Oilfield produced water and constructed wetlands technology: a comprehensive review Open Access

Oil and gas are among the most important commodities in the world (Waqar et al. 2023), yet their production generates substantial volumes of wastewater due to their coexistence with various underground fluids (Fakhru'l-Razi et al. 2009; Guerra et al. 2011; Igunnu & Chen 2012; Escobar Martínez et al. 2023; Kostina et al. 2023; Kundu et al. 2023). When oil and gas extraction starts, reservoir pressure reduces and water is injected into the reservoir water layer to maintain hydraulic pressure and enhance oil recovery (Igunnu & Chen 2012; Kundu et al. 2023). Additionally, water from outside the well breaks through the reservoir area (Guerra et al. 2011; Igunnu & Chen 2012; Chakraborty et al. 2022). As production continues, a time comes when formation water reaches the production well and this water is produced together with the injected water and the hydrocarbon mixture as shown in Figure 2 (Igunnu & Chen 2012; Abdulredha et al. 2020). Produced water is, therefore, a mixture of injected water, formation water, petroleum hydrocarbons, and treating chemicals that come along during oil and gas production (Fakhru'l-Razi et al. 2009; Abdulredha et al. 2020; Wenzlick & Siefert 2020; Bhatkar et al. 2023), and depending on the source, it may be classified as oilfield produced water, natural gas produced water, or coal bed methane produced water (Walters 2006; Clark & Veil 2009; Jiménez et al. 2018; Fetanat & Tayebi 2023). This waste stream is regarded as the largest and the most corrosive environment in oilfield operations because apart from chemical species that are natural to produced water, operators deliberately add treating chemicals to prevent operational problems that may occur at various stages in the production process and residues of these chemicals are left either in the oil or water phase of the produced water (Stephenson 1992a; Johnson & Coderre 2011; Al-Ghouti et al. 2019). Produced water is therefore not a single commodity, but a complex product of which the physical and chemical properties vary considerably depending on the geographical location, geological formation, type of hydrocarbons produced, operational conditions, and the type of chemicals used in the processing facilities (Veil et al. 2004; Clark & Veil 2009; Fakhru'l-Razi et al. 2009; Alkhowaildi et al. 2021). Discharge of untreated or poorly treated produced can affect the quality of the receiving water bodies, such as rivers, lakes, or oceans, and depending on the composition and concentration of the produced water, it can introduce contaminants such as salts, metals, hydrocarbons, radionuclides, and biocides into the environment (Panagopoulos 2022; Panagopoulos & Giannika 2022, 2023).

The methodology included five foundational stages: Identifying the research question (Stage 1), Searching for relevant studies (Stage 2), Screening and selecting eligible studies (Stage 3), Data charting (Stage 4), and Collating, summarizing, and reporting results (Stage 5). The current review did not include the optional sixth stage that incorporates stakeholder consultation toward the translation of knowledge.

The review seeks to answer the following research questions: what is produced water composition, effects, and management approaches? What is the most popular surface treatment approach for oilfield produced water? What is the place of CWs in produced water management, components, and design considerations?

The population, concept, and context (PCC) framework informed the inclusion criteria of this scoping review. The population involves all studies published in English on oilfield produced water. The concept entails defining attributes of produced water and the technologies developed to treat produced water. The context is any geographical setting across the globe and not time-delimited.

All published and unpublished studies outside the English language were excluded. Hence, there is the potential for language bias. Studies and documents reporting on wastewater treatment in general but not oilfield produced water specifically were also excluded.

The search strategy aimed to find only published studies. An initial limited search of Google Scholar was undertaken followed by the analysis of text words contained in the title and abstract and of the index terms used to describe the article. A second search using all identified keywords and index terms was undertaken across information sources. Thirdly, the reference lists of all identified articles and reports that met the inclusion criteria of the review were searched for additional articles or reports. A manual supplementary search (hand search) was also done to identify relevant additional articles or reports.

The electronic search engine explored was Google Scholar. This omnibus system allowed for articles to be searched without any stringent database specific restrictions.

After the search, all references to published data that satisfied the requirements for inclusion in the review were gathered and uploaded into the Endnote 20 software to eliminate duplicate references. A Rayyan QCRI review manager received an export of all citation information for use in screening and selecting publications. Following a pilot test, titles and abstracts were checked against the review's inclusion criteria by two independent reviewers. Two impartial reviewers also evaluated the full-text papers that may fit the inclusion requirements. For full-text items that did not match the inclusion requirements and were excluded, explanations were given. At any point throughout the selection process, disputes between reviewers were settled by conversation or by consulting a third reviewer.

The relevant studies included were charted using the following characteristics and domains: author and year of publication, title of study, aim of study, study design, study setting, produced water characteristics discussed, produced water treatment technologies mechanisms, advantages and disadvantages, treatment technology-specific challenges, and other significant findings.

This review synthesized evidence from existing literature from peer-reviewed studies on web search engines; therefore, ethical approval was not required.

Oilfield wastewater has been in contact with hydrocarbon-bearing formation for centuries. As a result, it bears some of the chemical properties of the formation, the hydrocarbons as well as chemicals that were added during the drilling, production, and treatment processes (Speight 2016; Melchert 2022). The authors believe that the quality and the quantity of chemicals used in the production and processing of oil and gas contribute significantly to changing the physical and chemical properties of oilfield produced water as observed by other authors such as Nyieku et al. (2022) and Al-Ghouti et al. (2019). Though the constituents of produced water vary greatly, they can broadly be categorized as dissolved and dispersed oil, dissolved formation minerals, heavy metals, radionuclides, production chemicals, dissolved gases, formation solids, scale products, waxes, and microorganisms (Roach et al. 1994; Utvik et al. 2004; Veil et al. 2004; Speight 2016). Discharge of poorly treated or untreated produced water can have significant impacts on the ecosystems and biodiversity of the affected areas. For example, produced water discharge into aquatic habitats can alter the salinity, temperature, pH, and oxygen levels of the water body. This can affect the growth, reproduction, and survival of organisms in and around that environment.

Oil and grease is a term applied to all organic materials which are suspended or dissolved in the produced water at the time of discharge (Stephenson 1992a; Hedar 2018). Though there are many constituents in produced water, oil and grease are given maximum attention in both onshore and offshore operations because of their devastating effect on the environment (Veil et al. 2004). Oil in produced water is usually measured as oil and grease, and this is a common test method that measures many types of organic chemicals that collectively lend an ‘oily’ property to water (Johnsen et al. 2004). The concentration varies, and not all produced water has the same constituents of oil even when they contain the same amount of oil and grease (Fakhru'l-Razi et al. 2009; Al-Ghouti et al. 2019). Measuring the oil content of produced water is important for both process control and reporting to regulatory authorities (Fakhru'l-Razi et al. 2009; Yang 2011). Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) measurements give an indication of the organic content of produced water (Kiepper et al. 2014). Oil pollution creates a film layer on the surface water which prevents oxygen penetration to aquatic life (Kingston 2002).

