mercury lamp (125 W, UV-C at 254 nm) Special low-pressure mercury-vapor lamp (254 nm) Low-pressure mercury lamps ( 254 nm, 2.2 W) Low-pressure mercury lamp (15 W, 254 nm) Low-pressure mercury lamps Mercury lamp (125 W, UV-C at 254 nm) UV Logic model 02AM15 High-pressure mercury vapor lamp (185 nm) 2-propanol Aniline Winery wastewater DEP Phenol Ethylbenzene and chlorobenzene Tert-butyl formate and its intermediates Board paper industry Sulfamethoxazole Phenol Bacteria inactivation in a freshwater Olive oil mills wastewater Low-pressure portable UV lamp (254 nm, 8–32 W) 9H Low-pressure mercury UV lamp (254 nm) Biotreated municipal wastewater Rhodamine B 60 Low-pressure mercury lamps (254 nm) MMA 15 60 180 60 150 70 s 60 50 300 90 200 150 120 90 Mercury lamp (125 W, UV-C at 254 nm) Organic aqueous solution UV-irradiation time (min) UV-lamps Pollutant Table 6: Summary of studies on the removal of pollutant by (UV/O3). TOC: 44.3 k (ozone): 7.86 × 10-3 g/min pH value: 3–12 [Ozone]: 10 mg/l [IUV]: 35.96 W/m2 Temp.: 25°C [Ozone]: 9.10 mg/l Air flow rate: 150 l/h pH value: 7 [IPA] = 1000 mg/l, Ozone dosage = 18.4 mg/min Temp. = 25°C [Aniline]: 0.04 mm [Ozone]: 0.5–0.85 mg/l Natural pH: 10 Ozone: 0.68 g/min Ozone dosage: 1.5 and 4 mg/l min Flow rate: 0.5 l/min pH: 11 k (ozone): 7.86 × 10-3 g/min [Pollutant]: 100 mg/l Ozone dosage: 50–450 mg/l Influent [ozone]: 39 mg/l Effluent [ozone]: 15 mg/l pH: 7 Flows of ozone: 0.8–8 g/h pH value: 9 [Ozone] = 1.0 mg/l pH: 7 [SMX] = 1.0 mg/l pH: 11 k (ozone): 7.86 × 10–3 g/min Ozone dosages: 0.1–0.2 min mg/l UV dosage: 50 mJ/cm2 Temp.: 10, 20, 30 and 40°C pH: 5, 7 and 7 Ozone partial pressure: 0.54, 0.9 and 1.67 kPa pH: 3 Ozone dosage: 10–40 ml/min Kusic et al. (2006a) TOC: > 40 Phenol: Complete Degradation: < 70 Liu et al. (2012) k: (0.60–3.38)±0.13 × 105 m-1 s-1 CR: 97.76 COD: 39.72 82.4–97.5 Cuiping et al. (2011) Sharrer and Summerfelt (2007) Benitez et al. (1997) Kusic et al. (2006a) Amat et al. (2005) k: 184 min-1 TOC: > 40 Phenol: Complete Bacteria removal: 100 Garoma et al. (2008) TOC: 99 Cheng et al. (2013) Oh et al. (2006) Lucas et al. (2010) Zhao et al. (2013) Wu et al. (2008b) Gong et al. (2008) Shang et al. (2007) Kusic et al. (2006a) References Degradation: 50 TOC: 26 Degradation: 100 TOC: 23 COD: 85 COD: 51 Treatment efficiency (%) Experimental conditions 282 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Based on the results obtained, it is clear that the degradation efficiency depends on the types of pollutants and the dosage of ozone used. Lucas et al. (2010) used O3/UV at the natural pH to treat winery wastewater of initial pH = 4, 7, and 10, to investigate the effect of radical formation on the rate of COD removal. The authors observed the fastest COD removal at pH 10 as a result of fast reaction between the organic matter and the radical species (i.e., ·OH, HO2·, O2·-, and O3·-). Ozone self-decomposition to radicals occurs at alkaline pH due to initiation reaction (Tomiyasu et al. 1985). Besides, a sharp decrease in the pH of the solution showed that strong oxidation took place. The reduction in pH was a result of the formation of dicarboxylic acids, CO2, and carbonic acids. The result obtained by Arslan et al. (2014) was in agreement with that reported by the previous researchers. In this study, the experimental ranges of the factors were selected as initial pH of 2–11, ozone concentration of 0–15 mg/l, and reaction time of 20–60 min. The authors observed that O3/UV yielded the highest degradation efficiency of raw hospital wastewater at initial pH of 8.0 and O3 concentration of 4.2 mg/l within 27 min. Kusic et al. (2006a,b) discussed the effect of UV light, pH, and ozone dosage on the degradation of phenol in UV/O3 systems. The highest calculated rate constant for phenol degradation, k = 0.1936 min-1, was obtained. The highest phenol degradation rates were observed in strong alkaline media. The result showed that degradation efficiency increased with pH up to pH 8 and then it reduced at pH 9 and 10. However, the maximum TOC removal of 44.3% was observed at pH 11. In addition, Gurol and Vatistas (1987) also demonstrated that based on their experimental results, molecular ozone was dominant only at low pH level (acidic). Free radicals such as OH radicals were the main influencing factor at neutral or basic pH. Such observation supports that UV could help enhance the degradation efficiency by generating additional free radicals. Besides, ozone concentration can also significantly influence the oxidation process. Ozone molecules should dissolve in the aqueous solution and diffuse into the system for continuous oxidation process for an efficient oxidation process (Gurol and Vatistas 1987). It was proven by Amat et al. (2005) in their control experiment. The result showed that the COD and TOC removal efficiency under UV light treatment alone was much lower than that under the combination of ozone and UV. The ozone/ UV combination has shown significant synergetic effect. Beltrán et al. (1997) observed that O3/UV radiation was the best oxidation method to remove COD and TOCD regardless of wastewater type. UV light intensity is another operating parameter that can influence oxidation rates. 283 Wu et al. (2008a,b) observed that ozonation process was strongly enhanced by the presence of UV light for degradation of 2-propanol. The enhanced degradation is due to abundant generation of hydroxyl radicals. Ozone absorbs UV radiation and generates hydrogen peroxide. The subsequent photolysis of hydrogen peroxide generates hydroxyl radicals as shown in Eq. (24). Hydrogen peroxide could quicken ozone decomposition into OH radicals. Their study showed that OH radicals play an important role as an active species in the photolytic ozonation process. Considering the aforementioned discussion, the combination of UV with O3, at optimum design and optimized conditions, will yield higher degradation and removal rate compared to using ozonation alone regardless of type of wastewater. The synergistic effects of this combination have been discussed in the literature. It is believed that the synergistic effects are attributed to intensive generation of highly oxidative and non-selective reagents such as hydroxyl radicals. Based on Table 6, to date, COD removal of more than 90% has been recorded for the treatment system utilizing UV with O3. Besides, the simplicity of the process makes it a suitable alternative compared to the other oxidation methods. Choosing a proper irradiation source, initial pH value, and optimum concentration and developing new operative designs are important to make the process more successful. Among them, an appropriate reactor design could considerably stimulate the synergistic effects of the system for industrial scale-up. 2.5 UV/Persulfate More recently, sulfate radical (SO4·-)-based AOPs have been identified as a promising technique for degradation of recalcitrant pollutants. Sulfate radical is one of the strongest aqueous oxidizing species with a high redox potential of 2.5–3.1 V (Cai et al. 2014) similar to that of hydroxyl radical which is of 2.8 V. Sulfate radicals also offer several advantages over other oxidants such as longer half-life, fast kinetics, higher stability than hydroxyl radical, greater transport distances in the subsurface level, and ability to work in a wide range of pH, and it can be activated by low-cost oxidant precursors. However, although it is more stable than hydroxyl radical, the narrow selectivity of sulfate radical toward organic matters makes it less efficient compared to hydroxyl radical. Sulfate radical reacts through electron transfer or addition or hydrogen abstraction. The high redox potential of sulfate radical causes it to produce radicals from many anions through electron transfer (Criquet and Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 284 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Leitner 2009). These attributes collectively make sulfate a viable option for chemical oxidation of a broad range of contaminants. Persulfate anions with a redox potential of 2.1 V are widely used as sulfate-based precursor oxidants, and they demonstrate a great ability in degradation of refractory compound (Watts and Teel 2006, Rastogi et al. 2009, Lin et al. 2014). Peroxydisulfate (PDS, S2O82-) and peroxymonosulfate (PMS, HSO5-) are two types of oxidants that have been used for degradation of synthetic dyes, pharmaceuticals, chlorophenols, PAHs, and pesticides. PDS is commonly used as a source of sulfate radical because of its high aqueous solubility and stability at room temperature and relatively low cost (Cai et al. 2014). Besides, persulfate reduction is relatively harmless and considered environmentally friendly (Cai et al. 2014). The commonly used salts as a source of sulfate radicals for organic matter decompositions are sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), and ammonium persulfate (NH4)2S2O8 (Huang et al. 2005). It has been reported in the previous work that potassium persulfate yields better organic matter removal efficiency at its natural pH, and it is much cheaper compared to other salts. Although it is a strong oxidant, the reactivity of persulfate ions is mostly slow at room temperature. It has been postulated that persulfate anion (S2O82-) can be activated by heat, UV, electron transfer, and transition metals to generate a stronger oxidant sulfate radical (SO4·-). (Liang et al. 2003, Liang et al. 2004a,b, Huang et al. 2005, Criquet and Leitner 2009, Mora et al. 2009) [Eqs. (27)–(38)]. The formed radical has a redox potential of 2.6 V and is kinetically improved. In accordance with Eqs. (1)–(15), in the reaction accelerated by UV light, the oxidation process is begun by generation of the sulfate radicals followed by hydroxyl radical. Photolysis of S2O82- results in the formation of two SO4·- radicals [Eq. (1)] (Tsao and Wilmarth 1959). It is noted that SO4·- has a maximum optical absorption spectrum at 440–450 nm and a molar extinction coefficient between 460 and 1600 m-1 cm-1. The formed radicals are powerful oxidizing agents which attack the recalcitrant compounds in the contaminated water and cause a complete decomposition of those compounds into carbon dioxide and water. Although sulfate ion is formed as an end product, it is inert and not considered as a secondary pollutant. The sulfate ions formed help decrease the pH and increase the salt content of the effluents. S2O8 2- + hv→ 2SO4⋅- (27) SO4⋅- + RH 2 → SO4 2- + H + + RH ⋅ (28) RH ⋅ + S2O8 2- → R + SO4 2- + H + + SO4⋅- (29) SO4⋅- + RH → R⋅ + SO4 2- + H + (30) 2R⋅ → RR ( dimer ) (31) SO4⋅- + H 2O → HSO4- + OH ⋅ (32) HSO4- → H + + SO4 2- (33) OH ⋅ + S2O8 2- → HSO4- + SO4⋅- + ½O2 (34) SO4⋅- + OH ⋅ → HSO4- + ½O2 (35) 2OH ⋅ → H 2O2 (36) OH ⋅+ H 2O2 → H 2O + HO2⋅ (37) S2O8 2- + H 2O2 → 2H + + SO4 2- + O2 (38) S2O8 2- + e → SO4⋅- + SO4 2- (39) S2O8 2- + heat → 2SO4⋅- (40) S2O8 2- + M n+ → SO4⋅- + SO4 2- + M ( n+1 )+ (41) (Mn+ (Transition metals) = Ag+, Cu+, Co2+, Fe2+, Mn2+) R = organic compounds Among the AOPs, the homogeneous AOPs employing PDS and UV/PDS have been found to be very effective in degrading refractory pollutants. Regardless of the type of precursor used, persulfate oxidation is highly sensitive to the process conditions. Therefore, the optimization of operating parameters is very important in order to determine the influence of each parameter on the process efficiency. Based on the literature study, it is noted that operating parameters including initial pH, initial concentration of pollutant, initial concentration of persulfate, and temperature have an impact on the system efficiency. Khataee and Mirzajani (2010) are the pioneer authors who have investigated the photo-oxidative decolorization of the textile dye C.I. Basic Blue 3 (BB3) through UV/PDS process. In their work, they reported the effects of operating parameters on photochemical treatment of a dye solution. The decolorization efficiency was investigated in the presence of potassium PDS (K2S2O8), irradiated by a 30 W UV-C lamp. The result showed that the decolorization efficiency was affected by operating parameters such as the reaction time, UV light intensity, initial concentration of BB3, and amount of PDS. Among other parameters, initial concentration of PDS is very important as it supplies free active radicals for decolorization and degradation process. The proper addition of PDS is necessary to improve the degradation rate. At higher concentration of PDS ion, more sulfate and hydroxyl radicals are available to attack the aromatic Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT