Separation and Purification Technology 85 (2012) 45–53

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Separation and Purification Technology

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Ultrafiltration as pretreatment of seawater desalination: Critical flux, rejectionand resistance analysis

Jia Xu a, Ling G. Ruan b, Xue Wang a, Ye Y. Jiang a, Li X. Gao a, Jie C. Gao a,⇑a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China,Qingdao, Shandong 266100, Chinab Institute of Seawater Desalination and Multipurpose Utilization, Tianjin 300192, China

a r t i c l e i n f o

Article history:Received 8 July 2011Received in revised form 21 September 2011Accepted 22 September 2011Available online 1 October 2011

Keywords:UltrafiltrationCritical fluxRejectionResistanceSeawater desalination

1383-5866/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.seppur.2011.09.038

⇑ Corresponding author. Tel./fax: +86 532 6678187E-mail address: gaocjie@mail.hz.zj.cn (J.C. Gao).

a b s t r a c t

In this study, the concept ‘‘critical flux’’ was used in the seawater desalination system, which could usedto enhance the performance of ultrafiltration (UF) process regarding membrane fouling. The impacts ofvarious factors, such as seawater properties (natural organic matter 0.012–0.082 cm�1 of UV254, ionicstrength 409–470 mmol L�1, calcium concentration 19–29 mmol L�1), membrane property (molecularweight cut-off, 20–30 and 80–100 KDa) and hydrodynamic conditions (cross-flow velocity 0.02–0.17 ms�1), on the UF performance were systematically investigated in terms of the critical flux, rejectionand filtration resistance under Jcrit conditions. The results showed that the critical flux declined with anincrease in NOM, calcium concentration and membrane MWCO, but the critical flux was improved by ele-vated feed turbidity and cross-flow velocity. Ionic strength was found to have less impact on the criticalflux than divalent ion concentration, and both impacts of ionic strength and divalent ion concentrationdepended on the NOM concentration in the feed. The turbidity removal efficiency maintained at approx-imately 100% under all the testing conditions, while the NOM removal efficiency increased with the NOMconcentration and feed turbidity, and decreased with the membrane MWCO, regardless of ionic strengthand calcium concentration in the feed. As for the filtration resistance under Jcrit conditions, it showed thatthe resistance caused by fouling (adsorption) was minimal and could be ignored under all the testingconditions except for ultra low cross-flow velocity (0.02 ms�1).

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ultrafiltration (UF) is being widely used as an absolute barrierto suspended particles, colloids, macromolecules, algae and bacte-ria. As an effective pretreatment process, it provides the consistentand high quality water for further treatment such as seawaterreverse osmosis (SWRO) [1]. Compared to the conventional pre-treatment system, UF has several prominent advantages such ashaving a smaller footprint, less chemical dosage, full-automaticoperation, superior permeate quality, etc. However, the mostchallenging problem interfering with the UF performance is mem-brane fouling.

To address this problem, an enormous amount of researchworks have been carried out to identify robust membrane andappropriate operating conditions, but there is still a long way togo to completely solve the problem. In order to have the optimumof UF operating conditions, membrane flux is considered as themost important parameter in UF design, and it is closely relatedto membrane filtration characteristics and membrane fouling. As

ll rights reserved.

2.

a result, the concept ‘‘critical flux’’ (Jcrit) was proposed in 1995and it provides a new and efficient operational approach to mini-mize the membrane fouling. Since then, the research on criticalflux has been expanding and now represents a significant fractionof works on membrane fouling. The most recent studies, however,emphasized colloidal suspensions such as silica particles and latex,organic solutions such as humic substances and proteins, and sur-face water such as rivers and lakes [2–7], all of which only presentless complex natures and chemistry compared to seawater whichis characterized by relatively high content of natural organic mat-ter (NOM), turbidity and salinity. In addition, most studies unfortu-nately did not pay attention to the rejection performance of UFrelated to the ‘‘critical flux’’ operating conditions. Till now, thereare a limited number of systematic research on investigating thecritical flux of UF during the pretreatment of seawater and thefactors that influence the UF performance in terms of rejectionand filtration resistance under Jcrit conditions.

The UF performance can be influenced by many differentfactors, especially by the properties of the solution phase, such asnatural organic matter (NOM), turbidity, ionic strength and cal-cium concentration [8], but also by the properties of the membraneitself, such as molecular weight cut-off (MWCO) and operating

Table 1Characteristics and properties of the UF modules.

Properties of membrane A module B module

Material Modified PES Modified PESPore size, lm 0.008 0.003MWCO, kDa 80–100 20–30Fiber length, mm 1410 1410Fiber outer/inner diameter, mm 1.3/0.8 1.3/0.8Module diameter, mm 125 125Fiber number each module 4900 4900Effective membrane area, m2 11 10Operating pressure, bar <2 <2

�1

46 J. Xu et al. / Separation and Purification Technology 85 (2012) 45–53

parameters, i.e. cross-flow velocity [9]. Therefore, the objective ofthis study is to understand the effects of these above factors onthe UF performance in terms of critical flux, rejection and filtrationresistance under Jcrit conditions. A cross-flow UF membrane systemwas used in treating real seawater without pretreatment by coag-ulation. In this work, the natural flux mode was applied for themeasurement of critical flux. The results obtained in this studyare expected to provide a sound understanding of the effects ofvarious operating factors on the UF performance under Jcrit condi-tions during treatment of seawater so as to have the optimum ofUF operating conditions.

