Nanofiltration of seawater: fractionation of mono- and multi-valent cations

DESALINATION ELSEVIER

Desalination 140 (2001) 67-77 www.elsevier.com/Iocate/desal

Nanofiltration of seawater: fractionation of monoand multi-valent cations C. Oumar Anne, Dominique Tr6bouet, Pascal Jaouen, Francis Qu6m6neur* Laboratoire de Gdnie des Procddes-Institut des Substances et Organisraes de la Met (ISOMer) CRTZ Boulevard de l'Universitd, BP 406, F-44 602 Saint-Nazaire Cede.x, France Tel. +33 (2) 40 17 26 15; Fax +33 (2) 40 17 26 18; eraail:francis.quemeneur@lgp, univ-nantes.fr

Received 5 October 2000; accepted 2 January 2001

Abstract

First the capabilities of six nanofiltration membranes (cut-offs of between 100 to 1000 daltons) to selectively demineralize salt water containing the same cations as seawater (monovalent: Na÷, K÷; divalent: Ca2+, Mg2+) were assessed and compared. The AFC-30 membrane displaying monovalent cation rejection rates of about 50% and divalent cations rejection rates higher than 90% was chosen for the second part of the study dealing with seawater. The second part shows that water with the desired overall cation content and desired monovalent over divalent cation ratio can be obtained from raw seawater, without using foreign water, through a series of processes: (1) diafiltration, using osmosed seawater as washing solution, with a diavolumes number of 3; (2) concentration, with a volume reduction factor of 1.5; and (3) light final dilution of the aforementioned osmosed water. Potential applications of the selectively demineralized water are in the field of health (preparation of nasal sprays, medical dietetics, etc.). Keywords: Membrane process; Nanofiltration; Seawater; Cation separation; Ion charge

1. Introduction

Membrane separation processes have been used for several years to concentrate or fractionate suspended particles and dissolved substances. Reverse osmosis (RO), now in widespread use to prepare drinking or irrigation water [1] from briny waters or seawater, seriously challenges *Corresponding author.

distillation [2]. Ultrafiltration constitutes a valuable aid for the fractionation and concentration of colloidal substances contained in seawater [3,4]. Nanofiltration (NF), which occupies an intermediate position between the two techniques mentioned above, both as regards the size o f separated species and pressures involved, is a more recent development in this field. The ability of NF membranes to demineralize salty solutions

0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 1 ) 0 0 3 5 5 - 1

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C Oumar Anne et al. /Desalination 140 (2001) 67-77

partly or selectively (natural waters, food liquids), based, moreover, on the presence of chemical functions capable of carrying positive or negative charges [5], makes it an interesting technique in many respects. Thus, when drinking water is prepared from seawater, it may constitute a preliminary treatment upstream from RO or multi-stage distillation, which, by reducing the load of these processes, allows the decrease of overall energy consumption and an increase in the conversion ratio of salt water to fresh water [6]. The possibility of NF reducing both the overall salinity of natural salt waters and the ratio r: "the number of monovalent cations over the number of divalent cations" leads the way to potential applications in the field of health (e.g., preparation of nasal sprays, medical dietetics, hot mineral springs). The purpose of the present study is to assess the possibilities of NF in this field, to examine and suggest a series of processes (diafiltration, concentration and dilution) allowing, from seawater, to adjust overall salinity (final Na +content desired: 120 moles.m -3) and the ratio r (starting with r = 8) to a desired level ( 2 . 2 < r < 2.4). In the first step, the ability of various membranes to desalt and to modify the proportions ofmonovalent and multi-valent cations was evaluated on salt waters prepared in the laboratory. Next, the membrane displaying the best capabilities was used in the study on seawater. So as to avoid any contact with chemical reactants including ion-exchange resins, diafiltration and dilution operations were carried out using seawater treated physically by RO.