The quantity of dissolved, dispersed, and suspended oil present in produced water before treatment is influenced by factors such as oil composition, pH, salinity, total dissolved solids (TDS), temperature, oil/water ratio, type, and the quantity of oilfield treatment chemicals used (Stephenson 1992a; Duraisamy et al. 2013). The type and quantity of stable compounds such as waxes, asphaltenes, and fine solids present in produced water may also affect the amount of oil and grease (Fakhru'l-Razi et al. 2009).

Oil in produced water can be classified as suspended, dispersed, and dissolved (Dickhout et al. 2017). Suspended oil consists of free, dispersed, and sometimes emulsified oil, while dispersed oils comprises small discrete droplets such as higher molecular weight poly aromatic hydrocarbons (PAHs), and heavy alkylated phenols suspended in the aqueous phase with size varying from 2 to 30 μm (Meijer & Madin 2010; Samuel et al. 2022). Oil in the dispersed form contributes significantly to BOD and can create potential toxic effects near the discharge point within the aquatic ecosystem (Stephenson 1992a; Duraisamy et al. 2013). The concentration of PAHs with six to nine carbon alkylated phenols is strongly associated with dispersed oil in produced water (Faksness et al. 2004). The dispersed oil content of produced water varies depending on factors such as oil density, shear history of the oil droplet, and interfacial tension between the water and the oil (Miadonye & Amadu 2023). Miadonye & Amadu (2023) and Stephenson (1992a) explain that if the difference between the density of oil and that of water is not significant, there will be little driving force to move the oil droplet to the top for separation and collection. Again, if the oil has been through several shear devices at high speed or pressure, then the oil droplets entering the water treatment system will be smaller than it should be for effective separation to occur during treatment, leaving a lot of oil in the wastewater (Hadjiefstathiou et al. 2023). Also, improper use of production chemicals such as surfactants in oil–water treatment systems also has the potential to reduce the interfacial tension between the oil and water phase, thereby causing small amounts of oil droplets to be stabilized in the water phase in the produced water (Stephenson 1992a). Oil in dispersed form is more difficult to remove with the gravity-based method employed in API treatment (Neff et al. 2011). These instances may require further treatment of the produced water to polish it to meet the regulatory discharge limit.

Dissolved organic compounds are distributed between low carbon to medium carbon ranges; therefore, the oil in the dissolved phase contains alkanoic acids such as methanoic acid, propanoic acids, and mono-aromatic nuclear compounds, including benzene, toluene, ethylbenzene, and xylene (popularly referred to as BTEX). There are also low molecular weight phenols, PAHs, aliphatic hydrocarbons present in dissolved organic compounds (Hayes & Arthur 2004). Additionally, the concentration of soluble organics in produced water increases with a decrease in pH and an increase in temperature; however, it is not significantly affected by salinity (Fakhru'l-Razi et al. 2009).

The quality and quantity of dissolved non-hydrocarbon organic materials in produced water may vary depending on the type of oil produced (that is paraffinic, asphaltenic, or gas condensate) and the presence of soluble aromatic compounds in the wastewater (Stephenson 1992a). Produced water from gas condensate-producing platforms has higher concentrations of dissolved oil compounds than produced water from oil-producing platforms (Veil et al. 2004; Duraisamy et al. 2013; Irawan et al. 2015). Therefore, the treatment of dispersed and dissolved oil in produced water poses a great challenge and it is a major issue of concern in the oil and gas industry, given that alkylated phenols and polyaromatic hydrocarbons can easily bio-accumulate in aquatic organisms such as cod and blue mussels (Saththasivam et al. 2016; Aravind & Kamaraj 2022). Alkylated phenols and polyaromatic hydrocarbons may also affect reproduction, altering several biochemical and genetic biomarkers within living organisms when ingested (Harvey et al. 2012; Bakke et al. 2013; Aravind & Kamaraj 2022). Gearheart et al. (1999) observed that non-BOD₅ organic contaminants could degrade in CWs when given sufficient hydraulic retention time (HRT). The authors believe that these findings could be appropriate in the treatment of dissolved oil in produced water through the establishment of a vegetated FWSFCW in conjunction with an oily wastewater treatment plant (WWTP). The nature-based technology could offer additional treatment to dissolved oil and other recalcitrant chemical species which escaped the action of the gravity-based treatment method.

Dissolved inorganic compounds in oilfields wastewater encompass cations, anions, heavy metals, salts, and naturally occurring radioactive materials (NORMs; Brusseau & Artiola 2019). The chemistry of produced water is significantly influenced by the presence of cations and anions (Al-Ghouti et al. 2019). For instance, sodium cation (Na⁺) and chlorine anion (Cl⁻) determine produced water salinity, which can range from a few mg/L to 300,000 mg/L (Igunnu & Chen 2012; Melchert 2022).

The salt content of produced water is measured in terms of electrical conductivity, salinity, and TDS (Mary et al. 2015), however, it varies widely from that of freshwater to salt levels to about 10 times higher than seawater and TDS in produced water ranging from 100 mg/L to more than 400,000 mg/L (Al-Ghouti et al. 2019). Produced water containing dissolved solids in excess of 100 g/L is usually classified as brine (Guerra et al. 2011; Nicot et al. 2020). Additionally, TDS comprising inorganic salts, mainly calcium, magnesium, potassium, sodium, bicarbonates, chlorides, sulfates, and some amounts of dissolved organic matter in the water phase can increase the produced water conductivity and scale forming potential (Faksness et al. 2004; Boyd & Boyd 2015; Li et al. 2016; Herawati et al. 2023). The most dominant ion found in produced water is sodium cation (Na⁺), whose high levels compete with potassium, calcium, and magnesium for uptake in plants (Benko & Drewes 2008; Dong et al. 2022). Therefore, excess sodium can prompt deficiencies of other cations in plants and other living organisms (Davis et al. 2012; Evelin et al. 2019), excess Na⁺ is also likely to cause poor soil structure and inhibit water infiltration in soils (Massey et al. 2007). Although heavy metals' concentration in produced water is often greater than that which is found in seawater, it is quickly diluted and does not have much adverse effect on the marine ecosystem (Otton 2006; Azetsu-Scott et al. 2007). Metals in produced water are generally less harmful compared with organic components, however, they precipitate to form undesirable solids which pose a great challenge in the treatment processes (Bakke et al. 2013; Ghafoori et al. 2022). High concentrations of trace elements such as boron, bromine, fluorine, and lithium may also be found in some produced waters (Nielsen 1998; del Villar et al. 2023). These elements are considered phytotoxic and when adsorbed, they may remain in the soil even after the saline water has been flushed away (Neff et al. 2011).

Large quantities of dissolved inorganic (carbon dioxide, nitrogen, and hydrogen sulfide) and organic (methane, ethane, propane, and butane gases) are contained in oilfield produced water (Kulongoski et al. 2018). The concentration of these gases increases with depth in any given formation (Veil et al. 2004), and their solubility in water generally decreases as salinity and temperature increase (Hansen & Davies 1994; Al-Anezi & Hilal 2007). Hence, the presence of these gases in produced water can exacerbate corrosion (Veil et al. 2011).

