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Reversible and irreversible compaction of ultrafiltration membranes S. Stade a,1 , M. Kallioinen a,, A. Mikkola b,2 , T. Tuuva c,3 , M. Mänttäri a,4 a Laboratory of Separation Technology, Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta FIN-53851, Finland b Laboratory of Machine Design, Department of Mechanical Engineering, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta FIN-53851, Finland c Laboratory of Physics, Department of Mathematics and Physics, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta FIN-53851, Finland article info Article history: Received 15 March 2013 Received in revised form 21 June 2013 Accepted 24 June 2013 Available online 3 July 2013 Keywords: Ultrasonic time-domain reflectometry Irreversible compaction Reversible compaction Regenerated cellulose Polyethersulphone abstract This study evaluates differences in reversible (after relaxation) and irreversible compaction and the effect of compaction on the performance of three different ultrafiltration membranes. The evaluation is based on results from both off-line and on-line measurements of compaction. The on-line measurements were done with an ultrasonic time-domain reflectrometry (UTDR) tool with improved resolution compared to tools used in earlier studies. The results reveal that the regenerated cellulose membrane compacted significantly more than the tested polyethersulphone membranes. This dissimilarity originates from the different membrane mate- rials used and from significant differences in the membrane structures. It is also found that measure- ments of membrane compaction, whether made on-line or off-line, are not predictive for membrane performance. For instance, compaction of the UH030 membrane was negligible but its permeability decrease and retention increase due to the compaction were significant. Compaction decreased the cut-off values of the 30 kDa membranes to lower than 8 kDa. The results thus indicate that the skin layers of the membranes compact significantly causing remarkable changes in membrane performance. Thick- ness changes occurring in the scale of skin layer thicknesses are out of the resolution limits of methods thus far available for monitoring of membrane compaction in real-time. Real-time measured information on compaction phenomena is further needed to be able to distinguish flux decrease caused by concentra- tion polarization and the effects of reversible compaction. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Membrane compaction, or creep, is a well-known phenomenon in polymeric membranes. It can be divided into time-dependent recoverable elastic deformation and time-dependent unrecover- able viscous deformation [1]. In this article, these deformation phe- nomena are henceforth called reversible and irreversible compaction, respectively. Membrane compaction has been shown to lead to irreversible flux decline even at relatively low filtration pressure. For instance, Tessaro and Jonsson [2] reported that a polysulphone (PSU) ultra- filtration (UF) membrane lost part of its filtration capacity under constant transmembrane pressure of 1.8 bar, Kallioinen et al. [3] found that regenerated cellulose (RC) UF membranes compacted and lost part of their filtration capacity after use at 3 bar, and Pers- son et al. [4] noticed compaction and decline in the filtration capacity of polysulphone (PSU) and cellulose acetate (CA) UF mem- branes when they were used at pressures in the range of 0–3 bar. In addition to flux decline, membrane compaction can also af- fect the separation efficiency. Membrane compaction can increase the retention of molecules which are of the same magnitude as the membrane cut-off value or which are smaller than the cut-off va- lue, because the membrane compaction might cause a decrease in the membrane pore size or a deformation of the pore geometry. However, Tarnawski and Jelen [5] found that even though the per- meability decrease of a PSU UF membrane due to compaction in a pressure range of 0–10 bar was significant, the selectivity of the membrane remained unchanged. Kallioinen et al. [3] also reported that compaction of the mem- brane had no significant effect on the observed PEG (35,000 g/mol) retention of regenerated cellulose membranes. It should be noted, however, that the PEG molecules (35,000 g/mol) used in their experiments were large compared to the cut-off values of the tested membranes (12,000 g/mol measured with PEGs [6,7]). Retention of the 35,000 g/mol PEG was already very high before the membrane compaction and, thus, in this case, membrane com- paction could not increase the PEG retention remarkably. 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.039 Corresponding author. Tel.: +358 40 5939 881; fax: +358 (5) 621 2199. E-mail addresses: stade@lut.fi (S. Stade), mari.kallioinen@lut.fi (M. Kallioinen), mikkola@lut.fi (A. Mikkola), tuure.tuuva@lut.fi (T. Tuuva), mika.manttari@lut.fi (M. Mänttäri). 1 Tel.: +358 40 5063 609; fax: +358 (5) 621 2199. 2 Tel.: +358 40 736 3095; fax: +358 (5) 621 2499. 3 Tel.: +358 40 550 0205; fax: +358 (5) 621 2898. 4 Tel.: +358 40 734 2192; fax: +358 (5) 621 2199. Separation and Purification Technology 118 (2013) 127–134 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

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Separation and Purification Technology 118 (2013) 127–134

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology

journal homepage: www.elsevier .com/ locate /seppur

Reversible and irreversible compaction of ultrafiltration membranes

1383-5866/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2013.06.039

⇑ Corresponding author. Tel.: +358 40 5939 881; fax: +358 (5) 621 2199.E-mail addresses: [email protected] (S. Stade), [email protected] (M. Kallioinen),

[email protected] (A. Mikkola), [email protected] (T. Tuuva), [email protected] (M.Mänttäri).

