membrane autopsy as a sustainable management of fouling ... · tection (fluorimeter f-1050, merck)....

Desalination 204 (2007) 155–169

0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved

Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by theEuropean Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006.

*Corresponding author.

Membrane autopsy as a sustainable management of foulingphenomena occurring in MF, UF and NF processes

M. Pontiéa*, A. Thekkedatha, K. Kecilia,b,c, H. Habarouc, H. Sutyc, J.P. Crouéd

aUniversité d’Angers, UMR-MA105 Paysages et Biodiversité, Laboratoire des Sciences de l’Environnement et del’Aménagement (LSEA), ACI-JC n°4052 (2002), 2, Bd. Lavoisier, 49045 Angers cedex 01, France

Tel. +33 (2) 41 73 52 07; Fax +33 (2) 41 73 53 52; email: [email protected] Nationale Supérieure de Chimie de Paris, UMR-CNRS n°7575, ACI-JC n°4052 (2002),

11, rue P. et M. Curie, 75231 Paris cedex 05, FrancecAnjou-Recherche, Veolia Water, Chemin de la Digue, 78603 Maisons-Laffitte, France

dESIP, LCEE, 40 avenue du Recteur Pineau, 86022 Poitiers cedex, France

Received 28 February 2006; accepted 15 March 2006

Abstract

Our paper is focused on membrane autopsies conducted on hollow-fibre MF/UF membranes operated with rawsurface water and with two selected and well-characterized hydrophobic (denoted HPOA) and hydrophilic (denotedTPIA) NOM fractions for a polyamide nanofiltration (NF) membrane, using a large panel of analytical tools includingnovel technologies such as electron spectroscopy for chemical analysis (ESCA) and pyrolysis gas chromatography/mass spectrometry in addition to field emission gun scanning electronic microscopy (FESEM), atomic forcemicroscopy (AFM), contact angle and streaming potential measurements. Complementary information obtainedfrom the analyses of the feed water, the permeate and the backwash water solution (DOC, UV254, high pressureSEC, total amino acids (AA) and amino sugar (AS), hardness, alkalinity) are also discussed. Our results showedmore severe flux decline with MF membranes as compared to UF membranes. Also, MF PVDF which appeared tobe more hydrophobic, more negatively charged with a higher roughness was more susceptible to fouling than MFPS. Identification of proteins and amino sugars in the organic foulants (i.e. bacterial cell wall residues) as shown bythe pyrolysis GC/MS analysis permitted to suspect biofouling presence and FESEM and AFM images recordedfrom MF PVDF after backwash confirmed the presence of bacteria. The goal of our work was also to study the

156 M. Pontié et al. / Desalination 204 (2007) 155–169

fouling properties of 2 humic fractions (i.e. hydrophobic, denoted HPOA) and non-humic (i.e. hydrophilic, denotedTPIA), NOM fractions isolated from the Blavet River (France) toward a commercial polyamide (PA) NF membrane(denoted NF-55). The comparison of the affinity of both selected NOM fractions with the PA NF membrane wasconducted using contact angle, hydraulic permeability, streaming potential (SP) and NaCl aqueous solutionpermeation measurements. HPOA is more retained inside the pores as compared to the TPIA that was mainlysorbed on the membrane surface. Furthermore the membranes acidic–basic properties were amplified after foulantsdeposition in comparison to the cleaned membrane where a dominant specific sorption of monovalents and divalentions occurred.

Keywords: MF/UF/NF; Fouling; Autopsy

1. Introduction

Successful utilization of membrane technologyhas been greatly limited by membrane fouling.The fouling phenomenon increases operation andmaintenance costs by deteriorating membraneperformances (flux decline vs. time, zeta potentialchanging during time, etc.) and ultimately short-ening the membrane life. The membrane processperformances in terms of flux are strongly corre-lated to the raw water quality. Accumulation ofmaterial on or in the membrane skin leads to a re-duction of the water flux through the membrane.Natural organic matter (NOM) plays an importantrole in membrane fouling. For MF/UF membranesthis fouling is partially reversible by applying abackwash, which removes part of the accumulatedmatters. After being backwashed, the membranepartially regains its initial hydraulic permeabilitydepending on the nature of the foulants. Unfor-tunately, in nanofiltration (NF) process, pure waterbackwashing operations cannot be applied. NOMfouling increases operation and maintenance costsby deteriorating the membrane performance (fluxdecline vs. time) and ultimately shortening themembrane life.

