1. Introduction

Water pollution seriously affects the ecosystems and availability of healthy freshwater [1]. The preservation of water resources is one of the most important issues of the twenty-first century [2]. Nowadays, sources of safe drinking water are limited and under stress [3]. The main problems encountered are population growth, rapid decline of forest areas, urbanization, climate change due to global warming, and industrialization [1]. Large amounts of raw water were consumed by industrial processes for various purposes [4]. Consequently, wastewater is

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

produced in large quantities [4]. Wastewaters are composed of a complex mixture of different organic and inorganic compounds, which may be toxic and difficult to degrade [4]. The most common inorganic water pollutants are heavy metals, nitrates, sulfates, phosphates, fluorides, and chlorides, which have serious hazardous effects [3]. The organic pollutants found in wastewaters may be generalized as insecticides, herbicides, fungicides, polycyclic aromatic hydrocarbons, phenols, biphenyls, halogenated aromatic hydrocarbons, formaldehyde, detergents, oils, greases, normal hydrocarbons, alcohols, aldehydes, ketones, proteins, lignin, and pharmaceuticals [3]. These pollutants remain either in dissolved, in colloidal, or in suspended form [3]. Mostly, wastewater treatment is carried out by conventional wastewater treatment processes [5]. Conventional wastewater treatment technologies could be categorized as physical processes including screening, flotation, filtration, and sedimentation [6]; chemical processes such as coagulation/flocculation, chlorination, adsorption, and ion exchange [6]; biological processes, i.e., activated sludge, aerated lagoons, and membrane bioreactors [2].

colorless, odorless, and clear; the amount of the sludge formation is low which can be easily stabilized and dehydrated; compared with chemical coagulation, the effluent contains less total dissolved solids; and the gas bubbles produced in the cathode allow the pollutants to be separated easily by floating them to the surface. However, weaknesses of EC method can be listed as follows: sacrificial electrodes need to be replaced regularly, formation of an impermeable film layer on the cathode may reduce the efficiency of the process and it is inefficient for the removal

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters

http://dx.doi.org/10.5772/intechopen.75460

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With regard to the treatment of effluents polluted with organic compounds, biological oxidation is the most economical process, but the presence of toxic or biorefractive molecules may prevent this approach [25]. Despite chemical oxidation using chlorine, ozone or hydrogen peroxide is currently used for the oxidation of biorefractive contaminants to harmless or biodegradable products; in some reactions, the intermediate products remain in the solution and may cause similar or higher toxicity than the initial compounds [25]. EOx can completely degrade many harmful organic pollutants before they reach the receiving environment [4]. EOx is based on in situ production of oxidants either directly at the surface of the electrodes or indirectly from the chemical compounds in the treated water during treatment [4]. EOx may occur either through direct oxidation with hydroxyl radicals produced on the anode surface or indirectly through oxidants such as chlorine, hypochlorous acid, and hypochlorite or hydrogen peroxide/ozone formed at electrodes [13]. The hydroxyl radicals can oxidize substantially all of the organic compounds with an oxidation rate 109 times higher than that of ozone [26]. The oxidation power of chlorine and other anode-formed oxidants remains low compared to the hydroxyl radicals and as a result it does not allow many pollutants to be effectively oxidized to carbon dioxide and water [13]. Electrode material selection is very important because it affects process selectivity and efficiency [12]. Electrode material must have the properties such as resistance to erosion, corrosion and formation of passivation layers, high electrical conductivity, selectivity and catalytic activity, low cost, and durability. Noble metal electrodes; metal alloys electrodes; mixed metal oxide electrodes which are also referred as dimensionally stable anodes (DSAs) such as Ti/Ta2O5- IrO2, Ti/SnO2-IrO2, Ti/RuO2-IrO2, Ti/Sb-SnO2, Ti/SnO2-Sb2O5-RuO2, and Ti/TiO2-IrO2; carbon and graphite electrodes; and boron-doped diamond (BDD) electrodes are some examples of the electrode materials used in recent wastewater treatment studies [27]. Strengths of EOx method can be summarized as follows: it requires simple equipment and operating conditions, totally mineralizes persistent organic pollutants, has low electrode maintenance cost, and forms disinfecting compounds. However, EOx method has some weaknesses such as: it is inefficient for the removal of suspended solids, and formation of an impermeable film

of persistent dissolved organic pollutants.

layer on the cathode may reduce the efficiency of the process.

their sequential and simultaneous combinations.

