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0043-1354/$ - see frodoi:10.1016/j.watres

�Corresponding auE-mail addresses

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Complexity of ultrafiltration membrane fouling caused bymacromolecular dissolved organic compounds insecondary effluents

Jens Haberkampa,�, Mathias Ernstb, Uta Bockelmannc, Ulrich Szewzykc, Martin Jekela

aTechnische Universitat Berlin, Chair of Water Quality Control, Sekr. KF 4, Str. des 17. Juni 135, 10623 Berlin, GermanybTechnische Universitat Berlin, Centre for Water in Urban Areas, Sekr. KF 4, Str. des 17. Juni 135, 10623 Berlin, GermanycTechnische Universitat Berlin, Chair of Environmental Microbiology, Sekr. FR 1-2, Franklinstr. 29, 10587 Berlin, Germany

a r t i c l e i n f o

Article history:

Received 14 January 2008

Received in revised form

12 March 2008

Accepted 13 March 2008

Available online 26 March 2008

Keywords:

Cross-flow ultrafiltration

EPS extraction

Extracellular polymeric substances

Membrane fouling

Secondary effluent

Size exclusion chromatography

nt matter & 2008 Elsevie.2008.03.007

thor. Tel.: +49 30 314 25367: jens.haberkamp@tu-beu-berlin.de (U. Bockelma

a b s t r a c t

Recent investigations indicate the relevance of extracellular polymeric substances (EPS) in

terms of fouling of low-pressure membranes in advanced wastewater treatment. In this

study, the high impact of the macromolecular fraction of effluent organic matter on fouling

was confirmed in cross-flow ultrafiltration experiments using secondary effluent with and

without autochthonous biopolymers. A method for the extraction of a natural mixture of

EPS derived from the bacterium Sinorhizobium sp. is presented. Ultrafiltration of solutions of

this bacterial EPS extract revealed a correlation between the concentration of EPS and the

loss of permeate flux. However, in ultrafiltration tests using extracted bacterial EPS in a

model solution as well as in secondary effluent without autochthonous biopolymers, the

extent of membrane fouling was not identical with the fouling provoked by secondary

effluent organic matter, although the biopolymer concentrations were comparable. The

differences in the fouling behaviour of the extracted bacterial EPS and effluent organic

matter are considered to be due to different compositions of the biopolymer fraction in

terms of proteins, polysaccharides, and other organic colloids, indicating a particular

impact of proteins on ultrafiltration membrane fouling.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Wastewater treatment is currently facing increasing demands

regarding the enhanced protection of receiving water bodies

and the reuse of secondary effluents as resource for drinking

water production in scarcity areas. Low-pressure membrane

filtration provides a potential alternative for advanced treat-

ment of municipal sewage, e.g., in membrane bioreactors

(MBRs) or tertiary treatment of secondary effluent. Their

application in such systems has significantly increased with-

in the last decade. However, membrane fouling is still a

r Ltd. All rights reserved.

; fax: +49 30 314 23850.rlin.de (J. Haberkamp), mnn), ulrich.szewzyk@tu-b

fundamental drawback, necessitating periodical chemical

cleanings and eventually forcing the replacement of irrever-

sibly fouled membranes. Membrane fouling can be caused by

particles, dissolved or colloidal organic and inorganic sub-

stances, as well as by the attachment of microorganisms onto

the membrane surface. While the formation of a filter cake

due to the deposition of particulate matter is controllable by

appropriate hydrodynamic conditions and backwashing, the

character and size of fouling-causing substances being

smaller than 0.45mm are not yet completely identified. te

Poele (2005) indicates the significance of colloids of the size

athias.ernst@tu-berlin.de (M. Ernst),erlin.de (U. Szewzyk), martin.jekel@tu-berlin.de (M. Jekel).

