A Comprehensive Review of Advanced Treatment Technologies for the Enhanced Reuse of Produced Water

Abstract

Produced water (PW) is considered to be the largest source of industrial wastewater associated with oil and gas extraction operations for industrial production. It is a mixture of organic and inorganic compounds that has high complexity in terms of various characteristics. Globally, the volume of PW is increasing along with the expansion of gas and oil fields, leading to major impacts on the environment. Existing treatment technologies involve partially treating the PW through removing the suspended solids, heavy metals, without removing organic components and re-injecting the water underground using water disposal injection wells. The treatment process consists of a primary treatment unit to remove the particles, followed a secondary biological or chemical processing treatment, while the final treatment stage involves the use of a tertiary treatment unit to improve the water quality and remove the remainder of the undesired components. Moreover, while PW is considered one of the available options to be utilized as a water source, no alternate advanced treatment options on a commercial scale are available at present due to the limitations of existing PW treatment technologies, associated with their maintainability, sustainability, cost, and level of quality improvement. As such, research focused on finding an optimal treatment approach to improve the overall process continues to be conducted, with the aim of reusing the water instead of injecting it underground. This literature review discusses the latest advanced technologies for PW treatment aimed at reusing the full stream capacity of PW and eliminating the need for wastewater disposal via injection. It is concluded that researchers should focus on hybrid treatment technologies in order to remove the pollutants from PW, effectively allowing for its reuse.

1. Introduction

Produced water (PW) is defined as “water that is generated from the reservoirs during recovery operations of gas and oil production”. This type of wastewater contains organic and toxic materials such as benzene, toluene, ethylbenzene, and xylenes (collectively known as BTEX); inorganic components such as heavy metals, polyaromatic hydrocarbons (PAHs), and alkylphenols (AP) [1]; and other pollutants as well as dissolved and suspended solids, along with chemicals that are separated in processing units. The water-to-gas ratio is reported to generally be above 3 and may reach a value of 20, depending on the location and deployment conditions of the used wells [2]. Conventional treatments involve the use of general treatment technology that mainly includes physical separation, followed by chemical and/or biological treatment, allowing for reuse of the water [3]. This is carried out due to the presence of complex pollutants in PW resulting in environmental concerns.

Typically, partial treatment of PW under certain specifications is carried out, allowing the PW to be re-injected underground via injection into disposal wells, due to the lack of an appropriate treatment enabling water reuse. Partial treatment mainly involves significantly reducing the total suspended solids (TSS) through a series of filtration processes, ensuring the integrity of the wells over time and ensuring that the PW injection pathway does not become blocked. However, this has raised concerns regarding its environmental impacts, which may cause chronic toxicity and affect ecosystems. However, it is anticipated that the volume of PW will increase in the near future due to the expansion of oil and gas extraction operations and deployed reservoirs, causing the ratio of PW to oil to increase dramatically. Therefore, attention should be paid to the treatment of PW for reuse purposes, allowing for the disposal of PW to be (at least partially) eliminated, especially in countries with scarce water resources.

Despite its expense, a large number of studies have focused on PW treatment, in order to find a suitable treatment technology through the use of an optimization process. These technologies involve advanced systems such as physical, chemical, biological, and hybrid treatments. These studies involve the use of advanced oxidation processes, electro-coagulants, membranes, micro-organisms, Fenton reactions, hydrates, and other technologies, with some reporting that further investigations are required in order to improve the overall systems in terms of their performance, cost, and limitations.

This study reviews the existing technologies for the treatment of produced water and the latest advances in processes aimed at facilitating reuse of the full stream capacity of PW, thus eliminating wastewater disposal via injection. Furthermore, the challenges associated with existing treatment technologies, including restrictions, limitations, and efficiency, are also discussed.

This study provides an overview of recent findings related to PW technologies between 2019 and 2023. Approximately 96 articles were searched using Google Scholar, of which 75 papers were considered and 21 were discarded. Some of the articles were considered in detail that contain information related to the State of Qatar and its PW parameters. The main words used in the search were “advanced produced water technologies”, “produced water treatment”, biological treatment for produced water”, and “Hybrid technology for produced water”. The focus on produced water technologies increased after 2016 due to the increasing volume of produced water and enormous expansions of oil and gas fields. Furthermore, while the number of articles on produced water has increased, an ultimate design or novel approach has still not yet been achieved, due to various limitations (either technical or directly involved with extreme operating costs). In addition to the published articles, the current review focuses on understanding the advanced technologies for produced water proposed to date, and the possible techniques to de-bottleneck the challenges impeding the commercial use of PW in the State of Qatar.

2. Characteristics of PW

PW is classified as the source of wastewater with the largest volume resulting from the production of oil and gas fields, which has been estimated at 23,000 barrels/day [4]. The volume of PW is directly proportional to the production rate and lifetime of a reservoir where, for nearly depleted fields, it has been reported that the water cut can reach 98% [3].

Globally, it has been estimated that the production of PW is nearly 250 million bbl/day, with around 80 bbl/day of oil production, resulting in a water cut of 80%. Furthermore, it has been recently reported that the water cut reached 85%. However, the volume of PW from gas fields is higher than that from oil fields, due to the dissolved acidic gasses used in the former [3]. Moreover, the water cut also depends on other factors such as the used production technologies, history of the reservoir, geographical location, and age of the wells. The water volume keeps increasing for reservoirs that are nearly depleted, where the oil production reaches as low as 2% at the point of the project where it is considered economically undesirable if further continued.

The disposal of untreated PW that contains hazardous components could lead to serious environmental impacts. Thus, appropriate treatment should be conducted in order to reduce the harmful materials to the acceptable range for disposal or treatment for its reuse [3].

A comprehensive study has been carried out by Al-Ghouti et al., in order to evaluate the existing PW treatment technologies and their performance. They concluded that the choice of a proper technology depends mainly on the location of the PW and the comparison of costs and benefits [5].

Table 1. Concentration (parts per million) of elemental components in produced water and seawater [6].

Elemental Component/Ion	Maximum Concentration in Seawater	Concentration Range in Produced Water
Salinity	35,000	23,000–67,300
Sodium	10,565	46,100–141,000
Chloride	18,982	2530–25,800
Calcium	406	530–4300
Magnesium	1258	130–3100
Potassium	384	210–1170
Sulfate	2716	46–1200
Bromide	88	7–1000
Ammonium Bicarbonate		77–560
Iodide	147	3–210
Boron	166	8–40
Carbonate	4.42	30–450
Lithium	0.17	3–50
TDS	35,490	463,000
Strontium	12	23–300

compares the concentration ranges of the elemental components in PW and seawater.

The chemical and physical characteristics of PW differ extensively, due to many factors such as well age, location, depth, and the geochemistry of the production source. PW has a similar salinity to that of seawater but higher values for sodium and chloride ions, as shown in the Table 1. The salinity of PW varies from around 300 parts per thousand [6] to approximately 36% higher, when compared to seawater. Moreover, seawater also consists of various components and elements, such as molecular gases, organic matter, and solids. The salinity of seawater is 35% typically, which is approximately 200 times that of freshwater. The above figures for PW indicate that almost all of its components are significantly higher than those of seawater; for instance, the highest level of calcium in seawater is less than the minimum value in PW, while the highest figure for the latter is nearly 10 times that in seawater. This is also applicable for magnesium, bromide, and potassium. However, sulfate and boron are two times higher in seawater.

Most of the ions in seawater are present in PW, while PW contains additional complex components due to it originating from oil and gas fields, leading to the presence of hazardous materials that have high potential to damage the marine life if discharged into the sea. An experiment on discharging PW into seawater has been carried out at laboratory scale, which recorded an obvious risk due to the enormous accumulation of concentrated chemicals that definitely lead to damage to marine life and biological groups [6]. However, the concentrations of PW pollutants vary in oil and gas fields, as illustrated in

Table 2. Typical PW constituents for oil and gas fields [7].

Parameter	Oil Field	Gas Field
Total oil/grease (mg/L)	2–565	2.3–60
Total organic carbon (mg/L)	500–2000	67–38,000
Total suspended solids (mg/L)	1.2–1000	8–5484
Total dissolved solids (mg/L)	247,000	2600–360,000
Chemical oxygen demand (mg/L)	1220	2600–120,000
Sodium (mg/L)	132–97,000	520–120,000
Chloride (mg/L)	80–200,000	1400–190,000
pH	4.3–10	3.1–7.0

.

The pollutant parameters in PW are almost the same for both gas and oil fields, with different value ranges (Table 2); for instance, PW from gas fields typically contains higher total organic carbon (TOC), total suspended solids (TSS), total dissolved solids (TDS), and chemical oxygen demand (COD), when compared to PW from oil fields. However, the pollutant values of PW from oil fields are still significantly higher than the levels indicated in environmental regulations and will lead to environmental impacts if discharged into the sea. Chloride is one of the major ions of concern in PW, which has a small radius with strong permeability and, so, can destroy the oxide film on the surface of steel structures, resulting in intensive corrosion. There are several proven technologies for the removal of chloride, including adsorption, membrane separation, oxidation, and chemical precipitation [8].

3. Usage of PW for Irrigation

Worldwide environmental standard limits have been defined for either agriculture irrigation, disposal, or re-injection. The quality of wastewater used for irrigation must not exceed the limits, in order to protect eco-environmental systems. For example, the ion content limits in PW (expressed in mmol) for sodium, potassium, calcium, magnesium, nitrate, and sulfate are 52, 2.6, 7.1, 1.9, 0.04, and 0.6 mmol, respectively [9]. Generally, the total pollution in all water samples must be below the permissive limits, such as COD of 40 mg/L and biological oxygen demand (BOD) of 20 mg/L, except when used for irrigation purposes. M. Elkholy et al. have investigated water samples for agriculture irrigation, and observed COD and BOD values of 10 and 5 mg/L, respectively [10].

Alban et al. [9] investigated the possibility of using PW for irrigation purposes in certain environments that offer potential competitive advantages for PW management, in order to support oil and gas firms. A soil water model was prepared to examine the use of PW for irrigation of sugar beet in Qatar under typical climatic conditions [9]. Furthermore, PW was blended with desalination and treated sewage water, in order to avoid extensive damage to the soil (e.g., sodification and salinization) during the experiment. The experiment was conducted and resulted in several challenges regarding the reuse of PW in irrigation, mainly due to high salinity, heavy metal, and alkaline ion concentrations, which exceeded the regulatory thresholds for irrigation water. Therefore, essential PW treatment is highly recommended prior to its reuse for irrigation purposes, even when utilizing the blending method [9].

Bridget et al. [11] further investigated the reuse of PW for irrigation purposes in the United States, as the demand of irrigation exceeds the volume of PW by five-fold [11]. Theoretical research evaluated the demand and concluded that untreated PW is not fit for purpose, as its salinity is seven times that of seawater and intensive treatment is required, which is not cost-effective [11]. The authors referred to the need to conduct feasibility studies regarding the treatment of PW and reusing it outside the energy sector, based on the local demand and environmental benefits.