Feed flow rate, L h <4000 <4000

2. Experimental

2.1. Testing membrane system

The pilot-scale UF system with capacity of 72 m3 d�1 used inthis study located near the Yellow Sea of China is shown inFig. 1, and the membrane properties of the two UF modules areshown in Table 1. In this study, module A was used for investigat-ing the effects of various operating factors on the UF performanceunder critical flux conditions. And, module B was applied when theeffects of molecular weight cut-off was evaluated.

The UF membrane system mainly consisted of feed diving pump(Gaobang glass fiber reinforced plastic centrifugal pump), 1 m3

cylindrical feed tank, feed pump with pressure transducer (Grund-fos multiple centrifugal pump), hollow fiber UF module (Lanlv, Chi-na), flux and pressure measurement system and backwash systemwith solenoid pilot actuated valve. The feed tank was fed with pre-filtered seawater (temperature 26–28 �C) by feed diving pump im-mersed in shore of Jiaozhou Bay. Continuous feed was pumped tothe UF module by the feed pump which was switched on and offautomatically by the water level gauge installed in the feed tank.The TMP and flux were measured by rotameter and pressure gaugeand periodically recorded manually. Fully automated backwashand in situ cleaning procedures were controlled by a programma-ble logic controller (PLC). All water sources such as UF permeate,concentrate and backwash waste were collected and injected backto the sea via the injection well on site.

2.2. Critical flux measurement

The critical flux (Jcrit) was measured using natural flux modewhich was introduced in detail in our previous work [10]. The

(a)

Fig. 1. Pictures of UF pilot system. (a) co

elevation of pressure or reduction of flux with the extent of mem-brane fouling is in a natural state during UF operation without per-meate pump, neither imposing a fixed flux to measure a change inpressure (constant flux mode) nor imposing a fixed pressure tomeasure a change in flux (constant pressure mode). The obviousdistinction between the nature flux mode and other modes usedin the previous literature [11–13] is that whether an additionalexternal force exists except for hydraulic force and lateral forceduring cross-flow operation. The process of the critical flux mea-surement using natural flux mode is illustrated in Fig. 2, and the re-sults indicate that critical flux equals to 105.6 L m�2 h�1.

2.3. Ultrafiltration process

The pilot-scale filtration experiments were conducted in a totalcontinuous mode, whereby feed was pumped into the feed tankcontinuously and both retentate and permeate were returned backto sea. Conventional one-factor-at-a-time analysis was employedin four tasks to investigate the influence of concentrations ofNOM, inorganic salts (Na+, Ca2+), membrane MWCO and cross-flowvelocity on the UF performance in terms of critical flux, rejectionand filtration resistance under Jcrit conditions. Critical flux wasdetermined according to the measurement described in Section2.2. Rejection (R) and the total filtration resistance (rt) [2] wereevaluated from:

R ¼ CF � CP

CF

� �� 100% ð1Þ

where CF and CP are the solute concentrations in the feed and per-meate, respectively.

(b)

ntainer of UF system; (b) UF system.

Table 2Chemistry of raw seawater of the Yellow Sea.

UV254 (cm�1) Turbidity (NTU) Na+ (mmol L�1) Mg2+ (mmol L�1) Ca2+ (mmol L�1) Cl� (mmol L�1) SO2�4 (mmol L�1)

0.010–0.025 2.30–20.91 406.16–410.98 93.55–97.27 18.91–21.23 490.14–499.12 50.09–52.18

Note: Each value was obtained by more than five measurements.

Table 3Chemistry of the pretreated seawater by pre-filter.

UV254 (cm�1) Turbidity (NTU) Na+ (mmol L�1) Mg2+ (mmol L�1) Ca2+ (mmol L�1) Cl� (mmol L�1) SO2�4 (mmol L�1)

0.010–0.020 2.15–3.25 406.16–410.98 93.55–97.27 18.91–21.23 490.14–499.12 50.09–52.18

Note: Each value was obtained by more than five measurements.

0.000 0.015 0.030 0.045 0.060 0.075 0.0900

2

4

6

8

10

12

TO

C (

mgL

-1)

UV254

(cm-1)

R2=0.9648

Fig. 3. Correlation of UV254 and TOC in seawater.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

TMP

J(L/

m2 h)

T(min)

TM

P (

×0.1

MP

a)

20

40

60

80

100

120

140

160

Flux

0 40 80 120 160 200 240 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

20

40

60

80

100

120

140

160

180

200

TMP(×0.1 MPa)

J(L/

m2 h)

Fig. 2. Measurement of Jcrit in UF treating seawater process by nature flux method (Operating conditions: cross-flow velocity around 0.11 ms�1, feed turbidity 3.01–4.50 NTU,feed UV254 0.016–0.021 cm�1).

