Petroleum Wastewater Treatment

Abstract

Petroleum hydrocarbons in refinery wastewater are considered the main cause of pollution. Wastewater from oil refineries contains large amounts of oil and fat in the form of suspended particles, light and heavy hydrocarbons, phenol, and other dissolved organic substances, which cause environmental pollution if they are discharged into the environment without treatment. Usually, conventional methods of treating petroleum wastes have a lot of costs; due to the existence of sufficient area for the construction of solar distillation ponds and suitable sunlight, as well as a large number of sunny days near the equator, the solar distillation method can be used. Membrane bioreactors based on biological decomposition and biological transformation of oils and waste oil materials have provided new solutions for the biological treatment of these wastewater. In addition to these methods, Fenton’s advanced oxidation methods, electrochemical coagulation method, and membrane filtration method are mentioned in this chapter.

1. Introduction

Water is one of the most important resources in the development of countries. During the twentieth century, the world’s population tripled, and water use increased six-fold. The world’s available water is only sufficient for the current population with minimal access to clean water [1]. Improper distribution in terms of space and time and an increase in population and per capita consumption of water have aggravated this problem. The world’s population is increasing, and drinking water resources are decreasing, so the world may face the problem of water shortage in the future. Destructive activities and inefficient use of water resources, along with the increase in population and increase in water demand, have severely limited water resources in the last few decades. The United Nations states that the lack of water resources has caused the reduction of agricultural land, and the production of food in recent decades has been extremely risky. The lack of water resources is a serious threat to human life [2].

Climate change and the reduction of rain forests and the reduction of the thickness of the ozone layer all aggravate the water shortage. Lack of water also has side and indirect effects, such as increasing poverty and hunger, ecosystem destruction, desertification, climate change, and even world peace [3].

The per capita standard of drinking water consumption in different countries rarely exceeds 200 liters per day. But the results of researchers’ studies show that per capita water consumption is much higher than the standards set in the countries [4]. That is because besides the direct consumption of water, humans consume water through the consumption of food, fruits, and even services and goods, and its amount is on average about 3400 L per day per person in the world, which is called virtual water [5]. Climatic conditions, place and time of production, management and planning, and culture and habits of people are some of the effective factors in the amount of virtual water [6].

The security of water resources has become another challenge with increasing demand. Harvesting and purifying water from surface and underground sources, as well as treating wastewater produced in underground aquifers, in addition to polluting aquifers, will also disrupt the natural water cycle [7]. On the other hand, due to the possibility of the spread of many diseases caused by the contamination of water and sewage, in order to preserve the health of human societies and prevent disruptions in the water cycle, sewage must be properly collected, treated, and returned to the natural water cycle [8]. The most important goals are to build urban and industrial wastewater treatment systems, maintain public health, protect the environment, prevent the pollution of water sources, and reuse treated wastewater in industry and agriculture [9].

Wastewater provides a valuable source of recoverable water. Although this source can contain dangerous compounds that endanger public health and the environment, at least 90% of the wastewater is water [10]. New technologies to treat wastewater and return it to water supply networks are an important factor in increasing limited water resources. Water treatment plants are an important part of the water recovery process. The main goal of water purification processes is to reduce the concentration of water pollutants by separation, destruction, and disinfection [11]. Many efforts have been made to maintain the quality of treated industrial wastewater, recover them, and prevent them from jeopardizing public health [12].

From another point of view, the disposal of untreated effluents from factories and industries creates many health risks for human societies [13]. In order to reduce these risks, wastewater treatment plans for factories must also be developed. Therefore, the use of water obtained from sewage treatment in agriculture can not only make an important contribution to the water supply of the society but is also considered a solution for environmental preservation and sustainable development [14].

The production fluid of oil wells is usually a combination of gas, oil, and water. Water with oil can be observed as free water or fine suspended droplets or both in the fluid. Since the production of excess water is an integral part of the production and preliminary processing of crude oil, in order to prevent environmental pollution, maintain reservoir pressure, and increase extraction from oil production wells, these waters, after preliminary treatment in the treatment systems of desalination units, are again sent to disposal wells that are intended to be injected for this purpose [15].

