Membrane fouling mechanism in ultrafiltration of succinic acid fermentation broth

Bioresource Technology 116 (2012) 366–371

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Membrane fouling mechanism in ultrafiltration of succinic acid fermentation broth Caixia Wang a,b, Qiang Li a, Huang Tang a,b, Daojiang Yan a,b, Wei Zhou a,b, Jianmin Xing a,⇑, Yinhua Wan a a b

National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e

i n f o

Article history: Received 29 December 2011 Received in revised form 28 March 2012 Accepted 29 March 2012 Available online 4 April 2012 Keywords: Succinic acid Fermentation broth Separation Ultrafiltration Fouling mechanism

a b s t r a c t The membrane fouling mechanism was studied in treating succinic acid fermentation broth during deadend ultrafiltration. Different membranes were used and two models were applied to analyze the fouling mechanism. Resistance-in-series model was applied to determine the main factor that caused the operation resistance. Results indicated that most membranes tended to be fouled by cake layer or concentration polarization. Hermia’s model, which is composed of four individual sub-models, was used to analyze the predominant fouling mechanism. Results showed that the fouling of RC 10 kDa and PES 30 kDa was controlled by the complete blocking mechanism, while PES 100 kDa was controlled by the intermediate blocking and PES 10 kDa was controlled by cake layer. This conclusion was also proved by SEM photos. Membrane characteristics were monitored before and after ultrafiltration by AFM and goniometer. Both contact angle and roughness of most membranes increased after ultrafiltration. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Succinic acid (1,4-butanedioic acid) is identified as an important building-block chemical which can be used for the synthesis of high value-added derivatives, such as 1,4-butanediol (BDO), gbutyrolactone (GBL), tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), 2-pyrrolidone (2-Pyrr), succinimide, succinic esters, etc. (Delhomme et al., 2009; McKinlay et al., 2007). Traditionally it was synthesized through petrochemical process. Great efforts have been made to develop biotechnology methods to produce succinic acid from renewable resources since last decade. However, fermentation process is still not economically feasible, mainly due to the low succinic acid concentration in the fermentation broth, and the high cost of succinic acid separation and purification (Zeikus et al., 1999). Up to now, considerable efforts have been made to improve the succinic acid concentration in the fermentation broth (Li et al., 2010; Lin et al., 2011), and several possible operation units such as adsorption (Li et al., 2009), extraction (Lee, 2011) and electrolysis (Yi et al., 2008) have been investigated for the recovery of succinic acid. Currently, ultrafiltration has been widely used in various physicochemical and biochemical processes for the separation of solids from liquid with characters of low operation cost, low energy consumption and elimination of filter aids (Shao et al., 2011; Zhang et al., 2010; Chaiklahan et al., 2011). However, like other pressure ⇑ Corresponding author. Address: P.O. Box 353#, Zhongguancun Bei-er-tiao 1, Haidian District, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China. Tel./fax: +86 10 62550913. E-mail address: [email protected] (J.M. Xing). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.03.099

driven membrane processes, membrane fouling is one of the biggest obstacles for the wide application of ultrafiltration. And effects of basic parameters, such as transmembrane pressure, size of particles in feed solution and the membrane characteristic upon the membrane fouling are not completely understood. Previous studies showed that the particles or solids in the feed solution were blocked both inside and on the surface of membrane, which lead to the decline of flux (Kujundzic et al., 2010). Obvious evidence has proved that the predominant mechanism which accounts for flux decline vary with different feed solutions and operation conditions. And models have been developed to reveal the membrane fouling mechanism or to predict the flux decline. Resistance-in-series model has been applied in the ultrafiltration of mixtures of macromolecule solutes (Leberknight et al., 2011; Dizge et al., 2011). In this model, it supposes that four major factors, membrane hydraulic resistance (Rm), concentration polarization resistance (Rc), cake or gel layer resistance (Rg) and adsorption resistance (Ra), cause the flux decline. By quantifying the four factors through mathematical calculation, we can figure out which is the main factor. This model is quite helpful in understanding the reasons or factors that mainly cause the flux decline. Among all models reported in literature, empirical models were precise in flux monitoring. But it could not explain the mechanism adequately. Complete theoretical model seldom predicted the flux decline well while it can contribute to the understanding of fouling phenomena (Bhattacharjee and Datta, 2003). Hermia (Hermia, 1982) developed a semi-empirical model to describe the fouling mechanism, in which four mechanistic sub-models were included, i.e., complete blocking model, standard blocking model, intermediate blocking model and the cake layer model. The model which

