Membrane fouling and resistance analysis in dead-end ultrafiltration of Bacillus subtilis fermentation broths

Separation and Purification Technology 63 (2008) 531–538

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Membrane fouling and resistance analysis in dead-end ultrafiltration of Bacillus subtilis fermentation broths Ruey-Shin Juang a,b,∗ , Huei-Li Chen a , Ying-Shr Chen a a b

Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 32003, Taiwan Fuel Cell Center, Yuan Ze University, Chung-Li 32003, Taiwan

a r t i c l e

i n f o

Article history: Received 31 March 2008 Received in revised form 11 June 2008 Accepted 12 June 2008 Keywords: Dead-end ultrafiltration Fermentation broths Flux decline Membrane fouling Resistance-in-series model

a b s t r a c t Flux decline during dead-end ultrafiltration (UF) of the fermentation broths of Bacillus subtilis ATCC (American Type Culture Collection) 21332 culture was studied, in which polyethersulfone membrane with a molecular weight cut-off of 100 kDa was used. Prior to UF, the broth was treated by centrifugation at 10,000 × g. All experiments were performed at a feed pH of 7, a feed surfactin concentration of 0.56 g L−1 , and a stirring speed of 300 rpm but at different applied pressures (P, 86–430 kPa). The resistance-inseries model was used to analyze flux behavior, which involves the resistances of membrane itself and the cake as well as those due to adsorption and solute concentration polarization. It was shown that the resistance due to solute concentration polarization dominated the flux decline under the conditions studied. The resistances due to cake formation and solute adsorption were comparable, and their sum contributed below 3% of the overall resistance. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Pressure-driven membrane processes such as ultrafiltration (UF) and microfiltration (MF) have been widely applied in various chemical and biochemical processes for the separation of dissolved and suspended matter according to the size and molecular scale [1]. This is because this process basically involves no phase change and chemical agents and is more environmentally friendly and economic [2,3]. Particularly, it is effective for the separation of bioproducts and harvesting of microorganisms from fermentation broths. The characteristics of UF process that makes it excellent for many applications include the minimized physical damage of biomolecules from shear effects, minimal denaturation, high recovery, and the avoidance of re-solubilization problems because the solutes can be retained in the solution phase, high throughput, and cost effectiveness. Although cross-flow mode of UF process is often used in continuous operations for the separation of bioproducts such as peptides, proteases, proteins and antibiotics [4], the dead-end mode is still applied in laboratory-scale tests. In this work, the fermentation broth of Bacillus subtilis ATCC 21332 culture, treated by centrifugation, was selected as the model feed. Such broth will produce

∗ Corresponding author at: Department of Chemical Engineering and Materials Science, Yuan Ze University, 135 Yuan-Tung Road, Chung-Li 32003, Taiwan. Tel.: +886 3 4638800x2555; fax: +886 3 4559373. E-mail address: [email protected] (R.-S. Juang). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.06.011

surfactin, which is a heptapeptide linked to a ␤-hydroxy fatty acid and comprises mainly 14 or 15 carbon atoms [5]. It is one of very powerful biosurfactants that has exceptional surface-active power; e.g., it can lower the surface tension of water from 72 to 27 mN m−1 at a level even as low as 20 ␮M. Also, surfactin has the advantages of biodegradability, low toxicity, and biocompatibility over other chemically synthesized surfactants [6]. At concentrations above the critical micelle concentration, surfactant molecules readily associate to form supramolecular structures such as micelles or vesicles, with nominal molecular diameters up to two to three orders of the magnitude larger than that of single unassociated molecules [2]. Hence, surfactin micelles can easily be retained by UF membranes with sufficiently low molecular-weight cut-off (MWCO). In our previous work [3], the fermentation broth of B. subtilis culture has been treated by dead-end UF using polyethersulfone membrane with a MWCO of 100 kDa (PES 100) for surfactin recovery. This process gave an acceptably high rejection of surfactin micelles (86%) and high steady-state flux (e.g., 92.4 L h−1 m−2 ) at an applied pressure P of 86 kPa. Although both the recovery of surfactin from fermentation broth [3] and the use of resistance-in-series model in describing UF performance [7–10] have been studied in the past, understanding the mechanism of flux decline can lead not only to reduce membrane fouling, but also to develop better operation mode and possibly select more efficient membrane elements. Moreover, a deeper analysis on the UF of fermentation broths is of practical interest and importance in biotechnological applications. Experiments were performed at different applied pressures (P, 86–430 kPa) but at