The overall objective of our work is to bettercharacterize the nature of the fouling material thatis responsible for the reduction of permeabilityobserved during low-pressure membrane filtration(MF and UF) of raw and chemically treated sur-face waters using hollow-fibre modules and flat-sheet NF polyamide membranes. Our paper fo-

cuses on the membrane autopsy conducted on hol-low-fibre MF/UF membranes operated with rawsurface water and with two selected and well-characterized hydrophobic (denoted HPOA) andhydrophilic (denoted TPIA) NOM fractions forthe polyamide nanofiltration membrane, using alarge panel of analytical tools including noveltechnologies such as ESCA (electron spectroscopyfor chemical analysis) and pyrolysis gas chromato-graphy/mass spectrometry in addition to FESEM(field emission gun scanning electronic micro-scopy), AFM (atomic force microscopy), contactangle and streaming potential measurements.Complementary information obtained from theanalyses of the feed water, the permeate and thebackwash water solution (DOC, UV254, high pres-sure SEC, total amino acids (AA) and amino sugar(AS), hardness, alkalinity) are also discussed.

2. Materials and methods

2.1. Hollow-fibre modules (MF, UF)

Two MF membranes (modified polysulfone —PS and polyvinylidifluoride — PVDF) withsimilar molecular weight cut off (0.20 µm) andone modified PS UF membrane were used for ourstudy. The general characteristics of the hollow-fibre modules are given in Table 1. Filtrationmodules with PS hollow fibres were purchasedfrom the manufacturer whereas PVDF hollow-fibre modules were home-potted.

M. Pontié et al. / Desalination 204 (2007) 155–169 157

Table 1General characteristics of the hollow fibre modules

Membranes MF PVDF MF PS UF PS Material PVDF PS PS Inside diameter, µm 390 350 380 Outside diameter, µm 650 720 720 Fibre length, cm 43 43 50 Number of fibres 21 15 15 Surface, m2 0.0184 0.02 0.02 Mean pore size MF, µm/ MWCO UF, kDa

0.2 µm 0.2 µm 100 kDa

pH range 2–12 1–13 1–13 Filtration Outside → Inside

2.2. NF Membrane and module

The NF membrane under study is a thin-filmcomposite membrane built up of two layers — athin polyamide film denoted active surface and alarge mesoporous polysulfone denoted supportlayer. The membrane was received as a flat sheet.It was purchased from Filmtec (Dow, Denmark)and its commercialized name is NF-55. Thechemical structures of the support and activelayers materials are reported in a recent article[1]. Mean pores radius is 0.65 nm. Before all ex-periments the membrane was cleaned by meansof standard procedures to remove preservativesand rinsed with ultra-pure (UP) water from aMilliQ water system until the conductivity of thepermeate remained below 1 µS/cm. The effectivemembrane surface area was 138 cm2.

2.3. Feed water

The experiment was conducted with theprefiltered (1 µm) Marne River raw water (France)sampled in March 2003 during a very shiny periodwhich favoured the increase of the biological ac-tivity. The physical chemical characteristics of thestudied water are given in Table 2. The MarneRiver can be considered as a low DOC, low humiccontent, high alkalinity surface water.

Table 2Main characteristics of the feed water (raw surface waterprefiltered on 1 µm)

Parameters Feed water DOC, mg C/L 1.6–1.9 UV254nm, m–1 3.2–3.7 SUVA, L/mg C.m 1.7–2.3 Calcium, mg/L 85 Alkalinity, mg/L CaCO3 200 Silica, mg SiO2/L 1.5

2.4. Filtration protocol

Prior to any filtration, module integrity waschecked by soaking the module into ultrapurewater and flowing compressed air into the per-meate channel. Filtration tests were operated withlab-scale pilot test units equipped with hollow-fibre modules in a dead-end filtration mode at con-stant pressure and constant temperature (20°C).MF membranes were performed at –350 mbarwhereas UF experiments were carried out at–450 mbar. Transmembrane pressure, permeateflow rate as well as feed water temperature wererecorded with data logger every minute.

The membrane autopsy was conducted onmembranes that showed a 40% flux reduction


obtained after several days of operation. Back-wash (MilliQ water) was performed every day anda final chemical cleaning (NaOCl solution of200 ppm of free chlorine at 20°C followed by a20 g/L citric acid solution at 35°C) was operatedat the end of the experiment. Hollow fibres wereextracted before and after the chemical cleaningfor membrane autopsy. To ensure module integrityafter taking half of the fibres, holes were pluggedusing pins. Integrity of the module was checkedagain so that ultrapure water permeability couldbe measured. Fig. 1 presents the experimental pro-tocol scheme followed during our studies.

