Boron removal in new generation reverse osmosis (RO) membranes using two-pass RO without pH adjustment

Desalination 310 (2013) 50–59

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Boron removal in new generation reverse osmosis (RO) membranes using two-pass RO without pH adjustment Ali Farhat a, Farrukh Ahmad a, Nidal Hilal a, b, Hassan A. Arafat a,⁎ a b

Water and Environmental Engineering Program, Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emirates Centre for Water Advanced technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, UK

H I G H L I G H T S ► ► ► ► ►

A study on boron removal in 2-pass RO configuration without pH adjustment New generation high boron-rejection membranes from leading manufacturers were tested. Tested membranes under similar conditions using the same feed water High boron rejections, up to 96%, were achieved using 2-pass configuration, no pH adjustment. Effects of feed salinity, flow rate, pressure, and temperature on boron removal were also tested.

a r t i c l e

i n f o

Article history: Received 3 August 2012 Received in revised form 4 October 2012 Accepted 7 October 2012 Available online 29 October 2012 Keywords: Boron rejection Reverse osmosis Desalination Membranes Two-pass systems

a b s t r a c t Boron removal using new generation RO membranes from several leading manufacturers under a secondpass configuration and without pH adjustment was studied. The study was conducted using seawater from the Arabian Gulf (higher salinity and temperatures than average seawater). Membranes from several manufacturers were tested under similar operational conditions and the same feed water source. It was found that significant boron rejections, as high as 96%, were successfully achieved, using readily-available commercial RO membranes under a two-pass configuration and without any pH adjustment. Moreover, single-pass configurations exhibited high salt and boron rejection results reaching 99% and 91%, respectively. First pass permeates had boron levels below 1.4 ppm, which are adequate to comply with the new WHO guidelines (2.4 ppm) and those of other countries such as Australia, Canada, and UAE, whose boron guideline thresholds are above 1.4 ppm. The paper also assesses the influence of several operational parameters such as feed water salinity, flow velocity, temperature and feed pressure in second pass on boron removal in this process. It was found that higher boron removals were obtained with higher feed velocity, higher secondpass pressures, and lower feed temperatures. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Boron is a nonmetallic element that exists in the form of boric acid and borates in surface and groundwater. Boron seawater concentration ranges from 0.5 to 9.6 ppm [1] averaging at a value of 4.5 ppm [2] to 4.6 ppm [3]. In the Arabian Gulf, boron levels have been reported to be as high as 7 ppm [4]. While boric acid and borates are two pharmaceutical necessities [5,6], and considered to be medicinally important compounds as antibacterial and antifungal agents [7–10], high boron concentrations have been observed to constitute a threat to crops and several animal species [11]. The US Environmental Protection Agency (USEPA) and the World Health Organization (WHO) have established regulatory guidelines

⁎ Corresponding author. E-mail address: [email protected] (H.A. Arafat). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.003

for boron in drinking water, where the USEPA's maximum boron concentration is 0.6 ppm [12], and the WHO's is 0.5 ppm set in 1998 [13], which was revised from its 1993 value of 0.3 ppm [14]. Since the WHO guideline value of 0.5 ppm is a guideline value and not mandatory, different countries have set their own guideline levels. For example, the limit was set at 1.0 ppm in the EU and Singapore [15], 1.5 ppm in Abu Dhabi, UAE [16], 4.0 ppm in Australia [17], and 5.0 ppm in Canada [18]. In the 4th edition of the Guidelines for Drinking Water Quality published by WHO in 2011, the boron guideline value was revised to 2.4 ppm from 0.5 ppm [19], due to the lack of toxicty data on humans. This was encouraging news to water desalination utilities which had to incur higher capital and operational costs for boron removal. However, the new 2.4 ppm-B guideline may be considered high for several types of crops such as potato, carrot, cucumber, wheat, sunflower, onion, garlic, cherry, and berry, which are sensitive to boron levels above 2.0 ppm [20], resulting in premature ripening,

A. Farhat et al. / Desalination 310 (2013) 50–59

leaf damage, lower yields, and spots on the fruits [21]. Therefore, some countries and water treatment utilities still maintain their maximum boron concentrations at 0.5 ppm for agricultural purposes. In order to reach the 0.5 ppm level, boron needs to be rejected at levels of 90% or above from feed waters with 5 ppm-B. Attaining the 0.5 ppm boron level in seawater RO plant product water is not easily achievable [22,23]. Until recently, RO membranes failed to efficiently prevent boron from passing through, particularly when boron was present in its uncharged boric acid form (pKa = 9.2 at 25 °C [24]). The increase in pH above the boric acid pKa converts boric acid into the negatively charged borate anions, enhancing boron rejection across RO membranes. Several major adjustments in RO plant configurations have been investigated. The most common one in practice today is a second-pass RO stage added after the first-pass, where the pH of first-pass permeate is adjusted (increased) before being fed to the second pass [25,26]. This configuration consumes energy and chemicals, augmenting capital and operational costs. In an early investigation of boron removal in RO plants (in 1990s), Magara et al. [22,26] investigated the performance of single-stage SWRO in a pilot-scale experiment and observed boron rejection of 43–78%, concluding that a two-stage SWRO is a minimum requirement for producing potable water complying with the EU drinking water guidelines of 1 ppm. Later in 2000, Prats et al. [27] tested the Hydranautics 4040-LHACPA2, Toray SU-710, and Toray SUL-G10 membranes, obtaining boron rejections of 40%, 58%, and 47%, respectively. In 2001, Pastor et al. [28] and Taniguchi et al. [29] reported higher rejections of 60% and 75–90%, respectively, using Toray membranes. In 2003, Redondo et al. [25] observed boron rejections of 88– 91% using DOW's SW30HR-380, SW30HR-320, and SW30-380 membranes, producing permeates with boron levels of 0.79–0.86 ppm. Later generations of RO membranes with High Rejection (HR) and Low Energy (LE) were introduced by several manufacturers to