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Nomenclature A b C C0 Cm CP D J Jw N m n P Ra Rc Rm Rp SD t V0 Y0

membrane filtration area (m2 ) characteristic constant defined in Eq. (5) (s−1 ) surfactin concentration in the retentate (g L−1 ) surfactin concentration in the feed (g L−1 ) surfactin concentrations at the membrane wall (g L−1 ) surfactin concentration in the permeate (g L−1 ) shear-induced diffusivity (m2 s−1 ) UF flux of the feed broth (L h−1 m−2 ) UF flux of pure water (L h−1 m−2 ) number of data points mass of surfactin deposited on the membrane (kg) compressibility index of the cake defined in Eq. (4) applied pressure (Pa) resistance due to solute adsorption defined in Eq. (1) (m−1 ) resistance of cake defined in Eq. (1) (m−1 ) intrinsic membrane resistance defined in Eq. (1) (m−1 ) resistance due to concentration polarization layer defined in Eq. (1) (m−1 ) standard deviation defined in Eq. (29) (%) filtration time (s) initial volume of the feed (m3 ) pressure-dependent quantity defined in Eq. (25) (m−1 )

Greek letters ˛ resistance coefficient of the cake defined in Eq. (4) (m kg−1 Pa−0.8 ) specific resistance of the cake defined in Eq. (3) ˛0 (m kg−1 ) ˇ rejection of surfactin in dead-end UF defined in Eq. (11) ı cake thickness (m) viscosity of the permeate (Pa s) p ϕ constant defined in Eq. (20) Subscripts calc calculated value expt experimental value ss steady-state value

In this regard, the permeate flux of UF process (J) is thus expressed by [7–10]: J=

P p (Rm + Rc + Ra + Rp )

(1)

where P is the applied pressure (Pa), p is the viscosity of permeate (Pa s), Rm is the hydraulic resistance of membrane itself (m−1 ), Rc is the resistance of cake (m−1 ), Ra is the resistance due to solute adsorption (m−1 ), and Rp is the resistance due to concentration polarization (m−1 ). In general, Rm can be obtained when only the pure solvent is filtered. That is, the pure water flux (Jw ) is given by Jw =

P p Rm

(2)

Next, Rc is the resistance of the deposited cake formed at the membrane surface by suspended matter, that is, surfactin micelles present in the broth. It is accepted that Rc is proportional to the thickness of the cake and applied pressure [11], thus we have Rc =

m A

˛0

(3)

where m is the mass of surfactin deposited on the membrane and ˛0 is specific resistance of the cake [12]. The pressure dependence of specific resistance is proposed by [11]: ˛0 = ˛(P)

n

(4)

where n is the cake compressibility index and ˛ is the resistance coefficient depending on particle size and shape. The resistance due to solute adsorption is recognized to be timedependent and tends towards a steady value, Ra,ss , corresponding to adsorption equilibrium [13,14]. The following expression is obtained: Ra = Ra,ss (1 − exp(−bt))

(5)

where b is the characteristic constant of the membrane and feed solution. The resistance Rp is ascribable to the accumulation of any soluble solutes present in the feed broth at the membrane/solution interface. Once the permeate flux and solute rejection are known, the variation of solute concentration in the retentate at each time interval can be calculated from Eq. (6) in a batch system [13]: C=

C0 [V0 − (1 − ˇ)A V0 − A

t 0

t 0

J dt]

