**4. The treatment of petroleum wastewater with the advanced oxidation Fenton method**

Advanced oxidation basically refers to methods that destroy organic compounds by producing oxygen radicals. In these methods, they use strong oxidants, catalysts, radiation, and ozone to treat wastewater [4]. Fenton process, due to low operational cost compared to other advanced oxidation processes, low toxicity of iron ion and hydrogen peroxide, its simple technology, the possibility of its application in ambient temperature and pressure, high biocompatibility, short process duration, and low energy consumption, should be widely considered to reduce high pollution levels. Fenton's reaction is carried out in an acidic environment, and the optimal pH for this

reaction is 2.8–3 [27]. Fenton process is defined on the basis of electron transfer between H2O2 and a metal ion (generally iron ion), which acts as a homogeneous catalyst [28]. According to the mechanism of research in an acidic environment due to the reaction of hydrogen peroxide with Fe(II) or Fe(III) ions, the oxidation-reduction mechanism of the Fenton process is as follows. Based on this mechanism, the OH radical produced by attacking organic materials (RH) causes their destruction [29]. The chemical relationships of the Fenton process are shown in relationships (2)–(5) [30]:

$$\mathrm{H\_2O\_2} + \mathrm{Fe^{2+}} \rightarrow \mathrm{Fe^{3+}} + \mathrm{HO^-} + \mathrm{HO^\*} \tag{2}$$

$$\rm HO^{\*} + \rm RH \to H\_{2}O + \rm R^{\*} \tag{3}$$

$$\mathbf{R^{\*}} + \mathbf{Fe^{3+}} \rightarrow \mathbf{R^{+}} + \mathbf{Fe^{2+}} \tag{4}$$

$$\rm{R^{\*}} + \rm{H\_{2}O\_{2}} \rightarrow \rm{ROH} + \rm{OH^{\*}} \tag{5}$$

Hydroxyl radical is able to decompose organic pollutants in a short period of time and non-selectively [31]. Among the methods of producing hydroxyl radicals, the use of ultrasonic waves in advanced oxidation methods is considered one of the new methods [32]. Usually, the destruction of organic pollutants using acoustic and thermal decomposition is attributed to the activity of radicals. The increase in the effect of thermal decomposition and the reaction with radicals cause an increase in sound decomposition. Water molecules are broken in this method, and hydroxyl and hydrogen radicals are released [33]. This phenomenon includes the formation and destruction of gas bubbles, which results in the generation of very high pressure and temperature, which includes the thermal decomposition of dissolved organic compounds and the production of free radicals such as O� and OH� , H� or some oxidants such as peroxide [34]. It is hydrogen that can react with organic compounds. The destruction of gas bubbles causes the formation of very high temperatures and pressure, which leads to the separation of water vapor in the reactive hydroxyl radical and hydrogen atoms with the presence of other species (H2O, O2) [35]. Although advanced oxidation processes alone are not effective, the sono-chemical oxidation process can be done by adding chemicals such as persulfate and catalytic particles and increasing the efficiency of the process [36]. In recent years, a compound called persulfate (S2O8 <sup>2</sup>�) with an oxidation potential of 2.01 V has been known and introduced, which is capable of oxidizing toxic and resistant organic compounds, the advantages of which are cheapness and non-selective oxidation of organic compounds [37]. The high stability of the radical produced from it in different conditions, high solubility, a solid form, and, as a result, the ease of moving and storing, have shown the use of this substance in many research [38]. Regardless of these advantages, extensive studies on the use of persulfate show [13] that at room temperature, the ability of persulfate to decompose organic substances is low and slow; therefore, in order to accelerate the oxidation process with persulfate, it is necessary to perform an activation operation [12].

