**Abstract**

Various persistent organic pollutants (POPs) are increasingly being detected in numerous environmental matrices, including water. Even though there are currently some technologies for the elimination of these pollutants, it is necessary to evaluate their advantages, disadvantages, process time, and cost to find the optimal treatment depending on the characteristics of the pollutants and the matrix to remediate. This work was carried out to compare phase change technologies, advanced oxidation processes, and biological treatments for the elimination of POPs. In this chapter, a recent literature review of the aforementioned methods was performed. Studies are still being carried out to find the best way to eliminate POPs, as this depends on the treatment conditions, the type of water and the policies of each country, but biological treatments seem to be the best option so far.

**Keywords:** persistent organic pollutants, wastewater, treatment processes, phase change technologies, advanced oxidation processes, biological processes

## **1. Introduction**

In recent decades, the world population has grown exponentially, generating many negative ecological impacts, derived from residues such as drugs, pesticides, dyes, hormones, and personal care products. These wastes have persistent organic pollutants (POPs) in their composition [1–3].

POPs are usually halogenated and mostly chlorinated compounds that normally have nitro, sulfo, halogen, and/or aromatic residues that are responsible for their recalcitrance [4]. Its carbon-chlorine bonds are very stable against hydrolysis and at a higher number of these bonds, the higher is the resistance to degradation by biological or photolytic action. These POPs usually have ring structures with a single or branched chain. Due to their low solubility in water and high in lipids, they can pass through biological membranes and accumulate in the fat deposits of organisms [5]. Therefore, they can negatively affect human health and several other living organisms, causing mutagenicity, carcinogenicity, reproductive instability, as well as acute and chronic toxicity [3, 6].

The amounts of POPs used around the world have been considered a major concern, which is why 127 countries signed, on May 22, 2001, in Sweden, the Stockholm Convention, as part of the United Nations Environment Program (UNEP). This agreement initially generated a list with 12 of the most used priority toxic substances to prohibit and minimize their use [5, 7]. The number of chemical groups found on

this list continues to grow as new compounds that are part of this classification are identified, such as pentachlorophenol, nonylphenol, octylphenol, and dicofol [8].

The toxic properties of these substances persist for a long time in the environment and can travel long distances before being stored in fatty tissues [5, 7]. POPs are distinguished by being semi-volatile, which allows them to appear in the form of vapor and be present in the atmosphere. These POPs are transported long distances by air, soil, and water, affecting particularly fish and marine mammals. Due to the bioaccumulation properties of such pollutants, they can be transmitted through trophic chains [5, 9].

POPs can be found in large numbers of water bodies in concentrations ranging from ngL−1 to μgL−1, even in drinking water, but mainly in industrial, domestic, agricultural, and hospital discharges. These POPs are difficult to degrade with conventional wastewater treatments due to its physicochemical characteristics [1, 9].

Although there are current technologies for advanced wastewater treatment, they have limitations such as their high cost, the formation of toxic by-products and damage to the environment. Therefore, it is important to find, which method is optimal depending on the water characteristics. This chapter reviews the literature of existing processes and emerging technologies for the elimination of persistent organic pollutants in wastewater, analyzing their advantages, disadvantages, comparing time, cost, and degradation conditions of the treatment.

### **2. Treatment processes**

Unconventional wastewater treatment technologies have changed over time, developing a wide range of approaches to pollutant removal. Treatment processes are broadly divided into phase change technologies, advanced oxidation processes, and biological treatments.

Persistent organic pollutants comprise a wide range of various compounds and transformation products, but only those found mostly in the literature will be mentioned. It should be noted that the removal efficiencies were taken directly from the bibliography without any additional modification.

### **2.1 Phase change technologies**

These are processes capable of moving pollutants from one phase to another. They are commonly used in POPs removal and are divided into adsorption processes and membrane technologies filtration.

### *2.1.1 Adsorption processes*

Different adsorption processes have been studied for the removal of different pollutants, but the use of activated carbon (AC) is the most frequent due to its high porosity and specific surface [10, 11]. In general, good results of POPs removal have been obtained with AC, up to 99% depending on the contaminant and the application time [12, 13]. This type of treatment lasts approximately 90–150 minutes depending on the amount of the adsorbent and the pollutant [14, 15].