ARTICLE IN PRESS

Table 1 – Composition of R2A culture medium agar(following Reasoner and Geldreich, 1985)

Substance Amount per litre of ultra-pure water

Yeast extract 0.5 g

Proteose peptone no. 3 0.5 g

Casamino acid 0.5 g

Glucose 0.5 g

Sodium pyruvate 0.3 g

Dipotassium hydrogen

phosphate (K2HPO4)

0.3 g

Magnesium sulphate

(MgSO4 � 7H2O)

0.05 g

Agar 15 g

Tween 80 (fatty acid ester) 1 mL

WAT E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 1 5 3 – 3 1 6 13154

fraction between 0.45 and 0.1 mm in secondary effluent,

whereas Laabs et al. (2006) suggest the relevance of organic

macromolecules with sizes between 0.1 and 0.01mm regarding

the fouling of low-pressure membranes. Further studies have

pointed out the particular impact of dissolved organic

macromolecules on the loss of filtration performance in

micro- and ultrafiltration (Jarusutthirak and Amy, 2006),

especially in terms of irreversible long-term fouling (Rosen-

berger et al., 2006). The dissolved organic matter of secondary

effluent includes non-biodegradable substances deriving

from the raw wastewater, as well as compounds released

during the treatment process. The macromolecular fraction is

mainly composed of extracellular polymeric substances (EPS),

i.e., biopolymers of microbial origin. These are especially

polysaccharides, which are excreted for the adhesion of

bacteria onto surfaces (biofilm formation) or the cohesion to

other bacteria (formation of microbial aggregates), and

proteins, which possibly act as exo-enzymes, but also nucleic

acids and lipids (Flemming and Wingender, 2001).

Several studies focussing on the influence of the EPS

concentration on the extent of membrane fouling have

recently been published (Rosenberger et al., 2005; Ye et al.,

2005a; Garcia-Molina et al., 2006; Katsoufidou et al., 2007; van

de Ven et al., 2008). While te Poele and van der Graaf (2005)

indicate the relevance of proteins in ultrafiltration of second-

ary effluent, other authors point out the impact of poly-

saccharides on membrane fouling (Ye et al., 2005b;

Rosenberger et al., 2006; Fonseca et al., 2007). However, most

of the fouling studies are conducted either by means of

bench-scale tests, using model solutions of commercially

available proteins and polysaccharides with limited compar-

ability to natural wastewaters; or by observation of pilot

plants or full-scale filtration systems, which are fed by real

effluents with complex and varying composition, making it

difficult to draw explicit conclusions between the water

constituents and the flux decline in the filtration process.

The objective of this study was to investigate the impact of

dissolved organic matter (defined as substances o0.45mm) on

the extent of membrane fouling in cross-flow ultrafiltration.

Apart from ultrafiltration tests using secondary effluent, the

fouling behaviour of natural EPS in a model solution and in

secondary effluent with and without autochthonous biopoly-

mers was examined. For this purpose, a method for the

extraction of bacterial EPS was developed, allowing controlled

variations of the concentration of natural EPS in secondary

effluent.

2. Materials and methods

2.1. Extraction of bacterial EPS

Natural bacterial EPS were obtained from the bacterium

Sinorhizobium sp. This originally soil-borne, gram-negative

microorganism had previously been isolated from a slow sand

filter used for infiltration of surface water in Berlin-Marien-

felde (Germany) where it has been identified as predominant

bacterium, excreting high quantities of EPS.

For the extraction of bacterial EPS, an overnight culture of

Sinorhizobium sp. was plated onto petri dishes containing the

solid oligotrophic medium R2A (Table 1). After 72 h of

incubation at 28 1C, the bacterial cells together with the

produced EPS were scraped off from the agar plates

and resuspended in phosphate buffer solution (4 mmol L�1

NaH2PO4, 2 mmol L�1 Na3PO4, 9 mmol L�1 NaCl, 1 mmol L�1

KCl, pH ¼ 7; according to Frolund et al., 1996). In order to

detach the EPS bound to the bacterial cells, the suspension

was stirred for 2 h in contact with the cation exchange resin

Dowexs Marathons C (6 g per 0.1 L of suspension), which had

previously been equilibrated for 1 h in the phosphate buffer

solution. Thus, stabilising calcium ions were removed and

bound EPS were released into the solution (Jahn and Nielsen,

1995). Cation exchange resin and bacterial cells were subse-

quently separated from the EPS solution by centrifugation

(35 min at 3500 rpm) and filtration through 0.45mm cellulose

nitrate filters.