J dt

(6)

a fixed initial surfactin concentration of 0.56 g L−1 and a stirring speed of 300 rpm. Resistance-in-series model was adopted for this purpose because it is widely applied in the UF of the mixtures of macromolecule solutes [7,8].

where ˇ is the rejection of surfactin defined in Section 3.3 (Eq. (11)), C is the solute concentration in retentate (g L−1 ), V0 is the initial volume of the feed (m3 ), and A is the membrane area (m2 ). The mass of surfactin deposited m can be obtained by integrating the mass balance equation [14]:

2. Resistance-in-series model

dm = dt

Flux decline in either dead-end or cross-flow UF processes can be caused by several factors including concentration polarization, cake formation, solute adsorption, as well as plugging of the pores [9]. All these introduce additional resistances on the feed side to the transport across the membrane. Resistance-in-series model that considers membrane resistance, adsorption resistance, concentration polarization resistance, and cake resistance has been applied to describe such processes. This model is particularly applicable for the analysis of flux decline in UF or MF of the present broths because it contains many macromolecules such as surfactin micelles, proteins, polysaccharides, and peptides [7,8].

where Cm is surfactin concentration at the membrane wall (g L−1 ), D is the shear-induced diffusivity (m2 s−1 ), and ı is the cake thickness (m). In Eq. (7), JCA represents the convective flow towards the membrane and D(Cm − C)A/ı is the opposite diffusive flow. It was reported that the back diffusion is mainly due to shear-induced diffusion for a particle with a size near 1 ␮m [15]. In such cases, D is proportional to the wall shear rate. Because Cm  C, Eq. (7) can be simplified as [11]: dm = dt





JC −

JC −



D (Cm − C) A ı

(7)



D Cm A ı

(8)

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When the flux reaches pseudo-steady state, dm/dt equals zero. Thus we have [13,14]: Jss C =

D ı

Cm

(9)

Substitution of Eq. (9) into Eq. (8) yields: dm = (J − Jss )CA dt

(10)

3. Materials and methods 3.1. Microorganisms and culture condition B. subtilis ATCC 21332 was selected to produce surfactin. The nutrient broth (NB) medium contained 3 g L−1 beef extract, 5 g L−1 peptone, and the mineral salt (MS) medium at pH 7. The MS medium consisted of 40 g L−1 glucose, 50 mM NH4 NO3 , 30 mM KH2 PO4 , 40 mM Na2 HPO4 , 7 ␮M CaCl2 , 0.8 mM MgSO4 , 4 ␮M FeSO4 , and 4 ␮M tetrasodium salt of EDTA [3]. The MS medium pH was regulated at 7.0 by adding 0.1 M HCl or NaOH. Prior to use, the MS medium and deionized water (Millipore, Milli-Q) were sterilized in autoclave at 121 ◦ C for 15 min. All inorganic chemicals (Merck Co.) were as analytical reagent grade. The culture of B. subtilis was taken from −80 ◦ C frozen stock and transferred onto agar medium for pre-culture. B. subtilis culture (1 mL) was inoculated into 250-mL flask containing 100 mL of NB medium at 30 ◦ C with 200 rpm of agitation. After growing up to late exponential phase (near 9 h), the NB medium containing B. subtilis was inoculated (optical density at 600 nm, OD600 1.2) and fermented in 5-L fermenter with 4-L working volume at 25 ◦ C and 200 rpm for another 4 days (final OD600 , 16.0). When liquor inside the fermenter was centrifuged at 10,000 × g to remove microorganism impurities, the supernatant was the feed broth. Before dead-end UF, the pH of the broth finally was adjusted to be 7.0 by adding 0.1 M NaOH solution. 3.2. Analysis of surfactin concentration Culture samples were taken after centrifugation at 12,000 × g for 15 min to remove the biomass, and surfactin concentration in the clarified supernatant was measured by reverse phase C18 HPLC equipped with a Merck C18 column (5 ␮m) at 30 ◦ C [3,16]. Samples were subjected to filtration through a Millipore filter (0.45 ␮m) before analysis. A mixture of 3.8 mM trifluoroacetic acid (20 vol%) and acetonitrile was used as the mobile phase, and the flow rate was 1 mL/min. An aliquot of the sample (20 ␮L) was injected and analyzed using an UV detector (Jasco 975, Japan). The wavelength was set at 205 nm. Surfactin powder purchased from Sigma Co. served as the standard (98% purity as per label claim). Each concentration analysis was at least duplicated under identical conditions. The reproducibility is mostly within 5%. 3.3. Membranes, apparatus, and filtration experiments The UF experiments were carried out in batch stirred cell with a capacity of 300 mL (Millipore, Model XFUF07601). Because it was reported that surfactin readily forms micelles at concentrations above 15 mg L−1 (i.e., critical micelle concentration) [3] and the size of surfactin micelles was in the range 50–100 kDa [17], polyethersulfone (Millipore Co.) membrane with a MWCO of 100 kDa (PES 100) was tested. In this case, the acceptably high rejection of surfactin micelles and high permeate flux could be simultaneously achieved. The disc membrane had a diameter of 76 mm with a geometric (flat surface) area of 41.8 cm2 .

