Potential tertiary treatment of produced water using highly hydrophilic nanofiltration and reverse osmosis membranes

Potential tertiary treatment of produced water using highly hydrophilic nanofiltration and reverse osmosis membranes

Journal of Environmental Chemical Engineering 1 (2013) 1341–1349 Contents lists available at ScienceDirect Journal of Environmental Chemical Enginee...

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Journal of Environmental Chemical Engineering 1 (2013) 1341–1349

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Potential tertiary treatment of produced water using highly hydrophilic nanofiltration and reverse osmosis membranes Salem Alzahrani a, Abdul Wahab Mohammad b,*, Pauzi Abdullah c, Othman Jaafar a a

Department of Civil and Structural Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Research Centre for Sustainable Process Technology (CESPRO), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia c School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 June 2013 Received in revised form 28 September 2013 Accepted 2 October 2013

This study characterized the potential of new, highly hydrophilic nanofiltration (NF) and reverse osmosis (RO) membranes for the beneficial reuse of produced water. It was found that both NF and RO membranes were hydrophilic at 238  0.90 and 378  0.49, respectively. The findings of the permeation tests revealed that the NF membrane exhibited a higher permeability (7.3 L m2 h1) in pure water than the RO membrane (3.4 L m2 h1). The NF membrane was effective at rejecting certain monovalent salt ions (in 2000 mg/L, 97% Na2SO4, 95% MgSO4, 94.8% CaSO4, 94% K2SO4, and 87% Na2CO3), whereas the RO membrane was more effective at rejecting hard salts (96% Na2CO3, 88% NaCl, 85% KCl, 85.4% BaCl2, 83% NaHCO3, and 80– 81% for Na2SO4, MgCl2, SrCl2, and K2SO4). A primary assessment of the post-treatment potential of the NF and RO membranes for produced water showed that the critical component in produced water was characterized mainly by TDS and TOC at 854 and 26.3 mg/L, respectively. The RO membrane was more efficient at rejecting these components, in quantities of 244 mg/L of TDS and 6.7 mg/L of TOC, whereas the NF membrane attained 520 mg/L of TDS and 22.9 mg/L of TOC. Both membranes reduced the initial oil concentrations (2 mg/L), turbidity (21 NTU) and TSS (10 mg/L) to less than 1 mg/L. Conclusively, the findings on the treated water quality substantiated the possibility of utilizing RO-treated water as a future source of water. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: Produced water Membrane characterization Hydrophilic membrane Tertiary treatment Wastewater reuse

Introduction A significant amount of research has been conducted in the field of produced water treatment using membrane technology [1–6]. The output of such research has highlighted the economic and environmental aspects of creating solutions for produced water management. However, far fewer researchers have considered the beneficial recycling of produced water [7–10]. To date, studies conducted in this field have not achieved the optimal reuse of produced water as a strategic option based on the principles of sustainable development. Current scientific efforts have remained focused on developing the most economical and environmentally compliant solutions for the management and disposal of untreated produced water. In comparison, the reuse of municipal wastewater has become a common global practice. For example, the direct use of treated municipal wastewater has been recorded at 1.8 billion m3 for irrigation purposes and 1.23 billion m3 for other applications between 2008 and 2012. [11] However, the recycling

* Corresponding author. Tel.: +60 3 89216410; fax: +60 3 89216148. E-mail addresses: [email protected], [email protected], [email protected] (A.W. Mohammad). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.10.002

of industrial wastewater has not become widespread because of the associated technical and health issues. In the past, the costs of treating industrial wastewater, such as discharged produced water from the petroleum industry, with the goal of beneficial reuse have been high. However, these costs have dropped with the development of and advances in effective technologies such as nanotechnology [12]. Accordingly, the reuse of produced water in arid areas suffering from an acute shortage of water resources could be a promising practice, especially in countries where the amounts of withdrawn and consumed water are not balanced [13]. Saudi Arabia is one of the largest countries subject to extreme water shortage. Specifically, its internal renewable freshwater resources are estimated at 2 billion m3 each year, but 23.7 billion m3 is withdrawn annually. In other words, the withdrawn water exceeds the rate of natural water replenishment (NWR) by 986%. Similarly high excesses have been observed in other arid countries such as Iraq (187.5%), Libya (721%), Qatar 870.6%, and the United Arab Emirates (2665.3%) [13]. All of the aforementioned countries are top global oil and gas producers and producers of discharge produced water that has been treated only to meet environmental regulations [14]. However, massive amounts reaching 56.23 million m3 are generated and discharged daily by the petroleum industry [15]. This discharged water would require only one

