Desalination 189 (2006) 110–118

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

Presented at the 10th Aachen Membrane Colloquium, 16–17 March 2005, Aachen, Germany.

*Corresponding author.

Ultrafiltration of aqueous solutions containing organic polymers

V. García-Molinaa,b*, S. Lykoa, S. Esplugasb, Th. Wintgensa, Th. Melina

aInstitut für Verfahrenstechnik, RWTH Aachen, Turmstraße 46, D-52056, GermanyTel. +49 (2) 4180 95428; Fax +49 (2) 4180 92252; email: garcia@ivt.rwth-aachen.de

bDepartment of Chemical Engineering, University of Barcelona, Martí i Franquès 1, 08028, Barcelona, SpainTel. +34 (93) 402 13 12; Fax +34 (93) 402 12 91

Received 20 April 2005; accepted 8 November 2005

Abstract

In the last decades, the research done in the field of membrane technology applied to wastewater has drasticallyincreased . However, the applicability of this technology is limited due to the fouling of membranes, directlyaffecting its performance. In this study, several parameters thought to be in some extent responsible for membranefouling are evaluated. Ultrafiltration was the selected membrane process and membranes with different molecularweight cut off (MWCO) have been employed. The feed of the experiments consisted of solutions containing humicand fulvic acids, natural organic matter (NOM), alginic acid and cellulose. The influence of different sets of operatingparameters, such as applied pressure, concentration of polymer, content of calcium and pH of the feed solution inthe performance of the process has been investigated.

Keywords: Ultrafiltration; Organic polymers; Water treatment

1. Introduction

Ultrafiltration (UF) is a relatively low-pressuremembrane process whose use as a water treatmenthas rapidly increased in the last decade due tostricter regulations for water quality, decreasedcost, improved membrane materials and modules,simplicity of installation and improved reliabilitywhen compared with conventional treatment

processes such as sedimentation and rapid sandfiltration [1]. The applications of UF can be ana-lyzed from two different points of view. In somecases, the aim of the process is to recover or con-centrate some valuable substances from a solution— this is common practice in the chemical andpharmaceutical industries [2]. Another applicationrelated to this is the concentration of a wastewaterstream for further treatment where a higher con-centration is required, as seen in some advancedoxidation processes. The second principal

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application comprises the removal of undesirableproducts such as particles, colloids, high molec-ular weight materials, bacteria and viruses [1,3]from an effluent stream in order to obtain morepurified water. Another example is the utilizationof membrane systems for potable water treatment,which are already in use in several countries [4].

Despite the wide range of advantages that UFpresents compared with some other traditionalwastewater treatments, its industrial applicationis limited due to deterioration of the membraneperformance and the flux decline over time, main-ly caused by concentration polarization and foul-ing. Among fouling, membrane surface foulingand internal pore fouling can be distinguished. Theformer is usually reversible and can be remediedwith the use of chemical cleaners or back flushing.On the other hand, pore fouling might be irrever-sible and therefore, efforts should concentrate inavoiding it or at least minimizing it.

Due to the importance of fouling, before start-ing an ultrafiltration process design, emphasisshould be put on studying all the parameters affect-ing its performance. These can be divided intothe factors related to the membrane itself and thefactors related to the filtration process conditions.Membrane parameters can be classified into twocategories: physical-chemical parameters (poresize, porosity, skin thickness, surface roughnessand charge, zeta potential and contact angle) andfunctional parameters such as membrane perme-ability, solute sieving and mechanical strength [5].From a practical perspective, the functional para-meters determine the suitability of a membranefor a particular application [6]. On the other hand,the main filtration parameters are the applied pres-sure and the initial concentration and the molecu-lar weight of the substances contained in the feedsolution, all of which affect the permeability andthe retention of the membrane process. Regardingthe properties of the feed solution, a classificationof the fouling provoked can be establish depend-ing on the compounds it contains. Thus, organicfouling, biological fouling, inorganic fouling or