2. Materials and methods 2.1. Saline solutions

Preliminary experiments (comparison of six membranes) were conducted with a saline

Table 1 Characteristics of experimental saline solution for preliminary experiments, seawater from Saint Nazaire and osmosed seawater used for dialfiltration steps (concentrations in mol.m-3) Saline Seawater of Osmosed solution Saint Nazaire seawater [Na+] [K+] [Caz+] [Mg2+] [monovalent]=a [multivalent]=b r = a/b

79.8 1.7 1.7 8.9 81.4 10.6 7.7

470.0 11.0 9.0 51.0 481.0 60.0 8.0

32.7 0.1 0.I 0.5 32.8 0.6 54.0

solution consisting of ultrapure water (Elga, quality II, ISO 3696) and the following salts: NaC1, KCI, CaC12 and MgSO4. The goal of this first part of the study was to select the most appropriate membrane for the fractionation of mono- and multi-valent cations. Then, with the selected membrane, the second part of the experimental work consisted of evaluating performances of the NF process (diafiltration, concentration steps) with an application to prefiltered (0.2 ~tm) seawater from Saint Nazaire (Atlantic shore, France). The composition of the aforementioned waters is given in Table 1.

2.2. Analysis

The analytical procedures utilized in this study were those recommended by Afnor [7]. Concentrations of Na +, Ca 2+and Mga+ were determined by ionic chromatography with Dionex DX 120 equipped with a CS 15 column, and K + analysis was performed using flame atomic absorption spectrophotometry (Varian Spectra 220).

69

C. Oumar Anne et al. /Desalination 140 (2001) 67-77 2. 3. Experimental set-up

2. 5. Experimental methods

Experiments were carried out on a pilot plant presented in Fig. 1. It was equipped with tubular modules and was capable of operating under pressure from 10.105 to 40.105 Pa. The system consists of a centrifugal pump allowing liquid recirculation and a volumetric pump in charge feeding the recirculation loop. In this way pressure and tangential velocity adjustments are independent. The cross-flow velocity (U) was varied from 1 to 3 m.s -t (Reynolds criterion: 12,700
Preliminary experiments (with pure water, saline solution and the six tested membranes) were carried out at constant concentration; concentrate and permeate were circulated back to the feed tank. Samples of permeate collected every 10 min allowed us to follow variation of permeate flux for different applied pressures. When the equilibrium was reached (after 90 min), steadystate flux and rejection rates of the membranes were evaluated. Additional experiments with saline solution in concentration mode at volume reduction factor of two were performed to separate the cations. In the second part of the study (experiments with seawater and the pre-selected membrane), NF was performed in diafiltration mode (Fig. 2). The experimental volume was 31 and the number of added diavolumes varied from 1 to 5. Concentration mode (the retentate of the initial step was circulated back to the feed tank, thereby increasing salt concentration while the permeate was collected in another reservoir) was

2.4. Membranes

Organic tubular membranes from KochWeizmann and PCI were 1.20m in length and 127.10 .4 m in diameter. The effective membrane area was 0.05 m 2 per tube. Their characteristics are described in Table 2. More selectively, a spiral-wound RO membrane (Helicon RO cartridge-CDRN 95-004) was also used (with Millipore Pro-Lab 1 pilot-plant operating at 40.105 Pa) for the production of osmosed seawater for diafiltration.

Diwater afiltration ~1 R~Iuentate

*--7 (or

VR)

Vp

Permeate Fig. 1. Schematic representation of the pilot plant.

Fig. 2. Schematicprocedure of diafiltration step.

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C. Oumar Anne et al. / Desalination 140 (2001) 67-77

Table 2 Characteristics of the six tested membranes Pure water flux, Jo (m3.m-2.s-l) T = 25oC; Ap = 25.105 Pa

Data from manufacturer Reference

Material

Cut-off (dalton)

AFC 30 MPT 10 MPT 08 MPT 34 MPT 31 MPT 04

Polyamide on polyethersulfone Polysulfone on polypropylene Polyamide on polyethersulfone Polysulfone on polypropylene Polysulfone on polypropylene Polyethersulfone

100 200 200 300 400 >1000

also evaluated as a complementary technique to the diafiltration step with the aim o f obtaining an acceptable fractionation of mono- and multivalent cations. In this case the operating pressure was maintained constant at 25.105 Pa.

30.10-6 170.10-6 15.10-6 26.10 -6 51.10 -6 38.10 -6

Since volume V0 remains constant during processing, the number o f diavolumes added is equal to the output withdrawn, which can be written as d(DV) _ jv S v°

2. 5.1. Mass balance in dialfiltration

When diafiltration is performed using water containing residual cation concentration Cr, which is the case in the present study, overall mass balance in the course of a diafiltration process can be written:

(2)

dt

By combining Eqs. (1) and (2), we get

-Zo dCR -

(3)

dt -

dCR - J v S C e - Vo 1I° dt

d(DO Cr dt

(1)

The terms in Eq. (1) are defined in the list of symbols. The left-hand member of the equation represents the time variation of the amount of solutes in the load. The first term of the righthand member expresses the molar (or mass) output o f solutes flowing through the membrane, while the second term corresponds to the amount o f solutes provided per unit of time in the load by additions o f diavolumes.