1 Tel.: +358 40 5063 609; fax: +358 (5) 621 2199.2 Tel.: +358 40 736 3095; fax: +358 (5) 621 2499.3 Tel.: +358 40 550 0205; fax: +358 (5) 621 2898.4 Tel.: +358 40 734 2192; fax: +358 (5) 621 2199.

S. Stade a,1, M. Kallioinen a,⇑, A. Mikkola b,2, T. Tuuva c,3, M. Mänttäri a,4

a Laboratory of Separation Technology, Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta FIN-53851, Finlandb Laboratory of Machine Design, Department of Mechanical Engineering, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta FIN-53851, Finlandc Laboratory of Physics, Department of Mathematics and Physics, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta FIN-53851, Finland

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 March 2013Received in revised form 21 June 2013Accepted 24 June 2013Available online 3 July 2013

Keywords:Ultrasonic time-domain reflectometryIrreversible compactionReversible compactionRegenerated cellulosePolyethersulphone

This study evaluates differences in reversible (after relaxation) and irreversible compaction and the effectof compaction on the performance of three different ultrafiltration membranes. The evaluation is basedon results from both off-line and on-line measurements of compaction. The on-line measurements weredone with an ultrasonic time-domain reflectrometry (UTDR) tool with improved resolution compared totools used in earlier studies.

The results reveal that the regenerated cellulose membrane compacted significantly more than thetested polyethersulphone membranes. This dissimilarity originates from the different membrane mate-rials used and from significant differences in the membrane structures. It is also found that measure-ments of membrane compaction, whether made on-line or off-line, are not predictive for membraneperformance. For instance, compaction of the UH030 membrane was negligible but its permeabilitydecrease and retention increase due to the compaction were significant. Compaction decreased thecut-off values of the 30 kDa membranes to lower than 8 kDa. The results thus indicate that the skin layersof the membranes compact significantly causing remarkable changes in membrane performance. Thick-ness changes occurring in the scale of skin layer thicknesses are out of the resolution limits of methodsthus far available for monitoring of membrane compaction in real-time. Real-time measured informationon compaction phenomena is further needed to be able to distinguish flux decrease caused by concentra-tion polarization and the effects of reversible compaction.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Membrane compaction, or creep, is a well-known phenomenonin polymeric membranes. It can be divided into time-dependentrecoverable elastic deformation and time-dependent unrecover-able viscous deformation [1]. In this article, these deformation phe-nomena are henceforth called reversible and irreversiblecompaction, respectively.

Membrane compaction has been shown to lead to irreversibleflux decline even at relatively low filtration pressure. For instance,Tessaro and Jonsson [2] reported that a polysulphone (PSU) ultra-filtration (UF) membrane lost part of its filtration capacity underconstant transmembrane pressure of 1.8 bar, Kallioinen et al. [3]found that regenerated cellulose (RC) UF membranes compacted

and lost part of their filtration capacity after use at 3 bar, and Pers-son et al. [4] noticed compaction and decline in the filtrationcapacity of polysulphone (PSU) and cellulose acetate (CA) UF mem-branes when they were used at pressures in the range of 0–3 bar.

In addition to flux decline, membrane compaction can also af-fect the separation efficiency. Membrane compaction can increasethe retention of molecules which are of the same magnitude as themembrane cut-off value or which are smaller than the cut-off va-lue, because the membrane compaction might cause a decreasein the membrane pore size or a deformation of the pore geometry.However, Tarnawski and Jelen [5] found that even though the per-meability decrease of a PSU UF membrane due to compaction in apressure range of 0–10 bar was significant, the selectivity of themembrane remained unchanged.

Kallioinen et al. [3] also reported that compaction of the mem-brane had no significant effect on the observed PEG (35,000 g/mol)retention of regenerated cellulose membranes. It should be noted,however, that the PEG molecules (35,000 g/mol) used in theirexperiments were large compared to the cut-off values of thetested membranes (12,000 g/mol measured with PEGs [6,7]).Retention of the 35,000 g/mol PEG was already very high beforethe membrane compaction and, thus, in this case, membrane com-paction could not increase the PEG retention remarkably.