J. Xu et al. / Separation and Purification Technology 85 (2012) 45–53 47

rt ¼DP

lJcritð2Þ

where DP is the trans-membrane pressure; l is the seawater viscos-ity. In addition, to further assess the effects of various operating fac-tors on the extent of membrane fouling, the contribution (Cf) of thefouling resistance to the total filtration resistance was calculatedfrom:

Cf ¼rt � rm

rt

� �� 100% ð3Þ

where rm is the intrinsic membrane resistance and was determinedby the pure water permeability of a virgin UF membrane. The re-sults showed that rm of module A equals to 1.61 � 1012 m�1.

Seawater was drained off after each run and the membrane wasrinsed three times using deionized water for 30 min at a TMP of0.12 MPa and a cross-flow velocity of 0.09–0.12 ms�1. Next, freshdeionized water was recirculated at 0.01 MPa and 0.2 ms�1, untilthe initial clean water permeability was nearly restored.

2.4. Chemistry of raw seawater and analytical methods

The characteristics of raw seawater of Yellow Sea and pre-treated seawater by pre-filter are listed in Tables 2 and 3, respec-tively. Turbidity characterizing inorganic particles and UV254

directly related to the aromatic fraction of NOM in seawater weremeasured using a turbidity meter (LP2000-11, HANNA, Italy) andan ultraviolet spectrophotometer (UV-2540, SHIMADZU, Japan),respectively. To make sure whether or not UV254 can be a goodindicator to characterize NOM concentration in seawater, the cor-relation of UV254 and TOC was evaluated and the results are shown

in Fig. 3. The good linear relationship confirms the reliability ofUV254. Ion concentrations (Na+, Mg2+, Ca2+, Cl� and SO2�

4 ) were ana-lyzed using an ion chromatograph (Dionex90, DIONEX, USA).

3. Results and discussion

3.1. Effect of NOM concentration in seawater on the UF performance

Natural organic matter, such as humic acid (HA), has been iden-tified as one of the major foulants for UF membranes [14,15]. Toinvestigate the effect of NOM concentration in the feed seawateron the UF performance, four experimental runs were designed to

Table 4Feed chemistry of UF of the experimental runs.

Runs Feed HA dosage (g) UV254 (cm�1) Turbidity (NTU) Conductivity (ms cm�1)

1 Raw seawater 0 0.012 2.15–2.73 44.32 Raw seawater 0 0.021 2.75–2.96 44.43 Seawater with HA addition 1.1 0.034 2.66–3.23 44.44 Seawater with HA addition 1.9 0.047 2.87–3.15 44.05 Seawater with HA addition 3.4 0.082 3.12–3.31 44.2

48 J. Xu et al. / Separation and Purification Technology 85 (2012) 45–53

dose commercial HA (supplied by Sigma) into seawater for varyingthe NOM concentration (feed UV254 of 0.012–0.082 cm�1). The feedchemistries are shown in Table 4, where the values of turbidity andconductivity are almost the same for each run, but values of UV254

are totally different.The trend of critical flux of UF with the NOM concentration in

seawater is illustrated in Fig. 4(a). The critical flux monotonouslydeclines with UV254 to represent a 15.2% reduction when UV254 isincreased from 0.012 to 0.082 cm�1. The extent of adsorption byNOM molecules accumulated on the membrane surface is deter-mined primarily by the collision frequency of NOM molecules ontothe membrane surface, which increases at higher NOM concentra-tions [16]. As a result, more NOM molecules are generally adsorbedonto a membrane when the feed NOM concentration increases. Thesimilar trend was observed for latex suspensions by many previousstudies [11,17], where a drop in the critical flux with the increasingconcentration of latex suspensions was shown. In addition, dosingof HA leads to a considerable decrease in the critical flux, indicatingthat HA might be one of the dominant constituents of NOM in sea-water for the membrane fouling. As well known, HA with its typi-cal molecular weight spanning from a few thousand to a fewhundred thousand Daltons [18], comprises a group of heteroge-neous recalcitrant and more hydrophobic polymeric organics

0.000 0.015 0.030 0.045 0.060 0.075 0.09080

90

100

110

120

130

Crit

ical

flux

(Lm

-2h-1

)

Feed UV254

(cm-1)

(a)

0.000 0.015 0.030 0.045 0.060 0.075 0.09020

40

60

80

100

Feed UV254

(cm-1)

UV254

Turbidity

Rem

oval

effi

cien

cy (

%)

(b)

Fig. 4. Effect of feed UV254 on the critical flux (a) and the removal efficiencies ofturbidity and UV254 under Jcrit conditions (b).

compared to fulvic acid (FA), resulting in a more serious flux reduc-tion of 78% while it is 15% for FA [19]. It demonstrates that highhydrophobility and high molecular weight are the main propertiesof NOM foulant, resulting in the elevation in the filtration resis-tance as well as the membrane fouling during UF filtration.