rings, and thus the decolorization efficiency increases. Khataee and Mirzajani (2010) reported that the initial concentration of PDS was found to be an important parameter for the photooxidative decolorization of BB3 in the UV/S2O82- process. It was noted that the increase of initial S2O82- concentration from 0.1 to 1.8 mm increased the pseudo-first-order rate constant from 0.0036 to 0.1933 min-1. However, once it exceeded a certain value (1.8 mm), a decrease in the photodegradation rate was observed. It is because as the concentration of S2O82- increases, more sulfate and hydroxyl radicals are available to attack the aromatic rings and thus the rate of reaction increases. However, at higher S2O82- amounts, excessive amount of hydroxyl radicals are generated, which will recombine to form less reactive H2O2 [Eq. (10)] to become scavenger of OH· radical as well as the sulphate radical. This observation was in agreement with the result of the other studied reactions (Khataee 2010, Yoon et al. 2011). In addition, initial pH of the solution also plays an important role in degrading pollutants. Saien et al. (2011) reported that the efficiency of dye decolorization generally decreased mildly with either an increase or decrease relative to the system’s natural pH of 6.0. It was because SO4·species may undergo reactions with H2O or -OH to generate ·OH under neutral or alkaline pH conditions, according to Eqs. (16) and (17). Presence of SO42- anion can inhibit the reactivity of ·OH or SO4·-. HSO4- is with pKa value of 2.0, and above this SO42- is the predominant species rather than HSO4-. Therefore, they cause hydroxyl radical scavenging and slow down the decolorization efficiency. SO4⋅- + H 2O → H + + SO4 2- +⋅ OH (42) SO4⋅- + - OH → SO4 2- +⋅ OH (43) However, in an acidic media, additional SO4·- could be formed with acid catalyzation, according to Eqs. (18) and (19). Higher generation of SO4·- could also cause radical scavenging reaction instead of reaction with organic matter. However, the researchers did not observe any variation in the degradation efficiency in the cases of acidic initial pH values, and they attributed it to less generation of H+ during the reaction. Therefore, in this study, the natural initial pH of 6.0 is considered an optimum pH with the highest discoloration efficiency obtained. S2O8 - + H + → HS2O8 - (44) HS2O8 - → SO4⋅- + SO4 2- + H + (45) Similar trends were also reported by Criquet and Leitner (2009). The authors investigated the degradation of acetic acid by UV/PDS. They reported that the reaction rate constant decreased with increasing pH. In an 285 alkaline solution, OH- acts as a scavenger of sulfate radicals to slow down the degradation reaction by forming hydroxyl radical through reaction with sulfate radicals. This decreases the reaction rates. At acidic conditions, additional sulfate radicals can be formed from acid catalyzation, which aids in the degradation process as shown in Eqs. (42) and (43). The influence of pollutant concentration on degradation is very important from the mechanistic and application point of views. The effect of initial concentration of dye on decolorization of C.I. basic blue 3 was investigated by Khataee and Mirzajani (2010). They observed a decreased decolorization efficiency with increase in initial BB3 concentration. This is because increase in the concentration causes a rise in the internal optical density, and the solution becomes more impermeable to UV radiation. As a result, lesser amount of UV radiation reaches PDS to form radicals, and degradation efficiency decreases. Besides, higher concentration of pollutant leads to formation of more radical scavengers, which causes competition among the free radical scavengers and carbonaceous substances. It is important to determine the chemicals such as hydrogen peroxide based on the COD or concentration of pollutant. Their finding was in agreement with the finding of Salari et al. (2009) that photooxidation efficiency decreased as initial dye concentration of C.I. Basic Yellow decreased. The initial dye concentration increased at the same concentration of S2O82- by using UV/S2O82- process in a rectangular continuous photoreactor. Besides, the study conducted by Yoon et al. (2011) on methylated arsenic species using UV/S2O82- process showed that pH had an impact on degradation efficiency. They observed that the oxidation efficiency of arsenic species was the highest at pHi = 3 than pHi = 7 and pHi = 10. In addition, removal efficiency of the pollutant is also affected by UV light intensity. The study conducted by Salari et al. (2009) showed that no noticeable color removal was observed when the irradiation was applied in the absence of S2O82-. However, significant dye degradation was noticed by the authors using S2O82- with the presence of UV radiation. This is due to formation of hydroxyl radical during the reaction. Khataee and Mirzajani (2010) also observed that removal rate increased from 0.0769 to 0.2243 min-1 with increase in UV irradiation intensity from 9.5 to 33.1 W/m2. It is believed that increase in UV radiation enhances the production of sulfate and hydroxyl radicals, which further increases the removal efficiency. Based on the above discussion, it should be noted that UV/S2O82- process is very sensitive to the operating parameters such as initial concentration of pollutants, initial concentration S2O82-, pH, and UV radiation. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 286 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Therefore, it is necessary to optimize the parameters to enhance the removal efficiency of each reaction. Besides, due to the complexity of the reaction involved in the UV/ S2O82- process, researchers should focus more on the kinetics involved in the reactions. The kinetics study can also be used to identify the intermediates formed, and it is helpful for toxicity determination. Since the photochemical system is quite complex and it involves radiant energy balance, the spatial distribution, mass transfer, and mechanisms of a photochemical degradation involving radical species such as hydroxyl radical, a proper mathematic modeling, or simulation technique is important in order to accurately predict the system efficiency. Furthermore, mechanisms and chemical phenomena are useful for scaling up the photochemical systems. 2.6 Various combinations of UV-AOPs It can be noted that combination of UV radiation with different techniques could enhance the treatment efficiency. But appropriate techniques must be combined together to give economically and technically feasible options. Therefore, a few factors must be taken into consideration when combining the system such as the cost of the treatment system, flexibility of the treatment, type of pollutants, and biodegradability of the wastewater. Based on the previous studies conducted, the combined systems have been found to significantly improve the efficiency of treatment. Table 7 summarizes some of the work done using those processes with the type of equipment along with experimental conditions and important results obtained in the work. The efficiency of wastewater treatment system is commonly measured by the COD, TOC, biochemical oxygen demand (BOD), dissolved organic carbon, BOD5/COD ratio, and concentration of specified pollutants by using HPLC, GC-MS, and ion chromatography to confirm the oxidation of inorganic species (such as CI- and NO3-), as well as toxicity analysis and measurement of decolorization by UV-spectrophotometric method. The main driving mechanism of a treatment system is the generation of a free radical through the chemical reaction which can increase the rate of reaction. The combination of UV-radiation with H2O2, O3, and other catalysts is promising to increase the production of free radical, and recently few studies have been attempted to study the efficiency of using the combined system (Lester et al. 2011). In a recent study, Zhang et al. (2013) have reported the efficiency of using combined UV/TiO2/H2O2 under high UV photon flux irradiation for decolorization of methylene blue (MB). The photodegradation of dye using UV/TiO2/ H2O2 process was much more effective than UV/TiO2 process alone. UV/TiO2/H2O2 process achieved 98% decolorization of 20 mg/l MB under optimal conditions in 10 s. The authors have observed that the addition of H2O2 enhanced the photocatalytical reaction rate constant by almost three times. The photonic efficiency calculated based on the various experimental conditions showed that H2O2 could improve the light utilization efficiency of photocatalytic process. Experiments with different dosages of H2O2 were carried out to investigate the effect of H2O2 dosage on the photocatlytical process. The reaction rate constant showed remarkable increase with the concentration of H2O2 up to 100 mg/l; a very small increase was observed at H2O2 concentration of 200 mg/l, and no changes in rate constant was observed beyond 200 mg/l. since hydrogen peroxide is one of the prime factors contributing to the cost of photocatalytical process, it is important to minimize the amount, so authors have used H2O2 concentration of 100 mg/l as an optimum in this study. Kuo et al. (2013) studied the decolorization of C.I. Reactive Red using TiO2/powered activated carbon (PAC) under UV and visible light irradiation. The authors have studied the effects of the C/Ti ratio, calcination temperature, photocatalyst dose, CI reactive red 2 (RR2) concentration, wavelength of light, and pH on the decolorization efficiency of RR2 by TiO2/PAC/UV. Based on the result obtained, it is confirmed that PAC increased the photocatalytic efficiency of PAC/TiO2/UV than TiO2/UV by acting as a good adsorbent and photo-induced electron acceptor. The TOC removal efficiency utilizing UV (254 nm)/TiO2 and UV (254 nm)/TiO2/PAC systems were 40% and 56%, respectively, after 1 g/l photocatalyst was added, and the reaction time was fixed at 240 min. Result showed that efficiencies of TiO2/PAC in both decolorization and TOC removal were better than using TiO2. Kim et al. (2011) applied microwave/UV/O3/H2O2/TiO2 photocatalyst hybrid process system to investigate the photocatalytic decomposition characteristics of three different single-component organic dyes and their mixture. The authors have evaluated different combinations of microwave, ozone, hydrogen peroxide, and UV to find the optimal value to enhance the photocatalytic decomposition efficiency of organic dyes. In this study, microwave irradiation was added to accelerate the decomposition reaction by activating pollutants and photocatalysis. Moreover, oxidants such as ozone and hydrogen peroxide were added to enhance the decomposition efficiency of the system. When microwave irradiation was used alone with photocatalyst, the effect was not significant, and remarkable increase of decomposition rate was observed when it was used together with other auxiliary oxidants. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM Amoxicillin, ampicillin and cloxacillin antibiotics Real textile effluents Disperse blue 1 UV/TiO2/H2O2 UV/TiO2/H2O2 UV/TiO2/H2O2 C.I. Reactive Red 2 MB High photon flux/ UV/TiO2/H2O2 UV/TiO2/PAC Pollutant Method – Irradiation source was a highpressure lamp (250 W) assembled with a reflector, shutter, timer and air cooler in a closed box – The UV photon flux was adjusted by transmission filters of different density – The source of UV irradiation was a UV lamp with a nominal power of 6 W, emitting radiation at 365 nm and it was placed above the reactor – 600 ml reactor – The lamps were vertically fixed onto the top wall of a wooden box (80 cm × 80 cm × 50 cm) – Four fans were placed in different positions on the side walls of the reactor to minimize the heating effect produced by the lamps – The internal walls were covered with aluminum foil to avoid radiation – Photochemical reactor made of Pyrex glass equipped with a magnetic stirring bar, water circulating jacket and an opening for supply of air bubble was used – Irradiations were carried out using a 125 W medium-pressure mercury lamp – An 8 W lamp (254, 365, or 410 nm) was placed inside a quartz tube as the light source – The reaction temperature was 25°C in all experiments Equipment details – pH was 7 – The optimal calcination temperature for forming TiO2 – TiO2/PAC = 400°C; surface areas of TiO2 = 42 m2/g – TiO2/PAC = 108 m2/g – Irradiation time: 60 min – [Dye]: 0.25 mm – [H2O2]: 0.3 ml – TiO2: 1 g/l – V: 250 ml – pH: 3.0 – TiO2: 0.25 g/l – H2O2: 10 mm – Fe2+: 3.5971 × 10-2 mm – TiO2: 1.0 g/l – pH∼5 – Initial COD: 520 mg/l – [H2O2]: 2.9412 mm –U V photon flux ranging: 3.13 × 10-8 to 3.13 × 10-6 einstein cm2/s – H2O2 dosages: 5.8824 mm Optimum operating conditions Table 7: Summary of studies on the removal of pollutant by various combination UV-AOPs. – Percentage of sanatase increased with the amount of PAC in TiO2/PAC – The spectra indicate that C doping of TiO2 shifted the absorption edge from 418 nm to a longer wavelength of 471 nm – TOC removal percentage in the UV(254 nm)/TiO2 and UV (254 nm)/ TiO2/PAC systems was 40 and 56%, respectively – Photocatalytic reactions approximately followed a pseudofirst-order kinetics – Complete degradation of amoxicillin, ampicillin and cloxacillin in 30 min – Fenton reactions based treatment proved to be slower and exhibited more complicated kinetics than the ones using TiO2 – UV/TiO2/H2O2 reached reduction levels higher than 90% – The use of only peroxide or Fenton reagent resulted in COD reductions of 60% and 80% – More efficient generation of hydroxyl radical and inhibition of electron/ hole pair recombination – Optimum catalyst concentration has to be found to avoid excess catalyst and ensure total absorption of efficient photons – H2O2 could reduce energy consumption remarkably – 98% decolorization of 20 mg/l MB could be achieved in 10 s Important results Kuo et al. (2013) Saquib et al. (2008) Garcia et al. (2007) Elmolla and Chaudhuri (2010) Zhang et al. (2013) References A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT 287 Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM Pollutant Humic BPA Dyeing wastewater Clofibric acid Method UV/TiO2/oxidants UV/TiO2/zeolitebased systems TiO2/H2O2/UV VUV/H2O2 (Table 7: Continued) – Cylindrical glass reactor (inner diameter 8.0 cm, height 25 cm, volume 800 ml) with tap water running through a jacket with temperature controlled at 10°C. – The VUV lamp (10 W, combining 90% UV254 and 10% UV185) – The UV chamber consisted of 16 UV lamps – The effective volume: 1.88 l, – Total system volume (reactor plus mixing tank and piping): 10 l – The system was composed of a 64 W UV lamp, pH meterand an oxygen/ ozone generation device – The working volume of the photocatalytic reactor was 3.1 l – A maximum light intensity output of 254 nm –The photoreactor consisted of four 30 cm-long quartz tubesand five lamps – The external diameter of each tube was 1.2 cm and the internal diameter was 1 cm – The column photoreactor was irradiated with 254 and 365 nmUV(8 W, Philips) at room temperature – The intensities of 254 and 365 nm UV were 4.44 and 0.7 mW/cm2, respectively Equipment details [C]0 = 10.0 mg/l; [H2O2]0 = 2.9412 mm T = 10°C. TiO2 dosage = 1.82 g/l [H2O2] = 28.824 mm Reaction time = 20 min – The flow rate of the BPA solution was1.4 ml/min –P hotocatalyst dosage in each quartz tube was 6.5±0.5 g/l – The optimal experimental pH for the degradation of BPA in the TiO2 system was 6 [TiO2] dosage = 0.3 g/l [H2O2] = 1.4706 mm O3: 20 g/m [K2S2O8] = 50 mg/l Optimum operating conditions – Removal of humic acid and hazardous heavy metals was much greater when H2O2 was used as the oxidant – The SO2- and OH radical produced are responsible for the rapid photodegradation – Supporting TiO2 on zeolite increased its adsorption capacity – Reduced the extent of photogeneratedelectron-hole recombination in TiO2, enhancing photocatalytic activity – The deposition of Cu2O on TZ modified its photoconductive properties by increasing the charge separation efficiency between electrons and holes – Cu2O also acted as a trap for electrons, inhibiting the recombinationof electrons and holes – The removal of TOC increased with increasing concentration of FeCl3 upto600 mg/l. – Maximum removal efficiency (85%) of dye wastewater was obtained at the optimal value of TiO2 and H2O2 CA removal efficiency reached over 99% after 40 min Important results Li et al. (2011a) Lee et al. (2005) Kuo et al. (2014) Jung et al. (2009) References 288 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT So it suggests that microwaves produce active species to be used for degradation when it combines with auxiliary oxidants. The authors have also mentioned that the effect of H2O2 is prominent since further addition of H2O2 above the optimum hampered the reaction. This is due to the blocking of surface sites by H2O2 and the OH radical scavenging by H2O2. Recently, Patel et al. (2013) have studied the photochemical decolorization and mineralization of Reactive red 241 (RR241) in aqueous solution using a photo-Fenton process and its combination with activated charcoal and titanium dioxide. According to the COD and TOC removal efficiency, authors suggested that the UV/Fenton/TiO2 was more effective than homogeneous photo-Fenton and UV/ Fenton/activated carbon (AC). UV/Fenton/TiO2 process required 120 min for complete decolorization and yielded a maximum TOC reduction of 43.9% after 240 min in contrast with UV/Fenton/AC, which only gave 14.9% of TOC reduction after 240 min. Photo-Fenton/TiO2 is more effective than other processes possibly due to the fact that absorption of UV radiation with TiO2 generates conduction band (CB) electrons and valence band (VB) holes. These electrons (e) may react with Fe3+ to convert Fe2+ rapidly and increase the concentration of Fe2+. The preceding discussion shows that a number of studies have been directed to overcome the limitation of existing UV-based single process AOPS. Recently, AOPs by adding more than one oxidants and catalyst with UV radiation is gaining great attention from researchers. Based on previous studies, it was noted that a combination of AOPS may give advantages such as good photochemical efficiency, increase in the surface area of catalyst, and thus increases in the incident light absorption for the photocatalyst which produces more OH radical and accelerates auxiliary oxidants to produce more active species for degradation purposes. However, very limited number of studies have been conducted to prove the advantages over other AOPs. So, more precise and quantitative analyses in this area are required for more reliable evaluation and application. 3 M odels for the design and optimization of UV-AOPs system Referring to this context, UV-integrated AOPs have been proven to be efficient for pollutant destruction, and there are few full-scale applications already developed. Among others, UV/H2O2 system is the most often studied fullscale system for the destruction of organic compounds 289 by the combined mechanisms of direct UV photolysis and hydroxyl radical reactions (Kruithof et al. 2007, Swaim et al. 2008, Audenaert et al. 2011). However, publications on full-scale application of UV-AOPs are still scarce to date. Even though many researchers have proven the effectiveness of the technology in small-scale batch of UV-AOPS for degradation of a variety of pollutants, there are some factors that stymie the application of UV-AOPs at large scale. Lack of proper modeling and simulation tools for predicting and analyzing the system’s performance are among the major factors hindering their practical implementation (Wols and Hofman-Caris 2012). The modeling of a system offers advantages such as assessment of the performance of UV-AOPs based on the optimized value and to give a great assistance to scale up the systems. The model being developed to account for varying operating conditions depends on the properties of the wastewater and the UV-AOPS system. Designing a large-scale UV-AOPs system requires knowledge on system configuration as well as the chemical kinetics (reaction mechanisms and kinetic rate constants). System configurations including reactor design, pipe and fittings, lamp number, and lamp orientation are important (Wang et al. 2012). Though many studies have been conducted on efficiency of UV-AOPs, the existing information is not sufficient for designing a full-scale treatment system. Therefore, it is important to identify appropriate numerical tools to design and optimize an UV-AOPs system which is capable of combining hydrodynamic reactor models, fluence rate models, and chemical kinetic models. As mentioned earlier in the paper, UV-AOPs systems strongly depend on the composition and properties of the wastewater, type and concentration of oxidants, intensity of UV radiation, and type and dosage of catalyst. Optimization of these parameters is necessary in order to develop a full-scale system. CFD is a powerful numerical tool that has recently been widely used to design UV reactors and assess performance of UV reactors (Verbruggen et al. 2015). CFD is capable of numerically solving the fluid dynamics equations through space and time, including conservation of mass, conservation of momentum, and conservation of energy (Boyjoo et al. 2014). The developed equation can describe both the physical and chemical changes within a reactor by combining appropriate boundary and initial conditions. A CFD model for UV-AOPS also includes the spatial variations of fluence rate within the UV reactor. Many researchers have demonstrated the importance of combining hydrodynamics of UV reactor with fluence rate models to predict the effectiveness of the degradation process. The combination allows optimization of lamp Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 290 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT placement, minimization of UV screening, and improved prediction of contaminant removal in different UV reactors. Therefore, an effective CFD simulation must include turbulence models, fluence rate models, and accurate reaction mechanisms (direct and indirect photolysis) describing the oxidation of the contaminant (Verbruggen et al. 2015). UV-AOPs must incorporate the impact of turbulent mixing into any chemical reactions that occur within the system during the simulation (Ghafoori et al. 2013). Standard k-ε turbulence model is often used for investigating the hydraulics within a flow-through reactor. UV fluence rate depends on the distance of pollutant from the lamp and is a function of absorptive characteristics of the media through which the light is irradiated. In AOPs, UV light is absorbed by organic pollutant, oxidants, and catalyst. Therefore, the transmitted fluence rate is reduced, and consequently, the generation of hydroxyl radical throughout the UV reactor is affected. The reaction pathways involved in the degradation of pollutants to its intermediates and final products have been carefully studied in numerous studies. The reaction pathways are specific to the parent compound and chemical constituents in the water. Such studies have provided detailed information on oxidation process and numerical kinetic models. Previous works have clearly shown that the kinetic model process is important for predicting the conversion rate of reactants into the products. For example, kinetic models based on Langmuir-Hinshelwood equations can adequately describe reactivity results and provide kinetic constant and equilibrium adsorption constant for degradation of organic compound. The defined reaction kinetics of a UV-AOPs system can be combined with simulation models (CFD) to identify irradiance distribution profiles and hydrodynamic and turbulent characteristics of a UV reactor system. The developed kinetic reaction, in combination with the simulation model, may possibly describe the performance of a system based on the irradiation distribution profiles and hydrodynamic and turbulent characteristics of an UV reactor system (Verbruggen et al. 2015). There is a number of publications focusing on application of CFD models for AOPs to design and optimize the system based on the degradation of targeted pollutants (Chen et al. 2011, Wang et al. 2012, Boyjoo et al. 2014). However, it is noted that a very limited number of studies have been performed with CFD to stimulate UV/ AOPs. Alpert et al. (2010) have evaluated the performance of comprehensive CFD/UV/AOP models for the degradation of an indicator organic contaminant. The CFD results were validated with pilot-scale experiments. Besides that, CFD also has been used for modeling a photo-Fenton-like process by Ghafoori et al. (2013). The developed CFD model could be used to combine with the kinetic models to obtain more accurate performance prediction. In this study, the authors have developed a valid kinetic model based on the photochemical reactions and rate constants. The authors observed a good agreement between CFD simulation results and the experimental data which indicate that the model is accurate. But it should be noted, since very little information is available on the CFD modeling of photo-Fenton-like process, that it is difficult to assess the suitability of this modeling for photo-Fenton application. Additionally, many researchers have applied CFD for modeling different photocatalytical reactors (Pareek et al. 2003, Mohseni and Taghipour 2004, Romero-Vargas Castrillón and de Lasa 2007). It showed the efficiency of the model by saving cost and time when it is coupled with photocatalytical reactor. Recently, Jatinder and Kumar have studied the application of CFD in combination with response surface methodology (RSM) to optimize the operating parameters and improve the performance of immobilized titania-based annular photocatalytic reactor for the removal of Rhodamine B from water. CFD modeling of the photocatalytical process in annular reactor was designed, and the output of the CFD model was evaluated with experimental results to validate the model’s predictions. Then, the developed CFD model was used in combination with RSM to optimize the process parameters. The aforementioned discussion showed that CFD models can be effective in predicting the degradation efficiency of UV-AOPs. The combination of turbulence models, fluence rate models, and kinetic rate equations is important to stimulate the reaction using CFD. CFD is also capable of optimizing the energy and chemical usage to achieve better degradation efficiency. In addition, the main characteristic of a CFD method is its ability to accurately calculate the spatial variation of flow rate, reaction rate, and concentrations at the reactive surface. CFD can also simultaneously estimate several parameters from one experiment. However, more studies have to be conducted to evaluate the efficiency of CFD to predict the removal efficiency of a variety of organic contaminants. To date, only a few researchers have used CFD to model their systems from batch to pilot scales. 