Several articles on the reuse of PW for agricultural irrigation due to the high level of water demand and to reduce the operating cost of energy-intensive technology, such as desalination units, are available in the literature. Hannah et al. [12] reported that a major impact was observed after irrigation with PW. The crop yield decreased and the impact on the crop physiological parameters was evident [12]. Moreover, blending PW with more than 90% of treated water had a significant impact on the soil and crop yield. However, the accumulative effect on the soil has not yet been examined [12]. Many inquiries exist and need to be evaluated further to assess the feasibility of utilization of PW for irrigation, such as the excess sodium content and its impact on the soil, as well as excesses of salts and their impacts on plants and long-term impacts. Therefore, in order to address these critical gaps for the agriculture sector, the author suggested that further assessments at the field scale were conduct, with a focus on long-term impacts [12].

Rhizo-filtration is another phytoremediation technology that uses plant roots in order to absorb and precipitate heavy metals from contaminated effluents. This strategy has been enhanced through hydroponic experiments using accumulator and tolerant plants. Rhizo-filtration is used to examine the efficiency and reliance of phytoremediation organisms. This idea has been evaluated theoretically, and further experimental evaluations using plants in the field setting was recommended [13].

The utilization of PW for agricultural activities requires the consideration of associated health regulations due to the presence of toxic chemical species in PW that can have potential harmful impacts on humans and livestock [14].

4. Conventional Treatment Process

Most regulatory policies target the oil and gas contents of PW. This imposes strict guidelines on the industrial sector related to discharging water into the environment, which become more stringent depending on the potential impacts on the environment. Moreover, the treatment methods depend on the water quality, which determines its utility for the purpose of reuse or well re-injection disposal. An oil and grease concentration of 40 mg/L after a treatment process meets the discharge limits, according to the most recent international guideline for water disposal. For example, in the case of the environmental standard limits reported by the USA EPA for wastewater effluent discharge from the industrial sector, the standard pH is 6–8.5 and the maximum COD, BOD, O&G, and TSS are 125, 15, 5, and 20 mg/L, respectively. According to H. Eldos et al., many studies have been conducted in Qatar using different pH values, and it was concluded that the regulation limits are 1500 mg/L, 490 mg/L, 50 mg/L, and 20 mg/L for COD, BOD, oil and grease (O&G), and TSS, respectively [15]. The treatment approaches for PW can be classified into biological, physical, and chemical methods, as illustrated in Figure 1.

Physical treatment processes aim to remove the suspended solid particles with sizes ranging from 5 to 15 μm [17], as well as hydrocarbon components from produced water that are generated from the oil and gas reserves [3]. The regulatory permitted limits of hydrocarbon components vary by country, and generally range from 20 to 100 mg/L [17]. First, the water passes through a filtration process to remove large particles such as total suspended solids (TSS). Then, a gravity separator is used to separate oil from the lighter components, with the oily sludge particles accumulated in the bottom of separator prior to being transferred for further sludge treatment processes [3]. The oil will be routed to a liquid–liquid separation process in order to remove the trace oil from the PW. The most effective physical treatment method at this stage involves the use of a hydrocyclone, using fluid pressure to produce a centrifugal force which separates the oil droplets and particles from the produced water. This process can reduce the oil concentration from a maximum of 2000 mg/L to 20 mg/L with an overall average efficiency of 99% [17].

Coagulation and flocculation systems are used to adjust the pH and destabilize the particles in order to remove them using an air-dissolved flotation process. Typically, coagulation can be achieved through injecting chemicals such as ferric sulphate, ferric chloride, or polymers which neutralize the charge, in order to bond the small particles together for subsequent removal. This process occurs when the right stirring parameters are achieved. Flocculation is then used to mix the particles in order to increase the size from microflocs to as large a size as possible. This treatment is a result of van der Waals forces and is a well-established and cost-effective process. Experiments were conducted with various pH values ranging between 6 and 9, and the results demonstrated that 84% oil removal from produced water can typically be achieved [17]. However, safety concerns are the main issue associated with this system, due to corrosivity of the chemicals, and the variation in the reaction based on the dosage rate [18].

Dissolved air flotation (DAF) is conducted to remove the suspended solids, greases, oil, biochemical oxygen demand (BOD), and insoluble particles generated in the coagulation process [18].

The system consists of pressurized air supplied from the base of the tank, which creates bubbles that contact the wastewater, forcing the particles to the surface and allowing for their removal by skimming. This system is widely used and is a proven technology to improve water quality. However, the DAF design mainly depends on the produced water flowrate, in order to develop an appropriate and highly efficient air system [18].

The efficiency of a DAF system under the effects of pH and the removal of ionic species has been investigated by Mariana et al. [18]. An experiment was conducted, and they observed that the highly efficient system was able to remove most ions. Several runs were examined using DAF associated with coagulation and flocculation, where the best efficiency was obtained with coagulant and flocculant concentrations of 50 and 2 mg/L, respectively. The calcium removal efficiency was measured to be nearly 92%, with a magnesium removal efficiency of 94% and turbidity removal efficiency of 95%. The results of this study provide an opportunity to develop further studies in order to remove the macromolecules, fibers, dyes, and other materials that are present in PW.

The biological treatment of wastewater relies on the use of microbes and bacteria to degrade the organic components in the water. Wastewater can be biologically treated using an activated sludge process for the removal of various pollutants, such as organic contaminants including hydrocarbon components, BTEX, and O&G, from petroleum products such as those present in produced water [1].

Typically, three main processes are utilized for the secondary treatment, as described in the following [19].

An aerobic system uses oxygen for the micro-organisms, which degrade the organic pollutants in the wastewater and produce carbon dioxide and biomass. This is a very sensitive process that requires specific operating conditions in terms of dissolved oxygen and maintaining the solids in suspension. The process is associated with a risk of toxic materials, such as phenols, which poses limitations for conventional activated sludge treatment. Furthermore, the process is costly and energy-consuming, requiring about 0.18 kWh per ${\mathrm{m}}^3$. This process has been reported to have a removal efficiency of 60% for COD [20].

Anaerobic systems, also known as fermentation, do not use oxygen and are associated with carbon dioxide and methane as end products. Moreover, the system requires a high sludge concentration and a long retention time (about 3 to 5 days) in order to degrade organic matter. The removal efficiency of COD, O&G, and BOD has been reported to be 42%, 62%, and 38%, respectively [21].

Biological treatment is an effective technology for wastewater treatment using micro-organisms. However, it is an extremely sensitive process and requires specific conditions to accelerate the decomposition of organic matter, in terms of factors such as the pH, temperature, and aeration level [19].

The final stage of produced water treatment is designed based on the final intended use (i.e., utilization or disposal). Sand filtration is a well-established technology in which the water passes through sand for the removal of particles and the remaining organic matter, with an overall removal efficiency of 95%. After that, ultraviolet (UV) light disinfection can be utilized as a polisher, in order to ensure that any micro-organisms are disinfected at this stage [22].

Membrane filtration is the latest proven technology, which is generally used at the final stage of the tertiary treatment process to improve the water quality.

The previously discussed technologies do not reach the required level of water quality for the reuse of PW. Therefore, is it proposed that further investigations are carried out in order to increase the efficiency of full-scale produced water treatment systems [23]. This is mainly related to the complexity of treatment, as produced water typically contains around 100 types of organic compounds, which are practically required to be reduced to 8 types in the effluent at 90% efficiency of the separation process [23].

Produced water was investigated in the State of Qatar by Al-Kaabi in 2016 [24], with a focus on PW content, as illustrated in

Table 3. Chemical and physical characterizations of produced water derived from natural gas production in the north field of the State of Qatar [24].

Parameter	Unit	Produced Water (Run-1)	Produced Water (Run-2)	Produced Water (Run-3)	Mean	SD
Test Parameter
PH	NA	4.43	4.43	4.44	4.43	0.01
COD	ppm	10,370	10,440	10,680	10,496.67	162.58
TOC	ppm	2424	2401	2392	2405.67	16.50
BOD	ppm	1034	1076	992	1034.00	42.00
Salinity	ppm	4528	4460.8	4518.4	4502.40	36.35
Conductivity	µs/cm	7075	6970	7060	7035.00	56.79
TSS	ppm	25	21	18	21.33	3.51
HEM	ppm	36.4	40.4	44.8	40.53	4.20
Ions and organics
Sulfide	ppm	349	324	306	326.33	21.59
Silica	ppm	1.9	2.0	2.09	2.00	0.10
Phosphate	ppm	2.13	2.07	1.98	2.06	0.08
Sulphate	ppm	46.3	45.92	46.16	46.13	0.19
Chloride	ppm	2913	2933	2917	2921.00	10.58
Formate	ppm	0.39	0.32	0.33	0.35	0.04
Acetate	ppm	373	368	365	368.67	4.04
Propionate	ppm	18.2	16.2	17.7	17.37	1.04
Phenol	ppm	1.92	1.905	2.04	1.96	0.07
Metals
Aluminum	ppb	4.16	9.17	17.52	10.28	6.75
Arsenic	ppb	5.47	7.00	9.23	7.23	1.89
Barium	ppb	60.93	60.03	60.58	60.51	0.45
Boron	ppb	5665.38	5717.93	5850.66	5744.66	95.49
Cadmium	ppb	0.05	0.05	0.05	0.05	0.00
Calcium	ppb	283,547.66	285,227.87	287,920.52	285,565.35	2205.88
Cobalt	ppb	7.54	7.34	6.24	7.04	0.70
Chromium	ppb	30.46	29.89	30.59	30.31	0.37
Copper	ppb	0.66	0.57	0.64	0.62	0.05
Iron	ppb	4262.88	4035.3	4134.41	4144.20	114.11
Manganese	ppb	259.04	255.21	260.53	258.26	2.74
Magnesium	ppb	44,354.25	46,476.78	44,362.22	45,064.42	1223.15
Molybdenum	ppb	5.53	5.53	5.5	5.52	0.02
Nickel	ppb	7.35	7.09	6.8	7.08	0.28
Potassium	ppb	101,024.28	100,956.16	100,786.56	100,922.33	122.42
Sodium	ppb	121,5547.0	1,182,652.96	1,196,301.16	1,198,167.04	16,526.21
Strontium	ppb	13,128.02	13,103.48	13,313.63	13,181.71	114.90
Vanadium	ppb	2.58	2.52	ND	2.55	0.04
Zinc	ppb	5.25	4.98	4.7	4.98	0.28
Glycol and inhibitors
Corrosion Inhibitor	ppm	609.6	620.1	640.23	623.31	15.57
KHI	%	0.27	0.27	0.27	0.27	0.00
MEG	%	0.33	0.33	0.33	0.33	0.00
BTEX and TN
Benzene	ppb	8031	16,069	9410	11,170.00	4298.32
Ethyl benzene	ppb	4084	5415.5	4446.5	4648.67	688.39
Toluene	ppb	262	289.5	283	278.17	14.37
Xylene	ppb	1055.5	1201.5	1213.5	1156.83	87.96
TN	ppm	47.6	47.51	47.13	47.41	0.25

Note: ND: not detected.

. The experimental runs involved the utilization of a conventional treatment to purify the produced water for domestic uses (e.g., irrigation) using activated carbon and a filtration method. The report concluded a significant reduction in TSS (by 81%), with removal efficiencies for boron, cobalt, iron, manganese, and potassium metals of 68, 92, 99, 95, and 14%, respectively, as detailed in

Table 4. Chemical and physical characterization of produced water after sand filtration using conventional treatment [24].

