A Flux Enhancing Pretreatment for the Ultrafiltration of Acid Whey

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A flux enhancing pretreatment for the ultrafiltrationof acid whey

M.C. Almcija*, A. Guadix, A. Martinez-Ferez, P. Gonzlez-Tello, E.M. Guadix

Department of Chemical Engineering, University of Granada, 18071 Granada, SpainTel. +34 958 243 308; Fax +34 958 248 992; email: [email protected]

Received 30 June 2008; revised 03 February 2009; accepted 09 February 2009

Abstract

The reduction in permeate flux caused by fouling is a drawback in the operation of membrane systems that canbe alleviated using mechanical and electrical methods. Alternatively, physicochemical treatments provoking theelimination of fouling agents have been proposed as effective strategies for the enhancement of flux. A procedure(comprising salt addition, pH and temperature modification and centrifugation) that removes calcium and phosphatesalts from acid whey without altering the profile of valuable proteins is presented in this research work. It has

been demonstrated through the paper that this pretreatment enhances significantly the permeate flux of whey whencross-flow ultrafiltered through a 50 kDa tubular ceramic membrane both in total recycle and continuous diafil-tration mode. For modelling purposes, a resistances in series model was applied to explain the effect of transmem-

brane pressure and a complete-standard blocking model fitted the experimental data of the variation of permeateflux with time.

Keywords: Ultrafiltration; Acid whey; Ceramic membranes; Flux enhancement

1. Introduction

Membrane fouling during whey filtration

has been extensively studied in the scientific lit-erature [1,2]. This undesirable phenomenoninvolves a reduction in permeate flux and, there-fore, a lower productivity of the filtration process.In order to mitigate fouling, several strategies

have been devised. Some mechanical methods [3]include: (a) turbulence promoters (such as bafflesor moving balls) that reduce holdup in the feed

channel by increasing velocities and wall shearrates; (b) periodic backflushing by pumping the

permeate back to remove material absorbed ontothe membrane; (c) uniform transmembrane

pressure with a permeate pumping loop and (d)pulsatile flow obtained with pistons or rotatingdiscs.

Desalination 245 (2009) 737742

*Corresponding author.

Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and

ModellingFrom the Nano to the Macro Scale(EWM 2008), May 2528, 2008, Vale do Lobo, Algarve, Portugal.

0011-9164/09/$ See front matter 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.desal.2009.02.045


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M.C. Almcija et al. / Desalination 245 (2009) 737742

Other approaches apply an electrical field toa conventional membrane filtration unit, which isknown as electrically-enhanced membrane filtra-tion. Here, the electrical field acts as an additional

driving force to the transmembrane pressure andreduces concentration polarisation. Brisson et al.[4] applied up to 3333 V/m during the microfil-tration of whey protein solutions, where perme-ation was increased by a factor 3. Low frequencyultrasound has also been used to facilitate cross-flow filtration of whey [5]. An enhancement fac-tor of between 1.2 and 1.7 can be achieved at lowirradiation power levels, which can be improvedfurther with the combined use of turbulence pro-moters. Importantly, the membrane integrity is

not altered by the ultrasonic irradiation, accordingto electron micrographs.

Although proteins have been indicated as themain component of the fouling deposit formedduring the processing of whey, calcium and

phosphates have been also directly implicateddue to the formation of insoluble calcium saltsand as possible bridging agents between mem-

brane and proteins or proteins themselves [6].Therefore, precipitation processes involving the

removal of calcium and phosphates, while pre-serving the protein composition, have beendescribed for the palliation of membrane foul-ing. A simple procedure was proposed byMusale and Kulkarni [7] consisting of a pH raiseto 7.5, 10 min holding at 28C and filtrationthrough Whatman paper (No. 1). Kim et al. [8](and posteriorly Rinn et al. [9] at a pilot plantscale) obtained clarified whey by cooling below5C, adding CaCl

2, adjusting to pH 7.3, warm-

ing to 50C and decanting/centrifuging the

resulting precipitate. Gesan et al. [10] modifiedthis pretreatment by increasing the temperatureto 55C and maintaining the pH constant duringthe heat treatment. In this paper, a combinationof these methods is presented with two mainobjectives: To maintain the original protein pro-file and to evaluate its capability of enhancingthe permeate flux when whey is cross-flow

ultrafiltered with a 50 kDa tubular ceramicmembrane.