NORMs are also found in low concentrations within oil and gas holding formations (Chriss & Bursh 2002; Rosenblum et al. 2017). These include Radium-226 and Radium-228 (daughter products of Uranium-238 and Thorium-232) (Veil et al. 2011). Small quantities of NORM can be transferred into produced water which deposits radioactive scale in the piping at the well over time (White & Rood 2001; Fakhru'l-Razi et al. 2009). Radium-bearing scale and sludge found in oilfield equipment and discarded on soils pose hazards to human health and ecosystems (Otton 2006; Landa 2007; Guerra et al. 2011). Although the levels in produced water are very low and pose no risk, the authors share a similar view to Torres et al. (2016) and Ali et al. (2017) that scale from pipes and sludge from tanks holding produced water can concentrate over a long period of time to create health problems such as cancer and cardiovascular diseases, the need to handle these constituents with maximum caution.

Treating chemicals are deliberately added during oil and gas production to enhance recovery and overcome operational challenges (Kokal 2005; Prabha et al. 2014; Kelland 2016; Nikookar et al. 2023). These are mixtures of complex chemical additives of various molecular compounds used in drilling, production, and oil/water separation (Fakhru'l-Razi et al. 2009; Samuel et al. 2022). Examples include corrosion inhibitors and oxygen scavengers, scale inhibitors, biocides, emulsion breakers and clarifiers, coagulants, flocculants, and clarifiers' solvents (Cline 1998; Veil et al. 2004; Kelland 2016; Pichtel 2016). Production chemicals soluble in water do not have toxicity effects in the aqueous phase compared with those soluble in the oil phase, although some production chemicals can increase the partitioning of oil compounds into the aqueous phase at high concentrations (Georgie et al. 2001; Saravanan et al. 2021). Corrosion is a serious issue in oil and gas production and transportation systems because of the presence of chloride salts which are either dissolved in the formation water or emulsified in the crude oil (Popoola et al. 2013; Tamalmani & Husin 2020; Nikookar et al. 2023). Corrosion inhibitors and oxygen scavengers in the form of organic compounds such as monoethylene glycol (MEG) or methanol are utilized to provide hydrate and corrosion control in oil and gas production systems to reduce equipment corrosion (Lehmann et al. 2014; Nikookar et al. 2023). These compounds may partition into the oil phase to form more stable emulsions, thus making oil/water separation less efficient in gravity differential separation devices (Mohamed et al. 2017).

Additionally, produced water contains sand, silt, carbonates, clays, proppant, corrosion products, and other suspended solids derived from the producing formation and well bore operations and these can precipitate to form production solids (scale) (Kaufman et al. 2008). These scales can range from small, insignificant to big slurry solids which can influence the fate of oil/water separation in API oil/water separating systems (Cline 1998; Tian et al. 2022). Moreover, during production (at a reduced temperature and pressure) ions such as Ca²⁺, Ba²⁺, Fe²⁺, ⁠, and in supersaturated produced water react to form precipitates in the form of salts (scales), contributing to the production of solids content of produced water (Kelland 2016; Kermani et al. 2023). However, scale inhibitors, coagulants flocculants, and clarifiers are used to deal with the problems (Veil et al. 2004; Yan et al. 2021), but the quantity and quality of these chemicals have consequences on the produced water characteristics which subsequently affect the larger ecosystem (Gomes et al. 2009; Gruszka & Nagy 2023). Treatment chemicals vis biocides, reverse emulsion breakers, and corrosion inhibitors pose the greatest concerns because of their potential to cause aquatic toxicity (Veil et al. 2004; Stringfellow et al. 2014; Pichtel 2016; Peers De Nieuwburgh 2020). Treatment chemicals have the capability to precipitate and accumulate in marine sediments affecting the toxicity, bioavailability, and biodegradability of produced water (Brendehaug et al. 1992; Neff 2002; Gambino et al. 2021). Regulators in this industry will have to keep an eye on the quality and the quantity of the use of these chemicals to save the ecosystem from further destruction since a lot of the formulations are kept as trademark secrets.

The tremendous volume of water used in extracting oil and gas creates the environment for microorganisms to thrive (Yemashova et al. 2007; Ahmed & Fakhruddin 2018), especially when the water is non-potable and contains a lot of nutrients for bacterial growth (de Figueredo et al. 2014). It gives rise to a wide range of microorganisms including fermentative organisms, methanogens, acetogens, sulfate reducers, anaerobic organisms, nitrate reducers, manganese, and iron reducers in produced water samples (Dahle et al. 2008). The bacteriological quality of oil and gas produced water is a function of the interaction of a complex set of factors, including the chemistry of the wastewater (Basso et al. 2005). The presence of hydrogen sulfide-producing bacteria could negatively impact the success of gas extraction (Basso et al. 2005), with their metabolic byproduct causing souring of oil wells and corrosion of drilling equipment in oil reservoirs (Engle & Rowan 2014; Akob et al. 2015). One major problem faced by oil-producing companies is the control of hydrogen sulfide gas produced as a result of the metabolism of sulfate by anaerobic microorganisms (Kaur et al. 2009). Hydrogen sulfide is one of the major toxic and corrosive gases responsible for various environmental and economic losses including reservoir souring, contamination of natural gas and oil, corrosion of metal surfaces, and the plugging of reservoirs and consequent low production of oil (Haratian & Meybodi 2021). Since microorganisms can be used to enhance oil and gas production and mitigate potential contamination (Dahle et al. 2008), more attention in terms of research should be given to their beneficial use in the oil and gas industry.

The relatively large volumes and partially hazardous nature of produced water make its management challenging (Khatib & Verbeek 2002; Colella et al. 2021). Regulatory constraints often limit oil and gas operators to management methods that may not be necessary for environmental protection (Gunningham et al. 2004; Davis 2012; Ite et al. 2013). In many onshore areas of the world, produced water is injected into underground formations that contain water unsuitable for human consumption and wetlands (Stephenson 1992b; Fakhru'l-Razi et al. 2009; Veil 2015; Nwosi-Anele & Iledare 2016; Stefanakis 2019). In order to meet stringent environmental regulations, physical, chemical, and biological methods are employed to treat oilfields produced water (Al-Ghouti et al. 2019; Ghafoori et al. 2022). Bhatkar et al. (2023) and Fakhru'l-Razi et al. (2009) point out that the cost involved as well as the chemicals used for the treatment of hazardous sludge generated is expensive, hence limiting the application of physical and chemical methods. For many operators, the preferred disposal method is the one that adequately protects the environment at the lowest cost (Stephenson 1992a; Veil et al. 2004; Arthur et al. 2009). Authors such as da Silva Almeida et al. (2019) and Igunnu & Chen (2012) have found that more than 40% of the global daily oilfield wastewater is treated and discharged into the environment. This is an issue of concern to many environmental activists, given the partially hazardous nature of oily wastewater. The practice leaves room for many speculations on the potential environmental consequences (Hammer et al. 2012). The authors agree with the views of sundry scholars such as Patni & Ragunathan (2023), Amakiri et al. (2022), Li et al. (2021), and Arthur et al. (2005) that approaches for managing produced water should be determined by the predominant characteristics and the quantity of water generated to save the ecosystem from further deterioration. Some approaches outlined by Arthur et al. (2009) include:

• Avoidance of water production onto the surface through the use of polymer gel that blocks water-contributing fissures or fractures (Maitland 2000; Veil 2011). Injected gels are mainly made of water, a small volume of polymers and crosslinking chemical agents to shut off unwanted water production (Taha & Amani 2019). Gel treatments can completely seal off layers; therefore, they are considered aggressive and risky conformance control operations (Bai et al. 2015). Down hole water separators can also be used to separate water from oil or gas streams down hole and re-inject it into suitable formations (Amini et al. 2012). This approach is ideal because it allows the produced water to remain down the hole, however, it is not always possible because of the cost implications (Veil et al. 2004; Arthur et al. 2005).

