Boron in Produced Water: Challenges and Improvements: A Comprehensive Review

INTRODUCTION

Produced water includes water trapped in underground formations and water injected into the stratum to drive out the crude oil (Deng et al., 2002). It is separated from crude oil above ground in an oil/water separator (Murray-Gulde et al., 2003). In early stages of oil production, water content is usually low but can rise to as high as 80% during the later years of the well (Lu et al., 2006). Produced water is the largest waste-stream of oil and gas exploration. Global produced water production is estimated at about 250 million barrels per day compared with about 80 million barrels per day of oil (Fakhru’l-Razia et al., 2009). The chemical composition and behavior of produced water varies when compared with the surface waters because they are constrained within an aquifer (Wemedo et al., 2009). Produced water has distinctive characteristics due to the presence of organic and inorganic matters, high salinity, BTEX, PAH, etc. which can cause toxicity to the environment. Naturally, produced water contains various microorganisms which result in microbial corrosion of the inner surfaces of pipes and related systems conveying the water. Such microbial corrosion process occurs by formation of biofilms on the metal surfaces (Puyate and Rim-Rukeh, 2008). The constituents of produced water vary and can differ from well to well (Cakmakce et al., 2008). The pH of produced water is about 6-8.5 (Cakmakce et al., 2008; Veil et al., 2004) while boron concentration in produced water is about 26-28 ppm (Cakmakce et al., 2008). Produced water is increasingly being considered as a way to supplement limited freshwater resources in many parts of the US as well as other countries (Xu et al., 2008). Therefore, effective treatment method should be employed to treat this essential water for reuse and irrigation purposes especially in arid areas where farmers experience water scarcity.

Boron is a commonly known drinking water contaminant that affects the reproductability of living organisms (Dydo et al., 2005). In nature boron appears mostly as boric acid (H₃BO₃) and borax, (Na₂B₄O₇.10H₂O). In aquatic systems, it exists primarily as undissociated boric acid and borate ions (Bryjak et al., 2008). The main sources of boron in surface water are urban wastewater containing detergents and cleaning products, industrial effluents and chemical products used in agriculture (Liu et al., 2009a). When water with high boron concentration is used for irrigation, boron compounds form complexes with heavy metals like Pb, Cu, Co, Ni, Cd etc. and increase the potential toxicity of these heavy metals.

Table 1:	Health hazard data of boric acid (Bryjak et al., 2008)

Serious health and environmental problems are caused when these complexes pass to groundwater (Sayiner et al., 2008; Seki et al., 2006; Seyhan et al., 2007). Among all the elements of the periodic table, only carbon surpasses boron in variety of applications (Melnyk et al., 2005). The production of boron compounds has substantially increased recently due to increasing demand for these compounds in nuclear technology, rocket engines as fuels, production of heat resistant materials such as refractories and ceramics, high quality steel, heat-resistant polymers, catalysts, manufacture of glass, pharmaceuticals, corrosion inhibitors in anti-freeze formulations for motor vehicle and other cooling systems, dyestuff production, cosmetics, flame retardants, mild antiseptics, soaps, detergents, neutron absorber for nuclear installations, fertilizers, disinfectants, food preservatives etc. (Yilmaz et al., 2008a; Cengeloglu et al., 2007; Fujita et al., 2005; Melnyk et al., 2005; Yilmaz et al., 2005; Magara et al., 1998).

In soil, boron is available to plants as boric acid (H₃BO₃), the form in which it is absorbed by root and transported. Boron plays an important role in plant carbohydrates metabolism, sugar translocation, hormonal action, functioning of the apical meristem, biological membrane structure and function, neutron capture therapy and other industrial products. Boron deficiency in plants may result in reduced growth, yield loss and even death of plant depending on the severity of the deficiency (Yilmaz et al., 2008b). The range between deficiency and toxicity of boron is very narrow and influences the total uptake of anions in plants which also affect plant growth and development (Tariq and Mott, 2006). Boron is readily leached by rainfall and needs to be regularly replaced in a programme commensurate with irrigation, retentive properties of the soil and the requirements of the crops (Gupta, 1979). For humans, boron can represent reproductive dangers and has suspected teratogenetic properties as shown in Table 1 (Bryjak et al., 2008). Linder et al. (1990) and Redondo et al. (2003) reported that boron has presented impediments to male reproduction in studies carried out in laboratories. Studies in rats, mice and rabbits have demonstrated several developmental and teratogenetic effects.