scaling and particulate fouling can be distin-guished. Inorganic fouling is attributed to preci-pitate formations on the membrane surface, whileparticulate fouling is due to colloidal depositionin sub-micron size ranges. Biological fouling iscaused by growth and adhesion of micro-organisms, while organic fouling is thought to becaused by NOM including humic and fulvic acidsas well as proteins and polysaccharides associatedwith microbial activity [7]. Inherent characteristicsof the feed solution, such as the pH and the ionicstrength also appear to play a significant role inthe process [3,8]. In this sense, calcium and phos-phates have been directly implicated with mem-brane fouling due to the formation of insolublecalcium salts, and as possible catalysts or bridgingagents between the membrane and solutes [9]. Onthe other hand, the pH of the solution can have aninfluence in both, the structure of the moleculescontained in the aqueous solution and the surfacemembrane charge. Ghosh and Schnitzer [10]pointed out that while at high pH and low ionicstrength, humic acids have a large, flexible andlinear shape, at low pH and high ionic strengththey turn into a small, rigid and spherical shape.Regarding the effect of pH on the membrane, inthe case of a membrane consisting of amine groups,at low pH it will become positively charged, whilea membrane containing carboxilic acids will benegatively charged at mid to high pH conditions[11].

2. Materials and methods

2.1. Membrane filtration test cell

Ultrafiltration experiments were developed ina low-pressure filtration cell GN 10-400 suppliedby Berghof (Germany). It contains a PTFE supportdisk where two cylinders are placed. The innercylinder is made of glass and it contains the feedsolution, and the external cylinder is made ofPlexiglas and serves as burst shield. The Plexiglascylinder has a security valve to ensure an

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immediate pressure release in case of overpres-sure. On the surface of the PTFE support disk, aTyvec paper should be laid in order to protect themembrane (76 mm diameter).

The test cell is capable of working with a feedsolution volume of 400 mL and at maximumconditions of 10 bar pressure and 100°C tempera-ture. Experiments were effectuated at room temp-erature and the cell was pressurized by air. In orderto reduce the fouling of the membrane, the testcell is also equipped with a stirrer, which was usedat a stirring rate of 300 rpm. A scheme of the testcell used in these experiments can be found else-where [12].

2.2. Chemicals

The feed solution for the tests consisted ofstock solutions Suwannee River humic acidStandard II, Suwannee River fulvic acid Standard,1R101N Suwannee River NOM (RO isolation)and 1R108N Nordic Reservoir NOM (RO isola-tion). These chemicals were supplied by the Inter-national Humic Substances Society (Minnesota,USA). NOM consists of a polydisperse mixtureof individual particles in natural water like humicand fulvic acids, hydrophilic acids, carbohydrates,etc. Its natural concentration is low, usually below20 mg/L [13], but it gives coloration to water,which makes it unsuitable for drinking. Solutionscontaining cellulose powder microcrystalline andalginic acid were tested as well. These productswere supplied by MP Biomedicals, LLC (Esch-wege, Germany).

The rest of the chemicals employed for the con-duction of this investigation were purchased fromMerck (Därmstadt, Germany): KH2PO4 (crystal-line, extra pure), hydrogenphtalat of potassium(for analysis), and CaCl2.2H2O (crystalline gradefor analysis).

2.3. Membranes

The membranes used for this investigationwere supplied by Microdyn-Nadir (Wuppertal,

Germany). The first one was a C030FM mem-brane with a molecular weight cut off (MWCO)of 30 kDalton (kDa) and made of cellulose. It canbe used in a pH range from 1 to 12 and up to amaximum temperature of 55°C. The second onewas a P005F membrane with a MWCO of 5 kDa,made of polyethersulfone, which can be used inall pHs and at a maximum temperature of 95°C.

One important feature of membranes is the zetapotential since it gives information about thecharge of the membrane. When it is lower thanzero, the membrane is negatively charged, andwhen it is higher than zero, the membrane ispositively charged. The zeta potential of the 30membrane vs. the pH is depicted in Fig. 1. Fromthis chart it can be concluded that the membraneis negatively charged, and that the charge becomesmore negative with the decrease of the pH.