Fig. 4(b) illustrates the removal efficiencies versus the NOMconcentration in the feed under Jcrit conditions. In this case, the re-moval efficiency of turbidity is really high (nearly 100%), indicatingthat UF can efficiently remove suspended particles and its removalefficiency is independent from the NOM concentration in the feed.For the UV254 removal, as the feed UV254 increases from 0.012 to0.047 cm�1, the removal efficiency of UV254 gradually increasesfrom 50% to 66%. However, when the NOM concentration in thefeed is sufficiently high (feed UV254 > 0.050 cm�1), a further in-crease in the feed UV254 has little impact on the UV254 removal,and the corresponding value was approximately 67%. The resultsare different from those obtained by Choi [20] who concluded thatdissolved organic carbon (DOC) removal changed slightly with theincreasing NOM concentration in the feed when treating syntheticwater, which might be due to the different nature of feed water. Inthis study, seawater was the UF feed which is characterized by ahigh ionic strength with Na+ of 406.16–410.98 mmol L�1 and Cl�

of 490.14–499.12 mmol L�1. NOM, at such high ionic strength,tends to curl due to the thin double layer and spherical macromol-ecules are formed, which is enhanced as the increase in NOM con-centration. It leads to a partial adsorption between NOM andmembrane, which could be observed even under Jcrit conditiondue to the weak form of Jcrit [10]. It develops a secondary physicalbarrier to the organic matter molecules due to the adsorption ofNOM or other foulants on the membrane surface or pores, resultingin the improved retention of these molecules. When an increase inthe NOM concentration cannot influence the curling extent of NOMmacromolecules, the UV254 removal efficiency will maintainunchanged.

Fig. 5 presents the relationship between the feed UV254 and thefiltration resistance. It was expected that higher NOM concentra-tion would increase filtration resistance due to the stronger

0.000 0.015 0.030 0.045 0.060 0.075 0.09015.6

16.0

16.4

16.8

17.2

17.6

18.0

Feed UV254

(cm-1)

rt

Cf

Tot

al r

esis

tanc

e r t (

1011

m-1)

0.00

0.03

0.06

0.09

0.12

0.15

0.18

Cf

Fig. 5. Effect of feed UV254 on filtration resistance during UF in treating seawaterunder Jcrit conditions.

Table 5Feed chemistry of the experimental runs for evaluating the effect of inorganic salts.

Runs Feed UV254 (cm�1) Turbidity (NTU) Na+ (mmol L�1) Ca2+ (mmol L�1)

2 Raw seawater 0.021 2.75–2.96 409.25 19.106 Seawater with NaCl addition 0.023 2.75–3.19 �430a 19.107 Seawater with NaCl addition 0.024 2.81–3.21 �450a 19.108 Seawater with NaCl addition 0.019 2.51–2.79 �470a 19.109 Seawater with NaCl and HA addition 0.051 2.11–2.13 �470a 19.1010 Seawater with CaCl2 addition 0.021 2.65–2.76 409.25 �23a

11 Seawater with CaCl2 addition 0.022 2.91–3.25 409.25 �26a

12 Seawater with CaCl2 addition 0.022 2.27–2.34 409.25 �29a

13 Seawater with CaCl2 and HA addition 0.050 2.91–2.99 409.25 �29a

Note: a‘‘�’’ represents the sum of ion concentration in raw seawater and ion concentration dosed into seawater.

J. Xu et al. / Separation and Purification Technology 85 (2012) 45–53 49

adsorption of NOM on the membrane surface even under Jcrit con-ditions. The results shown in Fig. 5 confirm the hypothesis with re-spect to the total filtration resistance, the corresponding data of1.62, 1.67, 1.70, 1.74 and 1.76 � 1012 m�1 in Runs 1–5, respec-tively. It might be explained by the formation of spherical NOMmolecules with smaller diameters. And, although the increase inthe total filtration resistance seems to be more obvious in therange of the relatively low NOM concentrations, the total filtrationresistance is always relatively low, particularly the resistance ow-ing to adsorption or fouling. The ratios of adsorption (fouling)resistance to rm (1.61 � 1012 m�1) ranges from 0.58% to 9.72%,which also confirms the concept ‘‘critical flux’’ for optimizing theUF operation due to the low and even negligible extent of mem-brane fouling.

5 6 7 8 9 10 11 12 1380

90

100

110

120

Crit

ical

flux

(Lm

-2h-1

)

Run number2

(a)

5 6 7 8 9 10 11 12 1340

50

60

70

80

90

100

2Run number

Rem

oval

effi

cien

cy (

%)

UV254

Turbidity

(b)

Fig. 6. Effect of salt concentration in the feed on the critical flux (a) and the removalefficiency under Jcrit conditions (b).