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advanced post-treatment step, such as filtration through nanofiltration (NF) and reverse osmosis (RO) membranes, to enable its indirect reuse as a possible source of potable water. This use would occur through the augmentation of surface water basins or dams, mixing with other water taken from surface water or groundwater, recharging or the use of groundwater aquifers or storage as underground water for future use [16]. In this context, the reuse of produced water for beneficial uses has become possible thanks to the emergence of nanotechnology membranes, which have enabled the rejection of nano-sized and dissolved molecules and ions. However, an effective solution to membrane fouling, a major drawback of membrane technology, has not been forthcoming [17]. Even in the absence of long-term solutions for diminishing membrane fouling, however, significant achievements have been made toward understanding the fate and essence of fouling by identifying its mechanisms, foulants and characteristics, and other relevant chemical and hydrodynamic factors in various applied solutions and operation parameters [17,18]. One such advancement in this field has been the employment of highly hydrophilic membranes for the filtration of industrial wastewater, such as produced water, that have a low fouling tendency. As explained by Tang et al. [17], this characteristic may be attributed to membrane properties such as smooth surfaces that affect the hydrodynamics and surface interactions near the liquid–membrane interface, electrostatic interactions, and acid–base interactions. For example, Bowen et al. [19] have employed an atomic force microscope (AFM) to analyze the interaction force between a colloidal silica probe and a rough membrane surface, and they found that a membrane with a low surface roughness exhibits a smaller interaction between the electrostatic repulsion, colloid and membrane surface. Therefore, the selection of a hydrophilic membrane to treat produced water (e.g., in a way that meets allowable discharge standards) for reuse in beneficial applications must show a lower tendency for potential membrane fouling by the foulants present in produced water. The content of discharged produced water is complex, and based on substantial peer-reviewed publications on discharged produced water characteristics, Table 1 represents an overview of the global composition of discharged produced water originating from upstream and downstream processes in the petroleum industry. An analysis of 429 samples of such discharged water from

7 countries, including India, China, Brazil, the UK, Norway, the US and the UAE has revealed that the critical components in discharged produced water that could create technical challenges during the application of membrane technology are predominantly TDS and TOC. The range of TDS in discharged produced water, as featured in Table 1, has been identified as 4380–49,971 mg/L, whereas the TOC ranges from 45–800 mg/L [20–33]. The gap between the presence of high levels of inorganic content (TDS) compared with the lower occurrence of organic content (TOC) has stemmed from the fact that most global environmental regulations for discharged produced water focus on organic components, specifically hydrocarbon compounds. Their concentrations are evaluated by an oil parameter, with the allowable limit for discharge in the Northeast Atlantic and North Sea area (at offshore oil production platforms) set at 30 mg/L of oil. At onshore petroleum facilities, this level may be as low as 5–10 mg/L [34– 36]. Therefore, advanced tertiary treatment, such as the application of a hydrophilic membrane, is necessary to remove these contaminants when recycling produced water with such a high contaminate content, as presented in Table 1, and to meet the regulated standards for reuse. Therefore, this study explored the potential for applying two new, hydrophilic NF and RO membranes to tertiary produced water treatment through the evaluation of their characteristics. Materials and methods Membrane characterization Two new commercial membranes supplied by Amfore Inc. (Amei Ande Membrane Technology Ltd., Beijing, China), the BW30 RO and the NF1, were studied. The selected membranes were characterized using different techniques, including contact angle, field emission scanning electron microscope (FESEM), attenuated total reflection Fourier transform infrared spectroscopy ATR-FTIR and the membrane permeation test. The hydrophobicity of the membrane surface was analyzed through contact angle measurements, performed with a static sessile drop method using the Goniometer contact angle (Model 290, Rame´-Hart, Netcong, USA). A 2-mL droplet of deionized water was placed on the membrane

Table 1 A typical composition of discharged produced water from petroleum industry. Parameter

Unit

Min

Max

No. of samples

TPH TOC Oil Chloride Sulfate Barium Zinc Iron Mercury Cadmium Lead Copper Manganese Nickel Arsenic Chromium Calcium Magnesium Sodium TDS pH COD 226-Radium 228-Radium

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L – mg/L Bq/L Bq/L

2.4 45 8 17 6.2 0 0.0068 4.2 0.000017 0.00005 0.0001 0.0002 1 0.001 0.0005 0.01 987 369.1 12,878 4380 7.5 <90 0.002 0.02

32 800 360 25,502 621.3 342 4 37 0.0002 3.3 0.08 0.35 7 0 0.05 10 2669 678 18,876 49,971 8.4 1588 58 59

43 16 8 7 2 31 31 30 30 17 13 12 10 10 10 3 2 2 2 11 3 14 63 59

Data source: [20–33]

