Complexity of ultrafiltration membrane fouling caused by macromolecular dissolved organic compounds in secondary effluents

Complexity of ultrafiltration membrane fouling caused by macromolecular dissolved organic compounds in secondary effluents

ARTICLE IN PRESS WAT E R R E S E A R C H 42 (2008) 3153 – 3161 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres ...

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ARTICLE IN PRESS WAT E R R E S E A R C H

42 (2008) 3153 – 3161

Available at www.sciencedirect.com

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

Complexity of ultrafiltration membrane fouling caused by macromolecular dissolved organic compounds in secondary effluents Jens Haberkampa,, Mathias Ernstb, Uta Bo¨ckelmannc, Ulrich Szewzykc, Martin Jekela a

Technische Universita¨t Berlin, Chair of Water Quality Control, Sekr. KF 4, Str. des 17. Juni 135, 10623 Berlin, Germany Technische Universita¨t Berlin, Centre for Water in Urban Areas, Sekr. KF 4, Str. des 17. Juni 135, 10623 Berlin, Germany c Technische Universita¨t Berlin, Chair of Environmental Microbiology, Sekr. FR 1-2, Franklinstr. 29, 10587 Berlin, Germany b

art i cle info

ab st rac t

Article history:

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

Received 14 January 2008

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

Received in revised form

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

12 March 2008

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

Accepted 13 March 2008

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

Available online 26 March 2008

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

Keywords: Cross-flow ultrafiltration EPS extraction Extracellular polymeric substances Membrane fouling Secondary effluent Size exclusion chromatography

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 treatment of municipal sewage, e.g., in membrane bioreactors (MBRs) or tertiary treatment of secondary effluent. Their application in such systems has significantly increased within the last decade. However, membrane fouling is still a

fundamental drawback, necessitating periodical chemical cleanings and eventually forcing the replacement of irreversibly fouled membranes. Membrane fouling can be caused by particles, dissolved or colloidal organic and inorganic substances, 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 0.45 mm are not yet completely identified. te Poele (2005) indicates the significance of colloids of the size

Corresponding author. Tel.: +49 30 314 25367; fax: +49 30 314 23850.

E-mail addresses: [email protected] (J. Haberkamp), [email protected] (M. Ernst), [email protected] (U. Bo¨ckelmann), [email protected] (U. Szewzyk), [email protected] (M. Jekel). 0043-1354/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.03.007

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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 0.01 mm 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 (Rosenberger 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 secondary effluent, other authors point out the impact of polysaccharides 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 comparability 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 o0.45 mm) 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 biopolymers 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-Marienfelde (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

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

Amount per litre of ultrapure water

Yeast extract Proteose peptone no. 3 Casamino acid Glucose Sodium pyruvate Dipotassium hydrogen phosphate (K2HPO4) Magnesium sulphate (MgSO4  7H2O) Agar Tween 80 (fatty acid ester)

0.5 g 0.5 g 0.5 g 0.5 g 0.3 g 0.3 g 0.05 g 15 g 1 mL

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 L1 NaH2PO4, 2 mmol L1 Na3PO4, 9 mmol L1 NaCl, 1 mmol L1 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 subsequently separated from the EPS solution by centrifugation (35 min at 3500 rpm) and filtration through 0.45 mm cellulose nitrate filters.

2.2.

Analytical methods

2.2.1.

Size exclusion chromatography

Size exclusion chromatography with continuous UV254 nm and organic carbon (OC) detection was used to characterise the DOC composition of EPS solutions and secondary effluent (LCOCD 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 10 mg L1, the standard deviation is less than 1% of the measured value (measurement range: 1–5 mg L1 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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cut-off (MWCO) of 150 kg mol1 (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 L1 NaCl and 3 mmol L1 CaCl2, resulting in initial permeabilities of 326740 L m2 h1 bar1 at 1 bar transmembrane 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-compaction 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 s1, 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:

2.2.2. Determination of polysaccharides, proteins, and total nitrogen content The polysaccharide concentration (as glucose equivalents) of the bacterial EPS solution and secondary effluent was determined using the photometrical method following Dubois et al. (1956). The protein concentration (as BSA equivalents) 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 setup 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

8 low molecular weight acids

humic substances OC signal [AU]

6 low molecular weight neutrals

4

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biopolymers (EPS)



TMP mJ

2

2.4.

Ultrafiltration test solutions

2.4.1.

Secondary effluent

0 20

40

60 80 elution time [min]

100

120

Secondary effluent was obtained from the sewage treatment plant Berlin-Ruhleben (accomplishing mechanical and

Fig. 1 – LC-OCD chromatogram of secondary effluent (threefold dilution).

flow meter

feed pressure

cross-flow

gauge and valve

test cell

retentate retentate pressure gauge and valve

gear

peristaltic

pump

pump

permeate

data acquisition

feed tank balance Fig. 2 – Ultrafiltration test set-up.

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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 0.45 mm 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 L1, of which the biopolymer fraction comprised 0.4 mg L1 (as measured by the LC-OCD system). The pH value was between 7.5 and 8.0. Polysaccharide concentrations were 572 mg L1 (as glucose equivalents), protein concentrations were 1575 mg L1 (as BSA equivalents).

2.4.2.

