61 ADVANCED OXIDATION PROCESSES FOR FOOD INDUSTRIAL WASTEWATER DECONTAMINATION

High organic matter content is a basic problem in food industry wastewaters. Typically, the amount and composition of the effluent varies considerably. In the article four groups of advanced processes and their combination of food industry wastewater treatment have been reviewed: electrochemical oxidation (EC), Fenton’s process, ozonation of water and photocatalytic processes. All advanced oxidation processes (AOP`s) are characterized by a common chemical feature: the capability of exploiting high reactivity of HO• radicals in driving oxidation processes which are suitable for achieving decolonization and odour reduction, and the complete mineralization or increase of bioavailability of recalcitrant organic pollutants.


INTRODUCTION
Industrial wastewater characteristics vary not only between the industries that generate them, but also within each industry.These characteristics are also much more diverse than domestic wastewater, which is usually qualitatively and quantitatively similar in its composition.On the contrary, industry produces large quantities of highly polluted wastewater containing toxic substances, organic and inorganic compounds such as: heavy metals, pesticides, phenols and derivatives thereof, aromatic and aliphatic hydrocarbons, halogenated compounds, etc., which are generally resistant to destruction by biological treatment methods [Meriç et al. 2005, Shu 2006, Ledakowicz et al. 2001, Gogate & Pandit 2004].
Compared to other industrial sectors, food industry requires great amounts of water, since it is used throughout most of plant operations, such as production, cleaning, sanitizing, cooling and materials transport, among others [Mavrov & Be1ieres 2000, Cicek 2003, Álvarez et al. 2011].
As a result, meat, poultry, dairy, olive mill etc. processing plants are the facilities producing "difficult" wastewater with large total load of organic pollutants like proteins or fats and chemicals used for cleaning and sanitizing processing equipment [Álvarez et al. 2011[Álvarez et al. , Zhukova et al. 2011].
The wastewater streams with different levels of pollution load (low, medium and high contamination) are collected and treated in an on-site installation or in a municipal sewage treatment plant.Increasing food production will increase the volume of sewage and the cost of disposal for food processing plants and present difficult challenges for municipal wastewater treatment plant operators [Mavrov & Be1ieres 2000, Cicek 2003].Currently, in accordance with the legislation of the European Union introduced more stringent controls and rules concerning pollution of industrial wastewater [Marcucci et al. 2002, Mason 2000].Due to the adverse impact of even small quantities of these compounds on the organoleptic characteristics of the discharge of waste waters such as colour, odour, taste, etc., as well as a threat to living organisms began to look for effective, efficient and economically viable methods to remove them.
In order to achieve these aims the potential and promising methods need to include advanced oxidation methods (AOP's), which include the Fenton reaction, UV photolysis, sonication, ozonation, electrochemical oxidation, etc.These processes involve the generation of highly free radicals, mainly hydroxyl radical (HO • ) via chemical, photochemical and photocatalytic reactions.Their application is unavoidable for the treatment of refractory organic pollutants.The application of AOP`s in wastewater treatment leads to the degradation of the pollutant rather than transferring it to another phase, making the relevant technologies effective in the removal of organic pollutants in solution.In recent years, one of the main objectives of these processes mainly with highly polluted effluents has been not to mineralise the pollutant totally (AOP`s for complete mineralization are very expensive), but to improve the biodegradability for a possible coupling of the AOP`s with a conventional biological treatment process [Sanz et al. 2003, Muñoz et al. 2005].

PROCESSES FOR FOOD INDUSTRY WASTEWATER TREATMENT
The AOP`s has been used to reduce the organic load or toxicity of wastewaters from different industries.They are based on the generation of hydroxyl free radicals, which have a high electrochemical oxidant potential (Table 1).The generation of hydroxyl radicals involves the combination of classical oxidants, such as H 2 O 2 or O 3 with UV radiation or a catalyst.The formed radicals react with organic materials breaking them down gradually in a stepwise process.The generation of hydroxyl radicals can be achieved by a variety of reactions, such as O 3 /UV, H 2 O 2 /UV, Fenton reaction, photo-Fenton, TiO 2 / H 2 O 2 /UV etc.Such integrated solutions can lead to more efficient use of chemical oxidants while reducing the effect of toxic or inhibitory compounds in bioreactors, leading to more robust and stable biological treatment [