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enhance the RO membranes performance and efficiency for both salt and boron rejection, and to lower the process' energy and cost. For instance, several models of Toray TM-820 membranes and DOW-SW-30 membranes achieved boron rejections of 87–93% [30]. Table 1 shows a timeline of literature reporting on boron rejection in RO membranes, along with types of membranes used, feed water characteristics, as well as other relevant operational conditions. It can be noticed from Table 1 that newer RO membranes became more capable of higher boron rejection. In recent years, most membrane manufacturers have introduced RO membranes that achieve considerable salt and boron rejection. The latter can potentially be achieved in a two-pass system without pH adjustment. While this two-pass configuration without pH adjustment has already been implemented by the more recent RO plants (e.g., Fujairah F2 plant, UAE, using Toray membranes), to the best of the authors' knowledge, no thorough independent studies have been reported to assess the performance of RO membranes under such a setup. Hence, there is a need for such independent assessment, using a variety of new membranes from different manufacturers but under similar test conditions. The research presented in this paper studies whether boron concentration can be sufficiently reduced using the new generation of commercial high-boron-rejection RO membranes from most leading manufacturers under a two-pass configuration without pH adjustment. The study was conducted using real seawater from the Arabian Gulf. The study also assesses the influence of several operational parameters (salinity, feed flow velocity, feed pressure in second pass, and temperature) on boron removal. It is important to mention that past reported studies (many presented in Table 1) were conducted by different groups using different operational parameters and different feed waters. Thus, the research presented in this paper has the advantage of testing RO membranes from several manufacturers under similar operational conditions and while using the same feed water.

Table 1 Timeline of boron rejection results in RO membranes by several researchers since 1996. Year

Membrane type

pH

Temp. (°C)

Experimental conditions

% B-rejection

Study medium

Ref.

1996

Polyaromaticamide (Model not specified) ES10-D4 Hydranautics 4040-LHACPA2 Toray SU-710 Toray SULG10 Toray SUL-C10 Toray UTC-80 SW30HR-380 SW30HR-320 SW30-380 BW30-400 BW30LE DOW-SW-30-HR

5–7.8

20–35 °C

0.4–0.65 bar, 0.07–4.3 ppm Boron

43–78%

RO Plant

[22]

8–9 6.5–8.5

– –

8 bar, 1 ppm Boron BW

RO Plant RO pilot plant (7.2 m3/d)

[26] [27]

9.5 – 8

– 25–30 °C 25 °C

BW with 4 ppm Boron 4 ppm Boron 32,000 ppm NaCl, 800 psi

Pilot Plant Lab-scale Field data

[28] [29] [25]

8.2 10.5 8.2 10.5 5.5

23 °C

2000 ppm NaCl, 225 psi 2000 ppm NaCl, 150 psi 700 psi, 5.1 ppm Boron

Lab-scale test unit

[33]

Pilot plant

[37]

8

25 °C

32,000 ppm NaCl, 800 psi, 5 ppm Boron

Lab scale test unit

[16]

8

25 °C

32,000 ppm NaCl, 800 psi, 5 ppm Boron

Reporting manufacturing companies data

[30]

8–8.2

16 °C

39,000 ppm NaCl, 800 psi

60–70% 40% 58% 47% 60% 75–90% 90% 90% 88% 65% 55% 88% 99% 89% 99% 55% 75% 80% 91% 91% 90% 90% 93% 93% 91% 93% 91% 88% 87% 85% 88%

Small scale RO plant

[32]

1998 2000

2001 2001 2003

2008

UTC-80AB 2008

2010

2011

2011

DOW-SW-30 DOW-BW-30 GE-AG SU-820 TM-820-370 UTC-80 DOW-SW-30-HR-380 TM-820A-400 TM-820C-400 TM-820E-400 DOW-SW-30-XHR-400i DOW-SW-30-HRLE-400i DOW-SW-30-XLE-400i DOW-SW-30-ULE-400i DOW-SW-30-2540 DOW-SW-30-XHR-2540

290 psi, 5.1 ppm Boron

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A. Farhat et al. / Desalination 310 (2013) 50–59