Table 2:	Boron solubility in (g L-1) of cold water (Tariq and Mott, 2007)

Table 3:	Different plant tolerance for boron (Yilmaz et al., 2008b)

In recent years, boron toxicity has gained an increasing interest because of the greater demand for desalinated water, in which boron concentration may be very high for healthy irrigation (Kaya et al., 2009). Different authors have reported the role of salinity on toxicity of boron to plants. Yermiyahu et al. (2008) reported that salinity may reduce or increase boron toxic effect when both occur together while Ferreyra et al. (1997) reported that increased salinity decreases boron toxicity in numerous vegetables, rootstocks, wheat and chickpeas. Very low boron concentration is required in irrigation water for certain metabolic activities. However, if it is present in amounts higher than required, it becomes toxic (Fujita et al., 2005; Ozturk and Kavak, 2008). Several factors affect boron availability to plants. Shaaban (2010) in his review, listed soil pH, soil texture, soil temperature/moisture, soil calcium carbonate and soil organic matter as factors affecting boron availability to plants. The solubility and retention of boron in soil depends on the various soil components and ions, specifically cations (K, Ca, Mg and Na) which readily combine with boron to form metaborates. Table 2 outlines the solubility of various boron compounds (Tariq and Mott, 2007). Therefore care is required in the management of this essential micronutrient in plants because the range between deficiency and toxicity is relatively narrow (Smith et al., 2010). Plants tolerance to boron varies as outlined in Table 3.

There is no easy method available for the removal of boron from water and wastewater (Bick and Oron, 2005; Liu et al., 2009b; Melnyk et al., 2005). Structural studies have indicated that in borates, the boron atom usually combines with either three or four oxygen atoms forming [BO₃] or [BO₄] groups. Accordingly, the electronic orbitals are hybridized to a planar SP² or a three-dimensional SP³ structure (Xue et al., 2000). The commonly used Reverse Osmosis (RO) desalination systems are not efficient enough in boron removal since boric acid might be transported through RO membranes in a manner similar to water (Kabay et al., 2008a; Oo and Song, 2009; Turek et al., 2007a). Conventional ion exchange is also inapplicable due to poor ionization of boron acid and requires periodical regeneration of resins when the ion exchange capacity becomes saturated (Melnyk et al., 2005; Park et al., 2007). Biological treatment is inefficient because of the complex boron chemistry. The objective of this review is to (a) Identify the factors affecting different treatment methods in removing boron from produced water (b) Examine the importance of integrated treatment process in removing boron from water (c) Suggest pre-treatment and post-treatment process for boron removal.

CHEMISTRY OF BORON

Boron is a metalloid and behaves as a Lewis acid (Arias et al., 2011). The borate monovalent anion B(OH)₄- dominates at higher pH while non-ionized boric acid B(OH)₃ dominate at lower pH (Kabay et al., 2010). The dissociation of boric acid in water can be described as follows:

(1)

Further dissociation of boric acid in water occurs at higher temperature:

(2)

(3)

In aqueous solution, boric acid does not dissociate as a Bronsted acid but as a Lewis acid which interacts with water molecules to form tetrahydroxyborate ion (Arias et al., 2011):

(4)

Between pH 7 and 10 and at high concentration (>0.025 mol L^-1), polyborate anions are formed:

(5)

At lower pH, the dominant boron species is boric acid while at high pH, borate compounds predominate. This property of boron has been an obstacle to many treatment methodologies.