Test Parameters	Unit	Sand Filter (Run-1)	Sand Filter (Run-2)	Sand Filter (Run-3)	Mean	SD	SF%
pH	NA	7.54	7.79	7.43	7.59	0.18	−71.13
COD	ppm	9400	9600	9300	9433.33	152.75	10.13
TOC	ppm	2383	2433	2456	2424.00	37.32	−0.76
Conductivity	µs/cm	8500	8810	8820	8710.00	181.93	−23.81
TSS	ppm	5	5	5	5.00	0.00	76.56
HIM	ppm	1.2	1.3	1.1	1.20	0.10	97.04
Ions and organics
Sulfide	ppm	0.03	0.03	0.03	0.03	0.00	99.99
Silica	ppm	0.848	0.848	0.94	0.88	0.05	55.99
Phosphate	ppm	2.5	1.7	1.7	1.97	0.46	4.53
Sulphate	ppm	379	312	419	370.00	54.06	−702.14
Chloride	ppm	2582	2583	2587.7	2584.23	3.04	11.53
Formate	ppm	0.39	0.32	0.33	0.35	0.04	0.00
Acetate	ppm	313.298	323.98	382.215	339.83	37.09	7.82
Propionate	ppm	15.247	16.773	17.7	16.57	1.24	4.57
Phenol	ppm	1.392	1.416	1.368	1.39	0.02	28.80
Metals
Cadmium	ppb	0.15	0.23	0.03	0.14	0.10	−173.33
Aluminum	ppb	ND	ND	ND	ND	ND	ND
Barium	ppb	655.41	415.6	495.73	522.25	122.08	−763.04
Boron	ppb	4850	5002	4869	4907.00	82.82	14.58
Arsenic	ppb	5.95	6.98	5.12	6.02	0.93	16.82
Calcium	ppb	449,454	503,736	534,554	495,914.67	43,085.76	−73.66
Cobalt	ppb	0.58	0.59	0.52	0.56	0.04	92.00
Chromium	ppb	0.43	1.34	0.48	0.75	0.51	97.53
Copper	ppb	132.37	124.74	84.65	113.92	25.63	−18,175.94
Iron	ppb	17.46	28.14	21.45	22.35	5.40	99.46
Manganese	ppb	7.67	12.21	12.46	10.78	2.70	95.83
Magnesium	ppb	53,366	55,000	50,412	52,926.00	2325.43	−17.45
Molybdenum	ppb	0.55	0.55	0.55	0.55	0.00	90.04
Nickel	ppb	10.49	12.53	8.93	10.65	1.81	−50.42
Potassium	ppb	87,793	87,550	85,247	86,863.33	1405.05	13.93
Sodium	ppb	1,138,533	1,140,287	1,262,204	1,180,341.33	70,900.57	1.49
Strontium	ppb	13,854	14,257	14,378	14,163.00	274.36	−7.44
Vanadium	ppb	1.62	2	2.21	1.94	0.30	23.79
Zinc	ppb	154.18	156.79	132.16	147.71	13.53	−2868.05
Glycol and inhibitors
Corrosion Inhibitor	ppm	35.1	36.73	38.16	36.66	1.53	94.12
KHI	%	0.1	0.1	0.1	0.10	0.00	62.96
MEG	%	0.01	0.01	0.01	0.01	0.00	96.97
BTEX and TN
Benzene	ppb	1006	519	773	766.00	243.58	93.14
Ethyl benzene	ppb	7.66	4.3	7.43	6.46	1.88	99.86
Toluene	ppb	353	158.64	315	275.55	103.01	0.94
Xylene	ppb	43.04	24.56	46.95	38.18	11.96	96.70
TN	ppm	17.2	17.5	16.84	17.18	0.33	63.77

Note: ND: not detected.

. Moreover, they achieved slight reductions in COD and chloride, with removal efficiencies of 10 and 11%, respectively, but no reduction in TOC or BTEX. Therefore, it was recommended to conduct further evaluations using appropriate advanced technology in order to further develop PW treatment approaches for the enhanced removal of COD, chloride, TOC, and BTEX.

Economic and environmental studies have reported that the treatment effect is dominated by the degree of hydrocarbon content in PW. Thus, the development of treatment processes for produced water management to achieve the highest level of treatment, either for reuse or injection, remains challenging [23].

The treatments for PW are continually advancing, and the selection of an optimal approach depends on the compounds present in the produced water and the effluent quality requirements for reuse options, as well as considering several factors such as operating costs and sustainability [11].

Advanced treatment technologies that could be used to facilitate the reuse of produced water, instead of well re-injection, are discussed in the following sections.

5. Chemical Treatments Enabling Reuse of Produced Water

This section explores the treatment options to enhance the reuse potential of treated PW.

5.1. Advanced Oxidation Processes (AOPs)

Advance oxidation processes, including ozonation, Fenton reactions, and photocatalysis have been heavily used as advanced technologies to treat and polish produced water, as has been reported by Silvia et al. [25]. Most of these works were carried out using synthetic wastewater. The researchers prepared synthetic seawater by adding BTEX, malonic acids, phenol, and naphthalene, and implemented all three AOP treatment options. The results varied greatly in terms of total organic carbon (TOC) removal efficiency. Photocatalysis was found to be the least efficient PW treatment, with TOC removal of 20%. In contrast, ozonation combined with H₂O₂ showed the best results, with TOC removal efficiency of 74% and acetic acid elimination of 78% [25]. However, the high efficiency of organic removal alone is inadequate for the reuse of water, as the other matters (e.g., dissolved organic components) were not removed, as stated by the researchers [25]. Bioprocessing seems inadequate, due to the highly toxic components in PW significantly lowering the removal efficiency of components such as phenol and BTEX. This complexity renders the single treatment of PW for reuse purposes impossible.

AOP technology is mainly dependent on the characteristics of hydroxyl radicals, as they are highly reactive and have the potential to oxidize organic matter to carbon dioxide and water. This is the advantage of AOPs compared with other treatment technologies, which mostly aim to capture the contaminants via processes such as membrane and active carbon filtering, as reported by Silvia et al. [25].

Fenton-based reactions consist of an aqueous combination of hydrogen peroxide (H₂O₂) and Ferrous ions Fe²⁺, which lead to the decomposition of hydrogen peroxide to hydroxyl radicals and ions and the oxidation of ferrous ions, as represented in reaction (1) [26]:

Fe²⁺ + H₂O₂ → Fe³⁺ + · OH + OH⁻

(1)

Hydrogen peroxide and ozone are the strongest oxidants, which are used in combination with either homogeneous or heterogeneous catalysts for AOP treatment, in order to enhance the generation of radicals.

Most of the latest studies on AOP treatment have focused on evaluating the removal efficiency of single organic components. However, PW is complex and contains multiple organic components, thus requiring the use of an integrated system to degrade them all in a high salt concentration with higher efficiency. Furthermore, acetic acid is considered one of the factors that lowers the overall efficiency.

Fenton-based processes are effective for individual TOC removal, and can rapidly remove phenols, naphthalene, and BTEX; however, they are not effective for the removal of acetic acid [25,27]. The highest elimination of total organic carbon was achieved using a combination of ozonation with H₂O₂, with removal efficiencies for TOC and acetic acid of 74% and 78%, respectively. H₂O₂ is consumed during this reaction and the final pH is nearly 8, which is within the discharge limit range.

Despite the remaining percentage of acetic acid in the PW, AOPs are a promising treatment approach which require further research and pilot-scale experiments, as reported by Silvia et al. [25]. Additional optimization is required for H₂O₂ consumption at large scale during the process of ozonation treatment. Moreover, RO is another option to be tested in combination with Fenton process, in order to remove acetic acid [25].

Marco et al. [28] further evaluated AOPs using Fenton-based, electro-oxidation, photo-assisted, heterogeneous catalysis and homogeneous processes for the removal of organic components from PW. Commonly used technologies for PW treatment include ozonation, anodic oxidation, and heterogeneous photocatalysis. However, electrochemistry and photo-assisted processes have demonstrated significant improvements in terms of organic removal efficiency, ranging up to 96%. Such an approach could also be used prior to the primary or secondary treatment, in order to enhance the biological treatment and membrane process at the final stage of PW treatment. The combination of AOPs is a promising methodology for fulfilling the requirements of both secondary and tertiary treatments. However, it should be subjected to further examination and implementation regarding real PW systems [28].

AOPs have some restrictions for the treatment of PW streams that include radical scavenging species such carbonate and chloride. Consequently, AOPs require pre-treatment process in order to increase their sustainability and reliability [28].

Moreover, Priscila et al. [29] conducted two experiments using seawater including phenols and real oil-derived produced water. The two AOPs used included photo-Fenton and photocatalysis processes. The catalyst types for the AOPs were TiO₂ for photocatalysis and FeSO₄.7H₂O for photo-Fenton [29].

The experiment revealed an effective removal of dissolved organic matter when using both processes and heating to 75 °C with effluent pH at 7. However, this experiment was not performed at large scale and, so, requires further evaluation [29].

Various types of AOPs and mechanisms are highly competitive for PW treatment and are capable of removing organic pollutants with high efficiency. AOPs can be divided into two main groups: conventional and emerging technologies. Conventional technologies, such as coagulation, ozone-based, Fenton-based, and adsorption processes, can only change the phase of the organic compounds and do not transform them into other components. Meanwhile, emerging technologies are enhanced AOPs, which are integrated for better economic and degradation efficiency [30].

Ozone-based AOPs combined with UV or H₂O₂ have been used extensively for water treatment since their discovery, due to their high rate of contamination degradation and disinfection effect [31]. However, these processes have several limitations, including slow mass transfer of O₃, high energy consumption to produce O₃, low solubility of O₃, and instability of ozone in the solution [31]. To overcome these challenges, a catalytic ozonation process was developed to enhance the treatment through the use of dissolved metals, such as zinc, copper, magnesium, and iron, to catalyze the O₃ decomposition reaction and produce hydroperoxyl radical and hydroxide to degrade the contaminants [31]. The literature indicated that a pilot-scale catalytical ozonation reactor was used to treat 22 m³/h of wastewater and achieved nearly 70% COD removal after 30 min of processing [31].

A new AOP for water purification has recently been evaluated by Yangju L. et al. [32] that uses percarbonate (SPC-AOP) and is considered an environmentally friendly, safe-to-handle, stable, and cost-effective treatment method for water purification. This process enables the joint formation of multiple reactive species, such as superoxide radicals, hydroxyl radicals, carbonate radicals, and peroxymonocarbonate, in order to simultaneously degrade the pollutants [32]. The study concluded that the combined SPC-AOP is an efficient treatment method that removes more than 90% of most of the contaminant species. It was also highlighted that future evaluations should focus on the agents and transformation products, as well as the combination of theoretical and experimental methodologies in depth, in order to define the target pollutants for water treatment [32].

5.2. Electrocoagulation

The electrocoagulation (EC) process is another advanced PW treatment technology. Manilal et al. [33] reported a novel method for PW treatment based on EC designed with shift polarity, which removes the pollutants from PW via electrode passivation. The performance of this system was evaluated in terms of pollutant removal, energy consumption, and operating cost. The experimental outcome indicated the removal of 99% COD and 98% oil and grease effectively from PW. The sludge generation was much lower than that observed in other electro-chemical treatment methods, such as coagulation and flocculation [33]. Many studies have evaluated EC and related areas for improvement, such as the pH, electrolysis time, electrode gap, and electrolyte concentration [33]. However, the main bottleneck of the EC process has been reported as the passivation of electrodes over time during continuous electrochemical processes [33].

