Boron removal from seawater using high rejection SWRO membranes — impact of pH, feed concentration, pressure, and cross-flow velocity

0011-9164/08/$– See front matter © 2008 Elsevier B.V. All rights reserved

Desalination 227 (2008) 253–263

Boron removal from seawater using high rejection SWROmembranes — impact of pH, feed concentration, pressure,

and cross-flow velocity

H. Koseoglua, N. Kabayb, M. Yükselb, S. Sarpb, Ö. Ararc, M. Kitisa*

aDepartment of Environmental Engineering, Suleyman Demirel University, 32260 Isparta, TurkeyTel. +90 (246) 211-1855; Fax: +90 (246) 237-0859; email: [email protected]

bDepartment of Chemical Engineering, cDepartment of Chemistry, Ege University, 35100 Izmir, Turkey

Received 3 December 2006; Accepted 21 June 2007

Abstract

The main objective of this work was to investigate boron removal from seawater using two commercial highrejection SWRO membranes. The impact of solution pH, feed concentration, pressure, and cross-flow velocity onboron rejection and permeate flux was determined. The membranes used were the TorayTM UTC-80-AB andFilmtecTM SW30HR. A lab-scale cross-flow flat-sheet configuration test unit was used for all RO experiments.Seawater sample was collected from the Mediterranean Sea, Alanya-Kızılot shores, south Turkey. For all experi-ments, mass balances were between 91% and 107%, suggesting relatively low loss of boron on membrane surfacesduring 14 h of operation. Operation modes did not have any impact on boron rejection, indicating that boron rejectionwere independent of feedwater boron concentrations up to 6.6 mg/L. For both membranes, much higher boronrejection were obtained at pH of 10.5 (>98%) than those at original seawater pH of 8.2 (about 85–90%). Permeateboron concentrations less than 0.1 mg/L were easily achieved at pH 10.5 by both membranes. The dissociated boronspecies are dominant at this pH, thus both electrostatic repulsion and size exclusion mechanisms are responsible forthe higher boron rejection. The rejection of salts in seawater did not correlate with boron rejection at constantconditions. For each membrane type, permeate fluxes at constant pressure were generally lower at pH of 10.5, whichmay be partially explained by membrane fouling and enhanced scale formation by Mg and Ca compounds fromconcentration polarization effect at higher pH values. While somewhat higher boron rejection was found for onemembrane type as the pressure was increased from 600 to 800 psi, increasing pressure did not affect boron rejectionfor the other membrane. Feed flowrate thus the cross-flow velocity (0.5–1.0 m/s) did not exert any significant impacton boron rejection at constant conditions.

Keywords: Boron; Desalination; Membrane; Reverse osmosis; Seawater; SWRO

*Corresponding author.

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1. Introduction

Production of drinking water through seawaterdesalination using reverse osmosis (RO) mem-branes has become increasingly attractive, espec-ially in coastal areas with limited freshwatersources. Many seawater desalination RO(SWRO) plants have been operating successfullyfor nearly 30 years [1–4]. The number and capa-city of RO plants have also increased signi-ficantly, e.g., systems with permeate capacity upto 300,000 m3/d will be in operation [5]. The costof desalination by SWRO plants has beendecreasing over the last decade. Greater compe-tition, improved technology, optimized processdesign, the use of higher efficiency energyrecovery devices, and the development of newmembranes with higher permeability and rejec-tions have all contributed to this reduction indesalted water prices [3–6]. However, onechallenge in conventional SWRO plants is thedifficulty of meeting boron standards in productwaters. Therefore, some of the plants employadditional treatment steps including pH adjust-ment of feedwater or permeate from the initialstages, dilution of RO permeate with othersources, ion exchange post-treatment of ROpermeate, and passing the desalinated waterthrough extra RO stages. In addition, variousprocess configurations were proposed to achievelower boron concentrations in product waters ofSWRO plants [3,5,7–10]. All these further treat-ment options increase the cost of desalination.Furthermore, all these additional treatmentrequirements affect system configuration. On theother hand, process developments such as two-pass, split partial permeate treatment, one-stagearray configuration, and high pH feed for boronremoval were proven to be cost effective [5].