2. Materials and methods 2.1. RO system and test procedure A lab-scale reverse osmosis system, custom-built by Sterlitech Co. (Kent, WA, USA) to test flatsheet membranes, was installed in the labs of Masdar Institute of Science and Technology (Abu Dhabi, United Arab Emirates). The system consists of a feed tank fitted with a high pressure pump to pump the water into a flat-sheet membrane cell (membrane area is 8.5 cm × 4 cm, channel flow depth is 2.75 mm). No spacers were used. Fig. 1 shows a schematic diagram of the lab-scale RO system with its parts and connections. Seawater was collected from the Arabian Gulf, off the shores of Abu Dhabi, and was used as feed water in all the first pass tests. The feed water came from one homogeneous batch of seawater which was collected at once from a stagnant bay area and was given enough time in the lab to settle, after which it was kept mixed for homogeneity and consistency. Visually, the settled sample appeared very clear and no additional pre-treatment was performed. The salinity of this feed seawater, used as the first pass feed, was in the range of 44,300– 47,400 ppm. The feed water was pumped while passing along a feed bypass valve (for feed flow rate control) and a pressure relief valve (for safety purposes). The applied pressure was controlled and monitored using a brine valve/pressure regulator and a pressure gage, respectively. The pressure applied was consistently fixed at 800 psi (~ 54 bar) in all first pass runs and either at 150 (~10 bar) or 220 psi (~ 15 bar) in the second pass runs. The brine generated was recycled into the feed tank, allowing feed salinity buildup over time to simulate the variation in feed salinity between the first and last RO element in a plant stage. At the end of the test, concentration buildup in the feed tank was up to about 75%, corresponding to a recovery ratio of about 40–45%. After every run, the system was thoroughly flushed with tap water followed by Milli-Q water (Millipore Corporation, Billerica, MA, USA) for several minutes and was completely drained afterwards. Two tests were conducted to simulate the two-pass system. In the first test, a seawater RO (SWRO) membrane was used. The permeate was continuously collected and sampled (every 1 h) over approximately 12 h in non-glass containers to avoid the chelation of boron from the borosilicate-glass containers into the permeate. Permeate

collected from the first test was then used (without pH, or any other, alteration) as feed in the second test, simulating the second RO pass. A brackish water RO (BWRO) membrane, from the same manufacturer of the first pass membrane, was used in the second pass test. As the high-pressure pump used in the system generated significant heat, which led to a buildup in feed temperature, the feed tank was fitted with a cooling coil circulating chiller water to maintain the feed water temperature constant. In few tests, the cooling rate was adjusted to allow the feed temperature to increase in order to study the effect of the latter on boron rejection. Water samples of around 20 mL were collected simultaneously every hour from the feed and permeate. The temperature, pH, and conductivity of each of these samples, in addition to permeate flow rate (from which flux was calculated) were measured. Also, the boron concentrations in these samples were measured using a DR2800 Spectrophotometer (HACH Co., Loveland, Colorado, USA). The boron reagent for this measurement was prepared by dissolving one BoroVer®3 Boron Reagent Powder Pillows (HACH Company) in 75 mL of concentrated sulfuric acid. In a non-glass vial, 2.0 mL of the water sample was pipetted to 35 mL of the reagent solution and mixed well and left to react for at least 25 min before being measured using a spectrophotometer. A blank was prepared and measured similarly but using 2.0 mL of deionized water instead. 2.2. Membranes Ten membranes (5 SWRO membranes and 5 BWRO) from four leading manufacturing companies were studied in this work. The studied SWRO membranes were Toray's UTC-SW, GE's GE-AD, KOCH's KOCH-TFC-8040-SW, DOW's DOW-SW-30HR, and DOW-SW-30-XLE. The studied BWRO membranes were UTC-BW, GE-AK, KOCH-TFC8040-XR, DOW-BW-30, and DOW-BW-30-LE. The Toray UTC-SW and UTC-BW were supplied courtesy of the Toray Company, the KOCHTFC-8040-SW and KOCH-TFC-8040-XR were supplied courtesy of the KOCH Company, while the remaining membranes were purchased from Sterlitech Co. All of these membranes, except the DOW-BW-30, were suggested by the suppliers as new-generation membranes with higher boron rejection than the older versions. Membranes from another leading manufacturer, Hydranautics, were planned to be tested,

Fig. 1. Schematic diagram of the lab-scale SWRO test setup used.

A. Farhat et al. / Desalination 310 (2013) 50–59

however, the authors could not obtain any new generation Hydranautics flat sheet membrane samples in time for this study. The membranes were pre-conditioned by soaking in DI water for 2 days, followed by thorough rinsing using DI water prior to testing. 3. Results and discussions 3.1. Boron rejection performance Table 2 summarizes the average boron concentrations of the different tested membrane combinations, as well as all operational parameters and feed characteristics such as pH, temperature, TDS, and boron concentrations of the feed water for both the 1st and 2nd passes. Also shown are the overall (combined) two-pass boron rejection results. The initial feed volume was 3.0 L in all the first pass (SWRO) tests, except for the DOW-SW30HR which started with 7.0 L; whereas the initial feed volume for the second pass (BWRO) tests was 1.0 L, except for the DOW-BW30 whose initial volume was 1.37 L. The feed temperature was controlled between 28 °C and 33 °C in the 1st pass and between 22 °C and 25 °C for the 2nd pass. Unlike the temperature, feed pH was not controlled and was found to be 7.5–8.4 in the 1st pass, which was similar to the condition of the actual seawater used. However, it was observed that the feed pH in the 2nd pass (6.6–8.1) was slightly lower than that of the 1st pass. In RO desalination, it is known that the pH of the permeate (the desalinated water) is usually lower than the feed. This is because carbon dioxide passes through the membranes and bicarbonate is rejected. Since the pH is governed by the logarithm of the ratio of bicarbonate to carbon dioxide in these systems, the pH of the permeate water is lower than the feed [31]. The average boron rejections in the SWRO membranes (1st pass) ranged from 80% to 91% with an average value of 85%, which was higher than the average boron rejections of 55 to 87% (averaging at around 70%) in the 2nd pass, where BWRO membranes were used. In the 1st pass, boron levels were reduced below 1.4 ppm with the highest rejection by Toray UTC-SW, producing a 1st-pass-permeate with 0.63 ppm-B (91% rejection), followed by DOW-SW30HR with 0.89 ppm permeate (87% rejection). The membranes of GE-AD, KOCH-SW, and DOW-SW30XLE produced permeates that had boron concentrations of 1.21, 1.37, and 1.09 ppm, respectively. Furthermore, as expected, DOW's membrane designed for higher rejections (DOW-SW30HR) yielded higher rejection than DOW's low energy membrane (DOW-SW30LE), where the DOWSW30HR reduced boron to 0.89 ppm-B compared to 1.22 ppm-B by DOW-SW30LE's (85% rejection). The 2nd pass, accomplished without pH adjustment, rejected boron to levels of 0.60 ppm or lower in all membrane combinations, a value that complies with almost all health regulatory guidelines for boron. Moreover, boron levels as low as 0.27 ppm in the 2nd-passpermeates, as it was in the case of DOW-BW30 (87% rejection) and Toray UTC-BW (73% rejection), could be achieved. It is noted that the difference in the percentages of rejections in these two membranes,