• In the past, freshwater was commonly used in water flooding but because of increasing scarcity, freshwater is typically no longer used as a viable source for water flooding (Davarpanah 2018). Produced water can be treated to meet the desired quality for use in drilling, stimulation, and workover operations and injected into the same formation or another suitable formation (Arthur et al. 2005; Eyitayo et al. 2023). This option is expensive because it involves transporting the wastewater from the producing site to the injection site as well as treatment of the injectate to reduce possible fouling and scaling agents and bacteria (Arthur et al. 2005). For a water flooding operation to be successful, the water used for injection must be of a quality that will not damage the reservoir rock (Veil et al. 2004). This option is advantageous because the wastewater produced is placed back underground (Arthur et al. 2005; Goel et al. 2012).

• Significant treatment can be applied to produced water to meet quality standards for consumption in beneficial uses such as irrigation, rangeland restoration, cattle and animal consumption, and drinking water for private use or in public water systems (Arthur et al. 2005; Hagström et al. 2016).

• Produced water could also be treated to meet regulated standards and discharged (Hedar 2018; Jiménez et al. 2018). The last two approaches may require that corrosion and microbial issues be addressed because once on the surface, oxygen is introduced into the produced water treatment environment (Veil et al. 2004; Fakhru'l-Razi et al. 2009).

Surface treatment for produced water discharge involves the use of gravity-based separation, followed by discharge into the environment. This seems to be affordable for many operators, including those in Ghana. Unfortunately, this approach can potentially pollute soil, surface water, and underground water if it is not properly carried out (Neff et al. 2011; Igunnu & Chen 2012; Helmy & Kardena 2015; Hedar 2018). The authors are of the view that the approach could be used in conjunction with the tertiary treatment method to polish the treated effluent to meet regulatory standards before it is discharged into the environment.

The gravity-based oil–water separators or gravity differential separation device, namely API, is the most economical state-of-the-art method for managing oily wastewater for discharge (Pintor et al. 2016; Ghafoori et al. 2022). The device is capable of handling large flow rates with varying oil concentrations (Kokal 2005; Frising et al. 2006). However, the treatment processes are slow and may require large equipment support (Jaworski & Meng 2009). API oil–water separator devices operate by taking advantage of the differences in density between oil droplets and water, hence the main focus is to remove free, dispersed, and large oil globules and non-oil-coated solids suspended materials from produced oilfields wastewater (Stephenson 1992a; Chu et al. 2015; Rasouli et al. 2021). The efficiency of oil removal in gravity-based oil–water separators depends on several factors, namely flow rate, oil particle size, the density of oil, temperature, characteristics of wastewater, and separator design (Kajitvichyanukul et al. 2006; Saththasivam et al. 2016). By employing gravity-driven settling, the separators remove large quantities of oil and suspended solids from the produced water prior to subsequent downstream treatment processes and polishing (Schultz 2005; Arabi et al. 2020).

The API device works on the principle of Stokes' Law (Schultz 2005; Pintor et al. 2014; Liu et al. 2022a,b) which allows the oil to rise to the surface due to the difference between the specific gravity of the oil to be separated and that of the produced water (Kundu & Mishra 2013). This difference between the two is typically much less than the difference between the total suspended solids (TSS) and wastewater, so the majority of TSS settle in the unit (Wahi et al. 2013). Thus, the oil and TSS phases alike are removed in the API separator device (Schultz 2005). API separators are able to remove oil particles of 150-μm size or larger (Huang et al. 2015; Pintor et al. 2016). Unless sizing adjustments are made to compensate for the removal of smaller oil droplets, particles smaller than 150 μm will normally exit the separator with the wastewater, and will need to be removed through additional treatment processes (Kajitvichyanukul et al. 2006). It is generally expected that API separators should be able to achieve the effluent quality desired by their users; however, this is not always the case, as the performance of the device is affected by the influent oil, TSS concentrations, and other site-specific issues (Schultz 2005; Cooper et al. 2021). Proper design and selection of support equipment are crucial for the efficient operation of API devices (Buitinck et al. 2013). Operational activities, such as changes in crude oil slates or the introduction of spent caustic materials into the separators, can affect the separator's treatment efficiency (Schultz 2005; O'Donnell 2022).

Given the environmental and economic implications for produced water management, nature-based solutions such as the use of CWs can be explored to assess its viability in the management of oilfield produced water in conjunction with the API oil–water separator (Mustafa et al. 2024; Stefanakis 2022; Waly et al. 2022). Many authors, namely Wu et al. (2023), Gorgoglione & Torretta (2018), Wu et al. (2015), and Vymazal (2005), are of the strong conviction that the emerging technology for the treatment of a variety of wastewater is CWs because it has been successfully applied for the treatment of industrial wastewater, agricultural/aquaculture, and stormwater runoff with more than 50% reduction in initial, operation, and maintenance cost.

The use of natural systems for the treatment of waste was spawned because of several factors including lack of data to be able to predict the performance of the technology. However, there is now a realization that not all pollution problems can be resolved through technological means; in many cases, pollutants are just moved from one form/state to another (Mahmood et al. 2013; Bianchini et al. 2023). Conventional approaches usually require massive amounts of resources apart from the perpetuation of carbon and nitrogen cycle problems in the ecosystem (Mahmood et al. 2013; Shekhar et al. 2023). Spellman (2023), Mahmood et al. (2013), and Mitsch & Jørgensen (2003) strongly advocate for the use of ecological engineering for pollution and waste management. They think it is advantageous because apart from the fact that natural treatment systems are able to integrate human society with its natural environment for the benefit of both, it also has the potential to restore the disturbed ecosystem and develop new sustainable ones with both human and ecological values (Table 1).

Natural wetlands are intermediate between terrestrial and aquatic ecosystems (Hammer & Bastian 2020). They are used for various purposes, including wastewater treatment, stormwater management, and carbon sequestration (Tan et al. 2023). Wetlands have been used for water purification for centuries in many parts of the world, although in most instances, the reasoning behind the usage was more of disposal than treatment (Verhoeven & Meuleman 1999; Haberl et al. 2003; Vymazal 2011; Zurita & Vymazal 2023). Wetlands naturally absorb and store carbon from the atmosphere, and CWs can replicate this process (Vymazal 2005; Tan et al. 2023). The system has a rich diversity of plants and animals which can act as filtering systems, removing sediment, nutrients, and other pollutants to improve water quality (Brisson et al. 2020).