TREATMENT TECHNOLOGIES

Reverse Osmosis (RO): Reverse osmosis is a major technology in wastewater treatment and have been used in removing different water contaminants. Newly produced seawater reverse osmosis membranes are claimed to have boron removal efficiency of 91-93% (Oo and Song, 2009). Conventional RO membranes reject boron to a level of about 40-78%. Single stage RO membranes are able to turn seawater of boron concentration (4-5 mg L^-1) into permeate water with boron concentration of about 0.9-1.8 mg L^-1 (Sagiv and Semiat, 2004). This is because boric acid can diffuse through RO membrane in non-ionic form similar to that of carbonic acid or water. Rejection of boron by RO is better for the borate ion due to its charge while the rejection of non-ionized boric acid is low due to its smaller size and lack of electrical charge. Due to the absence of ionic charges at low pH, the hydration of the molecule results in a smaller size and less rejection of the molecule by a membrane. The dissociated form on the other hand will be fully hydrated, resulting in a larger radius and an enhancement of the negative charge of the ion. This results in higher rejection both by exclusion and repulsion by the negatively charged membrane (Redondo et al., 2003). Because boric acid dominate at low pH, it is able to form bridges of hydrogen with the active groups of the membrane and diffuse in a similar way to that of carbonic acid or water (Fig. 1) (Pastor et al., 2001). Several factors affect RO process. These factors include temperature, operating pressure and pH. Studies have shown that characterized parameters for selection of RO membranes should include zeta potential, contact angle, roughness and pore size distribution. Membrane surface roughness is also important in trapping of pollutants (Sarp et al., 2008).

Effect of pH: Boron rejection depends greatly on pH (Magara et al., 1998). With high salinity feeds such as seawater, boron rejection as high as 90% has been demonstrated but for low salinity waters, boron rejection very much depends on the pH level since pH determines the form of boron in water due to the equilibrium reaction between boric acid and borate ions (Bick and Oron, 2005). Higher pH levels result in appreciable decrease of boron in the permeate.

Fig. 1:	Mechanism of H2O transport through reverse osmosis membrane by forming bridges of hydrogen (Pastor et al., 2001)

Fig. 2:	Impact of solution pH on boron rejection by A and B membranes (batch concentration mode, pressure: 700 psi, Feed flow rate: 3.8 L min-1, Feed temp.: 22-24 EC) (Koseoglu et al., 2008a)

This is because uncharged boron in the form of boric acid diffuses in the membrane for pH lower than 9.5. At high pH, boric acid transforms into borate ions (Bouguerra et al., 2008). Potential precipitation of calcium carbonate and magnesium hydroxide at high pH in seawater reverse osmosis applications is a problem and must be avoided (Cengeloglu et al., 2008). Koseoglu et al. (2008a) reported that a much higher boron rejection was obtained at pH of 10.5 (>98%) than those at natural seawater pH of 8.2 (about 85-90%) (Fig. 2). At high pH, there is stronger repulsion between membrane active surface and the charged borate ion which leads to the enhanced boron rejection (Bonnelye et al., 2007). Figure 3 shows dissociation of boric acid at various pH.

Effect of operating pressure: Increase in applied pressure increases the net pressure and consequently the permeate flux as well as rejection of particle across membrane (Binyam et al., 2010). An increase in applied pressure results in an increase in water flux which leads to higher recoveries (Bonnelye et al., 2007). Applied pressure also increases boron and salt rejections (Bouguerra et al., 2008; Guler et al., 2011). The higher the applied pressure, the higher the boron rejection (Magara et al., 1998; Koseoglu et al., 2008b).

Fig. 3:	Different forms of boron depending on pH, (Bonnelye et al., 2007)

Fig. 4:	Dependancy of boron rejection on the operating pressure (Cengeloglu et al., 2008)

Figure 4 shows an upward trend in boron rejection as the applied pressure was simultaneously increased.