2.2. Analytical methods

2.2.1. Size exclusion chromatographySize exclusion chromatography with continuous UV254 nm and

organic carbon (OC) detection was used to characterise the

DOC composition of EPS solutions and secondary effluent (LC-

OCD system by DOC-Labor Dr. Huber, Karlsruhe, Germany;

SEC column: Toyopearls HW-50S by Tosoh Bioscience, Tokyo,

Japan). A characteristic LC-OCD chromatogram of secondary

effluent is presented in Fig. 1. Proteins, polysaccharides, and

further organic colloids elute within the so-called biopolymer

peak and are quantified by the calibrated infrared detector of

the LC-OCD system. The detection limit of the LC-OCD

system is 10mg L�1, the standard deviation is less than 1% of

the measured value (measurement range: 1–5 mg L�1 C;

samples containing higher DOC concentrations are diluted).

An additional UV detector allows the qualitative estimation of

organic nitrogen (ON) contents of the separate fractions by

measuring the absorbance of nitrate (at l ¼ 220 nm), which is

formed by oxidation of organic compounds inside the

oxidation reactor. A method for the quantitative analysis of

the ON concentration of the different fractions is currently

being developed.

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WA T E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 1 5 3 – 3 1 6 1 3155

2.2.2. Determination of polysaccharides, proteins, and totalnitrogen contentThe polysaccharide concentration (as glucose equivalents) of

the bacterial EPS solution and secondary effluent was

determined using the photometrical method following Du-

bois et al. (1956). The protein concentration (as BSA equiva-

lents) was measured by the modified photometrical Lowry

method according to Frolund et al. (1996). The total nitrogen

concentration of the EPS solution was determined using a

Multi N/C 3100 high-temperature analyser (Analytik Jena AG,

Jena/Germany).

2.3. Experimental set-up

The fouling tests were conducted using the experimental set-

up illustrated in Fig. 2. The applied ultrafiltration flat-sheet

membrane UP 150 is made of permanently hydrophilised

polyethersulfone (PES) and has a nominal molecular weight

200

2

4

6

8

low molecularweight neutrals

low molecularweight acidshumic

substances

biopolymers(EPS)O

C s

igna

l [A

U]

elution time [min]120100806040

Fig. 1 – LC-OCD chromatogram of secondary effluent (three-

fold dilution).

flowmeter

feedtank

cross-flowtest cell

gearpump

peristalticpump

ba

feed pressuregauge and valve

Fig. 2 – Ultrafiltrati

cut-off (MWCO) of 150 kg mol�1 (supplied by Microdyn-Nadir

GmbH, Wiesbaden, Germany). For each filtration experiment,

a new membrane sheet was inserted into the Plexiglass

cross-flow test cell (effective membrane surface area: 0.02 m2)

and rinsed with 12 L of deionised water in order to remove

solvent residues originating from the production process. The

membrane was subsequently pre-compacted for 24 h using a

solution of 6 mmol L�1 NaCl and 3 mmol L�1 CaCl2, resulting in

initial permeabilities of 326740 L m�2 h�1 bar�1 at 1 bar trans-

membrane pressure (TMP). Thereafter, the 24-h fouling test

was started either by addition of the EPS concentrate to the

NaCl/CaCl2 solution, or by positioning the suction tube of the

gear pump into a feed tank containing 10 L of secondary

effluent with or without additional bacterial EPS. In the

following, only the filtration curves after 24 h of pre-compac-

tion are shown and discussed. The ultrafiltration tests were

carried out at a constant TMP of 1 bar, a cross-flow velocity of

0.2 m s�1, and T ¼ 25 1C. The membrane flux was continuously

measured by an electronic balance; data were recorded by a

computer. The experiments were conducted in recycle mode,

returning the retentate continuously and the permeate

periodically (after accumulation of 0.8 L of permeate) back

into the feed tank, resulting in a nearly constant feed

concentration throughout a filtration run.

