Application of Advanced Oxidation Processes for Domestic and Industrial Wastewater Treatment

Abstract

Domestic and industrial wastewaters are complex matrices that contain a large variety of pollutants. Consequently, the conventional processes at wastewater treatment plants (WWTPs) cannot remove these. These pollutants remain in the effluent and are discharged into different environmental compartments worldwide, generating a range of negative impacts on the environment and human health. In this chapter, general features and the application of the most common advanced oxidation processes (AOPs) for the treatment of domestic and industrial wastewater are described. Also, the feasibility of scaling up advanced oxidation processes for pollutants removal (emerging and conventional) and the advantages and complications of each type of advanced oxidation process when applied to wastewater treatment (domestic and industrial) are shown.

1. Introduction

Most of the human activities (daily household activities, urban runoff, manufacturing, and service processes) generate wastewater [1], which is a very complex matrix due to the mixture of pathogens, heavy metals, organic matter, particulate solids, and other toxic substances. Due to this complexity of municipal and industrial wastewater, conventional wastewater treatment processes usually cannot provide high-quality treatment, so advanced oxidation processes are used [2].

Advanced oxidation processes are powerful techniques that can remove (a general term used in this chapter to define the mineralization, removal, transformation, or degradation of the conventional or emerging pollutants mentioned in the text) pathogens and harmful organic compounds in a liquid medium by generating hydroxyl radicals (HO^•) through various chemical pathways [3, 4]. Hydroxyl radicals (HO^•) have a high oxidative power amounting to 2.8 eV [5].

Among the various techniques of advanced oxidation processes employed to generate hydroxyl radicals (HO^•), photo-peroxidation (UV-H₂O₂), Fenton, photo-Fenton, indirect ozonation, photocatalysis, and sonolysis are the most common [6].

The main advantages of advanced oxidation processes over other methods are that they are entirely environmentally friendly, are carried out in situ, and do not produce large quantities of toxic wastes [7]. In addition, it has been shown that some advanced oxidation processes tend to increase the biodegradability of certain pollutants in the treated wastewater [8].

On the other hand, advanced oxidation processes nowadays have been used for the removal of persistent organic pollutants in wastewater, such as emerging pollutants [9, 10, 11], because the persistent or emerging pollutants are those that remain in the environment and can cause problems to human health [12].

Generally, advanced oxidation processes are coupled with tertiary processes to treat domestic wastewater to remove emerging pollutants in WWTPs that handle conventional processes since they cannot degrade or remove them [12, 13]. In contrast, in the industrial sector, advanced oxidation processes are used as the principal wastewater treatment [14, 15] since the industry generates more toxic waters [16] than the commonly called dump, which is the domestic wastewater found in dwellings.

However, the purpose of using advanced oxidation processes, either as tertiary or primary processes for domestic and industrial wastewater treatment, is to produce a high-quality effluent that can be reintroduced into the environment without adversely affecting flora, fauna, or human health.

Therefore, this chapter aims to outline, in general terms, the most common advanced oxidation processes for removing conventional and emerging pollutants from domestic (as a tertiary or coupled process) and industrial wastewater. It also discusses the advantages and disadvantages of each process and their potential for scale up.

2. Domestic and industrial wastewater

As water is used for most human activities, it becomes polluted, making it unsafe for human consumption, which must be treated to be returned to the environment in the best possible way [17]. In general, wastewater can be classified into domestic wastewater and industrial wastewater.

2.1 Domestic wastewater

Domestic wastewater is the water generated by human activities at home, including washing clothes, dishes, hands, body, vehicles, and household cleaning. Domestic wastewater can be categorized (Figure 1) as yellow (contains urine), brown (contains feces as well as cleaning water), black (contains urine, feces, and bacterial activity), and gray wastewater (contains water from cooking, laundry, and human cleaning). Domestic wastewater contains millions of human enteric bacteria and other microorganisms [18]. Like other types of wastewater (industrial wastewater), domestic wastewater may contain conventional pollutants or substances that affect its quality, such as organic carbon (organic matter), nitrogen-containing organic matter, inorganic matter, suspended and dissolved solids, microorganisms, and heavy metals [19].

Figure 1.

General classification of domestic wastewater.