According to **Figure 5**, the activation of persulfate is carried out as an advanced oxidation process with heat, UV light, and transition metal (Me2þ). The final product of the activation process is the production of sulfate radical (SO*:*� <sup>4</sup> ) with an oxidation potential of 2.6 V. Relations (6) and (7) show thermal and chemical activation of S2O8 <sup>2</sup>� [38]:

$$\text{SO}\_{8}\text{O}\_{8}^{2-} + \text{heat or UV} \rightarrow \text{SO}\_{4}^{-} \tag{6}$$

**Figure 5.** *Schematic of the Fenton process [32].*

$$\rm{S\_2O\_8}^{2-} + \rm{Me^{n+}} \rightarrow \rm{Me^{(n+1)+}} + \rm{SO\_4^{2-}} \tag{7}$$

Persulfate anion is considered a strong oxidant, and when activated, it can produce stronger oxidants such as sulfate radical. Since persulfate is produced slowly at normal room temperature, it is converted into radical sulfate by active photochemical or thermal decompositions, and it is used as a fast method in chemical decomposition processes [11]. During a study, Bing et al. investigated the effect of hydrogen peroxide, persulfate, and periodate in the oxidation of TiO2 photocatalyst. The results showed that adding 2–10 moles of mineral oxidants, persulfate, periodate, and hydrogen peroxide had a faster decomposition rate compared to TiO2/UV. In another study in which Hosseini et al. [39] investigated the decomposition of Blue 25 acid in aqueous media using Fe<sup>2</sup>þ/ultra-sonic and H2O2/ultra-sonic, the results showed that these two processes had a higher removal efficiency than the ultra-sonic process alone.

#### **5. The treatment of petroleum wastewater with the electrocoagulation**

Considering that spilling and leaking of oil into water is unavoidable in most cases and considering the adverse effects of water contaminated with petroleum derivatives on humans and the environment, so far, there have been various methods for purifying water, and separating these two important substances (water and oil) has been suggested by the researchers [40]; among them, methods of gravity separation, types of filters, reverse osmosis, biological processes, flotation with dissolved air, membrane bioreactors, adsorption with activated carbon, chemical coagulation, and electric flotation have been reported [16].

The electrocoagulation method is one of the effective separation methods of oil from the emulsion. It is the water that is optimal and affordable both technically and economically. In this method, as shown in **Figure 6**, the purification process is done in three stages: (I) The reaction of the electrolyte on the surface of the electrode and the

**Figure 6.** *Schematic diagram of the electrocoagulation process [41].*

formation of coagulants by electrolytic oxidation in the aqueous phase. (II) Adsorption of colloidal particles on coagulants. (III) Removal of clots by sedimentation or flotation [42].

In this method, at the same time as the anode is corroded, electrolyte gases (generally H2) are produced in the cathode, which leads to more flotation [43]. In electrocoagulation, metals such as iron and aluminum are usually used as anodes, which produce hydroxides, oxyhydroxides, and polymeric hydroxides when oxidized [44]. The metal hydroxides formed act as coagulants of liquid impurities, and the hydrogen bubbles formed on the cathode side provide foam formation. These products are usually much more effective than added chemicals and are able to destabilize colloidal suspensions and emulsions [45]. The electrocoagulation method has several advantages, including no need to add chemicals, simple equipment, convenient operation, low initial cost and low operating cost, short reaction time, rapid sedimentation of the created flocs, low sludge production [46], high safety, no need to transport and move chemicals, and coagulant production on site, and can act as an efficient method in separating petroleum compounds from water [21]. This method has been very efficient for purifying water contaminated with solids, dyes, heavy metals, and soluble organic and inorganic substances. As mentioned, iron and aluminum electrodes are used as anodes, and the reaction mechanism by iron is as follows [47]:

In the Andes:

$$\mathbf{4Fe} \rightarrow \mathbf{4Fe}^{2+} + \mathbf{8e}^- \tag{8}$$

$$\text{4Fe}^{2+} + \text{10H}\_2\text{O} + \text{O}\_2 \rightarrow \text{4Fe(OH)}\_3 + \text{8H}^+ \tag{9}$$

*Petroleum Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109853*

In the Cathode:

$$\text{8H}^+ + \text{8e}^- \rightarrow \text{4H}\_2\tag{10}$$

Total reactions:

$$\text{4Fe} + \text{10H}\_2\text{O} + \text{O}\_2 \rightarrow \text{4Fe(OH)}\_3 + \text{8H}^+\tag{11}$$