C.X. Wang et al. / Bioresource Technology 116 (2012) 366–371

best fits the experimental data was identified as the predominant fouling mechanism. Researches have been done to study the fouling mechanism by using model solutions such as bovine serum albumin and human IgG, and find that this model provided good match to the experimental data (Mondal et al., 2011). Ho Chia-Chi and Zydney (2000) developed the combined pore blocking and cake layer model. The data obtained from this model were in good agreement with the experimental data from microfiltration of different protein solutions. In succinic acid feed solution system, residual cell debris and magnesium carbonate still existed in the fermentation broth, thus there was no doubt that in the end of ultrafiltration a cake layer will form and result in fouling. In this study, fouling mechanisms of different membranes were firstly studied in treating succinic acid fermentation broth during ultrafiltration process. Resistance-in-series model was applied to determine the main factors that caused the operation resistance, i.e., flux decline. Hermia’s model was used to analyze the predominant fouling mechanism for each membrane. Membrane surface characteristics before and after ultrafiltration were monitored to verify the determined fouling mechanism for each membrane. The contribution of this paper is to provide a better understanding of the fouling formation process for the four membranes. Identification and quantification of the predominant fouling mechanism is very important for the prediction of membrane flux, membrane selection, fouling control and membrane cleaning strategy. This paper also provides a better understanding of how membrane characteristics influence the membrane flux. The foulants during this process were identified preliminarily. Thus, this work will be helpful for the application of ultrafiltration in the treatment of succinic acid fermentation broth. 2. Fouling models and analysis In the dead-end filtration, flux decline can be caused by several factors, such as the adsorption between membrane and solutions, cake or gel formation, concentration polarization, and membrane hydraulic resistance. Resistance-in-series model (Juang et al., 2008) has been described in this process. This mode is particularly applicable to the analysis of flux decline of fermentation broth. It was described as follows:

J¼

dv DP ¼ dt lRtot

DP lJ

ð2Þ

ð3Þ

After this, the fermentation broth was filtrated and the permeate flux (Jtot) was recorded during the whole process. According to Eq. (2), Rtot could be calculated during this process. Then the feed

ð4Þ

Subsequently, the membrane was flushed with DI water, cleaned by removing the gel layer, and then the water flux Jirr was determined. At this point, the concentration polarization resistance Rc and the gel or cake layer resistance Rg could be assumed to be zero. Therefore:

Ra ¼ DP=lJ irr  Rm

ð5Þ

Thus, all membrane resistances can be quantified by Eqs. (2)–(5). Besides the quantification of membrane fouling, identification of fouling mechanism also provides a good understanding on the membrane capacity, defined as permeate volume per membrane area which can be processed until the flux declines to a certain fraction of the initial flux, or until the formed cake starts to increase the filtration resistance dramatically. Hermia developed a semi-empirical model for dead-end filtration based on constantpressure condition in order to predict the fouling mechanism that predominate the flux decline during the filtration. Hermia’s model (Hermia, 1982; Vela et al., 2008) which includes four basic types of fouling: complete blocking model, intermediate blocking model, standard blocking model and cake layer model, was used to analyze the fouling mechanism for each membrane in this study. In Hermia’s model (Eq. (6)), t is the filtration time, V is the permeate volume, k is constant and n takes different discrete constants for different types of fouling: n = 2 indicates the complete blocking model, n = 1.5 represents the standard blocking model, n = 1 stands for the intermediate blocking model, and n = 0 indicates a cake layer model. Complete blocking model assumes that every molecule that reaches the membrane surface completely blocks the pore entrance of the membrane. More importantly, molecule never settles on another molecule that previously deposits on the membrane surface. The blocking takes place on the membrane surface but not the inside pores of membrane. For n = 2, Eq. (6) linearizes and expresses in terms of permeate volume and filtration time in Eq. (7): (Hermia, 1982; Vela et al., 2008).