The above-mentioned conventional pollutants, referring to the Mexican standard NOM-001-SEMARNAT-2021 (which establishes limits for conventional pollutants in wastewater), can be evaluated in domestic and industrial wastewater, for example, as chemical oxygen demand (COD), total suspended solids (TS), total organic carbon (TOC), and others [20].

2.2 Industrial wastewater

On the other hand, regarding industrial wastewater, different industries rely on water for their processes, producing wastewater. Some industries that generate wastewater from their processes include textile, paper, oil, pharmaceutical, and others (Figure 2) [21].

Figure 2.

Examples of types of industries that generate wastewater in their processes.

Industrial wastewater, unlike domestic, is very different in its pollutant load depending on the sector (based on the process being handled) that generates it (e.g., textile or paper industry). It is known that industrial wastewater usually contains a mixture of several pollutants, such as persistent organic pollutants, endocrine-disrupting chemicals, and many others [22].

Furthermore, another characteristic of industrial wastewater is that it is difficult to treat, and an individual analysis has to be made beforehand to determine a specific treatment based on what is required [23].

2.3 General characteristics of domestic and industrial wastewater

Domestic and industrial wastewater characteristics (conventional pollutants) can be divided into physical, chemical, organic, and biological [24], as seen in Figure 3.

Figure 3.

General characteristics of domestic and industrial wastewater.

The physical, chemical, organic, and biological characteristics (Figure 3) that may be present in domestic and industrial wastewater, depending on the regulations used in a given country, may be considered as conventional pollutants.

However, in recent decades, in domestic and industrial wastewater, not only the presence of conventional pollutants (general characteristics of wastewater) has been reported. Other types of pollutants, called emerging pollutants, have also been detected [25].

2.4 Emerging pollutants in wastewater

Emerging pollutants are natural or synthetic compounds that are not subject to legislation. Thus, they are not monitored in the environment and have the potential to cause damage to the environment and human health [26]. One of the characteristics of emerging pollutants is that they are commonly found and quantified in the environment in concentrations varying from ng L⁻¹ to μg L⁻¹ [27]. Emerging pollutants can be classified as personal care products, hormones, flame retardants, industrial additives, endocrine-disrupting chemicals, pharmaceuticals, nanomaterials, and pesticides. Due to their presence, the emerging pollutants can also convincingly diminish the water quality [28].

In addition, current research has shown that certain emerging pollutants can cause harm to humans, such as thyroid disruption, DNA damage, and liver metabolism impairment [29].

It should be noted that conventional wastewater treatment plants, which cannot remove emerging pollutants [30], through raw sewage and effluent generate the main entry of emerging pollutants into the water cycle. Figure 4 shows how emerging pollutants are widely distributed through the water cycle in different environmental compartments until they reach human consumption (e.g., drinking water). Emerging pollutants can reach human consumption through contaminated drinking water or the trophic chain. Many animals or crops can interact with emerging pollutants as they are distributed, as shown in Figure 4 [31].

Figure 4.

Distribution of emerging pollutants through the water cycle in the environment [31].

Therefore, it is essential to use methods such as advanced oxidation processes to eliminate emerging and conventional pollutants (general characteristics of wastewater) so that they cannot be distributed in the environment through the water cycle.

3. Advanced oxidation processes for the removal of conventional and emerging pollutants in domestic and industrial wastewater

Advanced oxidation processes were first proposed for drinking water treatment in the 1980s. Subsequently, they have been adopted by the scientific community for the treatment of various wastewater due to their oxidative capacity for removing organic matter [15].

Moreover, the advanced oxidation processes are defined as those that can be carried out at an ambient temperature and pressure and generate hydroxyl radicals (HO^•) as oxidative species for pollutant removal [15, 32, 33].

It is important to note that advanced oxidation processes require chemicals as raw materials and sometimes energy (e.g., UV radiation) to generate hydroxyl radicals (HO^•) [34], for example, Fenton (Fe²⁺/H₂O₂) and photo-Fenton (Fe²⁺/H₂O₂/UV radiation). In general, a catalyst (e.g., Fe²⁺) and an oxidant (e.g., H₂O₂) are the chemicals used to perform advanced oxidation processes to generate hydroxyl radicals (HO^•) [35].

Hydroxyl radicals (HO^•) are non-selective radicals that can remove pollutants through mechanisms such as electron transfer, hydrogen abstraction, and radical addition [34].