Fig. 1. The schematic experimental setup for dead-end UF process.

Fig. 1 illustrates the experimental setup. Temperature was controlled at around 25 ◦ C by air conditioner. The feed volume was 300 mL and the cell was stirred at 300 rpm using a magnetic motor. This stirring speed was selected because it could lead to effective agitation but prevent formation of a serious vortex in the cell. The applied pressure (P) was monitored with pressurized N2 gas by means of a transducer. Because the compositions of the retentate and permeate varied with time, particularly at the early stage of the process, the rejection of surfactin (ˇ) was calculated at pseudo-steady state in flux by ˇ =1−

C  p

C0

(11)

where Cp and C0 are the concentrations of surfactin in permeate at steady state and the feed, respectively. The typical time profiles of filtration flux J over the entire process can be empirically expressed in the following exponential dependence [18]: J=

r 

(Ji−1 − Ji )exp(−ki t) + Ji

(12)

i=1

Here, the steady-state flux was obtained at t → ∞ through the selection of r such that the percent of standard deviation between the fitted and measured flux was less than 1% (in most cases, r = 2–4). When the experiment was completed, the used membrane was immediately flushed with NaOH at pH 13, 1 wt.% Terg-a-zyme solution, and deionized water in sequence for 30 min each in order to restore the hydraulic permeability. It was finally stored in 0.1 M NaOH solution overnight at 4 ◦ C. Only the cleaned membrane was repeatedly employed here if the difference of pure water flux between the cleaned and fresh membranes were smaller than 5%. 4. Results and discussion 4.1. Flux decline in dead-end UF process Although composition of the broth treated after centrifugation could not be properly characterized, the results observed were useful because we used the practical broths in the experiments. It was reported that it may contain some macromolecules (proteins, polysaccharides, peptides) and small molecules (amino acids, glycine, serine, threonine, alanine, etc.), as shown in Table 1 [17]. The effects of applied pressure (P) on the flux and rejection in dead-end UF with PES 100 membrane are shown in Fig. 2a. It is noted that the percentage in the parentheses of symbol legend refers to the steady-state rejection of surfactin defined in Eq. (11). Howell and Velicangil [19] have divided the UF process into three time intervals: first few seconds (a quasi-steady-state concentration polarization layer is set up), 1–10 min (solute adsorption),

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Fig. 2. Effects of applied pressure on (a) time-dependent flux and (b) steady-state flux with PES 100 membrane.