Elektrochemical oxidation
It was observed that electrochemical oxidation (EC) has the potential to be a distinct economic and environmental choice for treatment of wastewater and due to strict environmental regulations [Kobya & Delipinar 2008, Gengec et al. 2012].EC involves the formation of hydroxyl radicals at the active sites of anode and has been used for the decontamination of various inorganic and organic pollutants [Rizzo 2011].One of the major advantages of electrochemistry is that on the surface of the electrodes only electrons are produced and consumed, thereby "pure reagents" and do not contribute to a further increase in the number of chemical compounds in the environment, which often takes place in other processes [Zaleska & Grabowska 2008].Moreover, advantages of the EC compared to conventional chemical coagulation include reduced wastewater acidification and salinity, low dosage of coagulant, superior coagulant dispersion and intrinsic electroflotation separation capability [Gengec et al. 2012].
The efficiency of electrochemical process depends on electrode and supporting electrolyte types, applied current, solution pH, nature of target contaminant/water matrix and initial concentration of the pollutants [Rizzo 2011].In the EC process different anodes have been investigated: graphite, Pt, TiO 2 , IrO 2 , PbO 2 , several Ti-based alloys and boron-doped diamond electrodes but the most generally employed as a electrode material is iron (Fe) or aluminium (Al) [Gengec et al. 2012, Rizzo 2011].
Generally, six main processes occur during EC: (1) migration to an oppositely charged electrode (electrophoresis) and aggregation due to

Fenton process
Many AOPs use hydrogen peroxide as the main oxidizing agent, which is a more efficient reagent than gaseous oxygen concerning the contaminants mineralization.Fenton's process has its origin in the discovery reported in 1894 that ferrous ion strongly promotes the oxidation of tartaric acid by hydrogen peroxide.However, only much later the oxidation activity has been ascribed to the hydroxyl radical [Herney-Ramirez et al. 2010].
Oxidation with Fenton's reagent is accomplished with a mixture of ferrous ions and hydrogen peroxide, and it takes advantage of the reactivity of the free hydroxyl radicals produced in acidic solution by the catalytic decomposition of hydrogen peroxide and of the coagulation produced by the ferric hydroxide precipitates [Gogate The mechanism of the Fenton's process is quite complex, and some papers can be found in the literature where tens of equations are used for its description (Eq. 1) [Mert et al. 2010].
Fe 2+ + H 2 O 2 → Fe 3+ + OH − + OH • (1) As iron(II) acts as a catalyst, it has to be regenerated, which seems to occur through the following scheme [Pérez et al. 2002, Rivas et al. 2003]: The photo-Fenton process, as its name suggests, is rather similar to the Fenton one, but employing also radiation [Tokumura et al. 2006].
Fe(OH) 2 + + hʋ → Fe 2+ + HO • (4) Its effectiveness is attributed to the photolysis of Fe(III) cations in acidic media yielding Fe(II) cations (Eq.4).In this process, the regeneration of Fe 2+ by photo-reduction of Fe 3+ is accelerated, this photo-reduction being an additional source of highly oxidative hydroxyl radicals, as compared with the "simple" Fenton's process [ The main advantage of the process is degradation organic as well as inorganic pollutants that will leading to high mineralization levels [Sanz et al. 2003

Ozonation
Ozonation of water is a well-known technology and the strong oxidative properties of O 3 and its ability to effectively oxidize many organic compounds in aqueous solution have been well documented.Unlike other oxidizing agents such as Cl 2 , oxidation with O 3 leaves no toxic residues that have to be removed or disposed [Sarayu et  (7) This arises from the fact that pH affects the double action of ozone on the organic matter which may be a direct or an indirect (free radical) oxidation pathway.At low pH, ozone exclusively reacts with compounds with specific functional groups through selective reactions, such as electrophilic, nucleophilic or dipolar addition reactions (i.e.direct pathway).At basic pH, ozone decomposes yielding hydroxyl radicals, a highly oxidizing species which reacts nonselectively with a wide range of organic and inorganic com- Ozonation has been successfully applied to the treatment of winery and distillery wastewater [Zaleska & Grabowska 2008], olive mile wastewater [Oller et al. 2011], meat industry wastewater [Millamena 1992], molasses wastewater [Coca et al. 2007] etc.