The production amount of salt water along with oil in desalination units from crude oil is significant. These effluents have created a big problem for the environment due to their specific quantitative and qualitative characteristics, which include soluble salts, the presence of petroleum substances, volatile and non-volatile organic substances, and other hazardous pollutants in large volumes [16]. Due to the presence of supersaturated soluble salts and suspended particles and corrosive agents, these wastewaters have a strong tendency to deposit, and if they are injected into the well without preliminary treatment, it may cause clogging of the flow path in the underground flow pipes and/or the opening of the well or cause the corrosion of the flow pipes as a result of the effluent leaks into the environment.

Oil refineries, as one of the complex process industries, consume significant amounts of water based on the size and configuration of the process for multiple operations (65 to 90 gallons of water per barrel of crude oil) and a large volume of wastewater with diverse natures from 1.6 to 0 [17]. They produce 4 times the amount of processed crude oil. The recycling and reuse of this significant amount of wastewater for various purposes, including the supply of water needed for cooling systems, process units, irrigation, and firefighting after the implementation of purification based on quality standards in oil refineries, is a significant matter [18]. Several post-refining approaches, depending on the nature of the type and size of process units in oil refineries, have evolved over the past decades with the aim of improving water and wastewater management [19]. These approaches include the investigation and implementation of traditional techniques such as distillation, evaporation, active carbon filtration, sand filtration, and chemical oxidation and advanced cases such as membrane separation under pressure, electrodialysis, ion exchange, and advanced oxidation processes [20].

The necessity of treatment includes engineering investigations and the use of appropriate technology, measuring the quantitative and qualitative parameters of wastewater in the outlet pool of the treatment plant, comparing with the declared standards, the fate of excess sludge, and ensuring the absence of unpleasant odors and floating objects in the outlet wastewater and no change in the color and turbidity of receiving water at the place of discharge [21].

2. The treatment of crude oil desalination unit wastewater with the solar evaporation method

Effluents from crude oil desalination units due to their specific qualitative and quantitative characteristics, which include highly soluble salts of 50 grams per liter, and the presence of petroleum substances, volatile and non-volatile organic substances, and other hazardous pollutants for the environment, as well as a large volume, create a big problem in the vicinity of oil units and the environment around them. At present, these wastewaters are usually re-injected into operating wells or abandoned wells without treatment or after initial treatment, including the removal of suspended particles and major associated oil substances, with the aim of increasing harvest or preserving the environment. Also, these wastewaters are sent to the evaporation ponds adjacent to these units after degreasing the crude oil desalination units to protect the environment, so that over time, with the solar evaporation method, their amount is reduced, and the volume of the pond to enter the production wastes is emptied again.

This method creates the risk of pollutant leakage into groundwater and release to air and harm to humans, birds, and other creatures around the pond. Considering the sufficient temperature and the high intensity of radiation most days of the year, the use of solar energy seems appropriate. Solar distillation is a relatively simple solution for brackish water sources. Distillation is one of the processes used to purify water, and for this purpose, any heat source can be used. In the solar distillation method, using the energy of the sun, evaporated water and pure water vapor after condensation are used as pure water. The use of solar distillation method is a solution for water supply in remote areas that face a shortage of drinking water and common resources such as heat and electricity grid.

The possibility of building in small capacities, no need or minimal need for fuel and electricity, and the absence of environmental pollution caused by fuel consumption are among the advantages that make the use of this system in areas with significant renewable energy potential and, at the same time, where electricity and fuel transmission is difficult justifiable. The first and simplest solar device built is a single pond solar still (solar still). The building of this device consists of a wooden pond whose floor is blackened by safe pigments.

In a general and apparent classification, pond solar desalination plants can be divided into two groups with a one-way slope and a two-way slope liquefaction surface (Figure 1). According to the investigations, the solar distillation pond with a liquefaction surface with a one-way slope has a higher efficiency, because the incoming radiation is more.

Figure 1.

Pond solar desalination plants with a one-way slope (a) and a two-way slope, and (b) liquefaction surface.