In order to compare the results of one test series, the

absolute permeate volume was graphically related to the total

filtration resistance R, which was calculated by dividing the

TMP by the dynamic viscosity of the permeate m and the

permeate flux J:

R ¼TMPm J

2.4. Ultrafiltration test solutions

2.4.1. Secondary effluentSecondary effluent was obtained from the sewage treat-

ment plant Berlin-Ruhleben (accomplishing mechanical and

permeate

retentate

lance

retentate pressuregauge and valve

dataacquisition

on test set-up.

ARTICLE IN PRESS

WAT E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 1 5 3 – 3 1 6 13156

biological treatment with biological nitrogen and phosphorus

removal). Due to the focus of this study on the impact of

dissolved effluent organic matter on membrane fouling, the

secondary effluent was filtered through 0.45mm cellulose

nitrate filters prior to filtration tests in order to remove any

particulate matter. The DOC concentration of secondary

effluent samples was 11.170.2 mg L�1, of which the biopoly-

mer fraction comprised 0.4 mg L�1 (as measured by the

LC-OCD system). The pH value was between 7.5 and 8.0.

Polysaccharide concentrations were 572 mg L�1 (as glucose

equivalents), protein concentrations were 1575 mg L�1

(as BSA equivalents).

2.4.2. EPS solutionsThe fouling potential of dissolved organic matter is depend-

ing on ionic strength, calcium concentration, and pH value of

the solution due to the influence of these parameters on the

effective net charge and the spherical extension of organic

macromolecules (Kilduff et al., 2004). In order to approach the

respective conditions of secondary effluent (from the sewage

treatment plant Berlin-Ruhleben) in filtration tests, the

extracted EPS were dissolved in a model solution of

6 mmol L�1 NaCl and 3 mmol L�1 CaCl2, resulting in an ionic

strength of 15 mmol L�1. The pH value was adjusted to

approximately 7.5 using sodium hydroxide solution. Since

the biopolymer concentration of secondary effluent was

0.4 mg L�1, the DOC concentration of EPS solutions for

ultrafiltration tests was adjusted to 0.4, 0.8, and 1.6 mg L�1,

respectively, in order to obtain solutions containing a

comparable amount of EPS, but no other fouling-relevant

DOC.

3. Results and discussion

3.1. Extraction of bacterial EPS

During 72 h of incubation, the low molecular weight OC

sources of the R2A culture medium were metabolised and

transformed into macromolecular EPS (Fig. 3a). While the EPS

concentration in the liquid culture medium was relatively low

200

2

4

6

8

10

formationof EPS

liquid culture medium R2A liquid R2A after 72 h incubation

of Sinorhizobium sp.

OC

sig

nal [

AU

]

elution time [min]120100806040

Fig. 3 – (a) Formation of bacterial EPS in liquid culture medium R

bacterial EPS concentrate (50-fold dilution) after extraction from

cation exchange resin Dowexs Marathons C.

and the solution contained comparatively high amounts of

other organic substances that might have interfered in

ultrafiltration tests, the incubation of Sinorhizobium sp. on

R2A agar plates and subsequent extraction (as described

above) resulted in a concentrated, viscous, and relatively pure

EPS solution (Fig. 3b). Due to the mobilisation of bound EPS

using a cation exchange resin, the EPS yield was about 20%

higher compared to the extraction procedure without cation

exchange resin.

The EPS concentration of the extracted solutions was

11378 mg L�1 (quantification of the biopolymer peak in LC-

OCD chromatograms). Photometrical quantification of poly-

saccharides and proteins yielded 100715 mg L�1 of polysac-

charides (as glucose equivalents) and 2078 mg L�1 of proteins

(as BSA equivalents); the total nitrogen concentration was

2.670.2 mg L�1. Therefore, the bacterial EPS concentrate

consisted mainly of polysaccharides and contained only low

amounts of proteins. Flemming and Wingender (2002) also

found evidence that within the EPS produced by bacteria in

pure cultures, polysaccharides are more likely to be the

predominant fraction than in mixed cultures occurring under

environmental conditions. By contrast, the secondary effluent

used in the present study contains higher proportions of

proteins (see above).