Electrodialysis (ED): Electrodialysis (ED) is a commonly used treatment technology. ED is an electrochemical process in which ions migrate through ion selective semi permeable membranes as a result of attraction to two electrically charged electrodes. It has high rejection for total dissolved solids, ions and colloids (Bick and Oron, 2005). Conventional ED is only capable of removing about 42-75% of boron (Yazicigil and Oztekin, 2006). Advantages of ED include high rejection of contaminants, low pressure requirement (lower than RO), requires minimal supervision in a remote setting, reject can meet effluent disposal limits and prevents scaling while its disadvantages include high capital and operating cost, high level of pre-treatment and frequent electrode replacement. Several factors affect the efficiency of boron removal by ED in produced water. They include interference of ions, pH of the sample and nature of the ED membrane. Studies have shown that chloride and sulfate affect boron transport in ED (Bandura-Zalska et al., 2009; Kabay et al., 2008b).

Fig. 5:	Effect of chloride ions on boron removal (B:Cl equivalent ratio = 1:1; D: Dilute compartment, C: Concentrated compartment) (Kabay et al., 2008b)

Fig. 6:	A typical ED experimental diagram (Bandura-Zalska et al., 2009)

Salinity affects boron removal through electrodialysis. In a study of boron removal by electrodialysis, Kabay et al. (2008a) observed that chloride transported more quickly than borate ions (Fig. 5). Boron transport depends on ion charges and the hydratized radius of ions. Therefore, transport of boron becomes harder with increasing radius of ions and ion charges (Bandura-Zalska et al., 2009). Increase in solution pH results in increase in transfer of boron. Also, increase in current density results in an increase in transfer of boron from the cathode chamber to the anode chamber. A typical ED diagram is represented in Fig. 6.

Effect of interfering ions: Transfer of boron by electrodialysis using anion exchange membranes is influenced by salinity. It is reported that in the presence of chloride, percent removal of boron decreases and operation time increases while in the presence of sulfate, percent removal of boron did not change but operation time increased. This difference in transport property of boron in the presence of sulfate/chloride may be attributed to the hydrated radius of several anions present in the aqueous medium (Banasiak and Schafer, 2009; Turek et al., 2007b). Bandura-Zalska et al. (2009) proposed an integrated process (desalination and ED) in boron removal because of the effect of interfering ions. Desalination is applied at low pH to remove ions at no boron transport while ED is applied at high pH to remove boron in the second stage since borate ions are present at high pH. The main goal of the preliminary stage is to reduce salinity at negligible boron transport as well as maximize desalination degree and rate at final concentrate boron level. Turek et al. (2007a) also used desalination and ED in removing boron from wastewater. It was observed that high initial dilute pH values do not enable reliable boron flux through the membrane. This may be due to high mobility and relative high content of hydroxide ions. The dissociation constant of B(OH)₃ decreases with increasing NaCl concentration. NaCl concentration affects removal of other ions. Ions with smaller intrinsic crystal radii have higher hydration numbers, larger hydrated radii and hold their hydration shells more strongly. The larger the crystal ionic radius, the more diffuse the electric charge and the fewer water molecules surround the ion (Banasiak and Schafer, 2009). Kabay et al. (2008b) in their study, observed that more than 90% of chloride ions was removed from the solution in 18 min while only 20% of boron was removed along with chloride. After 42 min of operation time, almost all chloride ions were removed from the solution while about 40% of boron still exists in the solution (Fig. 7).

