4. Combined electrocoagulation and electrooxidation mechanism and applications

#### 4.1. Mechanism

As stated in Section 1, EC is a fast but incomplete process, while EOx is a complete but slow process; coupling the two processes proposes a functional hybrid. COD removal mechanism of a combined EC and EOx process can be summarized as follows: production of metal ions; hydrolysis of metal ions to generate metal hydroxides and/or polyhydroxides in appropriate pH; generation of bubbles of oxygen and hydrogen gases at the anode and the cathode, respectively, which provides the flotation of coagulated pollutants; destabilization of colloidal pollutants and trapping of destabilized colloidal particles to form flocks; removal of pollutants by precipitation [46]; formation of adsorbed hydroxyl radicals on anode surface [39]; degradation of organic pollutants by direct oxidation of adsorbed hydroxyl radicals; generation of indirect oxidizing agents such as chlorine, hypochlorite, and hydrogen peroxide near the anode; and electrochemical combustion of organic pollutants by indirect oxidizing agents [26].

#### 4.2. Applications of combined electrocoagulation and electrooxidation

In the recent years, application of combined treatment has been successfully achieved in the removal of different kinds of pollutants either by sequential EOx treatment of the electrocoagulated Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters http://dx.doi.org/10.5772/intechopen.75460 81


treatment. By evaluating the previously published studies, it has been concluded that EOx is a remarkable treatment method in terms of organic matter, turbidity, color, phenol, total

Current density (mA/cm<sup>2</sup>

Current density (mA/cm<sup>2</sup>

Anode material Ti/PbO2,

ranges for parameters

Ti/RuO2, Ti/ RuO2-IrO2

) 2–15 5

Optimum conditions

) 1.56–6.25 6.25 TOC removal:

Results at optimum conditions

80% SEEC: 2.34 kWh/g TOC

60 mg/L Color: 20 CU SEEC: 4.12 kWh/ton Operating cost: 0.57 \$/ton

Ti/PbO2 COD:

4. Combined electrocoagulation and electrooxidation mechanism and

Table 2. EOx treatment results of various wastewaters for selected examples in the literature.

As stated in Section 1, EC is a fast but incomplete process, while EOx is a complete but slow process; coupling the two processes proposes a functional hybrid. COD removal mechanism of a combined EC and EOx process can be summarized as follows: production of metal ions; hydrolysis of metal ions to generate metal hydroxides and/or polyhydroxides in appropriate pH; generation of bubbles of oxygen and hydrogen gases at the anode and the cathode, respectively, which provides the flotation of coagulated pollutants; destabilization of colloidal pollutants and trapping of destabilized colloidal particles to form flocks; removal of pollutants by precipitation [46]; formation of adsorbed hydroxyl radicals on anode surface [39]; degradation of organic pollutants by direct oxidation of adsorbed hydroxyl radicals; generation of indirect oxidizing agents such as chlorine, hypochlorite, and hydrogen peroxide near the anode; and electrochemical combustion of organic pollutants by indirect oxidizing agents [26].

In the recent years, application of combined treatment has been successfully achieved in the removal of different kinds of pollutants either by sequential EOx treatment of the electrocoagulated

4.2. Applications of combined electrocoagulation and electrooxidation

Kjeldahl nitrogen (TKN), and O&G removal efficiencies.

Ref. Wastewater Electrode material Parameters Research

applications

[44] Pharmaceutical (synthetic)

80 Wastewater and Water Quality

[45] Industrial park sewage (real)

Anode: BDD Cathode: Stainless steel

Anode:

Cathode: Mesh-structure Ti

plate

Mesh-structure plate

4.1. Mechanism


Table 3. Simultaneous EC-EOx treatment results of various wastewaters for selected examples in the literature.

effluent or simultaneous EC and EOx treatment with an appropriate electrode configuration. The aim of sequential EC-EOx is to propose a new approach that combines the synergistic effects of the two methods [47]. Simultaneous EC-EOx offers a hybrid system integrating monopolar (MP) EOx and bipolar (BP) EC in a single reactor that also provides all the advantages of sequential EC-EOx [19]. Recent studies on combined EC-EOx treatment of different kinds of wastewater are presented in Tables 3 and 4.


5. Conclusion

Ref. Wastewater Electrode

[52] Cardboard (real) EC:

Dairy (real)

[53] Soft drink (real)

[54] Soluble coffee industrial plant (real)

material

EC: Al

EOx: platinized titanium

Al

EOx: Anode: BDD Cathode: Stainless steel

EC: Copper

EOx: Anode: Niobium substrate BDD Cathode: Copper

EC: Anode: Al Cathode: Graphite

EOx: Anode: Graphite Cathode: Al

Parameters Optimum

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters

Operating time (min) 6

Operating time (min) 15

Operating time (min) 60 Initial pH 7

Operating time (min) 180 Initial pH 7.2 Na2SO4 concentration (g/L) 5

Current density (A/dm<sup>2</sup>

Current density (mA/cm<sup>2</sup>

Current density (mA/cm<sup>2</sup>

Current density (mA/cm<sup>2</sup>

Current density (A/m<sup>2</sup>

Current density (A/m<sup>2</sup>

Current density (A/m<sup>2</sup>

Initial pH 8 Time (min) 20

pH 6 Time (h) 6

Initial pH 7.98 Treatment time (min) 62

Treatment time (min) 53

conditions

) 1.4

) 100

) 30

) 500

Results at optimum conditions

83

Turbidity removal:

100% Color removal: 90.4% COD removal: 66.4%

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

) 7.5 Turbidity removal: 98.7% TOC removal: 86.4% COD removal: 85.3%

) 51 COD removal:

) 149.2 Color removal:

85% TOC removal: 75% TP removal: 89% Total nitrogen: 84% Color removal: 100%

100% COD removal: 89% TOC removal: 72% SEEC: 45.28 kWh/m<sup>3</sup> BOD5/COD: 0.6

Due to the increasing need for freshwater resources and the large number of pollutants produced by the industries, it becomes more and more important to treat and reuse wastewater. For this purpose, effective, economical, and easy-to-operate wastewater treatment technologies need to be developed. Because of the fact that electrochemical treatment methods have various

Table 4. Sequential EC-EOx treatment results of various wastewaters for selected examples in the literature.

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters http://dx.doi.org/10.5772/intechopen.75460 83


Table 4. Sequential EC-EOx treatment results of various wastewaters for selected examples in the literature.