Photocatalytic process
The photocatalytic or photochemical degradation processes are gaining importance in the area of wastewater treatment, since these processes result in complete mineralization with operation at mild conditions of temperature and pressure [Tarek et  The selection of the adequate catalyst must consider the following properties: chemical activity, stability, availability and handiness, cost and lack of toxicity [Navarro et al. 2005].The surface area and the number of active sites offered by the catalyst (thus nature of catalyst, i.e. crystalline or amorphous is important) for the adsorption of pollutants plays an important role in deciding [Gogate & Pandit 2004].Several catalytic materials have been studied in photocatalysis (various oxides such as TiO 2 , ZnO, SnO 2 ,WO 3 , ZrO 2 , CeO etc. or sulfides such as CdS, ZnS etc.) [Gogate & Pandit 2004, Sakthivel et al. 2003, Habibi et al. 2001].Among the semiconductors reported so far, outstanding stability and oxidative power makes TiO 2 , the best semiconductor photocatalyst for environmental remediation and energy conversion processes [ However, there are two potential drawbacks associated with the use of TiO 2 , namely: (a) its possible toxic effects on human health and (b) reduced activity due to the complexity of water matrix (i.e., presence of solids or inorganic ions) [Chatzisymeon et al. 2008].
TiO 2 in its anatase form has an energy bandgap of 3.2 eV and can be activated by UV radiation with a wavelength up to 387.5 nm.It only requires 1 W/m 2 of light [Navarro et al. 2005, Navgire 2012].Photocatalytic degradation occurs through a multistep process that involves the formation of reactive species on the surface of the photocatalyst and the subsequent generation of hydroxyl radicals that result in the mineralization of most organic compounds [Navarro et The UV radiation required for the photocatalytic processes can be obtained from artificial sources or the sun.There is a significant economic incentive for solar light based photocatalytic degradations [Banu et al. 2008].
The major factors affecting TiO 2 /UV light process are: initial organic load, amount of catalyst, reactor's design, UV irradiation time, temperature, solution's pH, light intensity and the presence of ionic species.The use of excessive amounts of catalyst may reduce the amount of energy being transferred into the medium due to the opacity offered by the catalyst particles [Stasinakis 2008].Organic compounds can then undergo both oxidative degradation through their reactions with valence band holes, hydroxyl and peroxide radicals and reductive cleavage through their reactions with electrons yielding various by-products and eventually mineral end-products  Several applications are presented below (Table 5).

THE COMBINATION OF AOP`S FOR WASTEWATER TREATMENT
Differently combined AOP's have been developed and investigated by several research groups as alternatives for treating food industrial wastewater containing organic pollutants.This may commonly cause a reduction of toxicity or elimination of a specific pollutant or reduce the reaction time and economic cost [Oller et

CONCLUSION
Food industry uses large amounts of water for many different purposes including cooling and cleaning, as a raw material, as sanitary water for food processing, for transportation, cooking and dissolving, as auxiliary water etc.In principle, the water used in the food industry may be used as process and cooling water or boiler feed water.As a consequence of diverse consumption, the amount and composition of food industry wastewaters varies considerably.Characteristics of the effluent consist of large amounts of suspended solids, nitrogen in several chemical forms, fats and oils, phosphorus, chlorides and organic matter.
AOP's constitute a promising technology for the treatment of food industry wastewaters containing difficult to biodegradable organic contaminants.It involves the generation of free hydroxyl radical (HO • ), a powerful, nonselective chemical oxidant to change organic compounds to a more biodegradable form or of carbon dioxide and water.These processes can reduce a broad spectrum of chemical and biological contaminants which are otherwise difficult to remove with conventional treatment processes of food industry wastewater.The study clearly shows the suitability of ozone and ultrasound as a pre-treatment step for the thermally pretreated wastewater for aerobic treatment by increasing the COD removal efficiency.Ozone was more efficient in COD removal with a 25-times increase in the rate of biodegradation of ozonated sample along with discoloration of the effluent sample. [ Numerous researches have evaluated on the treatment of refractory compounds by different AOP`s [Ledakowicz et al. 2001, Oller et al. 2011, Tarek et al. 2011, Pera-Titus et al. 2004].

