Demineralization of whey and milk ultrafiltration permeateby means of nanofiltration

E. Su�areza, A. Loboa, S. Alvarezb*, F.A. Rieraa, R. �Alvareza

aDepartment of Chemical and Environmental Engineering, University of Oviedo,C/ Juli�an Clavería, 8, 33006 Oviedo, Spain

bDepartment of Chemical and Nuclear Engineering, Polytechnic University of Valencia,Camino de Vera s/n, 46022 Valencia, Spain

Tel. +34 963879630; Fax +34 963877639; email: sialvare@iqn.upv.es

Received 8 July 2007; revised 25 October 2007; accepted 1 November 2007

Abstract

In this work, the membrane technique of nanofiltration (NF) was used to carry out partial demineralization ofwhey and milk ultrafiltration permeate (MUP). The experiments were performed in a completely automated NFpilot plant. An aromatic polyamide spiral wound membrane supplied by Osmonics (USA) was selected(DK2540C model). Batch and discontinuous diafiltration (DF) configurations were compared. Permeate fluxeswere higher for MUP due to the lower amount of proteins, which can cause severe membrane fouling. The degreeof demineralization for the divalent cations was negligible for both feed streams. Protein and lactose permeationwere also observed to be very low. Ion permeation was higher for MUP probably due to the lower amount ofproteins. When discontinuous DF was performed, ion removal increased in the case of whey, but it did notsignificantly improve in the case of MUP. The degree of ion removal was higher than 30%. Finally, the DonnanSteric Partitioning pore Model was used to predict lactose and ion rejection. A good agreement between predictedand experimental data was observed.

Keywords: Nanofiltration; Whey; Milk ultrafiltration permeate; Demineralization; Modelling

1. Introduction

Nanofiltration (NF) is a membrane techniquethat is mainly used to remove ions from a feed

solution. In this work, it has been used toperform partial demineralization of whey andmilk ultrafiltration permeate (MUP). Both arewastewaters obtained during the industrial pro-duction of cheese. Whey is the liquid that isseparated from the cheese after the coagulation

Presented at the Third Membrane Science and Technology Conference of Visegrad Countries (PERMEA), Siofok,Hungary, 2–6 September 2007.

*Corresponding author.

Desalination 241 (2009) 272�280

0011-9164/09/$– See front matter # 2009 Published by Elsevier B.V.doi:10.1016/j.desal.2007.11.087

of the milk. It contains proteins (8.4 g/L), lactose(79 g/L), fat (B/2.5 g/L) and mineral salts (5.6�8.4 g/L). MUP is obtained when milk is concen-trated prior to cheese production. Its compositionis similar to that of skim milk except for thenegligible amount of proteins that it has. Bothstreams show a very high COD value (about60,000mg O2/L) and therefore they cannot bedrained without a treatment. Moreover, valuablecompounds (protein, lactose) can be found intheir composition. Thus they could be used forthe manufacture of food products such as ice-creams, drinks, desserts or cooked products.However, they show a very high mineral saltcontent that should be reduced, as it negativelyaffects the quality of the product [1,2].

Whey demineralization can also be performedby means of ion exchange and electrodialysis.However, both processes require high capital andoperating costs and show regeneration and foulingproblems, respectively [2]. By means of NFdemineralization and pre-concentration can becarried out at the same time. NF membranes areoften used for the separation of charged solutes[3]. This process has been sometimes used to treatdairy products [4,5]. However, several authorsreported selectivity (losses of lactose) and pro-ductivity (fouling) problems [6].

Several models have been proposed to prediction rejection inNF.Most of theworks are based onthe extended Nernst�Planck (ENP) equation. Oneof those models is the Donnan Steric PartitioningporeModel (DSPM) [7�11]. Thismodel describesthe transport of ions in terms of the effectivemembrane thickness�porosity ratio and the effec-tive membrane charge density. According to themodel, solute retention is explained by the combi-nation of steric hindrance, solvation energy,Donnan exclusion and dielectric exclusion effects.