2. Materials and methods2.1. Obtention and pretreatment of whey

Acid whey was obtained from pasteurizedwhole milk. First, the milk was centrifuged at4C for 30 min in order to remove fat. Then, the

pH was adjusted to 4.2 by adding 2 N HCl forcasein coagulation, which was separated fromthe whey by further centrifugation at the sameconditions. The pretreatment process applied tothe acid whey was as follows: First, CaCl

2up to

a concentration of 1.2 g/L was added to the wheyat 25C. Then, pH was raised to 7.3 using 6 N

NaOH and temperature was increased to 55C.These conditions were held for 8 min, involvingthe aggregation of complex lipidcalcium phos-

phate particles. Finally, the whey was cooleddown to 10C and centrifuged for 30 min. The

precipitate contained the insoluble calcium phos-phate aggregates and the supernatant was theclarified whey.

For both unpretreated and pretreated whey,individual protein concentrations (-lactalbumin,-lactoglobulin, BSA, lactoferrin and Ig-G) weredetermined by reversed-phase high-performanceliquid chromatography (RP-HPLC) using themethod described by Elgar et al. [11] and modi-fied by Palmano and Elgar [12]. The HPLC sys-tem (Waters, Milford MA, USA) consisted of anAlliance Separation Module 2690 interfaced witha M-474 absorbance detector (214 nm) and a Mil-lenium data acquisition and manipulation system.

A 1-mL Resource RPC column (Amersham Bio-sciences, Uppsala, Sweden) was operated at roomtemperature at a flow-rate of 1 mL/min. Solvent Awas 0.1% v/v trifluoroacetic acid (TFA) in Milli-Q water and solvent B was 0.09 % v/v TFA, 90%v/v acetonitrile in Milli-Q water. The column wasequilibrated in 80% solvent A. The gradient usedwas: 01 min, 20% B; 16 min, 2040% B; 616

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min, 4045% B; 1619 min, 4550% B; 1920min, 50% B; 2023 min, 5070% B; 2324 min,70100% B; 2425 min, 100% B; 2527 min,

10020% B; 2730 min, 20% B.

2.2. Ultrafiltration experiments

In order to evaluate the influence of the pre-treatment on the flux enhancement through ultra-filtration membranes, clarified acid whey andunpretreated acid whey were ultrafiltered

employing a 50 kDa tubular ceramic membrane(Tami, France). The pH of unpretreated whey wasset at 7.3, which was the resulting value for the

final pretreated whey.First, the influence of transmembrane pres-

sure on permeate flux was analysed by operat-ing the ultrafiltration rig in the total recyclemode at 30C, a cross-flow velocity of 3.5 m/sand transmembrane pressures between 0 and2.5 bar. Permeate flux was calculated frommass measurements in an analytical balance.

-LA-LGBSAIgG

LF

0.99 g/L3.07 g/L0.12 g/L0.39 g/L

0.04 g/L

BSAIgGLF

1.03 g/L3.09 g/L0.10 g/L0.41 g/L0.04 g/L

(a)

(b)-LA-LG

Fig. 1. Reverse phase HPLC chromatograms of unpretreated (a) and pretreated (b) whey. The concentration of the indi-vidual proteins is shown in the text box.

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0

10

20

30

40

50

60

70

0 1 2 3 4

t (h)

JF,

L/(m2

h)

0

25

50

75

100

0.0 0.5 1.0 1.5 2.0 2.5

P (bar)

JF,

L/(m

2h)

Fig. 2. Permeate flux of unpretreated () and pretreated() whey as a function of transmembrane pressure at

30C for a 50 kDa tubular ceramic membrane.

Fig. 3. Fluxtime profiles for the continuous diafiltrationof unpretreated () and pretreated () whey at 30C fora 50 kDa tubular ceramic membrane. Transmembrane

pressure was fixed at 1.5 bar.Then, the time evolution of permeate flux was

observed in the following operating conditions:temperature 30C, transmembrane pressure 1.5

bar and cross-flow velocity 3.5 m/s. The ultrafil-tration mode was continuous diafiltration, whichwas conducted for 4 h with a constant retentatevolume of 2 L.

3. Results and discussion

With respect to a possible variation in thecomposition of individual proteins between theoriginal acid whey and the pretreated one, accord-ing to the chromatograms depicted in Fig. 1, itcan be concluded that no significant alterationswere caused by the processing steps comprisedin the clarification. Nearly identical concentra-tions were determined for the five proteinsassayed.