53

despite the similar permeate boron concentrations, was due to the different TDS (ppm) content of the feed water to the 2nd pass, i.e., different permeates of the 1st passes of their corresponding SWRO membranes. Similarly, as was observed in the 1st pass tests, the membranes designed for higher rejections revealed better boron rejection. When comparing DOW's DOW-BW30 and DOW-BW30LE, higher boron rejection was obtained by the former (0.27 ppm-B, 87% rejection) than by the latter (0.60 ppm-B, 55% rejection). The average overall performance of the combined two-passes yielded boron rejection percentages as high as 96%, produced by the combination of DOW-SW30HR/DOW-BW30 membranes, as well as Toray's UTC-SW/UTC-BW membranes. To the authors' knowledge, boron rejection of 96% presented in this paper is one of the highest boron rejections reported in two-pass RO systems using Arabian Gulf seawater, notorious for its higher-than-average salinity (41,700– 47,400 ppm). This important outcome could potentially lead to reducing the utilization of large amounts of chemicals for increasing pH, thus reducing environmental impacts and operational costs. It is also noteworthy that all of the new generation membranes tested here showed considerable capabilities in significantly removing boron to levels below 1.4 ppm in the 1st pass alone. Therefore, one pass might possibly be adequate to comply with the new boron guidelines of WHO (2.4 ppm) in countries whose boron limit is 1.5 ppm-B or higher (e.g., Australia, Canada, and UAE), especially if the produced water is not to be used for agricultural purposes. Table 3 summarizes the average salt rejection (%) and average fluxes (L.m−2.h−1) of all tested SWRO membranes in the 1st pass (at 800 psi) and BWRO membranes in the 2nd pass (at 150 psi). The average fluxes were calculated from ranges of values for each of the membranes. The ranges are 21–27, 36–54, 10–21, 52–70, 10–21, 22–34, 16–24, 56– 65, 21–27, and 36–54 L.m−2.h−1 for DOW-SW-30HR, DOW-BW-30, GE-AD, GE-AK, KOCH-TFC-8040-SW, KOCH-TFC-8040-XR, DOW-SW30XLE, DOW-BW-30-LE, UTC-SW, and UTC-BW, respectively. The results of the tested membranes show substantial salt rejections in the 1st pass, where rejection was as high as 99% for DOW-SW-30HR and 98.7% for both KOCH-SW and GE-AD, producing a permeate with less than 600 ppm TDS. The permeates of the 2nd pass registered TDS readings lower than 200 ppm, corresponding to a salt rejection of 90 to 97% (typical of BWRO membranes), and an overall two-pass salt rejection exceeding 99.6%, which is within the range reported in the literature. It is important to mention here that, two of the manufacturers of the membranes utilized in this study, Koch and Toray, provided us with projections (generated using their own in-house simulation software) of the expected flux, salt rejection, and boron rejection values expected of their membranes under our average test conditions. These projections are shown in Table 4. While our experimental values (Tables 2 and 3) were, in most cases, slightly lower than the manufacturer projections, they were very reasonably close. When relating boron rejections with salt rejections for the studied membranes, it was noticed that in the SWRO membranes, boron rejections increase as salt rejections decrease, with the exception of

Table 2 Average boron rejection of the first pass SWRO membranes (at feed pressure of 800 psi and cross flow velocity of 0.45 m/s), the second pass BWRO membranes (at 150 psi and 0.45 m/s), and the overall two-pass boron rejection. 1st pass membrane

2nd pass membrane

1st pass (at 800 psi and 0.45 m/s) Feed pH

Feed temp Feed TDS (°C) (ppm)a

Feed boron (ppm)b

Average boron rejection in permeate

Feed pH

Feed temp (°C)

DOW-SW30HR GE-AD KOCH-SW DOW-SW30XLE Toray UTC-SW

DOW-BW30 GE-AK KOCH-XR DOW-BW30LE Toray UTC-BW

7.9–8.1 7.8–8.2 8.1–8.4 8.0–8.2 7.5–8.0

27.8–29.6 28.2–29.7 27.8–28.7 28.7–29.5 29.8–33.3

6.2–7.3 4.5–7.8 6.5–8.4 5.2–8.6 5.5–7.8

87% 80% 82% 85% 91%

7.5–8.1 7.6–7.8 7.2–7.4 6.6–7.0 7.3–7.9

23.5–23.8 828–1773 1.5–2.6 22.5–24.9 1293–1644 1.0–1.6 22.4–23.4 917–1409 1.2–1.6 23.5–24.6 1801–2090 1.1–1.5 23.0–23.9 1587–2005 0.8–1.1

a b

Feed TDS increased with brine recycling. Boron concentration increased with brine recycling.