Recall that the passivation of electrodes is defined as the formation of an inhibiting oxide layer on the electrode’s surface during the electrochemical reaction. This layer is sufficient to inhibit metal dissolution from the anode and electron transfer from the cathode, which ultimately increases the resistance and results in increased power consumption [33]. This concern has been reported frequently by many researchers.

Experiments involving a polarity changing switch, AC current, and ultra-sonic waves have been conducted by two researchers so far, in order to mitigate passivation concerns. In this regard, Manilal et al. [33] studied an EC batch process designed with a polarity changeover switch in order to remove the pollutants from prepared wastewater with a similar composition to PW. It was noticed the pollutant removal was significantly affected with the use of the changing polarity EC treatment. However, the pollutant removal rate achieved using this design highly depends on the current density, and it must be further investigated in terms of continuous-flow operations and reduction in its footprint.

Moeen et al. [34] studied different EC systems to evaluate their system performance and efficiency using electro-activation and thermal activation for real PW using iron electrodes. Their experiments illustrated the significant removal of almost all hydrocarbon pollutants in PW, with an operating cost much lower than that of AOPs [34]. According to the laboratory pollutant removal results, 96% of H2S, 99% of oil and grease, 84% COD, and 97% turbidity were removed. However, the system is insufficient for the removal of soluble organic materials and ammonia, and should be subjected to further investigations [34].

Researchers have focused on the enhancement of EC processes in recent years; for example, Ezerie et al. [35] investigated the continuous electrocoagulation of boron for PW treatment using an aluminum and iron plate electrode system with variation in the electrode distance and time. The overall reported boron removal efficiency was nearly 50%.

Boron is an inorganic chemical element that has a high level of removal complexity from PW due to the difficulty of removal of ionic forms based on the pH of the solution. At higher pH levels, boron exists as borate ion, whereas it exists in the form of boric acid at lower pH. Moreover, it has a significant impact on downstream process such as RO, biological treatment, and electrodialysis application; as such, the demand for suitable boron removal technology is increasing [35].

Electrocoagulation is one of the promising technological alternatives to conventional industrial treatment technologies for PW and wastewater, due to its low operating cost, high efficiency, and basic mechanism that involve oxidation of the electrode under the influence of an electric field. Once the anode oxidizes, the wastewater pollutants are neutralized by the influence of the metal ion and are released into the aqueous phase [35].

The study concluded that the boron content decreased by 80–60% for aluminum and nearly 70–50% for the iron electrode, with a retention time from 45 to 30 min. Moreover, it decreased further when increasing the electrode distance from 0.5 to 1.5 cm. This study demonstrated an initial suitable application for boron removal, with further investigation being required to enhance the pollutant removal process [35].

The electrode material is the key element when seeking to enhance the performance of EC technology. The selection of the material highly depends on the influent components and oxidation potential. Aluminum and iron are the most used electrode materials, due to their low cost, availability, and potential. Using the mentioned materials in a continuous electrocoagulation approach for boron removal should be further evaluated, as there is no existing study investigating the use of iron and aluminum for EC processing [35].

Moreover, another study, conducted by Golnoosh et al. [36], aimed to soften produced water using a high-pH catholyte from brine electrolysis. The benefits of this approach include generating caustic soda and chlorine in order to reduce the environmental footprint and need for chemical transportation. The degree of pollutant removal varies based on the PW pH and the catholyte [36]. Mg removal above 90% was achieved when the pH was 11, compared to lower Ca removal (below 10%) due to the higher bicarbonate alkalinity of PW. Most researchers have recommended that a combined PW treatment system be designed, as it is not possible to remove all PW components with a single treatment technology [36]. However, membrane fouling poses a challenge for this combination, due to the high concentrations of cations and organic matter in PW. Consequently, pre-treatment is needed in combination with membrane technology [36].

Alkalization technology is a cost-effective method using alkaline chemicals such as caustic soda to promote the chemical precipitation of divalent cations. Therefore, utilizing PW for brine electrolysis is the best option for alkaline production from treated PW after the removal of multi-valent ions and organic matter. The conclusion of this study focused on future work that is necessary to validate this theoretical option for the real treatment of PW via an electrolysis process [36].

Chia et al. [37] performed a comparison of chemical and electrochemical coagulation in terms of performance, energy consumption, and cost. The essential step of water treatment is to remove the emulsified hydrocarbons, which requires the use of chemical agents for destabilization of the emulsified oil.

The conventional chemical coagulation (CC) approach involves the rapid mixing of polymeric and metal salt coagulants with water to induce charge destabilization. Meanwhile, slow mixing is needed to enhance the aggregation of emulsified oil droplets for further separation. Likewise, electrochemical coagulation uses sacrificial electrodes (e.g., aluminum or iron) to supply coagulants. In this process, a DC current is passed through water, where the anode generates metal ions to form metal hydroxide precipitates which destabilize particles for subsequent flocculation processing [37]. It has been reported that EC is a more effective and economic process than CC. In addition, there is no direct comparison between the two technologies, as CC involves intermittent chemical dosing while EC involves the continuous supply of coagulants through the sacrificial electrodes. Therefore, the operating conditions play a key role in these processes.

The results for EC and CC were compared, and it was observed that CC is capable of removing more COD than EC with iron electrodes. However, CC is not able to remove oil and grease effectively, even at a higher coagulant dosage. It was also reported that EC with iron at higher concentrations is able to remove O&G and COD, and is a simpler treatment that does not requires further treatments such as filtration or flotation, as is the case for CC [37].

Further investigations are needed to improve both treatment technologies, as overdosage in CC results in poor performance. Other challenges are related to the operating costs and footprints of chemicals used in the CC process. EC is a promising technology and researchers need to address its constraints to improve the overall efficiency for PW treatment [37].

The use of EC treatment in combination with degradation processes has been reported by Nidheesh et al. [38]. The selection of the degradation process, such as photo-Fenton or ozonation electro-Fenton, purely depends on the characteristics of the PW in order to obtain the highest removal rate of all pollutants from the PW. There are many types of AOPs that can be investigated further to determine highly effective technologies, such as sulfate radical-based processes and anodic oxidation for advanced oxidation processes [38].

The authors reported that the combination of AOP and EC is very effective and does not require further investigation in order to debottleneck the limitations of real PW. It was recommended to set up a pilot-scale instead of lab-scale plant to obtain an accurate performance evaluation. Moreover, real PW should be investigated at the experimental stage, as most of the existing studies have focused on synthetic wastewater. This is a promising treatment technology which requires more attention, as very few articles have referred to the combination of EC and Fenton-based processes [38].

Mahdieh et al. [39] have reported that EC is an effective mechanism for removal of oil and heavy metals from PW. An experiment was conducted using synthetic PW and different types of electrodes, such as aluminum, iron, zinc, and copper. It was observed that, when using iron electrodes, the COD was removed under low and high saline concentrations at around 90% and 80%, respectively. The average COD removal rates using zinc and copper electrodes were 87% and 85%, respectively. Meanwhile, a higher removal rate of 95% was achieved using aluminum. When the report was concluded, the authors did not refer to further investigations to improve the performance [39].

Coalescence is another treatment technology for PW that must be considered, as reported by Yiqian et al. [23]. Novel technologies can be obtained by combining two or more proven and well-established PW treatment approaches, such as separation, sedimentation, compact flotation, and hydrocyclone methods.

Coalescence refers to the process of oil droplets coalescing in a filter cartridge for purification of PW. This type of adsorption or separation process is not accurate and effective, similar to membrane filtration, mainly due to the large holes among the fibers. Therefore, coalescence technology should be subjected to further investigation for technological improvement [23].

Many investigations have been conducted to evaluate large-scale PW treatment approaches. Milinkumar et al. [40] have reported a novel technology for produced water treatment using a settling tank. However, it is impossible to have a single treatment for the complete processing of a complex solution such as PW, and most researchers acknowledge that this type of water requires a combined treatment technology in order to enable its reuse.

Yiqian et al. [2] considered PW from economic and management perspectives in order to evaluate the constraints associated with PW treatment. It was reported that about 80% of the content in oil fields is water, with this value increasing significantly to about 98% in some wells.

5.3. Desalination

Typically, thermal distillation and RO are the most used techniques to remove dissolved salts from wastewater. However, these technologies are energy-intensive and require energy-based assessments to achieve cost-effective technology in order to meet water regulations. Moreover, a hydrate-based desalination process has been examined by Ponnivalavan et al. [41]. In this process, thermodynamics play a key role in freezing water; the salts do not participate in the formation of hydrates and act as inhibitors. The authors used an additive and reported that, when the hydrate melted, the salts and guest molecules can be recycled, with only pure water moving to a separate tank [41]. However, this process has only been tested at the pilot scale, with commercial-scale experiments not having been performed due to delays associated with challenges related to separation and slow kinetics. A detailed analysis was also not provided in the study [41].

5.4. Alkali Precipitation

Treating PW using an alkali precipitation technique has been investigated by Meicheng et al. [42]. The authors compared it with flocculation, in terms of agitating and settling times and the dosage of coagulant. It was reported that the alkaline pre-treatment process is a treatment for PW with high potential and the capability for pollutant removal, as well as being simple and cost-effective. During their experiment, three types of additives were selected for the alkaline test: sodium hydroxide, sodium carbonate, and calcium oxide. Figure 2 illustrates the test result, where suspended solids and hardness ions were removed from the PW. Suspended solids were removed up to 90% when sodium hydroxide was added to the solution. However, hardness was removed only partially (about 35%) and the resulting effluent did not meet the discharge regulations [42].

Generally, alkaline precipitation is combined with another treatment technology that has a higher ability for removal. Thus, it should be subjected to further research [42].

Tamires et al. [43] stated that a comprehensive PW treatment has not yet been proven, and more focus from both the academic and industrial sectors is required. The authors emphasized the adsorption process as a polisher step, as it is the most effective technique to date. However, the removal of soluble organic compounds is also challenging, as they cannot be removed using traditional treatments such hydrocyclone, settling, or flotation processes. Thus, advanced treatments could solve this problem.

Figure 3 illustrates the number of studies in the published literature related to produced water. Adsorption is involved in the highest number of studies, due to the sustainability and effectiveness of treatment process, as stated by the authors.

In 2020, Mona et al. [44] presented a PW treatment technique that was effective without forming scales during the oil recovery process. This study involved treating oil-free petroleum water (PPW) without scale tendency using water treatment sludge (WTS) produced from a drinking water system. They also assessed the characteristics of the WTS to determine the possibility of its reuse. It was evident that scale deposits are the main problems in production and injection wells in the petroleum field. Aluminum is the main material used in drinking water treatment plants (DWTPs). However, the process produces large amounts of solid waste materials, which are challenging to dispose of. The study also reported that there are cost and environmental benefits related to the reuse of WTS for the treatment of wastewater. Chemical coagulation is also important in wastewater treatment, as it alters most dissolved and suspended materials into precipitates. They also explained that coagulation consists of several mechanisms that work together to form insoluble precipitates. This process was evaluated as being cost-effective, environmentally friendly, and enhanced the reuse of PPW without issues related to scale formation. However, the authors did not mention whether any limitations exist or if further assessment is required [44].