Fig. 2 shows a picture of one of the pilot unitsused in this study. Permeate and feed waters weresampled every day for analysis (detailed in theAnalytical methods section).

2.4. Membrane autopsy

Membrane autopsies were performed usingthree approaches:• Characterization of back wash solutions using

total amino acids/aminosugars, and HPSEC/UV analyses

• Characterization by pyrolysis GC/MS of thefouling material isolated from the hollow fibersusing sonication (hollow fibres that were cutinto small pieces were subjected to sonicationin the presence of 50 mL of MQ water. Afterfiltration through a 1.2 µm porosity glass fibremembrane, the solution was lyophilized).

• Characterization of the membrane surfaceusing contact angle measurement, AFM,FESEM, ESCA and TOF SIMS analyses.

2.5. Analytical methods

Dissolved organic carbon (DOC) mea-surements were performed using a Shimadzu TOC5000A. UV absorbance at 254 nm was conductedusing a UV-VIS SAFAS Double Energy System190(UP) spectrophotometer. The specific UV ab-sorbance (SUVA) was calculated as the ratio ofUV absorbance and DOC content.

Total amino acids and total amino sugars wereanalyzed by HPLC after acid hydrolysis (6 N HCl,6 h) and precolumn derivatization using the AQCderivatization agent (6-aminoquinolyl-N-hydro-xysuccinimidyl carbamate). The reaction betweenAQC and amino acids or aminosugars leads tothe formation of highly fluorescent complexes thatcan be analyzed using fluorescence emission de-tection (fluorimeter F-1050, Merck).

High pressure size exclusion chromatography(HPSEC) analyses were conducted using a Protein-PakTM 125 column (ID: 7.8 mm; length: 30 cm)coupled with a UV detector (Waters 996 diodearray detector) according to the method proposedby Chin et al. [2].

Foulant material (approximately 1 mg of drysample deposited into 100 µL quartz tubes) weresubmitted to flash pyrolysis–GC/MS (finaltemperature of 625°C at a rate of 20°C/ms, witha final hold for 20 s) according to the methoddescribed by Bruchet et al. [3] using a Pyroprobe2000 (Chemical Data Systems, Oxford, Pa.) fila-ment pyrolyzer coupled with a Hewlett PackardG1800 A. After flash pyrolysis, the thermal degra-dation fragments were separated by GC. A 30-mAT-Wax (Alltech) fused-silica capillary columnwas temperature-programmed from 30 to 220°Cat a rate of 3°C/min. The fragments were thenidentified by a mass spectrometer operated at70 eV scanning from 20 to 450 amu at 1 scan/s.

Zeta potential values are calculated from ahomemade streaming potential apparatus, aspreviously described [4]. A pair of silver-chlorideelectrodes (Radiometer-analytical, France) werejudiciously introduced, one in the permeatechannel and the other one in the retentate com-partment. The electrodes are connected to a highimpedance voltmeter. The interest of this appa-ratus is its ability to characterize only the chargebrought by the pore walls [5].

The contact angle measurements were carriedout by the sessile drop technique: a liquid dropletof 1–2 µL is placed onto a flat homogeneous sur-face by means of a microsyringe. The contact


Feed water sampling point

Permeate channel

Permeate sampling

Fig. 1. Scheme of the experimental protocol.

Fig. 2. MF/UF filtration unit.


angle of the droplet with the surface is measuredwith KG1 apparatus purchased from Kruss (Ger-many). The angles are measured using the gonio-meter of the apparatus. These measurements werecarried out on dried fibre membrane samples(length 5 cm), fixed to the sample plate by doubleface sticky tape. During the short measurementtime (less than 1 min), no change in the contactangle was observed. Contact angles do not giveabsolute values of the hydrophilic/hydrophobiccharacter of the surface but allow a comparisonbetween clean and fouled membranes. A variationof 5° in the angle is needed in order to differentiatetwo samples.

Atomic force microscopy (AFM) analyses wereperformed with membrane samples that werepreliminary dried at room temperature. Thesamples were fixed by a self-adhesive tape andthe external side of the membrane fibre wasanalysed with a Nanoscope IIIA from Digital In-struments (multimode). All samples were analysedin tapping mode and phase contract in order tocharacterize the samples morphology and to mea-sure the mean surface roughness (RMS).