Carbon nanotube adsorption (CNT) is another technology, consisting of an allotrope of carbon that has different adsorption characteristics depending on the degree of waviness, diameter, internal geometry, physicochemical properties, and the treatment process used to its synthesis. CNTs are defined as single-walled nanotubes (SWNT), which have an internal diameter of approximately 1.0 nm, and multiwalled nanotubes (MWNT), which consist of several concentric tubes or layers of

### *Methods for Persistent Organic Pollutants Removal in Wastewater: A Review DOI: http://dx.doi.org/10.5772/intechopen.99973*

laminated graphene. Currently, there are only limited studies of this technology, but single-walled nanotubes are known to be more effective in removing POPs [16, 17].

There are countless studies of the use of clay minerals in adsorption processes, of which it has been observed that, depending on the type of clay, the amount of nitrogen, iron or other present minerals, different removal efficiencies can be produced. Although these approaches have shown very promising results, more research is required as the fate of the contaminants and the removal mechanisms involved remain largely unknown [18, 19].

Other adsorbent materials for POPs removal include zeolites, mesoporous, microporous materials, resins, and metal oxides. In which the nature of the pollutant significantly influences. One limitation of the use of these materials is the sustainability of their production since the use of soils, clays or other natural materials can be unsustainable in the long term. On the other hand, as in the adsorption methods mentioned above, there has been a limited application of these materials for the removal of POPs [20, 21].

**Table 1** shows a comparison of some of the adsorption processes, in terms of removal percentages, degradation conditions and an approximation of the application costs. There is a wide range between the removal percentages depending on the type of contaminant; this may be due to the carboxyl and hydroxyl groups they contain. On the other hand, these processes worked with acidic to neutral pH and with temperatures ranging between 22–30°C. Operating costs vary depending on the type of material used and can range from 0.01 to 7 USD per m3 .


### **Table 1.**

*POPs removal characteristics by adsorption processes.*

In general, for adsorption processes, the characteristics of the adsorbent material dictate the efficiency of the removal process, mainly because this determines other properties such as pore size, metallic or non-metallic nature, and the ability to couple with a second treatment [22]. It should be noted that a significant limitation is that most research studies discuss laboratory-scale tests and do not provide information for the expansion or large-scale viability of the processes [17].

### *2.1.2 Membrane technologies*

Membrane processes are another type of phase change technology with great POPs removal capacities; they are based on the use of hydrostatic pressure to eliminate suspended solids and high or low molecular weight solutes, allowing the water passage through. The duration of these processes is approximately 2–8 hours [15]. As in adsorption processes, membranes are produced from different materials with specific filtering characteristics such as pore size, surface charge, and hydrophobicity, which will determine the type of contaminant that can be retained [17, 23].

Ultrafiltration (UF) has been used for the removal of a significant variety of POPs since this process has a pore size in the range of 0.001–0.1 mm. The removal efficiency may vary according to the type of membrane and the contaminant. Generally, highly water-soluble polar pollutants are efficiently removed by ultrafiltration compared to non-polar compounds, poorly soluble in water [24–26].

Nanofiltration (NF) can be used for the removal of some POPs due to its small pore size (10 to 100 Å). This process could be considered more efficient than ultrafiltration for contaminants removal. Another advantage is the lower cost since it operates at low water pressure [24, 25]. However, the half-life of the membranes and their cost should be considered.

Microfiltration (MF) is a technique that has many advantages, but unfortunately, it is not useful for the removal of POPs as it cannot remove contaminants smaller than 1.0 mm [27, 28].

Reverse osmosis (RO) and forward osmosis (FO) use a semi-permeable membrane to separate water from dissolved solids, especially ions, by osmotic pressure. In comparison, reverse osmosis is more effective for the removal of persistent organic pollutants, since it can remove particles up to 10 Å, as well as colloidal particles [29, 30].