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surface at ambient temperature, and the contact angle image was captured from the membrane surface via an optical camera connected to DROPimage software. Three series of measurements at 5 different locations were taken, and 50 readings were recorded every 0.05 s in each run. Visual information on the top surface and the cross-sectional morphology of the membranes was provided by a Zeiss SUPRA 55 VP FESEM. After freezing the membranes in liquid nitrogen, each membrane was broken and then sputtered with gold prior to FESEM analysis. ATR-FTIR was used to characterize the functional group of the clean NF and RO membranes. The clean membrane specimens were recorded using the attenuated total reflection (ATR) technique with a Nicolet 6700 FTIR spectrometer (Thermo Scientific, USA). Several scans were taken at a 4 cm1 resolution between wavelengths of 4000 and 500 cm1. All samples were wetted and dried in a desiccator to prevent the interference of preservative materials effects in the membrane specimens. Membrane permeation test Permeation tests were performed to characterize the permeability, flux and rejection of the NF and RO membranes using pure water and samples of produced water. The deionized water used in the experiments, with a conductivity of 1 mS/cm, was obtained by demineralization using ion exchange followed by reverse osmosis (Heal Force1). Two types of produced water samples were used during the permeation tests: (i) actual samples of discharged produced water obtained from the produced water treatment plant of the Melaka refinery in Malaysia that were in the last stage of treatment prior to being discharged to the environment, and (ii) synthetic samples of salt solutions prepared to simulate the global concentrations of TDS present in discharged produced water, as shown in Table 1. The purity of the salts used in the experiment was more than 99%, except CaSO4, which had a purity of 98%. The salts were obtained from R&M1 Chemicals (CaCl2, CaSO4, Na2CO3, NaHCO3 and MgCl2), F.S.1 Chemicals (BaCl2, K2SO4 and SrCl2) and J. Kollin1 Chemicals (KCl, NaCl, Na2SO4 and MgSO4). The permeation tests were conducted in a dead-end filtration cell (Sterlitech, Model HP4750) with a diameter of 0.0047 m2 and an effective membrane area of 0.00146 m2. All membranes were soaked in ultrapure water for 24 h and dried at room temperature to remove preservatives, and this soaking also acted as a wetting process for the membrane. Next, a new wetted piece of flat-sheet membrane was placed at the bottom of a dead-end stirrer cell supported by a stainless steel porous support plate. The membrane was compacted without stirring for 1–2 h by pressurizing the stirred cell with nitrogen from a gas cylinder at a high level of pressure until the permeate flux became constant. After compacting and conditioning the membrane, the pure water permeability through the membrane was determined according to Eq. (1). A sample of feed solutions (real produced water samples and the 12 single salts CaCl2, CaSO4, K2SO4, KCl, MgCl2, MgSO4, Na2CO3, Na2SO4, NaCl, NaHCO3, SrCl2 and BaCl2) was then placed into the dead-end stirred cell to determine either the flux or the rejection (in accordance with Eqs. (2)–(4)) by equilibrating the passage of the first 20 mL of permeate. The following 20 mL of permeate was collected for flux, rejection and concentration analysis of the produced water’s post-filtration quality. The concentration of each salt solution was set at 2000 mg/L, and the stirring speed was set at 400 rpm when the salt solutions and actual samples of produced water were applied. The applied pressure of the salt solution during the permeation tests ranged from 2 to 20 bar. One level of applied pressure was set at 6 bar to test the potential of the NF and RO membranes to remove pollutants from the produced water by analyzing the following physico-chemical parameters: water turbidity (measured using a Hach Instruments Turbidimeter

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2100P), conductivity, total dissolved solids (measured by a Eutech Instruments Cyberscan CON11 conductivity/TDS meter), chemical oxygen demand (analyzed by a Varian Cary 50 UV-Vis Spectrophotometer), total suspended solids, oil and grease (measured by a Sartorius BP210S balance) and total organic carbon (quantified by an OI Analytical Aurora 1030 TOC analyzer). All parameters were analyzed according to the APHA standard methods [37], and the following equations were used to determine permeability, salt solutions flux and rejection. The permeabilities of the pure water and salt solutions were calculated over measured time intervals using (Eq. (1)): P¼

Q ADT

(1)

where P is the permeability (L m2 h1), Q is the quantity of pure water or salt solutions permeated, A is the membrane area (m2), and DT is the sampling time (h). The flux of the salt solutions was identified as (Eq. (2)) J ¼ P m ðDP  DpÞ

(2)

where J is the salt solution flux, Pm is the permeability (L m1 s1 bar1), DP is the applied pressure (bar), and Dp is the osmotic pressure (bar) [38]. Because the osmotic pressure of salt solutions may affect membrane walls during the calculation of flux, Dp was calculated based on the Van’t Hoff law (Eq. (3)):