2

Where Rm is the membrane hydraulic resistance, which can be determined by water flux of the clean membrane, Rg is the cake layer resistance, Rc is the concentration polarization resistance and Ra is the adsorption resistance. All resistances are in m1. To determine all the resistances, the following procedure was adopted: First, the membrane hydraulic resistance Rm was calculated by measuring the flux of DI water through a clean membrane (Jmem). In this situation, Rc, Rg, and Ra were zero, so Eq. (2) became Eq. (3):

Rm ¼ DP=lJ mem

Rg ¼ DP=lJ pore  Rm  Ra

d t

Where J is the permeate flux (L/m h), DP is the transmembrane pressure (TMP, Pa), l is the dynamic viscosity of permeate (Pa s), and Rtot is the total filtration resistance. The resistance-in-series model combines various resistances that cause flux decline as follows:

Rtot ¼ Rm þ Rg þ Rc þ Ra ¼

broth was replaced by the DI water and the water flux in this situation Jpore was recorded. At this point, the concentration polarization could be assumed to be zero and the sum of Rg, Ra and Rm was determined. So we have:

ð1Þ 2

367

dv

2

¼k

dt dv

n

InJ ¼ InJ 0  kc t

ð6Þ ð7Þ

In a standard blocking (n = 1.5), molecule size is smaller than membrane pore size, thus molecules can easily go inside membrane pores and the blocking occurs inside the membrane pores. Therefore, the volume of membrane pores decreases proportionally to the filtrated permeates volume. This model has an important hypothesis that all membrane pores have a constant length and diameter on the whole membrane. For n = 1.5, Eq. (6) linearizes and expresses in terms of permeate volume and filtration time in Eq. (8): (Hermia, 1982; Vela et al., 2008).

1 J 0:5

¼

1 J 0:5 0

þ ks t

ð8Þ

The intermediate blocking model considers that the molecule that reaches the pore entrance also blocks the pore entrance, which is similar to the complete model. However, this model is less restrictive than the complete blocking model. It assumes that some molecules may deposit on previously settled ones, which means not all molecules that reach membrane surface can seal a separate

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C.X. Wang et al. / Bioresource Technology 116 (2012) 366–371

membrane pore. Under this hypothesis, the n equals to 1, thus Eq. (6) linearizes and expresses in terms of permeate volume and filtration time in Eq. (9): (Hermia, 1982; Vela et al., 2008).

1 1 ¼ þ ki t J J0

3.3. Membrane characteristic analysis

ð9Þ

Cake layer formation model is based on the mechanism that molecules have bigger size than the membrane pores, thus molecules will accumulate on the membrane surface and form a permeable cake layer. The linear equation for the cake layer formation model is shown as follows: (Hermia, 1982; Vela et al., 2008).

1 J2

¼

1 J 20

þ kg t

guarantee that membrane fouling was not changed by artificial water flushing.