Consequently, various research projects have continuously proposed advanced oxidation processes to remove difficult-to-remove organic pollutants (emerging and conventional) from domestic and industrial wastewater [32]. It has been shown that advanced oxidation processes entirely or partially remove the target pollutants (emerging and conventional pollutants) and do not only concentrate them in one phase or transfer them to another phase compared to other treatment processes (e.g., coagulation, flocculation, and membranes) of domestic and industrial wastewater [34].

Therefore, the following subsections describe the most common advanced oxidation processes for wastewater treatment. These are photo-peroxidation (UV-H₂O₂), Fenton, photo-Fenton, indirect ozonation, photocatalysis, and sonolysis [6]. The advantages and disadvantages of their application, as well as their potential scale up, are also discussed.

3.1 Photo-peroxidation (UV-H₂O₂) process

To effect the photo-peroxidation (UV-H₂O₂) process, hydrogen peroxide (H₂O₂) must be photolyzed by UV radiation at wavelengths between 200 and 300 nm, causing homolytic fission of the O-O bond belonging to the hydrogen peroxide (H₂O₂) molecule and hydroxyl radicals (HO^•) are produced as shown in Eq. (1), as well as other oxidative species such as perhydroxyl radicals (HO₂^•), as shown in Eq. (2) [36, 37].

$${\text{H}}_2{\text{O}}_2+hv\to 2{\text{HO}}^{•}$$E1

$${\text{HO}}^{•}+{\text{H}}_2{\text{O}}_2\to {\text{H}}_2\text{O}+{\text{HO}}_2^{•}$$E2

The reaction rate for the generation of hydroxyl radicals (HO^•) is faster at pH > 10 (alkaline medium), so the process must be carried out in alkaline wastewater for the removal of the target pollutants (conventional and emerging pollutants). This, in turn, can be a disadvantage since chemicals for pH adjustment in certain types of wastewaters must be used as inputs before performing this advanced oxidation process. Another drawback of photo-peroxidation (UV-H₂O₂) for wastewater treatment is that the molar absorption of hydrogen peroxide (H₂O₂) in the UV region is insufficient, so high concentrations of hydrogen peroxide (H₂O₂) must be used for the removal of the target pollutants (conventional and emerging pollutants) [36].

In summary, the photo-peroxidation (UV-H₂O₂) process is dependent on the initial concentration of hydrogen peroxide (H₂O₂), the intensity of the UV light, the pH of the wastewater, and the constituents contained in the wastewater [38].

Regarding the removal of conventional pollutants, one example is the research of Philippopoulos and Poulopoulos [39] where the photo-peroxidation (UV-H₂O₂) process was applied for the oil industry wastewater treatment. This work shows that low percentages of chemical oxygen demand (COD) removal are obtained in the treated wastewater using this oxidation process, since the maximum removal percentage obtained was 45%, at a hydraulic retention time of 150 minutes, with an initial hydrogen peroxide (H₂O₂) concentration of 6660 mg L⁻¹. However, the removal of some pollutants present in the treated wastewater, such as isobutane, n-pentanol, phenol, o-cresol, and m-cresol amounted to almost 100%, although there was a low removal of ethylene glycol of almost 20%.

Similarly, research by Della Roca et al. [40] showed that the percentages of total organic carbon (TOC) removal provided by the photo-peroxidation (UV-H₂O₂) method are very low since only approximately 5% of the total organic carbon (conventional pollutant) was removed.

Also, it has been noted in several investigations that the scaling of reactors to carry out photocatalytic processes belonging to advanced oxidation processes (e.g., photo-peroxidation (UV-H₂O₂)) is still a challenge, due to complications in the light source, as a linear model is usually used and has many drawbacks despite its widespread use in several reactors, which makes it a challenge at present [41].

Then, based on the information gathered, the photo-peroxidation (UV-H₂O₂) process tends to have diverse complications, making it a problematic advanced oxidation process to scale up. Furthermore, the process has low removal yields for conventional pollutants such as chemical oxygen demand (COD) and total organic carbon (TOC).

3.2 Fenton process

Meanwhile, the advanced oxidation process, called Fenton, was reported in 1894 by Henry John Horstman Fenton [42], when one of his students found that a mixture of hydrogen peroxide (H₂O₂), iron salts, and tartaric acid dyeing an orange color to the water or solution containing them. This led to the discovery of the Fenton reaction shown in Eq. (3) [43].