Dp ¼ Rg TðC i;w  C i; p Þ

(3)

where Rg is the universal gas constant (L/bar), T is the absolute temperature (Kelvin), and Ci,w and Ci,p are the concentrations of feed (mol/L) and permeate (mol/L), respectively [39]. It should be noted that all concentrations of salt solutions were below 2000 mg/L in this study, so the osmotic pressure was not overestimated, as it can be when (Eq. (3)) is applied to highly concentrated salt solutions [40]. The calculation of an observed rejection for the salt solutions was based on the feed and permeates, using the conductivity of electrolyte solutions as measured with a conductivity meter (Mi 306, Martin instruments, Romania). All conductivity readings were first converted into concentrations according to the calibration curve. The rejection of the salt solutions and the removal efficiency of the NF and RO membranes during the filtration of the produced water were calculated using (Eq. (4)):   1  Cp R ð%Þ ¼  100 (4) Cw where R is the rejection and Cp and Cw are the concentrations of permeate (mg/L) and feed (mg/L), respectively. Results and discussion Membrane hydrophobicity In this study, the surface hydrophobicity of each selected NF and RO membrane was determined using the contact angle technique, which characterizes the behavior between a water drop and a membrane surface interface by estimating a contact angle (u). If the measured contact angle fell within the range of 08
[(Fig._1)TD$IG]

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Fig. 1. Water drop contact angle (u8) as a function of membrane surface hydrophobicity.

contact angles and their images, under clean conditions, are illustrated in Fig. 1. Hydrophilic membranes are preferred in industrial applications of wastewater treatment, particularly for water recycling purposes, owing to their low tendency toward fouling [17,19]. In this study, the findings on contact angles revealed that the selected membranes were highly hydrophilic compared to other commercial membranes, as shown in Table 2. Visualization of morphological membrane characteristics The morphological structures of the clean NF and RO membranes were modeled using field emission scanning electron microscopy (FESEM) imaging. Fig. 2 shows the FESEM images of the clean NF and RO membrane surfaces, while Fig. 3 shows FESEM micrographs of the membranes’ cross-sectional morphologies. Such morphologies are typically one of two types, a thin-film layer or selective layer and a supporter layer. These membranes were characterized as thin-film composite (TFC) membranes. Functional group analysis using ATR-FTIR Fig. 4 shows the FTIR spectra of the clean NF and RO membranes. The similar spectra of both membranes were congruent with their similar polyethersulfone (PES) construction. The absorption peaks at 1714 cm1 (ether linkage) and 1240 cm1 (organic phosphates) were ascribed to the symmetry of the O5 5S5 5O stretching vibrations in sulfonic groups, while the absorption peaks at 1015 cm1 (S5 5O) represented the presence of a sulfonic group in the NF and RO membranes. The peaks at 722 cm1 (halogen group), 1505 cm1 (isothiocyanate, N5 5C5 5S), and 1986 cm1 (carboxylic acids) represented the miscellaneous

chemical bonds of polyethersulfone, which constituted all the studied membranes [42,43]. Pure water permeability The pure water permeability of the NF and RO membranes was obtained from the slope of a plot of flux vs. trans-membrane pressure using Eq. (1). A line correlation was obtained with high coefficients (R2) of 0.89 and 0.96 for the NF and RO membranes, respectively. The results showed that the NF membrane exhibited a higher permeability (7.3 L m2 h1 bar1) than the RO membrane (3.4 L m2 h1 bar1), and the NF membrane was therefore expected to have a higher flux than the RO membrane. A comparison of the membranes’ permeability values with those of other commercial membranes is detailed in Table 3. Membrane flux analysis with salt solutions The salt solutions flux of both the NF and RO membranes increased linearly with trans-membrane pressure. As seen in Figs. 5 and 6, the flux of the NF membrane was higher than that of the RO membrane, which was consistent with the permeability values because the greater permeation of the NF membrane was expected to yield a higher flux, and vice versa for the RO membrane. Figs. 5 and 6 also revealed that the fluxes of some salt solutions were higher than that of the pure water permeability in the NF and RO membranes, especially at higher pressures. This behavior was explained in terms of the salt transport through the membrane that resulted from the opening of a number of closed pores into active pores, which led to an increase in the effective number of pores. Such an increase was subject to variations in

Table 2 A comparative assessments of contact angle measurements for commercial NF and RO membranes with pure water using the sessile drop method. No

Membrane type

Manufacturer

Contact angle

Reference

NF membrane 1 2 3 4 5 6 7 8 9 10 11 12 13 14

NF1 MPF-44 UTC20 NF-270 DK DL NF-90 HL RE2521TL BW30XLE NTR7450 N30F NFPES010 MPF-50

AMFOR INC., China Koch, USA Toray, Tokyo, Japan Dow FilmTec Corp. Edina, MN, USA GE Osmonics, FL, USA GE Osmonics, FL, USA Dow FilmTec GE Osmonics, FL, USA Woongjin Chemical Co., Ltd., Korea Dow FilmTec Corp. Edina, MN, USA Nitto-Denko,Somicon AG, Basel, Switzerland Nadir, Wiesbaden, Germany Nadir, Wiesbaden, Germany Koch, USA