ð10Þ

In Eqs. (7)–(10), J is the permeate volume (L m2 h1), t is the filtration time (min), J0 is the certain permeate volume (m3) when t = 0 and kc, ks, ki, kg are the model constants. 3. Material and methods 3.1. Succinic acid fermentation Actinobacillus succinogenes BE-1, which was isolated from bovine rumen in our laboratory and collected in China General Microbiological Culture Collection Center (No. CGMCC2650), was used for succinic acid production. The fermentation media contained per liter: 30 g glucose, 10 g yeast extract, 20 g MgSO4, 1.37 g K2HPO4, 1.53 g KH2PO4, 1.5 g NaCl, 0.05 g MnCl2, 0.38 g CaCl2, 30 g MgCO3. Fermentation media was sterilized at 115 °C for 30 min and glucose was sterilized separately. A 500 mL flask with 200 mL fresh broth was used for succinic acid fermentation, and the concentration of seed inoculum was 5%. The incubation condition was at 37 °C for 48 h. 3.2. Ultrafiltration Membranes made of polyethersulfone (PES, SEPRO, USA) with molecular weight cut-offs (MWCO) of 100, 30, and 10 kDa and regenerated cellulose (RC, Millipore, USA) with the molecular weight cut-off of 10 kDa were used in this study. A batch stirred cell equipped successively with various flat ultrafiltration membranes, whose surface area were 45 cm2, was used for the ultrafiltration. The applied pressure was 0.2 MPa constantly controlled by nitrogen gas. Feed broth was stirred at 300 rpm to minimize gel/cake formation during the ultrafiltration. The schematic diagram of UF equipment was similar to the equipment in Wan’s research (Wan et al., 2004). All membranes were soaked in 25% ethanol for 12 h to remove the conserving agent (glycerin layer), followed by measuring the initial water flux of each membrane. To determine the fluxes (Jmem, Jtot, Jpore, and Jirr) needed in resistance-in-series model, the following four procedures were adopted. Initial water flux was obtained by measuring the volume of permeate water collected in 100 mL flask over a recorded time. Then 150 mL fermentation broth was filtrated without recycling, during which samples were collected at certain permeate volumes for the following analysis. After that, the deionized (DI) water was filled with the stirred cell without removing the gel layer that was formed on the surface of the membrane, and then the water flux with gel layer was measured. After removing the gel layer gently and flushing the fouled membrane, water flux was measured again without the gel layer. Permeability of all membranes was shown in Table 1. To determine the fluxes needed in Hermia’s model, the first two procedures mentioned above were performed, that is, the initial water flux (J0) and flux (J) were measured. Membranes were not operated in cycles to

The surface of the membranes before and after ultrafiltration was observed using a JSM-6700F Field Emission Scanning Electron Microscope (JEOL, Japan). A small piece of dry membrane cut off from the objective membrane was placed on the sample stage, and was gold coated before scanned. Contact angle (h) of membranes was measured using the pendant drop method aided by video image Digitization technique. The water phase was injected slowly by a needle to the membrane surface under a constant pressure and monitored by contact angle system OCA (Dataphysics, Germany), A drop of pure water was placed on the porous membrane using a syringe, and the contact angle (h) was then determined from digital silhouette drop images. This was repeated five times at different points of the membrane with measurements taken from both sides of the drop, producing a total of 10 measurements which were averaged. The roughness of the membranes was measured by BioScope II AFM (Veeco, NY, USA) in contact mode. A small piece of membrane cut off from the objective membrane was stuck on a metal disc by double sided sticky tape, and then was placed on the AFM stage. The roughness (Rq) was determined as an average value that calculated from three different areas (5  5 lm). 4. Results and discussions 4.1. Resistance-in-series model analysis Resistance-in-series model (Eq. (2)) was applied to analyze resistances that lead to flux decline during the ultrafiltration process. Fouling resistances and their ratios to the total resistance are presented in Table 2. Fig. 1 showed the percentages of four resistances in the total resistance of each membrane. From Fig. 1, for the membrane PES 100 kDa, the flux decline should be largely attributed to the gel layer which deposit on the membrane surface. The hydraulic resistance of this membrane was the smallest among the selected membranes. For the membrane PES 30 kDa, it had a higher adsorption resistance (46.31%) and membrane hydraulic resistance (22.32%). The membrane PES 10 kDa appeared similar characteristic with the membrane PES 100 kDa, that the flux decline was mainly caused by the cake layer. For the membrane RC 10 kDa, concentration polarization was the major reason for the flux decline, and its adsorption resistance took up the smallest part. According to Table 1 and Fig. 1, it can also be seen that the type of the membrane resistance is influenced by the corresponding steady-state flux or each membrane. For example, the flux decline of membranes with higher flux (RC 10 kDa, PES 100 kDa and PES 10 kDa) were dominated by concentration polarization or cake layer resistance, while the flux decline of membrane with the

Table 1 Permeability of all selected membrane. Permeability (L m2 h1) at room temperature and 0.2 MPa

Membrane MWCO (kDa) 100 30 10 10 a

a

Materialsa

Initial water permeability (before ultrafiltration)

Steady permeability (during ultrafiltration)

PES PES PES RC

446.36 55.33 120.00 127.62

17.11 14.74 16.89 23.58

Provided by the manufacturer.