The main objective of this work was toreduce the mineral salt content of whey andMUP by means of NF, while the amount ofvaluable compounds (protein, lactose) was

desired to remain constant. Batch and diafiltra-tion (DF) configurations were compared in termsof ion removal. Moreover, the DSPM was usedto predict lactose and ion rejection.

2. Materials and methods

Whey and MUP supplied by CAPSA (Spain)were used as feed streams. Their composition isshown in Table 1.

The experiments were carried out in an auto-mated NF pilot plant specially designed for thiswork, which was described in a previous paperandwas equippedwith a temperature and pressurecontrol system [12]. A DK2540C aromatic poly-amide spiral wound membrane supplied byOsmonics (USA) was selected. The molecularweight cut-off of the membrane was 200 g/moland the total effective area 1.8m2. Feed, permeateand retentate were analysed for lactose, protein,

Table 1Composition of the whey and MUP used as feeda

Parameter Whey MUP

pH 6.21�6.70 6.44�6.77Conductivity (mS) 4.67�6.04 5.46�5.77Density (g/L) 1021�1023 1025�1027Dornic acidity (8D) 6.9�11.6 4.0�8.5Total dry extract (%) 4.63�5.71 5.27�5.36Lactose (g/L) 35.90�44.26 45.83�48.12Total protein (%) 0.536�0.742 0.011�0.072Serum protein (g/L) 4.96 0.053�0.286a -Lactalbumin (g/L) 1.09 0.053�0.225BSA (g/L) 0.21 NDb -Lactoglobulin (g/L) 3.17 ND�0.061Immunoglobulin (g/L) 0.49 NDNPN (%) 0.0372 0.033Ashes (%) 0.377�0.539 0.44�0.47Ca (ppm) 231.6�384.8 254�277Na (ppm) 313�493 405�461K (ppm) 1055�1535 1625�1742Mg (ppm) 54.4�73 75�82P (mg/100 g) 23.71�27.50 32.96�44.35Cl� (ppm) 1000�1418 1050�1108

aND, non-detectable; NPN, non-protein nitrogen.

E. Su�arez et al. / Desalination 241 (2009) 272�280 273

ashes, ions (Ca2�, Na�, K�, Mg2�, P, Cl�) andtotal dry extract content. The analytical methodsused are indicated in Table 2 [12]. Differentoperating conditions were tested. Batch NF anddiscontinuous DF were compared in terms of ionremoval and permeate flux. To perform the DFexperiments, unionised water was added to thefeed tank when the volume concentration ratio(VCR) reached 4.0, so that the initial volume wasreconstituted. Those experiments devoted to testthe DSPM were carried out with model solutionsof lactose (1 g/L) and KCl (0.1M).

3. The model

The main equation of the DSPM is the ENPequation, which is represented by the followingexpression [7�11]:

ji ¼ �Di;p

dcmi

dx� zic

mi Di;p

F

RT

dC

dxþ Ki;cc

mi Jv ð1Þ

where Di,p�/Di,�Ki,d and Jv�/ji/ci,p. In theabove equations, ji is the flux of component i(mol/m2 s), /cmi and ci,p are the concentration ofcomponent i in the membrane and in thepermeate, respectively (mol/m3), zi is the valenceof component i , F the Faraday constant (C/mol),R the ideal gas constant (J/mol K), T the absolutetemperature (K), C the electric potential (V) inthe axial direction (x), Ki,c and Ki,d the hindrancefactors for convection and diffusion, respectively,Jv the permeate flux (m/s), Di,� the diffusivity ofcomponent i in the bulk solution and Di,p is thehindrance diffusivity of component i inside thepores (m2 s−1).