The correlation between permeate flux (JF)and transmembrane pressure (P) for both clari-fied and unclarified whey is shown in Fig. 2. Itcan be seen that permeate flow increased 3.03.5-fold when the clarification procedure wasemployed, which demonstrates the applicabilityof the approach proposed.

A linear relationship was obtained in bothcases for transmembrane pressures up to 1 bar,

which is usually referred as the pressure-con-trolled region. For higher pressures, a decreasingslope was observed due to the partial control ofthe mass transfer. This behaviour can beexplained by the resistances in series model:

(1)

where the parameter a takes into account themembrane and fouling resistances and b isinvolved in the computation of the polarisationlayer resistance (which is considered to be pro-

portional to the transmembrane pressure). Thevalues of these parameters for the unpretreated (a= 6.13 102bar m2 h/L, b = 1.24 102 m2 h/L,r2 = 0.999) and treated whey (a = 2.03 102barm2 h/L, b = 2.66 103 m2 h/L, r2 = 0.999) were

employed to calculate the solid lines representedin Fig. 2. The results obtained for the parameterb reflect that a reduction in the resistance associ-ated to the polarisation layer of 4.5 was achievedafter the pretreatment.

In Fig. 3, it is represented the time evolutionof permeate flux when a transmembrane pressureof 1.5 bar was applied. During the continuousdiafiltration, the fluxes droped from the initial

JP

a b PF=

+

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values of 18 and 61 to the final 7 and 19 L/(m2 h)for unpretreated and pretreated whey, respec-tively. At the end of the 4 h of operation, only 0.9diavolumes of unpretreated whey were filtered,while this amount was triplicated in the case ofthe pretreated one.

In order to model the influence of the pretreat-ment in the time variation of flux, the experimen-tal data were successfully fitted to a mathematicalexpression [13] combining complete blocking(provoked by particles arriving to the membranethat block some pores, without superposition, andresponsible of a sharp decrease in flux at the

beginning of the filtration) and standard blocking(caused by particles deposited onto the internal

pore walls, which decrease the pore volume andexplain a slighter flux drop during the rest of theoperation):

(2)

whereJF0 is the initial flux, is the fraction ofpores susceptible of being completely blockedandA andB are the parameters of the completeand standard blocking, respectively. The valuesof these parameters are shown in Table 1.

It can be seen that the removal of phosphateparticles reduced one tenth the percentage ofblockable pores from 97% to 87%. The high val-ues obtained for the exponential parameters in thecomplete blocking term result in a fast kineticsfor this process, in which the whole fraction of

blockable pores was effectively blocked after 11and 4 min of filtration for unpretreated and pre-treated whey, respectively. After these times, the

amount of permeate flux is contributed only bythe standard blocking term. Under these condi-tions, the value for unpretreated whey of the stan-dard blocking parameter was responsible for aflux drop of 60%. Since the same parameter wasdoubled for pretreated whey, a higher decrease of

69% was achieved. In summary, although the pre-treatment increased the kinetics parameters offouling, an overall enhancement in flux wasobtained due to the decrease in the fraction .

4. Conclusion

A pretreatment procedure has been proposedfor the clarification of acid whey in order toenhance the permeate flux through ceramic mem-

branes. Its effectiveness was tested employing a50 kDa tubular membrane in cross-flow opera-tion. This process was able to remove foulingagents but did not affect the composition of indi-vidual proteins. As a result, when considering atransmembrane pressure range up to 2 bar, fluxwas improved by a factor of 3.03.5 with respectto the original whey, which could be related to aresistances in series model.

With respect to the behaviour of pretreatedwhey under continuous diafiltration, the final vol-

ume of permeate collected after 4 h of operationwas triplicated when compared to the unpre-treated whey. The time evolution of flux wasexplained by a mathematical model combiningcomplete and standard blocking. The fitting

parameters showed that the pretreatment step pro-voked a decrease in the fraction of pores suscep-tible of being completely blocked but acceleratedthe fouling mechanisms.

J

J

A t

B t

F

F0e= +

( )

+ ( )

1

1 2

Table 1Fitting parameters of the complete-standard blocking model for the continuous diafiltration of unpretreated and treatedwhey at 30C through a 50 kDa tubular ceramic membrane

A (h1) B (h1) r2

Unpretreated 0.9691 52.40 0.0953 0.9994Treated 0.8655 142.55 0.2088 0.9980

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Acknowledgement

This research work was funded by the SpanishPlan Nacional I+D+I under the ProjectCTQ2005-026/PPQ.

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A Flux Enhancing Pretreatment for the Ultrafiltration of Acid Whey

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