4 Cost of UV-combined AOPs AOPs have emerged as a technically feasible treatment method for various refractory industrial wastewater, but a proper economical analysis is needed for an industrial Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Capital cost 291 • Design’s flow rate • Total energy requirement in the AOP reactor • The energy supplied by single unit of AOP • The cost of AOPreactor (UV/US/Microwave) • The cost of the devices used • The power density used for UVenergy in the treatability study Part replacement cost • Lamp r for UV system • Transducer for US system • Ozone generator parts for ozone system • Catalyst holder for catalytic systems • Electronic circuit replacements • Replacement for microwave system • Replacement of membrane column Maintenance and operational cost Chemical cost • Hydrogen peroxide • Iron (II) sulfate • Sulfuric acid • Sodium hydroxide • Catalyst such as TiO2, CuO, CuSO4, ZiO2 Electrical cost • H2O2 pumping • Lamp operation • Heater resistances • pH-meter • Compressor • Syringe pump for H2O2 dosification • US device Labor cost and analytical cost • Water sampling cost • System inspection, replacement and repair • Sampling frequency the labor required to conduct • Cost of chemicals required for analysis Figure 2: The elements to be included in treatment system cost estimation based on capital, maintenance, and operational cost for UVbased AOPs. scale-up. It is hard to find any studies which address both economic feasibility and technical vitality of the UV-based AOPs. An economically feasible process is one of the most important aspects of a treatment system to be adopted in industrial environment. The cost of treatment can be represented by the sum of the capital, operational, and maintenance costs. Figure 2 illustrates the example of a few important components that are involved in cost estimation based on capital, operational, and maintenance cost for UV-based AOPs. The cost of the processes used on the UV system depends on the type of contaminants, properties of wastewater, flow rate of the effluents, and also the design of the reactor (Rodrigues et al. 2014). The costs can be calculated based upon the k ­ inetics of degradation. From the literature studies, there are two main orders proposed for the kinetics of AOPs: pseudo-first-order and zero-order kinetics (Beak et al. 2009). UV combined with AOPs is an energy-intensive process and has significant contribution to the operating cost of the process. The information on the energy consumption by the treatment system can be very useful for researcher to build the AOP-based wastewater treatment plant system. The figures of merit for AOPs based on the electrical energy consumption (system use electric energy) or area collector (solar energy system) were proposed by the International Union of Pure and Applied Chemistry (IUPAC) for the evaluation and comparison of wastewater treatment system (Bolton et al. 2001). Two figures of merit were proposed for electrical driven system. Figure of merit is a numerical quantity used to measure the efficiency of the system based on the more characteristic system. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 292 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT A number of researchers were attempting this technique to calculate the energy consumption based on the possible factors that may affect the efficiency such as average UV radiation over the time, the volume of the water to be treated, and the flow rate of the system. According to figures of merit, first-order kinetics is used for low concentration of pollutants and zero-order kinetics for the high concentration of pollutant. The figure of merit used for highly concentrated wastewater is electrical energy per mass (EEM). Zeroorder kinetic is applicable here, since the rate of removal is directly proportional to the rate of electrical energy consumption. EEM is defined by the amount of electrical energy in kilowatt hours (kW h) required to degrade 1 kg of the pollutant (Bolton et al. 2001). The simple formulas to calculate EEM (kW h/kg) are EEM = EEM = P × t × 1000 Batch scale V × M ×( Ci -C f ) (46) P × t × 1000 Continuous operation V × F ×( Ci -C f ) (47) Besides that, EEM is inversely proportional to factors such as the photon flow, the fraction of light absorbed by the reactor, and the quantum yield of the generated intermediates. The maximum efficiency and minimum feasible value of EEM could be achieved when the system has a larger amount of quantum yield and larger amount of light absorbed in the system according to Eq. (48). EEM = P × 1000 M × 3600 ×( Gx-Z ) (48) Electrical energy per order (EE/O) is defined as electrical energy (kW h) required to reduce the concentration of a pollutant by one order of magnitude in 1 m3 (1000 ml) of contaminated water (Bolton et al. 2001). This figure of merit is best applied for the pollutant with low concentration. The high value of EE/O resembles a low energy efficiency of a system. This figure of merit is assumed to be first-order kinetic as Eq. (18), t (min) is the reaction time in the reactor, and k1′ is the first-order rate constant (min-1). EEO = EEO = P × t × 1000 Batch scale V × ln×( Ci -C f ) P × t × 1000 Continuous operation F × ln×( Ci -C f ) ln Ci /C f = k1't (49) (50) (51) where P is the rated power (kW), M is the molar mass (g/mol), t is the irradiation time (min), V is the volume (l) of the wastewater in the reactor, Ci and Cf are the initial and final wastewater concentrations, and k1 is the pseudofirst-order rate constant (min-1) for the degradation of wastewater. It is obvious that figure of merit concept allows the calculation of capital cost required by any UV-based AOP system. This will be a great help for researchers to compare the efficiency of the various systems based on the electrical consumption to an individual system based on the order of reaction (zero- or first-order kinetics). Although AOPs are extensively studied for the degradation of a variety of refractory wastewaters, only a number of studies focus on the economic analysis. It is noted that most researchers focus on the technical feasibility of the process by studying parameters optimization, better configuration of a system, and design of a reactor, for a maximum efficiency. A very prominent study was conducted to evaluate the cost of UV-based AOPs based on the concept proposed by IUPAC in this paper. As mentioned earlier, the economic analysis is necessary to evaluate the system efficiency; a good system should minimize the cost and maximize the efficiency. Mahamuni and Adewuyi (2010) have conducted a very impressive review on the cost estimation for AOPbased ultrasound wastewater treatment. The authors have reviewed the cost of the following UV system, although the work was focused on the US for a comparison purpose: (1) UV alone, (2) UV+US, and (3) US combined with UV and O3. The costs have been calculated for flow rate of 1000 l/min. Rate constant was used as a basis to calculate the cost. Time taken for the 90% degradation was considered as a residence time. The amount of energy required to achieve 90% removal was calculated from the energy density (W/ml) in this study. The authors have calculated the cost of the treatment by taking into consideration the capital cost and operating cost involved. There are few conclusions that have been made based on the type of pollutants and UV+US studies in this work. From the cost estimation analysis provided in Table 8, it can be summarized that US combined with UV and H2O2 is more efficient and more economical compare to other combined systems. The higher cost of US compare to UV is observed in this study due to higher electrical energy and capital cost required by the system for the treatment. So as a conclusion, energy consumption by a particular system is one of the major factors contributing to the increase in cost of treatment system besides other operating costs such as chemical and maintenance costs. Lucas et al. (2010) have conducted an estimation of the operating costs of the, O3, O3/UV, and O3/UV/H2O2 processes based on the experiments carried out in the pilot-scale reactor. The authors excluded maintenance, Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT 293 Table 8: Cost estimation of various type of UV-Based AOPs (Mahamuni and Adewuyi 2010). Method K (min-1) Pollutant UV 0.0021 0.0589 0.0124 0.0524 0.4418 0.02064 0.0869 0.0055 0.005 0.181 0.02171 0.1793 0.0207 0.433 0.0357 0.712 0.0757 Phenol TCE Dye Phenol TCE Dye Phenol Dye Phenol TCE Dye Phenol Dye Phenol Dye Dye Phenol UV+H2O2 UV+O3 UV+US US+UV+O3 Photocatalysis UV+US+H2O2 US+photocatalysis Capital cost ($/1000 gallons) Operating and maintenance ($/1000 gallons) Total cost ($/1000 gallons) 5.48E+08 5.21E+06 9.09E+07 2.93E+07 6.96E+05 1.12E+07 1.35E+07 5.86E+09 3.43E+09 2.99E+07 1.50E+09 9.68E+07 2.67E+07 3.12E+09 7.99E+07 1.11E+08 4.55E+09 1.67E+08 2.29E+05 3.05E+06 4.05E+07 4.06E+05 3.57E+06 4.25E+06 1.73E+08 1.58E+08 1.18E+06 4.43E+07 4.65E+06 8.14E+07 9.51E+08 2.62E+06 2.36E+07 6.54E+08 7.15E+08 5.44E+06 9.40E+07 6.98E+07 1.10E+06 1.48E+07 1.78E+07 6.03E+09 3.59E+09 3.11E+07 1.54E+09 1.01E+08 1.08E+08 4.07E+09 8.25E+07 1.35E+08 5.20E+09 TCE, trichloroethylene. capital, labor, and depreciation costs in their study. The operational cost is evaluated based on the costs for electricity, oxygen production, lamp replacement, and H2O2 to operate the pilot plant. Based on the economic analysis of the investigated AOPs in this study, it revealed the O3/UV/H2O2 to be the most economical process (1.31 euro m-3 g-1 of TOC mineralized at pH 4 and a COD/H2O2 ratio of 2). The operation cost was calculated based on the TOC removal efficiency of different process applied in this investigation; O3/UV/H2O2 was found to be the most efficient, and although it used more energy compared to O3 or O3/UV, O3/UV/H2O2 is still considered as the most economical in this study. Durán et al. (2012) have evaluated the costs of treating real effluents from an integrated gasification combined cycle power station. The operational costs based on the consumption of electrical energy, reagents, and catalysts were calculated from the optimal conditions of each process studied. The authors have studied the following process and estimated the cost of treatment: (i) a photo-Fenton process at an artificial UV pilot plant, (ii) a modified photo-Fenton process with continuous addition of H2O2 and O2to the system, and (iii) a ferrioxalate-assisted solar photo-Fenton process at a compound parabolic collector pilot plant. The economic analysis was carried out by analyzing the degradation of TOC present in wastewater. The operational costs were compared with the amount of TOC removed in grams. Based on this study, it was found that the cost increases with the amount of TOC removed due to the energy consumption by an artificial UV lamp. Although modified photo-­Fenton process is capable of reaching higher mineralization degrees over a shorter period of time, solar photo-Fenton/ferrioxalate-assisted process was found to be a more profitable system with the treatment cost of 6 €/m3 (for 75% mineralization). The cost estimations of various AOPs in combination with UV is crucial for maximizing the degradation efficiency and minimizing the overall cost of the treatment system. In order to evaluate the efficiency of the process, more pilot-scale studies need to be carried out using different processes such as UV-Fenton, UV/H2O2/O3, and UV/TiO2. This is important for an industrial scale-up of the system. 