Boron is a naturally occurring elementthroughout the environment. The majority of theearth’s boron normally occurs in the ocean andnatural weathering of sedimentary rocks on landsurface is known to account for a large portion of

the total boron mobilized into soils and the aqua-tic environment [11]. Boron has been found to bepotentially harmful in drinking water and hassuspected teratogenetic properties. It was shownthat oral exposure of boric acid and borax inlaboratory animals induced male reproductiveimpediments [11–13]. Although boron is one ofthe most important micronutrient for plants, it isbeneficial to them only in small quantities. Regu-lar use of irrigating water with more than 1 mg/Lof boron is harmful for most plants [14,15]. Sen-sitive crops, including most citrus species, have aboron tolerance of only 0.40–0.75 mg/L, whilevegetables are more boron tolerant with maxi-mum thresholds of 1–4 mg/L [16]. Furthermore,there is a narrow concentration range betweenlevels essential for crop growth and levels that aretoxic. The World Health Organization (WHO) seta limit of 0.5 mg/L for boron in drinking waterand mentioned values between 0.3 and 0.5 mg/Lfor medium and large membrane desalinationplants. Similarly, the European Union (EU) clas-sified boron as a pollutant in potable waterregulations.

Depending upon the location and seasonalvariations, seawater contains about 4–6 mg/L ofboron, which is dominantly in the non-ionic boricacid form at the natural pH level of seawaters(7.9–8.2) [8,18–20]. Boric acid has a pKa ofabout 9.2; thus, above this pH level, concentra-tions of borate and other ionic forms of boronbecome dominant compared to the non-ionicform, which is very critical from the point ofboron rejection in RO membranes [8,16–18]. Attypical seawater pH values, regular RO mem-branes reject boron to a level about 40–80% (withpermeate boron concentrations of 0.9–2.0 mg/L)while the rejection of seawater salt is above 99%.In some cases even a two-stage RO process, atnormal operation conditions, is insufficient forreducing boron concentrations to meet thedrinking water quality requirements [7,18–20].The reason of the low rejection of the boric acidis because of its ability to diffuse through the

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membranes in a non-ionic form in the way similarto carboxylic acids or water [8,14,20]. Thus,boron rejection by RO membranes is a functionof pH, closely following the dissociation ratio ofboric acid. Dissociation ratio increases with thefeed salinity (ionic strength) and temperature [2,5,16,21]. With the design of a RO plant with asecond stage where the permeate is alkalinizedwith NaOH up to a pH of 9.5, it is stated that theboron of the product water is eliminated almost100%, involving an extra cost of 0.06 €/m3 [18].

In recent years, membranes with boron rejec-tion of 91–96% even at natural seawater pH aretaking place on the market [4,9]. The new mem-brane elements are being developed mainly forreduction of affinity with boron, reinforcement ofaffinity with water, and tighter molecular struc-ture (smaller pores) of membrane layer for thesize exclusion of the boric acid molecule [4]. Themain objective of this work was to determine theimpact of solution pH, feed concentration, pres-sure, and cross-flow velocity on boron rejectionand permeate flux in seawater using two com-mercial high rejection SWRO membranes(TorayTM UTC-80-AB and FilmtecTM SW30HR).

2. Materials and methods

A lab-scale cross-flow flat-sheet configurationtest unit (SEPA CF II, Osmonics) was used for allmembrane separation experiments (Fig. 1), whichsimulates the flow dynamics of larger, commer-cially available spiral-wound membrane ele-ments. Operating conditions and fluid dynamicscan be varied over broad ranges. The membranetest unit accommodates any 19 cm × 14 cm (7.5"×5.5") flat-sheet membrane for a full 140 cm2

(22"2) of effective membrane area. Maximumoperating pressure of the unit is 69 bar (1000 psi).Permeate flowrate is approximately proportionalto driving pressure. The membrane test systemconsists of a high pressure pump (Hydra-CellG13) equipped with digital variable frequency

drive (ABB ACS-140) to adjust feed flowrate andits 1.1 kW motor, pressure relief valve, membranecell, membrane cell holder, high pressure con-centrate control valve, hydraulic hand pump,pressure indicators in the membrane cell, cellholder and high pressure pump outlet, feed tank(37 L max. solution volume, stainless steel), per-meate collection tank, and connections/tubingsmade of either stainless steel or nylon-seal plasticand/or teflon. The required pressure in the mem-brane cell and permeate and concentrate flowrateswere adjusted by the concentrate control valve.Temperature of the feed tank was kept constantby circulating cool tap water through the jacketaround the feed tank outer walls during themembrane tests. All experiments were performedat feed water temperatures of 22±2EC.