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128 S. Stade et al. / Separation and Purification Technology 118 (2013) 127–134

As discussed above, membrane compaction has been found tolead to changes in membrane performance and affects process effi-ciency. For this reason, membrane experiments are often started bypressurizing membranes at a pressure higher than will be used inthe real filtration application to ensure that the compaction phe-nomenon does not cause unstable capacity in the experiments.However, pressurization at a pressure that is too high for the mem-branes can cause significant capacity losses or performancechange, thus affecting filtration results. Capacity losses and perfor-mance change might also occur as a result of utilization of a mem-brane at a too high pressure. Therefore, knowledge of thecompaction tendency of membranes is of value in the optimizationof filtration conditions.

Compaction tendency depends on the precise physical andchemical structure of the membrane and is, thus, challenging toevaluate based on information presented in membrane brochures,which tend to give limited details about issues such as how tem-perature and pressure impact on membrane compaction andcapacity. Comprehensive knowledge on compaction tendency inthe applied conditions is best achieved through experimentalwork.

The simplest way to evaluate membrane compaction is to mea-sure pure water flux at fixed operation conditions (e.g. pressureand temperature) until constant water flux is reached, as for in-stance Tessaro and Jonsson [2] have done. This method can be usedfor comparison of the compaction tendencies of different mem-branes at certain conditions but it does not give information onthe reversibility of compaction. Kallioinen et al. [3] evaluated com-paction by measuring the pure water flux before and after theexposure of the membranes to higher pressures during filtrationof water. They also examined compaction of ultrafiltration mem-branes with SEM and micrometer measurements after filtrationexperiments. The weakness of these methods is that they do notdistinguish reversible from irreversible compaction. Moreover,off-line examinations such as SEM analysis or micrometer mea-surements cannot show to what extent compaction affects filtra-tion capacity. The influence of compaction can also be seen whenpure water permeability measurements are done before and afterthe filtration of a process stream but the information such mea-surements provide does not necessarily show only the effect ofcompaction but also that of membrane fouling. Membrane com-paction has also been evaluated through data from differentmechanical testing arrangements [8–11]. The conditions duringthese experiments, however, differ significantly from those pre-vailing in a filter cell during filtration. Thus, the results do not giveaccurate information on membrane compaction during filtrationand its effects on filtration performance.

The best information on compaction during filtration at certainconditions would appear to be provided by real-time measure-ments. Peterson et al. [12,13] have demonstrated the applicabilityof ultrasonic time-domain reflectometry (UTDR) in real-time mon-itoring of compaction of reverse osmosis membranes and Zirfon�

composite membranes. Reinsch et al. [14] have demonstrated theuse of UTDR in monitoring of compaction of gas separation mem-branes. These studies show that UTDR is a feasible non-invasivetechnique that can be successfully applied in real-time monitoringof membrane compaction. However, membrane compaction dataof UF membranes measured in real-time has not been widely pub-lished, although it is valuable information for optimization of fil-tration processes.

This study examines compaction of three different UF mem-branes with both off-line and on-line methods. The on-line exam-ination is performed with an ultrasonic time-domain reflectometry(UTDR) tool that differs from those used in earlier studies in thatthe primary ultrasound transducer is mounted inside the filter inthe cover of the cell in a way which does not disturb the cross-flow

in the channel. Additionally, a secondary transducer is used fordetermination of sonic speed, which is needed in calculation ofthe distance from the primary transducer to the membrane. Theseimprovements enable more accurate measurement than in previ-ous studies. The objective of this study is to assess differences inthe reversible and irreversible compaction and decrease in perme-ability of three commercial ultrafiltration membranes after expo-sure to elevated pressures. A further aim is to explore if theeffect of compaction on membrane retention could be used to tai-lor the separation efficiency of membranes.

2. Experimental

2.1. Theory

Ultrasonic measurements are based on the mechanical pressurewaves caused by a high frequency oscillator. When the ultrasonicwave encounters an interface between two media with differentdensities, the energy of the wave is partially reflected back. This re-flected wave can be seen in an oscilloscope as an ‘‘echo’’. If thevelocity in the media and the arrival time of the reflected waveare known, the distance between the transducer and interfacecan be calculated by the following equation:

Ds ¼ 12

CDt ð1Þ

where Ds is distance from the transducer to the membrane, C is so-nic speed in the media, and Dt is arrival time (the time delay be-tween the emitted and received transducer signals) of theacoustic wave. Sonic speed of the media is typically assumed tobe constant if no parameters (pressure, temperature, flow condi-tions, concentration etc.) change. However, in real life, sonic speedmight vary significantly, because it is really challenging to keepsome parameters, for instance, temperature, totally constant duringfiltration. In the UTDR measurement system used in this study,changes in sonic speed are easily taken into account with a simplereference transducer which measures the sonic speed at the sametime as the distance from the transducer to the membrane is mea-sured. The reference transducer measures through the same mediabut with a fixed distance, which is constant (Fig. 1).