Sundry scholars such as Al-Jabri et al. (2020), Ezennubia & Vilcáez (2023), Waly et al. (2022), Kabutey et al. (2019), Stefanakis (2019), Wu et al. (2015), Saravanan et al. (2011), Fakhru'l-Razi et al. (2009), Kadlec & Wallace (2008), Vymazal (2005), Haberl et al. (2003), Place Jr (1991), and Cooper & Findlater (1990) have all pointed out that bio-treatment is a proven technique to degrade organic dissolved compounds in aqueous solutions and for that matter different types of macrophyte-based wastewater treatment systems can be combined with conventional treatment plants to exploit the advantages of each system to polish effluents from conventional WWTPs.

CWs, also known as treatment wetlands, are an example of a bio-treatment system built on sound ecological principles at a low cost, in terms of capital, energy demand, and mechanical technology requirement (Mahmood et al. 2013; Tan et al. 2023). They are constructed to utilize the natural processes involving wetland vegetation, soils, and their associated microbial assemblages in treating wastewater (Vymazal & Kröpfelová 2008; Ferreira et al. 2023). CW systems take advantage of the many processes that occur in natural wetlands, but in a more controlled environment (Vymazal & Kröpfelová 2008; Sabater et al. 2023) and can represent an alternative or an add-on to conventional wastewater treatment systems for treatment of various types of wastewaters including municipal wastewater, household wastewater, and industrial wastewater (Brix 2003; Cooper 2005; Crites et al. 2005; Licata et al. 2021; Tan et al. 2023). Engineered wetlands offer better opportunities for wastewater treatment than natural wetlands because they are built with a much greater degree of flexibility in terms of site selection, sizing, hydraulic pathways, and retention time as well as the well-defined composition of substrate, type of vegetation, and flow patterns (Brix 2020; Verhoeven & Meuleman 1999; Raza et al. 2023). Authors such as Rehman et al. (2018), Stefanakis (2019), and Eke et al. (2007) emphasized the potential of treatment wetlands for managing oilfields wastewater. Jain et al. (2023) used a novel composite baffled horizontal flow CW to efficiently remove about 93.93, 87.20, and 66.25% of turbidity, phenol, and COD, respectively, from real petrochemical wastewater which made the effluent quality reusable for irrigation, industrial, and other environmental purposes. A review by Alnaser et al. (2023) also concluded that oily contaminated water could be converted into usable water using CWs. Stefanakis (2019) have also recommended the technology by highlighting the tremendous advantages CW brings to the oil and gas sector.

Eke et al. (2007) used CW to treat simulated produced water and the outcome was very promising. Not that the technology also has the potential to reduce greenhouse gas (GHG) emissions relative to conventional WWTPs and this was confirmed by Tan et al. (2023) in a study which found that GHG emissions from a conventional WWTP were approximately 3.75 times higher than the hybrid CW system with the same treatment capacity.

Each type has its strengths and depending on the target contaminants, a particular type may be constructed (Wu et al. 2023). In the case of free water surface flow systems, plants are rooted in the sediment layer, and water flow is above the ground (Vymazal 2005; Brix 2020). In subsurface flow systems, plants are rooted in porous media such as gravels or aggregates through which water flows and treatment are accomplished (Mahmood et al. 2013). Subsurface flow systems are further divided into horizontal subsurface and vertical subsurface. Vertical flow subsurface CW systems are more effective for the mineralization of biodegradable organic matter and have greater oxygen transport ability (Achak et al. 2023a,b). Vertical flow CW is more efficient for the removal of suspended solids, carbon, and nitrification processes because of the aerobic conditions (Mahmood et al. 2013; Ji et al. 2023).

Although most FWSFCWs have been built to provide advanced wastewater treatment for municipal wastewaters, the range of applications has swiftly expanded to include the treatment of animal wastes, agricultural runoff, and industrial effluents (Brix 2020; Kadlec 2009a). Free water surface flow treatment wetlands are useful when the targeted wastewater for treatment has high unpredictable flow rates and large volumes (Verhoeven & Meuleman 1999; Ingrao et al. 2020). This may be advantageous for use as an add-on to a commercial conventional treatment facility where there are possibilities of occasional ‘peak periods’ with high treatment demands (Tilmans 2014).

A typical FWSFCWs with emergent macrophytes is a shallow sealed basin or sequence of basins, containing 20–30 cm of rooting soil, with a water depth of 20–40 cm (Vymazal 2010; Patel 2023). It has dense emergent vegetation which usually covers more than 50% of the surface with naturally occurring species also being present (Wu et al. 2015; Brix 2020). Plants are usually not harvested and the litter provides organic carbon necessary for denitrification which may proceed in anaerobic pockets within the litter layer (Vymazal 2010; Ramm et al. 2022). The flow is directed into a cell along a line comprising the inlet, and upstream embankment, and is intended to proceed across all portions of the macrophyte to the outlet structures. It is interesting to note that apart from the planted macrophyte, naturally occurring species may be present in a CW (Vymazal 2008; Kataki et al. 2021). Surface flow CWs closely mimic natural wetlands because plants are rooted in a submerged layer of sand (Kadlec 1995; Kadlec & Wallace 2008). Aeration of the sediment takes place by the unique property of wetland vegetation which act as oxygen pumps providing dissolved oxygen (DO) with their roots to a wide variety of microorganisms (Kadlec & Wallace 2008). The treatment potential of free water surface flow systems is very good for a large suite of pollutants such as TSS, fecal coliforms, BOD, trace metals, and organics in the range of 90%. For instance, recorded high mean removal efficiencies (81% for COD, 89% for BOD₅, and 89% for mineral oil) in reed beds treating heavily oil-polluted wastewater. Studies by authors such as Calheiros et al. (2007), Norton (2014), Vymazal (2005), and Weber & Legge (2008) have also found wetlands to reduce pathogens including total coliform counts to varying degrees above 50%. Weber & Legge (2008) have observed pathogen removal in upwards of 90% of CWs. However, nutrient reduction in FWSFCWs has been found to be a little low (Verhoeven & Meuleman 1999; Patel 2023).

Purification processes in FWSFCW include settlement of suspended solids, diffusion of dissolved nutrients into the sediment, mineralization of organic material, nutrient uptake by microorganisms and vegetation, microbial transformations into gaseous components, physicochemical adsorption, and precipitation in sediment (Verhoeven & Meuleman 1999; Brix 2020; Wu et al. 2023). Suspended matter in wastewater such as nutrients, heavy metals, and organic compounds (Debusk 1999; Weragoda et al. 2023) are effectively removed via settling and filtration through dense vegetation in free water surface flow treatment wetland (Brix 2020). Botseva et al. (2023) point out that there are surface forces such as van der Waal's forces and electric forces which also contribute toward the reduction of suspended solids.