2.4. Experimental procedure

The experimental procedure started with thenecessary preparation of the membranes. The dayprevious to the experiment, every membrane waswashed for a couple of minutes with tap water.They were then kept submerged in distilled waterfor 24 h. Once the membranes were placed in thetest cell, 400 mL of distilled water were introducedinto the cell. Then the pressure was adjusted (from

Fig. 1. Zeta potential vs. pH 30 kDa membrane.

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1 to 3 bar depending on the experiment) and thepermeate was collected. These experiments withwater were performed in order to be able to com-pare the performance of the membrane with waterand with the solution containing the polymer.

Within each experiment the time to collect acertain volume of permeate was measured in orderto determine the flux and permeability evolutionthroughout the filtration. After the test with water,the same procedure was applied with the solutionscontaining polymers. A final sample namedretentate was withdrawn from the test cell afterthe experiment.

Total organic carbon (TOC) of the samples wasmeasured in an external laboratory, ALA Analy-tisches Labor (Aachen, Germany).

3. Experimental results and discussion

The experimental part can be divided into fourdifferent sections. The influence of the initial con-centration of organics, the pH and the amount ofCa2+ in the feed, as well as the effect of the trans-membrane pressure applied were investigated ineach one of these sections. From the data measuredduring the course of the experiments the evolutionof flux, permeability, retention and normalizedflux were calculated as follows:

2 Volume of permeateFlux (L/m h)Time Membrane area

=×

(1)

2 FluxPermeability (L/m hbar)p

=∆

(2)

permeate

feed

TOCRetention 1

TOC= − (3)

polymer solution

water

FluxNormalized flux

Flux= (4)

3.1. Influence of the polymer concentration in thefeed solution

To examine whether the initial concentrationin the feed solution has an influence on the ultra-filtration performance, solutions containing dif-ferent amounts of cellulose and alginic acid wereprepared and ultrafiltrated using the 30 kDa mem-brane. The pressure applied in this set of experi-ments was 1 bar.

Ultrafiltration tests were carried out with an initialconcentration of cellulose of 1, 3, 5 and 10 mg/mL.In Fig. 2 the results of these ultrafiltration experi-ments are depicted. Regarding the effect of theinitial concentration, it can be affirmed that it isnot of major importance when working underthese operating parameters since the variations be-tween the different cellulose feed solutions are nottoo noticeable. However, attention must be drawnto the fact that during the course of the filtrationthe permeability decreased from an initial valuein the vicinity of 340 Lm–2h–1bar–1 to a final valuein the range between 240 and 260 Lm–2h–1bar–1. Itwould appear that this reduction in the perme-ability is a consequence of the fouling of the mem-brane.

Solutions containing alginic acid were alsoevaluated concerning the influence of its initialconcentration in the feed solution on the UF

Fig. 2. Permeability vs. volume of permeate. UF of cel-lulose solutions with 30 kDa membrane.

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process. The selected concentrations for theseexperiments were 3 and 0.3 mg/mL. The resultsof these tests are depicted in Fig. 3. As oppositeto UF of cellulose solutions, differences regardingthe permeability were observed. In this case, aremarkable decrease in the permeability wasobserved when increasing the concentration of thefeed solution from 0.3 to 3 mg/mL. This fact canbe explained considering that the phenomenoncalled concentration polarization, i.e. cake layerformation, is more probable to happen when work-ing with highly concentrated solutions. Accordingto this, the more concentrated solutions, the morepronounced decay in the flux. In addition, theretentions achieved by these experiments werecalculated according to Eq. (3). The retentionattained when working with the highest con-centrated solution was in the vicinity of 100%whereas with the lowest, the retention was 96%.The increase in the retention when increasing theinitial concentration can be explained taking intoaccount that some molecules adsorb at the mem-brane material, provoking a decrease in the porediameter and consequently, making the membranemore selective.

On the other hand, and taking into con-sideration the reduction in the permeability duringthe filtration, it is necessary to point out that as ithappened when filtrating cellulose solutions, the

Fig. 3. Permeability vs. volume of permeate. UF of alginicacid solutions with 30 kDa membrane.

fouling of the membrane might be the responsibleof this decay in the flux.