By introducing the rejection rate (R) o f the membrane for the solute, we get

R -- 1 -

c~ CR

(4)

The equation o f overall mass balance becomes

(l-R)

CR-C r = -

J.Sdt

=

-d(DV)

(5)

C OumarAnne et al./Desalination 140 (2001) 67-77 By expressing it as a function of the number of diavohmes (DV) over process duration,

ck

dC R

fCo (1 -R)

CR-C ,

The expression of the concentration factor of a solute as a function of the resulting membrane rejection rate and the volume reduction factor may be written as:

DV

=

-fo

(6)

CF= 1 - ( 1 - R 1V R)F

If the operation is carried out with high operating pressure and ionic charge, the rejection rate may be considered constant [8-10], and after integration, the equation becomes

DV-

1 ln[ (1-R) C°-C'] 1-R [ ( l - R ) CR-C r

(7)

2.5.2. Mass balance in concentration mode In concentration mode, the molar (or mass) balance on a solute may be expressed as:

VoCo= VRCR+V Cp

(8)

On the other hand, volumetric balance can be written as: Vo = VR + Vp

(9)

Introducing the concentration factor of a solute

(CF):

CF_CR Co

(10)

and the volume reduction factor (VRF):

VRF- Vo

V,,

71

(11)

(12)

3. Results and discussion

3.1. Preliminary testing to select the membrane 3.1.1. Permeability to pure water Fig. 3 shows that water flux increases linearly with pressure (verification of Darcy's Law) for the different membranes except MPT 10. Therefore, the intrinsic resistance to flow of the latter, probably slightly compressible, increases with pressure. 3.1.2. Study with experimental saltwater Variations of permeate flux with pressure (constant concentration) - - By comparing results of Fig. 4 to those of Fig. 3, variation of permeate flux with pressure followed approximately the same trend as in the case of pure water but with flow rates about 10% lower. Variation of rejection rate (constant concentration) - - Figs. 5 and 6, regarding respectively, Na ÷ and Ca 2÷ ions, show that rejection rates usually increase with applied pressure, thereby indicating that the transfer mechanism is partly diffusional. Table 3 shows that membrane AFC 30 allows for better separation of monovalent ions (represented by Na ÷) from divalent ions (represented by Ca2÷), with a rejection rate higher than 98% for the latter and about 56% for the former. Other membranes present either insufficient rejection rates for both aforementioned cations (for

72

C. Oumar Anne et al. /Desalination 140 (2001) 67-77 20

20 -...e.-- MPT I0 ....o.--MPT 31

18 16

&. •

MPT04 AFC 30

•t

MPT 08

S

%

- . . o - - MPT Io - - - o - - M P T 31 .k MPT04 l AFC 30 • MPT34 .k MPT08

15

14 '~

10

~'E 10 s

4

o

2 0

20

10

30

A P (10 5 Pa) --

T

f

t

0

10

20

30

A P (10 s Pa)

Fig. 3. Variation of membrane pure water flux with operating pressure.

Fig. 4. Variation of membranes permeation flux with operating pressure (saline solution).

100

I00 ,,L

MpT08

.t

MPTO8

II

AFC 30

•

~FC 30

A

MPT04

80

• MPT34 ---o--- M pT 3 ! ~ M P T

~lk

/

1 MPT 04 ---O-- MPT ] 1 • MPT34 ~MPT I0

10

~,

[]

t

r'l

[]

6O

A

r,,

r'l

~

f

40

20 20.

10 '

2o

30

A P (10 s Pa) ~0

20

30

P 110 s Pa I

Fig. 5. Variation of membranes rejection rate (for Na+) vs. pressure (saline solution, constant concentration).