The first, second and third terms in Eq. (1)represent the transport due to diffusion, electro-migration and convection, respectively. In orderto predict solute retention, the procedure pro-posed by Labbez et al. [14] was followed. Thehindrance factors Ki,d and Ki,c can be calculatedfrom the ratio of the Stokes solute radius to thepore radius, li, and the steric partition term thataccounts the finite size of component i , fi ,respectively, using Eqs. (2) and (3), where 0 B/

l B/ 0.95 [15].

Ki;d¼1:0� 2:3li þ 1:154l2i þ 0:0224l3i ð2ÞKi;c¼ð2� �iÞ

� ð1:0þ 0:054li � 0:998l2i þ 0:44l3i Þ ð3Þ�i ¼ 1� lið Þ2 ð4Þ

The Stokes radius of component i is given by:

ri ¼kBT

6��Di;1ð5Þ

where kB is the Boltzmann constant and m

represents the dynamic viscosity of the solution.The concentration gradient of component i

over the membrane can be derived from the ENPequation:

Table 2Analytical methods

Parameter Method

Dornic acidity Titration with 0.111 N NaOHpH pH-meterAshes Incineration in an oven at 5238CConductivity Conductivity meterTotal dry extract(TDE)

Drying in an oven at 1028C until theweight is constant

Lactose Reaction with dinitrosalicylic acidand measurement of absorption at540 nm

NPN Kjeldahl nitrogen determinationafter protein precipitation withtrichloroacetic acid

Serum protein HPLC using a reverse phase PLRP-S 300 column of 150�/7.5mm(Polymer Laboratories Inc., USA)and a Hewlett Packard (USA) HP1050 HPLC chromatograph [13]

Ca, Na, K, Mg Atomic absorption (Model 2100,Perkin Elmer, EEUU)

Cl� Chloride ion-selective electrode(Inolab, Merck, EEUU)

Total phosphorus Reaction with ammonic molibdova-nadate and measurement ofabsorption at 400 nm

274 E. Su�arez et al. / Desalination 241 (2009) 272�280

dcmi

dx¼ Jv

Di;p

Ki;ccmi � ci;p

� �� ziF

RTcmi

dc

dxð6Þ

The potential gradient through the membranecan also be derived from the ENP equation andcan be expressed as:

dc

dx¼Pn

i¼1ziJvDi;p

Ki;ccmi � ci;p

� �FRT

Pn

i¼1 z2i cmið Þ ð7Þ

where n is the number of solutes.The electroneutrality conditions inside and

outside the membrane are expressed, respec-tively, by means of the following equations:

Xni¼1

zicmi ¼ �X ð8Þ

Xni¼1

zici ¼ 0 ð9Þ

where ci is the bulk concentration of componenti and X the effective volume charge of themembrane. The zero electric current conditioninside the membrane can be written as

Ic ¼Xni¼1

F zijið Þ ¼ 0 ð10Þ

were Ic is the current density.The concentration of component i at both

membrane/solution interfaces is determinedusing the Donnan equilibrium condition:

cmi

ci

!¼ �i exp

�ziF

RTDcD

� �ð11Þ

where DcD is the Donnan potential. The bound-ary conditions are defined as:

ci ¼ ci;f at x ¼ 0 and ci ¼ ci;p at x ¼ Dx

where ci,f is the feed concentration of componenti at the interface membrane/solution and Dx isthe membrane thickness. The terms ci,p andci,f represent concentrations just outside themembrane.

The true retention of component i is definedas follows:

Rreali ¼ 1� ci;p

ci;fð12Þ

The apparent (measured) rejection (Ri) isdescribed by the following equation:

Ri ¼ 1� ci;p

ci;bð13Þ

where ci,b is the bulk concentration of compo-nent i , which differs from the concentrationdirectly in contact with the membrane (ci,f) dueto concentration polarisation. By taking intoaccount the film layer model for concentrationpolarisation

ci;f ¼ ci;p þ ci;b � ci;p

� �eJvki ð14Þ

Where ki is the mass transfer coefficient ofcomponent i , which was calculated according tothe Da Costa et al. correlation (1994) [16]:

Sh ¼ kidh

Di

¼ 0:664� kdcRe0:5Sc0:33

2dh

lm

!ð15Þ

In this equation Sh, Re and Sc are Sherwood,Reynolds and Schmidt numbers, respectively, dhthe hydraulic diameter of the membrane, Di thediffusion coefficient, kdc a parameter character-istic of the spiral wound membrane spacer and lmis the length of the fibres that form themembrane spacer.