3.2. Influence of the transmembrane pressure

To conduct an investigation about the effectof the transmembrane pressure, UF experimentsof solutions containing humic and fulvic acids,NOM 1R101N and NOM 1R108N were per-formed. The membrane used was the 5 kDa andthe concentration of these substances in the feedsolution was 10 mg/L. Three different transmem-brane pressures were applied to each one of thefeed solutions, 1, 2 and 3 bar. As expected, thehigher the pressure, the higher the flux. However,the permeabilities decreased when increasing theapplied pressure. This fact can be understood bytaking into account that the definition of perme-ability includes a pressure term whereas the fluxdoes not.

In Figs. 4 and 5 the flux and the permeabilitiesvs. the volume of permeate collected during thecourse of the UF experiments of solutions contain-ing fulvic acids are depicted.

Table 1 shows the influence of the applied pres-sure on the retention achieved at the end of theultrafiltration experiments of the solutions con-taining the two different NOM. In both cases itcan be observed that the highest retention isattained when working at the lowest pressure. Thisis due to the fact that an increase in the trans-membrane pressure implies an increase of the flux,which at the same time favors the concentrationpolarization on the membrane surface. As a resultof this phenomenon, the concentration differencebetween both sides of the membrane increases,

Table 1Retention values of the ultrafiltration experiments withdifferent feeds and at three transmembrane pressures

1 bar 2 bar 3 bar NOM 1R101N 0.65 0.57 0.45 NOM 1R108N 0.62 0.44 0.27

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Fig. 4. Flux vs. permeate volume. UF of 10 mg/L fulvicacids solutions with 5 kDa membrane at different trans-membrane pressures.

therefore, the diffusion driven force increases aswell and more particles cross the membrane.

3.3. Influence of the pH of the feed solution

The pH conditions under which UF proceedshas been reported to be a determining aspect undercertain operating parameters. To study whetherpH has an influence or not during the course ofthe UF of solutions containing polymers sometests have been effectuated. Experiments withsolutions containing polymers at different pHs,maintaining the applied pressure at 1 bar andworking with the 5 kDa membrane were per-formed. A buffer solution containing KH2PO4 andNa2HPO4 was used to set the pH at 7 and hydro-genphtalat of potassium and HCl were used tobuffer the solution at pH 3. Feed solutions con-sisted of 10 mg/L of humic acid, fulvic acid, NOM1R101N or NOM 1R108N.

In Fig. 6, the normalized flux [calculated as inEq. (4)] of the ultrafiltration of NOM solutions atdifferent pH values vs. the volume of permeatecollected is depicted. The normalized flux wascalculated as in Eq. (4). In this plot it can be ob-served that when working at pH 7, both NOMsolutions had an approximately constant fluxduring the experiment, meaning consequently, that

Fig. 5. Permeability vs. permeate volume. UF of 10 mg/Lfulvic acids solutions with 5 kDa membrane at differenttransmembrane pressures.

the performance of the process was similar thanwhen ultrafiltrating distilled water. However,when working at pH 3 the normalized flux showeda tendency to decrease continuously, making re-markable the differences between filtrating dis-tilled water and the solutions at pH 3. The lowerflux of the acidic solutions, when compared withthe same solutions at pH 7, might be mainly dueto the fouling of the membrane at low pH. Thefact that the membrane is more easily fouled under

Fig. 6. Permeability vs. permeate volume. UF of NOMsolutions with 5 kDa membrane at pH 7 and pH 3.

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acidic conditions can be understood taking intoaccount that at low pH the functional groups fromthe molecules are non-dissociated and their sur-face charge is weak. Consequently, electrostaticrepulsion is poor, not only between the moleculesbut also between the molecules and the membranesurface. This provokes NOM deposition and accu-mulation on the membrane which results in foul-ing and later, in a flux decay [14].