3.2. Effect of inorganic salts in seawater on the UF performance

Ionic strength of the feed water to membrane units is anotherimportant factor to influence the membrane performance, whichis often altered in conjunction with pretreatment practices aimedat protecting the membrane or enhancing the membrane perfor-mance [21]. However, the ionic strength in the previous workwas generally focused on 0.01–10.0 mmol L�1 [22–24], which ismuch lower compared to seawater that is characterized essentiallyby high salinity and high ionic strength, as shown in Table 2. There-fore, although numerous relative observations have been reportedin the literature [25], it is meaningful to investigate the effect of io-nic strength of the feed water with a background of seawater onthe UF performance under Jcrit conditions because few studies havebeen undertaken. In this study, inorganic salts such as NaCl andCaCl2 were dosed into seawater to alter the salt concentration.The feed chemistries of several experimental Runs 6–13 are pre-sented in Table 5.

The effects of feed salt concentration on the critical flux of UF intreating seawater are illustrated in Fig. 6(a), where the critical fluxof UF for the raw seawater (containing Na+ 409 mmol L�1) is shownfor comparison. In Runs 6–8, NaCl was added alone to alter Na+

concentration from 430 to 470 mmol L�1 in the feed. It can be seenthat ionic strength of 409–470 mmol L�1 with a seawater back-ground has not any significant effect on the critical flux of UFand the corresponding values are approximately 105 L m�2 h�1.This is different from the previous work by Kwon and Espinsasse[17,23,26], where an increase in ionic strength below the criticalconcentration could decrease the critical flux. It is related to abroad range of ionic strength (multiple orders of magnitude) andthe role of the ionic strength on the repulsive surface interactionwhich can be cleared through DLVO theory [27]. However, in thisstudy, there is an increase of 8% on an already high salinity water(seawater). The change in ionic strength might not effectivelyshield the charges due to the increased concentration of counterions, and not lead to a net reduction in the electrostatic repulsion

between NOM molecules. Consequently, the elevation in ionicstrength alone in the seawater will barely affect the adsorptionof NOM on the membrane surface as well as the critical flux.

In addition, it was assumed that the relatively low concentra-tion of NOM in the seawater might also be responsible for theabove phenomenon. Hence, in Run 9, both HA and NaCl were dosedinto seawater to investigate the effect of the high ionic strength onthe UF performance under a higher NOM concentration condition.Comparing the data in Runs 2, 4, 8 and 9, the corresponding valuesof critical flux are 105.6, 95.3, 105.2 and 92.1 L m�2 h�1 under thefeed conditions of raw seawater (UV254 0.020–0.021 cm�1, Na+

409 mmol L�1), raw seawater with HA addition (UV254 0.047–0.051 cm�1, Na+ 409 mmol L�1), raw seawater with NaCl addition(UV254 0.019 cm�1, Na+ 470 mmol L�1) and raw seawater with HAand NaCl addition (UV254 0.051 cm�1, Na+ 471 mmol L�1), respec-tively. Greater reduction of 12.4% in the critical flux was observedin Run 9 compared to that in Run 2 (raw seawater as feed), indicat-ing that a more pronounced influence of NOM concentration on theUF performance can be detected in treating seawater compared to

5 6 7 8 9 10 11 12 1314

15

16

17

18

19

rt

Cf

Cf

Tot

al r

esis

tanc

e r t

(1011

m-1)

2Run number

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

Fig. 7. Effect of salt concentration in the feed on filtration resistance during UF intreating seawater under Jcrit conditions.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

30

60

90

120

150

180

210

240 Pure water (module A) Pure water (module B) seawater (module A) seawater (module B)

J (L

m-2h-1

)

TMP(bar)

Fig. 8. Flux–TMP curve of pure water and seawater for module A and B.

50 J. Xu et al. / Separation and Purification Technology 85 (2012) 45–53

that of ionic strength. It might be because more NOM moleculestend to produce thin double layers at high ionic strength and formlarger quantity of spherical macromolecules that will better plugthe membrane pores.

In general, electrolyte addition can decrease the critical fluxwith a more significant effect for CaCl2 than for NaCl [27]. To inves-tigate the effect of divalent ion Ca2+ on the UF performance intreating seawater, various experiments in Runs 10–13 were con-ducted by addition of CaCl2. It shows that Ca2+ exhibits a detrimen-tal effect on the critical flux from 105.5 to 100.4 L m�2 h�1 whenCa2+ concentration increases from 19 to 29 mmol L�1. This resultagrees with those obtained by Seidel [28], where the critical fluxdecreased as the Ca2+ concentration increased during NOM nanofil-tration. It can be explained considering by the destabilization ofNOM macromolecules. Ca2+ has a strong affinity for carboxylicgroups [29] to complex with NOM and partially neutralize the neg-ative charge on NOM molecules [30]. The formation of Ca–NOMbond through the bridging effect [31] can decrease the stabilityof NOM molecules [27] so as to greatly enhance the collision effi-ciency of approaching destabilized NOM molecules onto the mem-brane surfaces [16], which leads to a higher filtration resistance. Asa consequence a decrease in the critical flux is observed. Note thatthere was no precipitation or crystallization occurred during theexperiments, although it was reported that Ca2+ concentration over4 mmol L�1 would cause the precipitation of Ca–NOM in a simpleionic system. It might be due to the synergistic effect of differentions in a more complicated ionic system such as seawater.