Since EC is a fast but incomplete process and EOx is a complete but slow process, coupling the two processes offers a practical hybrid [26]. EC has the ability to remove pollutants quickly but not completely. EOx is able to remove pollutants slowly but consistently. Sequential and simultaneous operations are combining the best abilities of EC and EOx processes with fast and complete pollutant removal [26]. This chapter focuses on effects of operating parameters on EC and EOx, pollutant removal mechanisms, and recent applications of these methods and

However, it is difficult to degrade the complex refractory organic pollutants in the wastewater by biological methods [7, 8]. In addition, physical–chemical methods are not always effective due to formation of additional pollution caused by the unreacted chemicals and difficulties of treatment of large amount of toxic sludge produced during conventional wastewater treatment [3–5, 9, 10]. With the strict environmental regulations on wastewater discharge, there is a need to develop efficient technologies and approaches at large scale for the treatment and management of industrial wastewater so that clean water quality can be maintained and amount increased while environmental protection and sustainability are achieved [1–5, 8, 11–13]. There has been an increasing interest in the use of electrochemical wastewater treatment technologies, such as electrodeposition [14], electrodisinfection [15], electro-Fenton [16], electrosorption [17], EOx [4] and EC [1] and their sequential [8, 18], and simultaneous [19, 20] combinations for the treatment of industrial wastewaters in recent years.

The foundations of electrochemical water treatment were laid in 1889 [11]. Electrocoagulation for drinking water was first carried out by Fred E. Stuart in 1946 [11, 21]. With the increasing interest in the second half of the twentieth century, many investigations were made on the topic [11]. Among others, EC and EOx methods are the electrochemical technologies that researchers are most interested in regarding water and wastewater treatment [2, 5, 11]. EC is based on the principle that coagulant species including hydroxide precipitates are produced in situ by electrolytic oxidation of the sacrificial anodic material, which is dissolved as ions by electric current applied through metal electrodes such as aluminum and iron [1, 22]. Thus, the EC method is more advantageous than the coagulation/flocculation method in which metal salts and polyelectrolytes are used as coagulants/flocculants in terms of sludge formation [23, 24]. EC aims to remove particles from the wastewater by destabilizing/neutralizing the repulsive forces that keep the particles suspended in the water [2]. When the repulsive forces are neutralized, suspended particles can be separated more easily from water by forming larger particles that can precipitate [2]. EC also provides the removal of pollutants by simultaneous cathodic reactions, either by deposition on the cathode or by flotation based on the formation of hydrogen gas at the cathode [1]. In recent years, EC was reported as an easy-to-operate, efficient, and economical method. Strengths of EC method can be summarized as follows; it requires simple equipment and operating conditions and does not require additional chemicals; treated water is colorless, odorless, and clear; the amount of the sludge formation is low which can be easily stabilized and dehydrated; compared with chemical coagulation, the effluent contains less total dissolved solids; and the gas bubbles produced in the cathode allow the pollutants to be separated easily by floating them to the surface. However, weaknesses of EC method can be listed as follows: sacrificial electrodes need to be replaced regularly, formation of an impermeable film layer on the cathode may reduce the efficiency of the process and it is inefficient for the removal of persistent dissolved organic pollutants.

produced in large quantities [4]. Wastewaters are composed of a complex mixture of different organic and inorganic compounds, which may be toxic and difficult to degrade [4]. The most common inorganic water pollutants are heavy metals, nitrates, sulfates, phosphates, fluorides, and chlorides, which have serious hazardous effects [3]. The organic pollutants found in wastewaters may be generalized as insecticides, herbicides, fungicides, polycyclic aromatic hydrocarbons, phenols, biphenyls, halogenated aromatic hydrocarbons, formaldehyde, detergents, oils, greases, normal hydrocarbons, alcohols, aldehydes, ketones, proteins, lignin, and pharmaceuticals [3]. These pollutants remain either in dissolved, in colloidal, or in suspended form [3]. Mostly, wastewater treatment is carried out by conventional wastewater treatment processes [5]. Conventional wastewater treatment technologies could be categorized as physical processes including screening, flotation, filtration, and sedimentation [6]; chemical processes such as coagulation/flocculation, chlorination, adsorption, and ion exchange [6]; biological processes, i.e., activated sludge, aerated lagoons, and membrane bioreactors [2].

