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Assessment and optimization of electrospun nanofiber-membranes in a membrane bioreactor (MBR)
Journal of Membrane Science 380 (2011) 181–191
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Assessment and optimization of electrospun nanofiber-membranes in a membrane bioreactor (MBR) M.R. Bilad a , P. Westbroek b , Ivo F.J. Vankelecom a,∗ a b
Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, Box 2461, 3001 Leuven, Belgium Gent University, Department of Textiles, Technologiepark 907, B-9052 Gent, Belgium
a r t i c l e
i n f o
Article history: Received 7 March 2011 Received in revised form 5 June 2011 Accepted 1 July 2011 Available online 7 July 2011 Keywords: Membrane bioreactor Membrane fouling High-throughput Nanofiber Electrospinning Non-woven Electrospun
a b s t r a c t The feasibility and optimization of low-cost nanofiber-membranes as potential MBR-membranes is studied. The nanofiber-membranes have a unique surface architecture, a high surface porosity and permeability and adjustable pore sizes. The materials, their heat treatment as well as the diameters and area-weights of nanofiber sheets were optimized. The comparative performance of a nanofibermembrane to lab-made polysulfone (PSFL ) and commercial polyvinylidene fluoride (PVDFT ) and polyethylene (PEK ) membranes was performed. The critical flux (CF) and trans membrane pressure (TMP) profiles were used as evaluation parameters. A heat treatment was efficient to prevent layered fouling on the nanofiber-sheets. The electrospun nanofiber-membranes showed performances comparable to those of the tested commercial membranes at short and long term. Further developments on nanofibermembranes are still required to further improve their performance and enhance their competitiveness in MBRs applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Membrane bioreactors (MBRs) are becoming more popular for wastewater treatment, especially when an excellent effluent quality is required [1]. However, membrane fouling and high membrane investment costs remain major obstacles [2]. Membrane bioreactors can only compete with, e.g. activated sludge system, by applying a low cost filter to substitute the traditional membrane and tackle the fouling problem by mean of low cost technology [3]. In MBRs, membranes principally act as a selective barrier retaining suspended solids and allowing the treated water to pass through it. Low cost filters such as non-wovens, meshes, filter cloths or nanofiber have sometimes found to be suitable to substitute the function of membranes in MBRs [3]. Recent studies proved the feasibility of non-wovens to substitute the traditional membranes in the lab scale or the pilot-scale textile bioreactors where no significant difference in effluent quality was found compared to MBRs [4–7]. However, the major problem limiting the application of nonwovens is the acute fouling due to their rough surface and a rather large pore sizes and wide pore size distributions [8–10]. The sludge flock can then be entrapped in the voids inside the fiber matrix
∗ Corresponding author. Tel.: +32 16 321594; fax: +32 16 321998. E-mail address: [email protected] (I.F.J. Vankelecom). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.07.003
and block. Such blockage is very difficult to be removed by shear stresses from the feed side, as applied currently in MBRs. To minimize the fouling, application of nanofiber seems to be more effective. Nanofiber-membranes have become an interesting option because of their acceptable selectivity, high permeability and low production cost. Fatarella et al. [11] estimated the production cost of nanofiber-membrane at 5 D /m2 . Most of the cost (75%) is from non-woven support. Application of cheaper supports indeed will further reduce it. This value is much lower than the 14–50 D /m2 production cost of traditional membranes [1,11,12]. A nanofiber is a fiber having a diameter smaller than one micron, and is commonly produced via electrospinning [13]. It can be used to produce many types of membrane for microfiltration [14–16], ultrafiltration [17], nanofiltration [18,19] and membrane distillation [20]. To form the nanofiber-membranes, nanofibers are used as a coating material onto non-wovens [11], as a sheet that is later fixed onto a non-woven at both feed and permeate sides [21], as a blending material together with micron fiber [22] or after heattreatment as a self-supporting membrane [16]. Fatarella et al. [11] coated a nanofiber onto a non-woven and combined a plasma treatment to modify the surface properties. Initial testing showed that this nanofiber-membrane had a CF of 35 l/m2 h, comparable to the traditional polyvinylidene fluoride (PVDF) membranes. Bjorge et al. [12] fixed a thin nanofiber layer on a non-woven support to form a nanofiber-membrane. They also revealed the feasibility of the nanofiber-membranes in