Adsorption: Adsorption is a process through which boron is adsorbed unto different kinds of particles. Many adsorbents have been used in adsorption process. They include cerium oxide (Ozturk and Kavak, 2008), activated carbon prepared from coconut shell impregnated with calcium and barium chlorides, citric and tartaric acids (Rajakovic and Ristic, 1996), activated alumina (Bouguerra et al., 2008), Al₂O₃ based materials (siral 30 and pural) (Seki et al., 2006), activated carbon impregnated with salicylic acid (Celik et al., 2008), iron-rich natural clays (Seyhan et al., 2007), activated sludge (Fujita et al., 2005), neutralized red mud (Cengeloglu et al., 2007) and composite magnetic particles (Liu et al., 2009b). Several factors affect adsorption of boron. They include loss of adsorbent, geometry of the system, the flow of water solution through the pores of the system, the diffusion rate, adsorbent dose and the kinetics of the surface reactions (Ozturk and Kavak, 2008).

Fig. 7:	Separation of boron in the presence of chloride ions (B:Cl : 1:1 (Kabay et al., 2008b)

Fig. 8:	Effect of pH on the adsorption of boron on cotton cellulose (C0 = 500 mg L-1, m = 0.1 g, V = 50 mL) (Liu et al., 2007)

Sample pH is a determinant of boron removal in adsorption study (Garcia-Soto and Camacho, 2006). Depending upon the nature of adsorbents, Irawan et al. (2011) reported that borate concentration in the solution and its adsorption are both pH-dependent and the optimum pH of boron adsorption is 2 for fly ash, 6.0 for composite magnetic particles, 7 for neutralized red mud and 9 for layered double hydroxide. Solution pH affects both boron speciation and surface properties of sorbent (Wei et al., 2011). The amount of removed boron increases with increasing adsorbent dose due to the increase in the total available surface area of the adsorbent particles (Yazicigil and Oztekin, 2006). Studies have shown that different adsorbents have varying adsorption capacities (Bouguerra et al., 2008; Ozturk and Kavak, 2008; Seki et al., 2006; Yilmaz et al., 2005). Figure 8 represents the effect of pH on boron adsorption on cotton cellulose. Different adsorbents have different adsorption capacities. Table 4 is a compilation of different adsorbent materials with their adsorption capacities.

Effect of adsorbent dose: It is essential to determine the amount of adsorbent that is required for an experiment (Garcia-Soto and Camacho, 2006). Increase in adsorbent dose increases the surface area of the adsorbent particles. Boron removal increased with increase in red mud dosage as a result of the increase in total available surface area of the adsorbent particles. Therefore, the higher the adsorbent concentration, the lower the boron level in the permeate (Bryjak et al., 2008). Figure 9 represents variation of boron removal with red mud dose (Cengeloglu et al., 2007).

Table 4:	Different adsorbents and their adsorption capacities

Fig. 9:	Variation of boron removal with red mud dose. Initial boron concentration: 43 mg L-1, pH 7, agitation speed: 500 rpm and temperature: 25±1°C. (Cengeloglu et al., 2007)

Ion exchange: Ion exchange is one treatment process that has gained wider application in boron treatment. The use of selective ion exchange resins (Purolite and Diaion) have not only been efficient in removing boron from wastewater but also in recovering boron from wastewater (Kabay et al., 2004). The most commonly used resins are the Amberlite IRA-743, otherwise known as Amberlite XE-243 and Diaion CRB 02 and Dowex XUS 43594.00 (Kabay et al., 2008c). These resins are macroporous polystyrene based resin, with functional groups specially designed for the selective removal of salts of boron from aqueous solutions. They are effective for solutions over a wide range of pH values and over a wide range of boron concentrations. These boron selective resins show great elimination performance and the sorption kinetics of both resins fit well to pseudo-second-order kinetics model (Shpiner et al., 2009). This complex formation is not pure ion exchange and therefore does not require ionization of boric acid. The resin performance is not affected by temperature variations, by pH value or by the background salinity of the water to be treated. Selective sorption of boron by these sorbents is as a result of formation of stable complexes like ethers or complex anions with polyoxicompounds. These resins used in ion exchange system have a macroporous polystyrene backbone and a very specific functional group based on N-methyl glucamine which has a tertiary amine end and a polyol end and makes a very stable complex with boric acid (Jacob, 2007). Boron removal efficiency here is reported to be about 93-98% (Melnyk et al., 2005). A typical ion exchange selective resin is shown in Fig. 10.