Depending on the type of process and the type of contaminants, removal efficiencies are highly variable with values between 11 and 99%. This is associated with the molecular weight of the contaminants, the lower the better removal and vice versa. On the other hand, these technologies are of average cost, ranging from 0.3 to 0.5 USD per m3 , as shown in **Table 2**.

According to the membrane technologies discussed in this section, as the size of the pores decreases, the efficiency of the POPs removal process improves significantly. However, membrane plugging can occur due to particles and colloids present in the feed streams. These processes are still being updated to achieve greater elimination in terms of quantity and quality of contaminants.

In summary, phase change processes can be effective in removing some persistent organic pollutants. However, the final disposal of the contaminants is challenging, because they pass into the solid phase after treatment, in the case of adsorption, or flow with the rejected effluent, in the case of membrane processes. Therefore, POPs will continue to be a problem for the environment [17].

### **2.2 Advanced oxidation processes**

In recent years there has been great interest in advanced oxidation processes (AOP), due to their great capacity to decompose pollutants, which is associated with


*Methods for Persistent Organic Pollutants Removal in Wastewater: A Review DOI: http://dx.doi.org/10.5772/intechopen.99973*

### **Table 2.**

*Removal properties of POPs by membrane technologies.*

the production of hydroxyl radicals in situ (oxidation potential, 2.8 V) that mineralizes pollutants [17].

AOPs are processes with different routes of free radical production, with specific working conditions and they can involve different materials. These processes have been applied for the elimination of different POPs and their effectiveness depends on the concentration of the oxidizing agent, the pH of the reaction mixture, the chemical structure, the initial concentration of the target contaminant, the wavelength, the intensity of the source, and presence of other organic matter. Therefore, there is no single AOP capable of eliminating all persistent organic pollutants [17].

Some of the most used and best-performing AOPs for POPs removal are UV/ H2O2 treatment, Fenton, wet air oxidation, and ozonation. The combined UV/H2O2 process is more effective in degrading POPs in water than UV irradiation or H2O2 oxidation alone, due to photolysis of hydroxyl radicals that generate H2O2 [32].

UV/H2O2 wastewater treatment complemented with microwave irradiation is very effective because of its short reaction time, reduction in activation energy, smaller equipment size, ease of operation and high product performance [15].

The Fenton process is capable of oxidizing aromatic contaminants. Iron (II) reacts with hydrogen peroxide to form iron (III) and hydroxyl radicals. Iron (III) is regenerated back to Fe (II) by hydrogen peroxide in an acidic environment. This technology can be used as a pretreatment method to reduce the toxicity of contaminants. In addition, the Fenton process has variants, such as the Fenton-like and photo-Fenton processes. The Fenton-like process uses iron (III) as a catalyst to convert the reaction from homogeneous to heterogeneous process and is more economical and efficient compared to the classical Fenton process, while having a similar mechanism. On the other hand, the photo-Fenton process is a more efficient and less pH-dependent treatment method, in which hydrogen peroxide can generate hydroxyl radicals under ultraviolet light and iron (III) can accept an ultraviolet photon to regenerate iron (II) [33, 34].

Wet Air Oxidation (WAO) can be used to treat toxic organic wastewater, with high temperature and pressure alone or with catalysts. In this process, the microorganisms mix with gaseous oxygen at temperatures ranging from 150 to 400°C and with pressures of 2 to 40 MPa [35].

Ozonization is a good alternative to degrade POPs at a very fast rate. The method involves a direct reaction between molecular ozone and dissolved compounds or by transformation to oxidants, hydroperoxyl radicals and other species that react with the target compounds [36].

As shown in **Table 3**, AOPs have a high degradation performance for all studies, for which pH is important, with values between 3.5 and 7. Another great advantage is the short time of operation that goes from 0.5 to 8 hours. The main difference between these processes is the source of energy provided, which raises the cost


### **Table 3.**

*Advanced oxidation processes for POPs removal.*

between 7.5 and 9 USD per m3 , subsequently, solar radiation is a potential alternative to reduce it [15, 17].