23 34.8 36 42.7 45.1 51 54.6 56.7 56.8 60.8 70 75.5 79.9 114.6

Present study [49] [50] [1] [51] [51] [1] [51] [52] [1] [50] [49] [49] [49]

RO membrane 1 2 3 4 5

SW30HR BW30RO RO membrane RE2521TL BW30XLE

Dow FilmTech, IMCD Limited, Australia Amfor Inc., China Hangzhou Beidouxing Mem. Co., Ltd., China Woongjin Chemical Co., Ltd., Korea Dow FilmTec Corp. Edina, MN, USA

32 37 53 56.8 60.8

[53] Present study [54] [52] [1]

[(Fig._2)TD$IG]

[(Fig._3)TD$IG]

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Fig. 2. FESEM images of the membrane surfaces of clean NF and RO membranes.

applied pressure compression, which in turn affected the porosity of the membrane. This phenomenon derives from the correlation of the pore structure and applied pressure factor [44]. Rejection of single salts In this study, the selected membranes were tested to identify their characteristics in terms of rejection in the presence of salts. To this end, twelve single salts were synthesized to simulate the content of TDS in discharged produced water, as presented in Table 1. These salt solutions included CaCl2, CaSO4, K2SO4, KCl, MgCl2, MgSO4, Na2CO3, Na2SO4, NaCl, NaHCO3, SrCl2 and BaCl2. Because the composition of produced water differs around the

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Fig. 3. FESEM images of the cross-sectional morphologies of clean NF and RO membranes.

world based on geological and operation differences [45], a variety of salts were applied to characterize the potential of NF and RO membranes in reusing discharged produced water. The concentrations of these salts’ feed solutions were maintained at approximately 2000 ppm, and the rejection results obtained as a function of the permeate flux in the NF and RO membranes are summarized in Tables 4–7. The results were interpreted in terms of the observed rejection or the lowest and highest rejection values obtained by applying trans-membrane pressures ranging from 2 to 20 bar. For the NF membrane, the order of rejection was identified as Na 2 SO 4 > MgSO 4 > CaSO 4 > K 2 SO 4 > Na 2 CO 3 > NaHCO 3 > SrCl2 > KCl > NaCl > MgCl2 > CaCl2 > BaCl2, as shown in Table 4. In comparison, the RO membrane demonstrated the order of

Table 3 A comparative assessment of permeability value for the commercial NF and RO membranes with pure water. No

Membrane name

Manufacturer

Permeability (L m2 h1 bar1)

Reference

NF membrane 1 2 3 4 5 6

NFPES010 (PES) NF270 NF1 (PES) NTR7450 (PES) NF90 N30F (PES)

Nadir, Wiesbaden, Germany Dow/FilmTec Corp. Edina, MN, USA Amfor Inc., China Nitto-Denko,Somicon AG, Basel, Switzerland Dow/FilmTec Corp. Edina, MN, USA Nadir, Wiesbaden, Germany

15.4 8.5 7.3 5.7 5.2 3.8

[50] [55] Present study [50] [55] [50]

RO membrane 7 8

BW30XLE BW30RO

Dow/FilmTec Corp. Edina, MN, USA Amfor Inc., China

6.1 3.4

[55] Present study

[(Fig._4)TD$IG]

[(Fig._5)TD$IG]

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NaCl

Flux J (L. m-2. h-1)

240

BaCl2

210

CaCl2

180

CaSO4

150

K2SO4 KCl

120

MgSO4

90

MgCl2

60

Na2CO3 Na2SO4

30

NaHCO3

0

SrCl2

0

2

4

6

8

10

12

14

16

18

20

22

Applied Pressure/bar Fig. 5. Salts solutions flux vs. applied pressure for NF membrane.

[(Fig._6)TD$IG]

rejection as Na2CO3 > MgSO4 > NaCl > KCl > BaCl2 > NaHCO3 > Na2SO4 > MgCl2 > SrCl2 > K2SO4 > CaCl2 > CaSO4, as shown in Table 6. Table 5 states that the NF membrane demonstrated the lowest rejection for BaCl2 and the highest rejection for Na2SO4. In comparison, the rejection sequence for the RO membrane was of a reversed order, as Table 7 demonstrates. This finding was attributed to the NF membrane’s characteristic low rejection of monovalent ions, high rejection of divalent ions and higher flux than the RO membrane, which demonstrated high rejection of those ions at low flux. These contrasting characteristics of the two membranes can be explained by the Donnan exclusion mechanism, which results from the charge interactions between ions and a membrane’s surface, wherein the fixed-charge ions of the membrane, in principle, repulse the co-ions in solution [38]. This behavior is particularly evident in Fig. 7(a–d), which illustrates the rejection of salts at 10 levels of applied pressure. The results shown in Fig. 7 collectively indicated that the NF membrane’s rejection performance was better than that of the RO for all contextual ionic species, except those that defined salinity, wherein the RO membrane was better than the NF for desalination. Illustratively, Fig. 7(a) shows that the RO membrane demonstrated higher rejection than the NF membrane for NaCl, KCl, and BaCl2. Next, Fig. 7(b) highlights the NF membrane’s steadier Table 4 Highest values of rejection by the NF membrane for 12 single salts. Salt