The main problem of this type of water desalination plant, like most solar water desalination plants, is the relatively low production of desalinated water. One of the other obstacles of desalters using the solar distillation method is the absorption of less solar energy in areas far from the equator, because in these areas, the liquefaction surface of the device is parallel to the horizon and the oblique radiation of solar rays, and hence, the absorption of solar energy is very low. After the desalination of sea water by the solar distillation system, researchers have performed chemical analyses to check the possibility of using the water produced by this system as drinking water and compared the results with drinking water. The results showed that the resulting distilled water could be mixed with well water to obtain drinkable water, and the quality of this water was acceptable. The results showed that impurities such as nitrate, chloride, iron, and solids soluble in water were removed by solar distillation method. Most of the research related to this topic is often reported for seawater desalination; the treatment of petroleum effluents with this method is a new topic. One of the parameters affecting the efficiency of pond solar desalination plants is the optimal depth of salt water. Research has been done on the effect of water depth inside the pond. In addition to the geometrical parameters of the pond, in recent years, most researchers have focused on the construction of solar energy absorbent beds and heat transfer to increase evaporation.

Zhang et al. [22] reported that highly polluted saline wastewater was treated by carbonized lotus seedpod with the solar evaporation method (Figure 2). In their research, COD was removed by more than 84%.

Figure 2.

Lotus seedpod-based solar steam generation. Digital photograph of a lotus seedpod (a) before and (b) after carbonization; (c–j) SEM images of the flower disc, receptacle, and petioles of the lotus seedpod after carbonization, respectively [22].

Sun et al. [23] synthesized a new photothermal substrate based on a chitosan/bamboo fiber matrix with high efficiency for use in water evaporators. According to Figure 3, this ultra-light substrate had the ability to increase the local temperature up to 37$°\mathrm{C}$ in less than 40 min. The evaporation rate in their research was 6.72 $\mathrm{kg}{\mathrm{m}}^{-2}$ of purified water with a removal efficiency of 85%.

Figure 3.

Image of the ultra-light substrate based on a chitosan/bamboo fiber matrix and temperature rise in 40 min [23].

3. The treatment of petroleum wastewater with the membrane bioreactor

Membrane bioreactor technology or MBR refers to technologies in which wastewater is biologically treated, and then, the resulting biomass is physically separated from the mixed liquid using membrane processes. All these steps are performed in a single bioreactor (Figure 4). Therefore, in this method, the secondary sedimentation basin is removed from the system. Among the other advantages of this system we can mention the small amount of space they need, the lack of sludge production, and the high quality of the output effluent. These systems are used for purification; so far, urban and domestic wastewater as well as industrial wastewater such as food, pharmaceutical, oil and petrochemical industries have been used. A lot of research has been done on the treatment of petroleum industry wastewaters by MBR and methods of improving its performance. One of the most important wastewaters from oil industries, which has many adverse effects on the environment, is the water produced in oil fields, which contains large amounts of salt and oil.

Figure 4.

Configurations of membrane bioreactor with the submerged membrane [9].

Soltani et al. [24] used a submerged MBR with a hollow fiber membrane to treat this wastewater. Due to the fact that this wastewater contains large amounts of salt, due to the increase in osmotic pressure, it destroys the cell wall of the normal microorganisms in the MBR system. In this study, the purified bacteria that were obtained from the areas of oil deposits in the sea, after exposure to the main sewage, were able to decompose 50% of phenanthrene, which is a complex and difficult-to-decompose aromatic compound with three benzene rings, after 45 days. Based on these results, these bacteria can break down other compounds in crude oil. By reducing the salt concentration in this experiment, contrary to expectation, the performance of bacteria purified from the environment with high salt concentration did not decrease. This confirms that these bacteria belong to the halotolerant group.

In a similar study, Xianling et al. [7] studied the purification of petroleum hydrocarbons in a membrane bioreactor by purifying different bacteria from oil-contaminated areas. In this system, it was found that COD removal efficiency was different in steady state, and despite the gradual increase of COD, the efficiency increased from 93 to 96.5%. The reason for this can be seen in the increase in MLSS concentration in these types of systems, which reached 16.2 $\mathrm{g}{\mathrm{L}}^{-1}$ in this system.