Two commercial high rejection SWRO mem-branes, the TorayTM UTC-80-AB and FilmtecTM

SW30HR (Dow Chemical), were tested for boronrejection. Notations of “A” and “B” are usedthroughout this paper for the Toray and Filmtecmembranes, respectively. Membrane sheets wereobtained from the manufacturers and used as-received. The Filmtec membrane is a polyamidethin-film composite membrane with followingcharacteristics: maximum operating temperature,45EC (maximum temperature for continuous ope-ration above pH 10 is 35EC); pH range, con-tinuous operation, 2–11; maximum feed silt den-sity index (SDI), 5; free chlorine tolerance,<0.1 ppm; minimum salt rejection, 99.6%, stabi-lized salt rejection, 99.75%; stabilized boronrejection, 90%. The above benchmark values arebased on the following conditions: 32,000 ppmNaCl, 5 ppm boron, 55.2 bar (800 psi), 25EC,pH 8, 8% recovery (data obtained from Filmtecmembranes product information, Form No. 609-00426-804, Dow Chemical Company).

The seawater sample was collected from theMediterranean Sea, Alanya-Kızılot shores (southTurkey). The sample was filtered (1 µm cartridgefilters) and kept at dark in the laboratory prior touse for membrane tests. Table 1 shows the

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Fig. 1. Schematic of the membrane test system.

Table 1Physicochemical characteristics of the filtered seawatersample

Parameter

pH 8.2Conductivity (µS/cm) (T: 19EC) 57,400Salinity (‰) (T: 19EC) 37.5Turbidity (NTU) 1.2Boron (mg/L) 5.1Nitrate (mg/L) 1.4Nitrite (mg/L) 0.02Total nitrogen (mg/L) 1.45Dissolved organic carbon (mg/L) 1.7

Values are averages of triplicate measurements.

physicochemical characteristics of the filteredseawater sample. Membrane operating pressurestested were 41.3 bar (600 psi), 48.2 bar (700 psi)

and 55.2 bar (800 psi). The tested feed water pHvalues were 8.2 (seawater pH) and 10.5. Thetested membrane feed flowrates were 3.8 and2 L/min with corresponding cross-flow velocitiesof about 1.0 and 0.5 m/s, respectively. A newmembrane was used for each test after condition-ing the membrane for about 8–10 h of feedingwith distilled and deionized water (DDW) at theconditions of the experiment to be conducted.After conditioning, each new membrane waschecked for total dissolved solids (TDS) removalsin seawater before starting an actual experiment.Two operation modes were employed in theexperimental matrix: batch concentration andconstant feed tank volume. In the batch concen-tration mode, concentrate stream was returnedback to feed tank with an initial test solutionvolume of 20 L. Permeate was collected in thepermeate tank. Therefore, the boron concentration

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in the feed tank gradually increased during theoperation due to concentration effect and decreas-ing feed tank volumes. In the constant feed tankvolume operation, feed tank volume was keptconstant (20 L) by continuously feeding the feedtank with filtered seawater during all the opera-tion period. The flowrate of this continuousfeeding to the feed tank was same with that ofpermeate. The concentrate stream was alsoreturned back to the feed tank.

The duration of each membrane test was about14 h. Samples from feed tank and permeate weretaken each hour for boron, TDS, conductivity,pH, and temperature measurements. To maintainconstant pH in the feed tank depending on theexperimental matrix, pH was monitored andadjusted with various concentrations of HCland/or NaOH solutions, if necessary. Flowrates ofconcentrate and permeate streams, membrane unitand pump outlet pressures were also recordedeach hour. In an effort to calculate overall massbalance on boron, the volumes remaining in feedand permeate tanks and tubings after the experi-ments, sample volumes and the associated boronconcentrations in these volumes were measured.In other words, the mass of boron applied to thesystem was compared with that of obtained fromvarious points at the end of the experiment.

