**1. Introduction, definitions and concepts**

Water is an important renewable natural resource; however, it is also reusable. For humans, its employment becomes a problem due to demographic growth and its application in agricultural and industrial enterprises. Thus, the limited availability of water for an eventual reuse appears to be the unique solution. In the past few years, wastewater treatment is the adopted solution in the majority of countries. Traditionally, industrial effluents are frequently contaminated by

© 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 reproduction in any medium, provided the original work is properly cited. © 2017 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 reproduction in any medium, provided the original work is properly cited.

hazardous organic substances, such as phenolic compounds and aromatic intermediates, as well as other halogenated or volatile organic substances, metals (mainly, Sb, Cu, Pb, Zn, Cd, Cr, Ni, Hg) and other chemical species, such as cyanide (CN− ), benzene and chloroform [1]. The presence of these compounds in wastewater and drinking water, due to their toxicity, become a serious problem affecting the ecosystem and causing, for example, health problems [2].

The phenolic compounds are harmful to human health, causing necrosis, digestive problems, and liver and kidney damages. The presence of phenols in drinking water may cause serious public health problems, and the death of fishes, even at concentrations in the range of 1 mg L−1. At concentrations of less than 1 mg L−1 (ppm), they are also toxic to other biological species and destroy the aquatic environment [3].

Phenolic substances are widely employed as industrial chemicals for wood preservation; in petroleum refineries and petrochemical plants; coke gasifiers; in manufacturing of pulp and paper; pharmaceutical plants; food industry; minerals; plastics; metals and organic chemical plants, as well as in agricultural activities as pesticides. However, several research studies indicated that some of these phenolic organic substances are recalcitrant and persistent in treated water as they are refractory to conventional treatment [4, 5]. The most detected organic pollutants in those wastewater are presented in **Table 1** [1].

Phenols are compounds derived from aromatic hydrocarbons by replacing hydrogen atoms with hydroxyls. These compounds are generally solids and are obtained from coal tar distillation and heating of chloro-benzene with water [6]. Phenols can be classified according to the number of hydroxyls into monophenols, diphenols and triphenols. Phenol is less volatile than water and sparingly soluble in it, since the phenol-water system forms an azeotrope to 9.2% by mass of phenol [7].

The toxicity of these organic substances contaminants in different water bodies, including wastewater, surface water, groundwater and drinking water, at environmental levels of mg L−1


**Table 1.** The main phenolic pollutants more detected in effluents.

significantly affect the organoleptic properties of water [8]. Resolution 430 of the Conselho Nacional do Meio Ambiente—CONAMA (National Council on the Environmental), Brazil set a maximum of total phenols concentration at 0.5 mg L−1 for all effluents originating from any polluting source that can be disposed of in water bodies as of 13 May 2011 [9].

hazardous organic substances, such as phenolic compounds and aromatic intermediates, as well as other halogenated or volatile organic substances, metals (mainly, Sb, Cu, Pb, Zn, Cd,

presence of these compounds in wastewater and drinking water, due to their toxicity, become a

The phenolic compounds are harmful to human health, causing necrosis, digestive problems, and liver and kidney damages. The presence of phenols in drinking water may cause serious public health problems, and the death of fishes, even at concentrations in the range of 1 mg L−1. At concentrations of less than 1 mg L−1 (ppm), they are also toxic to other biological species and

Phenolic substances are widely employed as industrial chemicals for wood preservation; in petroleum refineries and petrochemical plants; coke gasifiers; in manufacturing of pulp and paper; pharmaceutical plants; food industry; minerals; plastics; metals and organic chemical plants, as well as in agricultural activities as pesticides. However, several research studies indicated that some of these phenolic organic substances are recalcitrant and persistent in treated water as they are refractory to conventional treatment [4, 5]. The most detected

Phenols are compounds derived from aromatic hydrocarbons by replacing hydrogen atoms with hydroxyls. These compounds are generally solids and are obtained from coal tar distillation and heating of chloro-benzene with water [6]. Phenols can be classified according to the number of hydroxyls into monophenols, diphenols and triphenols. Phenol is less volatile than water and sparingly soluble in it, since the phenol-water system forms an azeotrope to 9.2%

The toxicity of these organic substances contaminants in different water bodies, including wastewater, surface water, groundwater and drinking water, at environmental levels of mg L−1

OH) 2-Chloro-phenol; 2,4-di-chloro-phenol; 2,6-di-chloro-

O) 2-Methyl-phenol; 3-methyl-phenol; 4-methyl-phenol; 2,4-di-methyl-phenol

2,3,4,5-tetra-chloro-phenol;

penta-chloro-phenol

) 2-Nitro-phenol; 4-nitro-phenol; 2,4-di-nitro-phenol

phenol; 2,4,5-tri-chloro-phenol; 2,4,6-tri-chloro-phenol;

2,3,4,6-Tetra-chloro-phenol; 2,3,5,6-tetra-chloro-phenol;

2-methyl-4,6-di-nitro-phenol; 2-cycle-hexyl-4,6-di-nitro-phenol;

2-sec-butyl-4,6-di-nitro-phenol (Dinoseb)

serious problem affecting the ecosystem and causing, for example, health problems [2].

), benzene and chloroform [1]. The

Cr, Ni, Hg) and other chemical species, such as cyanide (CN−

324 Phenolic Compounds - Natural Sources, Importance and Applications

organic pollutants in those wastewater are presented in **Table 1** [1].

**Groups Phenolic compounds**

NO) 4-Amino-phenol

Others 2-Chloro-3-methyl-phenol; 4-chloro-3-methyl-phenol;

destroy the aquatic environment [3].

by mass of phenol [7].

Phenol (C6

Cresols (C7

Nitrophenols (C6

Aminophenols (C6

H5

H8

H5 NO3

H7

**Table 1.** The main phenolic pollutants more detected in effluents.

According to literature review, phenolic and petroleum wastewater are recalcitrant compounds. Wastewater is generally characterized by the biological oxygen demand (BOD), chemical oxygen demand (COD), pH and total organic carbon (TOC) [10]. Petroleum wastewater contains a COD range from 850 to 1020 mg L−1, a BOD range from 570 mg L−1, a TOC range from 300 to 440 mg L−1 and a pH range from 8 to 8.2, showing that it contains large amounts of non-biodegradable organic matter [11]. Petro-chemical wastewater contains an initial COD range of 300–600 mg L−1, a BOD range of 150–360 mg L−1 and a pH range of 7–8 [12]. Conventional treatment of these substances is difficult because biologically resistant organic compounds do not induce oxygen depletion in receiving water [13].

Several biological and chemical methods have shown a low efficiency to degrade the contaminants completely. As biological and chemical methods degrade only up to 60% of the recalcitrant components and in addition they require larger operation area and more chemical processes to reduce the sludge [14].

There are a number of studies of industrial effluents treatments via conventional methods that are combined with chemical, biological and physical methods and also advanced oxidation processes (AOPs), shared with reactors for complete degradation of highly recalcitrant industrial wastewater. Section 2 discusses the AOP theory and the possibility of enhancement in reactor performance when implemented in the processes.