At the same time as the anode is corroded, electrolyte gases (generally H2) are produced in the cathode. Metals such as iron and aluminum are commonly used as anodes, which produce hydroxides, oxyhydroxides, and polymeric hydroxides upon oxidation [48]. The formed metal hydroxides act as coagulants of liquid impurities, and the formed hydrogen bubbles on the cathode side provide foam formation. These products are usually much more effective than added chemicals and are capable of destabilizing colloidal suspensions and emulsions. Nidheesh et al. [49] used electrocoagulation process with iron, aluminum, and steel electrodes to treat oil refinery wastewater, and the results showed that this process could be a suitable method to reduce sulfate and COD concentration from oil refinery wastewater. Also, in the research where *Asselin* et al. used electrocoagulation process with aluminum and steel electrodes to treat the oil effluents of ships, it was found that the optimal state was obtained with steel electrodes, and 93% removal for BOD, more than 95% for oil removal, and about 68% for COD removal are achieved in optimal conditions [41].

### **6. The treatment of petroleum wastewater with the membrane filtration method**

Several separation processes, including ultrafiltration, nanofiltration, and reverse osmosis, have been employed for oil/water separation. Membrane ultrafiltration is one of the most important separation processes in the field of industrial petroleum wastewater treatment. When the solvent molecules are less than 0.5 microns, ultrafiltration is used [50]. Before oil emulsions enter the environment, it is necessary to remove the oil in them to an acceptable level, which is determined by the standards. Petroleum effluents and oil-water emulsions are two important environmental pollutants [21]. Unlike urban wastewater, industrial wastewater discharged into the environment does not have any fixed characteristics. The composition and characteristics of industrial effluents are significantly variable, and even in different parts of the industry, these flows are visibly different. Despite the physicality of the filtration process, chemical purification processes can also be used. A huge amount of oil refinery effluents are in the form of oil-in-water or water-in-oil emulsions, which are produced from different parts of the extraction, transportation, and refining processes [51].

Methods based on membrane separation include dehydration of oil emulsion by reverse osmosis, coagulation resulting from microfiltration, microfiltration, membrane distillation, and ultrafiltration. Among the benefits of membrane technology are lower cost, no need for any chemical additives, and the ability to create an acceptable quality flow. Ultrafiltration is used as an effective method to separate, purify, and saturate water-soluble solutes or water-dispersed substances. In any case, due to the deformation of oil droplets with operating pressure, oil droplets can pass through the holes with pressure and pollute the flow. Despite the reduction in the cost of energy consumption of the ultrafiltration process, the problems caused by washing in this process are very expensive [52].

By replacing membrane processes with traditional purification methods, product quality is improved, and process efficiency is increased. Microfiltration membranes purify colloidal particles and bacteria with a diameter of 0.1–10 μm. Ultrafiltration membranes can separate large soluble molecules such as proteins and petroleum substances from the solution. In reverse osmosis membranes, the solvents are dissolved in the membrane and penetrate through the membrane to a lower concentration and are mainly used in the field of desalination of underground water or sea water. The difference in cavity drops (or apparent cavity) creates significant differences in the field of membranes used. Reverse osmosis and ultrafiltration processes are often used in oil/water treatment. Tubular modules are used in the field of oily wastewater treatment due to their resistance to the clogging of emulsion particles, easy replacement of the membrane, and the ability to use the high linear speed of the oily emulsion on the membrane surface [53].