The increase in the concentration of MLSS in such MBR systems is due to the use of a membrane that prevents the exit of the mixed liquid, and this concentration increases over time. After entering the petroleum wastewater into the reactor and physiological adaptation with it, the bacteria responsible for the decomposition begin to decompose the hydrocarbons, the final product of which is carbon dioxide and water. First, light hydrocarbons such as alkanes and aromatics with low molecular weight and then heavy hydrocarbons such as polycyclic aromatics are decomposed. This rate of decomposition of petroleum materials in an ultrafiltration bioreactor with transverse flow was observed up to 0.82 g of hydrocarbon per gram of MLVSS per day. In this study, all hydrocarbons with carbon atom numbers from ${\mathrm{C}}_{10}$ to ${\mathrm{C}}_{24}$ were removed with almost the same efficiency. Due to the complexity and diversity in the quality of petroleum wastes, in some researches, phenol has been used as a suitable indicator to investigate the removal of biodegradable compounds.

Zhou and Hong, by investigating the treatability of oil refinery wastewater by a fixed film bioreactor at a hydraulic retention time of 8 h and an initial phenol concentration of 30 mg/liter, reported a COD removal efficiency between 85 and 90% and a phenol removal efficiency of 100% [25].

The results of Viero et al. study confirm the high efficiency of phenol removal from petroleum wastewater in a submerged membrane bioreactor. In this research, which was carried out in three stages, during the operation period, a high organic loading rate was applied in the long term by mixing the flow of petroleum wastewater with phenol-rich wastewater to the bioreactor, and it was shown that the treatment of petroleum wastewater in a submerged membrane bioreactor with specific hydraulic retention time, in addition to removing organic matter, also caused high efficiency of phenol removal. The presence of a membrane in this bioreactor increases COD removal efficiency by 17% [9].

The operating conditions of a membrane bioreactor, such as the pressure applied to both sides of the membrane (Transmembrane Pressure: TMP), the amount of aeration, the speed of the transverse flow, and so on, affect the purification efficiency in this system. Increasing the transverse flow speed increases the turbulence of the liquid flow inside the reactor and increases the mass transfer coefficient in the concentration polarization layer and consequently the output flux. The relationship between the output flux (J) and the transverse flow velocity (V) in a bioreactor with transverse flow, in the treatment of petroleum wastewater in a refinery, is obtained as the following power relation [26]:

$$J=k{V}^n$$E1

n and k are affected by MLSS, and these two factors decrease with the increase of MLSS. The COD removal rate in this bioreactor is reported to be more than 93%. Of course, increasing the speed of the transverse flow and creating a turbulent flow removes the cake layer of organic materials on the surface of the membrane, which plays the role of an additional filter and prevents the passage of impurities, and as a result, the possibility of oil particles passing through the membrane increases. The pressure applied to the membrane also has a double effect. Although increasing TMP increases the amount of output flux, on the other hand, it causes more accumulation of oil droplets on the surface of the membrane as well as inside its pores and intensifies the clogging of the membrane. If this pressure increases, oil and oil droplets will pass through the pores of the membrane and reduce the quality of the output effluent. Aeration also causes the mixing of wastewater inside the bioreactor and provides better contact between particles and microorganisms. On the other hand, it creates a shear force that causes the biofilm flocs to break and separate from the membrane surface. Also, aeration increases dissolved oxygen in the bioreactor, which has a positive effect on COD removal.