2.2. UTDR measurement system and filtration module

An ultrafiltration cross-flow membrane module was built fromstainless steel to study membrane compaction with the UTDR tool.Two 10 MHz ultrasound transducers were inserted inside the mod-ule (Fig. 1). The form of the sensor is roundish and its diameter is5 mm. At the frequency of 10 MHz the sound wave is highly direc-tive, thus the membrane measurement is averaging the same areaas the sensor. The primary transducer was mounted inside the fil-ter in the cover of the cell in a way which does not disturb thecross-flow in the channel, and the secondary (reference) trans-ducer was fixed on the side of the cross-flow channel in the upperpart of the module. Positioning the transducers in this way enablesmore accurate measurement than in previous studies, in which theultrasound transducers have been located outside the filter cell andthe sonic waves introduced into the filter chamber through thecover of the cell (Fig. 1). When the transducer is mounted insidethe filter, measurement error can be decreased, because sonicwaves are introduced straight into the feed water and reflectionsfrom the interface between the filter cover and feed water areavoided. The use of a secondary reference transducer further de-creases measurement error; it is used for determination of sonicspeed, which is needed in calculation of the distance from the pri-mary transducer to the membrane. Sonic speed depends on the

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Fig. 1. Schematic presentation of the location of transducers in the membrane module used. A: ultrasound transducer location in earlier studies [12–14], B: primaryultrasound transducer used in this study, C: secondary reference transducer used in this study to measure sonic speed.

S. Stade et al. / Separation and Purification Technology 118 (2013) 127–134 129

temperature, pressure and flow conditions prevailing in the filterand variation in these conditions during filtration causes changesin sonic speed. If sonic speed is not determined but evaluated,the variation increases measurement error in distance values[11]. The secondary reference transducer can also be used for otherpurposes. For example, when filtration pressure and flow condi-tions are constant, it can be calibrated to measure temperature.

In the filtration module used, the cross-flow channel overthe membrane was 31 cm long and 1.8 cm high and wide.The lower part of the module contained a 51 cm2 filtration areafor a flat sheet membrane on a porous laser-cut support metal.The pulse-generator used in the study was an Agilent 33250Awith a Tektronix TDS 3052B oscilloscope. The arrival times inthe UTDR measurements can be read from the oscilloscopescreen with ±0.1 ns accuracy, which equals to ±0.075 lm inthe membrane thickness. In the UTDR measurements performedin the membrane module using a metal sheet instead of amembrane the accuracy of the UTDR measurement systemwas determined to be ±0.05 lm. The oscilloscope was con-nected to a computer for data collection. A 10 l water-heated/cooled feed tank was connected to the module via a Grundfosmulti-stage vertical centrifugal pump. Labom Pascal CV-digitalpressure gauges were connected before and after the modulewith an analogue pressure valve to control the flow speedand pressure inside the module. Temperature was monitoredfrom the retentate line with a digital thermometer. The systemsetup is shown in Fig. 2.

Fig. 2. Cross-flow setup with

Pressure leads to deformation of the filtration module and ifthe filtration module is not well designed, mechanical deforma-tions may be significant in terms of UTDR measurement accu-racy. The filtration module under investigation is manufacturedusing an endmill cutter, and its minimum wall thickness is15 mm. Mechanical deformation of the filtration module wasevaluated using the finite element method [15]. The finite ele-ment mesh of the structure was constructed using 162463 solidelements. The analyzed maximum deformation due to pressureincrease to 10 bar was 0.16 lm. One can conclude that mechan-ical deformation did not disturb the measurements done at con-stant pressure and that the deformation caused due to thepressure increase at the applied pressure area was significantlysmaller than the variance of the measured membrane thicknesschanges.

2.3. Ultrafiltration membranes

Three different polymeric flat-sheet UF membranes from themanufacturer, Microdyn-Nadir, were used in this study. The selec-tive skin layers of these membranes were made from regeneratedcellulose (RC), permanently hydrophilic polyethersulphone (PESH)and polyethersulphone (PES). The backing layers in the PES andPESH membranes were polyethylene (PE) and polypropylene (PP)and in the RC membrane polyethylene terephthalate (PET). Mem-brane properties as presented by the manufacturer are listed inTable 1. No pressure limits are given in the membrane catalogue

the UTDR measurement.