Eq. (3) shows how the catalyst reaction (Fe²⁺) with the oxidizing agent (H₂O₂) generates hydroxyl radicals (HO^•) as the main product, and ferric ions (Fe³⁺), among other species, as by-products [44, 45]. The orange color of the water, when Fenton’s student performed the above experiment, is now attributed to the ferric ions (Fe³⁺) generated as by-products in Eq. (3) [46].

$$F{e}^{2+}+{\text{H}}_2{\text{O}}_2\to F{e}^{3+}+{\text{OH}}^{-}+{\text{HO}}^{•}$$E3

Currently, it has been reported utilizing Eq. (4) that the ferric ions (Fe³⁺), if there is any remaining hydrogen peroxide (H₂O₂), can react with the hydrogen peroxide (H₂O₂) and subsequently generate other oxidative species such as perhydroxyl radicals (HO₂^•) [44, 45].

$$F{e}^{3+}+{\text{H}}_2{\text{O}}_2\to F{e}^{2+}+{\text{H}}^{+}+{\text{HO}}_2^{•}$$E4

Nevertheless, it is necessary to say that to carry out the Fenton reaction (Eq. (3)) and generate the hydroxyl radicals (HO^•) (oxidative species of interest) for the treatment of domestic and industrial wastewater, an acidic pH in the aqueous medium to be treated of about pH 3 must be maintained [44, 46].

The compound commonly used in several studies as a supplier of ferrous ions (Fe²⁺) for the Fenton process is ferrous sulfate heptahydrate (FeSO₄•7H₂O) [46, 47, 48].

Eq. (3), Eq. (5), and Eq. (6) have been used for the dosing of both the oxidant (H₂O₂) and the supplier catalyst (FeSO₄•7H₂O) to perform the Fenton process and to treat wastewater, either industrial or domestic [31, 46].

$$\text{Quantity}\left(mg\right){\text{H}}_2{\text{O}}_2=\frac{17}{8}\left[COD\text{, }mg{\text{L}}^{-1}\right]\left(\text{V}\left(\text{L}\right)\right)$$E5

$$\text{Quantity}\left(mg\right)FeS{\text{O}}_4•7{\text{H}}_2\text{O}=\left(278,010\right)\left(\text{nmol}F{e}^{2+}\right)$$E6

The dosages of hydrogen peroxide (H₂O₂) and ferrous sulfate heptahydrate (Fe²⁺ supplier) for domestic and industrial wastewater treatment have been made according to the initial value of the wastewater’s total Chemical Oxygen Demand (COD) because hydroxyl radicals (HO^•) are non-selective oxidants, and if, for example, they were calculated according to the amount of conventional or emerging pollutants of interest to be eliminated, these quantities would be underestimated, since there is a large number of organic pollutants in the wastewater that could interfere with its removal due to the nature of the hydroxyl radicals (HO^•). Therefore, they are generally included in the total chemical oxygen demand (COD) determined for the wastewater.

In Eq. (5), the total chemical oxygen demand (COD) of the wastewater to be treated is calculated in mg L⁻¹ and the volume of wastewater to be treated in L. In Eq. (6), the moles of ferrous ion (Fe²⁺) are calculated by means of the (H₂O₂) 1:1 (Fe²⁺) molar ratio shown for the Fenton reaction in Eq. (3).

On the other hand, the Fenton process has shown good performance in treating domestic and industrial wastewater, as several studies have demonstrated, it removes conventional and emerging pollutants such as total suspended solids (TSS), chemical oxygen demand (COD), pathogens (e.g., fecal coliforms), tartrazine, tetrachloromethane, azo dyes, orange II, acetylsalicylic acid, tetracycline, 2,4-dichlorophene, bisphenol A, estrogens, pesticides, and so on [46, 49]. In addition, the Fenton process has been used and demonstrated its feasibility as a tertiary process at the laboratory level to treat and remove emerging pollutants from a conventional WWTP effluent from a secondary activated sludge process [46]. This demonstrates the viability of using this process for domestic and industrial wastewater treatment.