Wollborn et al. have reported that if the shear pressure is lower than the critical pressure, then the emulsion will reach the maximum possible volume [54]. Ma et al. showed that in porous hydrophobic membranes, due to the coagulation and sedimentation of oil on the membrane cavities, the separation of oil-in-water emulsions is reduced [55]. Using a microporous polytetrafluoroethylene flat membrane, Nittami et al. [56] have investigated the effect of emulsion droplet size, stirrer speed in penetration test, oil phase volume fraction, and surfactant concentration in the feed solution on oil flow flux. For industrial/petroleum effluents, the amount of oil in the flow passing through the membrane is higher than the acceptable amount of the standard discharge to the environment. Tong et al. [57] used commercial polyvinylidene fluoride to treat oil field effluents. At the beginning of membrane filtration, due to the lack of gelatin layer formation on the surface of the membrane, the quality of the water coming out of the membrane is not very favorable. As the process progresses and due to the concentration of pollutants and the polarization of the membrane surface, a gelatinous layer is formed on the membrane surface. The gelatin layer formed on the surface of the membrane prevents polluting particles from entering the membrane cavities and leads to a decrease in membrane flux. The flux recovery percentage of modified membranes reaches 100% after washing with 1% OP-10 surfactant solution. The relationship between flux and pore pressure is not completely linear due to resistance in addition to membrane resistance. When oil recovery has a downward trend with increasing pressure, the amount of flux reduction is greater. Porosity, pore size distribution, and membrane substrate structure play an important role in determining the flux through the membrane. Also, by increasing the concentration of titanium oxide nanoparticles in the polymer solution, the number of shell cavities increases [58]. Based on the observations of Sutrisna et al. [20], membrane fouling is a combination of pore-clogging by smaller oil droplets in the emulsion and sedimentation of the oil layer on the surface. To check the effect of membrane clogging, the permeability of pure water passing through each membrane is measured before and after washing the membrane. The results of Wang et al. [59] showed that liquid droplets passed through the pores of the membrane more easily with the increase in osmotic pressure. Of course, with the increase of the osmosis pressure, the operating cost and depreciation of the equipment increase. Also, membrane clogging occurs at high pressure due to the formation of a colloidal layer. As the colloidal layer increases, the resistance of the droplets passing through the membrane increases, so the membrane flux decreases. Organic-inorganic composite membranes such as Al2O3-PVDF are widely used in petroleum wastewater treatment. Flux recovery is better for membranes washed with alkaline solutions. Hashemi et al. [60]

#### *Petroleum Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109853*

investigated the ultrafiltration of oil effluent from engine houses using tubular modules (with a large diameter). Most of the constructed membranes reduced the oil content in the flow to less than 10 mg/DCM. The current passing through this membrane is suitable for discharge to the environment. Hollow fiber membranes are much more efficient than tubular and flat plate membranes due to their high surface area to volume [61]. Due to the high specific area and hydrophilicity of titanium oxide nanoparticles, the flux increases. Membrane wettability is one of the important factors that can affect the flux and anti-clogging ability of membranes. By increasing the titanium oxide particles, the contact angle of water with the membrane surface can be significantly reduced. As the repulsion decreases, the membrane flux increases. Therefore, a membrane with maximum porosity and hole size has maximum flux. Due to the inherent hydrophobicity of PVDF polymer, this type of membrane is used in petroleum wastewater treatment, organic/water separations, gas absorption and membrane distillation, and ultrafiltration [62]. Additives such as polyvinylpyrrolidone, polyethylene glycol, and lithium chloride are used to improve the morphology and performance of the membrane and its mechanical strength. Zhu et al. [19] used alumina to improve the hydrophilic property and antifouling ability of the polysulfone membrane. They added SZP particles to the porous polysulfone membrane, which ultimately led to the improvement of polysulfone membrane properties such as hydrophilicity, antifouling ability, and tensile strength [18]. Composite membranes are used to treat petroleum wastewater. Due to the increase in the hydrophilicity of the membrane with the increase of hydrophilic SZP particles, the hydrophilic layer formed on the surface of the membrane plays an important role in removing the gel-like layer [63].

One of the reasons for the reduction of the passing flux is concentration polarization, which is due to the increase in the concentration of oil particles on the surface of the membrane. As the membrane filtration continues, the concentration of the preservative on the membrane surface becomes higher than the feed concentration, which ultimately leads to concentration polarization (ultimately creating a gel layer on the membrane surface). Also, due to the presence of impermeable pores in the membrane for the passage of oil droplets, clogging occurs [64].