3.2. Ultrafiltration of secondary effluent

After 24 h of preconditioning and compaction, the permeate

flux of pure NaCl/CaCl2 model solution continued to decrease,

indicating that the compaction of the membrane was not

completed (Fig. 4a), which is confirmed by the slight and

linear increase in the filtration resistance in relation to the

cumulated permeate volume (Fig. 4b). However, ultrafiltration

of secondary effluent resulted in a significant decrease of the

permeate flux, especially during the first hours of the

filtration run (Fig. 4a). This flux loss is reflected by the high

increase in the filtration resistance (Fig. 4b) and indicates the

rapid blockage of a large number of membrane pores by

molecules of a molecular size comparable to the diameter of

the membrane pores, which is likely to be accompanied by

200

2

4

6

8

10 EPS concentrate (1:50)

with ion exchange resin EPS concentrate (1:50)

without ion exchange resin

OC

sig

nal [

AU

]

elution time [min]120100806040

2A (LC-OCD chromatograms). (b) LC-OCD chromatograms of

R2A agar petri dishes with and without application of the

ARTICLE IN PRESS

01

2

3

4

5

6

7

8model solutionUF permeatesec. effluent

norm

. res

ista

nce

R/R

0 [ ]

permeate volume [L]

00.0

0.2

0.4

0.6

0.8

1.0model solutionUF permeatesec. effluent

norm

. flu

x J/

J 0 [

]

filtration time [h]2420161284 5040302010

Fig. 4 – Ultrafiltration of secondary effluent, UF permeate of secondary effluent, and NaCl/CaCl2 model solution: (a) normalised

permeate flux (J0: permeate flux after 24 h of pre-compaction); (b) normalised total filtration resistance vs. cumulated

permeate volume (R0: total filtration resistance after 24 h of pre-compaction).

200

2

4

6

8 feed permeate

OC

sig

nal [

AU

]

elution time [min]20

0

2

4

6

8 feed permeateO

C s

igna

l [A

U]

elution time [min]120100806040 120100806040

Fig. 5 – LC-OCD chromatograms of feed and permeate samples of ultrafiltration tests using: (a) secondary effluent and (b) UF

permeate of secondary effluent (samples taken after 1 h; three-fold dilution).

Table 2 – Retention of biopolymers and humic substancesduring ultrafiltration of secondary effluent and UFpermeate of secondary effluent

Ultrafiltrationtest

Samplingtime

Retention (%)

Biopolymers Humicsubstances

Secondary

effluent

10 min 81 19

1 h 77 11

24 h 82 27

UF permeate of

secondary

effluent

10 min – 15

1 h – 3

24 h – 10

WA T E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 1 5 3 – 3 1 6 1 3157

the formation of a filter cake by substances that are retained

by the membrane. LC-OCD chromatograms of feed solution

and permeate illustrate the high retention of biopolymers and

the comparatively low retention of humic substances,

whereas smaller molecules are completely transmitted

(Fig. 5a). Since interactions between retained molecules and

the membrane are considered to be the reason for membrane

fouling, the high retention of biopolymers confirms the

relevance of this fraction in terms of fouling.

In order to obtain a solution containing nearly the same

organic matrix as secondary effluent, but only minor con-

centrations of biopolymers, secondary effluent was filtered

through the same membrane UP 150. Subsequent ultrafiltra-

tion of the UF permeate resulted in a distinctly decreased

decline of the permeate flux (Fig. 4a), accompanied by a linear

and comparatively low increase in the filtration resistance

(Fig. 4b). As would be expected due to the previous ultrafiltra-

tion step, the biopolymers remaining in this solution were

completely transmitted through the membrane during the

subsequent ultrafiltration test, and humic substances were

not retained either after 1 h of filtration (Fig. 5b). However, a

time-dependent retention of humic substances was observed

(Table 2), consisting of three phases: (1) partial retention (15%)