Table 2 .
A brief summary of research studies in which EC were used for treating food industry wastewater Under optimal conditions of pH 4 and 18.2 mA/m 2 current density, Treatment reduced chemical oxygen demand (COD) by 90%, biochemical oxygen demand (BOD 5 ) by 96%, total solids by 95% and fecal coliforms by 99.9%.[Roa-Moralesetal.2007]Electrocoagulation(electrode material Fe and Al) Cattle-slaughterhouse wastewater In the case of aluminium electrode, polyaluminum chloride (PAC) was used as the coagulant aid for the aforesaid purpose.COD removal of 94.4% was obtained by adding 0.75 g L/PAC.In the case of iron electrode, EC was conducted concurrent with the Fenton process.As a result, 81.1% COD removal was achieved by adding 9% H 2 O 2 .[Ünetal.2009]Anaerobicelectrocoagulation (AE) and (AAE) anaerobic-aerobic electrocoagulation Baker's yeast wastewaterThe maximum color, COD and TOC were 88%, 48% and 49% at 80 A/m 2 , pH 4 and 30 min for AE and 86%, 49% and 43% at 12.5 A/m 2 , pH 5 and 30 min for AAE, respectively.The electrolysis was carried out the in lack of additional electrolyte (except for carbonates) and in the presence of NaCl (ranging from 17.5 to 85 mM).As it can be observed, the electrolysis without addition of NaCl (it remains around 20% of initial COD) while electrolyses with NaCl electrolyte obtain the complete removal of COD.2,the discoloration of the effluent, the reduction of the COD and the reduction of polyphenols exceeded 70%, the electrodes consumption was 0.085 kg Al/kg COD removed .Under optimal experimental conditions of flow rates (i.e.300 L/h), temperature (T=50 °C) and current density (i.e. 10 mA/cm 2 ), 97% of COD was removed in 3h electrolysis, with 17 kWh/m 3 energy consumption.

Table 3 .
A brief summary of research studies in which Fenton process were used for treating food industry wastewaterThe optimum ratio of H 2 O 2 (mg/L) to the initial COD cr was 1.05.The optimum initial pH were 3.5-4, H 2 O 2 / Fe 2+ was 2 and the optimum reaction time 30 min.The removal ratios of COD cr and color of the supernatant after static precipitation of the produced sludge were 88 and 95.4%, respectively.
al. 2007, Ulson de Souza et al. 2010].Ozonation is one of the AOP processes used to food wastewater treatment, which is versatile and environmentally sound [Sarayu et al. 2007].Although, the cost of ozone production still is high interest in the use of ozone in wastewater treatment has increased considerably in last few years due to the numerous advantages of this process [Meriç et al. 2005, Ulson de Souza et al. 2010].Ozonation can eliminate toxic substances, increase the biodegradability of organic pollutants and has high potential in decolourization [Zayas et al. 2007].

Table 4 .
A brief summary of research studies in which ozonation were used for treating food industry wastewater The highest efficiencies were achieved at 40 °C (the ozone dose rate constants = 2.3 g/h/L).Color and COD reductions were about 90% and 37%, respectively.In no case, the percentage of TOC removed was higher than 10-15%.

Table 5 .
A brief summary of research studies in which photocatalytic process were used for treating food industry wastewater al. 2011, Stasinakis 2008, de Sena et al. 2009, Zayas et al. 2007].

Table 6 .
A brief summary of research studies in which combining various AOP's were used for treating food industry wastewater Operation conditions -CDEO: natural pH; T: 25 °C; j: 300 A•cm −2 .Ozone production: 1 g/h, T: 25 °C; pH 12. Fenton process: pH 3; T: 25°C; Fe 2+ : 667 mg/dm −3 .Complete mineralization of the waste is obtained and no refractory compounds remain at the end of the process in both cases.
Sangave et al. 2007] UV/H 2 O 2 /O 3 Coffee wastewater The UV/H 2 O 2 /O 3 process is capable of reducing the COD content of the wastewater by 87% in 35 min at pH 2.0.By comparison, the UV/H 2 O 2 and UV/O 3 treatments under the same conditions reduced the COD by approximately 84%.UV/H 2 O 2 results for BOD 5 and COD reduction was 82.9% and 91.1% respectively.For TS and VS, reductions of up to 72.5% and 77.0% were achieved, respectively.DAF/photo-Fenton results for BOD 5 and COD reduction was 95.7% and 97.6\% respectively.For TS and VS, reductions of up to 61.5% and 90.8% were achieved, respectively.