Max rejection (%)

Pressure

Salt

Max rejection (%)

Pressure

Na2SO4 MgSO4 CaSO4 K2SO4 Na2CO3 NaHCO3

97.01 95 94.86 94.2 87.1 72.26

4 6 12 4 2 4

SrCl2 KCl NaCl MgCl2 CaCl2 BaCl2

53.3 47.03 44.57 44.26 39.83 31.41

12 2 2 8 16 10

Table 5 Lowest values of rejection by the NF membrane for 12 single salts.

Flux J (L. m-2. h-1)

Fig. 4. FTIR spectra of clean NF (red line) and RO (blue line) membranes.

NaCl

100 90 80 70 60 50 40 30 20 10 0

CaCl2 CaSO4 K2SO4 KCl MgSO4 MgCl2 Na2SO4 NaHCO3 SrCl2

0

2

4

6

8

10

12

14

16

18

20

22

BaCl2

Applied Pressure/bar Fig. 6. Salts solutions flux vs. applied pressure for RO membrane.

performance at rejecting CaCl2, MgCl2 and SrCl2, especially at high levels of pressure, whereas the RO membrane exhibited high rejection of CaCl2 and MgCl2 at only low levels of pressure. The NF membrane’s relative advantage over its RO counterpart is further apparent in Fig. 7(c), wherein the NF membrane shows a much higher rejection of sulfate salt compounds than the RO membrane. Incidentally, this particular finding also explains why the oil industry now largely uses NF membranes to treat produced water in the production phase; desulfation prior to injection is one of the critical objectives and prevents the occurrence of multiple operational problems. The result also explains why there are more than 50 large-scale applications of NF membranes in onshore and offshore oil production platforms around the world today [46]. Finally, as shown in Fig. 7(d), despite the RO membrane’s low rejection of some divalent salt ions, it was characterized by a higher level of rejection of critical salt ions such as sodium and carbonate ions, which represent the major constitution of salinity and scale formation, especially at high levels of applied pressure. Overall, the results of the membrane performance in terms of rejection indicated that the percent rejection of all salts ranged from 96.6% to 37% in the RO membrane and from 97% to 36% in the NF membrane. Although these overall percentages did not seem disparate, a closer look at the rejection results for individual salts did reveal unique merits of the Table 6 Highest values of rejection by the RO membrane for 12 single salts.

Salt

Min rejection %

Pressure

Salt

Min rejection %

Pressure

Salt

Max rejection%

Pressure

Salt

Max% rejection

Pressure

BaCl2 NaCl SrCl2 CaCl2 MgCl2 KCl

25.36 32.64 33.6 34.4 36.72 37.09

2 20 2 6 2 20

NaHCO3 Na2CO3 CaSO4 K2SO4 MgSO4 Na2SO4

50.91 68.79 79.59 85.15 88.77 90.14

20 2 20 20 20 20

Na2CO3 MgSO4 NaCl KCl BaCl2 NaHCO3

96.01 95 88.33 86.13 85.42 83.02

18 6 16 2 6 2

Na2SO4 MgCl2 SrCl2 K2SO4 CaCl2 CaSO4

81.63 81.58 81.57 80.16 78.92 37.08

2 2 8 2 2 20

[(Fig._7)TD$IG]

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B

90 80 70 60 50 40 30 20 10 0

Rejection %

Rejection %

A 100

0

2

4

6

8 10 12 14 16 Applied Pressure / bars

NaCl-NF KCl-RO

NaCl-RO BaCl-NF

18

20

90 80 70 60 50 40 30 20 10 0

22

0

2

4

KCl-NF BaCl-RO

6

8 10 12 14 16 Applied Pressure / bars

CaCl-NF MgCl-RO

CaCl-RO SrCl-NF

18

20

22

MgCl-NF SrCl-RO

D

C 120 110 100 90 80 70 60 50 40 30 20 10 0

Rejection %

Rejection %

1347

0

2

4

6

8 10 12 14 16 Applied Pressure / bars

18

20

CaSO-NF

CaSO-RO

NaSO-NF

NaSO-RO

KSO-NF

KSO-RO

MgSO-NF

MgSO-RO

100 90 80 70 60 50 40 30 20 10 0

22

0

2

4

6

8 10 12 14 16 Applied Pressure / bars

18

NaCO-NF

NaCO-RO

NaHCO-NF

NaHCO-RO

20

22

Fig. 7. (a–d) Rejection of salt solutions by NF and RO membrane vs. applied pressure.