To evaluate the effect of Ca2+ on the UF performance in the caseof a higher NOM concentration in the feed seawater, the experi-ment (Run 13) was conducted by dosing HA into seawater andthe critical flux was determined to 89.6 L m�2 h�1. Two primaryconclusions can be drawn: firstly, the effect of divalent ion suchas Ca2+ on the UF performance is more significant than that ofmonovalent ion such as Na+ during treating seawater. Secondly,the decline in the critical flux with Ca2+ concentration from 19to 29 mmol L�1 when the feed UV254 is approximately0.047–0.051 cm�1 in seawater is moderately higher than the de-cline with Ca2+ concentration when the feed UV254 is about0.019–0.021 cm�1. It indicates that the NOM concentration in thefeed has a significant impact on the effect of Ca2+ on the criticalflux, especially in the case of the relatively high NOMconcentration.

The removal efficiencies corresponding to different salt concen-trations in seawater under Jcrit conditions are shown in Fig. 6(b). InRuns 2, 6–8 and 10–12, it can be seen that the removal efficienciesof both turbidity and UV254 are not influenced by changing Na+ orCa2+ concentrations alone, with the corresponding values of 100%and 50%, respectively. It might be due to the feed seawater withhigh salinity (ionic strength). In addition, when the feed NOM con-centration increases, the elevation in ionic strength or Ca2+ concen-tration can improve the UV254 removal efficiency with the values of65.96%, 67.01% and 70.24% in Runs 4, 9 and 13, respectively, espe-cially in the case of high Ca2+ concentration. This is consistent withthe results obtained by Grozes [32]. It might be explained thatmuch stronger adsorption between NOM and membrane happenswith the addition of CaCl2, resulting in the increasing sieving effectof UF membrane and consequently the higher NOM removal. An-other explanation might be that the precipitation owing to thecombination of Ca2+ and NOM can be rejected by UF membraneso as to enhance the NOM removal [33]. But no precipitation wasobserved in these experiments.

Fig. 7 shows the results of filtration resistance obtained at dif-ferent salt concentrations in the feed under Jcrit conditions. Theevolutions of the total filtration resistance as well as the ratios ofadsorption resistance to rm versus Run number are plotted. Regard-ing the total filtration resistance, more significant effect of Ca2+

concentration is observed. For example, rt remains stable in therange of 1.65–1.67 � 1012 m�1 when changing ionic strength from409 to 470 mmol L�1, while an increase of 6.6% in rt when changingCa2+ concentration from 19 to 29 mmol L�1. And, rt increases evi-dently when HA was dosed into seawater. Regarding Cf, all the val-ues are less than 7% at any concentration of Na+ and Ca2+ tested, solong as at a relatively lower NOM concentration (feed UV254

0.021 cm�1); while Cf increases to 12% at a higher NOM concentra-tion (feed UV254 0.050 cm�1). It also confirms the difference in theUF performance, which seems significant if NOM concentration inthe seawater is taken into account.

3.3. Effect of membrane MWCO on the UF performance

The geometric structure of UF membrane (porosity, MWCO,pore shape) has been proven to be essential for critical flux[34,35]. In this study, to investigate the effect of MWCO on the crit-ical flux and membrane fouling during treating seawater, module A(MWCO of 80–100 kDa) and B (MWCO of 20–30 kDa) with thesame membrane material and module size were chosen for com-parison. Other membrane parameters have been presented in Ta-ble 1.

Both the pure water line and the flux–TMP curve of seawater formodule A and B were determined using nature flux mode, and theresults are shown in Fig. 8. As regards the pure water lines, it isfound that different membrane MWCO results in the differencein the membrane permeability B, and more excellent permeabilityis yielded for module A. According to Eq. (2) in Section 2.3, rm was

J. Xu et al. / Separation and Purification Technology 85 (2012) 45–53 51

calculated and the corresponding values are 1.61 � 1012 and1.69 � 1012 m�1 for module A and B, respectively. It indicates thatthe module with larger MWCO poses a lower intrinsic membraneresistance. On the contrary, as shown in Fig. 8, larger MWCO isnot responsible for the higher critical flux of UF in treating seawa-ter, that is, the critical flux is higher for module B than module A,with the corresponding values of 113.6 and 105.6 L m�2 h�1,respectively. Different MWCO [27] could lead to a change in localpermeate velocity (filtration force) and could also be the result ofthe balance between the drag force and surface interaction dueto the filtration force. From a hydrodynamic view, the increasingMWCO enhances the surface interaction due to the larger perme-ation flux, which results in the more significant adsorption be-tween components (such as NOM, inorganic particles) inseawater and UF membrane so as to lower the critical flux. This re-sult is consistent with the results obtained by Wu [4–6], but dis-agrees with the results from Kwon [17] and Gesan-Guiziou [11]who found MWCO had little effect on the critical flux during UFof colloidal latex suspension. It might be explained consideringby the measurement of the critical flux.