Fig. 6. Variation of membranes rejection rate (for Caz+) vs. pressure (saline solution, constant concentration).

example MPT 31 and MPT 04), or rejection rates very close to each other for the different cations (case o f MPT 08). • Concentration tests - - In this part of the study, our purpose was first to compare for a volumetric reduction factor of two the values o f ratio r and o f the solutes concentration

factor (CF) for the different membranes (Table 4). Ratio r, the value o f which before concentration is about 7.8, is slightly modified (2-4% only) by membranes MPT 10, MPT 08, MPT 04 and MPT 34, while membranes AFC 30 and MPT 31 will reduce the same

73

C. Oumar Anne et al. / Desalination 140 (2001) 67-77

Table 3 Comparison of rejection rates R (%) for Na÷ and Ca2+of different membranes lAP = 25.105 Pa, T = 25°C] (saline solution)

AFC 30 MPT 10 MPT 34 MPT 31 MPT 08 MPT 04

Na÷

Ca2÷

56.40 12.06 38.09 20.39 74.23 44.72

98.30 37.51 52.40 69.29 96.67 78.47

AFC 30 MPT 10 MPT 34 MPT 31 MPT 08 MPT 04

Table 5 Concentration experiments. Variations of ratio r and of concentration factor with VRF. Membrane AFC 30 [AP=25.105 Pa, T=-25°C] (saline solution) VRF1 VRF2 VRF3 VRF4 Ratio r CF global CF (for monovalents) CF (for divalents)

Table 4 Concentration mode (VRF = 2). Values of ratio r and solute concentration factor (CF) obtained with different membranes [AP=25.10 s Pa, T=-25°C] (saline solution)

7.8 1.00 1.00

6.6 1.52 1.47

5.6 1.98 1.87

5.0 2.30 2.13

1.00

1.98

2.99

3.81

r

CF

6.60 7.61 7.48 6.81 7.56 6.88

1.52 1.09 1.22 1.18 1.65 1.33

performing a diafiltration o f the water so as to sharply reduce the content in monovalent cations. As it also results in a slight decrease o f divalent ions content, diafiltration will have to be followed with a concentration o f the solution and possibly a dilution in osmosed water to adjust contents to desired levels according to the procedure schematized in Fig. 7. 3.2. Testing on s e a w a t e r 3.2.1. Diafiltration

ratio o f about 15%. Membrane MPT 31, while allowing for a good separation o f cations with different valencies, presents a rejection rate of divalent cations too weak for sufficient a concentration factor with VRF= 2. Membrane AFC 30 is therefore best suited to the rest of the study. Thus, we examined the evolution of ratio r according to a volumetric reduction factor for the membrane (Table 5). However, the content in monovalent cations increases too sharply during concentration: it is actually multiplied by 2.13 for VRF = 4. The observation, as well as advice from other authors [11,12], suggest using, in the case o f seawater, a different procedure consisting of

As pointed out in the introduction, diafiltration is not performed with pure water, but with osmosed seawater, the composition o f which is shown in Table 1. In Fig. 8 we plotted the evolution o f concentration CR o f the different cations in the retentate, up to a number o f diavolumes DV=5, and on Fig. 9, that o f rejection rates R. One observes that R roughly remains constant for divalent cations, with an 86% average for Ca 2+ and 91% for M g 2÷. The difference in retention rate values o f cations with similar valences, however puzzling by simply considering molecular weights, may be due to the fact that magnesium ions, although smaller, display larger hydrated diameters (0.80 rim) than those o f calcium ions (0.46 nm) [13]. Regarding

74

C. Oumar Anne et al. / Desalination 140 (2001) 67-77 revcrsc osmosed seawater

reverse osmoscd seawater

Feed (scawatcr)

J °"°"°"°n

I,

PCrlllCatc

des~lllc,d wiilcr wilh desired niliO r

Pcrlll¢;ilc

Fig. 7. Schematic procedure for partial desalination of seawater by nanofiltration.

1000

• . . . . . • O

Mg2-lmod¢l Ca2-~exp erimental.

[]

Ca2-~aodel K+experimental.

- .... " ~"

~

-41,."~

-4k..

~.

~.

100

_

. . . .

~ .,IK...

J"

,k

,,=I 0

N a - ~ x p e r i m e n t al. N a ~ n o d el M g 2 - l e x p e r i r n e n t nl.

K4model

4b

JL

J.

0. -

[]

4

5

E

~ 10 "--I~-._ "IS- . . . . . . .