47,400–55,400 44,600–68,200 44,300–76,800 44,300–60,000 41,700–57,600

2nd pass (at 150 psi and 0.45 m/s)

(0.89 (1.21 (1.37 (1.09 (0.63

ppm) ppm) ppm) ppm) ppm)

Feed TDS (ppm)a

Feed boron (ppm)b

Average boron rejection in permeate 87% 58% 75% 55% 73%

(0.27 (0.53 (0.35 (0.60 (0.27

ppm) ppm) ppm) ppm) ppm)

Average overall two-pass boron rejection 96.0% 91.4% 95.5% 92.0% 96.0%

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A. Farhat et al. / Desalination 310 (2013) 50–59

Table 3 Average salt rejection and flux of the first pass SWRO membranes (at feed pressure of 800 psi and cross flow velocity of 0.45 m/s) and second pass BWRO membranes (at 150 psi and 0.45 m/s). 1st pass membrane

Average flux (L.m−2.h−1)

Average salt rejection

2nd pass membrane

Average flux (L.m−2.h−1)

Average salt rejection

Overall salt rejection

DOW-SW30HR GE-AD KOCH-SW DOW-SW30XLE Toray-SW

25 18 19 22 25

99.0% 98.7% 98.7% 97.2% 96.8%

DOW-BW30 GE-AK KOCH-XR DOW-BW30LE Toray-BW

47 59 27 60 47

97% 90% 98% 91% 97%

99.9% 99.7% 99.95% 99.6% 99.9%

the DOW-SW30HR. For instance, GE-AD and KOCH-SW achieved 80– 82% B-rejection and 98.7% salt rejections, compared to around 85– 91% B-rejection and 97% salt rejection by DOW-SW30XLE and TORAY-SW. On the other hand, in BWRO membranes, the opposite was observed; boron rejections increased as salt rejections increased. Here, the salt rejections of GE-AK and DOW-BW30LE (90–91%) were relatively lower than those of KOCH-XR, TORAY-SW, and DOW-BW30 (97–98%). The former two membranes (GE-AK and DOW-BW30LE) registered lower boron rejections of 55–58% compared to the latter two (KOCH-XR and TORAY-SW) (73–75%), or the DOW-BW30 (87%). Finally, the average fluxes of the 1st pass membranes (18–25 L.m −2.h−1) were found to be lower than those of the 2nd pass ones, at 27–60 L.m −2.h−1. When correlating the observed fluxes with the salt rejection of SWRO membranes (with the exception of DOW-SW30HR), it can be easily recognized that as the average flux increased (18 to 25 L.m−2.h−1) the salt rejection decreased (98.7% to 96.8%), a phenomenon common for RO membranes. The 2nd pass membranes exhibit the same behavior, where the salt rejection decreases from 98% to 90% as the average flux increases from 27 to 59 L.m−2.h−1. 3.2. Effects of operational parameters on boron rejection It is established in the literature that boron removal is affected by membrane type [32] and pH [33]. This study also evaluated the effects of other operational parameters on boron rejection, namely, feed salinity, cross flow velocity, applied pressure, and feed temperature, in the new generation RO membranes tested. 3.2.1. Influence of feed salinity on boron rejection The influence of the increased salinity on boron removal was investigated in this study since the brine of the treated water was re-circulated into the feed tank, gradually increasing the salinity in the feed tank. In the first pass tests, the SWRO membranes showed a reduced boron rejection as the feed salinity gradually increased from 41,700 ppm-TDS (fresh seawater feed) to a maximum of 76,000 ppm-TDS throughout the experimental runs. As observed for GE-AD (Fig. 2-A) and Toray UTC-SW (Fig. 3-A), the boron rejection decreased from 84% to 75% in the former and from 94% to 87% in the latter, as salinity increased. It should be noted that, from industrial experiences, the first few data points in the run can often be doubtful because the membranes would have not reached steady state performance by then. Additionally, the membrane compaction usually takes place during this period, and therefore, should be accounted for. Nonetheless, the decreasing trend of boron rejection with increasing salinity was more obvious in KOCH-SW (Fig. 4-A),