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water filtration: for pathogens removal, as a stand-alone filter and as a substitute for traditional membranes in an MBR. More recently, Daels et al. [21] reported the application of the nanofibermembrane to substitute the membrane in a bench-scale activated sludge MBR and a trickling filter MBR. The previous studies mainly focused on the applicability and feasibility of nanofiber-membranes as a low cost filter in MBRs in the preliminary stages. Bjorge et al. [12] applied nanofiber membranes directly as a stand-alone filter without addition of any support. The nanofiber faced an acute problem due to layered clogging. Further improvement on membrane by sandwiching it in-between two non-woven has been reported by Daels et al. [21]. This induced a loss of productivity in term of flux from ±21 to ±4 l/m2 h in a few days. Furthermore, their testing parameters, such as an MLSS of 1–3.9 g/l, are also far below the ones normally used in MBRs. Further optimization of these nanofiber-membranes is thus still required to achieve better performances, together with a more detailed study to investigate their competitiveness. In the present study, the preparation of electrospun sheets was optimized. The membranes were assessed for their feasibility as a substitute for traditional membranes in a lab-scale MBR. The nanofiber sheet was improved by adding a non-woven as a support and by introducing a heat treatment on the sheet surface at the feed side to prevent layered clogging. The study was performed in a lab-scale high-throughput MBR (HT-MBR) [23]. The CF and the TMP profile as evaluation parameters. The effect of nanofiber material, heat treatment on the nanofiber-sheets, fiber diameter and area-weight on the CF were studied. At the end, the performance of the optimum obtained nanofiber-membrane was compared to a lab-made membrane and two commercial membranes. 2. Materials and methods 2.1. Activated sludge and synthetic wastewater The activated sludge used to inoculate the lab-scale HT-MBR was obtained from a pilot-scale MBR treating molasses wastewater in the Waterleau wastewater laboratory (Wespelaar, Belgium). The acclimatization and biological performance of the sludge was discussed earlier [24]. For this study, the activated sludge was maintained by applying a fed-batch operation. To prevent the accumulation of slowly biodegradable substances, a part of the liquid in the reactor was discharged every week. The activated sludge was settled and a part of the supernatant was withdrawn and replaced with tap water. During the test, the mixed liquor suspended solid (MLSS) concentrations were kept at 10–12 g/l by partially withdrawing the sludge and the sludge volume index (SVI) was in the range of 55–75 l/g, measured according to the standard method [25]. This activated sludge was used as a feed for both CF and long-term filtration test and was performed batch-by-batch over a year along with the membrane optimization process. The feed solutions, further referred to as ‘synthetic wastewater’, were prepared by diluting 0.45 ml/l of molasses stock solution. The diluted molasses solution was chosen as feed wastewater because it does not require pre-fine screening, it has a good COD/N ratio and contains trace elements [26]. The characteristics of the feed solution are given in Table 1. All were measured using a HACH-Lange cuvette. 2.2. Membrane preparation, characterization and module potting Nanofiber-membranes were formed from nanofiber-sheets, which were fixed by glueing the edges onto polypropylene nonwoven supports (Novatexx 2471, kindly supplied by Freudenberg, Germany), so that only the central part acts as the active separation layer. The nanofiber-sheets were prepared at the Department
Table 1 Characteristics of the synthetic wastewater. Parameters Chemical oxygen demand (COD) Total Nitrogen (TN) Total phosphorous (TP) Ammonium-N (NH4 -N) Nitrate-N (NO3 -N) Orthophosphate-P (PO4 -P)
Average concentration (unit) 433 (mg/l) 11.3 (mg/l) 1.8 (mg/l) 1.8 (mg/l) 1.5 (mg/l) 0.7 (mg/l)
of Textiles University of Gent, Belgium. Bjorge et al. [12] and Daels et al. [21] explain the production and functionalization of these sheets in detail. Three polyamide polymers were used to prepare them, namely Polyamide 6 (PA6) with repeating unit C6 H11 ON, Polyamide 66 (PA66) with repeating unit C12 H22 O2 N2 , and Polyamide 69 (PA69) with repeating unit of C15 H28 O2 N. PA6 and PA66 were also prepared in the presence of tetrabuthyl ammonium chloride ([CH3 (CH2 )3 ]4 NCl) as an additive to further improve the hydrophilicty. Four batches of nanofiber-sheets with different properties were used in this study, and their specifications are summarized in Table 2. The term “area-weight” (g/m2 ) is used to quantify the amount of polymer used to prepare one square meter of nanofiber sheet. For comparative performance test, three different traditional flat sheet membranes were used; a lab-made polysulfone (PSFL ), a commercial Polyvinylidene Fluoride (PVDFT ) and