To evaluate the membrane fouling for different modules underJcrit conditions, the total filtration resistance versus different fluxwas plotted in Fig. 9(a). As the permeation flux increases from 30to 105/113.56 L m�2 h�1 (less than Jcrit), the total filtration resis-tances for module A and B maintain nearly steady with the valuesof 1.73 � 1012 and 1.82 � 1012 m�1, respectively. When the perme-ation flux is elevated over Jcrit, the total filtration resistance in-creases dramatically, indicating that the significant membranefouling occurs. Note that in the whole range of the permeationfluxes tested, the total filtration resistance for module A is lowerthan module B. To further make sure that the difference in the totalfiltration resistance is the result of the membrane fouling, the con-tribution of adsorption resistance to the total resistance should be

20 40 60 80 100 120 14012

16

20

24

28

Tot

al r

esis

tanc

e r t

(10

11 m

-1) Module A

Module B

J (Lm-2h-1)

(a)

0

20

40

60

80

100

120

Rem

oval

effi

cien

cy (

%)

UF Module

Turbidity UV

254

Module A Module B

(b)

Fig. 9. Filtration resistance (a) and removal efficiency of module A and B in treatingseawater under Jcrit conditions (b).

taken into account. The similar values of adsorption resistance of0.12–0.13 � 1012 m�1 are obtained for module A and B. It indicatesthat the extents of membrane fouling (adsorption) are nearly thesame, although different UF modules with different MWCO areoperated. It also shows that the critical flux depends not only onthe membrane fouling but also on the membrane geometric struc-ture. As a result, module with larger MWCO can yield a lower crit-ical flux as well as the moderate membrane fouling.

Fig. 9(b) presents the permeate quality for module A and B intreating seawater under Jcrit conditions. Although the similar val-ues of turbidity removal of 100% can be observed, the differencein NOM removal illustrates that module B can achieve better per-meate quality with an increase of 18% of NOM removal comparedto module A. Combining with the analysis in the above paragraph,this occurs due to the membrane geometric structure. UF mem-brane with smaller MWCO can serve as a more effective screenso as to enhance the removal efficiency of NOM.

3.4. Effect of hydrodynamics on the UF performance

Hydrodynamics at the membrane surface has a major influenceon the critical flux [27], which has typically been expressed as apower law of the Reynolds factor in numerous papers dealing withcritical flux. However, the study on the effect of cross-flow velocityon the critical flux of UF in treating seawater is lacking. In thisstudy, the experiments were conducted at cross-flow velocitiesfrom 0.02 to 0.17 ms�1, a cross-flow velocity range for the systemrecovery requirement. The UF performances under Jcrit conditionsare shown in Figs. 10 and 11.

Fig. 10(a) shows the effect of cross-flow velocity on the criticalflux of UF in treating seawater. The critical flux increases sharplyfrom 50.2 to 100.3 L m�2 h�1 when the cross-flow velocity is in-creased from 0.02 to 0.07 ms�1. It is assumed that the concentra-tion polarization layer and adsorption layer can be disrupted by

0.00 0.03 0.06 0.09 0.12 0.15 0.18 0.2140

60

80

100

120

140

Crit

ical

flux

(Lm

-2h-1

)

Cross-flow velocity (ms-1)

(a)

0.00 0.03 0.06 0.09 0.12 0.15 0.18 0.210

20

40

60

80

100

Cross-flow velocity (ms-1)

Rem

oval

effi

cien

cy (

%)

UV254

Turbidity

(b)

Fig. 10. Effect of cross-flow velocity on the critical flux (a) and removal efficiencyunder Jcrit conditions (b).

0.00 0.03 0.06 0.09 0.12 0.15 0.18 0.2110

15

20

25

30

35

40

rt

Cf

Cf

Tot

al r

esis

tanc

e r t (

1011

m-1)

Cross-flow velocity (ms-1)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Fig. 11. Effect of cross-flow velocity on filtration resistance under Jcrit conditions.