5 Conclusions This review constitutes reference documents in the field of UV coupled with AOPs to help researchers develop or design new technology utilizing both UV and AOPs to treat recalcitrant wastewaters. UV-based AOPs have been proven to be an efficient and a sustainable alternative for degradation of recalcitrant contaminates compared to the use of UV alone. But it should be noted that very few studies have been conducted to evaluate the economical feasibility of this imperative technology. The following are the main conclusions of this study: 1. UV/H2O2 is efficient because of rapid production of hydroxyl radical in the reaction medium. However, its Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 294 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT performance is strongly affected by experimental conditions such as UV light sources, pH, and the properties of targeted wastewater. 2. The UV-TiO2 process has proven to be an effective oxidation technique, but its application is limited and not effective for highly concentrated pollutants. Major drawbacks identified are low efficiency at high catalyst concentrations and complex separation and recycling of TiO2. 3. The combination of UV radiation with ozone is more efficient than using ozone alone for certain recalcitrant pollutants due to the formation of additional hydroxyl radical via photolysis. However, this method is not economically viable because of the high energy requirement for both ozone formation and UV light. 4. Utilizing UV light simultaneously with Fenton reagents is economical, viable, and efficient. This is because Fenton reagents are readily available, safe to handle, and non-toxic. Moreover, use of UV light can possibly reduce the consumption of H2O2 in comparison with conventional Fenton oxidation. However, in comparison with the TiO2/UV system, separation of catalyst is easier. But limitations linked with conventional Fenton oxidation such as the initial pH of the solution, scavenging of hydroxyl radicals by nontarget substances, and sludge formation still needs to be considered for future applications. 5. Sulfate radicals also offer several advantages over other oxidants such as longer half-life, fast ­kinetics, higher stability than hydroxyl radical, greater transport distances in the sub-surface level, ability to work in a wide range of pH, and ability to be activated by low-cost oxidant precursors. However, although it is more stable than hydroxyl radical, the narrow selectivity of sulfate radical toward organic matter makes it less efficient compared to hydroxyl radical. However, UV/S2O82- process is very sensitive to the operating parameters; therefore, it is necessary to optimize the parameters to enhance the removal efficiency. Besides, due to the complexity of the reaction involved in the UV/S2O82- process, researchers should focus more on the kinetics involved in the reactions. 6. Recently, researchers were actively involved to improve the efficiency of UV-based AOPs by combining with more than one energy-dissipating component (microwave, solar, and US), oxidants (H2O2 and O3), and catalyst (TiO2, Fe(II), Fe(III), and zero iron). This combined process shows better efficiency compared to a single system by maximizing the efficiency of degradation and minimizing the cost. More studies 7. need to be carried out to evaluate the reliability and application of this process. Additionally, the cost estimate of various AOPs in combination with UV is crucial for maximizing the degradation efficiency and minimizing the overall cost of the treatment system. The industrial scale-up of UV-based AOPs is complicated without sufficient studies on cost evaluation, so more pilot studies need to be carried out on economical feasibility of UV-AOPs. 6 Recommendations The foregoing discussion on wastewater treatment using UV-based AOPs concludes that its application for real wastewater treatment is challenging. There are a few limiting issues that are required to be explored for its largescale applications. Therefore, based on the literature reviewed in this study, the following recommendations should be considered for improving UV-AOP-based wastewater treatment: 1. Wastewater stream turbidity is one of the major factors that have not been elucidated much in the literature. The treatment system is less effective if other organic matter is predominantly present. This is because oxidant and catalyst requirement can be exceedingly high in order to achieve effective degradation of trace target pollutants. Higher turbidity is caused by the presence of particulate matter which results in less penetration of UV light through wastewater. As a consequence, process efficiency is dramatically decreased due to the less interaction of UV light with oxidants and catalyst. In addition, turbidity is also directly proportional to the concentration of the pollutant, so careful considerations are required to optimize the parameters such as dosage of catalyst and oxidant as well as intensity of UV radiation. Besides, turbidity also increases the energy requirements of the process, as higher/strong UV irradiation is required to sustain the overall efficiency of the process. Therefore, in order to increase the effectiveness of the process, it is recommended to use small volumes of the effluent streams. This requires a careful consideration during the design of treatment system, for example, use of stacked tube reactors with multiple UV lights at different locations. 2. Besides, it is important to study the kinetics of combined or hybrid UV-AOPs. CFD models can be used Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT to determine the performance of different UV-AOP systems. For scale-up, a proper and accurate kinetic model needs to be developed based on the types of pollutant as the reaction pathways solely depend on the parental compounds, intermediates, and final products. The intermediates formed during the reaction should also be analyzed. It is important because they can increase the toxicity of the reaction solution sometimes. Numerous analytical methods such as GC, GC/MS, and FTIR can be used to determine the organic or inorganic compounds. 3. A proper study on the scavenging effect is also suggested as it could adversely affect the treatment efficiency and cause the formation of by-products which could possibly increase the COD, TOC, total suspended solids (TSS), and toxicity value. 4. Use of various UV-based technologies for the degradation of emerging pollutants such as alachlor, atrazine, carbamazepine, sulfamethoxazole, and others should also be conducted with possible aid from CFD. 5. The ·OH exposure distribution should be evaluated for each UV-based system. 6. Since the mechanism of a combined system is very complex and dependent on the time and other operating parameters, it is very important to develop a reliable model that is capable of optimizing the system’s parameters. Therefore, more research should be carried out to design and develop comprehensive models that can accurately predict the performance of a pilot and full-scale system. 7. High water consumptions in industry cause the excessive and irresponsible use of ground water, so zero water discharge would contribute to the conservation and replenishment of groundwater resources. Since in the zero water discharge system no effluent will be produced, it could eliminate the cost required to process the discharge water after treatment. Since most of the UV-AOPs system managed to achieve higher COD and TOC value, it is suggested to look into the modification of UV-based AOPs system by integrating with filtration system and designing a hybrid system which could give no discharge of water as the treated water can be recycled back to the system. Acknowledgments: The authors are grateful to the University of Malaya High Impact Research Grant (HIR-MOHE-D000037-16001) from the Ministry of Higher Education Malaysia and University of Malaya Postgraduate Research Fund which financially supported this work. 295 References Abdessalem AK, Bellakhal N, Oturan N, Dachraoui M, Oturan MA. Treatment of a mixture of three pesticides by photo- and electro-Fenton processes. Desalination 2010; 250: 450–455. Afzal A, Drzewicz P, Martin JW, Gamal El-Din M. Decomposition of cyclohexanoic acid by the UV/H2O2 process under various conditions. Sci Total Environ 2012; 426: 387–392. Al-Bastaki NM. Performance of advanced methods for treatment of wastewater: UV/TiO2, RO and UF. Chem Eng Process 2004; 43: 935–940. Aleboyeh A, Moussa Y, Aleboyeh H. The effect of operational parameters on UV/H2O2 decolourisation of Acid Blue 74. Dyes Pigm 2005; 66: 129–134. AlHamedi FH, Rauf MA, Ashraf SS. Degradation studies of Rhodamine B in the presence of UV/H2O2. Desalination 2009; 239: 159–166. Alkan U, Teksoy A, Atesli A, Baskaya HS. Efficiency of the UV/H2O2 process for the disinfection of humic surface waters. J Environ Sci Health A Tox Hazard Subst Environ Eng 2007; 42: 497–506. Alnaizy R, Ibrahim TH. MTBE removal from contaminated water by the UV/H2O2process. Desalin Water Treat 2009; 10: 291–297. Alpert SM, Knappe DRU, Ducoste JJ. Modeling the UV/hydrogen peroxide advanced oxidation process using computational fluid dynamics. Water Res 2010; 44: 1797–1808. Amat AM, Arques A, Miranda MA, Lopez F. Use of ozone and/or UV in the treatment of effluents from board paper industry. Chemosphere 2005; 60: 1111–1117. Arslan I, Balcioglu IA, Tuhkanen T. Advanced oxidation of synthetic dyehouse effluent by O3, H2O2/O3 and H2O2/UV processes. Environ Technol 1999; 20: 921–931. Arslan A, Veli S, Bingöl D. Use of response surface methodology for pretreatment of hospital wastewater by O3/UV and O3/UV/ H2O2 processes. Sep Purif Technol 2014; 132: 561–567. Arslan-Alaton I, Tureli G, Olmez-Hanci T. Treatment of azo dye production wastewaters using Photo-Fenton-like advanced oxidation processes: optimization by response surface methodology. J Photochem Photobio A 2009; 202: 142–153. Audenaert WTM, Vermeersch Y, Van Hulle SWH, Dejans P, Dumoulin A, Nopens I. Application of a mechanistic UV/hydrogen peroxide model at full-scale: sensitivity analysis, calibration and performance evaluation. Chem Eng J 2011; 171:113–126. Autin O, Hart J, Jarvis P, MacAdam J, Parsons SA, Jefferson B. Comparison of UV/H2O2 and UV/TiO2 for the degradation of metaldehyde: kinetics and the impact of background organics. Water Res 2012; 46: 5655–5662. Avisar D, Lester Y, Mamane H. pH induced polychromatic UV treatment for the removal of a mixture of SMX, OTC and CIP from water. J Hazard Mater 2010; 175: 1068–1074. Ayoub K, van Hullebusch ED, Cassir M, Bermond A. Application of advanced oxidation processes for TNT removal: a review. J Hazard Mater 2010; 178: 10–28. Azimi Y, Allen DG, Farnood RR. Kinetics of UV inactivation of wastewater bioflocs. Water Res 2012; 46: 3827–3836. Babuponnusami A, Muthukumar K. A review on Fenton and improvements to the Fenton process for wastewater treatment. J Environ Chem Eng. 2013. Banat F, Al-Asheh S, Al-Rawashdeh MM, Nusair M. Photodegradation of methylene blue dye by the UV/H2O2 and UV/ Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 296 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT acetone oxidation processes. Desalination 2005; 181: 225–232. Barakat MA. Adsorption and photodegradation of Procion yellow H-EXL dye in textile wastewater over TiO2 suspension. J HydroEnviron Res 2011; 5: 137–142. Beak MH, Ijagbemi CO, Kim DS. Azo dye Acid Red 27 decomposition kinetics during ozone oxidation and adsorption processes. J Environ Sci Health A Tox Hazard Subst Environ Eng 2009; 44: 623–629. Beltrán FJ, Encinar J, González JF. Industrial wastewater advanced oxidation. Part 2. Ozone combined with hydrogen peroxide or UV radiation. Water Res 1997; 31: 2415–2428. Beltrán FJ, Aguinaco A, García-Araya JF. Application of ozone involving advanced