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Table 1Membrane properties of the three ultrafiltration membranes used in the study [16].

Membrane Selectiveskin layer

Backingmaterial

MWCO(g/mol)

Processingtemperature(�C)

OperatingpH-range

UC030 RC PET 30,000 5–55 1–11UH030 PESH PE/PP 30,000 5–95 0–14UP020 PES PE/PP 20,000 5–95 0–14

130 S. Stade et al. / Separation and Purification Technology 118 (2013) 127–134

but the water flux values given for the UC030, UH030 and UP020membranes have been measured at 3 bar at 20 �C [16].

2.4. Experimental methods

Three types of filtration experiments were carried out at 30 �Cto study membrane compaction, its reversibility, and its effect onretention (Table 2).

In the pre-treatment step, which was included in all the per-formed experiment types, the membrane samples were treatedwith alkaline water solution (pH = 12) for 20 min to remove pre-servatives from the membranes and completely wet the mem-branes. The membranes were then rinsed and stored with ROtreated water. A new membrane was used for each experiment.All the membrane pieces were from the same membrane batch.Three similar exposure experiments were made with each testedmembrane type. The thicknesses of the wet membranes were mea-sured with a micrometer after the pre-treatment and after theexposure to pressure. The accuracy of the micrometer measure-ments was ±5 lm.

In the compaction experiments, exposure of the membranesamples to different pressures was performed with RO treatedwater at 30 �C at 1, 3, 5 and 7 bar pressure, respectively. Duringthe exposure, pure water flux through the membranes was mea-sured, and arrival times of both ultrasonic transducers were col-lected after the conditions had stabilized. It is known thatcompaction increases rapidly at the beginning of membrane expo-sure to pressure. After this stage, the time-dependent deformationis gradual and it might, to some extent, continue indefinitelythroughout the life of the membrane [2,12]. In this study, expo-sures were continued until deformation changes were less than2% in 10 min. The exposure lasted at least 60 min with all thetested membrane samples. The cross-flow velocity in the mem-brane cell was in the laminar regime (Re = 1100) in all the experi-ments. Retentate and permeate were recycled back to the feedtank. To hold the membrane against the support metal at thebeginning and at the end of the filtration experiments, the refer-ence UTDR-values were taken at 0.15 bar pressure. This pressurewas expected to have no significant effect on compaction. Afterthe compaction experiments, the membranes recovered for 16 h(0 bar pressure, wet conditions) before the ‘‘after’’-values wereread. After completion of the experiments, thicknesses of the sam-ples were measured with a micrometer and the samples weredried at room temperature in preparation for the SEMexamination.

Table 2Experiments performed in this study.

Compaction experiments Recovery experiments

PretreatmentThickness measurement (micrometer) UTDR measurements (1

30 �C)UTDR measurements (0.15 ? 1 ? 3 ? 5 ? 7 ? 0.15 bar,

30 �C)Thickness measurement (micrometer)

Scanning electron microscopy

The recovery experiments focused on permeability and thereversibility of the compaction. In these experiments, membranecompaction and permeability were monitored for 1 h at 1 bar,2.5 h at 5 bars and 1 h at 1 bar pressure.

In addition, experiments were made to find out the effect ofmembrane compaction on retention. Retention for polyethyleneglycol (PEG) molecules was measured before and after the 15 hof compaction at 7 bar. PEG 6000 was used for 20 kDa and PEG8000 for 30 kDa MWCO (Molecular Weight Cut-off) membranes.

The retention experiments were performed with a cross-flowrectangular filtration module in which three cells can be used inparallel (cell dimensions: 1 mm � 20 mm � 230 mm). Fluxes andretentions were first measured at 0.5, 1.0 and 1.5 bar pressureand at a cross-flow velocity of 5 m/s. After that, PEG was washedout of the system and the membranes were exposed to 7 bar pres-sure for 15 h with pure water. After the exposure to higher pres-sure, retentions were measured again at the same fluxes asbefore the exposure.

2.5. Morphological characterization of membrane compaction

SEM was used in examination of the reference membrane sam-ples and membrane samples exposed to different pressures. Thepurpose of the SEM analysis was to confirm possible structuralchanges seen as thickness change in the UTDR and micrometermeasurements. The samples were analyzed with a JEOL JSM-5800 scanning electron microscope. Reference samples had thesame pre-treatment as the samples exposed to pressure. Afterthe membranes were dried, cross-sections of the membrane sam-ples were prepared by cutting them in liquid nitrogen. The mem-brane samples were coated with gold. Secondary electronimaging (SEI) pictures were taken with 10 kV accelerating voltagefrom a 10 mm working distance.