The spectrophotometric curcumine methodwas employed for boron analysis. In this method,curcumine solution forms an orange-red complexcompound with borate ions and the absorbance ofthis compound is measured at λmax of 543 nm.Conductivity and TDS were measured using aWTW Inolab Cond. Level 1 conductivity meter.The pH was measured using a bench-scale SchottHandylab 1 pH meter. All chemicals used werereagent grade. Distilled and deionized water wasused for stock solution preparations and dilutions.

3. Results and discussion

Table 2 presents the mass balance calculationsperformed on boron for each experiment. Percent

Table 2Mass balance calculations on boron in seawaterexperiments

Exp. no. % mass balance

123456789

91.094.699.293.8

107.7100.7

97.698.6

100.2

mass balance represents the ratio of the boronmass obtained from all points (i.e., feed tank,permeate collection tank, tubings and samples) atthe end of the experiment to the boron massinitially applied to the system. For all experi-ments, mass balances were between 91% and107%, suggesting relatively low loss of boron onmembrane surfaces during 14 h of operation. Itshould be noted that many independent para-meters, including volumes of various tanks/samples and boron concentrations, are measuredand take place in mass balance calculations.Therefore, experimental errors in volume andconcentration measurements are expected, whichmay result in mass balance lower or higher than100%. Boron measurements were conducted induplicates. Percent coefficients of variationsbetween duplicate measurements were generallyless than 12%, averaging about 5%. Thus, anerror degree of approximately ±5% is inherent inmass balance calculations.

Fig. 2 shows the impact of solution pH onboron rejection in seawater experiments. For boththe membranes, a much higher boron rejectionwas obtained at pH of 10.5 (>98%) than those atoriginal seawater pH (about 85–90%). Permeateboron concentrations less than 0.1 mg/L wereeasily achieved at pH 10.5 by both membranes.The acid dissociation constant (pKa) of boric acid

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Fig. 2. Impact of solution pH on boron rejection by A and B membranes (batch concentration mode, pressure: 700 psi, feedflowrate: 3.8 L/min, feed temp: 22–24EC).

is about 9.2. Therefore, the charged boron species(i.e., B(OH)4

!) are expected to be dominant at pHvalues >9.2 compared to the neutral boric acid. Ata pH of 8.2, about 80% of boron is in neutralform. Although not dominant as B(OH)4

!, othercharged boron species including B2(OH)7

! andB3(OH)10

! have maximum concentrations at pHvalues around 9. Boron is about 100% in borateform at a pH of 12 [14]. For many ionic com-pounds, it is known that charged species arerejected to a greater extent by many ROmembranes through electrostatic repulsion. Fur-thermore, the dissociated boron species are fullyhydrated, resulting in a larger radius and anenhancement of the negative charge of the ion.Thus, both electrostatic repulsion and sizeexclusion mechanisms are responsible for thehigher boron rejection at elevated pH values[2,8]. Our results are consistent with the work ofRedondo et al. [2] in which boron rejection bySWRO membranes increased with degree ofboron dissociation, rising from 80% to 97% and99% at pH 9, 10 and 11, respectively. Similarly,when the pH was increased to 10.5–11, the boronrejection was as high as 99% or above [8,11]. Theboron rejection of many current SWRO mem-branes at nominal test conditions and naturalseawater pH generally ranges between 85 and

90%, corresponding to about 70–80% boronrejection in operating SWRO plants [8–10]. Thus,for typical Mediterranean seawater containing 5–6 mg/L of boron, operation at 50% recovery mayproduce permeate containing 1.6-2.0 mg/L ofboron [9].

Fig. 3 shows the impact of operation mode(i.e., feedwater concentration) on boron rejection.In the batch concentration mode, feed tank boronconcentration increased with operation period.After 3 h of operation to reach steady-state rejec-tion, operation mode did not have any impact onboron rejection, which was consistently largerthan 98%. This result indicates that boron rejec-tion is independent of feedwater boron concen-trations resulting in similar permeate boronconcentrations, consistent with the literature[11,22]. However, it should also be noted that theoperation period was only 14 h and that concen-tration factors in batch mode operations wereonly about 1.1–1.3, making feedwater boronconcentrations in the range of 5.6–6.6 mg/L.Higher operation periods and much higher feedboron concentrations may lead to higher boronpassage through membranes due to concentrationpolarization thus enhanced diffusion effects.