Solute retention can be determined by solvingthe above set of equations. For that purpose it isnecessary to know the membrane pore radius(rp), the ratio of membrane thickness to porosity(Dx /Ak) and the effective volume charge of themembrane (X). Those parameters were experi-mentally determined in this work. The poreradius and the ratio of membrane thickness toporosity were estimated using a 1 g/L lactosesolution. For that purpose, the Hagen Poiseuilleequation (Eq. (16)) was used. In this equationDP (Pa) represents the transmembrane pressure.

E. Su�arez et al. / Desalination 241 (2009) 272�280 275

In the case of uncharged solutes, such as lactose,the second term of the ENP equation is equal tozero and the set of equations is simplified. Theeffective volume charge of the membrane wasafterwards experimentally determined from sev-eral runs performed with a 0.1M KCl solution atdifferent pH values.

Jv ¼r2pDP

8� Dx=Akð Þ ð16Þ

The procedure to simultaneously solveEqs. (6)�(15) is thoroughly described in Labbezet al. (2003) and in Otero et al. (2006) [14,11].The numerical integration of Eqs. (6) and (9)was carried out by means of a fourth orderRunge�Kutta method. To simultaneously solvethe set of equations, the permeate concentrationof each component i were arbitrarily chosen, asthey are unknown. Then by solving Eqs. (6) and(7) accounting for the zero electric currentcondition (Eq. (10)), concentrations across themembrane are calculated and new values of thepermeate concentrations for each component iare obtained. These new permeate concentrationvalues are compared with initial ones. If differ-ences between initial and new values are low, thenumerical calculation is stopped and the compo-nent retentions are calculated. Otherwise, newpermeate concentrations are set and the proce-dure is repeated.

4. Results and discussion

Fig. 1 shows permeate flux during the con-centration of whey and MUP by means of NF.Flux decreased with concentration, as expected,mainly due to the increase in the osmoticpressure. Permeate flux was slightly lower forthe experiments performed with whey due to thepresence of proteins, which may represent asignificant source of membrane fouling. If theconcentration is high enough, they can form agel layer on the membrane surface, which

represents an additional resistance to permeateflux [12].

Fig. 2 shows ion rejection during the con-centration of whey and MUP. It can be observedthat ion retention slightly decreased with con-centration. The effect of concentration on theretention coefficient of chloride was much moremarked. The retention of monovalent ions wasmuch lower than the retention of divalent ions,due to their lower molecular weight. The highestrejection was observed for Ca2�, followed byMg2�, P�, Na� and K�, which showed similarretention, and Cl�, respectively. Chlorideshowed negative retention for high values ofVCR due to the Donnan effect. At the pH valueof whey and MUP (6.2�6.8), the membrane isnegatively charged as well as the proteins thatform the gel layer. Due to the negative charge onthe membrane surface, the permeation of mono-valent cations (counter-ions) is favoured. Inorder to maintain the electro-neutrality of thesystem, an equivalent permeation of negativelycharged ions (co-ions) is observed. The mainanions that can be found in whey and MUPcomposition are chloride, phosphate, citrate andsulphate. Due to its small size, chloride is theonly anion that can easily cross the membrane,while the rest of the anions are highly rejected

05

1015202530354045

Whey MUP

J (L

/h m

2 )

VCR1 2 3 4 5

Fig. 1. Permeate flux during the concentration of wheyand MUP by NF at 158C, 2.0MPa and a flow rate of0.6m/s.