However, the Fenton process drawbacks are related to the scale up associated with the implementation costs [50] and to the fact that the process must be divided into two stages: one to adjust the wastewater to be treated to the acidic pH (pH 3) and add the Fenton reagents (Fe²⁺ and H₂O₂); and the other to increase the pH to neutral (pH 7) and separate the ferric ions (Fe³⁺ and orange color of wastewater) generated as by-products process from the effluent produced by sedimentation processes [46]. It is noteworthy to say that Fenton’s advanced oxidation process may involve additional costs, due to the amount of acid required to be added to the wastewater to be treated to adjust the pH 3 (ideal pH level to manage the removal of pollutants), since if the pH of the wastewater to be treated is not adjusted to 3 beforehand, the removal effectiveness of the Fenton process of the conventional and emerging pollutants of interest may be affected.

Considering what was mentioned above, Fenton’s advanced oxidation process shows the capacity to remove several conventional and emerging pollutants. Nevertheless, there are still areas of opportunity to improve and scale it up to treat domestic and industrial wastewater.

3.2.1 Photo-Fenton process

The photo-Fenton process is a modified version of the traditional Fenton process where an additional energy source, such as UV radiation, is added. The Fenton reaction is identified as a powerful reaction when UV radiation is incorporated into Fenton’s advanced oxidation process for the removal of organic pollutants. Several studies have shown that the UV radiation used in the Fenton process increases the removal of organic pollutants commonly achieved by the traditional Fenton process [51].

The UV radiation added to the traditional Fenton process generates a photoreduction of ferric ions (Fe³⁺) to ferrous ions (Fe²⁺) as shown in Eq. (7), where hydroxyl radicals (HO^•) are generated as compared to the Fenton process which generates perhydroxyl radicals (HO₂^•) (Eq. (4)) [52]. The reaction shown in Eq. (7), which is generated by UV radiation in the photo-Fenton process, significantly increases the removal efficiency of organic pollutants because the reaction of Eq. (7) is added to the traditional Fenton reaction (Eq. (3)), which is also carried out at the same time in this process.

$$F{e}^{3+}+{\text{H}}_2\text{O}+hv\to F{e}^{2+}+{\text{H}}^{+}+{\text{HO}}^{•}$$E7

The pH handled in the advanced oxidation process, photo-Fenton, for the treatment of domestic and industrial wastewater, is the same pH 3 commonly used in the traditional Fenton process, and also the catalyst (Fe²⁺) supplier used is ferrous sulfate heptahydrate (FeSO₄•7H₂O) [53, 54].

For the dosing of hydrogen peroxide (H₂O₂) and ferrous sulfate heptahydrate (FeSO₄•7H₂O), Eq. (3), Eq. (5), and Eq. (6) can also be used in the same way in the photo-Fenton process.

The photo-Fenton process has been proposed as a tertiary treatment of domestic wastewater to remove persistent organic pollutants, such as emerging pollutants [55].

The photo-Fenton process has demonstrated its ability in research to remove various recalcitrant pollutants (e.g., emerging pollutants) in domestic and industrial wastewater of different types (e.g., oil refinery wastewater, tea industry wastewater, livestock wastewater, among others). Examples of good removal of emerging pollutants by the photo-Fenton process have included polymers, pesticides, reactive dyes, penicillin, 2-chlorophenol, and others [56]. This shows, as an advantage, the remarkable capacity of the photo-Fenton process to treat different types of wastewater and pollutants, as well as the traditional Fenton process.

Furthermore, another advantage that can be included is that the photo-Fenton process can be carried out with sunlight to treat wastewater (domestic and industrial) [57, 58], compared to other advanced oxidation processes that employ lamps that work at specific wavelengths.

Nonetheless, the complications presented by the photo-Fenton advanced oxidation process are the same as those presented by the Fenton traditional advanced oxidation process shown in subsection 3.2 of this chapter.

Finally, another problem that arises, and which is a great challenge, when wanting to scale up the advanced oxidation process, photo-Fenton, if it is desired to use sunlight, is that the reactor must be made of a translucent material so that it captures as much sunlight as possible to maximize the performance of the photo-Fenton reaction. So, despite the excellent development of the photo-Fenton process and its good performance in wastewater treatment, some challenges remain.

This chapter emphasizes only the process of homogeneous Fenton and photo-Fenton.

3.3 Indirect ozonation

Indirect ozonation, for its part, is usually better than direct ozonation because direct ozonation is very selective. Indirect ozonation produces hydroxyl radicals (HO^•) by decomposition of the ozone molecule (O₃) [59]. To carry out indirect ozonation in the wastewater to be treated, pH > 8 must be managed [59, 60, 61].