However, it is difficult to degrade the complex refractory organic pollutants in the wastewater by biological methods [7, 8]. In addition, physical–chemical methods are not always effective due to formation of additional pollution caused by the unreacted chemicals and difficulties of treatment of large amount of toxic sludge produced during conventional wastewater treatment [3–5, 9, 10]. With the strict environmental regulations on wastewater discharge, there is a need to develop efficient technologies and approaches at large scale for the treatment and management of industrial wastewater so that clean water quality can be maintained and amount increased while environmental protection and sustainability are achieved [1–5, 8, 11–13]. There has been an increasing interest in the use of electrochemical wastewater treatment technologies, such as electrodeposition [14], electrodisinfection [15], electro-Fenton [16], electrosorption [17], EOx [4] and EC [1] and their sequential [8, 18], and simultaneous [19, 20] combinations for the

The foundations of electrochemical water treatment were laid in 1889 [11]. Electrocoagulation for drinking water was first carried out by Fred E. Stuart in 1946 [11, 21]. With the increasing interest in the second half of the twentieth century, many investigations were made on the topic [11]. Among others, EC and EOx methods are the electrochemical technologies that researchers are most interested in regarding water and wastewater treatment [2, 5, 11]. EC is based on the principle that coagulant species including hydroxide precipitates are produced in situ by electrolytic oxidation of the sacrificial anodic material, which is dissolved as ions by electric current applied through metal electrodes such as aluminum and iron [1, 22]. Thus, the EC method is more advantageous than the coagulation/flocculation method in which metal salts and polyelectrolytes are used as coagulants/flocculants in terms of sludge formation [23, 24]. EC aims to remove particles from the wastewater by destabilizing/neutralizing the repulsive forces that keep the particles suspended in the water [2]. When the repulsive forces are neutralized, suspended particles can be separated more easily from water by forming larger particles that can precipitate [2]. EC also provides the removal of pollutants by simultaneous cathodic reactions, either by deposition on the cathode or by flotation based on the formation of hydrogen gas at the cathode [1]. In recent years, EC was reported as an easy-to-operate, efficient, and economical method. Strengths of EC method can be summarized as follows; it requires simple equipment and operating conditions and does not require additional chemicals; treated water is

treatment of industrial wastewaters in recent years.

72 Wastewater and Water Quality

With regard to the treatment of effluents polluted with organic compounds, biological oxidation is the most economical process, but the presence of toxic or biorefractive molecules may prevent this approach [25]. Despite chemical oxidation using chlorine, ozone or hydrogen peroxide is currently used for the oxidation of biorefractive contaminants to harmless or biodegradable products; in some reactions, the intermediate products remain in the solution and may cause similar or higher toxicity than the initial compounds [25]. EOx can completely degrade many harmful organic pollutants before they reach the receiving environment [4]. EOx is based on in situ production of oxidants either directly at the surface of the electrodes or indirectly from the chemical compounds in the treated water during treatment [4]. EOx may occur either through direct oxidation with hydroxyl radicals produced on the anode surface or indirectly through oxidants such as chlorine, hypochlorous acid, and hypochlorite or hydrogen peroxide/ozone formed at electrodes [13]. The hydroxyl radicals can oxidize substantially all of the organic compounds with an oxidation rate 109 times higher than that of ozone [26]. The oxidation power of chlorine and other anode-formed oxidants remains low compared to the hydroxyl radicals and as a result it does not allow many pollutants to be effectively oxidized to carbon dioxide and water [13]. Electrode material selection is very important because it affects process selectivity and efficiency [12]. Electrode material must have the properties such as resistance to erosion, corrosion and formation of passivation layers, high electrical conductivity, selectivity and catalytic activity, low cost, and durability. Noble metal electrodes; metal alloys electrodes; mixed metal oxide electrodes which are also referred as dimensionally stable anodes (DSAs) such as Ti/Ta2O5- IrO2, Ti/SnO2-IrO2, Ti/RuO2-IrO2, Ti/Sb-SnO2, Ti/SnO2-Sb2O5-RuO2, and Ti/TiO2-IrO2; carbon and graphite electrodes; and boron-doped diamond (BDD) electrodes are some examples of the electrode materials used in recent wastewater treatment studies [27]. Strengths of EOx method can be summarized as follows: it requires simple equipment and operating conditions, totally mineralizes persistent organic pollutants, has low electrode maintenance cost, and forms disinfecting compounds. However, EOx method has some weaknesses such as: it is inefficient for the removal of suspended solids, and formation of an impermeable film layer on the cathode may reduce the efficiency of the process.

Since EC is a fast but incomplete process and EOx is a complete but slow process, coupling the two processes offers a practical hybrid [26]. EC has the ability to remove pollutants quickly but not completely. EOx is able to remove pollutants slowly but consistently. Sequential and simultaneous operations are combining the best abilities of EC and EOx processes with fast and complete pollutant removal [26]. This chapter focuses on effects of operating parameters on EC and EOx, pollutant removal mechanisms, and recent applications of these methods and their sequential and simultaneous combinations.