A similar effect was observed when filtratinghumic and fulvic acids, notwithstanding the factthat a higher difference in the flux at both pH con-ditions was noticed in the humic acid experiments.This may be a consequence of the different struc-ture that humic acids present at basic and acidicpH.

3.4. Influence of the calcium contained in theinitial solution

Ultrafiltration of solutions containing NOMand alginic acid were performed in the presenceand absence of calcium at a transmembrane pres-sure of 1 bar. Solutions containing the two NOMat a concentration of 10 mg/L were employed forthis experimental part. In both cases the membraneused was the one with a MWCO of 5 kDa and218.2 mg/L of CaCl2.2H2O were added to the feed

Fig. 7. Normalized flux vs. permeate volume. UF of NOMsolutions in presence and absence of Ca2+ with 5 kDamembrane.

solution in order to establish conclusions aboutthe influence of the divalent cation.

In Fig. 7, the normalized flux calculated as inEq. (4) is depicted versus the volume of permeatecollected during the course of the UF of NOMsolutions with and without calcium. It can beobserved that the presence of the cation results ina lower flux. This may be caused by the formationof insoluble calcium salts or colloids with thesolute, or due to the characteristics of the calciumto act as a bridge between the solutes and the mem-brane, which results in the fouling of the mem-brane and consequently, in the decay of the flux.

As reported by Hong and Elimelech [14], inthe presence of calcium ions the charge of theNOM is reduced significantly, not only becauseof effective charge screening but also due tocomplex formation. The decrease in the charge ofthe NOM results in an increased deposition rateof the polymer on the surface of the membrane.The calcium also creates bonds between the poly-mer and the membrane surface.

The influence of calcium on the performanceof UF of solutions containing alginic acid using30 kDa membranes was tested as well. Solutionswith a concentration of 0.3 mg/L of alginic acidwere prepared and CaCl2.2H2O was added to onethem, having a final concentration of 218.2 mg/L.In Fig. 8, the permeability vs. the permeate

Fig. 8. Permeability vs. volume of permeate. UF of alg-inic acid solutions in the presence and absence of Ca2+.

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collected during these experiments is depicted. Inthis figure the fouling effect of the addition ofcalcium is remarkable. In fact, when ultrafiltratingthe alginic acid, a reduction in the permeabilityfrom 160 to 120 Lm–2h–1bar–1 was attained, while,in the presence of calcium, this the permeabilitydecreased until 30 Lm–2h–1bar–1.

4. Conclusions

Regarding the influence of the polymer con-centration in the feed solution on the UF per-formance, it is necessary to point out that twodifferent cases have been observed. On one sideUF of cellulose did not show remarkable differ-ences when increasing the concentration, however,UF of alginic acid solutions appeared to be sen-sitive to concentration. In fact, as its concentrationwas increased from 0.3 to 3 mg/mL, the fluxdecreased to 100 Lm–2h–1. However, in both seriesof experiments, i.e., tests with alginic acid andcellulose, a decrease in the flux during the courseof the UF was noticed, mainly due to concentrationpolarization.

It can also be concluded that increasing trans-membrane pressure is related to an increase in fluxand a decrease in permeability. Moreover, an in-crease in the pressure gives rise to a more pro-nounced concentration polarization and as a con-sequence, the retention decreases.

Among the operating parameters evaluated thepH of the feed solution appeared to be a deter-mining factor for the retention and the flux.Actually when working with NOM the flux de-creases at low pH due to the fouling of the mem-brane. The NOM appear to be electrolytic andtherefore the pH can strongly affect the protona-tion and deprotonation of the functional groups.At low pH the molecules are not protonated andthe membranes are slightly negatively charged,consequently the repulsion between them is lowand the membrane can be more easily fouled be-cause of the deposition of the NOM.

The calcium content of the feed was found tobe influencing the flux when working with NOMand alginic acid, since lower fluxes were observedwhen adding Ca2+ to the feed. This fact isexplained by considering the bridging propertiesof the calcium ion which aids the formation ofcomplexes not only between the molecules butalso between the molecules and the membrane.This results in the fouling of the membrane andthe consequent decrease in the flux.

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