Due to the high specific area and hydrophilicity of titanium oxide nanoparticles, the flux of PVDF ultrafiltration hollow fiber membranes increases. By increasing the concentration of titanium oxide nanoparticles, the pores of the membrane are blocked due to the accumulation of particles, and the formation of a dense substrate decreases, and as a result, the average size of the cavity decreases [65].

As a hydrophilic surface modifier, Pluronic F127 can greatly reduce the water contact angle of the membrane. Due to the stability of oil droplets on the surface of the PES/Pluronic F127 membrane, the water contact angle for used membranes is higher than that for fresh membranes. During the ultrafiltration process, many oil droplets settle on the surface of the membrane or are adsorbed on the surface. After washing with water, the membrane surfaces are still hydrophobic, and the oil droplets are not removed from the membrane surface [17].

The effects of concentration polarization and membrane fouling at constant pressure are observed with a significant decrease in flux with time. In this case, the concentration polarization is omitted due to the large size of the emulsified oil particles. The decrease in the membrane flux is due to the clogging of the membrane through surface absorption or the settling of oil droplets on the surface of the membrane or inside the membrane cavities. Sodium dodecyl sulfate is used as detergent to wash the captured membranes. The membrane surface washed with SDS solution is

very hydrophilic [66]. During the washing process, some SDS molecules distributed in the aqueous solution are absorbed on the membrane surface and lead to a decrease in surface tension [67]. Therefore, the stability of oil droplets is improved and prevents their sticking and coagulation. Polyethersulfone membranes offer very high thermal stability in addition to mechanical properties, but they also have disadvantages. The main problem of these membranes is their relative hydrophobicity [50].

### **7. Management of petroleum wastewater treatment**

Wastewater or sewage refers to mainly liquid local, urban, or industrial wastes and discharges. The method of collecting and discarding it differs in each region, depending on the local awareness of the environment, and scientists believe that the future will belong to those who make the best use of water. One of the main axes of sustainable development in the petrochemical industry is the optimal use of resources, and the reuse of wastewater in terms of the increasing importance of water as a vital substance has been one of the goals of the management of petrochemical companies, so that even, if possible, the wastewater of the production units after performing purification can be used again in the green space irrigation sector or in the industrial sector.

The development that petrochemicals are trying to achieve is sustainable and allround, and we deeply believe in the fact that without environmental preservation and optimal use of resources, the development of any industry is one-dimensional and unstable. Therefore, the reuse of wastewater is one of the interesting options in the petrochemical industry. The only concern of using wastewater is environmental pollution in the long term. Therefore, in order to solve existing environmental challenges and provide suitable solutions for the sustainable use of wastewater, determining the type of pollution caused by irrigation with wastewater and the resulting environmental effects should be fully investigated.

Wastewater management is planning, organization, care, and executive operations related to the production, collection, storage, transportation, recycling, processing, and disposal of wastewater, as well as education and information in the field of wastewater.

Environmental monitoring is a continuous process of care, examination, comparison, and accurate evaluation of environmental qualities, which is developed and carried out before, during, and after the implementation of projects. The most key things required for an environmental monitoring program to choose an effective method of oil waste treatment are: organizational structure, monitoring operations, timing, reporting, and financial status. By considering the conditions of the operating area and the facilities and characteristics of the petroleum wastewater and by choosing the appropriate treatment method, petroleum wastewater can be managed. For example, if there is enough space and suitable sunlight, the best option is to use the solar evaporation method, because petroleum wastewater can be managed by using a free energy with the lowest cost. Of course, if the oil effluent mainly contains volatile hydrocarbons, this method is not recommended, because pollution enters the air, or valuable hydrocarbons that can be recovered get lost, and this issue is not economically justified. The management of petroleum wastes in densely populated cities is faced with a lack of space to install and operate equipment. Therefore, in conditions where space is limited, a membrane bioreactor using microorganisms is a better choice. If the volume of wastewater is large, membrane filtration can be used. Of

course, in filtration, membrane clogging is a major problem that limits the development of this method in petroleum wastewater management.