Boron rejection was relatively constant (amaximum change of ±5%) during the 14-h

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Fig. 3. Impact of operation mode (i.e., feedwater concentration) on boron rejection (membrane A, pressure: 700 psi, pH:10.5, feed flowrate: 3.8 L/min, feed temp: 23–24EC).

operation period for all experiments, suggestingthat steady state dynamic membrane conditionswere immediately achieved in seawater. How-ever, as indicated previously, before each test, anew membrane was conditioned for about 8–10 hof feeding with DDW at the conditions of theexperiment to be conducted. It was found in thepreliminary single solute (boron in DDW) modelsolution experiments that about 3–5 h of opera-tion was necessary to reach steady-state boronrejection. This time period had also varied withmembrane type and experimental conditions.Although boron rejection was relatively constantin seawater experiments during the 14-h opera-tion period, permeate fluxes gradually decreasedin all experiments. This observation suggests thatmembranes immediately reach steady-state condi-tions for boron rejection but not for permeateproduction rate in the first 24 h of use (10 h forconditioning). Membrane compaction and foulingis apparently causing a reduction in permeate fluxat constant operating pressure during this period.

Fig. 4 shows the impact of pH and membranetype on permeate flux. For each membrane type,permeate fluxes at constant pressure were gene-rally lower at pH of 10.5. This impact was morepronounced for membrane B, i.e., about 15–40%reduction in fluxes was observed at pH of 10.5

compared to pH of 8.2. This phenomenon can bepartially explained by membrane fouling andenhanced scale formation by Mg and Ca com-pounds at higher pH values [9,14]. Scale forma-tion on the membrane surfaces caused by supersaturation is the disastrous effect of the concen-tration polarization which results in considerableflux decreases. In addition, the use of NaOH forelevating the pH to 10.5 further increased thesolute concentration in the feed water, resulting inan increase in osmotic pressure and concentrationpolarization effect, which both gradually decreasepermeate flux. Potential precipitation of calciumcarbonate and magnesium hydroxide at high pHin SWRO applications is a problem and must beavoided [9]. Furthermore, due to the formation ofcalcium carbonate scaling and the cost associatedwith the injection of voluminous amounts ofbases (i.e., sodium hydroxide), it may not befeasible to increase the pH value of the seawater[14].

Fig. 5 shows the impact of pressure onpermeate flux. As expected, as the membranepressures were increased from 600 to 800 psi per-meate fluxes also increased for both membranetypes. Similar fluxes were obtained for bothmembranes at constant pressure. However, for allexperimental conditions, permeate fluxes grad-

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Fig. 4. Impact of pH and membrane type on permeate flux (batch concentration mode, pressure: 700 psi, feed flowrate:3.8 L/min, feed temp: 22–24EC).

Fig. 5. Impact of pressure on permeate flux (batch concentration mode, pH: 8.2, feed flowrate: 3.8 L/min, feed temp: 22–24EC).

ually decreased during the 14-h operation periodalthough a leveling off was observed after 12 h atall tests. This result suggests that longer operationperiod than 14 h may be required to reach asteady-state permeate production rate. As dis-cussed previously, fouling and compaction appearto be important factors in flux reductions even atthe beginning of experiments with new mem-branes. Typical permeate flux in SWRO desali-nation plants with spiral-wound elements rangesbetween 14 and 16 L/m2-h. The ranges of per-meate fluxes measured in all different experi-

mental conditions in this work were 11–15, 13–17 and 19–21 L/m2-h for 600, 700 and 800 psipressures, respectively, after an operation periodof 14 h. The range of flux values obtained in thiswork is somewhat higher than those in commer-cial SWRO systems. This can be explained bytwo facts: (1) flat-sheet test units as used in thisstudy exert lower degree of pressure loss com-pared to spiral-wound commercial elements, and(2) prolonged operation periods after 14 h mayfurther reduce the fluxes, based on the gradualdecrease trend observed in this study.

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Fig. 6. Impact of pressure on boron rejection (batch concentration mode, pH: 8.2, feed flowrate: 3.8 L/min, feed temp: 22–24EC).