Considering PW treatment processes, there are few studies focused on the generation of electricity. Salman et al. [45] evaluated the utility of microbial fuel cells in converting chemical energy to electrical energy. In this case, micro-organisms act as a biocatalyst to speed up the reaction and produce energy [45]. Their research showed that microbial fuel cells are a suitable treatment approach for PW treatment and electricity generation. The cells are widely used as they are cost-effective and environmentally friendly, compared to other methods. The study indicated that partial treatment of oil and grease in produced water was achieved, and a balance between PW treatment and electricity generation was obtained [45].

A microbial fuel cell (MFC) is a bioreactor that converts the chemical energy in organic compounds into electrical energy through catalytic reactions under anaerobic conditions. Organisms are placed in an anode chamber to oxidize the organic compounds in PW into hydrogen and carbon dioxide. Hydrogen is transferred to the cathode through a proton exchange membrane (PEM) and reacts with oxygen to form water, creating an electrical flow between the two terminals. However, the main limitation in the context of PW treatment is related to the proper selection of bacteria that could survive in such complex water. Thus, further analysis needs to be conducted for this process [45].

5.5. Microemulsion

Dennys et al. [46] have reported that, while the treatment technology for PW is feasible, the state-of-the-art systems for PW treatment to eliminate dispersed elements have high operating costs. The study indicated that the petroleum industry has emphasized the use of surfactants and microemulsion systems (MESs) for the production and treatment of liquid and solid waste products.

A microemulsion is a thermodynamically stable system which can solubilize polar and non-polar substances. These systems are formed by two immiscible fluids (normally water and oil). MESs can be used to remove metals and dispersed oil from PW, and this method is advantageous as it does not result in solid waste generation, as compared to flotation. The surfactants used in this system are non-ionic commercial demulsifiers, which are commonly used in the oil industry. The study used the Scheffé mixture model for experiments with different mass sizes, in order to optimize the process and the oil removal kinetics, speed of separation, and load capacity. However, the following paper published by the authors did not refer to any limitations [46].

Luana et al. [47] presented and suggested to develop a combined PW treatment technology, as the characteristics of PW are very complex and advanced treatment technology is required for its reuse. AOPs are the main alternative techniques used for PW treatment. The technique forms reactive elements that are able to react with organic and inorganic compounds, thus converting them to less harmful products. AOPs are effective and have proven efficacy in the degradation of effluents in wastewater treatment. The research also indicated that PW treatment mainly involves a combination of two or more chemical, biological, physical, and/or photochemical techniques. In fact, PW cannot be treated with a single method, and a combination system is required for reuse of the water [47].

J.S.B. Souza et al. [48] have indicated that it is possible to eliminate O&G and salinity from PW using a microemulsion. The water is treated using the region of Winsor IV (microemulsions) to reduce the contaminants, with acceptable effects in terms of time, microemulsion dosage, and temperature.

Chemicals are added in this process, but they require close monitoring to avoid complications. Treatment methods such as demulsification can achieve excellent results for O&G treatment but can be expensive. The use of a microemulsion is also an outstanding treatment process, due to its ability to reduces the interface tension between water and oil, thus promoting interfacial transfer between different stages in the extraction process [48].

5.6. Liquid–Liquid Extraction

Liquid–liquid PW treatment is an interesting area of study in the oil and gas industrial sector. As such, this study investigated innovative industrial liquid–liquid wastewater extraction techniques for the decontamination of PW. Despite the complexity of the PW mixture, no literature review has focused on this topic, as reported by Ana et al. [49].

Therefore, a literature review on this topic was carried out to find novel liquid–liquid extraction methods. One study used bromocresol green (BG) and phenol with surfactant, brine, oil, and alcohol to examine the elimination from water via liquid–liquid extraction. It was evident that BG removed more than 90% water, and the removal efficiency increased as the system’s salinity increased [49].

The authors noted that the use of ethyl oleate and ethyl butyrate can remove a significant amount of phenol from water. However, the amount of phenol removed depended on the hydrophilicity of the solvent. Therefore, ethyl butyrate extracted phenol more effectively than ethyl oleate [49].

It was also evident that, at low pH, phenol extraction was more dependent on its interaction with the solvent. About 99% of phenol was eliminated using octanol, whereas the overall extraction efficiency between organic acids and amines was between 50 and 70%. Moreover, a multistage separation process was recommended for liquid–liquid extraction, which increased the efficiency to nearly 85%, in contrast to 60% in a single extraction process [49].

The study concluded that the liquid–liquid extraction method is versatile, effective, simple, and can thus be applied in many industries. However, assessments of the extraction effects are challenging, due to the complexity of PW samples [49].

5.7. PdAu-Catalyzed Oxidation

Yiyuan et al. [50] investigated how alumina-supported bimetallic PdAu degrades organic compounds through the catalytic formation of H₂O₂ at atm pressure and room temperature. The PdAu catalyst generated hydroxyl radicals and H₂O₂ in the presence of formic acid and oxygen. It was evident that PdAu had the highest rate of degradation when phenol was introduced to the system. They also evaluated several technologies to reduce the total dissolved solids (TDS) such as ion exchange, RO, evaporative crystallization, and membrane distillation [50]. Moreover, organic compounds can be removed using activated carbon. However, this technique gave mixed results, depending on the efficiencies and deployment costs.

They concluded the study by reporting that PdAu was the most active catalyst type to degrade phenol contamination in PW, due to the rapid generation of OH radicals. The bimetallic composition improved the resistance to catalyst deactivation due to the high salinity of PW. However, the study was conducted using only one bimetallic composition (1 wt% Pd and 1 wt% Au), and further research is required to evaluate the phenol degradation rate when using different compositions [50].

5.8. Bioelectrochemical Reactor

It is possible to remove the pollutants from PW using single- and dual-chamber microbial fuel cells (MFCs), as reported by Gunda et al. [51]. Their study involved treating the recalcitrant contaminants from produced water, such as hydrocarbons, COD, and sulfates, while both MFCs kept operating under the same conditions, including retention time, electrode material, and temperature. It was evident that bioelectrochemical systems (BESs) are versatile and can generate energy during the treatment process. These systems can also be used to remove specific contaminants, based on the content of the PW.

It was experimentally demonstrated that the dual-chamber system achieved higher efficiency in terms of bioelectrogenesis, sulfate and hydrocarbon removal, and COD reduction. However, the authors also stated that PW has different physicochemical characteristics that require special attention. MFCs, therefore, should be operated under ambient conditions. The study also showed that the electrode materials and wastewater in the anode affect the performance of the process. It is necessary to evaluate the system on a large scale for detailed information regarding its efficiency, power generation, and operational cost [51].

5.9. Ferrate (VI) Oxidation

The PW stream contains polycyclic aromatic hydrocarbons (PAHs) made up of benzene rings. Tahir et al. [52] evaluated how to remove PAHs from PW using ferrate (VI) oxidation for the first time in a highly concentrated sample including 15 PAHs. The research demonstrated that it is possible to remove more than 90% of PAHs and 73% of COD.

It is evident that several other methods are used for the treatment of PW. However, some of these methods, such as electrocoagulation, are expensive, consume a lot of energy, and result in the generation of a large amount of sludge. Membranes can also be used but consume a lot of energy, are subject to fouling, and lack the ability to degrade refractory organic contaminants. At present, ferrate (VI) oxidation has gained attention due to its high oxidation/reduction potential. This method is environmentally friendly, cheap, and highly efficient for organic contaminant removal. Ferrate (VI) also acts as a coagulant, oxidizer, and disinfectant in wastewater treatment. It is considered one of the most efficient oxidants in wastewater treatment due to its strong oxidizing power. However, its effectiveness depends on operational parameters such as contact time, Fe (VI) concentration, and pH. The authors suggested using real PW to conduct continuous flow assessment for detailed analysis of the application of Fe (VI) for PW treatment [52].

5.10. Phytoremediation

Sustainable water resource management is one of the biggest challenges across the globe. This phenomenon is worse in countries that experience harsh arid environments, such as Gulf countries. Fathy et al. [53] have indicated that countries such as Qatar require a suitable water resource plan for their growing population. The population and remarkable industrialization in the country have put increasing pressure on the existing renewable water sources. Studies have shown that Qatar is among the highest water-consuming countries in the world. However, water is mainly substituted from desalination, placing stresses on water resources [53]. The country has developed desalination technologies powered by fossil fuels for the treatment of wastewater over the last few decades. It is evident that PW is mainly produced from industrial activities such as oil, gas, coal, and gas shale production. The study indicates that, while several PW treatment methods are available, the selection must depend on the water’s composition and the desired quality of the end-product. Most countries use phytoremediation as it is a cost-effective and solar-driven biotechnological treatment. It is also effective, especially when the wastewater contains low levels of pollutants. Finally, it is environmentally friendly and an important substitute for mechanical remediation [53]. The experiment involved many types of crop species, including alfalfa, zea mays, sorghum bicolor, and more plants, in order to study the removal of toxic pollutants from produced water. The test was conducted by diluting PW ten times with tap water, in order to evaluate the efficiency at different concentrations. It was shown that the PW has a significant impact on both soil and plant growth, where the crop species completely died at a level of 20% concentrated PW. Beyond this concentration, the accumulation of heavy metals and morphological structure were completely disturbed. At the soil level, the main impact is a significant increase in the sodium adsorption ratio level, which interferes with the physical characteristics of soil, such as water flow and permeability [53].

5.11. Gas Hydrate

Several advanced technologies have been developed for PW and seawater management in the last few years. In 2021, Sirisha et al. examined how gas hydrate is used, in terms of technological and evolutionary aspects, for PW treatment at different salinity concentrations [6].

Gas hydrate treatment technology could be the future treatment technology for PW in order to generate clean water that can be used for various purposes, including potable water, as suggested by the authors [6].

The salt contents in PW can be reduced to acceptable limits using several treatment technologies such physical, biological, chemical, or desalination treatment. However, physical processes have limitations in terms of dissolved organic and inorganic content removal [6].

Compared with all other innovative techniques for PW treatment, gas hydrate is one of the most appropriate technologies for seawater and PW treatment and should be further explored by researchers. Gas hydrate is a thermal processing technique that is attractive as the vital resource is readily available water such as seawater and PW. Furthermore, industries can produce potable water using this technique, which is 50% more economical than other technologies and does not require a pre-treatment method for support. It is also more environmentally friendly when compared to other conventional methods. Gas hydrate only requires low temperatures, high pressures, and the essential quantity of water, as illustrated in Figure 4 [6].

This technique involves the physical change using a hydrate former, which is introduced into the reactor in the presence of PW, resulting in the production of a hydrate and brine mixture. Salt is captured in the structure of hydrate and is removed by filtration at a later stage. However, the key challenge of the hydrate formation treatment process is related to the separation of brine from the hydrate crystals. Thus, it should be subjected to further investigation in order to improve the separation process [6].

The separation process for a highly complex stream such as PW requires significant effort to develop a system that can effectively stand continuous operations while ensuring the reliability and sustainability of water treatment. Traditional treatments involve the use of a chemical precipitation mechanism, whereas advanced technologies use membrane applications for the purpose of brine separation. Both methods are efficient but have different limitations [54].