Uptake of boron by this selective ion exchange resin is high. An evaluation study with Diaion (CRB 02) and Dowex(XUS 43594.00) resins by Kabay et al. (2006) shows that both resins have almost similar boron adsorption capacity and are efficient in removing boron from solution as shown in Fig. 11.

Fig. 10:	Chemical structure of boron selective resin containing N-methyl glucamine (Kabay et al., 2008c)

Fig. 11:	Boron adsorption isotherms of Diaion CRB 02 and Dowex (XUS 43594.00) resins (Kabay et al., 2006)

Fig. 12:	Effect of pH on boron removal (Yan et al., 2008)

The advantages of these selective ion exchange resins over the conventional ion exchange treatment are that they are not affected much by pH of solution or concentration.

The mobility and hydratized radius of ions affects boron transport in an ion exchange process (Kir et al., 2011). An increase in pH increases removal of boron by the resin since the resin can only extract the ionic form of boron (Fig. 12). An increase in temperature also increases the removal of boron from brine (Yan et al., 2008).

Biological treatment: Biological treatment of produced water is still in its infancy (Shpiner et al., 2009). Reported literatures on boron removal through biological treatment are scarce. Previous works have been mainly laboratory/pilot scale experiments of aerobic and anaerobic treatment such as activated sludge (Tizghadam et al., 2008; Dincer, 2004; Tellez et al., 2002), aerated lagoons (Boiran et al., 1996; Montalvo et al., 2010), SBR system (Li et al., 2003; Xiao et al., 2008), immobilized cells system (Li et al., 2005; Lu et al., 2009; Wang et al., 2008), Biodegradation (Rezaee et al., 2006), dinitrogen fixation (Perona et al., 1991), sulfate removal (Chen et al., 2009; Zhou et al., 2011; Zhao et al., 2009), activated sludge, sand filters and activated carbon (Al-Jlil, 2009), sulfidogenic bacteria (Agrawal et al., 2010; Kaura et al., 2009), geochemical influence and biokinetics (Orphan et al., 2004; Tellez et al., 1995), Semi-continuous and continuous anaerobic treatment (Hong et al., 2009).

Various biomass species have been used in a single and combined process to treat produced water but there have been marked inefficiencies from literature over their ability to feed on colloids. Single step biological treatment has proved ineffective in removing many pollutants of wastewater. Conventional biological treatments have not been effective enough to reduce boron to its standard limit level for irrigation (Linares-Hernandez et al., 2010). One of the biological treatment methods that have been used in the treatment of wastewater pollutants is Waste Stabilization Ponds (WSPs). WSPs incorporate the activity of phototrophic, autotrophic and heterotrophic microorganisms, requires little or no energy and play important role in the removal of pollutants. Oxidation ponds are characterized by a high surface area and retention times of a few days to a few weeks. The biodiversity in the ponds may also influence the potential of the process to degrade a wide range of petroleum-derived compounds including the full or partial degradation of slowly degradable and recalcitrant material. WSPs have a large volume which enables them to tolerate shock loads and efficiently remove heavy metals (Shpiner et al., 2009). In a study of biological treatment of boron containing wastewater by activated sludge conducted by Dincer, 2004, it was observed that boron concentration affected COD removal rate when compared with the COD removal rate of a boron free wastewater. Whereas the COD concentration of a boron free wastewater in the reactor reached a steady state level of 20 mg L^-1 after 9 h of operation, resulting in a COD removal rate of 75.39 mg L^-1 h^-1 and a removal efficiency of 98% at a loading rate of 76.92 mg L^-1 h^-1, the COD removal rate of a boron containing wastewater for feed boron levels lower than 2500 mg L^-1 has efficiencies of about 80%. However, when the feed boron level is higher than 2500 mg L^-1, the system did not reach steady state and the COD removal rate decreased. The rate of COD removal also decreased with increasing boron in the feed wastewater, as shown in Fig. 13. The COD removal rate was nearly 75.39 mg L^-1 h^-1 with boron free wastewater and decreased to a value of 45.39 mg L^-1 h^-1 for 5000 mg L^-1 boron concentration, indicating approximately 40% reduction.