3. Results

3.1. On-line monitoring of compaction

Fig. 3 shows permeabilities and membrane thicknesses mea-sured in real-time with the UTDR monitoring tool at 1, 3, 5 and7 bars for the UC030, UP020 and UH030 membranes, respectively(compaction experiments). ‘‘After’’-values indicating how muchthe membrane thickness recovers from 7 bar exposure after thepressure has been released to 0.15 bar are also presented inFig. 3. Results of the recovery experiments (1 bar ? 5 bar ? 1 bar)are shown in Fig. 4.

The measured compaction of the PES-membrane UP020 wasnegligible in all experiments at the pressure range from 1 to7 bar and the compaction measured for the PESH membraneUH030 was small compared to that of the RC-membrane UC030,which compacted by almost 60 lm. At 1 bar, permeability of thehydrophilic UC030 membrane was significantly higher than thepermeabilities of the UP020 and UH030 membranes. However,coexistent with due to the stronger compaction, the permeability

Retention experiments

? 5 ? 1 bar, Retention measurements (0.5 ? 1.0 ? 1.5 bar, 30 �C)

Exposure to 7 bar, 30 �C

Retention measurements (in same flux as before theexposure)

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Fig. 3. Compaction and permeability of UC030, UH030 and UP020 membranes at different pressures at 30 �C (compaction experiments, Table 2). Membrane thicknesses atthe beginning of the measurements are listed in Table 3.

Fig. 4. Compaction and permeability of UC030, UH030 and UP020 membranes when recovery of compaction was studied. The compaction period was 2.5 h at 5 bar (recoveryexperiments, Table 2). Membrane thicknesses at the beginning of the measurements are listed in Table 3.

Table 3Membrane thicknesses measured before and after the compaction experiments withmicrometer and UTDR-measurement system.

Membrane Before(lm)

After(lm)

Compactionmicrometer (lm)

CompactionUTDR (lm)

UC030 259 228 31 34UH030 224 224 0 2UP020 238 238 0 0

S. Stade et al. / Separation and Purification Technology 118 (2013) 127–134 131

of the UC030 membrane decreased to the same level as the perme-ability of the other tested membranes already at 3 bar. Moreover,at pressures higher than 3 bar, the permeability of the UC030membrane was lower compared to the permeability of theUH030 membrane. Although there were some differences in howmuch membrane thicknesses changed due to the 5 bar pressure(see Table 2 and Figs. 3 and 4) the same kind of phenomenon couldbe seen in all the experiments. All membranes recovered some-what after the pressure was reverted to 1 bar from 5 bar (Fig. 4).

It seems that most of the observed compaction caused to theUH030 and UP020 membranes at a pressure range from 1 to7 bar was reversible (Figs. 3 and 4). UP020 thickness recovered tothe same level as it was at the beginning of the experiments. Com-paction caused to the UC030 membrane was only partly reversible;its thickness recovered by about 40% after pressure release,whereas the UH030 and UP020 membranes recovered fully (com-paction experiments). In the recovery experiment, the permeabil-ity of the UC030 recovered by 40% after release of the pressurefrom 5 bar to 1 bar. The permeabilities of the PES membranes(UH030 and UP020) remained at a low level without showing sig-nificant recovery.

3.2. Off-line measurements of membrane compaction

Based on off-line compaction measurements performed with amicrometer it seems that only the UC030 membrane was com-pacted as a result of the exposure to 1, 3, 5 and 7 bar (Table 3). Only

the compaction of the UC030 membrane could be clearly seen inthe SEM images (Figs. 5–7).

The SEM images of the virgin membranes (Figs. 5–7) revealedsignificant differences in the membrane substructures. TheUC030 membrane has a supporting layer containing very porousmacrovoids just under the thin skin layer. In the UH030 mem-brane, the macrovoids in the supporting layer have similar shapeand their size is at similar level to the UC030 membrane. However,in the UH030 membrane there is a ‘‘primary supporting layer’’,which is sponge-like, between the skin layer and the secondarysupporting layer containing the macrovoids. A similar kind ofsponge-like primary supporting layer was also found in theUP020 membranes. The macrovoids in the secondary supportinglayer of the UP020 membrane are smaller than in the UH030 andUC030 membrane samples but the secondary supporting layercontains more of them than the corresponding layers in theUH030 and UC030 membranes.