with B-rejection decreasing with salinity buildup from 86% to 76% and in DOW-SW-30XLE (Fig. 5-A) with B-rejection decreasing from 92% to 74%. On the other hand, the second pass BWRO membranes showed inconclusive results, where no clear correlation was observed between feed salinity and boron rejection (Figs. 2B to 5B). In fact, the influence of salinity on boron removal was reported in the literature for different types of membranes but with contradicting results and under heterogeneous sets of test conditions [34,35]. Tu et al. [35] investigated the coupled effect of increased pH and ionic strength on boron rejection using the brackish water membrane DOW-BW30, and observed that an increased ionic strength will slightly increase the boron rejection, suggesting a marginal allowance of salt passage in the 1st pass to optimize boron rejection in the 2nd pass at elevated pH. However, Oo and Song [34] reported the opposite after testing three brackish water membranes from Hydranautics (ESPA1, LFC1 and CPA2) at pH= 9 and 10, and concluded that boron rejection decreases as the ionic strength increases at the same pH. The former group [35] attributed their observed phenomenon to the fact that the pKa of boric acid decreases as the salinity increases, resulting in an increase in the boron rejection. Whereas, the latter group [34] related the boron rejection to the electrostatic repulsion taking place between the negatively charged borate ions and the negatively charged membrane surfaces, speculating that membrane surface charge density is reduced at higher salinities. Unfortunately, it seems that our results on salinity's effect on boron rejection in BWRO membranes add to the inconclusiveness evident in the literature [34,35]. Yet, it can be inferred that the salinity influence on boron rejection is less pronounced in BWRO membranes, especially when compared to other operational parameters as shown next. But, more importantly, this work presents evidence of the negative influence of increased feed salinity on boron rejection in the first pass using SWRO membranes. 3.2.2. Effect of cross-flow velocity on boron rejection The effect of the cross-flow velocity on boron rejection was investigated using Toray's UTC-SW membranes at two different cross-flow velocities. The flow area within the membrane cell has a depth of 2.75 mm and a width of 4 cm. Hence, the two calculated cross-flow velocities applied were 0.45 m.s −1 (corresponding to a flow rate of 3.0 L per minute (LPM)) and 0.15 m.s −1 (corresponding to the flow rate of 1.0 LPM). As can be observed in Fig. 6, the higher cross velocity of 0.45 m.s −1 yields higher boron rejection (89–94%) compared to only 86–90% boron rejection in the case of cross velocity of 0.15 m.s−1. Another experiment conducted using two older generation Toray membranes (data not shown here), UTC-80B and UTC-70B, at the same cross-flow velocities (0.45 and 0.15 m.s −1) further deduced that higher cross-flow velocities consistently lead to higher born rejection.

Table 4 Projected flux and salt and boron rejections for Koch and Toray membranes used in this study, provided by the membrane manufacturing company at the specified simulation conditions. Membrane

Temperature (°C)

Feed TDS (ppm)

Feed pressure (psi)

Feed pH

Feed boron concentration (ppm)

Projected salt rejection (%)

Projected boron rejection (%)

Projected flux (L/m2.h)

KOCH-SW KOCH-XR Toray UTC-SW Toray UTC-BW

29 29 29 24

43,000 2000 43,000 2000

800 150 800 150

8 8 7.5 7.3

5 5 5.5 1

99.7 99.1 99.88% 99.81%

88.4 79.2 94.51% 74.73%

23 20.8 25 32

A. Farhat et al. / Desalination 310 (2013) 50–59

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Fig. 2. Effect of feed salinity on salt and boron rejection in first-pass seawater membrane GE-AD at 800 psi (A) and brackish water membrane GE-AK at 150 psi (B). Data points were taken 1 h apart.

Fig. 3. Effect of feed salinity on salt and boron rejection in first-pass seawater membrane UTC-SW at 800 psi (A) and brackish water membrane UTC-BW at 150 psi (B). Data points were taken 1 h apart.

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Fig. 4. Effect of feed salinity on salt and boron rejection in first-pass seawater membrane KOCH-SW at 800 psi (A) and brackish water membrane KOCH-XR at 150 psi (B). Data points were taken 1 h apart.

These results coincide with findings by Sagiv and Semiat [36], whose numerical analysis tool predicted an increase in boron rejection at higher feed flow velocities, unlike Koseoglu et al. [33], who reported

no significant impact of cross-flow velocity on boron rejection. In fact, it can be explained that higher flow rates (i.e., higher cross-flow velocities) lead to higher flow turbulence in the feed channel, breaking or

Fig. 5. Effect of feed salinity on salt and boron rejection in first-pass seawater membrane DOW-SW30-XLE at 800 psi (A) and brackish water membrane DOW-BW30-LE at 150 psi (B). Data points were taken 1 h apart.

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Fig. 6. Effect of feed flow velocity on boron rejection in first-pass seawater membrane UTC-SW at 800 psi. Data points were taken 1 h apart.

reducing the boundary layers, thus, minimizing the concentration polarization and leading to increased boron rejection. 3.2.3. Effect of feed pressure on boron rejection in 2nd pass A range of feed pressure can be applied in BWRO membranes, but most membrane manufacturers suggest 10–15 bar (~ 150–220 psi). Hence, a second pass RO can be operated under the same pressure range. In this work, two different pressures (220 psi and 150 psi) were tested during 2nd pass operation, using two BWRO membranes: KOCH-XR and GE-AK. Fig. 7-A shows that boron rejection of KOCH-XR ranged between 75% and 85% at 220 psi, compared to only 60–75% at 150 psi. The higher boron rejection at 220 psi was further proven using the GE-AK membrane, where boron rejection at 220 psi (50–75%) was higher than at 150 psi (35–55%) (Fig. 7-B). Similarly, the impact of feed pressure was previously investigated [33,37], showing that boron rejection increases from 84% to 88% if the feed pressure increases from 220 psi to 500 psi [37], and from 88% to 91% as pressure changes from

600 psi to 800 psi [33]. Therefore, this research work further confirms that higher pressure applied in the 2nd pass leads to higher boron rejection. This is mainly because the higher feed pressure allows higher water permeation through the membrane, accompanied by an almost fixed boron permeation (governed by other parameters), thus leading to a lower boron concentration in the permeate and the net result of a higher effective boron rejection. The effect of higher pressure on membrane compaction may be another explanation for the reduced boron passage through the membrane, although we question if that difference in compaction is significant enough between 10 and 15 bar. 3.2.4. Effect of feed temperature on boron rejection The effect of feed temperature on boron rejection was studied in four membranes and in both passes. DOW-SW, GE-AD, and KOCH-SW membranes were investigated (1st pass), as well as DOW-BW (2nd pass). As can be observed from Fig. 8, all the tested membranes showed that boron rejection decreased as feed temperature increased. Such a

Fig. 7. Effect of feed pressure on boron rejection in second-pass brackish membrane KOCH-XR (A) and second-pass brackish water membrane GE-AK (B), both at 0.455 m.s−1.