2

3 DV

Fig. 8. Diafiltration o f seawater: variation of cation concentration with the number o f diavolumes (DV). Comparison o f experimental results with calculated values. [AP--25.105 Pa, T=25°C], AFC 30 membrane.

monovalent ions, on the other hand, the retention rate markedly increases from D V = 2. In Fig. 8, we also plotted the theoretical curve obtained from relation (7), with R equal to the first measured value. We observed a correct similarity between measured values and those calculated from the model of relation (7) for divalent cations. For monovalent ions, experimental values diverge from those calculated from a

number of diavolumes D V = 3 . This may be explained because relation (7) is based on the assumption of a constant rejection rate R, whereas the latter actually increases with the number of diavolumes. Relating R to operating parameters (DV, salinity, r) would allow improving the validity of this predictive model. Furthermore, Fig. 10 shows that ratio r does not vary significantly beyond D V = 3 . From the

C. Oumar Anne et al. /Desalination 140 (2001) 67-77 I00

75

10

80

60

~e

40

• Nm+ ~Ci2+ £ Mg2+

20

0 0

I 1

q 2

I 3

I 4

1

2

3

4

5

DV

I 5

DV Fig. 9. Diafiltration of seawater: variation of rejection rate (R) with the number of diavolumes (DV). [AP = 25.105 Pa, T = 25°C], AFC 30 membrane.

Fig. 10: Diafiltration of seawater: variation of ratio r with the number of diavolumes (DV). [AP=25.105 Pa, T=25°C], AFC 30 membrane.

Table 6 Concentrations of cations at the beginning and end of the concentration step for seawater (VRF= 1.5); cation concentration factors and membrane rejection rates are measured and calculated from Eq. (12) (concentrations in mol.m- 3), lAP = 25.105 Pa, T = 25°C] Retentate Initial

Permeate Final

CF

Final concentration (mol.m-3)

1.33 1.33 1.43 1.45

51.08 -0.67 2.80

R (%), measured

R (%), theoretical from Eq. (12)

62.77 -90.55 95.15

74.43 74.43 90.21 93.10

Concentration (mol.m-3) Na ÷ K÷ Ca2÷ Mg 2+

103.00 3.78 4.94 37.63

137.20 5.04 7.09 57.75

e c o n o m i c point o f view, insofar as the efficiency o f diafiltration processes both depends on treatment duration and m e m b r a n e selectivity [9], it seems therefore inappropriate to pursue diafiltration further than D V = 3, considering the quality o f the diafiltration water used.

3.2.2. Concentration o f retentate after diafiltration (VRF = 1.5) and dilution with osmosed water Table 6 gives the initial and final concentrations o f the different cations in the retentate, the average concentrations in the permeate as

76

C. Oumar Anne et al. / DesaBnation 140 (2001) 67-77

Table 7 Solution characteristics obtained after the different steps of the process (separation of cations in seawater): diafiltrationconcentration-dilution (concentrations in mol.m3)

Initial seawater Diafiitration (DV=3) Concentration ( VRF = 1.5) Dilution (5 volumes for 1 volume of osmosed water)

[Na÷]

[K+]

[Ca2+]

[Mg2+]

Y', [cations]

r

470.00 103.00 137.20 119.78

11.00 3.78 5.04 4.23

9.00 4.94 7.09 5.94

51.00 37.63 57.75 48.21

541.00 149.35 207.08 178.16

8.00 2.51 2.19 2.29

well as the concentration factor and rejection rates derived from concentrations at the end o f processes. The table also indicates rejection rates calculated with relation (12). The agreement between measured and calculated values is satisfactory within about 2% for divalent cations. The 12% or so difference for monovalent cations shows that their rejection rates are far more sensitive to concentration variations, as we previously observed during diafiltration tests in which they sharply vary with the number of diavolumes (DV). The ratio r, calculated from concentrations indicated in Table 6, shifts from 2.51 at the beginning of processing to 2.19 at the end o f concentration. However, due to the relatively high rejection rate o f monovalent ions (R = 63% for Na÷), the percentage of the latter in the retentate becomes too high compared with the initial purpose (about 120 mol.m -3, r = 2.3). The method used to bring the content back to the desired level consisted of diluting the retentate in osmosed water used for diafiltration in the proportions o f 21 of osmosed water for 51 o f retentate. Table 7 sums up the evolution of global salinity, the contents in monovalent and divalent cations and ratio r during the different processing steps.