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Fig. 5. SEM-pictures of the virgin (left image) and compacted (right image) UC030-membrane.

Fig. 6. SEM-pictures of the virgin (left image) and compacted (right image) UH030-membrane.

Fig. 7. SEM-pictures of the virgin (left image) and compacted (right image) UP020-membrane.

Fig. 8. Effect of compaction (15 h at 7 bar, 30 �C) on the membrane retention forPEG 8000 g/mol (UH and UC membrane), PEG 6000 g/mol (UP membrane).Retentions were measured at 100 L/(m2h). The results are average results fromthree different membrane sheets.

132 S. Stade et al. / Separation and Purification Technology 118 (2013) 127–134

3.3. Effect of compaction on the retention

The PEG retentions of the tested membranes before and afterthe 15 h of compaction at 7 bar are presented in Fig. 8. The reten-tions were measured at a flux of 100 L/(m2h). The highest increasein retention was measured for the UC030 membrane: after thecompaction at 7 bar its PEG (8000 g/mol) retention was about14% units higher (94%) compared to the value measured beforethe compaction. Respectively, PEG (8000 g/mol) retention of theUH030 membrane increased 10% units following the compaction.No significant changes were seen in the retention of the UP020after the compaction.

4. Discussion

The results of the experiments revealed significant differencesin the compaction tendencies of the tested membranes. TheUP020 and UH030 membranes were significantly less compactedthan the UC030 membrane. The remarkable compaction tendencyof the UC030 has been shown also by Kallioinen et al. [3].

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S. Stade et al. / Separation and Purification Technology 118 (2013) 127–134 133

The different membrane materials partly explain the differentcompaction tendencies of the tested membranes. However, onecrucial factor determining the difference in the compaction ten-dencies is the difference in the membrane structures (Figs. 5–7).All of the tested membranes contained macrovoids in their sup-porting layers but only the most compacted, the UC030 membrane,had the macrovoid layer directly below the thin skin layer. In theUP020 and UH030 membranes, there is a sponge-like primary sup-porting layer between the skin layer and the secondary supportinglayer containing the macrovoids. It has been shown in earlier stud-ies that macrovoids in the supporting layer just below the skinlayer increase the compaction tendency of a membrane comparedto a membrane having a sponge-like supporting layer below itsskin layer [4] and that big pore volumes in the supporting layer in-crease the compaction tendency of a membrane [17]. It can, there-fore, be concluded that the sponge-like layer just below the skinlayer in the UH030 and UP020 membranes has protected the sec-ondary layer containing the macrovoids and, thus, has decreasedthe compaction tendency.

The results of this study indicate that compaction inducedstructural changes in the skin layers of the UC030 and UH030membranes. This could be found when the simultaneously mea-sured compaction data and water permeability data were exam-ined. Pure water permeability decreased clearly with increasingpressure, but pure water permeability decrease and membranethickness did not change proportionally. (Fig. 3) The decrease inmembrane thickness of the UH030 membranes was negligible,whereas the permeability of the membrane decreased by 61%.The corresponding value for the UC030 membrane was 91% whenthe membrane thickness decreased by 47 lm (examined at thepressure range from 1 to 7 bar). The pure water permeability de-crease might originate from an increase in hydrodynamic resis-tance due to compaction of the skin layer. Skin layer compactionoccurs at a level that is out of the resolution of the UTDR tool used,even though the resolution of this tool was improved by installingthe sensors inside the membrane module and using a referencesensor.

The same phenomenon was observed when the recovery ofcompaction was studied (results in Fig. 4). In the recovery tests,the PES membranes UH030 and UP020 almost fully recovered fromthe small compaction but permeability did not recover at all. Afterthe pressure was released to 1 bar, the UC030 membrane compac-tion and permeability partly recovered. Most of the compaction ofthe UC030 membrane seemed to happen under the very thin skinlayer. This could partly explain the increased hydrodynamic resis-tance of the membrane which was seen as decreased permeability.However, the skin layer might also be compacted. This finding sug-gests that the changes in the total thickness of the PES membranesdo not have great influence on membrane permeability but com-paction of the thin skin layer has to have occurred, which cannotbe recognized in the UTDR measurements.

Peterson et al. [12] found a similar phenomenon when theymonitored compressive strain of a reverse osmosis membrane withUTDR: the water permeability continued decreasing after the com-pressive strain had reached the stable conditions. They suggestedalso that the reason could be the densification of the skin layer.Due to the limitations of their UTDR tool, they could not, however,show any results supporting this theory.

