Desalination 199 (2006) 345–347

Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy.

0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved.

Utilization of nanofiltration membranes for whey and milk ultrafiltration permeate demineralization

Eva Suáreza, Alberto Loboa, Silvia Alvarez-Blancob*, Francisco A. Rieraa,Ricardo Álvareza

aDepartment of Chemical and Environmental Engineering, University of Oviedo, C/Julián 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 9638-79630; Fax + 34 9638-77639; email: sialvare@iqn.upv.es

Received 28 October 2005; accepted 3 March 2006

1. Introduction

Whey and milk ultrafiltration permeate (MUP)are both wastewaters obtained during the industrialproduction of cheese. Whey is the liquid that isseparated from the cheese after the coagulation ofthe milk and MUP is obtained when milkis concentrated prior to cheese production. Wheycontains proteins (8.4 g/L), lactose (79 g/L), fat(<2.5 g/L) and mineral salts (5.6–8.4 g/L), whileMUP composition is similar to that of skim milkexcept for the negligible amount of proteins that ithas. Both streams show a very high COD value(about 60,000 mg/L) and therefore they cannot bedrained without a treatment. Moreover, valuablecompounds (protein, lactose) can be found in theircomposition. Thus they could be used for themanufacture of food products such as ice-creamsor cooked products [1]. However, they show avery high mineral salt content that should bereduced. In this work, the membrane technique ofnanofiltration (NF) is used to carry out a partialdemineralization of whey and MUP.

2. Materials and methods

The experiments were carried out in a com-pletely automated NF pilot plant, which was spe-cially designed for this work at the University ofOviedo. An aromatic polyamide spiral woundmembrane supplied by Osmonics (USA) wasselected (DK2540C model). It has a molecularweight cut-off of 200 g/mol, a 48 mil spacer andan active surface of 1.8 m2. Feed streams weresupplied by CAPSA (Spain). Feed, permeate andretentate were analyzed for lactose, protein,ashes, ions (Ca2+, Na+, K+, Mg2+, P, Cl-) and totaldry extract content. Permeate flux, ion retentionand the degree of ion removal were measured.Different operating conditions were tested. Dis-continuous diafiltration was also considered toincrease ion removal.

3. Results and discussion

In Fig. 1 permeate flux is plotted vs. volumeconcentration ratio (VCR). It can be observed thatpermeate flux decreases with concentration dueto the increase in the osmotic pressure. Permeate*Corresponding author.

doi:10.1016/j.desal.2006.03.081

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flux was slightly lower for the experiments per-formed with whey due to the presence of proteins.When whey is nanofiltered, the concentration ofproteins near the membrane surface increases withpressure due to the formation of a concentrationpolarization layer. If the concentration is highenough a gel layer could be formed, which repre-sents an additional resistance to permeate flux.

Ion retention can be explained by steric aswell as charge effect [2]. The retention ofmonovalent ions was much lower than the reten-tion of divalent ions, due to their lower molecu-lar weight. The highest retention was observedfor Ca2+, followed by Mg2+, phosphorus, Na+

and K+, which showed similar retention, and Cl-,respectively. Chloride showed negative reten-tion for high values of VCR due to the Donnaneffect. At the pH of whey and MUP (6.2–6.8)the proteins that form the gel layer and themembrane are negatively charged. Therefore,the permeation of monovalent cations isfavoured. In order to maintain the electro-neu-trality of the system, an equivalent permeationof negatively charged ions is expected. Due toits small size, chloride is the only anion that caneasily cross the membrane, while the rest of theanions are highly rejected because of steric hin-drance. Therefore, due to this effect, chloride

ions could cross the membrane even against theconcentration gradient.

Ion retention slightly decreased with concen-tration. For both feed streams protein retentionwas observed to be approximately 100% for allthe operating conditions tested, while lactoseretention decreased from 99.8% to 97.5% as theconcentration increased. Retention was slightlyhigher in the case of whey due to the presence ofproteins.

The degree of demineralization significantlyincreased with VCR both, when whey and MUPwere nanofiltered. The degree of demineraliza-tion for the divalent cations was negligible forboth feed streams. For VCR 4, when whey wasconcentrated, the degree of chloride removalwas approximately 60%, while the degree ofmonovalent cations removal was 30–40%. Thedegree of overall ashes reduction was 27%.When MUP was nanofiltered, for VCR 4.7 thedegree of chloride removal was about 77%; thedegree of monovalent cations demineralizationwas 40–50% and the degree ashes removal was36%.

The degree of ion removal increased when adiafiltration step was considered, but lactoseloses increased as well, especially in the case ofMUP, where lactose rejection was slightlylower. When a discontinuous diafiltration stepwas performed ion removal increased up to 37%in the case of whey and to 38% in the case ofMUP. Therefore this step is not justified in thecase of MUP.

4. Conclusions

The selected membrane was able to signifi-cantly reduce the concentration of species thatnegatively affect the quality of the product, suchas NaCl, while ions with high nutritional value,such as Ca2+, were recovered in the process. Pro-tein and lactose losses were observed to be neg-ligible as well.

Fig. 1. Permeate flux vs. volume concentration ratio(VCR) at a cross-flow velocity of 0.6 m/s, a transmem-brane pressure of 2.0 MPa and a temperature of 15°C.

05

1015202530354045

1 2 3 4 5VCR

J (L

/h m

2 )

WheyMUP

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References

[1] R. Jeantet, J. Rodríguez and A. Garem, Nanofil-tration of sweet whey by spiral wound organicmembranes: impact of hydrodynamics, Lait, 80(2000) 155–163.

[2] Y. Xu and R.E. Lebrun, Investigation of the soluteseparation by charged NF membranes: effect ofpH, ionic strength and solute type, J. Membr. Sci.,158 (1999) 93–104.