EPS solutions

The fouling potential of dissolved organic matter is depending 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 L1 NaCl and 3 mmol L1 CaCl2, resulting in an ionic strength of 15 mmol L1. The pH value was adjusted to approximately 7.5 using sodium hydroxide solution. Since the biopolymer concentration of secondary effluent was 0.4 mg L1, the DOC concentration of EPS solutions for ultrafiltration tests was adjusted to 0.4, 0.8, and 1.6 mg L1, 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

6 formation of EPS

4

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

10

liquid culture medium R2A liquid R2A after 72 h incubation of Sinorhizobium sp.

8

3.2.

OC signal [AU]

OC signal [AU]

10

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 L1 (quantification of the biopolymer peak in LCOCD chromatograms). Photometrical quantification of polysaccharides and proteins yielded 100715 mg L1 of polysaccharides (as glucose equivalents) and 2078 mg L1 of proteins (as BSA equivalents); the total nitrogen concentration was 2.670.2 mg L1. 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).

2

EPS concentrate (1:50) with ion exchange resin EPS concentrate (1:50) without ion exchange resin

8 6 4 2 0

0 20

40

60 80 100 elution time [min]

120

20

40

60 80 100 elution time [min]

120

Fig. 3 – (a) Formation of bacterial EPS in liquid culture medium R2A (LC-OCD chromatograms). (b) LC-OCD chromatograms of bacterial EPS concentrate (50-fold dilution) after extraction from R2A agar petri dishes with and without application of the cation exchange resin Dowexs Marathons C.

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8

1.0 model solution UF permeate sec. effluent

0.8

norm. resistance R/R0 [ ]

norm. flux J/J0 [ ]

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0.6 0.4 0.2

model solution UF permeate sec. effluent

7 6 5 4 3 2 1

0.0 0

4

8 12 16 filtration time [h]

20

10

0

24

20

30

40

50

permeate volume [L]

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).

8 feed permeate

feed permeate

OC signal [AU]

OC signal [AU]

8 6 4 2 0 20

40

60 80 100 elution time [min]

120

6 4 2 0 20

40

60 80 100 elution time [min]

120

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).

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 concentrations of biopolymers, secondary effluent was filtered through the same membrane UP 150. Subsequent ultrafiltration 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 ultrafiltration 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)

Table 2 – Retention of biopolymers and humic substances during ultrafiltration of secondary effluent and UF permeate of secondary effluent Ultrafiltration test

Sampling time

Retention (%) Biopolymers

Humic substances

Secondary effluent

10 min 1h 24 h

81 77 82

19 11 27

UF permeate of secondary effluent

10 min 1h

– –

15 3

24 h



10

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) subsequent increase in the retention (10% at the end), presumably 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

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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 composition 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 L1 was higher compared to the test using 0.4 mg L1 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 L1 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 ¼

t¼24 Xh

ðpermeate volume ½L

t¼0

 EPS feed concentration½mg  L1 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.

3.4. Impact of biopolymers in secondary effluent and bacterial 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 previously 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 L1; 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 solutions regarding the biopolymer peak (Fig. 7b). The nominal upper size exclusion limits of the applied SEC column are 20,000 g mol1 for polysaccharides and 80,000 g mol1 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 EPSspiked 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

1.0 0.4 mg/L EPS 0.8 mg/L EPS 1.6 mg/L EPS

0.8

norm. flux J/J0 [ ]

norm. flux J/J0 [ ]

1.0

0.6 0.4 0.2

0.4 mg/L EPS 0.8 mg/L EPS 1.6 mg/L EPS

0.8 0.6 0.4 0.2 0.0

0.0 0

4

8 12 16 filtration time [h]

20

24

0

4

8

12

16

20

24

delivered EPS [mg C]

Fig. 6 – Ultrafiltration of bacterial EPS solutions: (a) normalised permeate flux vs. filtration time; (b) normalised permeate flux vs. cumulated mass of EPS delivered to the membrane.

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

0.8

OC signal [AU]

norm. flux J/J0 [ ]

1.0

0.6 0.4 0.2

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

8 6 4 2 0

0.0 0

4

8 12 16 filtration time [h]

20

20

24

0.6

40

80 60 100 elution time [min]

120

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

0.4

0.2

0.0

ON signal [AU]

UV254 nm signal [AU]

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

0.6

0.3

0.0 35

40 45 elution time [min]

50

35

40 45 elution time [min]

50

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).

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 determination 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 fragments 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 ultrafiltration membrane fouling.

A possible deposition of inorganic colloids on the fouled membranes was examined by energy-dispersive X-ray spectroscopy (LEO 1530 FE-SEM by Carl Zeiss SMT AG, Oberkochen, Germany). Deposits of the elements silicon, calcium, aluminium, 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 permeate or EPS in model solution). Therefore, the influence of inorganic colloids on the organic membrane fouling investigated in this study is considered to be negligible. In summary, despite comparable total biopolymer concentrations (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 assumption is supported by forward-flush tests using demineralised 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 EPSspiked UF permeate (data not shown).

Acknowledgement The laboratory work of Anne Ko¨nig, Angela Wu¨rtele, Hui Cheng, and Daniela Pallischeck is greatly acknowledged.

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 treatment. In cross-flow ultrafiltration experiments using model solutions of the extracted bacterial EPS at concentrations which are relevant in secondary effluents (0.4–1.6 mg L1), 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 ultrafiltration 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 appropriate 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 macromolecular composition of the solutions, a conclusion regarding 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.

R E F E R E N C E S

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