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C.X. Wang et al. / Bioresource Technology 116 (2012) 366–371 Table 2 Fouling resistances for different membranes. MWCO (kDa)

Material

Rtot (1012 m1)

Rm (1012 m1) (%)

Ra  1012 ( m1) (%)

Rc (1012 m1) (%)

Rg (1012 m1) (%)

100 30 10 10

PES PES PES RC

14.96 17.66 15.52 10.91

0.58 (3.87) 3.943 (22.32) 2.15 (13.86) 2.10 (19.25)

0.06 8.18 2.70 0.45

3.76 1.55 4.40 4.73

10.56 (70.58) 3.99 (22.59) 6.27 (40.40) 3.63 (30.79)

100

a

(0.42) (46.31) (17.39) (4.12)

(25.13) (8.78) (28.35) (43.35)

3.3

PES 100kDa PES 30kDa PES 10kDa RC 10kDa

3.2

PES 30kDa

80

PES 10kDa

3.1

RC 10kDa

ln[J(L/m2 h)]

Percentage of total resistance(%)

PES 100kDa

60

40

3.0 2.9 2.8 2.7

20

2.6 2.5 0

0

20

40

4.2. Hermia’s model analysis Hermia’s model was applied to interpret the fouling mechanism of each membrane during the ultrafiltration. By fitting the experimental data into these four models, fouling mechanism which was prevailing can be identified. Figs. 2 and 3a and b showed the fitting of experimental data to four types of Hermia’s models according to Eqs. (7)–(10), respectively. Table 3 showed all the corresponding correlation coefficients (R2), and the maximal R2 values were asterisked to indicate the besting fitting model. Fig. 2(a) showed the fitting of experimental data to the complete blocking model, and best fitting occurred in RC 10 kDa and PES 30 kDa, as the values of R2 were higher than other R2 values. Thus it indicates that both membrane RC 10 kDa and membrane PES 30 kDa were suitable for the complete blocking model. This could also be proved by the SEM photos of the two membranes before and after ultrafiltration. The complete blocking could lead to accumulation of particles on membrane surface, which would show severer membrane surface fouling. Fig. 4 (Supplementary figure P1) showed the SEM photos of all selected membranes before and after usage. By comparing all membranes, d and h presented the severest surface fouling, i.e., the foulant mostly

80

100

120

140

80

100

120

140

0.35

PES 100kDa PES 30kDa PES 10kDa RC 10kDa

0.30 1/2

lowest flux (PES 30 kDa) was mainly resulted from adsorption and hydraulic resistance. In this study, cake layer was formed by the biomass and the unutilized magnesium carbonate. So removal the unutilized magnesium carbonate can diminish this resistance. Results indicated that ultrafiltration was severely limited by the concentration polarization which was mostly caused by inorganic salts. This conclusion was in accordance with previous report (Macedo et al., 2011).

b

m h )

Fig. 1. Percentage of total membrane resistance for individual membrane, Rtot is total filtration resistance, Rm is the membrane hydraulic resistance, Rc is the concentration polarization resistance, Rg is the cake layer resistance, and Ra is the adsorption resistance.

60

Time(min)

Rg/Rtot

-1/2

Rc/Rtot

0.25

1/2

Ra/Rtot

1/J (L

Rm/Rtot

0.20

0.15

0

20

40

60

Time(min) Fig. 2. Fitting of experimental data to the complete blocking model and the intermediate blocking model. a: fitting of experimental data to the complete blocking model according to Eq. (7); b: fitting of experimental data to the standard blocking model according to Eq. (8).

accumulated on the surface of the membrane which is a feature of complete blocking model. Fig. 2(b) showed that none of the membrane’s experimental data fitted well with the standard blocking model, which can be explained by the fact that the sizes of most solute molecules in the fermentation were bigger than the pore size of the membranes. Fig. 3(a) showed the fitting of experimental data to the intermediate blocking model, and the membrane PES 100 kDa fitted this model the best, which indicated that the flux decline of PES 100 kDa was controlled by the intermediate blocking. When intermediate blocking occurred, particles go to both inside and outside pores of the membrane. From SEM photos (Supplementary figure

370

a

C.X. Wang et al. / Bioresource Technology 116 (2012) 366–371 Table 4 Contact angle for virgin and fouled membranes.