and long term (cake formation). The gradual, instead of quick, flux decline is not purely a result of concentration polarization; other factors such as adsorption of surfactin micelles and small impurities onto PES membrane surface as well as cake formation may play a certain role [4]. The moderate-to-high steady rejection of surfactin of 88–94% only with PES 100 membrane may support this argument. It has been reported that for the mixtures of all macromolecular solutes where concentration polarization can have a strong effect on solute retention because the solutes that are completely retained would form a kind of second or dynamic membrane [9]. Fig. 2b shows the effect of P on the steady-state flux with PES 100 membrane. The flux is pressure dependent when P < 80 kPa but is pressure independent when P > 80 kPa. This is the weak form of critical flux, which is lower than the corresponding pure water flux at the same P but still stable with time [4]. This type of critical flux is found in the case of some solutes that are small enough to go into the pores of the membrane and adsorb onto the Table 1 The reported components and their molar weights (in g mol−1 ) classified in the raw broths [17] Macromolecules

Mid-molecules

Small molecules

Surfactin micelle (50,000–100,000) Polysaccharides Peptides Proteins

Surfactin monomer (1036)

MS medium (80–400) Phthalic acid (150) Amino acid (200) Glycine (75) Serine (105) Threonine (119) Alanine (89)

Fig. 3. Determination of (a) the resistance of membrane itself (Rm ) and (b) the sum of the resistances of membrane and due to solute adsorption (Ra + Rm ).

pore walls, which is favored by attractive electrostatic forces and high solute concentrations [4]. In this system, the small amino acid molecules as well as protein, polysaccharide, and peptide macromolecules would cause such effect. 4.2. Resistance-in-series analysis Fig. 3a shows the linear relationship between 1/Jw and 1/P, indicating the validity of Eq. (2). The value of Rm is thus obtained as follows when p = 1.12 × 10−3 Pa s: Rm = 3.11 × 1011 (m−1 )

(13)

To obtain Rc , n and ˛ in Eq. (4) must be available. In principle, ˛ has to be determined from experiments for irregular shaped solutes [10] but can be obtained using the Carman-Kozeny equation for spherical particles such as latex particles [11] and yeast cells [12]. As usual, cake compressibility index n was determined from dead-end filtration experiments. Separate experiments were carried out with either the present feed broth (i.e., containing surfactin) or a previously ultrafiltered one [13,20]. Specific cake resistance is calculated by subtracting the resistance deduced from the UF of pre-filtered broth from that deduced from the UF of feed broth. The mass of cake is calculated by multiplying surfactin concentration by the permeate volume. Also, the resistance coefficient ˛ is determined from dead-end UF experiments carried out at a constant P, by fitting the modeled flux to the experimental data (Fig. 2a). In this work, a value of n of 0.8 as well as a value of ˛0 of 2 × 1012 m kg−1 when P = 49 kPa are alternatively adopted fro simplicity from the work of Tanaka et al. [21] on the cake formed in cross-flow filtration of the fermentation broth of B. subtilis. Therefore, a value of ˛ of 3.53 × 108 m kg−1 Pa−0.8 is obtained [10]

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Table 2 The expression of Rc at different applied pressures (where P in Pa and t in s) Rc (m−1 )

P (Pa) 8.6 × 10 1.72 × 105 2.58 × 105 3.44 × 105 4.30 × 105

1.83 × 10 (P) t 1.86 × 104 (P)0.8 t0.74 2.85 × 104 (P)0.8 t0.68 3.82 × 104 (P)0.8 t0.66 4.92 × 104 (P)0.8 t0.64

4

4

0.8 0.77

Equation

Correlation coefficient

(15) (16) (17) (18) (19)

0.9903 0.9952 0.9957 0.9948 0.9972

and we have m 0.8 Rc = 3.53 × 108 (P) A

(14)

On the other hand, the time-dependent relationship of (m/A) is obtained here by simultaneously solving the set of Eqs. (3), (4), (6), and (10) using the METLAB doftlab software. The expressions of Rc under various P values are shown in Table 2. The specific resistance ˛0 of gel layer equals 3.1 × 1012 m kg−1 as P = 86 kPa, for example, which is larger than those reported ranging from 1 to 1.7 × 1012 m kg−1 in cross-flow MF of lactic acid fermentation broths (P = 50–100 kPa) [13] but comparable to those from 2 to 11 × 1012 m kg−1 in cross-flow filtration of different kinds of bacteria (P = 49 kPa) [21]. An alternative way was used to evaluate experimental Ra value. Because both Rp and Rc are pressure dependent, their sum can be expressed by [8]: Rc + Rp = ϕ P