oxidation processes to remove some pharmaceutical compounds from urban wastewaters. Ozone Sci Eng 2012; 34: 3–15. Benitez FJ, Beltran-Heredia J, Torregrosa J, Acero JL. Treatments of wastewaters from olive oil mills by uv radiation and by combined ozone-UV radiation. Tox Environ Chem 1997; 61: 173–185. Bianco B, De Michelis I, Veglio F. Fenton treatment of complex industrial wastewater: optimization of process conditions by surface response method. J Hazard Mater 2011; 186: 1733–1738. Bin AK, Sobera-Madej S. Comparison of the advanced oxidation processes (UV, UV/H2O2and O3) for the removal of antibiotic substances during wastewater treatment. Ozone Sci Eng 2012; 34: 136–139. Bledzka D, Miller JS, Ledakowicz S. Kinetic studies ofn-Butylparaben degradation in H2O2/UV System. Ozone Sci Eng 2012; 34: 354–358. Bolton JR, Bircher KG, Tumas W, Tolman CA. Figures-of-merit for the technical development and application of advanced oxidation technologies for both electric- and solar-driven systems. Intern Union Pure Appl Chem 2001; 73: 627–637. Boyjoo Y, Ang M, Pareek V. CFD simulation of a pilot scale slurry photocatalytic reactor and design of multiple-lamp reactors. Chem Eng Sci 2014; 111: 266–277. Bustos YA, Vaca M, Lopez R, Torres LG. Disinfection of a wastewater flow treated by advanced primary treatment using O(3), UV and O(3)/UV combinations. J Environ Sci Health A Tox Hazard Subst Environ Eng 2010; 45: 1715–1719. Cai C, Zhang H, Zhong X, Hou L. Electrochemical enhanced heterogeneous activation of peroxydisulfate by Fe–Co/SBA-15 catalyst for the degradation of Orange II in water. Water Res 2014; 66: 473–485. Chang CN, Ma YS, Fang GC, Chao AC, Tsai MC, Sung HF. Decolorizing of lignin wastewater using the photochemical UV/TiO2 process. Chemosphere 2004; 56: 1011–1017. Chang M-W, Chung C-C, Chern J-M, Chen T-S. Dye decomposition kinetics by UV/H2O2: Initial rate analysis by effective kinetic modelling methodology. Chem Eng Sci 2010; 65: 135–140. Chaudhuri M, Wahap MZBA, Affam AC. Treatment of aqueous solution of antibiotics amoxicillin and cloxacillin by modified photo-Fenton process. Desalin Water Treat 2013; 51: 7255–7268. Chelme-Ayala P, El-Din MG, Smith DW. Degradation of bromoxynil and trifluralin in natural water by direct photolysis and UV plus H(2)O(2) advanced oxidation process. Water Res 2010; 44: 2221–2228. Chen J, Deng B, Kim CN. Computational fluid dynamics (CFD) modeling of UV disinfection in a closed-conduit reactor. Chem Eng Sci 2011; 66: 4983–4990. Cheng Z-W, Peng-fei S, Jiang Y-F, Yu J-M, Chen, J-M. Ozone-assisted UV254nm photodegradation of gaseous ethylbenzene and chlorobenzene: Effects of process parameters, degradation pathways, and kinetic analysis. Chem Eng J 2013; 228: 1003–1010. Cho I-H, Zoh K-D. Photocatalytic degradation of azo dye (Reactive Red 120) in TiO2/UV system: optimization and modeling using a response surface methodology (RSM) based on the central composite design. Dyes Pigm 2007; 75: 533–543. Chu W, Choy WK, So TY. The effect of solution pH and peroxide in the TiO2-induced photocatalysis of chlorinated aniline. J Hazard Mater 2007; 141: 86–91. Chun H, Yizhong W. Decolorization nd biodegradability of photocatalytic treated azo dyes and wool textile wastewater. Chemosphere 1999; 39: 2107–2115. Criquet J, Leitner NKV. Degradation of acetic acid with sulfate radical generated by persulfate ions photolysis. Chemosphere 2009; 77: 194–200. Cuiping B, Xianfeng X, Wenqi G, Dexin F, Mo X, Zhongxue G, Nian X. Removal of rhodamine B by ozone-based advanced oxidation process. Desalination 2011; 278: 84–90. Daneshvar N, Salari D, Khataee AR. Photocatalytic degradation of azo dye acid red 14 in water: investigation of the effect of operational parameters. J Photochem Photobio A 2003; 157: 111–116. Daneshvar N, Salari D, Khataee AR. Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2. J Photochem Photobio A 2004; 162: 317–322. Daneshvar N, Khataee AR. Removal of azo dye C.I. acid red 14 from contaminated water using Fenton, UV/H(2)O(2), UV/H(2)O(2)/ Fe(II), UV/H(2)O(2)/Fe(III) and UV/H(2)O(2)/Fe(III)/oxalate processes: a comparative study. J Environ Sci Health A Tox Hazard Subst Environ Eng 2006; 41: 315–328. De la Cruz N, Esquius L, Grandjean D, Magnet A, Tungler A, de Alencastro LF, Pulgarin C. Degradation of emergent contaminants by UV, UV/H2O2 and neutral photo-Fenton at pilot scale in a domestic wastewater treatment plant. Water Res 2013; 47: 5836–5845. Dimitrakopoulou D, Rethemiotaki I, Frontistis Z, Xekoukoulotakis NP, Venieri D, Mantzavinos D. Degradation, mineralization and antibiotic inactivation of amoxicillin by UV-A/TiO(2) photocatalysis. J Environ Manage 2012; 98: 168–174. Domínguez JR, Beltrán J, Rodríguez O. Vis and UV photocatalytic detoxification methods (using TiO2, TiO2/H2O2, TiO2/O3, TiO2/S2O82–, O3, H2O2, S2O82–, Fe3+/H2O2 and Fe3+/ H2O2/C2O42–) for dyes treatment. Catal Today 2005; 101: 389–395. Dopar M, Kusic H, Koprivanac N. 2011. Treatment of simulated industrial wastewater by photo-Fenton process. Part I: the optimization of process parameters using design of experiments (DOE). Chem Eng J 173: 267–279. Drouiche M, Le Mignot V, Lounici H, Belhocine D, Grib H, Pauss A, Mameri N. A compact process for the treatment of olive mill wastewater by combining OF and UV/H2O2 techniques. Desalination 2004; 169: 81–88. Duran A, Monteagudo JM, Carnicer A, Ruiz-Murillo M. Photo-Fenton mineralization of synthetic municipal wastewater effluent Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT containing acetaminophen in a pilot plant. Desalination 2011; 270: 124–129. Durán A, Monteagudo JM, San Martín I. Photocatalytic treatment of an industrial effluent using artificial and solar UV radiation: An operational cost study on a pilot plant scale. J Environ Manage 2012; 98: 1–4. El Hajjouji H, Barje F, Pinelli E, Bailly JR, Richard C, Winterton P, Revel JC, Hafidi M. Photochemical UV/TiO2 treatment of olive mill wastewater (OMW). Bioresour Technol 2008; 99: 7264–7269. Elmolla ES, Chaudhuri M. Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2O2/TiO2 photocatalysis. Desalination 2010; 252: 46–52. Elmorsi TM, Riyad YM, Mohamed ZH, Abd El Bary HM. Decolorization of Mordant red 73 azo dye in water using H2O2/UV and photo-Fenton treatment. J Hazard Mater 2010; 174: 352–358. Eren Z. Ultrasound as a basic and auxiliary process for dye remediation: a review. J Environ Manage 2012; 104: 127–141. Feng J, Hu X, Yue PL. Effect of initial solution pH on the degradation of Orange II using clay-based Fe nanocomposites as heterogeneous photo-Fenton catalyst. Water Res 2006; 40: 641–646. Fenoll J, Garrido I, Hellín P, Flores P, Vela N, Navarro S. Photocatalytic oxidation of pirimicarb in aqueous slurries containing binary and ternary oxides of zinc and titanium. J Photochem Photobio A: Chem 2015; 298: 24–32. Garcia JC, Oliveira JL, Silva AE, Oliveira CC, Nozaki J, de Souza NE. Comparative study of the degradation of real textile effluents by photocatalytic reactions involving UV/TiO2/H2O2 and UV/ Fe2+/H2O2 systems. J Hazard Mater 2007; 147: 105–110. Garoma T, Gurol MD, Thotakura L, Osibodu O. Degradation of tert-butyl formate and its intermediates by an ozone/UV process. Chemosphere 2008; 73: 1708–1715. Garrido-Ramírez EG, Theng BKG, Mora ML. Clays and oxide minerals as catalysts and nanocatalysts in Fenton-like reactions – a review. Appl Clay Sci 2010; 47: 182–192. Gaya UI, Abdullah AH. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problems. J Photochem Photobio C 2008; 9: 1–12. Ghafoori S, Mehrvar M, Chan PK. Photoassisted Fenton-like degradation of aqueous poly(acrylic acid): from mechanistic kinetic model to CFD modeling. Chem Eng Res Des 2013; 91: 2617–2629. Ghafoori S, Mehrvar M, Chan PK. Photoreactor scale-up for degradation of aqueous poly(vinyl alcohol) using UV/H2O2 process. Chem Eng J 2014; 245: 133–142. Ghiselli G, Jardim WF, Litter MI, Mansilla HD. Destruction of EDTA using Fenton and photo-Fenton-like reactions under UV-A irradiation. J Photochem Photobio A 2004; 167: 59–67. Ghodbane H, Hamdaoui O. Decolorization of antraquinonic dye, C.I. Acid Blue 25, in aqueous solution by direct UV irradiation, UV/ H2O2 and UV/Fe(II) processes. Chem Eng J 2010; 160: 226–231. Giroto JA, Teixeira ACSC, Nascimento CAO, Guardani R. Photo-Fenton removal of water-soluble polymers. Chem Eng Process Process Intensif 2008; 47: 2361–2369. Glaze WH, Kang J-W, Chapin DH. The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation. Ozone Sci Eng 1987; 9: 335–352. 297 Gong J, Liu Y, Sun X. O3 and UV/O3 oxidation of organic constituents of biotreated municipal wastewater. Water Res 2008; 42: 1238–1244. Gozzi F, Machulek A, Ferreira VS, Osugi ME, Santos APF, Nogueira JA, Dantas RF, Esplugas S, de Oliveira SC. Investigation of chlorimuron-ethyl degradation by Fenton, photo-Fenton and ozonation processes. Chem Eng J 2012; 210: 444–450. Gracia R, Aragües JL, Ovelleiro JL. Study of the catalytic ozonation of humic substances in water and their ozonation byproducts. Ozone Sci Eng 1996; 18: 195–208. Grcic I, Maljkovic M, Papic S, Koprivanac N. Low frequency US and UV-A assisted Fenton oxidation of simulated dyehouse wastewater. J Hazard Mater 2011; 197: 272–284. Guittonneau S, De Laat J, Duguet JP, Bonnel C, Doré M. Oxidation of parachloronitrobenzene in dilute aqueous solution by O3 + UV and H2O2+ UV: a comparative study. Ozone Sci Eng 1990; 12: 73–94. Gurol MD, Vatistas R. Oxidation of phenolic compounds by ozone and ozone + U.V. radiation: a comparative study. Water Res 1987; 21: 895–900. Haji S, Benstaali B, Al-Bastaki N. Degradation of methyl orange by UV/H2O2 advanced oxidation process. Chem Eng J 2011;168: 134–139. Hasan DuB, Abdul Aziz AR, Daud WMAW. Oxidative mineralisation of petroleum refinery effluent using Fenton-like process. Chem Eng Res Des 2012a; 90: 298–307. Hasan DuB, Aziz ARA, Daud WMAW. Using D-optimal experimental design to optimise remazol black B mineralisation by Fenton-like peroxidation. Environ Technol 2012b; 33: 1111–1121. Herrera-Melián JA, Tello Rendón E, Doña Rodríguez JM, Viera Suárez A, Valdés do Campo C, Pérez Peña J, Araña Mesa J. Incidence of pretreatment by potassium permanganate on hazardous laboratory wastes photodegradability. Water Res 2000; 34: 3967–3976. Hu Q, Zhang C, Wang Z, Chen Y, Mao K, Zhang X, Xiong Y, Zhu, M. Photodegradation of methyl tert-butyl ether (MTBE) by UV/ H2O2 and UV/TiO2. J Hazard Mater 2008; 154: 795–803. Hu X, Wang X, Ban Y, Ren B. A comparative study of UV-Fenton, UV-H2O2and Fenton reaction treatment of landfill leachate. Environ Technol 2011; 32: 945–951. Huang K-C, Zhao Z, Hoag GE, Dahmani A, Block PA. Degradation of volatile organic compounds with thermally activated persulfate oxidation. Chemosphere 2005; 61: 551–560. Huang YH, Huang YF, Chang PS, Chen CY. Comparative study of oxidation of dye-reactive black B by different advanced oxidation processes: Fenton, electro-Fenton and photo-Fenton. J Hazard Mater 2008; 154: 655–662. Huber MM, Canonica S, Park GY, von Gunten U. Oxidation of pharmaceuticals during ozonation and advanced oxidation processes. Environ Sci Technol 2003; 37: 1016–1024. Ince NH, Gönenç DT. Treatability of a textile azo dye by UV/H2O2. Environ Technol 1997; 18: 179–185. Irmak S, Erbatur O, Akgerman A. Degradation of 17beta-estradiol and bisphenol A in aqueous medium by using ozone and ozone/UV techniques. J Hazard Mater 2005; 126: 54–62. Jin-hui Z. Research on UV/TiO2 photocatalytic oxidation of organic matter in drinking water and its influencing factors. Procedia Environ Sci 2012; 12: 445–452. Jiraroj D, Unob F, Hagege A. Degradation of Pb--EDTA complex by a H(2)O(2)/UV process. Water Res 2006; 40: 107–112. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 298 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Jung JT, Choi JY, Chung J, Lee YW, Kim JO. UV/TiO2 and UV/TiO2/ chemical oxidant processes for the removal of humic acid, Cr and Cu in aqueous TiO2 suspensions. Environ Technol 2009; 30: 225–232. Jung YJ, Kim WG, Yoon Y, Kang JW, Hong YM, Kim HW. Removal of amoxicillin by UV and UV/H2O2 processes. Sci Total Environ 2012; 420: 160–167. Kartal OE, Turhan GD. Decolourization of C.I. Reactive Orange 16 via photocatalysis involving TiO2/UV and TiO2/UV/oxidant systems. Desalin Water Treat 2012; 48: 199–206. Kaur S, Singh V. TiO2 mediated photocatalytic degradation studies of Reactive Red 198 by UV irradiation. J Hazard Mater 2007; 141: 230–236. Kavitha V, Palanivelu K. Degradation of 2-Chlorophenol by Fenton and photo-Fenton processes – a comparative study. J Environ Sci Health A 2003;38: 1215–1231. Khataee AR. Optimization of UV-promoted peroxydisulphate oxidation of C.I. Basic Blue 3 using response surface methodology. Environ Technol 2010; 31: 73–86. Khataee AR, Mirzajani O. UV/peroxydisulfate oxidation of C. I. Basic Blue 3: modeling of key factors by artificial neural network. Desalination 2010; 251: 64–69. Kim D, Chen JK-C, Yen TF. Naval derusting wastewater containing high concentration of iron, treated in UV photo-Fenton-like oxidation. J Environ Sci 2010; 22: 991–997. Kim S-J, Kim S-C, Seo S-G, Lee D-J, Lee H, Park SH, Jung S-C. Photocatalyzed destruction of organic dyes using microwave/ UV/O3/H2O2/TiO2 oxidation system. Catal Today 2011; 164: 384–390. Kim I-Y, Kim M-K, Yoon Y, Im J-K, Zoh K-D. Kinetics and degradation mechanism of clofibric acid and diclofenac in UV photolysis and UV/H2O2reaction. Desalin Water Treat 2013; 1–8. Kralik P, Kusic H, Koprivanac N, Loncaric Bozic A. 2010. Degradation of chlorinated hydrocarbons by UV/H2O2: the application of experimental design and kinetic modeling approach. Chem Eng J 158: 154–166. Kruithof JC, Kamp PC, Martijn BJ. UV/H2O2Treatment: a practical solution for organic contaminant control and primary disinfection. Ozone Sci Eng 2007; 29: 273–280. Kuo C-Y, Wu C-H, Chen S-T. Decolorization of C.I. Reactive Red 2 by UV/TiO2/PAC and visible light/TiO2/PAC systems. Desalin Water Treat 2013; 52: 834–843. Kuo C-Y, Wu C-H, Lin H-Y. Synergistic effects of TiO2 and Cu2O in UV/ TiO2/zeolite-based systems on photodegradation of bisphenol A. Environ Technol 2014; 35: 1851–1857. Kusic H, Koprivanac N, Bozic AL. Minimization of organic pollutant content in aqueous solution by means of AOPs: UV- and ozone-based technologies. Chem Eng J 2006a; 123: 127–137. Kusic H, Koprivanac N, Bozic AL, Selanec I. Photo-assisted Fenton type processes for the degradation of phenol: a kinetic study. J Hazard Mater 2006b; 136: 632–644. Lee S-M, Kim Y-G, Cho I-H. Treatment of dyeing wastewater by TiO2/H2O2/UV process: experimental design approach for evaluating total organic carbon (TOC) removal efficiency. J Environ Sci Health A 2005; 40: 423–436. Leong SK, Bashah NAA. Kinetic study on COD removal of palm oil refinery effluent by UV-Fenton. APCBEE Procedia 2012; 3: 6–10. Lester Y, Gozlan I, Avisar D, Mamane H. Photodegradation of sulphadimethoxine in water by medium pressure UV lamp. Water Sci Technol 2008; 58: 1147–1154. Lester Y, Avisar D, Mamane H. Photodegradation of the antibiotic sulphamethoxazole in water with UV/H2O2 advanced oxidation process. Environ Technol 2010; 31: 175–183. Lester Y, Avisar D, Gozlan I, Mamane H. Removal of pharmaceuticals using combination of UV/H(2)O(2)/O(3) advanced oxidation process. Water Sci Technol 2011; 64: 2230–2238. Lester Y, Mamane H, Avisar D. Enhanced removal of micropollutants from groundwater, using pH modification coupled with photolysis. Water, Air, Soil Poll 2012; 223: 1639–1647. Lester Y, Avisar D, Mamane H. Ozone degradation of cyclophosphamide – Effect of alkalinity and key effluent organic matter constituents. Ozon Sci Eng 2013a; 35: 125–133. Lester Y, Mamane H, Zucker I, Avisar D. Treating wastewater from a pharmaceutical formulation facility by biological process and ozone. Water Res 2013b; 47: 4349–4356. Li K, Hokanson DR, Crittenden JC, Trussell RR, Minakata D. Evaluating UV/H2O2 processes for methyl tert-butyl ether and tertiary butyl alcohol removal: effect of pretreatment options and light sources. Water Res 2008; 42: 5045–5053. Li W, Lu S, Qiu Z, Lin K. UV and VUV photolysis vs. UV/H2O2 and VUV/H2O2 treatment for removal of clofibric acid from aqueous solution. Environ Technol 2011; 32: 1063–1071. Liang CJ, Bruell CJ, Marley MC, Sperry KL. Thermally activated persulfate oxidation of trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA) in aqueous systems and soil slurries. Soil Sediment Contam 2003; 12: 207–228. Liang C, Bruell CJ, Marley MC, Sperry KL. Persulfate oxidation for in situ remediation of TCE. I. Activated by ferrous ion with and without a persulfate–thiosulfate redox couple. Chemosphere 2004a; 55: 1213–1223. Liang C, Bruell CJ, Marley MC, Sperry KL. Persulfate oxidation for in situ remediation of TCE. II. Activated by chelated ferrous ion. Chemosphere 2004b; 55: 1225–1233. Lide DR. CRC Handbook of chemistry and physics, 85th ed., Taylor & Francis, 2004, ISBN: 0849304857, 9780849304859, pp. 1252–1274. Lin C-H, Yu R-F, Cheng W-P, Liu C-R. Monitoring and control of UV and UV-TiO2 disinfections for municipal wastewater reclamation using artificial neural networks. J Hazard Mater 2012; 209–210: 348–354. Lin H, Wu J, Zhang H. Degradation of clofibric acid in aqueous solution by an EC/Fe3+/PMS process. Chem Eng J 2014; 244: 514–521. Linden KG, Rosenfeldt EJ, Kullman SW. UV/H2O2 degradation of endocrine-disrupting chemicals in water evaluated via toxicity assays. Water Sci Technol 2007; 55: 313–319. Liu BW, Chou MS, Kao CM, Huang BJ. Evaluation of selected operational parameters for the decolorization of dye-finishing wastewater using UV/Ozone. Ozone Sci Eng 2004; 26: 239–245. Liu C-C, Hsieh Y-H, Lai P-F, Li C-H, Kao C-L. Photodegradation treatment of azo dye wastewater by UV/TiO2 process. Dyes Pigm 2006; 68: 191–195. Liu R, Chiu HM, Shiau C-S, Yeh RY-L, Hung Y-T. Degradation and sludge production of textile dyes by Fenton and photo-Fenton processes. Dyes Pigm 2007; 73: 1–6. Liu X, Garoma T, Chen Z, Wang L, Wu Y. SMX degradation by ozonation and UV radiation: a kinetic study. Chemosphere 2012; 87: 1134–1140. Liu P, Li C, Kong X, Lu G, Xu J, Ji F, Liang X. Photocatalytic degradation of EDTA with UV/Cu(II)/H2O2process. Desalin Water Treat 2013; 51: 7555–7561. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Lu LA, Ma YS, Daverey A, Lin JG. Optimization of photo-Fenton process parameters on carbofuran degradation using central composite design. J Environ Sci Health B 2012; 47: 553–561. Lucas M, Peres J. Decolorization of the azo dye reactive black 5 by Fenton and photo-Fenton oxidation. Dyes Pigm 2006; 71: 236–244. Lucas MS, Peres JA, Li Puma G. Treatment of winery wastewater by ozone-based advanced oxidation processes (O3, O3/UV and O3/UV/H2O2) in a pilot-scale bubble column reactor and process economics. Sep Purif Technol 2010; 72: 235–241. Lydakis-Simantiris N, Riga D, Katsivela E, Mantzavinos D, Xekoukoulotakis NP. Disinfection of spring water and secondary treated municipal wastewater by TiO2 photocatalysis. Desalination 2010; 250: 351–355. Mahamuni NN, Adewuyi YG. Advanced oxidation processes (AOPs) involving ultrasound for waste water treatment: a review with emphasis on cost estimation. Ultrason Sonochem 2010; 17: 990–1003. Malik P. Kinetics of decolourisation of azo dyes in wastewater by UV/H2O2 process. Sep Purif Technol 2004; 36: 167–175. Mansouri L, Bousselmi L. Degradation of diethyl phthalate (DEP) in aqueous solution using TiO2/UV process. Desalin Water Treat 2012; 40: 63–68. Matafonova G, Batoev V. Recent progress on application of UV excilamps for degradation of organic pollutants and microbial inactivation. Chemosphere 2012; 89: 637–647. Mendez-Arriaga F, Esplugas S, Gimenez J. Degradation of the emerging contaminant ibuprofen in water by photo-Fenton. Water Res 2010; 44: 589–595. Modirshahla N, Behnajady MA, Ghanbary F. Decolorization and mineralization of C.I. Acid Yellow 23 by Fenton and photo-Fenton processes. Dyes Pigm 2007; 73: 305–310. Mohammadi M, Sabbaghi S. Photo-catalytic degradation of 2,4-DCP wastewater using MWCNT/TiO2 nano-composite activated by UV and solar light. Environ Nanotechnol Monit Manage 2014; 1–2: 24–29. Mohseni M, Taghipour F. Experimental and CFD analysis of photocatalytic gas phase vinyl chloride (VC) oxidation. Chem Eng Sci 2004; 59: 1601–1609. Mora VC, Rosso JA, Carrillo Le Roux G, Mártire DO, Gonzalez MC. Thermally activated peroxydisulfate in the presence of additives: a clean method for the degradation of pollutants. Chemosphere 2009;75: 1405–1409. Muruganandham M, Swaminathan M. Advanced oxidative decolourisation of Reactive Yellow 14 azo dye by UV/TiO2, UV/H2O2, UV/H2O2/Fe2+ processes – a comparative study. Sep Purif Technol 2006a; 48: 297–303. Muruganandham M, Swaminathan M. Photocatalytic decolourisation and degradation of Reactive Orange 4 by TiO-UV process. Dye Pigm 2006b; 68: 133–142. Neamtu M, Yediler A, Siminiceanu I, Kettrup A. Oxidation of commercial reactive azo dye aqueous solutions by the photo-Fenton and Fenton-like processes. J Photochem Photobio A 2003; 161: 87–93. Oh BS, Jung YJ, Oh YJ, Yoo YS, Kang JW. Application of ozone, UV and ozone/UV processes to reduce diethyl phthalate and its estrogenic activity. Sci Total Environ 2006; 367: 681–693. Papic S, Vujevic D, Koprivanac N, Sinko D. Decolourization and mineralization of commercial reactive dyes by using 299 homogeneous and heterogeneous Fenton and UV/Fenton processes. J Hazard Mater 2009; 164: 1137–1145. Pareek VK, Cox SJ, Brungs MP, Young B, Adesina AA. Computational fluid dynamic (CFD) simulation of a pilot-scale annular bubble column photocatalytic reactor. Chem Eng Sci 2003; 58: 859–865. Park J-H, Choi E, Gil K-I. Removal of reactive dye using UV/TiO2 in circular type reactor. J Environ Sci Health A 2003; 38: 1389–1399. Park JH, Cho IH, Chang SW. Comparison of fenton and photo-fenton processes for livestock wastewater treatment. J Environ Sci Health B 2006; 41: 109–120. Patel SG, Yadav NR, Patel SK. Evaluation of degradation characteristics of reactive dyes by UV/Fenton, UV/Fenton/Activated Charcoal, and UV/Fenton/Tio2processes: a comparative study. Sep Sci Technol 2013; 48: 1788–1800. Peternel IT, Koprivanac N, Bozic AM, Kusic HM. Comparative study of UV/TiO2, UV/ZnO and photo-Fenton processes for the organic reactive dye degradation in aqueous solution. J Hazard Mater 2007; 148: 477–484. Philippopoulos CJ, Poulopoulos SG. Photo-assisted oxidation of an oily wastewater using hydrogen peroxide. J Hazard Mater 2003; 98: 201–210. Qiu M, Huang C. A comparative study of degradation of the azo dye C.I. Acid Blue 9 by Fenton and photo-Fenton oxidation. Desalin Water Treat 2010; 24: 273–277. Ramesh T, Vigneswaran S, Moon I. A review on UV/TiO2 photocatalytic oxidation process (Journal Review). Korean J Chem Eng 2008; 25: 64–72. Rastogi A, Al-Abed SR, Dionysiou DD. Effect of inorganic, synthetic and naturally occurring chelating agents on Fe(II) mediated advanced oxidation of chlorophenols. Water Res 2009; 43: 684–694. Rauf MA, Marzouki N, Korbahti BK. Photolytic decolorization of Rose Bengal by UV/H(2)O(2) and data optimization using response surface method. J Hazard Mater 2008; 159: 602–609. Riga A, Soutsas K, Ntampegliotis K, Karayannis V, Papapolymerou G. Effect of system parameters and of inorganic salts on the decolorization and degradation of Procion H-exl dyes. Comparison of H2O2/UV, Fenton, UV/Fenton, TiO2/UV and TiO2/UV/H2O2 processes. Desalination 2007; 211: 72–86. Rizzo L, Della Sala A, Fiorentino A, Li Puma G. Disinfection of urban wastewater by solar driven and UV lamp – TiO2 photocatalysis: Effect on a multi drug resistant Escherichia coli strain. Water Res 2014; 53: 145–152. Rodrigues CSD, Madeira LM, Boaventura RAR. Synthetic textile dyeing wastewater treatment by integration of advanced oxidation and biological processes – Performance analysis with costs reduction. J Environ Chem Eng 2014; 2: 1027–1039. Romero-Vargas Castrillón S, de Lasa HI. Performance evaluation of photocatalytic reactors for air purification using computational fluid dynamics (CFD). Ind Eng Chem Res 2007; 46: 5867–5880. Rosario-Ortiz FL, Wert EC, Snyder SA. 2010. Evaluation of UV/H2O2 treatment for the oxidation of pharmaceuticals in wastewater. Water Res 44: 1440–1448. Rosenfeldt EJ, Linden KG, Canonica S, von Gunten U. Comparison of the efficiency of *OH radical formation during ozonation and the advanced oxidation processes O3/H2O2 and UV/H2O2. Water Res 2006; 40: 3695–3704. Rubio D, Nebot E, Casanueva JF, Pulgarin C. Comparative effect of simulated solar light, UV, UV/H2O2 and photo-Fenton Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 300 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT treatment (UV–Vis/H2O2/Fe2+,3+) in the Escherichia coli inactivation in artificial seawater. Water Res 2013; 47: 6367–6379. Ryu H, Gerrity D, Crittenden JC, Abbaszadegan M. Photocatalytic inactivation of Cryptosporidium parvum with TiO(2) and low-pressure ultraviolet irradiation. Water Res 2008; 42: 1523–1530. Sabaikai W, Sekine M, Tokumura M, Kawase Y. UV light photo-Fenton degradation of polyphenols in oolong tea manufacturing wastewater. J Environ Sci Health A Tox Hazard Subst Environ Eng 2014; 49: 193–202. Saghafinia MS, Emadian SM, Vossoughi M. Performances evaluation of Photo-Fenton