a commercial polyethylene (PEK ). The PSFL was prepared from a 10% PSF (BASFUltrason)/N-Methyl-2-pyrrolidone (Acros Organics) solution by phase inversion [27]. The solution was cast on a polypropylene non-woven support (Novatexx 2471, was also kindly supplied by Freudenberg, Germany) and then brought into contact with a nonsolvent (water). In the non-solvent bath, the polymer solidifies to form a thin porous membrane. The PVDFT and PEK were purchased from membrane suppliers, Toray and Kubota respectively. Prior to use, all membranes were potted to form modules with an effective membrane area of 0.016 m2 . A flat sheet membrane was fixed to a PVC frame by glueing the edges together using two components epoxy glue (UHU-Plus endfest 300, Germany). The membrane sheet was folded to form a small envelope. Both membrane sides were separated by two sheets of spacer (Agfa, Belgium) in the interior of the module. Each spacer has a thickness of 2 mm giving the total thickness of the module ±5 mm. Permeate was sucked from the module interior through the permeate line. More detailed information about the module potting is available in Bilad et al. [23]. 2.3. Experimental setup The permeability of nanofiber-membranes was measured in the high-throughput (HT) filtration apparatus for pressure driven processes [28], while the CF test was performed in the labscale HT-MBR [23]. All HT-equipment was purchased from HTML (www.HTML-membrane.be, Belgium). The HT-MBR set-up has a working volume of 18.6 L and is equipped with a coarse and fine air bubble aeration mechanism for membrane and biological aeration respectively. To allow simultaneous filtration, the modules are fixed into two parallel holders, each consisting of three modules. This setup allows a correct comparison of membrane performances in MBR by applying an identical operating and mixed liquor conditions for all tested membranes. Parallel operation is essential to avoid discrepancies on the feed, due to the dynamic behavior of the activated sludge over the testing period. To ensure continuous and homogeneously distributed membrane aeration, each module holder was equipped with an individual, interconnected and adjustable aeration device, which was located beneath the modules. Inside the reactor, each permeate line was connected
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Table 2 The properties of the nanofiber-sheets applied. Sample batch
Sample code
Material
Heat treatment ◦
Temperature ( C) PA66 PA6 PA69 PA6 + additive PA66 + additive
Nanofiber diamater (nm)
Area-weight (g/m2 )
250
20
250
20
20
10 20 50 100
Time (s)
1
M1A M1B M1C M1D M1E
2
M2A M2B M2C M2D M2E M2F
3
M3A M3B M3C M3D M3E M3F
PA6
170
30
100 150 200 250 350 470
4
M4A M4B M4C M4D
PA6
170
30
250
150 160 170 170
30 10 30 60
to an individual line, an individual vacuum gauge and passed to a separate channel in the multi-channel peristaltic pump (WatsonMarlow 205U 16 Channel Pump, UK) using isoprene manifold tubes (Watson-Marlow, UK). The filtration flux could be adjusted by changing the rotational speed of the pump, changing the membrane area or by changing the diameter of the manifold pump tubing. 2.4. Analytical methods 2.4.1. Membrane characterization The microstructures of all fresh membranes that were used in the comparative performance test were observed with scanning electron microscopy (SEM, Philips SEM XL30 FEG with Edax dx4i system). The properties of the membranes, surface pore size and porosity, were identified with an image processing software, imageJ (NIH, USA) [29]. The performance of the long-term tests was evaluated based on the TMP profile. The flux (J) and the permeability (L) of the membrane were calculated by using Eqs. (1) and (2), respectively. J=
V At
L=
J TMP
(l/m2 h) (l/m2 h bar)
(1)
flux at which the TMP proportionality to flux ceases to exist [31]. All CF measurements were performed using fresh membranes after pre-treatment by soaking for 3 h in 30% ethanol/water solution and compaction by filtration with clean water at a flux of 90 l/m2 h for 3 h. 2.4.3. Long-term experiment For the exploratory tests in Sections 3.2 and 3.3, the filtration was performed in a cycle of 8 min filtration at 20 l/m2 h followed by a 2 min relaxation, resulting in a net flux of 16 l/m2 h. The relaxation was applied by simply stopping the filtration process. The cycle duration was arbitrarily chosen as being a standard for fullscale submerged flat sheet MBRs. For the comparative performance test in Section 3.6, the filtration at incremental fluxes without relaxation was applied. The filtration fluxes were gradually step-wise increased to speed up the fouling process, to perform a so-called extended flux step experiment. The initial flux was 8 l/m2 h, well below all CFs of the tested membranes. This flux was and step-wise increased to 16 l/m2 h. During the test, all permeates were recycled into the MBR to maintain the sludge concentration. The total recycle procedure was also applied for the CF tests.