two membranes’ rejection performance. The RO membrane showed a higher rejection of most of the salts, which was best explained by the membrane’s smaller porosity size compared to the NF membrane. It was also observed that the RO membrane’s salt rejection was independent of the applied pressure, while the NF membrane’s salt rejection increased gradually with increasing pressure, as shown in Fig. 7. Although it could be suggested that the variations in the rejection of charged and uncharged salt solutions may actually have arisen from differences in ionic transport mechanisms [47], membrane porosity appeared to be a far more predominant factor affecting the salt rejection process in this experiment than transport mechanisms such as diffusion and convection [44]. This assertion was particularly supported by this study’s observations on the flux of certain salts. Removal efficiency of the organic and inorganic constituents of produced water samples The potential of hydrophilic NF and RO membranes in the reuse of disposed produced water as a source of indirect potable water was tested by conducting a primary assessment of their removal efficiency for conductivity, turbidity, oil and grease, TDS, COD, TSS and TOC at the low applied pressure level of 6 bar. These parameters were selected to represent the major organic and inorganic components of such water because the current available guidelines for reusing wastewater as indirect potable water [16] suggest meeting the following requirements: to satisfy drinking water standards [16], the pH value should be in the range of

6.5–8.5, the turbidity should not exceed 2 NTU, no detectable total coli/100 mL should be observed, chlorine residue is limited to a minimum of 1 mg/L, the total organic carbon should not exceed 3 mg/L, and the total organic halogen (TOX) should not exceed 0.2 mg/L. Therefore, a mini assessment of the NF and RO membranes’ potential for removal efficiency used the major parameters listed in Table 8 to test their feasibility as tertiary treatment for the reuse of discharged produced water. The data set on removal efficiency was compared with the international standards of drinking water and wastewater reuse [16]. Table 8 provides data on the produced water quality assessment, pre- and post-filtration, with the use of hydrophilic NF and RO membranes. It should be noted that the quality of the discharged produced water before tertiary treatment using NF and RO membranes was highly acceptable for reuse, and could easily have met potable water standards upon filtration and been either injected in a groundwater aquifer, mixed with other surface water resources or Table 7 Lowest values of rejection by the RO membrane for 12 single salts. Salt

Min rejection %

Pressure

Salt

Min rejection %

Pressure

CaSO4 MgCl2 CaCl2 KCl Na2SO4 K2SO4

27.01 25.49 38.68 57.19 67.88 71.02

4 20 16 20 20 14

BaCl2 SrCl2 NaHCO3 NaCl MgSO4 Na2CO3

73.16 74.63 77.67 82.02 88.76 90.52

20 20 8 2 20 4

S. Alzahrani et al. / Journal of Environmental Chemical Engineering 1 (2013) 1341–1349

1348 Table 8 Assessment of produced water quality. No.

Test parameter

Units

Before filtration

1 2 3 4 5 6 7

Turbidity Total dissolved solids Oil and grease Conductivity Total suspended solids Chemical oxygen demand Total organic carbon

NTU mg/L mg/L mS/cm mg/L mg/L mg/L

21 854 2 1670 10 96 26.3

stored for future use. The results detailed the ways in which the NF and RO membranes differed greatly in the quality of water they produced, in terms of physico-chemical parameters. As seen from the results listed in Table 8, filtration using the NF membrane reduced the TDS in the produced water from 854 mg/L to 520 mg/ L. The RO membrane yielded an even greater TDS reduction, to below 244 mg/L, which implied a 71% total reduction in TDS. The RO membrane achieved a final TDS level below the maximum allowable limit of 500 mg/L set by the US Environmental Protection Agency (USEPA) [48]. For conductivity, which represents the levels of dissolved salts in a water sample, filtration with NF and RO membranes yielded final conductivity values of 945 mS/cm and 444 mS/cm, respectively, down from the original maximum of 1670 mS/cm in the produced water sample. The COD level of 96 mg/L in the raw produced water pre-filtration indicated the presence of organic particulates. Filtration with the NF membrane reduced the COD to 60 mg/L, while the RO membrane yielded an even lower post-filtration COD of 30 mg/L. The rather low levels of oil and grease in the pre-filtration samples of produced water were reduced to non-detectable levels, implying their total removal by both the NF and RO membranes. The pre-filtration sample’s turbidity value of 21 NTU was also effectively reduced to non-detectable levels by both the membranes, well below the maximum level of 1 NTU set by the USEPA for drinking water standards [48]. The TSS concentration prefiltration process was identified as 10 mg/L, which was reduced to non-detectable levels of less than 1 mg/L upon filtration through the NF and RO membranes. It should be noted here that the presence of suspended solids in produced water typically provides adsorption sites for foulants. Hence, produced water samples were prepared pre-filtration with 0.45 mm to prevent the clogging of pores and subsequent sluggish flux in the NF and RO membranes. After filtration, the deposition of particulate matter was observed on the surface of every membrane, suggesting that the TSS in the produced water posed the risk of potential membrane fouling, as implied by the observed flux decline incurred by the presence of small particles in the sample. The TOC was reduced from 26.3 mg/L before filtration to 22.9 mg/L by the NF membrane and 6.7 mg/L by the RO membrane, with the latter showing a higher efficiency at removing small organic particulates to the permissible levels set by the USEPA. Conclusion Two new commercial NF and RO membranes were characterized and tested to examine their potential as a tertiary treatment for the beneficial reuse of discharged produced water from the petroleum industry. The results of the characterization found that both membranes were hydrophilic. Their surface functionality, as identified from ATR-FTIR measurements, indicated that the structure of both membranes consisted of PES and a selective layer with carboxyl salts, whereas a morphological characterization by FESEM showed that the membranes consisted of thin-film