276 E. Su�arez et al. / Desalination 241 (2009) 272�280

because of steric hindrance. Therefore, due to theDonnan effect, chloride permeation is favoured.If concentration is high enough co-ions couldcross the membrane even against the concentra-tion gradient, thus explaining the negativeretention coefficients that can be observed forchloride in this figure.

Ion permeation was observed to be higher inthe case of MUP, probably due to the high amountof proteins present in whey composition. Theprotein gel layer could become an additionalresistance to ion permeation and could also causethe complexation of some ions. It should be notedthat whey concentration was performed at adifferent temperature than MUP concentration,and temperature could also affect permeation.Nevertheless, when whey and MUP NF wereperformed at similar temperatures, ion permeationwas also higher in the case of MUP [12].

Taking into account the apparent retentioncoefficient of each ion, the degree of deminer-alization was calculated as previously described[12]. The degree of demineralization for thedivalent cations was negligible for both feedstreams. For VCR 4, when whey was concen-trated, the degree of overall ashes reduction was27%. When MUP was nanofiltered, for VCR 4.7the degree of ashes removal was 36%.

For both feed streams protein retention wasobserved to be approximately 100% for all theoperating conditions tested; while lactose reten-tion was higher than 99.5% in the case of wheyand ranged between 97.3 and 99.8% in the caseof MUP. The lowest lactose rejection wasobserved for the greatest values of VCR.

In order to try to improve ion removal,discontinuous DF was performed. The permeateflux values obtained are shown in Fig. 3. In thecase of whey, at the beginning of the DFprocesses permeate flux was lower than in thecase of batch NF, because of membrane fouling.However, for VCR values higher than 2.0,permeate flux was higher in the case of dis-continuous DF, probably due to the lower ionconcentration. Ion rejection (especially mono-valent cation rejection) increased when DF wasperformed, due to the lower ion concentrationand the higher membrane fouling. When the firstdiscontinuous DF step was performed, overallion removal increased up to 37%; however, thesecond DF step did not cause a significantimprovement in ion removal. Therefore, as waterconsumption increased to a large extent, thesecond DF step is not justified.

In the case of MUP, permeate flux was verysimilar for batch NF and discontinuous DF. Ion

–20

0

20

40

60

80

100

(A) (B)

VCR

R (

%)

0 1 2 3 4

Cl– Na K Mg Ca P Ashes

–100–80–60–40–20

020406080

100

1 2 3 4 5R (

%)

VCR

0

Fig. 2. Rejection of ions during the concentration of whey and MUP by NF at 2.0MPa and 0.6m/s. Demineralization of(A) whey at 378C and (B) MUP at 158C.

E. Su�arez et al. / Desalination 241 (2009) 272�280 277

rejection was also observed to be higher whenDF was performed, and the overall ion removalincreased up to 38%. Moreover, lactose lossesincreased as well; therefore the DF step is notjustified in this case.

Finally, the DSPM was used to predict lactoseand KCl rejection. A good agreement betweenpredicted and experimental data was observed(Fig. 4). The highest discrepancies were found athigh permeate flux values, but the difference

between calculated and experimental rejectionwas not higher than 10%. Membrane charge wasobserved to be a function of pH. The DK2540Cmembrane was negatively charged at the pHvalues tested. The highest ion retention wasobserved for the pH values that corresponded tothe highestmembrane charge density. Thismay bedue to the electrostatic repulsion between thechloride ions and the membrane surface. Becauseelectroneutrality of the permeate solution must be

Without DF DF1 DF2

0

10

20

30

40

50

60

70

(A) (B)

VCR

J (L

/h m

2 )

1 1.5 2 2.5 3 3.5 4

Concentration Discontinuous DF

0 1 2 3 540

5

10

15

20

25

30

35

40

20

J (L

/h m

2 )

VCR

Fig. 3. Permeate flux during the demineralization of whey and MUP at 2.0MPa and 0.6m/s by means of batch NF anddiscontinuous DF. Demineralization of (A) whey at 378C and (B) MUP at 158C.