In this type of advanced oxidation process, an ozone (O₃) generator has to be considered [62], which can complicate the applicability and scalability of the process.

Another complication of indirect ozonation is that the ozone (O₃) dosages to treat domestic and industrial wastewater must be defined in the laboratory beforehand. Moreover, the number of ozone (O₃) generators used to treat the wastewater of interest is defined according to the volume to be treated, the ozone (O₃) generation capacity of the equipment used (generator), and the ozone (O₃) dosages previously defined in the tests carried out at laboratory level to treat the wastewater of interest.

Nevertheless, the advanced oxidation process, indirect ozonation, according to the information gathered, is inefficient for removing persistent pollutants (e.g., some emerging pollutants) [63].

Given the above information, the advanced oxidation process, indirect ozonation, presents problems such as applicability, scalability, and difficulty in removing persistent pollutants (e.g., emergency pollutants) from the wastewater (domestic and industrial) to be treated, which makes it an ineffective method.

3.4 Photocatalysis

Instead, photocatalysis is a series of chemical reactions that require light to carry out and it is difficult in most cases to perform them in the dark [64]. It should be noted that the most important energy source in advanced oxidation processes, photocatalytic, to which it owes part of its name, is light [65].

Although the reactions in photocatalysis vary per process for wastewater treatment, the general process that occurs in photocatalysis in the different processes can be specified globally in four steps [66]:

1. Absorption of light to generate electron–hole pairs.

2. Separation of excited charges.

3. Transfer of holes and electrons to the surface of the photocatalyst.

4. And finally, the use of charges to carry out redox reactions to remove pollutants (conventional and emerging) from interest.

The photocatalysts commonly used in these advanced oxidation processes are usually solids [67].

The dosages of the photocatalyst, light radiation, and pH to be handled in the advanced photocatalytic oxidation processes are previously established based on laboratory or pilot tests with the wastewater (domestic or industrial) to be treated. It is worth mentioning that radiation, pH, and photocatalyst dosages vary according to the nature and composition of the wastewater to be treated.

Nonetheless, although advanced photocatalysis oxidation processes have demonstrated good capacity to remove emerging pollutants, there are still many challenges to improve the processes, such as the separation of the photocatalyst from the actual treated wastewater for reuse and disposal, the development of photocatalytic materials, and the lack of real-world application of these photocatalysis processes for wastewater treatment [67].

To conclude, advanced photocatalysis oxidation processes still need to be further investigated and scaled up to real-world applications despite providing good pollutants removal in treated wastewater.

Note that due to the large number of photocatalysis processes, only a general description is given in this subsection of the present document.

3.5 Sonolysis

Sonolysis is the advanced oxidation process that uses ultrasound as the main tool to produce hydroxyl radicals (HO^•) in an aqueous medium (wastewater) to remove different types of conventional and emerging pollutants [68]. To be considered ultrasound, the waves generated must have a frequency greater than 20 kHz [69].

The high acoustic intensity generated by the ultrasound radiates to the liquids (e.g., domestic and industrial wastewater) and produces cavitation [70].

The general principle of producing hydroxyl radicals (HO^•) in sonolysis to treat wastewater of interest (domestic and industrial) is divided into the following steps: first, sound energy and dissolved bubbles lead to the growth of bubbles near adiabatic collapse, better known as cavitation, then cavitation generates extreme temperature and pressure conditions without bubbles collapsing, and finally, they reach a violent collapse.

In the wastewater to be treated, cavitation is the process that produces the hydroxyl radicals (HO^•), which are the oxidative species of interest that are used to remove the conventional and emerging pollutants of interest [68].

A drawback of the sonolysis process for the treatment of domestic and industrial wastewater is that if the wastewater to be treated contains a high load of solid particles (e.g., total suspended solids), these can dissipate the ultrasonic waves and weaken the dissipation energy of the process, decreasing the removal efficiency of conventional or emerging pollutants of importance. This indicates that wastewater with a high solids load can only be treated directly by sonolysis after first undergoing a solids removal process. Other parameters or factors that affect the removal provided by the sonolysis process are the frequency with which the ultrasonic waves are generated, the temperature of the wastewater to be treated, and the properties of the pollutants, whether conventional or emerging, to be removed [71].

In addition, the concentration of volatile compounds in the wastewater to be treated affects the concentration of cavitation bubbles and thus decreases the removal of pollutants provided by the process [72].