Fig. 7. Impact of pressure on salt (as conductivity) rejection (batch concentration mode, pH: 8.2, feed flow rate: 3.8 L/min,feed temp: 22–24EC).

The impact of pressure on boron rejection isshown in Fig. 6. Boron rejection varied between85 and 92% for all pressures at a pH of 8.2. As ageneral trend, membrane A exhibited somewhathigher rejection as the pressure was increasedfrom 600 to 800 psi although there was somescatter in the data. It was reported that the boronrejection tended to rise as pressure was increased[8,11]. Similar boron rejection was found formembrane B at pressures 700 and 800 psi; againthe data were somewhat scattered. Since the dif-ferences in boron rejection are relatively small theinherent experimental errors in permeate boronmeasurements should also be considered.

Fig. 7 shows the impact of pressure on saltrejection (as conductivity). After steady-stateconditions were achieved, similar salt rejectionswere found (97–99%) for both membranes. How-ever, while salt rejections were about 98–99% atseawater pH of 8.2 for both membranes, saltrejections decreased to 95.5–97% at pH of 10.5.These results indicated that the rejection of saltsin seawater does not correlate with boron rejec-tion at constant conditions, consistent with theliterature [2]. Boron rejection is principally con-trolled by the membrane chemistry (diffusion)and to a lesser degree by convective transport,whereas salt rejection is controlled by both [2].

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Fig. 8. Impact of feed flowrate on boron rejection (membrane A, batch concentration mode, pH: 8.2, pressure: 800 psi, feedtemp: 22–24E).

On the other hand, Taniguchi et al. [23] re-ported a specific relationship between salt andboron concentrations in permeate samples regard-less of various operating conditions, and this wasconfirmed by solving transport equations. Fig. 8shows the impact of feed flowrate (cross-flowvelocity) on boron rejection. At constant mem-brane pressure of 800 psi and pH of 8.2, feedflowrate thus the cross-flow velocity did not exertany significant impact on boron rejection.

4. Conclusions

Mass balances were between 91% and 107%for all experiments, suggesting relatively low lossof boron on membrane surfaces during 14 h ofoperation. Although membranes reached steady-state conditions for boron rejection within about4–5 h of operation, permeate fluxes graduallydecreased in the 14 h of operation. Membranecompaction and fouling apparently caused areduction in permeate flux at constant operatingpressure during this period. However, a generalleveling off was observed for fluxes after 12 h.Operation modes did not have any impact onboron rejection, indicating that boron rejectionwas independent of feedwater boron concen-trations up to 6.6 mg/L. For both membranes,much higher boron rejection was obtained at pHof 10.5 (>98%) than those at original seawater pHof 8.2 (about 85–90%). Permeate boron concen-

trations less than 0.1 mg/L were easily achievedat pH 10.5 by both membranes. The dissociatedboron species are dominant at this pH, thus bothelectrostatic repulsion and size exclusionmechanisms are responsible for the higher boronrejection.

While salt rejections were about 98–99% atseawater pH of 8.2 for both membranes, saltrejections decreased to 95.5–97% at pH of 10.5,indicating that the rejection of salts in seawaterdoes not correlate with boron rejection at constantconditions. For each membrane type, permeatefluxes at constant pressure were generally lowerat pH of 10.5, which may be partially explainedby membrane fouling and enhanced scale forma-tion by Mg and Ca compounds from a concen-tration polarization effect at higher pH values. Asthe membrane pressures were increased from 600to 800 psi, permeate fluxes also increased forboth membrane types, as expected. While some-what higher boron rejection was found for onemembrane type as the pressure was increasedfrom 600 to 800 psi, increasing pressure did notaffect boron rejection for the other membrane.After steady state conditions were achieved,similar salt rejections were found (97–99%) forboth membranes at pressures of 600, 700 and800 psi (pH 8.2). At constant membrane pressureof 800 psi and pH of 8.2, feed flowrate thus thecross-flow velocity did not exert any significantimpact on boron rejection.

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Acknowledgement

The financial support of the Middle EastDesalination Research Center (MEDRC) (ProjectNumber: 04-AS-004) for this work is well ack-nowledged. The authors thank Toray Industries,Inc. and the Dow Chemical Company (FilmTec)for providing the membrane samples.

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