Field emission scanning electron microscopy(FESEM) was conducted using a LEO1501 ap-paratus. Before the analysis all samples were fixedwith carbon self-adhesive tape and then recoveredof a ultra-thin layer of Pt/Pd (4 nm) using anevaporating technique. Beam energy was very low(under 3 keV).

Electron spectroscopy for chemical analysis(ESCA) is also called XPS (X-ray photoelectronspectroscopy) and provides information about theelemental composition of the membrane surface.

Table 3Some characteristics of the tested membranes

Membranes MF PVDF MF PS UF PS Pure water permeability, L/h.m2.bar 450–550 250–350 90–150 Contact angle, ° 79±2 0±5 0±5 Rms roughness, nm 190±40 33±10 7±5 Zeta potential, mV ( ±1) (MF KCl 210–4 MUF 10–3 M) –18 –11 –2

ESCA analysis were conducted with a Scienta 200(USA) under the following operating conditions:source: Al Kα monochromatic (450 Wt), analysedarea: around 0.5×4 mm2, detection angle: 90°, ana-lysis in the material occurred with a penetrationthickness of 10 nm. All samples were dried beforebeing analysed. The results presented correspondto the mean average of the analyses of three dif-ferent parts of the membrane: the top, the middleand the end of the fibre.

3. Results and discussion (MF–UF experi-ments)

3.1. Characterization of the tested membranes

Additional characteristics of the membranesstudied are given in Table 3. The membranesstudied bring negative charge as obtained fromzeta potential values. In the case of PS materialwe can assimilate this charge to residual SO3

–

functional groups. For the PVDF membrane thisfinding cannot be discussed due to the lack ofinformation. The two MF membranes showsignificant differences in terms of roughness (seeFig. 3) and hydrophobicity. The MF PS was foundto be highly hydrophilic in character as comparedto MF PVDF due to surface grafting of hydrophiliccomponents. It is important to notice that the highroughness observed for the MF PVDF membranemay significantly influence the contact angle mea-surements, as reported by Neumann and Good [6].On the contrary, contact angles determined for thePS membranes should be less influenced by sur-face roughness.


Fig. 3. 3D images of MF PVDF and UF PS membranes.

Fig. 4. 2D FESEM images of MF PVDF (left) and MF PS (right) membranes.

As shown in Fig. 4, the two MF membraneshave very different morphologies.

We can remark also for the same material (PS)but different pore sizes (MF and UF) that zeta po-tentials calculated from streaming potential mea-surements are different. Indeed overlapping of theelectrochemical double layer occurred in UF andnot in MF. Then, in UF, the zeta potentials obtain-ed are only apparent. Finally the two MF mem-branes exert very different hydraulic permeability,

450–550 L/h.m2.bar for the PVDF membrane ascompared to 250–350 L/h.m2.bar for the PS mem-brane.

3.2. Dramatic decrease of hydraulic permeabil-ity of the tested membranes

Fig. 5 shows as an example the decrease ofthe relative permeability of the MF-PVDF moduleas a function of time during a period of 4 days.


0,0

0,2

0,4

0,6

0,8

1,0

0 500 1000Time (min)

Lp/L

p0

Fig. 5. Relative permeability as a function of time for theMF PVDF modules.

Fig. 6. Membrane flux decline during filtration of the Marne River water (prefiltered on 1 µm): relative permeability vs.DOC delivered to the membrane.

The rapid reduction of the relative permeabilityas a function of time is a consequence of the strongfouling properties of the feed water. Whatever thebackflushing step operated after each day offiltration, one can observe a gradual increase ofthe kinetics of flux reduction day after day as aconsequence of irreversible fouling phenomenon(the flux was only partly recovered after back-flushing as shown by the gradual decrease of therelative permeability when the pilot unit was re-

started for a new period of filtration). After a pe-riod of 3 days, the hydraulic permeability wasreduced by approximately 80% due mainly toirreversible fouling (i.e. fouling that remained afterbackwash). Fig. 6 compares the relative perme-ability of the three modules tested as a functionof the mass of organic carbon delivered per m2 ofthe membrane. Flux declines were significantlymore important for MF membranes as comparedto the UF membranes. Also, the MF PS membraneexerted lower flux decline than the MF PVDFmembrane. However the two MF membranesshowed irreversible fouling.