5.12. Fibrous Coalescence Technique

Oily water removal is another concern in the treatment of PW, which requires an urgent focus in research as the use of oil and gas extraction is increasing. In 2019, Hao et al. [55] introduced a novel technology for oily PW removal using fibrous coalescer technology. Their experiments involved the use of metal and oleophilic polymer fibers in a fibrous bed with an appropriate geometric structure to separate the components of the PW. It was evident that the oil contaminants in the PW were reduced significantly to a certain percentage. This is one of the best applications of the fibrous coalescence technique in the gas field, providing a promising alternative that contributes to cleaner production and marine conservation [55].

Decreasing the pressure and temperature can lead to oil–water emulsification during phase transformation and fluid transportation of the mixture, consequently generating a large quantity of oil droplets less than 10 micrometers in size, an achievement beyond that possible with traditional separation treatment approaches [55].

The coalescing filter and fibrous coalescers could separate emulsified oil droplets less than 10 micrometers, as shown in Figure 5; however, the coalescing filter is not compatible with complicated operating conditions such high pressure, flow disturbances, and particle accumulation conditions. In contrast, the fibrous coalescer is more flexible and has a high tolerance for particle accumulation and flow fluctuations. Additionally, it is a proven novel technology and has been operating well at the industrial scale for more than a year with outstanding performance, discharging PW with an acceptable oil concentration (below the national regulatory limit) [55].

A Polit system was established in the South China Sea to evaluate a PW treatment that contains a liquid phase and organic and inorganic pollutants of high complexity, as reported by Yiqian et al. [56]. The test was conducted with multiple treatment units, as shown in Figure 6. The liquid phase was classified into six categories, where the typical situations are represented in Figure 6a–d, whereas Figure 6e,f depict extremely abnormal situations. Figure 6g shows the PW after oil removal, Figure 6h shows the PW discharged into the ocean, and Figure 6i shows the appearance of the oil and water contents.

The study concluded that the effluent concentration was less than 50% of the environment regulatory standard and, thus, it is acceptable to discharge the treated PW into the sea. However, the capacity of the pilot system is 0.5 cubic meters per hour, and it was suggested to test the unit at a larger scale on a continuous operation basis [56].

6. Membrane Technologies for Physical Treatment

Physical processing methods are a pillar of wastewater treatment, aiming to improve the quality of water. Several technologies are used in both the primary and tertiary stages such as particle removal using screening, filtration, and membranes, which have recently been used widely in industries for water treatment. This literature review aims to investigate the physical technologies developed in the past three years and their limitations for further consideration.

6.1. Membrane Bioreactors (MBRs)

A membrane bioreactor combines either ultrafiltration or microfiltration with a suspended growth bioreactor to treat various wastewaters (e.g., industrial or municipal). MBRs have the capability to degrade biomass in produced water from oilfields and could replace the conventional secondary-activated sludge treatment systems. Membrane filtration has become an essential treatment technology for wastewater and is well established and widely used at present. Membrane bioreactors (MBRs) are the latest technology for the removal of O&G and COD from wastewater; however, their ability to remove organic and inorganic matter requires further verification, as reported by Mohammad et al. [57]. Thus, the authors studied the possibility of removing phosphate ions with salt content. Furthermore, it was observed that the removal efficiency of O&G and COD reached 90%, while the phosphate ion removal rate was 15%. Fouling of the membrane is the main constraint of this treatment, and the removal efficiency varies with the salinity concentration in the PW. Thus, it was suggested to carry out further assessments using a combined system to increase the removal efficiency of salt content [57]. Generally, MBR operating conditions are highly influenced by characteristics such as the content, size, and growth rate of the micro-organisms [58]. Furthermore, the efficiency of micro-organisms can affect the performance of MBRs, mostly in a manner related to the effluent quality and fouling of the membrane. Therefore, it is necessary to determine the optimal operating conditions, microbial stoichiometry, and MBR kinetics to achieve maximum performance and optimum operating conditions. Nevertheless, the microbial community in an MBR varies from one plant to another and with the scale of a given MBR. In general, the major groups of micro-organisms used in MBRs are bacteria, filamentous bacteria, protozoa, fungi, and algae [58].

6.2. Hydrophilic Polyvinylidene Fluoride (PVDF) Membranes

Membrane technologies have undergone remarkable improvements recently, in terms of water quality, salt rejection, and water permeability. Hydrophilic polyvinylidene fluoride (PVDF) membranes are one of the most promising technologies, due to their excellent chemical stability, mechanical properties, and film-forming ability. However, their low permeability flux and poor anti-fouling ability are the main concerns that limit their application [59].

A type of hydrophilic polyvinylidene fluoride (PVDF) membrane was evaluated in 2020 by Normi et al. [60] for the separation of emulsified oil droplets from OPW, with the results showing a removal efficiency greater than 98%. However, the implementation of this technology on a large scale is limited due to fouling concerns reported at the lab scale.

Fouling phenomena extremely limit the sustainability and continuous operation of membrane technology. Developing new materials has been the main focus in the past few years. Of all polymeric-based membranes, those of the PVDF type are used widely, despite the limitations associated with fouling. Detailed investigations are required to improve their antifouling properties [60].

6.3. Fenton and Modified Fenton Oxidation Coupled with Membrane Distillation

An alternative advanced treatment for PW was evaluated by Giulio et al. [61]. The purpose of this study was to use a combined system that consists of a Fenton-based process followed by membrane distillation, with the objective of reducing the contaminants using the Fenton process as pre-treatment in order to remove the pollutants completely and to eliminate the fouling concerns regarding the membrane. It was observed that oxidation as a pre-treatment removed dissolved organic carbon (DOC) by almost 60% and almost completely degraded toxic organic matter with low molecular weights. While pre-treatment did not enhance the process of membrane distillation, it significantly improved the water quality. However, pre-treatment using Fenton-based processes has not yet been proven to eliminate fouling in membranes, which should be considered in further research, as reported by the authors [61].

6.4. Membrane Distillation (MD)

Membrane distillation is a well-established and proven treatment method for PW feeds, which serves to reject the dissolved solutes without a major impact on the process conditions. These parameters make this technology highly effective, and it is the best option for hypersaline water (i.e., with high levels of salinity) such as PW. Polymeric membranes have recently gained the attention for wastewater treatment over ceramic membranes, due to their performance and lower cost [7].

Using hybridized MD systems to treat PW shows great performance, either with PW feed for pre-treatment or post-cleaning. To achieve a cost-effective approach for beneficial PW re-use, polymeric MD membranes have been compared with typical treatment options, as evaluated by Tijjani et al. [7]. MD has recently become an area of focus, with several efforts being conducted towards its long-term stability, brine handling capacity, and cost reductions through the use of more sustainable energy sources.

MD is a thermal process that has the ability to recover high-purity water from high salinity wastewater such as PW. Despite the positive results of MD in terms of efficiency, it is necessary to improve the system in terms of synthesis process and material selection for sustainable and continuous PW treatment operations [7].

Amit et al. [62] studied membrane distillation treatment technology and described it as one of the most promising technologies for PW treatment, despite the fouling and scaling effects observed in real PW applications. There exist many MD configurations, with the sweeping gas MD being the latest technology that has the ability to treat high-salinity PW. There have been extensive studies on emerging desalination approaches for hypersaline PW treatment.

MD is a thermal-based process that uses microporous hydrophobic membranes for wastewater treatment. In this process, water treatment occurs when the membrane repels the liquid water and vapor diffuses through the pores of the membrane. The partial vapor pressure gradient is the driving force for mass transfer in an MD system. MD can be configured based on the PW characteristics, including air gap, direct contact, vacuum, or sweeping gas MD designs. For instance, sweeping gas MD involves the use of a gas stream to sweep the permeate side of the membrane [62].

The study stipulated that it is important to optimize the MD operating conditions. Particularly, the main challenge is the saturation of seeping gas on the permeate side. Moreover, research has also indicated that scaling may occur, due to low solubility in real PW mixtures, which can severely impact the membrane’s performance. This indicates that a well-optimized MD process can mitigate scaling and membrane wetting effectively [62].

Focus on membrane distillation treatment has increased recently. In 2020, Jingbo et al. [63] evaluated the performance of pervaporative vacuum membrane distillation to desalinate prepared saline and hypersaline water at the laboratory scale. The intention was to evaluate the highly contaminated water that could form fouling and scaling on membranes, such as PW. The traditional porous type of membrane suffers from these concerns, which makes it difficult to treat the feed water before desalination. The experiment revealed that PVM is a highly effective treatment for feed stream salt removal. In some cases, it is important to use chemicals to soften and reduce the content of scale-forming minerals in water. The main challenge in vacuum membrane distillation is that it is difficult to form a defect-free zeolite coating to facilitate the process [63].

6.5. Ceramic Membranes

Despite their various advantages, the main challenge associated with ceramic membranes for oil and water separation is low water flux, as reported by Sara et al. [64]. However, the study indicated that it is possible to overcome this limitation and increase water flux and oil rejection through the use of commercial membranes coated with different concentrations of silica, starting from 0.25 to 1 wt%. Silica nanoparticles were deposited on the ceramic membrane, and it was shown that the dip-coating approach was effective. This is an initial observation motivating further assessment to accommodate industrial conditions and decrease the gap between real life and an ideal laboratory setup. This strategy will help to optimize and exploit the advantages of ceramic membranes in oil–water extraction. The research concluded that coating commercial ceramic TiO₂ membranes with SiO₂ silica nanoparticles improves the potential to remove oil from industrial wastewater [64].

Weschenfeldera et al. [65] carried out an investigation by applying ceramic membranes in an ultrafiltration process to treat PW containing the cationic surfactant Dodecyltrimethylammonium bromide (DTAB). This study mainly aimed to evaluate the effects of sodium chloride (NaCl) with DTAB on interfacial properties such as membrane wetting and surface tension. It was noted that the surfactant of the oil phase bonded with the membrane surface, resulting in a reduction in permeate flux. In contrast, the presence of sodium chloride in oil containing DTAB increased the permeate flux, indicating that the ionic strength changed the interaction between the membrane surface and oil phase. It was concluded that the ultrafiltration process using a ceramic membrane is an effective treatment and has potential application value for PW treatment, as the efficiency of the membrane was not impacted even with high concentrations of the surfactant [65].

Furthermore, the use of a novel titanium-based ceramic ultrafiltration membrane to purify field produced water was investigated by Mohamed et al. [66]. Titanium dioxide is an attractive inorganic filtration material that is used for polymeric and ceramic membranes. TiO₂ was used and found to be stable under several operating conditions, and it is commonly available and simple to prepare. Thus, an investigation was conducted on the treatment of saline industrial water using a multistage system staring with pre-treatment using microfiltration (MF) with an aluminum dioxide membrane followed by ultrafiltration (UF) with a TiO₂ membrane, ultimately achieving 99.5% water purification.

Fouling resistance was the main challenge and the efficiency decreased as the blockage of pores increased. However, the overall efficiency was 98%, demonstrating the high efficacy of this system. It is a promising technology which requires further evaluation to address issues related to operating conditions, PW characteristics, membrane material selection and properties, and fouling concerns, in order to increase the efficiency beyond 99.5% [66].