4. Conclusions The present work aimed at studying the capabilities of NF membranes to selectively demineralize seawater and suggesting a series of processing steps to obtain a final salinity and a ratio r pre-established "number o f monovalent cations over number o f divalent cations". The goal was achieved using two methods: * concentrating seawater diluted beforehand to preferentially increase its contents in divalent cations; • diafiltering seawater to entrain monovalent ions; then readjusting contents in divalent cations by concentration and possible dilution. A first experimental study on synthetic water containing the same major cations (and in the same proportions) as seawater showed that, among the NF membranes tested, membrane AFC 30, displaying rejection rates higher than 90% with divalent cations and about 50% with monovalent cations, is the most appropriate to resolve the problem. However, when concentrating the water to a volumetric reduction factor o f 4, the content in monovalent ions increases too sharply to obtain the desired ratio. Therefore, the second part of the study was carried out directly on seawater with membrane

C. Oumar Anne et ai. / Desalination 140 (2001) 67-77

AFC 30, starting with a diafiltration step with o s m o s e d seawater. Starting with a number o f three diavolumes ( D V = 3), ratio r hardly varies at all, in keeping with the predictive model suggested. Therefore, it is not wise economically to pursue diafiltration beyond that limit. Diafiltration must be followed with a concentration at VRF = 1.5 and a slight dilution in osmosed seawater to obtain the desired water quality with 178 cations moles per m 3 and a ratio about 2.3. The present w o r k constitutes a first approach. Process durations might actually be reduced by using washwater with lower salinity.

5. Symbols cp

--

Co

--

c,

--

Jo

- -

Jv

- -

Ap R F

--

S

m

T t U Vo r'p

--

vR

--

--

Solute concentration in the permeate, mol.m -3 Solute concentration in the retentate, mol.m -3 Solute concentration in the initial bulk, mol.m -3 Solute concentration in osmosed sea water for diafiltration, mol.m-3 Deionized water flux o f a clean membrane, m3.m-2.s -1 Volumic permeation flux, m3.m-2.s- 1 Operating pressure, Pa M e m b r a n e rejection rate Ratio: number o f monovalent cations on number o f multivalent cations M e m b r a n e area, m 2 Temperature, °C Time, s Tangential velocity, re.sInitial volume, m 3 Permeate volumem m 3 Retentate volume, m 3

77

References [1] K. Marquardt, in: H.G. Heitmann, ed., Saline Water Processing: Desalination and Treatment of Seawater, Brackish Water and Industrial Waste Water, VCH Verlagsgeseilschatt, Weinheim, Germany, 1990, pp. 135-147. [2] A. Maurel, in: J.C. Charpentier, ed., Techniques de I'ing6nieur: trait6 de g~nie des proc&16s, J2790, Pads, 1993, pp. 1-24. [3] R. Semp6r~, G. Cauwet and J. Randon, Mar. Chem., 46 (1994) 49. [4] K. Mopper, Z. Feng, S.B. Bentjen and R.F. Chen, Mar. Chem., 55 (1996) 53. [5] C. Guizard, Proc., La nanofiltration darts l'industrie, Techno-Membranes, Montpellier, France, 1998, pp. B1-B17. [6] A.M. Hassan, M.A.K. A1-Sofi, A.S. AI-Amoudi, A.T.M. Jamaluddin, A.M. Farooque, A. Rowaili, A.G.I. Dalvi, N.M. Kither, G.M. Mustafa and I.A.R. AI-Tisan, Desalination, 118 (1998) 35. [7] Afnor: Agence Franceaise de Normalisation, ed., Qualit6 de l'eau, recueil des normes fran~}aises, Vols.l-4, 2nd eel., Pads, 1997. [8] J. Schaep, B. Van der Bruggen, C. Vandecasteele and D. Wilms, Sep. Purification Technol., 14 (I 998) 155. [9] B. Dutr6 and G. Tr~gardh, Desalination, 95 (1994) 227. [10] K. Mehiguene, Y. Garba, S. Taha, N. Gondrexon and G. Dorange, Sep. Purification Technol., 15 (1999) 181. [11] B.A. Asbi and M. Cheryan, Desalination, 86 (1986) 49. [12] F. Ren6, J.C. Leuliet, B. Chaufer, H. Bamier, C. Fonade and M.Y. Jaffrin, in: G. Daufin, F. Ren6 and P. Aimar, eds. Les s6parations par membrane darts les procAd6s de l'industrie alimentaire. Collection: Sciences et Techniques Agroalimentaires. Lavoisier Tee & Doe, Pads, 1998, pp. 139-218. [13] G.M. Rios, R. Joulie, S.J. Sarrade and M. Carl6s, AIChE J., 42 (1996) 2521.