It is worth noting that the removal of organic contaminants occurs through microbial degradation and settling of colloidal particles mediated by microorganisms and controlled by the availability of oxygen and redox condition of soil, both of which are governed by hydrology (Reddy et al. 2010; Vymazal 2010; Brix 2020). Soluble organics are removed by a number of separation processes and the rate at which this process will occur depends on the characteristics of the organic matter and the solid surfaces available for attachment (Choudhary et al. 2011; Candido et al. 2023). For most wastewater entering wetlands, a fraction of the carbon is in particulate form and another in dissolved form (Gearheart et al. 1999; Reineke & Schlömann 2023). Particulate settling at the inlet zone of the wetland provides one mechanism for the removal of particulate carbon and the dissolved form is processed by aerobic and anaerobic microbial communities in the wetland (Achak et al. 2023a,b). About 45–50% of carbon in organic matter is utilized by microorganisms as a source of energy in cell synthesis (Gearheart et al. 1999; Wang et al. 2023a, b). Microorganisms attached to the sediment, litter and plants in the water column decompose dissolved carbon through oxidation in the aerobic zone and methanogenesis in the anaerobic zone (Gearheart et al. 1999; Reineke & Schlömann 2023; Wu et al. 2023). In addition, dissolved carbon is also fixed into new biomass during photosynthesis, however, plant decomposition returns a significant proportion of this carbon into the water column (Hurd et al. 2022; Xiao et al. 2022).

COD measures easily degradable materials (BOD) and oxidizable, but not easily degradable compounds and the concentration of the easily degradable compounds is assumed to be equal to BOD₅ (Gearheart et al. 1999; Hurskainen 2020; Otieno 2020; Pitman 2022). The difference between COD and BOD₅ represents the oxidizable, but not easily degradable compounds in the wastewater (Wei et al. 2023). Spellman (2018), Stroo & Ward (2008), and Gearheart et al. (1999) indicated that the non-BOD₅ compounds can be degraded aerobically beyond a 5-day period or under anoxic conditions through anaerobic decomposition.

Nutrient reduction in free water surface flow wetland systems is a little low because most of the important processes involved in nutrient removal occur within the sediment and wastewater flows over the sediment so that dissolved nutrients have to penetrate through diffusion, which is a slow process (Verhoeven & Meuleman 1999; Vymazal 2007; Brix 2020; Patel 2023). Nitrogen occurs in several oxidation states such as organic N, NH₃, ⁠, ⁠, N₂, and N₂O in treatment wetlands because of the numerous biological and physicochemical transformations, but the dominant form depends on the type of wastewater and the pretreatment procedures which might have occurred prior to the wetland treatment (Xu et al. 2004, 2023; Dordio et al. 2008; Vymazal & Kröpfelová 2008). A study by Wang et al. (2019) found ammonium (mg/L) to range from 30 to 300 in produced water.

In free water surface flow wetland, a fraction of organic nitrogen is readily mineralized into ammonia (NH₃) and distributed between the ionic (⁠⁠) and unionic form (NH₃), depending on the pH and temperature of the water, but NH₃ volatilizes under higher pH values as a result of algal photosynthesis (Gearheart et al. 1999; Vymazal 2010; Wu et al. 2023). Ammonium (⁠⁠) is removed primarily through nitrification in the presence of oxygen under aerobic conditions (in the water column) and subsequent denitrification under anaerobic conditions (in the litter layer) (Kumwimba et al. 2023). It can also be assimilated by plants or adsorbed onto surfaces of sediments (Xu et al. 2004; Vymazal 2010; Kumwimba et al. 2023).

Phosphorus removal is usually not the primary target of CWs because phosphorus retention is found to be low in all the types of CWs (Vymazal 2010; Martín et al. 2013; Krzeminska et al. 2023). Soil or peat accumulation is shown to be the major long-term phosphorus sink in terms of phosphorus cycling in wetlands. This is because phosphorus interacts strongly with wetland soil (Vymazal 2010; Wu et al. 2023). Phosphorus retention is usually low because of limited contact of water with soil particles which adsorb and/or precipitate phosphorus and plant uptake represents only temporal storage because the nutrients are released to water after the plant decays (Kadlec & Wallace 2008; Ge et al. 2023). Low removal of nutrients in treatment wetlands is attributable to the fact that most of the important processes involved in nutrients removal occur within the sediment, whereas the wastewater flows over the sediment so that dissolved nutrients have to penetrate through diffusion, which is a slow process (Verhoeven & Meuleman 1999; Xu et al. 2004; Wu et al. 2023). Notwithstanding, the removal efficiency of nitrogen and phosphorus can be doubled if the water levels in the wetland are manipulated so that it has alternating wet and dry periods (Kadlec 1995; Verhoeven & Meuleman 1999; Wu et al. 2023; Zhou et al. 2023) based on the feeding mode employed.

Various feeding modes of influent such as continuous, batch, and intermittent have shown to be an important design parameter which can influence the redox conditions, oxygen transfer, and diffusion in wetland systems, thereby modifying the treatment efficiency (Wu et al. 2015; Alam & Khan 2023). Studies conducted to evaluate the effect of influent feeding modes on the removal efficiency of treatment wetlands generally show that the batch feeding mode has better performance because of its ability to promote more oxidized conditions (González et al. 2021; Lei et al. 2023). The impacts of continuous and intermittent feeding modes on nitrogen removal in free water surface flow and subsurface water flow treatment wetlands were evaluated and it was found that intermittent feeding mode enhanced the removal of ammonium more effectively in subsurface flow CWs without any significant effect on free water surface flow treatment wetland (Jia et al. 2011; Nguyen et al. 2021; Yang et al. 2023). In essence, the feeding strategy employed may depend on the type of treatment system and the dominant contaminant to be removed from the wetland system (Alam & Khan 2023). However, higher removal of organics and nutrients has been associated with the batch feeding mode because of its ability to create aerobic and anaerobic environments in the wetland bed (Fan et al. 2016; Lei et al. 2023; Wu et al. 2023). For wetland systems treating oilfield produced water, dominated by organic contaminants, the batch feeding mode may be preferred for effective treatment of the wastewater (Lei et al. 2023; Wu et al. 2023).

Wetland plants constitute an integral part of a CW (Kadlec & Wallace 2008; Brix 2020; Pinho & Mateus 2023). Many studies including those conducted by Mitsch et al. (2023), Wu et al. (2023), Gupta & Gandhi (2023), Carranza-Diaz et al. (2014), and Brix (2003) have shown that planted wetlands perform better than unplanted wetland, however, the full mechanisms by which planted wetlands boost performance of treatment wetlands is still being explored (Jethwa & Bajpai 2016). Feng et al. (2022) and Tanner (2001) point out that the ecosystem engineering role of plants in treatment wetlands is most obvious in free water surface flow treatment systems where the shoots and litter of emergent plants form the main physical structure in the water column moderating water flow, stabilizing sediments, shading and sheltering the water column, providing surfaces for biofilm attachment and growth, as well as providing refuge and habitat for other biota. Not that only wetland vegetation increases fluffy mass, deposition, and filtration of suspended solids through idealized hydrodynamic conditions (Jethwa & Bajpai 2016; Zhang et al. 2023). In temperate regions, emergent wetland plants insulate the water surface from cold temperatures during winter, trapping falling and drifting snow, and reducing the heat loss effects of wind (Kadlec & Wallace 2008; Brix 2020; Wu et al. 2023). It is therefore clear that the type of plant, plant tolerance, plant density, oxygen supply to root, and type of microbial growth on root surface are plant-related issues that influence the treatment efficiency of CWs (Jethwa & Bajpai 2016; Kulshreshtha et al. 2022).