(2)
where V is volume (l), t time (h), A effective filtration area (m2 ) and TMP trans-membrane pressure (bar) or (kPa, as presented in the figures). 2.4.2. Critical flux Critical flux is an important concept in MBRs that provides useful information for an operational point of view. In many cases, CF is also used as a parameter for a direct comparison of membrane performances in a short-term experiment [29]. In this study, CF was measured using the stepwise method suggested by Le-Clech et al. [30]. The applied initial flux, step height and step duration were 2 l/m2 h, 2 l/m2 h and 5 min, respectively. To determine the CF, the final TMP values of each step are plotted against the fluxes. Below the CF, a linear relationship exists between the TMP increment and the imposed fluxes. The CF was determined to be the minimum
Clean water permeability (l/m 2.h.bar)
PA6
–
–
14000 12000 10000 8000 6000 4000 2000 0 M1A
M1B
M1C
M1D
Nanofiber sheets Fig. 1. The CWP of nanofiber-sheets.
M1E
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Fig. 2. The microstructure of the membranes used for the comparative performance test: (A) PVDFT , (B) PEK , (C) PVDFL and (D) nanofiber-sheet.
2.5. Membrane cleaning
45 + additive
40
3. Results
35
Critical Flux (l/m 2.h)
After CF measurements, the membranes were physically cleaned prior to use for the long-term tests. First, the flat sheet module was removed from the bioreactor tank. A physical cleaning was applied with tap water for 10 min to remove cake layers from the membrane surfaces.
30 25 20 15 10
3.1. Membrane characterization Fig. 1 shows that the clean water permeabilities (CWPs) of nanofiber-sheets range from 5300 to 9200 l/m2 h bar with relatively high deviation. The CWPs obtained for all nanofiber-sheets are much higher than those of the commercial membranes currently used in MBRs. The properties and CWP of the typical commercial membranes in MBRs were given in detail by Judd [1]. Fig. 2 shows that the nanofiber-sheet has a unique structure compared to traditional membranes. The pores are formed from the interspaces between cross-linked nanofibers. The pore size of the nanofibersheets used in this study, more towards the upper side, is in the range of the pore sizes normally used in MBRs. They are in the magnitude of ultra- to micro-filtration. Table 3 summarizes the properties of membranes and nanofiber-sheets used for the long-term experiment in Section 3.6. The nanofiber-sheet has a significantly higher surface porosity, almost twice that of PEK and PVDFT membranes. 3.2. The effect of nanofiber material Fig. 3 shows that the CFs are in the range of 25–40 l/m2 h. These values are comparable to commercial membranes [32]. PA6 (M1A)
5 0 M1A
M1B
M1C
M1D
M1E
Nanofiber-membrane samples
Fig. 3. The CF of nanofiber-membranes with different nanofiber materials.