6.6. Pressure-Retarded Osmosis

Pressure-retarded osmosis (PRO) is one of the latest technologies that holds promise for PW purification. This type of osmotic application can be used for seawater and PW, allowing water energy to be recovered from hypersaline solutions. However, fouling is one of the major challenges associated with this application, as reported by Dareen et al. [67]. An investigation was carried out using real PW with different pre-treatment arrangements, as shown in Figure 7, to evaluate the PRO performance.

A pilot-scale seawater experiment was recently conducted for RO and PRO in order to evaluate the performance of each technology in terms of reducing the energy consumption of the desalination system. Moreover, the impact of fouling on PRO was not reported in this study.

The research concluded that PW can be treated using PRO with different treatment arrangements based on the requirements for the effluent. The extensive scheme was found to be the most suitable for reducing the pollutants significantly, enabling water reuse. However, further investigation is suggested, in order to assess the optimal combination with the best performance, as well as the selection of materials to eliminate or minimize the impacts of fouling [67].

6.7. Nanofiber Membrane

An advanced study was carried out using a nanofiber membrane (NFM), which has high potential for use in filtration processes for PW treatment due to its high permeability and porous structure, as reported by Nur et al. [68]. However, similar to most membrane technologies, the NFM faces the challenge of fouling, which reduces its performance and efficiency during continuous operations. In order to increase its performance and mechanical strength, the NFM was treated with solvent vapor to induce the fusion of fibers. A fabricated nylon polymer was selected for this process, as it has high hydrophilicity. The experiment demonstrated that the dispersed oil was removed completely from the PW. There are several techniques that can be used to improve the performance of membranes and eliminate fouling issues prior to their application on a commercial scale [68].

6.8. High-Temperature Membranes

A temperature-driven reverse osmosis membrane was tested in order to apply it in a PW treatment system. The investigation was carried out by Cheng et al. [69], considering different operating conditions in terms of temperature and pH. The results showed that 98% boron removal from PW was achieved at 11 pH and a temperature of 60 °C. The highest TDS and sodium rejection rates were achieved at a pH of 10, as illustrated in Figure 8.

The temperature of the PW was about 90 °C, requiring the use of cooling media before passing it to the normal RO membrane. However, silica precipitation could occur due to the high silica content in the PW, resulting in fouling. Thus, the use of a high-temperature RO membrane is an attractive option to evaluate for PW treatment purposes, given its novelty in the oil and gas industries. However, it was suggested that the membrane’s reliability and fouling behavior should be evaluated at the pilot scale [69].

6.9. Inclined Forward Osmosis

Forward osmosis (FO) is an attractive option for PW treatment due to its solute retention ability and low energy consumption. An investigation was carried out by Shafiq et al. [70], using seawater as a draw solution (DS) with an aeration and inclination module to evaluate the fouling and flux, hence improving the concentration factor. However, FO is significantly impacted by fouling and low flux, suggesting the need for further evaluations to optimize the application with respect to its energy concentration and inclination angle in order to reduce the level of fouling [70].

6.10. Membrane Technologies Challenges and Limitations

An investigation into advanced membrane technology for the treatment of produced water was carried out by Haneen et al. [71] using a novel membrane technology framework, taking into account key limitations and constraints from environmental and economic perspectives. The study focused on the main advanced PW treatment technologies, illustrating the advantages and disadvantages of each technology (as detailed in

Table 5. Advantages and drawbacks of PW treatment technologies [71].

Produced Water Treatment Methods
	Microfiltration	Ultrafiltration	Reverse Osmosis	Adsorption	Ion-Exchange
Advantages	▶ High recovery of fresh water	▶ High recovery of fresh water	▶ Compact module ▶ Removes dissolved contaminants	▶ Cheap ▶ Efficient ▶ Compact	▶ Low energy required ▶ Continuous treatment possible
Drawbacks	▶ High energy required ▶ Low efficiency	▶ High energy required ▶ High membrane fouling	▶ Requires high pressure ▶ Small traces of grease/oil may cause membrane fouling	▶ Low efficiency at high feed concentrations ▶ High retention time	▶ Requires pre-treatment ▶ Requires post-treatment

).

Furthermore, it has also been recently reported that a novel fabricated polystyrene membrane was used for PW treatment, using membrane distillation with direct contact on a bench-scale unit along with an activated carbon unit. The results indicated that the specific surface area is 20 times greater, when compared to undoped membranes. The present advanced membrane technologies were also discussed; in particular, thermal and pressure-driven processes. For instance, the approach can combine an osmotic process along with a microbial desalination cell in order to enhance the PW treatment effect; the feasibility of such an approach mainly depends on the geographical location, maintainability, and related operability costs [71].

6.11. Membrane Fouling Mitigation

In 2021, a study was conducted by Yinghong et al. [72], in order to investigate the possible mitigation of membrane fouling in the context of PW treatment. Microfiltration of synthetic produced water via backpulsing was tested, which was found to be an effective method to reduce the severity of membrane fouling. However, the efficiency varied based on operating conditions such as the duration, frequency, and amplitude, as well as the design of the membrane itself.

Backpulsing is a practical technology that involves reversing the pressure periodically to remove the accumulated foulants on the pores and surface of the membrane, hence recovering the membrane efficiency. This method has been employed in several applications, such as those relating to wastewater and food industries. However, few studies have shown that this method is also applicable in the context of high-salinity water treatment. The results indicated that the backpulsing technique with a microfiltration pore size of 0.1 µm is very effective, reducing the dispersed oil from an average of 200 mg/L to 5 mg/L. Further investigation is suggested to enhance the performance, frequency, and duration of wastewater treatment [72].

7. Biological Treatment

Biological treatment is a secondary method for wastewater treatment, aimed at removing the dissolved organic matter and improving the water quality. Several studies have been conducted to enhance biological processes and determine suitable micro-organisms for wastewater treatment [1]. This literature review aims to investigate the recent biological technologies that have been developed within the last three years and their limitations for further consideration.

7.1. Biological Effects of Elevated Major Ions

Produced water is the principal waste stream from offshore oil and gas production. In 2003, offshore plants discharged around 667 million metric tons of produced water into oceans around the world. Due to its complex blend of organic and inorganic substances, the environmental impacts associated with releasing produced water into the ocean are a significant concern. This water’s composition ranges from freshwater to severely saline brine. The primary organic components of produced water are water-soluble low-molecular-weight organic acids and monocyclic aromatic hydrocarbons. Toxicants include total PAH and higher-molecular-weight alkyl phenols, commonly ranging from 0.040 to 3 mg/L in produced water, as reported in [73]. Produced water frequently has higher quantities of metals, such as barium, iron, manganese, mercury, and zinc, than saltwater. When discharged into the ocean, produced water swiftly dilutes, frequently becoming 100 times less concentrated within 100 m of the discharge point. Aromatic hydrocarbons, alkylphenols, and some metals are particularly problematic due to their propensity for bioaccumulation and toxicity in marine creatures near discharge sites.

PW contains high concentrations of major ions that need to be removed in order to reuse the water, discharge it to sea, or re-inject it into disposal water wells. Ning et al. [74] evaluated the impacts of those ions on marine life after exposure for seven days. The results indicated that the survival of fish and aquatic organisms was four times lower, when compared with treated water.

While the International Produced Water Conference believes that produced water has minor impacts on particular offshore production sites, there are still concerns regarding its composition, destiny, and ecological implications. Comprehensive scientific research is required, using an ecosystem-based management (EBM) approach, in order to understand how the chemicals in produced water disperse, are altered (both biologically and abiotically), and the impacts of chronic low-level exposure to them on the environment.

7.2. Halotolerant Bacteria for Organic Compound Treatment

Seyed et al. [75] carried out a biological study to evaluate the capability of bacteria to treat synthetic and real PW. Their results showed that 97% of COD was reduced to 52% when the NaCl concentration increased to 60 g/L. Moreover, the removal efficiencies of total organic carbon and hydrocarbons were 85% and 90%, respectively.

The sodium chloride concentration level varies depending on the geographical location of the PW. A higher NaCl concentration impacts the biological process and reduces the efficiency of the activated sludge due to the plasmolysis of micro-organisms. Thus, for PW with higher salinity, halotolerant bacteria are the best option for treatment processes. This type of bacteria has the ability to survive in extreme and aggressive water types, such as PW. However, their grow rate is also significant. Halotolerant bacteria and aerobic systems are attractive potential options for advanced biological treatments. Moreover, further assessment is suggested to evaluate and improve the process, in terms of TOC degradation time and suitable processes for hardly degradable compounds in PW [75].

A halotolerant bacterial consortium was utilized by Ahmadi et al. [76] as the inoculum in a moving bed bioreactor designed to remediate saline petrochemical effluent. The isolated strains have halotolerant capacities of up to about 3.2%. The researchers investigated the effects of various organic loading rates and TDS concentrations on the performance and biokinetic coefficients of the bioreactor. At organic loading rates of less than 2.7 kg COD m⁻³ d⁻¹ and TDS concentrations of 25,000 to 30,000 mg L⁻¹, a significant COD removal efficiency of 77% was attained. The growth yield (Y) varied between 0.178 and 0.129 mg VSS mg COD-1 at various TDS concentrations.

7.3. Anaerobic Treatment

Anaerobic treatment technology for PW in the presence of extreme concentrations of COD and TOC has been evaluated by Zaixing et al. [77] using anaerobic micro-organisms. The results demonstrated high COD/TOC removal efficiency, reported at 93% and 90%, respectively. The recovery of methane during this process was nearly 35 mmol/g carbon.

PW effectively responds to anaerobic biological treatment. However, several challenges are associated with this type of treatment, when compared with aerobic processes. For instance, anaerobic processes require long treatment times and are extremely sensitive to variations in operating conditions such as temperature. Furthermore, high salinity could negatively impact the growth of anaerobic microbes [77].

Khong et al. used a hybrid upflow anaerobic sludge blanket (HUASB) reactor to investigate the anaerobic treatment of produced water obtained from an onshore crude oil terminal. The generated water was composed of COD, 1597 mg/L; NH3-N, 14.7 mg/L; phenol, 13.8 mg/L; BOD5, 862 mg/L; sodium, 6240 mg/L; and chloride, 9530 mg/L. The study aimed to address the issues related to the high salinity and hazardous chemicals in produced water, which could impair the efficiency of methanogens without biomass modification prior to anaerobic digestion. The experimental design included examining COD removal from generated water at various dilutions of produced water using tap water (TW). The reactor was run under mesophilic conditions (35 ± 2 °C) with a hydraulic retention time (HRT) of 5 days and a continuous feed duration of 250 days. The results demonstrated variable degrees of COD removal for the four dilution ratios, ranging between 76.1% and 46.3%, with final average effluent COD values ranging from 123.7 mg/L to 240 mg/L [78].

8. Hybrid Treatment Process

The use of hybrid methods for wastewater treatment is a promising principle in order to effectively improve the water quality. Several technologies can be used in both primary and tertiary stages, including advanced particle removal, filtration, membranes, and AOPs, which have recently been widely introduced in industrial settings for water treatment. This literature review aims to investigate the recent hybrid technologies that have been developed within the past three years and their limitations for further consideration.