at the beginning, presumably due to adsorption on membrane

surface and pore walls of the initially clean membrane; (2)

nearly complete transmission after 1 h, indicating that

despite the initial adsorption phase, the pores remain wide

enough to enable the passage of macromolecules; (3) sub-

sequent increase in the retention (10% at the end), presum-

ably due to the continuous reduction of the pore diameter due

to deposition of further humic substances inside the pores,

thus slightly changing the filtration characteristics of the

ARTICLE IN PRESS

WAT E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 1 5 3 – 3 1 6 13158

membrane and resulting in a constantly increasing filtration

resistance. A comparable time-dependent retention of humic

substances was observed during ultrafiltration of secondary

effluent, but due the high retention of biopolymers and the

resulting formation of a filter cake, influencing the separation

properties of the membrane, the increase in the retention was

higher towards the end of the filtration run. In summary,

these experiments confirm that within the complex composi-

tion of dissolved organic compounds in secondary effluent,

biopolymers play the crucial role regarding the fouling of

ultrafiltration membranes, whereas humic substances are of

minor relevance.

3.3. Ultrafiltration of EPS solutions

Within 2 h after addition of extracted bacterial EPS to the

NaCl/CaCl2 model solution, the permeate flux decreased to

less than 30% of the initial value (Fig. 6a). The initial flux

decline observed during ultrafiltration of 0.8 and 1.6 mg L�1

was higher compared to the test using 0.4 mg L�1 EPS. The

retention of EPS was between 92% and 97%, indicating that

the extracted EPS were too large to pass through the

membrane pores. It is noteworthy that there was a slight

recovery of the permeate flux in the second half of the

filtration tests using 0.8 and 1.6 mg L�1 EPS. This effect

was reproducible and might be due to structural changes of

the EPS fouling layer, and thus, an increased permeability

inside it.

For a better comparison of the different filtration curves,

the normalised permeate flux was related to the amount of

EPS delivered to the membrane surface, which was calculated

as follows:

delivered EPS½mg� ¼Xt¼24 h

t¼0

ðpermeate volume ½L�

� EPS feed concentration½mg � L�1�Þ

The decline of the filtration curves obtained this way is

nearly identical (Fig. 6b), confirming that the extent of

membrane fouling is proportional to the amount of bacterial

EPS delivered to the membrane surface.

00.0

0.2

0.4

0.6

0.8

1.00.4 mg/L EPS0.8 mg/L EPS1.6 mg/L EPS

norm

. flu

x J/

J 0 [

]

filtration time [h]2420161284

Fig. 6 – Ultrafiltration of bacterial EPS solutions: (a) normalised p

vs. cumulated mass of EPS delivered to the membrane.

3.4. Impact of biopolymers in secondary effluent andbacterial extract on fouling

Although the biopolymer concentration (related to carbon)

was comparable in EPS model solution and secondary

effluent, the flux decline observed during ultrafiltration of

the latter was more severe, indicating that the predominant

fouling mechanisms during ultrafiltration of these solutions

were not identical (Fig. 7a). In order to examine whether the

reduced extent of fouling induced by bacterial EPS was due to

the lack of background DOC (e.g., humic substances) in the

model solution, extracted EPS were added to UF permeate of

secondary effluent. Ultrafiltration of UF permeate spiked with

EPS increased the flux decline significantly over that pre-

viously observed in ultrafiltration of UF permeate (cf. Fig. 4).

However, the results in Fig. 7a also show that ultrafiltration of

the spiked UF permeate yielded less fouling than secondary

effluent, although the biopolymer concentration and the

composition of the background DOC were comparable in

both solutions. Even an increased EPS concentration by

further addition of bacterial EPS to UF permeate (0.8 and

1.6 mg L�1; data not shown) could not provoke a flux decline of

comparable intensity as observed during ultrafiltration of

secondary effluent, confirming the differences between the

fouling mechanisms induced by extracted bacterial EPS and

effluent organic matter, respectively.