3.3. Evidence of membrane fouling: modificationof physical and chemical properties of the mem-branes studied

Fig. 7 shows 2D FESEM images of the fouledMF PVDF and MF PS membranes obtained afterfiltration of the 1 µm prefiltered Marne River rawwater. As compared to Fig. 4, fouling materialsare visible not only at the surface but also in thepores .

Furthermore, evidence of the presence of foul-ants at the surface of the membrane was confirmedby the modification of the membrane charac-

0

0.5

1

1.5

2

0 750 1000 1250 1500250 500 1750 2000DOC delivered to the membrane (mgC/m 2 )

Lp

/ Lp0

MF PVDFMF PSUF PSUF PS


Fig. 7. 2D FESEM images of the fouled MF PVDF (left) and MF PS (right) membranes.

Table 4Evolution of the water permeability, contact angle, zeta potential and surface mean roughness (Rms roughness) of virgin,fouled (after the final backwash) and chemically cleaned membranes

Parameters Membranes Virgin Fouled After chemical cleaning MQ water permeability, L/h.m2.bar MF PVDF

MF PS UF PS

430 330 140

10 150 65

150 200 ND

Contact angle, ° MF PVDF MF PS UF PS

79±2 0±5 0±5

52 0 34

77 0 24

Zeta potential, mV (±1) MF PVDF MF PS UF PS

–18 –11 –2

–2 –7 ND

–8 –11 ND

Rms roughness, nm (20×20 µm2) MF PVDF MF PS UF PS

190±40 33±10 7±5

220±38 52±5 51±8

240±48 76±20 86±43

teristics. Table 4 compares the water permeability,contact angle, zeta potential and roughness ofvirgin, fouled (after the final backwash) and chem-ically cleaned membranes.

After several days of filtration and a final back-wash, the MilliQ water permeability was foundto be severely reduced for all membranes, the MFPVDF showing the most important modification.For the MF PVDF, a slight decrease of its contact

angle was noticed (reduction of the hydrophobiccharacter) while surprisingly no change wasobserved for the MF PS. On the contrary, the UFPS membrane became more hydrophobic (contactangle increases). The increase in roughness con-firmed the accumulation of materials at the surfaceof the three studied membranes. The attenuationof the charge (less negative) as deduced from thestreaming potential measurements also indicates


that foulants are also present in the membranepores. One can notice that only minor modifica-tions were observed for the MF PS by comparingwith the two other membranes.

After chemical cleaning the initial permeabilityof the MF membranes was only partially recover-ed. The cleaning procedure seemed to be howevermore efficient when applied to the MF PS mem-brane with regard to flux characteristics. The zetapotential of the MF PS was found to reach its ini-tial levels after cleaning while the contact anglefor the UF PS membrane remained unchanged.The opposite trend was observed for the MFPVDF (contact angle similar to its original value,zeta potential slightly modified). The cleaningprocedure did not modify the roughness of allmembranes that remained high as compared tothe original values (virgin module). The little in-crease after cleaning may correspond to analyticalerrors. The efficiency of the cleaning proceduredepends on the nature of the membrane. Also,these results seem to indicate that both alkalineand acid cleanings did not totally remove thefouling materials. This finding may indicate thatthe foulants may correspond to neutral type struc-tures and possibly positively charged components(decrease in zeta potential).

-1

1

3

5

7

2 5 8 11 14

Retention time (min)

UV

260

(Arb

itrar

y un

it)

raw waterMF PVDFMF PSUF PS

Fig. 8. HPSEC/UV chromatograms of raw, permeate and backwash waters (a) raw waters and permeates; (b) backwashwaters.

(a) (b)

3.4. Structural identification of the fouling mate-rial

3.4.1. Analyses of the backwash waters

The HPSEC/UV260 profiles of the raw waterand the corresponding MF and UF permeates areshown in Fig. 8a. All chromatograms are relativelysimilar which agrees as shown by others that bothMF and UF membranes do not signifcantly re-move organics. DOC analyses confirmed this find-ing which indicates that MF and UF foulants re-present a minor fraction of the organics presentin the solution.

Fig. 8b compares the HPSEC/UV profiles ofthe MF and UF backwash waters collected duringthe whole filtration experiment over several days.MF and UF backwash waters exhibit very distinctprofiles. The MF profiles are mainly characterizedby a more intense peak at 6 min that can be attri-buted to polysaccharide-type structures (high mo-lecular weight structures), whereas the UF profilethat is similar to both feed water and permeatesshows chromatographic bands (retention timesbetween 10 and 11 min) that relate to humic-likemoieties (humic substances and building blocks,as reported by Huber [7], and low molecularweight components. The HPSEC profiles demon-

-1

1

3

5

2 5 8 11 14

Retention time (min)

UV

260

(Arb

itrar

y un

it) MF PS

UF PS

MF PVDF

(b)


strate that the nature of MF and UF foulants issignificantly different, higher molecular weightstructures contributing to a large extent to MFfouling.