(20)

where ϕ is a constant and is proportional to the reciprocal of limiting flux [8]. Thus we have J=

P p (Rm + Ra + ϕ P)

or 1 = p J



ϕ+

Rm + Ra P

(21)

 (22)

As shown in Fig. 3b, the plot of 1/J against 1/P gives Rm + Ra = 3.82 × 1011 m−1 from the slope of the line. That is, an experimental Ra value of 7.1 × 1010 m−1 is obtained when p and Rm (Eq. (14)) are involved. As described in Section 2, the resistance due to solute adsorption Ra has exponential time dependence and tends towards a steady value corresponding to adsorption equilibrium. It is assumed that there is the same time dependence for the resistance due to solute concentration polarization Rp [13]. That is, the experimental results are treated using an expression of the same type as Eq. (5): Ra + Rp ∝ (Ra,ss + Rp,ss )(1 − exp(−bt))

(23)

Because Rm (Eq. (13)) and Rc (Eqs. 15–19) are known, we can obtain (Ra + Rp ) by solving Eq. (1) using the METLAB doftlab software. Figs. 4 and 5 show the comparisons of the fitted and measured (Ra + Rp ) under different applied pressures. An average value of parameter b = 0.0034 ± 0.0008 s−1 is found to fit the results of any experiment. The fitting is very good (in terms of correlation coefficients). That is, the changes of (Ra + Rp ) with time have an exponential dependence as indicated in Eq. (23). Fig. 6a shows the effect of P on steady-state resistances (Ra,ss + Rp,ss ), which are calculated when t → ∞. Thus, we have the following expression because linear relationship holds: Ra,ss + Rp,ss = 8.14 × 1010 + 2.36 × 107 P

(24)

where Ra,ss and Rp,ss are in m−1 and P in Pa. The calculated Ra value of 8.14 × 1010 m−1 is obtained when P = 0 from Eq. (24), which is slightly larger than the measured one of 7.1 × 1010 m−1 as indicated above.

Fig. 4. Comparisons of the fitted and measured time changes of (Ra + Rp ) at an applied pressure of (a) 86 kPa, (b) 172 kPa, and (c) 258 kPa.

On the other hand, the variations of the intercepts in Figs. 4 and 5 (obtained when t = 0), Y0 , with P are shown in Fig. 6b. Similarly, the following expression holds in the P range 86–430 kPa [13]: Y0 = 3.66 × 1011 + 9.89 × 106 P

(25)

Substitution of Eqs. (24) and (25) into Eq. (23) yields: Ra + Rp = (3.66 × 1011 + 9.89 × 106 P) + (8.14 × 1010 + 2.36 × 107 P)(1 − exp(−0.0034t))

(26)

As indicated in Eq. (5), the resistance due to solute adsorption Ra can be obtained from Eq. (23) when P = 0. Thus, we have Ra = 8.14 × 1010 (1 − exp(−0.0034t))

(27)

In this regard, the resistance due to solute concentration polarization Rp can be expressed by Rp = (3.66 × 1011 + 9.89 × 106 P) + 2.36 × 107 P × (1 − exp(−0.0034t))

(28)

It is noted that Eqs. (25), (26), and (28) are valid only in the P range 86–430 kPa.

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Fig. 5. Comparisons of the fitted and measured time changes of (Ra + Rp ) at an applied pressure of (a) 344 kPa and (b) 430 kPa.

Fig. 7. Comparisons of the calculated and measured time changes of the fluxes at an applied pressure of (a) 86 kPa, (b) 258 kPa, and (c) 430 kPa.