However, given all the complications presented by the sonolysis, which interferes with the production of hydroxyl radicals (HO^•) for the removal of conventional and emerging pollutants in domestic and industrial wastewater, it is suggested to use it as a tertiary or polishing process wastewater treatment because if the raw wastewater presents high loads of solids or volatile compounds, the removal capacity can be affected. It should be mentioned that the removal of conventional and emerging pollutants of interest by sonolysis is usually slow [73].

4. Overview and suggestions to carry out the advanced oxidation processes

Certain constituents present in the wastewater (domestic and industrial) to be treated can also inhibit advanced oxidation processes. Chloride (Cl⁻) and bromide (Br⁻) can act as scavengers of hydroxyl radicals (HO^•) [44] and, to some extent, inhibit the removal of pollutants in wastewater (conventional or emerging) provided such processes. So, for example, depending on the type of chlorination used in a conventional WWTP for water treatment and disinfection, an advanced oxidation process cannot be used as a tertiary or quaternary process because chlorine can act as a scavenger of the hydroxyl radicals (HO^•) and inhibit the advanced oxidation process to some extent. It should be emphasized that it is necessary to identify in which form the chlorine (e.g., chloride) is added for treating wastewater (domestic and industrial).

As a suggestion, if the wastewater (domestic and industrial) to be treated is highly loaded with the aforementioned anions (Cl⁻ and Br⁻), additional processes for their removal can be contemplated and combined with the advanced oxidation processes, but this would increase the cost of treating the wastewater (domestic and industrial) to be treated considerably.

Additionally, it is not advisable to use hydrochloric acid (HCl) in advanced oxidation processes that require pH adjustment to acidic levels of the wastewater (domestic and industrial) to be treated, since its nature and its dissociation into chlorides (Cl⁻) and protons (H⁺) in the wastewater could affect the removal of pollutants (conventional and emerging) of interest, because indirectly through the addition of hydrochloric acid (HCl), chlorides (Cl⁻) would be added to the wastewater (domestic and industrial).

As a final part, to define the amounts of oxidant to be added to the wastewater (domestic and industrial) to be treated by the advanced oxidation processes that employ hydrogen peroxide (H₂O₂) as the oxidant, as well as the Fenton and photo-Fenton processes, Eq. (5) reported in the present chapter can be used for that purpose. The amounts of catalyst can be established stoichiometrically using the reaction of the catalyst with the oxidant (H₂O₂), which it presents in the advanced oxidation process to be employed, in particular, for the generation of the hydroxyl radicals (HO^•) for the treatment of wastewater (domestic and industrial). An example of this is in subsection 3.2 relating to the Fenton process.

5. Conclusions

Because poorly treated wastewater is the main entry point for conventional and emerging pollutants into the water cycle and the environment, and it can have both indirect and direct effects on flora, fauna, and human health, processes such as advanced oxidation must be used to treat domestic and industrial wastewater, either as tertiary or primary processes.

On the other hand, among the advanced oxidation processes discussed in this chapter, Fenton’s advanced oxidation process and its variant (photo-Fenton) are, according to the information gathered, the best option for the treatment of domestic and industrial wastewater and the removal of conventional and emerging pollutants from wastewater. In addition, compared to the other advanced oxidation processes mentioned earlier in this chapter, the Fenton processes have the least difficulty scaling up. Photocatalysis, indirect ozonation, and cavitation are the advanced oxidation processes that lack scalability and should be further developed.

Finally, there is a need for further improvement of advanced oxidation processes to increase their applicability to the treatment of domestic and industrial wastewater at the macro level and to improve the quality of treated domestic and industrial wastewater by reducing the presence of conventional and emerging pollutants.

Acknowledgments

This research was funded by the National Council of Humanities, Science and Technology (CONAHCYT), with scholarship grant number 805457 and project number CF-2019/102967. Acknowledges to Dagoberto Rodríguez-Ortiz from CIMAV-DGO for his valuable technical support and collaboration.

Conflict of interest

The authors declare no conflict of interest.

Notes/thanks/other declarations

Firstly, thanks to my God and my family for their support and understanding. To Dr. Liliana Reynoso Cuevas (thesis director and mentor) for her support and advice to carry out my research project and different publications at national and international levels. This work is dedicated to your grandmother, who was like a mother to me (Beatriz Ruelas).