process and sonolysis for the treatment of Penicillin G formulation effluent. Procedia Environ Sci 2011; 8: 202–208. Saien J, Nejati H. Enhanced photocatalytic degradation of pollutants in petroleum refinery wastewater under mild conditions. J Hazard Mater 2007; 148: 491–495. Saien J, Soleymani AR, Sun JH. Parametric optimization of individual and hybridized AOPs of Fe2+/H2O2 and UV/S2O82– for rapid dye destruction in aqueous media. Desalination 2011; 279: 298–305. Salari D, Niaei A, Aber S, Rasoulifard MH. The photooxidative destruction of C.I. Basic Yellow 2 using UV/S2O82– process in a rectangular continuous photoreactor. J Hazard Mater 2009; 166: 61–66. Saquib M, Abu Tariq M, Haque MM, Muneer M. Photocatalytic degradation of disperse blue 1 using UV/TiO2/H2O2 process. J Environ Manage 2008; 88: 300–306. Schrank SG, Santos JNRD, Souza DS, Souza EES. Decolourisation effects of Vat Green 01 textile dye and textile wastewater using H2O2/UV process. J Photochem Photobio A 2007; 186: 125–129. Schulte P, Bayer A, Kuhn F, Luy T, Volkmer M. H2O2/O3, H2O2/ UV and H2O2/Fe2+processes for the oxidation of hazardous wastes. Ozone Sci Eng 1995; 17: 119–134. Shang NC, Chen YH, Ma HW, Lee CW, Chang CH, Yu YH, Lee CH. Oxidation of methyl methacrylate from semiconductor wastewater by O3 and O3/UV processes. J Hazard Mater 2007; 147: 307–312. Sharrer MJ, Summerfelt ST. Ozonation followed by ultraviolet irradiation provides effective bacteria inactivation in a freshwater recirculating system. Aquacult Eng 2007; 37: 180–191. Shavisi Y, Sharifnia S, Hosseini SN, Khadivi MA. Application of TiO2/perlite photocatalysis for degradation of ammonia in wastewater. J Ind EngChem 2014; 20: 278–283. Shu HY. Degradation of dyehouse effluent containing C.I. Direct Blue 199 by processes of ozonation, UV/H2O2 and in sequence of ozonation with UV/H2O2. J Hazard Mater 2006; 133: 92–98. Shu H-Y, Chang M-C. Decolorization and mineralization of a phthalocyanine dye C.I. Direct Blue 199 using UV/H2O2 process. J Hazard Mater 2005a; 125: 96–101. Shu H-Y, Chang M-C. Decolorization effects of six azo dyes by O3, UV/O3 and UV/H2O2 processes. Dyes Pigm 2005b; 65: 25–31. Shu HY, Chang MC. Pilot scale annular plug flow photoreactor by UV/H2O2 for the decolorization of azo dye wastewater. J Hazard Mater 2005c; 125: 244–251. Shu HY, Chang MC, Fan HJ. Effects of gap size and UV dosage on decolorization of C.I. Acid Blue 113 wastewater in the UV/H2O2 process. J Hazard Mater 2005; 118: 205–211. Shu HY, Chang MC, Hsieh WP. Remedy of dye manufacturing process effluent by UV/H2O2 process. J Hazard Mater 2006a; 128: 60–66. Shu HY, Fan HJ, Chang MC, Hsieh WP. Treatment of MSW landfill leachate by a thin gap annular UV/H2O2 photoreactor with multi-UV lamps. J Hazard Mater 2006b; 129: 73–79. Sohrabi MR, Ghavami M. Photocatalytic degradation of Direct Red 23 dye using UV/TiO2: Effect of operational parameters. J Hazard Mater 2008; 153: 1235–1239. Summerfelt ST. Ozonation and UV irradiation – an introduction and examples of current applications. Aquacult Eng 2003; 28: 21–36. Swaim P, Royce A, Smith T, Maloney T, Ehlen D, Carter B. Effectiveness of UV advanced oxidation for destruction of micro-pollutants. Ozone Sci Eng 2008; 30: 34–42. Tanaka K, Padermpole K, Hisanaga T. Photocatalytic degradation of commercial azo dyes. Water Res 2000; 34: 327–333. Tang WZ, Huren A. UV/TiO2 photocatalytic oxidation of commercial dyes in aqueous solutions. Chemosphere 1995; 31: 4157–4170. Tang WZ, Zhang Z, An H, Quintana MO, Torres DF. TiO2/UV photodegradation of azo dyes in aqueous solutions. EnvironTechnol 1997; 18: 1–12. Tang C, Chen V. The photocatalytic degradation of reactive black 5 using TiO2/UV in an annular photoreactor. Water Res 2004; 38: 2775–2781. Tezcanli-Guyer G, Ince NH. Individual and combined effects of ultrasound, ozone and UV irradiation: a case study with textile dyes. Ultrasonics 2004;42: 603–609. Thiruvenkatachari R, Ouk Kwon T, Shik Moon I. Degradation of phthalic acids and benzoic acid from terephthalic acid wastewater by advanced oxidation processes. J Environ Sci Health A Tox Hazard Subst Environ Eng 2006; 41: 1685–1697. Thiruvenkatachari R, Kwon TO, Jun JC, Balaji S, Matheswaran M, Moon IS. Application of several advanced oxidation processes for the destruction of terephthalic acid (TPA). J Hazard Mater 2007; 142: 308–314. Tokumura M, Morito R, Kawase Y. Photo-Fenton process for simultaneous colored wastewater treatment and electricity and hydrogen production. Chem Eng J 2013; 221: 81–89. Tomiyasu H, Fukutomi H, Gordon G. Kinetics and mechanism of ozone decomposition in basic aqueous solution. Inorg Chem 1985; 24: 2962–2966. Tony MA, Purcell PJ, Zhao Y. Oil refinery wastewater treatment using physicochemical, Fenton and photo-Fenton oxidation processes. J Environ Sci Health A Tox Hazard Subst Environ Eng 2012; 47: 435–440. Toor AP, Verma A, Jotshi CK, Bajpai PK, Singh V. Photocatalytic degradation of Direct Yellow 12 dye using UV/TiO2 in a shallow pond slurry reactor. Dyes Pigm 2006; 68: 53–60. Torrades F, García-Montaño J. Using central composite experimental design to optimize the degradation of real dye wastewater by Fenton and photo-Fenton reactions. Dyes Pigm 2014; 100: 184–189. Trabelsi-Souissi S, Oturan N, Bellakhal N, Oturan MA. Application of the photo-Fenton process to the mineralization of phthalic anhydride in aqueous medium. Desalin Water Treat 2011; 25: 210–215. Trapido M, Hirvonen A, Veressinina Y, Hentunen J, Munter R. Ozonation, ozone/UV and UV/H2O2degradation of chlorophenols. Ozon Sci Eng 1997; 19: 75–96. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Tsao M-S, Wilmarth WK. The aqueous chemistry of inorganic free radicals. I. The mechanism of the photolytic decomposition of aqueous persulfate ion and evidence regarding the ­sulfate-hydroxyl radical interconversion equilibrium. J Phys Chem 1959; 63: 346–353. Tureli G, Gursoy BH, Olmez-Hanci T, Arslan-Alaton I. H2O2/ UV-C and Fe2++/H2O2/UV-C treatment of a commercial naphthalene sulphonate (H-acid). Desalin Water Treat 2010; 23: 66–72. Verbruggen SW, Lenaerts S, Denys S. Analytic versus CFD approach for kinetic modeling of gas phase photocatalysis. Chem Eng J 2015; 262: 1–8. Vogna D, Marotta R, Napolitano A, Andreozzi R, d’Ischia M. Advanced oxidation of the pharmaceutical drug diclofenac with UV/H2O2 and ozone. Water Res 2004; 38: 414–422. Vujevic D, Papic S, Koprivanac N, Bozic AL. Decolorization and mineralization of reactive dye by UV/Fenton Process. Sep Sci Technol 2010; 45: 1637–1643. Wang JL, Xu LJ. Advanced oxidation processes for wastewater treatment: formation of hydroxyl radical and application. Crit Rev Environ Sci Technol 2012; 42: 251–325. Wang Z, Liu J, Dai Y, Dong W, Zhang S, Chen J. CFD modeling of a UV-LED photocatalytic odor abatement process in a continuous reactor. J Hazard Mater 2012; 215–216: 25–31. Watts R, Teel A. Treatment of contaminated soils and groundwater using ISCO. Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management 2006; 10: 2–9. Wols BA, Hofman-Caris CHM. Modelling micropollutant degradation in UV/H2O2 systems: Lagrangian versus Eulerian method. Chem Eng J 2012; 210: 289–297. Wu C. Effects of operational parameters on the decolorization of C.I. Reactive Red 198 in UV/TiO2-based systems. Dyes Pigm 2008; 77: 31–38. Wu JJ, Yang JS, Muruganandham M, Wu CC. The oxidation study of 2-propanol using ozone-based advanced oxidation processes. Sep Purif Technol 2008a; 62: 39–46. Wu C-H, Chang C-L, Kuo C-Y. Decolorization of Procion Red MX-5B in electrocoagulation (EC), UV/TiO2 and ozone-related systems. Dyes Pigm 2008b; 76: 187–194. Xu X-R, Li X-Y, Li X-Z, Li H-B. Degradation of melatonin by UV, UV/ H2O2, Fe2+/H2O2 and UV/Fe2+/H2O2 processes. Sep Purif Technol 2009; 68: 261–266. Xu H, Xu W, Wang J. Degradation kinetics of azo dye reactive Red SBE wastewater by complex ultraviolet and hydrogen peroxide process. Environ Prog Sustain Energy 2011; 30: 208–215. Yang W, Zhou H, Cicek N. Treatment of organic micropollutants in water and wastewater by UV-based processes: a literature review. Crit Rev Environ Sci Technol 2013.130906052408006. Yasar A, Ahmad N, Latif H, Amanat Ali Khan A. Pathogen re-growth in UASB effluent disinfected by UV, O3, H2O2, and advanced oxidation processes. Ozone Sci Eng 2007; 29: 485–492. Yeber MC, Cid JA. Oil removal from fishmeal mill wastewater by the Fe°/UV process: optimization by experimental design. Desalin Water Treat 2013; 51: 2102–2108. 301 Yonar T, Kestioglu K, Azbar N. Treatability studies on domestic wastewater using UV/H2O2 process. Appl Catal B, 2006; 67: 223–228. Yoon S-H, Lee S, Kim T-H, Lee M, Yu S. Oxidation of methylated arsenic species by UV/S2O82. Chem Eng J 2011; 173: 290–295. Zarora C, Segura C, Mansilla H, Mondaca MA, Gonzalez P. Kinetic study of imidacloprid removal by advanced oxidation based on photo-Fenton process. Environ Technol 2010; 31: 1411–1416. Zhan F, Li C, Zeng G, Tao S, Xiao Y, Zhang X, Zhao L, Zhang J, Ma J. Experimental study on oxidation of elemental mercury by UV/ Fenton system. Chem Eng J 2013; 232: 81–88. Zhang Y, Pagilla K. Treatment of malathion pesticide wastewater with nanofiltration and photo-Fenton oxidation. Desalination 2010; 263: 36–44. Zhang H, Choi HJ, Huang CP. Optimization of Fenton process for the treatment of landfill leachate. J Hazard Mater 2005; 125: 166–174. Zhang Q, Li C, Li T. Rapid photocatalytic decolorization of methylene blue using high photon flux UV/TiO2/H2O2 process. Chem Eng J 2013; 217: 407–413. Zhao G, Lu X, Zhou Y. Aniline degradation in aqueous solution by UV-aeration and UV-microO3 processes: efficiency, contribution of radicals and byproducts. Chem Eng J 2013; 229: 436–443. Zoschke K, Börnick H, Worch E. Vacuum-UV radiation at 185 nm in water treatment – a review. Water Res. 2014; 52: 131–145. Zuorro A, Lavecchia R. Evaluation of UV/H2O2 advanced oxidation process (AOP) for the degradation of diazo dye Reactive Green 19 in aqueous solution. Desalin Water Treat 2013; 1–7. Zuorro A, Fidaleo M, Fidaleo M, Lavecchia R. Degradation and antibiotic activity reduction of chloramphenicol in aqueous solution by UV/H2O2 process. J Environ Manage 2014; 133: 302–308. Bionotes Archina Buthiyappan Faculty of Engineering, Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Archina Buthiyappan graduated with a Bachelor’s degree in Industrial Chemistry in 2008 and a Master’s degree in Forensic Science in 2010 from University Technology of Malaysia. She joined the University of Malaya, Malaysia, as a doctoral candidate in 2012. Her research focuses include application of various types of advanced oxidation processes such as Fenton, photo-Fenton, and electroFenton to treat real textile effluents. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM 302 A. Buthiyappan et al.: Degradation and cost of UV IAOP for WT Abdul Raman Abdul Aziz Faculty of Engineering, Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia, azizraman@um.edu.my Abdul Raman Abdul Aziz completed his PhD in the area of threephase mixing. Currently, he is a Professor and holds the position of Deputy Dean at the Faculty of Engineering, University of Malaya, Malaysia. His research interests are in advanced wastewater treatment and mixing in stirred vessels. Prior to joining UM, he worked in the oil and gas and food industries from 1989 to 1993. He is also active in consultancy projects and is currently supervising many PhD candidates. He has to date published more than 100 papers in journals and conference proceedings both locally and internationally. He is also a member of professional and learned societies such as the Institution of Chemical Engineers (IChemE, UK) and the Institution of Engineers Malaysia (IEM). Wan Mohd Ashri Wan Daud Faculty of Engineering, Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Wan Mohd Ashri Bin Wan Daud is a Professor of Chemical Engineering, University of Malaya, Malaysia. He obtained his Bachelor’s degree in Chemical Engineering in 1991 from Leeds University, Leeds, UK, and his Master’s degree in Chemical Engineering in 1992 from the University of Sheffield, Sheffield, UK. He earned his PhD degree in Chemical Engineering in 1996 at the University of Sheffield. His research fields include fuel cell, energy, biomass conversion and the synthesis of catalyst materials, catalysis, zeolites, polymerization process, separation process (adsorption, activated carbon, and carbon molecular sieve), ordered mesoporous materials, and hydrogen storage materials. Professor Daud has published approximately 90 research papers. Brought to you by | University of Sussex Library Authenticated Download Date | 6/28/18 2:24 PM