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Fig. 8. The effect of temperature on boron rejection in four membranes at cross flow velocity of 0.45 m.s−1.

decrease can be attributed to the increase in boron permeability caused by the temperature increase [16]. Similarly, Dominguez et al. [30], Hyung and Kim [38] and Mane et al. [39] have all reported a lower boron rejection as the feed temperature increases. This effect of temperature can be deteremental for desalination plants operating off the coasts of the Arabian Peninsula where summer temperatures of 50 °C are not uncommon (seawater temperature easily reaching upper 30 °C). The results presented in Section 3.1 show that RO plants can satisfy regulatory boron concentration limits using a 2-pass setup without the need for pH adjustment using Arabian Gulf seawater as feedwater and standard operating conditions (28–30 °C, 800 psi in first pass, and 150 psi in second pass). However, the results presented in this section on the effects of feed temperature and salinity show a potential risk to achieving the lower targeted boron level, as a result of higher summer temperatures or salinity buildup within the RO stage. However, the results of this section also show that boron rejection can be increased by increasing the cross flow velocity or feed pressure (particularly in the second pass). Adjusting these operational parameters can be used to counter the undesired effects of temperature and salinity, while maintaining a no-pH-adjustment setup. The choice of parameters (and process for that matter) will ultimately be based on economical, logistical, and environmental considerations.

4. Conclusion This work reports significant boron rejections as high as 96% using real seawater from the Arabian Gulf (TDS: 41,700 ppm–47,400 ppm) while employing readily-available commercial new generation RO membranes under a two-pass configuration and without any pH adjustment. Most two-pass configurations studied in the work almost completely rejected both salt (> 99.9%) and boron (>95%) using Toray, KOCH, and DOW membranes. More importantly, single-pass configurations reported high salt and boron rejection results reaching as high as 99% and 91%, respectively, yielding permeates with boron levels below 1.4 ppm. Thus, single pass may possibly be adequate to comply with the new guidelines of WHO (2.4 ppm) and those of other countries such as Australia, Canada, and the UAE, whose boron guidelines are 1.5 ppm or higher, in cases where the produced water is not to be used for irrigation of sensitive crops. The influence of higher feed salinity on the boron rejection was investigated and was found to cause a mild reduction in the boron removal when using SW membranes, yet its effect using BW membranes was minor. Furthermore, other operational parameters and feed characteristics such as cross flow velocity, applied pressures and temperature were evaluated for their effect on boron rejection. It was found that higher boron removals were achieved with higher feed flow velocities. Moreover, with higher applied pressures in

BWRO membranes, boron removal was also augmented. Finally, lower boron rejections were observed as feed temperature increased. Acknowledgment The authors would like to thank Dr. Tarek Waly from Dow Membranes, Mr. Rahul Sardeshpande from Toray Company, and Mr. Peter Moss from Koch Membranes for providing membrane samples, conducting and providing us with membrane performance projections, and for their valuable discussions and feedback. References [1] W. Woods, An introduction to boron: history, sources, uses, and chemistry, Environ. Health Perspect. 102 (1994) 5. [2] Y. Xu, J. Jiang, Technologies for boron removal, Ind. Eng. Chem. Res 47 (2008) 16–24. [3] J. Kim, H. Hyung, M. Wilf, J.S. Park, J. Brown, in: Boron Rejection By Reverse Osmosis Membranes: National Reconnaissance and Mechanism Study, US Department of the Interior Bureau of Reclamation, Denver Colorado, 2009. [4] M. Busch, W. Mickols, S. Jons, J. Redondo, J. De Witte, Boron removal in sea water desalination, Int. Desalination Water Reuse Q. 13 (2004) 25. [5] United-States-Pharmacopeia-30/National-Formulary-25, Electric Version, Pharmacopeial Convention, Rockville, MD, United States, 2007. [6] British-Pharmacopoeia, Electronic version 11.0, Her Majesty's Stationery Office, London, 2007. [7] M. Chung, Handbook on Borates: Chemistry, Production, and Application, in, Nova Science Publishers Inc., New York, 2010. [8] L. Romano, F. Battaglia, L. Masucci, M. Sanguinetti, B. Posteraro, G. Plotti, S. Zanetti, G. Fadda, In vitro activity of bergamot natural essence and furocoumarin-free and distilled extracts, and their associations with boric acid, against clinical yeast isolates, J. Antimicrob. Chemother. 55 (2005) 110. [9] R. Jovanovic, E. Congema, H. Nguyen, Antifungal agents vs. boric acid for treating chronic mycotic vulvovaginitis, J. Reprod. Med. 36 (1991) 593. [10] K. Van Slyke, V. Michel, M. Rein, Treatment of vulvovaginal candidiasis with boric acid powder, Am. J. Obstet. Gynecol. 141 (1981) 145. [11] A. Farhat, F. Ahmad, H.A. Arafat, Analytical techniques for boron quantification supporting desalination processes: a review, Desalination (in press), http://dx. doi.org/10.1016/j.desal.2011.12.020. [12] J. Moore, An assessment of boric acid and borax using the IEHR evaluative process for assessing human developmental and reproductive toxicity of agents, Expert Sci. Comm. Reprod. Toxicol. 11 (1997) 123–160. [13] W.H. Organization, Boron (Environmental Health Criteria Monograph 204), World Health Organization, IPCS, Geneva, Switzerland, 1998. [14] L. Melnik, O. Vysotskaja, B. Kornilovich, Boron behavior during desalination of sea and underground water by electrodialysis* 1, Desalination 124 (1999) 125–130. [15] E. Weinthal, Y. Parag, A. Vengosh, A. Muti, W. Kloppmann, The EU drinking water directive: the boron standard and scientific uncertainty, Eur. Environ. 15 (2005) 1–12. [16] K.L. Tu, L.D. Nghiem, A.R. Chivas, Boron removal by reverse osmosis membranes in seawater desalination applications, Sep. Purif. Technol. 75 (2010) 87–101. [17] Australian Drinking Water Guidelines, In: N.H.a.M.R. Council (Ed.), 2011. [18] CDW, Guidelines for Canadian Drinking Water Quality Summary Table, 2008. [19] W.H. Organization, Boron in drinking-water. Background document for development of WHO guidelines for drinking-water quality, WHO/HSE/WSH/09.01/2, 2009. [20] N. Hilal, G. Kim, C. Somerfield, Boron removal from saline water: a comprehensive review, Desalination 273 (1) (1 June 2011) 23–35. [21] K.V. Peinemann, S.P. Nunes, Membranes for Water Treatment, WILEY-VCH Verlag GmbH &Co., KGaA, Weinheim, 2010.