Macrophytes frequently used in treatment wetlands include emergent, submerged, floating leaved, and free-floating plants (Wu et al. 2015; Kabutey et al. 2019; Liu et al. 2022a, b). However, out of more than 150 macrophyte species used globally in CWs, only a limited number of these plant species are very often planted in reality (Vymazal 2013; Liu et al. 2022a,b). A survey by Retta et al. (2023) and Vymazal (2011, 2013) on emergent plants showed that Phragmites australis, Typha latifolia, and Cyperus papyrus are the most frequently used species in FWSFCW in Europe, Asia, North America, and Africa, respectively.

P. australis, also known as a common reed, is the most frequently used plant around the globe, predominantly in Europe, Canada, Australia, and most parts of Asia and Africa with Typha species being the most second widely used plant in subsurface CWs in North America, Australia, Africa, and East Asia (Vymazal 2013; Kiniry et al. 2023). Moreover, some ornamental plants such as Iris pseudacorus are used in treatment wetlands in some tropical and subtropical countries (Sandoval et al. 2019; Liu et al. 2022a,b; Wu et al. 2023).

Wetland plants have certain features such as lenticels (that permit flow of air into the plants); aerenchymous tissues (that allow gaseous convection all over the length of the plant, providing air to plant roots); special morphological growth structures, such as supports, knees, or an air-filled root, that supply extra root aeration that allows aggressive growth in flooded soils (Vymazal 2013; Pang et al. 2023). Adventitious roots are for surface assimilation of gases and plant nutrients directly from the water column and extra natural tolerance to chemical byproducts resulting from growth in anaerobic conditions. These key morphological features enable wetland plants to perform their functions (Vymazal 2013; Brix 2020; Liu et al. 2022a,b). Wetland plants with special features such as thick waxy coating or cuticles on their leaves with stomata are especially found to be useful in treating oilfield produced water (Nyieku et al. 2020; Anderson et al. 2022; Rajendran et al. 2023; Shah 2023).

Hydrology is a very important feature in wastewater treatment by natural and CWs because it determines the timing and extent of soil saturation (Price & Waddington 2000; Ferreira et al. 2023; Jayathilake et al. 2023). Ecological processes in wetlands are strongly influenced by hydrological processes, which control the dynamics of surface water as well as the inputs and outputs of dissolved matter and sediments (Hayashi et al. 2016; Xin et al. 2022; McGinnis et al. 2023). The formation, persistence, size, and function of wetlands are controlled by hydrologic processes and this is considered the driving force in wetland formation (Carter 1996; Deemy et al. 2022). Hydrological factors are critical as they control the functions and potential optimization of wetland systems in relation to their treatment efficiency (Tchobanoglous & Culp 1980; Ghosh & Gopal 2010; Wu et al. 2015; Jayathilake et al. 2023). Although wetland hydrology is very important, it can sometimes be the most difficult factor to determine in the field because it can be highly variable (Kadlec & Johnson 2023). Setting the appropriate hydrological variables (hydraulic load and retention time) is important for establishing adequate contact time with the microbial community in the wetland for optimal contaminants removal (Wu et al. 2023). The removal efficiency of pollutants in CWs is found to depend on key hydrologic variables such as hydraulic loading rate (HLR) and HRT (Kadlec & Wallace 2008; Wang et al. 2023a, b). These two common variables are used in determining the size and nominal detention of wastewater in CWs (Carleton et al. 2001; Kadlec & Johnson 2023). HLR and HRT affect the duration of the contact between wastewater and the wetland system (Perrens 2023). A longer stay of the water in surface flow treatment wetlands may enhance pollutant removal by longer contact time with microbial-attached surfaces and higher sedimentation rates at lower current velocities, though different contaminants require different HRTs for their removal (De La Mora-Orozco et al. 2018; Pu et al. 2023). The effect of HL and HRT on the treatment efficiency of CWs can be studied in several ways; however, comparative dataset has to be interpreted with care because of possible differences that may exist in the wetland design, wastewater characteristics, and climatic conditions, other than HL and HRT (Toet et al. 2005; Flores et al. 2023). The best approach could be through the use of an experiment to evaluate the responses of pollutant removal to HL and HRT more directly by varying these hydraulic variables within the same wetland system (Patyal et al. 2023; Perrens 2023). The use of replicated treatment units that are simultaneously supplied with the same influent at the same HL, but sampled at HRTs could be applied to study the effect of HL and HRT on treatment efficiency (Toet et al. 2005; Kadlec & Wallace 2008).

The effect of HL and HRT on treatment efficiency is also influenced by plant type, temperature, and media type (Ghosh & Gopal 2010; Alam & Khan 2023). Authors such as Wu et al. (2015, 2023) and Wood & Steffen (1999) are of the view that permeability limitations of media, particularly soil, will ultimately be the deciding factor on the hydraulic loading that the wetland system can accommodate where pollutant adsorption is the desired treatment mechanism. The low permeability of the bed media tends to encourage surface flow rather than filtration through the bed for systems internationally designed for subsurface flow and surface flow systems demonstrate significant short-circuiting (Ahmed et al. 2008; Kandra et al. 2023). These factors minimize available residence times and contact opportunities for optimal treatment to be effected (Stentella et al. 2023). Management techniques such as regular maintenance must be developed to ensure optimal treatment; and these should allow for operational changes to be made in response to changes in the wastewater characteristics, effluent quality, climatic conditions, and effluent discharge requirements (Wood & Steffen 1999; Kadlec & Johnson 2023). A shallow hydrologic environment in treatment wetlands creates unique biogeochemical conditions which are necessary for improving water (Fennessy et al. 2008; Greenway 2003; Armstrong et al. 2023). Contaminants are generally removed either aerobically or anaerobically by complex oxidation/methanation processes with the help of diverse microorganisms in the system (Rajan et al. 2019; Nyieku et al. 2020). Microbial degradation and plant absorption are major mechanisms that act to eliminate and transform nutrients and waste loads in CWs (Stottmeister et al. 2003; Saeed & Sun 2012; Brix 2020). These processes that occur to remove contaminants are intended to be influenced by the environmental conditions of the CW (Wu et al. 2015; Nyieku et al. 2021; Niu et al. 2023).