Detailed analysis of the LC-OCD chromatograms indicates

differences between secondary effluent and EPS-spiked solu-

tions regarding the biopolymer peak (Fig. 7b). The nominal

upper size exclusion limits of the applied SEC column are

20,000 g mol�1 for polysaccharides and 80,000 g mol�1 for

globular proteins, respectively (manufacturer information by

Tosoh Bioscience, Tokyo, Japan). The steep increase and

subsequent distinct decrease of the biopolymer peak in the

LC-OCD chromatograms of EPS model solution and EPS-

spiked UF permeate indicate a higher amount of large

polysaccharides eluting with the void volume. By contrast,

the slighter increase and broader shape of the biopolymer

peak in the chromatogram of secondary effluent indicate

an increased proportion of more compact biopolymers

(e.g., proteins). The differences in the composition of the

00.0

0.2

0.4

0.6

0.8

1.00.4 mg/L EPS0.8 mg/L EPS1.6 mg/L EPS

norm

. flu

x J/

J 0 [

]

delivered EPS [mg C]2420161284

ermeate flux vs. filtration time; (b) normalised permeate flux

ARTICLE IN PRESS

00.0

0.2

0.4

0.6

0.8

1.0secondary effluent0.4 mg/L EPS in UF permeate0.4 mg/L EPS in model solution

norm

. flu

x J/

J 0 [

]

filtration time [h]

350.0

0.3

0.6

0.9 secondary effluent 0.4 mg/L EPS in UF permeate 0.4 mg/L EPS in model solution

ON

sig

nal [

AU

]

elution time [min]35

0.0

0.2

0.4

0.6 secondary effluent 0.4 mg/L EPS in UF permeate 0.4 mg/L EPS in model solution

UV

254

nm s

igna

l [A

U]

elution time [min]

200

2

4

6

8

10 secondary effluent 0.4 mg/L EPS in UF permeate 0.4 mg/L EPS in model solution

OC

sig

nal [

AU

]

elution time [min]2420161284 120100806040

504540 504540

Fig. 7 – Ultrafiltration of secondary effluent and bacterial EPS in UF permeate of secondary effluent and in NaCl/CaCl2 model

solution: (a) normalised permeate flux vs. filtration time; (b) LC-OCD chromatograms of the feed solutions (three-fold

dilution); (c) UV254 nm signal of the LC-OCD chromatograms (detail); (d) organic nitrogen (ON) signal of the LC-OCD

chromatograms (detail).

WA T E R R E S E A R C H 4 2 ( 2 0 0 8 ) 3 1 5 3 – 3 1 6 1 3159

biopolymer fraction are confirmed by detailed examination of

the UV254 nm and ON signals of the LC-OCD chromatograms

(Figs. 7c and d). In contrast to proteins, polysaccharide

molecules do neither contain UV-active components nor

nitrogen. Therefore, the significantly elevated UV254 nm and

ON signals are a qualitative evidence of higher proportions of

proteins in the secondary effluent. These chromatographic

results are in accordance with the photometrical determina-

tion of proteins and polysaccharides, revealing a significantly

increased polysaccharide concentration in the bacterial EPS

extract and a higher proportion of proteins in secondary

effluent (see above).

Apart from polysaccharides and proteins, bacterial cell

fragments represent another fraction of organic colloids in

secondary effluent eluting within the biopolymer peak (Laabs

et al., 2004) and containing nitrogen. Therefore, cell frag-

ments might also contribute to the ON signal of the

biopolymer peak of secondary effluent. Considering the

comparatively mild procedure applied for the extraction of

bacterial EPS, the content of cell fragments in the EPS extract

due to cell lysis is likely to be relatively low (Frolund et al.,

1996), and they are not contained in the UF permeate of

secondary effluent either due to their previous removal by

ultrafiltration. Since the proteins cannot be distinguished

from cell fragments using the LC-OCD method applied, the

latter have to be taken into consideration as a further fraction

of biopolymers with potential relevance in terms of ultra-

filtration membrane fouling.