The retention measurements performed in this study (Fig. 8)substantiate the hypothesis of compaction and densification ofthe skin layers of the tested membranes. When the retentions werecompared at equal flux value (100 L/(m2h)), it can be seen thatcompaction at 7 bar caused an increase of the PEG (8000 g/mol)retention from 81% to 94% and from 83% to 93% with the UC030and UH030 membranes, respectively. Over 90% retention valuesmeasured after compaction indicate that membrane cut-off has

changed to become less than 8000 g/mol. This suggests that bycontrolling the compaction it might be possible to increase theretention of the studied membranes. This result also shows thatto get more accurate information on the compaction phenomenaaffecting membrane performance, there is still a need for real-timethickness measurements enabling monitoring of the compaction inthe scale of the membrane skin layers.

The results of this study demonstrate that examination of mem-brane compaction is very challenging if only off-line methods, suchas micrometer measurements or SEM, are used. Comparison of theresults of the UTDR measurements and the micrometer measure-ments reveals that the results are in a good consistence. However,the micrometer measurements can be done only off-line.

The main disadvantage of off-line methods is that they canshow only irreversible compaction, although both reversible com-paction and irreversible compaction affect filtration capacity dur-ing the filtration. For instance, the slight reversible compaction ofthe UH030 and UP020 membranes would not have been noticedbased on the on-line and off-line results in this study, if compac-tion had been examined only with micrometer measurementsand SEM (Figs. 3 and 4). The flux decrease caused by reversiblecompaction can be confused with concentration polarization.Reversible compaction represented about 40% and 30% of the totalcompaction when the UC030 membrane was operated at 7 bar and5 bar (Figs. 3 and 4) respectively. In the recovery experiments, thepermeability decreased from 360 to 150 L/(m2 hbar) when pres-sure was increased from 1 to 5 bar. When the pressure was re-duced back to 1 bar the permeability increased to 230 L/(m2 hbar), i.e. a 80 L/(m2 hbar) increase in permeability. This corre-sponds to about 40% of the total decrease of permeability at 5 bar.Therefore, in this example, about 40% of the permeability decreasecould erroneously be assumed to be caused by concentrationpolarization, although it was due to reversible compaction. Optimi-zation of filtration is difficult without knowledge of the amount ofreversible compaction and its effect on permeability.

5. Conclusion

Compaction of three different UF membranes, made from differ-ent materials, was compared. Both off-line and on-line methodswere used in the experiments to monitor compaction. The resultsrevealed significant differences in compaction of the tested mem-branes. The UC030 membrane was clearly compacted already at3 bar, while the UH030 and UP020 membranes compacted lesseven though they were exposed to 7 bar pressure. The slight mea-surable compactions of the UH030 and UP020 membranes weremainly reversible; they could not be seen in the off-line measure-ments. Based on the micrometer and UTDR measurements, almosthalf of the compaction caused to the UC030 membrane at 7 barpressure was reversible.

Exposure of the membranes to elevated pressures had a signif-icant irreversible effect on water permeability and retention. Thisindicates that their selective layers were compacted, although thiswas not seen directly, even though a UTDR tool with improved res-olution was used. Compaction happened thus in both the selectiveskin layer and the layers below the skin layer.

The results indicated that it might be possible to control thepore size of ultrafiltration membranes with the controlled compac-tion. For instance, with the compaction at 7 bar, the cut-off valuesof the tested UH030 and UC030 membranes could be decreased be-low the value of 8000 g/mol.

The different compaction tendencies of the tested membranesoriginated partly from the different membrane materials but thedifferent structures also had a crucial effect. In the most compact-ing UC030 membrane, the supporting layer containing macrovoidswas just below the skin layer, whereas in the UP020 and UH030

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membranes, there was a sponge-like layer between the skin layerand the secondary supporting layer containing the macrovoids.The structure of the UC030 membrane increased its measurablecompaction tendency compared to the other tested membranes.

This study demonstrated that on-line measurement tools arerequired to obtain information about irreversible and reversiblecompaction phenomena. With off-line methods alone, the revers-ible part of the compaction might not have been observed at all.This reversible part represented about 40% of the total compactionwhen the UC030 membrane was used at 5 bar. Consequently, it canbe concluded that on-line compaction values are important infor-mation when membranes are planned for applications, e.g. biore-finery fractionations, where the applied pressures might behigher than in water treatment. At higher operation pressures,the membrane permeability decreases also due to reversible com-paction. During filtration this phenomenon can be easily confusedwith concentration polarization, which could lead to adoption ofinappropriate mitigation methods.

Acknowledgements

The authors are grateful to the Academy of Finland (Project SA/122181). Special thanks to Peter G. Jones for language checking.

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