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 ${\mathrm{H}}_2{\mathrm{O}}_2$ 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]:

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{Fe}}^{2+}\to {\mathrm{Fe}}^{3+}+{\mathrm{HO}}^{-}+{\mathrm{HO}}^{\bullet }$$E2

$${\mathrm{HO}}^{\bullet }+\mathrm{RH}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{R}}^{\bullet }$$E3

$${\mathrm{R}}^{\bullet }+{\mathrm{Fe}}^{3+}\to {\mathrm{R}}^{+}+{\mathrm{Fe}}^{2+}$$E4

$${\mathrm{R}}^{\bullet }+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{ROH}+\mathrm{O}{\mathrm{H}}^{\bullet }$$E5

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 ${\mathrm{O}}^{·}$ and ${\mathrm{OH}}^{·}$, ${\mathrm{H}}^{·}$ 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 (${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{O}}_2$) [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 (${{\mathrm{S}}_2{\mathrm{O}}_8}^{2-}$) 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 (${\mathrm{Me}}^{2+}$). The final product of the activation process is the production of sulfate radical (${\mathrm{SO}}_4^{.-}$) with an oxidation potential of 2.6 V. Relations (6) and (7) show thermal and chemical activation of ${{\mathrm{S}}_2{\mathrm{O}}_8}^{2-}$ [38]:

Figure 5.

Schematic of the Fenton process [32].

$${{\mathrm{S}}_2{\mathrm{O}}_8}^{2-}+\text{heat or}\mathrm{UV}\to {\mathrm{SO}}_4^{.-}$$E6

$${{\mathrm{S}}_2{\mathrm{O}}_8}^{2-}+{\mathrm{Me}}^{\mathrm{n}+}\to {\mathrm{Me}}^{\left(n+1\right)+}+{\mathrm{SO}}_4^{2-}$$E7

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 $\mathrm{Ti}{\mathrm{O}}_2$ photocatalyst. The results showed that adding 2–10 moles of mineral oxidants, persulfate, periodate, and hydrogen peroxide had a faster decomposition rate compared to $\mathrm{Ti}{\mathrm{O}}_2$/UV. In another study in which Hosseini et al. [39] investigated the decomposition of Blue 25 acid in aqueous media using ${\mathrm{Fe}}^{2+}$/ultra-sonic and ${\mathrm{H}}_2{\mathrm{O}}_2$/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 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].

Figure 6.

Schematic diagram of the electrocoagulation process [41].

In this method, at the same time as the anode is corroded, electrolyte gases (generally ${\mathrm{H}}_2$) 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:

$$4\mathrm{Fe}\to 4{\mathrm{Fe}}^{2+}+8{\mathrm{e}}^{-}$$E8

$$4{\mathrm{Fe}}^{2+}+10{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\to 4\mathrm{Fe}{\left(\mathrm{OH}\right)}_3+8{\mathrm{H}}^{+}$$E9

In the Cathode:

$$8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\to 4{\mathrm{H}}_2$$E10

Total reactions:

$$4\mathrm{Fe}+10{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\to 4\mathrm{Fe}{\left(\mathrm{OH}\right)}_3+8{\mathrm{H}}^{+}$$E11

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] 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 all-round, 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.

8. Conclusion

One of the problems of today’s world is the pollution of underground water sources due to the pollutants imported from various industries, especially refineries. These harmful particles enter the water in different ways and pollute the water.

The volume of production effluents is increasing, and there are many petroleum substances in these effluents. Due to the fact that these hydrocarbons are difficult to decompose biochemically and cause damage and destruction to the environment, they must be purified before being discharged into the environment.

Therefore, due to the harmful risks that they cause to human health, the environment, plants, and aquatic organisms, the treatment of petroleum effluents has been given a lot of attention. In addition, due to the large volume of petroleum effluents, a method that can be easily performed, is economically viable, and is able to separate oil pollution with high efficiency is very important.

A specific separation method should be used for each type of industrial waste according to the physical nature of the oil waste. These petrochemical effluents are mainly in the form of oil-in-water emulsions; as a result, breaking emulsions and separating oil require a correct understanding of their physical properties and chemical composition. The choice of each of the methods depends on the economic conditions and the type and form of the oily pollutant in the water. Petroleum wastewater management is done based on the type of pollutant and its volume. Choosing the right treatment method depends on many variables (pollutant type, wastewater volume, pollutant concentration, ability to recycle and extract valuable materials, local energy sources). For example, if the wastewater volume is small and sunlight is available, solar evaporation is the best choice.