A possible deposition of inorganic colloids on the fouled

membranes was examined by energy-dispersive X-ray spec-

troscopy (LEO 1530 FE-SEM by Carl Zeiss SMT AG, Oberkochen,

Germany). Deposits of the elements silicon, calcium, alumi-

nium, and iron were either not or only in traces detectable,

regardless of the type of feed water used in the previous

ultrafiltration experiment (i.e., secondary effluent, UF perme-

ate or EPS in model solution). Therefore, the influence of

inorganic colloids on the organic membrane fouling investi-

gated in this study is considered to be negligible.

In summary, despite comparable total biopolymer concen-

trations (related to carbon), qualitative differences regarding

the composition of the macromolecular fraction are likely to

be the reason for the different fouling behaviours of

secondary effluent and solutions spiked with extracted

bacterial EPS, indicating the high relevance of proteins and

possibly other organic colloids in terms of ultrafiltration

membrane fouling.

Regarding the predominant fouling mechanisms, the flux

decline induced by the extracted bacterial EPS is considered

to be influenced by the formation of a rather loosely bound

concentration polarisation layer of large polysaccharides,

whereas the filtration of secondary effluent results in the

tighter adhesion of macromolecular substances due to the

more complex variety of biopolymers within effluent organic

matter, including higher proportions of proteins. This as-

sumption is supported by forward-flush tests using deminer-

alised water after ultrafiltration of EPS-spiked secondary

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effluent and UF permeate, revealing that the recovery rate of

the permeate flux was higher after ultrafiltration of EPS-

spiked UF permeate (data not shown).

4. Conclusions

A method for the extraction of EPS derived from the

bacterium Sinorhizobium sp. has been developed. Addition of

the viscous and relatively pure natural EPS mixture to

different test solutions allows the systematic variation of

the EPS concentration in order to investigate the impact of

biopolymers on membrane fouling in tertiary sewage treat-

ment.

In cross-flow ultrafiltration experiments using model solu-

tions of the extracted bacterial EPS at concentrations which

are relevant in secondary effluents (0.4–1.6 mg L�1), the EPS

were almost completely retained by the membrane, thus

causing a severe permeate flux decline, especially in the

initial filtration phase. A correlation between the EPS

concentration in model solution and the extent of membrane

fouling was observed. However, ultrafiltration of secondary

effluent resulted in a higher flux decline than ultrafiltration

of the EPS model solutions. Selective removal of the

autochthonous biopolymers (which contributed 4% to the

total DOC) from secondary effluent by previous ultrafiltra-

tion revealed a significantly lower fouling potential of the

remaining organic compounds in subsequent ultrafiltration

tests. Therefore, biopolymers are considered to be the

predominant fouling-active fraction within the DOC of

secondary effluent, whereas humic substances and smaller

organic compounds play a minor role in ultrafiltration

membrane fouling.

Addition of extracted bacterial EPS to UF permeate of

secondary effluent (without autochthonous biopolymers)

caused significantly more fouling than ultrafiltration of

UF permeate of secondary effluent alone. However, the

bacterial EPS could not provoke the same fouling rate as

observed in ultrafiltration of secondary effluent, although the

composition of the background DOC was comparable. Thus,

extracting EPS from pure bacterial cultures is not an appro-

priate surrogate for organic foulants found in secondary

effluent and would therefore not be useful for bench-scale

membrane filtration studies aimed at finding ways to reduce

the fouling rate or to clean membranes more efficiently after

fouling. However, since the differences in the fouling

behaviour of the bacterial EPS extract and secondary effluent

are due to qualitative differences in terms of the macro-

molecular composition of the solutions, a conclusion regard-

ing the impact of the complex mixture of biopolymers on

membrane fouling can be drawn. The extracted bacterial EPS

contain significantly more polysaccharides than proteins,

whereas the secondary effluent used contains a larger

proportion of proteins, as well as other organic colloids

(i.e., fragments of bacterial cells). Therefore, the increased

flux decline observed during fouling tests using secondary

effluent indicates the relevance of proteins and possibly

further organic colloids in terms of ultrafiltration membrane

fouling.

Acknowledgement

The laboratory work of Anne Konig, Angela Wurtele, Hui

Cheng, and Daniela Pallischeck is greatly acknowledged.

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