Total dissolved amino acids and aminosugarsanalyses performed on the backwash watersindicated that the DOM recovered was proportion-ally more enriched in proteinaceous and amino-sugar type structures as compared to the feed water(2900–3000 nmol/mgC as compared to 300–400 nmol/mg C).

3.4.2. Analysis of the fouling material by py-rolysis GC/MS

Part of the foulant was recovered from thehollow fibres using sonication in the presence ofMilliQ water, lyophilized and subjected to flashpyrolysis GC/MS analysis. Fig. 9 shows the GC/MSpyrochromatogram of the foulant isolated fromthe MF PS after backwash.

The abundance of the acetamide peak (produc-ed from aminosugars) and the presence of the hyd-roxypropanone, furfural, and cyclopentenonepeaks (polysaccharide origin) in combination withthe acetonitrile, toluene, styrene, pyrrole and pyr-

Fig. 9. Pyrochromatogram of fouling material isolated from MF PS hollow fibre after backwash.

Acé

tam

ide

Pyrr

ole

Cré

nolAci

de a

cétiq

ueFu

rfur

al

Phen

ol

Ben

zoni

t rile

Styr

e ne

hydr

oxyp

ropa

none

2 C

yclo

pent

ene-

one

Met

h yl p

yrro

l e

Pyrid

i ne

Me t

hyl p

yrid

ine

Acé

ton i

trile

Tol u

ène

ridine peaks (protein origin), indicate that the foul-ing material is mainly of bacterial origin (livingand/or nonliving residues plus soluble microbialproducts). The identification of minor peaks ofphenol and cresol in the pyrochromatogram canbe related to the presence of a low amount of poly-hydroxyaromatic structures (i.e. humic-type mate-rial) in the foulant, however tyrosine is also knownto produce the same by-products during pyrolysis.This fingerprint is similar to the one obtained fromthe analysis of organic colloids isolated fromsurface water and identified as bacterial cell wallresidues.

3.4.3. Chemical analysis of the membrane sur-face

ESCA analyses were conducted on membranefibres after backwash and chemical cleaning.ESCA provides the elemental composition of the“extreme” surface (10 nm) of the membrane al-lowing the identification of a potential foulant. Italso provides insight in the chemical structure ofthe organics with the possible identification offunctional groups. Table 5 gives the elementalcomposition (expressed in %, sum of all the ele-


ments identified is equal to 100%) of virgin hollowfibres and hollow fibres isolated from the moduleafter backwash and chemical cleaning.

The elemental composition determined byESCA analysis revealed the presence of nitrogenfor all virgin membranes, the element that doesnot enter as a constituent of the membrane poly-mer. The same remark can be underlined with theidentification of oxygen in the virgin PVDF mem-brane. The identification of oxygen as well asnitrogen as part of the material composition maycorrespond to the presence of chemicals common-ly used by manufacturers to make the membranesurface more hydrophilic either to solvents used

Table 5Elemental composition (ESCA analysis) of clean and fouled membranes (Marne River)

MF PVDF MF PS UF PS Element Clean After

backwash After chemical cleaning

Clean After backwash

After chemical cleaning

Clean After backwash

After chemical cleaning

% C 49.61 49.69 49.16 76.23 52.15 65.69 72.32 52.85 65.93 % F 43.02 21.48 39.36 — — — — — — % O 6.11 22.40 9.02 17.00 34.26 24.08 19.82 33.98 23.58 % N 1.26 2.34 0.91 3.06 4.07 2.35 3.11 4.14 2.45 % S — — — 2.48 0.59 1.58 1.82 0.74 1.77

during hollow fibre production. Low temperaturepyrolysis GC/MS analyses conducted on virginMF and UF PS confirmed the presence of pyrro-lidinone and pyrrolidone type structures as partof the membrane constituents (non shown). Ele-mental compositions of the fouled membraneswere found to be significantly different as com-pared to the virgin membrane. This finding con-firms the presence of organic deposits. The de-crease of the relative abundance of fluoride andsulphur for PVDF and PS membranes respectivelyare good indicators of the presence of additionalorganic constituents (i.e. foulants). The averageelemental composition of aquatic NOM (i.e. 45–

Fig. 10. 2D AFM-FESEM images for the evidence of bacteria development under the MF PVDF membrane.