After filtration NF

RO

<1 520 <1 945 <1 60 22.9

<1 244 <1 444 <1 30 6.7

composite structures in selective and support layers. Membrane permeation tests demonstrated that the percent rejection of all the salts under study ranged from 36% to 97% for both the NF and RO membranes, respectively. The primary assessment of these membranes for their potential use in produced water treatment showed that the critical components of the discharged produced water were mainly TDS and TOC, identified at 854 and 26.3 mg/L, respectively. The RO membrane was more efficient at rejecting these components, bringing the TDS down to 244 mg/L and the TOC to 6.7 mg/L at a low pressure of 6 bar, whereas the NF membrane yielded 520 mg/L of TDS and 22.9 mg/L of TOC at a similar low pressure. Both membranes demonstrated a high reduction of initial oil concentrations (2 mg/L), turbidity (21 NTU) and total suspended solids (TSS), decreased from 10 mg/L to less than 1 mg/L. The study concluded that the selected hydrophilic membranes had the potential for use in tertiary produced water treatment and that the RO membrane was far more effective than the NF membrane in rejecting salts and organics present in produced water. However, the effectiveness of the NF membrane in treating organic content was essentially limited by the applied pressure of 6 bar. It was anticipated that the NF membrane’s performance could be improved by increasing the applied pressure to higher values. The findings on RO-treated water quality substantiated the possibility of utilizing produced water as a source of potable water in the future. Acknowledgments The authors of this work wish to gratefully acknowledge the financial support for this work by the UKM Research Grant (DIP) through the project no. DIP-2012-01. References [1] S. Mondal, S.R. Wickramasinghe, Produced water treatment by nanofiltration and reverse osmosis membranes, J. Membr. Sci. 322 (2008) 162–170. [2] P. Xu, J.E. Drewes, Viability of nanofiltration and ultra-low pressure reverse osmosis membranes for multi-beneficial use of methane produced water, Sep. Purif. Technol. 52 (2006) 67–76. [3] M. C¸akmakce, N. Kayaalp, I. Koyuncu, Desalination of produced water from oil production fields by membrane processes, Desalination 222 (2008) 176–186. [4] N. Ghaffour, M.W. Naceur, N. Drouiche, H. Mahmoudi, Use of ultrafiltration membranes in the treatment of refinery wastewaters, Desalin. Water Treat. 5 (2009) 159–166. [5] D. Wang, F. Tong, P. Aerts, Application of the combined ultrafiltration and reverse osmosis for refinery wastewater reuse in Sinopec Yanshan plant, Desalin. Water Treat. 25 (2011) 133–142. [6] S.H.D. Silalahi, T. Leiknes, High frequency back-pulsing for fouling development control in ceramic microfiltration for treatment of produced water, Desalin. Water Treat. 28 (2011) 137–152. [7] F. Tao, C. Stanley, R. Hobbs, J. Sides, J. Wieser, C. Dyke, T. David, P. Pliger, Conversion of oilfield produced water into an irrigation/drinking quality water, in: SPE/EPA Exploration and Production Environmental Conference, San Antonio, USA, 3–7 October, 1993. [8] M. Melo, H. Schluter, J. Ferreira, R. Magda, A. Junior, O. de Aquino, Advanced performance evaluation of a reverse osmosis treatment for oilfield produced water aiming reuse, Desalination 250 (2010) 1016–1018. [9] P. Xu, J.E. Drewes, D. Heil, Beneficial use of co-produced water through membrane treatment: technical-economic assessment, Desalination 225 (2008) 139–155.

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