CW is considered as a biological filter that utilizes the natural processes involving wetland vegetation, soil, and associated assemblage to treat wastewater under controlled conditions (Agaton & Guila 2023; Salah et al. 2023). However, biotic conditions of wetlands are intrinsically linked to the wetland hydrology which affects nutrient availability and physiochemical parameters (Scholz & Lee 2005; Mahoney et al. 2023). Removal processes are so important because they are responsible for turning over both carbon and nitrogen simultaneously in wetlands (Wießner et al. 2005; Zhao et al. 2022), however, internal environmental conditions of CWs such as temperature, pH, DO, and redox conditions influence and modify a variety of key pollutant removal processes such as sedimentation, filtration, precipitation, volatilization, absorption, plant uptake, and microbial processes in treatment wetlands (El Barkaoui et al. 2023; Overton et al. 2023). These factors can equally regulate water quality parameters of wetland effluents discharged to surface water (Scholz & Lee 2005; Stein & Hook 2005; El-Refaie 2010; Wu et al. 2015; Gorgoglione & Torretta 2018; Patel 2023).

DO in treatment wetlands usually gets depleted due to competition to meet respiration needs, carbonaceous BOD, nitrogenous oxygen demand, and sediment/litter oxygen demand needs (Gearheart et al. 2004; Kadlec et al. 2000). Microorganisms attached to solid surfaces mediate between the DO concentration and the oxygen-consuming reactions in the wetland (Li et al. 2022a, b; Salah-Tazdaït & Tazdaït 2023). Oxygen transferred from air and oxygen generated within the wetland supplement the DO concentration of the influent (Wang et al. 2020; Nyieku et al. 2021). Release of oxygen by the roots of wetland plants by diffusive and/or convective gas transport mechanisms from the atmosphere is also believed to play an important role in the supply of oxygen to the microorganisms in the rhizosphere which in turn contributes to DO generated within the system (Vymazal & Kröpfelová 2008; Salah-Tazdaït & Tazdaït 2023). Wetlands are mostly associated with waterlogged soils and the concentration of DO within sediments and the overlying water is of critical importance because most of the anaerobic reactions occur in the water–sediment interface zone of the wetland (Kadlec & Wallace 2008; Cunha-Santino & Bianchini 2023). Removal of most of the wastewater carbon occurs at the inlet oxidized air–water interface zone where DO concentration is comparatively high in the wetland system (Gearheart et al. 1999; Rosendo et al. 2022).

Closely linked to this is the oxidation–reduction potential which is determined by the amount of DO in the wetland (González et al. 2022). Oxidation–reduction, also known as redox, is a chemical reaction involving electron transfer from the donor to the acceptor. This affects chemical and microbial processes and has a large effect on the biological availability of major and trace nutrients (Wießner et al. 2005; Zhang & Furman 2021). Species with a higher reduction potential possess a higher tendency to acquire electrons and be reduced. On the other hand, a species with a higher oxidation potential possesses a higher tendency to lose electrons and be oxidized (Fang & Hu 2022). While deeper sediments in the wetlands are generally anoxic, a thin layer of oxidized soil usually exists at the soil–water interface (Scholz & Lee 2005; Scholz & Scholz 2011). The oxidized layer is important because it permits oxidized forms of various ions to exist in the wetland (Dong & Li 2023). The extent of reduction in the wetland sediment affects the chemical processes that occur above the water column (Scholz & Lee 2005). Zhao et al. (2022), Gupta et al. (2021), and Kuschk et al. (2003) indicate that the aerobic and anaerobic microbial processes allow different oxidation–reduction conditions to exist in the wetland's rhizosphere at the same time. Oxidation (decomposition of organic matter) occurs in the presence of an electron acceptor. However, oxygen is considered the preferred electron acceptor for aerobic microbial respiration, but once oxygen is consumed, the alternative electron acceptor for aerobic microbial respiration is nitrate followed in sequence by manganese oxide, iron oxide, sulfate, and finally carbon dioxide (Szögi et al. 2004). Physical and biological reactions responsible for BOD removal, nitrification, and denitrification depend on temperature (Kadlec & Wallace 2008).

This comprehensive review has delved into the multifaceted nature of oilfield produced water to explore its intricate composition and consequential environmental effects. Attention was given to components such as oil, dissolved minerals, treating chemicals, and bacteria. Several approaches employed in the management of oilfield wastewater were mentioned with much emphasis on the use of API oil–water separators and CWs. The review highlights that the quality and the quantity of chemicals used in the production and processing of oil and gas contribute significantly to changing the physical and chemical properties of oilfield produced water and for that matter judicious use of chemicals during the drilling, production, and treatment processes could reduce the environmental impacts. The authors agree that none of the approaches for measuring oil and grease in produced water are able to measure all the organic compounds therein, hence there should be changes in environmental policy to alter oil in water measurement needs to improve not only discharge regulation but also production and treatment problems. For this purpose, future studies could focus on oil concentrations in produced water and what each change in concentration indicates in terms of production and treatment problems to feed into developing robust environmental policies for the oil and gas sector.

Our review journey through the diverse types of CWs, removal mechanism, feeding mode, wetland plants, wetland hydraulics, and environmental conditions of CWs has shed light on their distinct characteristics and applications in treating oilfield produced water by showing that engineered wetlands offer better opportunities for wastewater treatment than natural wetlands because they are built with a much greater degree of flexibility in terms of site selection, sizing, hydraulic pathways, and retention time, as well as well-defined composition of substrate, type of vegetation, and flow patterns. These parameters are chosen based on the wastewater characteristics to ensure optimum pollutant removal. The use of CWs in conjunction with oily WWTPs could be an innovative environmental solution in mitigating the impact of industrial activities on the environment. The challenge of low nutrient removal in wetlands could be improved through the use of batch feeding mode which is able to create aerobic and anaerobic environments in the wetland bed. The adaptability of this technology is crucial in the context of oilfields wastewater treatment where the composition can be highly variable depending on the geographical location, geological formation, type of hydrocarbon, operational conditions, and the type of chemicals used in the process facilities and the configuration needs to be modified to suit the dominants wastewater characteristics. The review also reveals that achieving the delicate balance between hydraulic efficiency and sufficient retention time is paramount for successful treatment and this can be achieved through experimentation to evaluate the responses of pollutant removal to hydraulic load and retention time more directly by varying these hydraulic variables within the same wetland. Another critical layer which emerged from the review relates to the internal and external environmental conditions, showing the nuanced interplay between ecological components and hydraulic dynamics within these wetland systems.

The authors recommend further research and development to refine and optimize CW systems, with the introduction of an aeration system and configuration that are more effective for the treatment of saline and organic dominated wastewater types. Collaborative efforts between industry, academia, and regulatory bodies are imperative to drive innovation and ensure the widespread adoption of these sustainable technologies, which have largely been regarded as black boxes to ensure a harmonious blend of human ingenuity and ecological resilience.

This study was funded by the Regional Water and Environmental Sanitation Centre Kumasi (RWESCK) at the Kwame Nkrumah University of Science and Technology, Kumasi with funding from the Ghana Government through the World Bank under the Africa Centre of Excellence project. The views expressed in this paper do not reflect those of the World Bank, Ghana Government, and KNUST.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.