8.1. Integrated Electrocoagulation–Forward Osmosis–Membrane Distillation for PW

Forward osmosis (FO) and membrane distillation (MD) technologies have been introduced recently for the treatment of water with high salinity, such as PW. Several studies have evaluated the feasibility of integrated systems including FO and MD to recover water from industrial effluents. Moreover, the combined system can also be attached to an electrocoagulation (EC) system for pre-treatment in order to minimize fouling and increase the performance of the overall integrated system, as shown in Figure 9. EC is capable of removing total suspended solids and total organic carbon by 95% and 80%, respectively. Moreover, the impacts of operational and process conditions have been considered and studied by Kamyar et al. [79].

The results demonstrated the high efficiency of the combined system with an appropriate configuration, which was able to remove almost all pollutants from PW (with an average of 96%) and stable performance. It is necessary to enhance the system further, in order to increase its efficiency and address fouling issues to enable continuous-operation applications [79].

8.2. Electrocoagulation Process Through Hybrid Processes

Electrocoagulation (EC) is the main focus for PW treatment, due to its high removal rate of organic and inorganic matter and low energy consumption. Abdulkarim et al. [80] have investigated the efficiency of using EC combined with other processes such as oxidation, chemical, membranes, and electromagnetic treatment. An experiment was conducted at the pilot scale in order to assess the hybrid EC processes, and the results revealed several advantages in terms of pollutant removal efficiency, fouling reduction, operating costs, and energy consumption. Moreover, further EC hybrid processes can be evaluated in order to compare the overall benefits of each system and evaluate the feasibility of continuous flow operations for commercialization purposes [80].

A review was performed by Xinchao et al. [81] regarding the characterization of PW and suitable options to treat the water for reuse purposes. Oxidation, membrane, biological, and adsorption approaches were evaluated, and the possibility of combining them was assessed. A novel combined system which was tested using PW included integrated EC and FO systems. The system achieved the highest performance and was able to remove 99% of TSS; as such, it was recommended for this specific purpose. The study focused on the detailed processes of advanced technologies such as those used in adsorption, membrane, and oxidation approaches, and the author suggested that combined systems for PW treatment should be further evaluated [81].

8.3. Combined EC-MF-MD for PW Treatment

A combined electrocoagulation, microfiltration, and membrane distillation process was investigated by Mahmood et al. [82], as shown in Figure 10, for PW treatment. Pre-treatment methods were used to enhance the membrane performance when using a highly concentrated prepared sample. The experimental results demonstrated significant reductions from 245 g/L to 0.05 g/L for TDS and from 120 to 60 mg/L for TOC. Furthermore, a pore size of 0.1 μm was used of MF after the EC process, allowing for further reduction in the TOC to 40 mg/L, followed by MD to finally obtain 1 mg/L TOC, resulting in a high water quality.

Temperature is a key process parameter, which was manipulated during the test in order to minimize the fouling of membrane pores and surfaces. The experiment was completed successfully, and the authors intend to focus on performing tests using real PW in further investigations [82].

8.4. Electrochemical and Bioelectrochemical Systems

A novel approach has been developed, as reported by Gunda et al. [83], using an integrated electrochemical (EC) and microbial fuel cell (MFC) system in order to enable the treatment of PW. The experiment was conducted using the combined system shown in Figure 11, where different current densities were utilized in the EC approach, while similar conditions were maintained for the MFC.

The experiment proved the functionality of the combined system for PW treatment. The results indicated that COD and total hydrocarbon were removed at rates of 90% and 89%, respectively, while 43% of TDS was removed.

Figure 12 illustrates the change in COD removal efficiency with variation of the current density. The figure demonstrates that, as the current density increases, the COD removal increases as well. However, it is required to evaluate the optimum energy consumption and balance the net energy of the overall system, in which EC consumes and MFCs generate energy. This combination was proven to be useful for PW treatment, and it was suggested to further implement an experiment using real PW in the context of a continuous operation application [83].

8.5. Electro-Coagulation/Forward Osmosis System

A novel hybrid system was investigated by Basma et al. [84], who combined EC and FO applications for PW treatment. Multiple current densities were applied during EC in order to evaluate the pollutant removal efficiency before passing through the FO system using a polymeric membrane. The experiment was conducted using prepared concentrated water and the results showed that EC achieved 97% and 91% removal of O&G and TOC, respectively. Moreover, 99% of TSS was removed by the hybrid system, resulting in high-quality water.

The O&G removal efficiency increased when increasing the current density, reaching a maximum efficiency of 97% and then reducing by an average of 2% with further incremental current density increases, as shown in Figure 13, thus defining the optimal operational limits of the system. Moreover, the retention time is a key factor affecting this process. In contrast, Figure 14 illustrates different curves for TOC removal efficiency while increasing the current density. The highest TOC removal efficiency was recorded at 91% at a residence time of 10 min, after which it decreases with an increase in the current density. Similarly, at a residence time of 30 min, the maximum efficiency was nearly 80%, which then declined significantly when further increasing the current density [84].

The residence time plays a vital role in this process, as it decreases the overall efficiency of TOC removal due to the excess aluminum ions produced via the electrocoagulation process, which reverses the negative surface charge of TOC and prevents its agglomeration. Finally, it was suggested that further investigations should be performed using real PW, in order to improve the system [84].

8.6. Organophosphonate Draw Solution

Fouling is the major concern associated with membranes when using extremely concentrated water such as PW. Chun et al. [85] investigated the effectiveness of scale inhibitor in preventing foiling on membranes. Ethylenediamine tetra (methylenephosphonic acid) sodium salt (EDTMPA-Na) was used as a scale inhibitor and, during the experiment, it was observed that it works effectively as an anti-fouling agent and increases the recovery of the membrane. However, it was also noted that further mitigation is required specifically for FO membrane fouling accumulation.

8.7. Bioelectrochemical Systems

The sustainability of wastewater treatment is a focal area for many researchers. It is well known that bioelectrochemical systems (BESs) are attractive systems in this area, as reported by Jonnathan et al. [86]. Synthetic PW was prepared to examine the performance of BESs including parameters such as external resistance, temperature, and retention time. It was reported that microbial fuel cells (MFCs) are the most effective BESs for removing contaminants from PW.

Electrochemical reactions are catalyzed by micro-organisms in order to form biofilms on the electrode surfaces and enhance the transmission of electrons. BESs consist mainly of anodes, cathodes, and electrolytes. The anodic and cathodic chambers could be separated by a membrane, based on the design intent and PW characteristics (see Figure 15).

Generally, this process uses microbes in the anodic chamber to oxidize substrates and produce protons and electrons, where the protons pass through the membrane to the cathode chamber and combine with oxygen to form water, while electrons transfer through an external circuit to produce electricity [86].

From the experiment, it was concluded that the BES could achieve a high removal efficiency for organic matter, heavy metals, and several contaminants in synthetic PW, thus making it a suitable environmentally friendly option for wastewater treatment. In particular, 97% of COD, 61% of sulfates, and nearly 91% of hydrocarbons were removed in the process. However, it is suggested that real PW is used with this system, in order to evaluate bacterial growth and enhance the overall removal efficiency [86].

9. Future Outlook and Challenges

The oil and gas industry produces a substantial amount of wastewater—around 39.5 million cubic meters per day—as a result of extraction and refining processes. Due to the hazardous nature of produced water, environmental concerns, and worldwide water scarcity, there has been a renewed emphasis on creating solutions for the treatment of PW. Various rules and limits have been implemented to address these concerns and reduce the environmental and economic implications of PW.

Initially, regulations primarily targeted the oil and gas (O&G) components in PW; however, recent studies have shifted towards considering all constituents of PW, in consideration of stricter regulations. Some water-stressed regions with challenging environments are exploring the treatment of PW for irrigation and other uses. Different approaches such as minimization, recycling/reuse, and disposal are being considered to tackle PW-related issues, with treatment being a crucial step before reuse or disposal, depending on the desired end-use.

The characterization of PW is essential for developing effective treatment strategies, as its composition can vary significantly across locations. Selecting a suitable treatment method depends heavily on the pollution sources present in the water. Energy-efficient and environmentally friendly methods are preferred for PW management in order to reduce the consumption of chemicals and environmental impacts.

Various treatment technologies, including those based on physical, chemical, and biological processes, advanced oxidation processes (AOPs), thermal treatment, and membrane filtration approaches, have been used to address PW challenges. Each method has its advantages and limitations, necessitating a holistic approach that integrates multiple treatment processes for optimal water quality and cost-effectiveness.

To enhance the efficiency of biological treatments, addressing challenges such as microbial competition, substrate availability, and environmental conditions is crucial. Membrane filtration, despite its advantages, faces issues including fouling and high energy consumption, thus requiring advanced pre-treatment methods. The integration of different processes and technologies is often necessary to meet regulatory requirements and ensure water reuse or the safe disposal of PW.

Most of the advanced PW treatment technologies are able to reduce the contaminants present in PW to acceptable environmental levels, with some of them producing water that is able to be reused. However, the main concern is the sustainability and performance of such systems over time during continuous operation due to the complexity associated with the extreme salinity of PW along with its hydrocarbon components. This literature review revealed that the traditional primary and secondary treatments are capable of reducing the undesirable components in PW to within certain limits and may be sustainable in the long term. However, the tertiary stage, defined as the “polisher” stage, requires the removal of the remaining pollutants beyond the ability of methods used in previous stages, as well as purifying the water to meet the specifications for its reuse.

Hybrid systems are a key area of focus, which have the potential to treat PW to reusable water specifications and reduce the energy consumption of the desalination system, thus protecting the environment from water re-injection via disposal wells. Electro-coagulation combined with an osmotic system for PW treatment appears to be an attractive system when comparing its results and performance with those of other technologies such as AOPs and bioelectrochemical and biological approaches, or even a single treatment in the tertiary phase. However, several limitations need to be addressed to de-bottleneck the existing challenges and improve the overall system to make it suitable for commercial use. Therefore, future works should aim to focus on the design, performance, and limitations of combined electro-coagulation and osmotic processes in order to find a suitable method for the effective treatment of PW.

Research into risk assessments and management strategies for PW is ongoing, aiming to weigh the risks associated with various contaminants and treatment approaches against established standards and thus guiding decision making and optimizing PW management practices. Comprehensive studies are required in order to better understand the potential risks and benefits of different PW management approaches.

10. Conclusions

In conclusion, the rate of generation of produced water has increased over time with the expansion of the oil and gas industry, and conventional water treatments are not capable of effectively treating this type of water due to its complexity. In this review, comprehensive information on conventional and advanced oxidation mechanisms and the associated challenges, as well as recommendations for future research, were discussed in order to enhance the performance and practical commercial implementation of systems designed for the treatment of produced water. It was clearly illustrated that integrated systems involving multiple technologies are required for the purification of produced water to such a standard that it can be reused safely, instead of re-injecting it into water disposal wells. There are still challenges to be addressed, where the common challenges are related to fouling, material degradation, and maintainability, with associated impacts on sustainability and the degradation rate of pollutants. The selection of combined treatment technologies should be considered as a key focal area from both theoretical and experimental standpoints in order to develop a robust treatment method for this type of water. In summary, integrating AOPs with other water treatment methods not only improves the pollutant removal rates but also constitutes progress toward more efficient and sustainable water treatment strategies.