52 J. Xu et al. / Separation and Purification Technology 85 (2012) 45–53

strongly hydrodynamic shear stress due to the higher cross-flowvelocity, and thus the critical flux is enhanced. However, no signif-icant improvement in the critical flux is observed between 0.07and 0.17 ms�1, indicating that a further increase in cross-flowvelocity beyond 0.07 ms�1 has litter impact on the critical flux. Thisis different from the results obtained by Gésan-Guiziou [36] whoexpressed the hydrodynamic criteria in terms of a critical wallshear stress and found a linear variation of critical flux and wallshear stress of ultrafiltration in treating skimmed milk. The differ-ent results observed might be due to the feed nature. Skimmedmilk is an unicomponent system, where the adsorption due to anearly uniform interaction can be gradually slacked down at theincreasing cross-flow velocities so as to cause a step-by-stepchange in critical flux. While in seawater, NOM (with a varietyrange of molecule weights and organic types) and inorganic parti-cles (with different particle sizes) can result in a more complicatedinteraction between different foulants and membrane. The adsorp-tion layer due to a weak and less strong interaction can be removedby changing the cross-flow velocity, but the strong adsorption can-not be disrupted effectively at any cross-flow velocity. As a result,any enhancement in the critical flux is not obtained even at cross-flow velocity of 0.17 ms�1. In addition to water recovery, the valuesof 73.42%, 64.79%, 53.75% and 44.14% are obtained in correspond-ing to the cross-flow velocity of 0.02, 0.07, 0.11 and 0.17 ms�1,respectively. According to the analysis mentioned above, cross-flow velocity of 0.07 ms�1 appears to be appropriate for UF in treat-ing seawater from a critical flux view.

Fig. 10(b) shows the permeate quality of UF under Jcrit condi-tions. As shown in Fig. 10(b), the turbidity removal maintainsapproximately 100% at all tested cross-flow velocities. Regardingthe UV254 removal, it declines significantly from 59.4% to 55.1%at the cross-flow velocity from 0.02 to 0.07 ms�1 but keeps a stea-dy state (about 55%) at the cross-flow velocity beyond 0.07 ms�1. Itmight be explained considering by the thickness of adsorptionlayer on the membrane surface under Jcrit conditions. Highercross-flow velocity can result in a thinner adsorption layer, whichleads to the increasing NOM leakage to cause permeate qualitydegradation. When the cross-flow velocity (over 0.07 ms�1) cannoteffectively change the thickness of adsorption layer, similar UV254

removals are obtained. However, results of another work [37] alsousing a ceramic UF membrane for seawater, showed that CODMn

removal efficiency increased from 16.2% to 37.1% with the cross-flow velocity from 2.7 to 4.9 ms�1. The distinguishing results mightbe due to the different operating conditions (Jcrit or super-Jcrit con-ditions) which can result in the different type of membrane fouling(mainly adsorption layer or cake layer).

The behavior of the filtration resistance for different cross-flowvelocities under Jcrit conditions is shown in Fig. 11. The total

filtration resistance is up to 2.57 � 1012 m�1 at 0.02 ms�1 and de-clines slightly from 1.73 � 1012 to 1.65 � 1012 m�1 at 0.07 and0.17 ms�1, which demonstrates that the cross-flow velocity hasan obvious impact on the membrane fouling even under Jcrit condi-tions and the relatively obvious fouling happens at a lower cross-flow velocity. In addition to Cf, the values of 60.19%, 7.72%, 3.78%and 2.64% are observed at 0.02, 0.07, 0.11 and 0.17 ms�1, respec-tively. It indicates that the resistance due to membrane fouling(adsorption) cannot be ignored at ultra low cross-flow velocity of0.02 ms�1 even under Jcrit conditions.

4. Conclusion

This study evaluated the effects of various factors on the UF per-formance in terms of the critical flux, rejection and filtration resis-tance under Jcrit conditions during the pretreatment of seawater.Some outcomes of this study are summarized below:

(1) Dosing of HA leads to a considerable decrease in the criticalflux. This is due to the higher collision frequency of NOMmolecules onto the membrane surface at higher NOM con-centrations. And, the UV254 removal is enhanced with theincreasing NOM concentration in the feed, which might bedue to the spherical molecules formed at high ionic strengthand high NOM concentration.

(2) The experiments with addition of CaCl2 and NaCl demon-strate that both Ca2+ and Na+ have little impact on theremoval efficiency under Jcrit conditions. Filtration resistancedue to fouling (adsorption) can be omitted. The critical fluxdeclines with a more significantly by adding Ca2+ than byNa+, especially under a feed seawater condition of highNOM concentration.

(3) The experiments with different MWCO UF membranes showthat lower total resistance is not responsible for the highercritical flux. Lower critical flux and lower total resistanceare obtained by UF membrane with larger MWCO. ButMWCO has little impact on the adsorption resistance whenUF is operated under Jcrit conditions.

(4) The fouling behavior is strongly dependent upon the hydro-dynamic conditions. Enhanced critical flux and lowered fil-tration resistance are observed at higher cross-flowvelocities, while the change is very little when the cross-flowvelocity exceeds the optimum value. In addition, membranefouling (adsorption) cannot be ignored at ultra low cross-flow velocity of 0.02 ms�1 even under Jcrit conditions. Theeffect on the removal efficiency is limited under thatcondition.

Acknowledgments

This work was supported by ‘‘Specialized Research Fund for theDoctoral Program of Higher Education of China’’ (No.20090132120007) and the Fundamental Research Funds for Cen-tral Universities of the Ministry of Education of China (No.201113023).

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