The complete form of flux decline as a function of time and P can be finally obtained by substituting Eqs. (13), (15)–(19), (27), and (28) into Eq. (1). Fig. 7 compares the calculated and measured time changes of the permeate fluxes. A good agreement is obtained. The standard deviation (SD), defined in Eq. (29), is within 9.6% under the P ranges studied:



N 2 [(J − J )/J ] expt expt calc 1 SD (%) = 100 ×

N−1

Fig. 6. Effect of applied pressure on (a) the sum of steady-state resistances due to solute adsorption and concentration polarization (Ra,ss + Rp,ss ) and (b) the value of Y0 defined in Eq. (25).

(29)

where N is the number of data points. In a summary, some parameters in this standard resistancein-series model cannot be determined from separate experiments including m, Ra , Rc , and Rp . They must be obtained by optimizing the measured and modeled results. However, there are several parameters in this model that can be determined independently such as Rm , Ra,ss and Rp,ss , (D/ı)Cm (Eq. (8) or (9)), n, ˛, ˛0 , and b [13].

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(>82%) under the P ranges studied. Moreover, increasing P leads to a decrease of the contribution of Rm and Ra but an increase of the contribution of Rp and Rc . In a word, the key role of Rp on flux decline makes the present dead-end process unattractive although membrane materials appear adequate. However, the flux of this operation can be enhanced by hydrodynamic methods such as the use of cross-flow mode [4,9]. 5. Conclusions We have analyzed the flux decline behavior in dead-end UF of fermentation broth of B. subtilis ATCC 21332 culture at pH 7 and 25 ◦ C using PES membrane with a MWCO of 100 kDa by resistancein-series model. The following results were obtained: (1) Weak form of critical flux behavior was observed from the effect of applied pressures (P) on steady-state fluxes. The small amino acid molecules as well as some macromolecules of protein, polysaccharide, and peptide in the feed would cause such effect. (2) Under the conditions studied, the resistance due to concentration polarization Rp played a crucial role in flux decline and it contributed more than 82% of the total resistance during the whole UF process. In addition, Rp increased with increasing P, and resistance of cake Rc was basically comparable to resistance due to solute adsorption Ra . (3) The significant contribution of Rp in the present dead-end UF process indicated that the membrane used was adequate because the flux could be enhanced by hydrodynamic methods alone (including the use of cross-flow mode). This is one of the advantages in flux decline analysis by resistance-in-series model, which allowed us to know how the flux was improved. Fig. 8. Comparisons of time changes of various filtration resistances calculated at an applied pressure of (a) 86 kPa and (b) 344 kPa.

4.3. Relative contribution of various resistances Fig. 8 shows the calculated results of various resistances. As expected, Ra , Rc , and Rp increase with time, and finally levers off [22]. Under the conditions studied, Rp plays a crucial role in flux decline, and Rc is comparable to Ra when P = 430 kPa. However, at low P (e.g., 86 kPa) the contribution decreases in the order Rp  Rm > Ra > Rc during the whole UF process. The low adsorptive fouling of PES 100 membrane observed in this work is presumably due to quick modification of the membrane by surfactin (reasonable via adsorption to hydrophobic membrane material), in which the surface of the membrane is hydrophilized and further adsorption of biomacromolecules or adhesion of micelles is effectively reduced. Once various resistances are obtained, we can know the relative contribution of each resistance to the flux decline in the present UF process. Table 3 shows the relative contribution (in percentage), defined by dividing each resistance to the overall resistance, at different filtration times (50 and 300 s). It is found that the resistance due to solute concentration polarization Rp is absolutely dominant Table 3 Relative contribution of various resistances to the flux decline in dead-end UF process P (kPa)

86 172 258 344 430

t = 50 s

t = 300 s

Rm (%)

Rc (%)

Ra (%)

Rp (%)

Rm (%)

Rc (%)

Ra (%)

Rp (%)

16.7 10.2 7.4 5.8 4.7

0.2 0.2 0.3 0.3 0.3

0.7 0.4 0.3 0.2 0.2

82.4 89.2 92.0 93.7 94.8

8.7 4.8 3.5 2.6 2.0

0.6 0.8 0.9 1.1 1.3

2.5 1.2 0.8 0.7 0.5

88.2 93.2 94.8 95.6 96.2

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