FESEM AFM


50% C; 35–40% O; 3–5% N; 4–5% H; 1–2% S)agrees with the observed changes of the elementalcomposition of the membrane that are an increaseof N (85% for MF PVDF and 33% for both PSmembranes) and O (366%, 200% and 170% forMF PVDF, MF PS and UF PS) for all membranes,a decrease of C for PS and constant C content forPVDF. ESCA analyses also clearly underlined thatN was present at the surface of the membrane asamine and amide functional groups. This result isin accordance with the identification of proteinsand aminosugars in the organic foulants (i.e. bac-terial cell wall residues) as shown by the pyrolysisGC/MS analysis. As a support of our hypothesis,the presence of bacteria at the surface of the mem-brane was clearly observed on FESEM and AFMimages recorded from MF PVDF after backwash(Fig. 10).

After chemical cleaning, the elemental com-position of the all membranes remained slightlydifferent than the one determined for the virginmembrane. Traces of organic foulants still re-mained on the membrane after cleaning as shownby the deviation noticed for C and O (MF and UFPS) and O and F (MF PVDF) with the virgin mem-brane. However these results confirmed the bene-ficial impact of the cleaning procedure.

4. Results and discussion (NF experiments)

The goal of our work was to study the foulingproperties of 2 humic fractions (i.e. hydrophobic,denoted HPOA) and non-humic (i.e. hydrophilic,denoted TPIA) NOM fractions isolated from theBlavet River (France) toward a commercial

Table 6 Structural characteristics of the Blavet River NOM isolates

Elemental analysis (%)

Acid base titration (meq/g of TOC)

Molecular weight (Da)

UV254/TOC (m–1.L/mg C)

Fraction C H N O Ashes COOH OH phenolic Mw Mn SUVA HPOA 47.0 4.5 2.0 38.8 6.2 8.8 4.1 1570 900 4.3 TPIA 43.3 4.6 2.9 40.6 2.7 11.7 4.3 1040 700 2.6

polyamide (PA) NF membrane (denoted NF-55).For the characterization of NOM fractions,different analytical tools were used: elementalanalyses using microanalyses, specific UVabsorbance, solid-state cross-polarization magicangle spinning (CPMAS) 13C-NMR, high pressuresize exclusion (HPSEC) chromatography andacid/base titration [1,8]. The comparison of theaffinity of both selected NOM fractions with thePA NF membrane was conducted using contactangle, hydraulic permeability, streaming potential(SP) and NaCl aqueous solution permeationmeasurements.

Table 6 gives some structural characteristicsof the NOM fractions isolated from the BlavetRiver.

The elemental analysis indicates that theHPOA and TPIA fractions are almost pure NOMwith a very low ash content. The HPOA fractionshows higher C/O (more oxygenated functionalgroups), C/N (more proteinaceous-type struc-tures), and C/H (more unsatured carbon bonds)ratios as compared to the TPIA fraction. TheHPOA fraction also incorporates a larger pro-portion of high molecular weight (Mw) structures(i.e. higher Mw determined by HPSEC/UV254analysis) and a larger proportion of UV absorbingmoieties (i.e. aromatic moieties) as indicated byits higher SUVA value (i.e. specific UV absorb-ance, UV absorbance at 254 nm divided by DOC).

The PA material was found to be more sensitiveto hydrophobic NOM adsorption leading toirreversible fouling with drastic modifications ofthe initial physico-chemical properties of themembrane: (i) increase of its hydrophobicity,


(ii) decrease of its hydraulic permeability asso-ciated with a decrease in its pore size and con-sequently (iii) increase of the observed rejectionof salty solutions. The higher decrease in theperformances of this PA NF membrane is observedfor the more hydrophobic foulant, HPOA, asshown in Fig. 11.

At the same time, a displacement of theisolectric point (IEP) of the membrane materialwas observed from 4.5 for the clean membrane(KCl 10–4mol.L–1) to 3.4 after HPOA sorption. Ata lower pH range than the IEP, the effects ofcations and H+ on the charge properties of themembranes increase near the shear plane, yieldingmore positive SP values, as recently reported byPontié et al. [1].