Fig. 13:	Variations of total COD removal rate with H3BO3-B concentration in feed wastewater at steady-state (Dincer, 2004)

Electrocoagulation (EC): Electrocoagulation (EC) is the process of destabilizing suspended, emulsified or dissolved contaminants in an aqueous medium by introducing an electric current into the medium (Emamjomeh and Sivakumar, 2009). EC is based on the valid scientific principle of water response to strong electric field. EC is an emerging treatment technology which has been applied with success in wastewater treatment (Linares-Hernandez et al., 2010). Electrocoagulation involves the generation of coagulants in situ by electrical dissolution of the sacrificial anode and involves the following three mechanisms; electrode oxidation, gas bubble generation, flotation and sedimentation of flocs formed (Emamjomeh and Sivakumar, 2009).

EC presents similar advantages as chemical coagulation and reduces its disadvantages which results in higher yields and less waste sludge (Essadki et al., 2009).The advantages of EC include high particulate removal efficiency, compact treatment facility, relatively low cost and possibility of complete automation (Aoudj et al., 2010; Chen, 2004). EC is characterized by a fast rate of pollutant removal, simplicity in operation and low capital and operating costs (Chen et al., 2000). EC has been used in reducing boron to the World Health Organization (WHO) standard limit of 0.5 mg L^-1. The efficiency of EC depends on sample pH, current density, temperature, boron concentration, electrode spacing and treatment time. pH is an important parameter influencing the performance of the electrochemical process because pH determines the ionic form of boron (borate ions) (Balasubramanian et al., 2009). At higher inter-electrode distance, rate of aggregation of suspended particles as well as adsorption of contaminants will be low. At minimum inter electrode distance, the resistance for current flow in the solution medium is lower and that facilitates the electrolytic process for enhanced removal (Ghosh et al., 2008).

Fig. 14:	Changing of borate ions species depending

Fig. 15:	Effect of current density on boron removal pH in aqueous media (Yilmaz et al., 2007) (Yilmaz et al., 2008a)

The current density is the current per unit surface; it is a parameter that controls the anode dissolution speed on the one hand and that of hydrogen formation on the other hand (Nanseu-Njiki et al., 2009). Effect of pH and current density on EC is represented in Fig. 14 and 15.

Boron removal efficiency of 95% with electrocoagulation was reported by Yilmaz et al. (2008b) (Fig. 16). Electrocoagulation can be an efficient means of removing boron from wastewater in a cost effective way when the operating parameters are favorably combined.

Integrated system: Different treatment techniques can be combined to treat produced water. Some studies have used integrated process to treat produced water.

Fig. 16:	The effect of temperature on boron removal (3.0 mA m-2 current density, pH 8.0 and 150 rpm stirring speed) (Yilmaz et al., 2008b)

Ebrahimi et al. (2010) used Microfiltration (MF) and Ultrafiltration (UF) as a pretreatment step and Nanofiltration (NF) as the final step in the treatment of oilfield produced water. Turek et al. (2005) studied adsorption/co-precipitation process coupled with reverse osmosis. Li et al. (2010) investigated the effects of the combination of anaerobic process and micro-electrolysis in the removal of COD and biodegradables. The integrated system can be used to remove boron from produced water. Each component of the integrated system should be operated at its best operating condition. However, the cost of such combination should also be taken into consideration to ascertain its feasibility. Various membrane methods like MF, UF, NF and RO can be combined, RO and ED; Micro-electrolysis and biological treatment; EC and biological treatment/RO; adsorption/co-precipitation/RO; ion exchange and RO, 2 pass RO with increased pH especially in the 2nd pass etc.