0.09

PES 100kDa PES 30kDa PES 10kDa RC10kDa

0.08

Virgin membrane Contact angle (°)

Single fouled membrane Contact angle (°)

PES 100 PES 30 PES 10 RC 10

67.8 61.3 61.7 19.9

70.9 63.9 64.8 22.7

0.06

-1

2

1/J(L m h)

0.07

Membrane type MWCO (kDa)

0.05

0.04

0.03 0

20

40

60

80

100

120

140

100

120

140

Time(min)

b

0.007

PES 100kDa PES 30kDa PES 10kDa RC 10kDa

0.006

0.004 0.003

2

-2

4

2

1/J (L m h )

0.005

0.002 0.001 0.000 0

20

40

60

80

Time(min) Fig. 3. Fitting of experimental data to the standard blocking model and the cake layer model. a: fitting of experimental data to the intermediate blocking model according to Eq. (9); b: fitting of experimental data to the cake layer model according to Eq. (10).

Table 3 Measures of fitting experimental data to Hermia’s model linear equations.

w

MWCO (kDa)

Complete blocking

Standard blocking

Intermediate blocking

caker layer

PES100 PES30 PES10 RC10

0.89575 0.82141w 0.77378 0.94768w

0.89657 0.80956 0.78281 0.93701

0.89664w 0.79754 0.79139 0.92418

0.89456 0.77311 0.80708w 0.89308

Indicates the best fitting model of membrane in that row.

P1) a and b, we could see pores on the clean membrane, while these pores were blocked by the foulants after using, and the foulants on the surface were less than those on the membrane PES 30 kDa. However, this result contradicted the resistance-in-series model analysis, where the prevailing resistance of membrane PES 100 kDa was cake layer resistance. An explanation is that the experimental data of PES 100 kDa could fit all four model types similarly (Table 3), which indicates the possibility that the flux decline could be controlled by the combination of cake layer model and intermediate blocking model.

Fig. 3(b) showed that the experimental data of membrane PES 10 kDa fitted best to the cake layer model, which was also consistent with the results of resistance analysis shown in Fig. 1. The formation of cake layer impedes the entrance of molecules into the membrane pore and keeps them back on the cake layer. If the cake layer has a relatively loosen structure, the flux of membrane can be higher than the flux of membrane that is controlled by pore blocking model, which could explain that the membrane PES 10 kDa had a higher flux (16.89 L/m2 h) than PES 30 kDa (14.74 L/m2 h) in this study. Because the cake layer was easily to remove after filtration and gently water flushing, the foulants on the membrane surface were not as many as before which could be observed from the SEM photos e and f (Supplementary figure P1). Among all fouled membranes, the foulants of f (fouled PES 10 kDa) was not as severe as other membranes. Previous studies reported that membrane fouling depended both on the membrane itself and the feed solution (Grenier et al., 2008). In this study, RC 10 kDa and PES 30 kDa were fouled by complete blocking which indicated that particles in the feed broth sealed the membrane pores and accumulated on the membrane surfaces. The flux decline of membrane PES 100 kDa was controlled by intermediate blocking. So we can deduce that particles in the feed broth have similar size with PES 100 kDa membrane pore size. It sealed part of the pores and accumulated on the surface, which was in accordance with the higher flux it showed. The fouling mechanism of membrane PES 10 kDa was cake layer fouling. Thus particles in the crude broth were much greater than the pore size of PES 10 kDa, and they accumulated on the surface at the very initial moment of ultrafiltration without sealing the pores. These results were verified by SEM photos. For membrane with same material, such as PES 100 kDa, PES 30 kDa and PES 10 kDa, fouling mechanism varied mainly because of the difference of their pore sizes. In this study, RC 10 kDa is more hydrophilic than PES 10 kDa which can be seen from Table 4. Flux and fouling mechanism of the two membranes differ because of membrane characteristics difference such as the hydrophily, pore size or porosity and should be elevated how these characteristics influence the ultrafiltration process of this crude broth. It is obvious that cake layer fouling can be easily removed by water flushing than complete blocking model and intermediate blocking fouling. So for succinic acid fermentation broth ultrafiltration, it is better to choose PES 10 kDa other than PES 30 kDa. And for PES 10 kDa, a cross-flow model ultrafiltration may be adopted because flux decline caused by cake layer can be diminished by using cross flow filtration. Membrane flux in scale-up equipment under similar conditions can be predicted according to the determined fouling mechanism in this study. 4.3. Membrane characteristic variation The measurements of contact angle reflect membrane hydrophobicity. All the membranes selected in this paper are hydrophilic as their contact angles are all less than 90°. In this study, the RC membrane is more hydrophilic than PES membranes, which can be seen from its smaller contact angle. Table 4 presented the contact angles of clean and fouled membranes (without water