A. Farhat et al. / Desalination 310 (2013) 50–59 [22] Y. Magara, T. Aizawa, S. Kunikane, M. Itoh, M. Kohki, M. Kawasaki, H. Takeuti, The behavior of inorganic constituents and disinfection by products in reverse osmosis water desalination process, Water Sci. Technol. 34 (1996) 141–148. [23] M. Wilf, Membrane Desalination Technology, Balaban Publications, 2007. [24] N. Greenwood, Boron, Compr. Inorg. Chem. 1 (1973) 665–991. [25] J. Redondo, M. Busch, J. De Witte, Boron removal from seawater using FILMTECTM high rejection SWRO membranes, Desalination 156 (2003) 229–238. [26] Y. Magara, A. Tabata, M. Kohki, M. Kawasaki, M. Hirose, Development of boron reduction system for sea water desalination* 1, Desalination 118 (1998) 25–33. [27] D. Prats, M. Chillon-Arias, M. Rodriguez-Pastor, Analysis of the influence of pH and pressure on the elimination of boron in reverse osmosis, Desalination 128 (2000) 269–273. [28] M. Rodríguez Pastor, A. Ferrándiz Ruiz, M. Chillon, D. Prats Rico, Influence of pH in the elimination of boron by means of reverse osmosis, Desalination 140 (2001) 145–152. [29] M. Taniguchi, M. Kurihara, S. Kimura, Boron reduction performance of reverse osmosis seawater desalination process, J. Membr. Sci. 183 (2001) 259–267. [30] C. Dominguez-Tagle, V.J. Romero-Ternero, A.M. Delgado-Torres, Boron removal efficiency in small seawater reverse osmosis systems, Desalination 265 (2011) 43–48. [31] I.A. Al-Khatib, H.A. Arafat, Chemical and microbiological quality of desalinated water, groundwater and rain-fed cisterns in the Gaza strip, Palestine, Desalination 249 (2009) 1165–1170.

59

[32] E. Güler, N. Kabay, M. Yüksel, E. Yavuz, Ü. Yüksel, A comparative study for boron removal from seawater by two types of polyamide thin film composite SWRO membranes, Desalination 273 (2011) 81–84. [33] H. Koseoglu, N. Kabay, M. Yüksel, S. Sarp, Ö. Arar, M. Kitis, Boron removal from seawater using high rejection SWRO membranes—impact of pH, feed concentration, pressure, and cross-flow velocity, Desalination 227 (2008) 253–263. [34] M.H. Oo, L. Song, Effect of pH and ionic strength on boron removal by RO membranes, Desalination 246 (2009) 605–612. [35] K.L. Tu, L.D. Nghiem, A.R. Chivas, Coupling effects of feed solution pH and ionic strength on the rejection of boron by NF/RO membranes, Chem. Eng. J. 168 (2011) 700–706. [36] A. Sagiv, R. Semiat, Analysis of parameters affecting boron permeation through reverse osmosis membranes, J. Membr. Sci. 243 (2004) 79–87. [37] Y. Cengeloglu, G. Arslan, A. Tor, I. Kocak, N. Dursun, Removal of boron from water by using reverse osmosis, Sep. Purif. Technol. 64 (2008) 141–146. [38] H. Hyung, J.H. Kim, A mechanistic study on boron rejection by sea water reverse osmosis membranes, J. Membr. Sci. 286 (2006) 269–278. [39] P.P. Mane, P.K. Park, H. Hyung, J.C. Brown, J.H. Kim, Modeling boron rejection in pilot-and full-scale reverse osmosis desalination processes, J. Membr. Sci. 338 (2009) 119–127.