Rreal Rcalculated

Jv (m/s × 106)

0.9

0.92

0.94

0.96

0.98

1

(A) (B)

R

0 10 20 30 40 50

pH 3.0pH 3.4

pH 4.0pH 5.0

pH 6.0pH 8.0

0

0.1

0.2

0.3

0.4

0.5

0.6

R

Jv (m/s × 106)0 10 20 30 40

Fig. 4. Comparison between experimental (symbols) and predicted (lines) rejection when a 1 g/L lactose solution (A)and a 0.1M KCl solution (B) were nanofiltered.

278 E. Su�arez et al. / Desalination 241 (2009) 272�280

maintained, the Na� will also be rejected. More-over, several authors demonstrated that the size ofpolyamide membrane pores is reduced as themembrane charge increases due to conformationalchanges as a result of repulsion between chargedCOO� groups. Therefore, as pore size decreasessolute rejection increases [17].

5. Conclusions

NF was observed to be an effective method toperform partial demineralization of whey andMUP. Permeate flux was lower when whey wasnanofiltered probably due to the presence ofproteins, which can form a gel layer on themembrane surface. For the same reason, thedegree of mineral salt removal was observed tobe higher in the case of MUP. Losses of lactoseand protein in the permeate stream were very low.Divalent ion permeationwas very low aswell. Thedegree of mineral salt removal increased withVCR. Bymeans of batch NF, the degree of overallion removal increased up to 27%atVCR4.0 in thecase of whey; when MUP was nanofiltered atVCR 4.7, the degree of ashes removal was 36%.By means of DF, the degree of ashes reductionincreased up to 37% in the case of whey and to38% in the case of MUP. Therefore, as waterconsumption and lactose losses increases, the DFstep is not justified in the case of MUP.

The DSPM was able to predict lactose andsalt rejection when model solutions were used asfeed streams. The divergence between predictedand experimental rejection was lower than 10%for all the operating conditions tested.

Acknowledgements

This work was supported by the SpanishMinistry of Education (Project number:PPQ2000-1112). The authors also wish to thankthe dairy company CAPSA for the whey andMUP supplied.

Nomenclature

ci Concentration of component i(mol/m3)

ci,b bulk concentration of component i(mol/m3)

/cmi concentration of component i in themembrane (mol/m3)

ci,p concentration of component i in thepermeate (mol/m3)

ci,f concentration of component i in theretentate (mol/m3)

dh hydraulic diameter (m)Di diffusion coefficient of component i

(m2/s)Di,� diffusivity of component i in the bulk

solution (m2/s)Di,p diffusivity of component i inside the

pores (m2/s)F Faraday constant (C/mol)Ic current density (A/m2)ji flux of component i (mol/(m2 s))Jv permeate flux (m/s)kB Boltzmann Constant (J/K)kdc parameter characteristic of the spiral

wound membrane spacerki mass transfer coefficient (m/s)Ki,c hindrance factor for convectionKi,d hindrance factor for diffusionlm length of the fibres that form the

spacer (m)R ideal gas constant (J/(mol K))Ri apparent retention coefficient of com-

ponent iRreali real retention coefficient of component iri Stokes radius of component i (m)rp pore radius (m)T absolute temperature (K)x axial directionzi valence of component ifi steric partition termDP transmembrane pressure (Pa)Dx /Ak ratio of membrane thickness

to porosity (m)

E. Su�arez et al. / Desalination 241 (2009) 272�280 279

X effective membrane volume charge(mol/m3)

li ratio of the Stokes solute radius to thepore radius

m dynamic viscosity of the solution(kg/(m s))

c electric potential in axial direction (V)DcD Donnan potential (V)

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