C.X. Wang et al. / Bioresource Technology 116 (2012) 366–371 Table 5 The mean roughness (Rq) values of virgin and fouled membranes (5.0  5.0 lm surface area).

371

2011AA02A203) and the Knowledge Innovation Program of the Chinese Academy of Sciences (no. KSCX2-EW-G-2).

Membrane type MWCO (kDa)

Virgin membrane Roughness (nm)

Fouled membrane Roughness (nm)

Appendix A. Supplementary data

PES 100 PES 30 PES 10

41 23.4 17.8

69 18.9 51.8

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2012. 03.099.

flushing). As can be seen from Table 4, for different PES membranes, contact angles of PES 30 kDa (61.3°) and PES 10 kDa (61.7°) were much similar, and they were both slightly smaller than the angle of PES 100 kDa (67.8°), which is more hydrophobic. All membranes were more hydrophobic after ultrafiltration, which indicated that some hydrophobic foulants adhered on the membrane after fouled. Contact angle of PES 100 kDa increased from 67.8° to 70.9° after single fouled. For PES 30 kDa and PES 10 kDa, contact angles after ultrafiltration were also slightly increased. The hydrophobic foulants should be further elevated because if these membrane surface foulants were not removed after single usage by physical or chemical cleaning, it would directly affect the lifetime of the membrane. According to the contact angles of the membranes and their flux, membrane with hydrophilic characteristic tend to have a higher flux for this succinic acid crude broth solution system. Fig. 5 (supplementary figure P2) showed the AFM photos of PES 100 kDa before and after ultrafiltration (without water flushing), and the roughness of membranes before and after fouling was shown in Table 5. It can be seen that as the MWCO decreased, the surface roughness increased. The roughness of PES 100 kDa increased from 41° to 69° after fouling, which could also be seen in the AFM 3d photos. The increased roughness of membrane after fouling indicated that macromolecules or foulants with bigger size adhered on the membrane surface, which was consistent with the result of contact angle variation. These foulants may cause the concentration polarization and have negative influence on the lifetime of membrane. Similar and contrary conclusions were both reported in literatures (Dizge et al., 2011; Hashino et al., 2011). One possible explanation is that the roughness variation was closely related to both the feed solution system and the characteristics of membrane used in the process.

5. Conclusion Resistance-in-series model analysis showed that membranes with higher flux tend to be fouled by cake layer or concentration polarization. Hermia’s model analysis indicated that RC 10 kDa and PES 30 kDa were controlled by complete blocking, PES 100 kDa by intermediate blocking, and PES 10 kDa by cake layer fouling. These results were verified by SEM photos. Membrane with hydrophilic characteristic tends to have a higher flux. The increase of contact angles and roughness were caused by macromolecules foulants. Membrane flux in scale-up ultrafiltration equipment under similar conditions can be predicted according to the fouling mechanism determined in this study. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 Project, no.

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