For the hydrophilic TPIA foulant no displace-ment of the IEP was observed. Then the results ofSP experiments conducted through the membranehave indicated that HPOA is more retained insidethe pores as compared to the TPIA that was mainlysorbed on the membrane surface. Furthermore, the

Jp/ ∆

P (L

.h-1

.bar

-1)

0.22

0.2

0.18

0.16

0.14

0.12

2 pH4 6 8 10 120.1

Clean NF-55

NF-55 fouled HPOA

NF-55 fouled TPIA

Fig. 11. Hydraulic permeabilities vs. pH for NF-55: clean,fouled with HPOA and fouled with TPIA (pH = 6.7, UPwater solutions, T = 20°C, membrane contact time withNOM = 6 h).

membranes acidic–basis properties were amplifiedafter foulant deposit in comparison to the cleanedmembrane where a dominant specific sorption ofmonovalents and divalent ions occurred.

5. Conclusion

Our results showed more severe flux declinewith MF membranes as compared to the UF mem-brane. Also, MF PVDF which appeared to be morehydrophobic, more negatively charged with ahigher roughness was more susceptible to foulthan MF PS. As has already been observed byothers, DOC and UV254 remained almost unchang-ed after MF and UF membrane filtration whichconfirmed that foulants represent a minor part ofthe organic constituents of the feed water.

HPSEC/UV conducted on backwash watersindicated that MF fouling materials mainly incor-porate non-humic structures of high and low mole-cular weights, while the presence of humic com-ponents was clearly identified in UF fouling mate-rials. Total dissolved aminoacid and aminosugaranalyses showed that backwash waters were moreenriched in proteinaceous type structures ascompared to the raw water.

The presence of fouling material at the surfaceof the membrane or in the pores after backwashwas confirmed based on the contact angle, AFMand zeta potential measurements. Pyrolysis GC/MS analysis operated on organics isolated (sonica-tion) from the membrane after backwash indicatedthat the fouling material made of polysaccharides,aminosugars and proteinaceous structures wasmainly of bacterial origin (living and/or nonlivingresidues plus soluble microbial products). ESCAanalyses conducted at the membrane surfaceconfirmed the presence of organic materials withamine and amide functional groups. Part of thefoulants remained after chemical cleaning.

In order to simulate specific events, such asrainfalls or algal bloom which lead to a higherDOC content in surface waters, and in order tobetter understand and characterize how each of


these organic fractions behaves regarding MF, UFand NF membranes fouling, future tests withothers synthetic solutions containing proteins, su-gars, polysaccharides, alone and in melting solu-tions will be performed.

References[1] D. Violleau, H. Habarou, H. Essis-Tome, J.P. Croué

and M. Pontié, Fouling studies of a polyamide nano-filtration (NF) membrane by selected natural organicmatters (NOM): an analytical approach, Desalina-tion, 173 (2005) 223–238.

[2] Y. Chin, G.R. Aiken and E. O’Loughlin, Molecularweight, polydispersity and spectroscopic propertiesof aquatic humic substances. Environ. Sci. Technol.,28 (1994) 1853.

[3] A. Bruchet, C. Rousseau and J. Mallevialle, Pyro-lysis–GC–MS for investigating high-molecular-weight THM precursors and other refractory orga-nics. J. AWWA, 82(9) (1990) 66–74.

[4] M. Pontié, X. Chasseray, D. Lemordant and J.M.Lainé, The streaming potential method for the chara-cterization of ultrafiltration organic membranes andthe control of cleaning treatments, J. Membr. Sci.,129 (1997) 125–133.

[5] M. Pontié, Effect of aging on UF membranes by astreaming potential (SP) method, J. Membr. Sci., 154(1999) 213–220.

[6] A.W. Neumann and R.J. Good, Thermodynamics ofcontact angles: I-Heteregeneous solid surfaces, J.Coll. Int. Sci., 38(2) (1972) 341–358.

[7] S.A. Huber, Evidence for membrane fouling byspecific TOC constituents, Desalination, 119 (1998)229–234.

[8] J.A. Leenheer, J.P. Croué, M. Benjamin, G.V. Kor-shin, C.J. Hwang, A. Bruchet A. and G. Aiken, Com-prehensive isolation of natural organic matter fromwater for spectral characterizations and reactivitytesting in natural organic matter and disinfection by-products: characterization and control in drinkingwater, S.E. Barrett, S.W. Krasner and G.L. Amy, eds.,ACS Symposium Series, 761 (2000) 68–83.

membrane autopsy as a sustainable management of fouling ... · tection (fluorimeter f-1050, merck)....

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