gives the highest CF, followed by PA66 and PA69. The order for the 3 polymer types reflects their hydrophilic character. By nature, nanofibers exhibit a very high contact angle when compared to a polymer film made from the same material. For instance, membranes and nanofibers prepared from PSF have contact angles of 70–80◦ and 140◦ respectively [14]. Despite the discrepancy on Table 3 The summary of the characteristics of the membranes used for the comparative performance test, as obtained from SEM image analysis using ImageJ. Parameter
PVDFT
PEK
PVDFL
Nanofiber-sheet
Pore size (m) Pore size from manufacturer (%) Surface porosity (%)
0.03 0.08 0.2
0.22 0.4 11
0.1 – 8.2
0.21 – 16.2
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due to defective potting. Surprisingly, a very constant and low TMP is found for M1A, which corresponds to the absence of fouling. After eight days of filtration, the samples were removed from the module holder for visual observation. Almost all membrane surfaces are covered with activated sludge, except for M1A. Closer investigation showed that the sludge is entrapped inside the nanofiber-sheets to form a sludge pocket. This way, the sheet acts as a depth filter, not a screen filter as expected. Daels et al. [21] also found a similar phenomenon in the lab-scale nanofiber-membrane bioreactor, and they defined this as “layered” fouling. This type of fouling is about impossible to be cleaned by air bubble scouring. The use of tap water to clean these membranes was thus not effective. This kind of fouling is not found in traditional membranes, since they consist of an integrated polymer matrix. To prevent this severe fouling of nanofiber based membranes, a heat treatment was applied to the nanofiber-sheet to improve their integrity and mechanical strength, as discussed in Section 3.3. The M1A membrane was visually found to be very clean. This explains its constant and low TMP shown in Fig. 6A. However, several open pores due to sheet scratching from particle movement could be observed on the sheet surfaces. In this condition, eventually this membrane should also experience the layered fouling. Without the treatment, the sheet layers are too weak to withstand the shear stress from air bubbles and particles scouring. 3.3. The effect of heat-treatment To improve the properties of nanofiber-membranes and especially their resistance to layered fouling, heat treatments were applied. Right after electrospinning, the nanofiber-sheets were heat-treated by exposing them to a temperature just below their
100
Trans membrane presure (kPa)
Trans membrane presure (kPa)
the effect of hydrophilicity of the membrane materials on fouling, recent studies are in an agreement that the hydrophilic materials are more favorable to a better fouling control [29]. Tetrabutyl ammonium chloride as an additive to the polymer did not give any advantages with respect to the CF. For both PA6 and PA66, CFs decrease in the presence of additives in the polymer/solvent spinning solutions. The CF is strongly depending on the applied operational parameters and the sludge conditions [33]. Therefore, it is very difficult to make a direct comparison between the CFs among the results obtained under different tests and setups. This is also the reason for the comparative performance test in parallel with the traditional membranes, discussed in Section 3.6. Nevertheless, this result gives an initial indication that the developed nanofiber-membranes have potential as a low cost filter in MBR applications. The CF of commercial membranes is in the range of 10–50 l/m2 h, as summarized by Tewari et al. [32]. To observe the feasibility of nanofiber-membrane to be applied in MBR, an exploratory long-term filtration was conducted. Five modules were placed in parallel at the module holder, and the filtration was performed continuously at a fixed flux of 20 l/m2 h for eight days. The ‘TMP-on’ and ‘TMP-off’ is given as the TMP measured just before the relaxation and filtration started, respectively. Fig. 4 shows the resistance profile of this exploratory test. The results show a severe fouling on M1B to M1E, which is indicated by a very high TMP of up to 50 kPa within the first three days of operation. This value is far beyond the critical TMP used for large scale MBR, which is normally 20 kPa. The relaxation is also found inefficient to restore the fouling, represented by a high TMP-off, very close to the TMP-on. For M1C, after three days of filtration, the TMP went down to zero. The only possible reason for this is the membrane leaking
M1A-On 80
M1A-Off
60 40 20
A 0 0
1
2
3
4
5
6
7
8
100 M1B-On M1B-Off
80 60 40 20
B
0
1
0
2
3
100 M1C-On M1C-Off
60 40 20
C 0 0
1
2
3
4
4
5
6
7
8
5
6
7
8
Time (days)
Trans membrane presure (kPa)
Trans membrane presure (kPa)
Time (days)
80
185
5
6
7
100 M1D-On M1D-Off
80 60 40 20
D
0
8
1
0
2
3
Trans membrane presure (kPa)
Time (days)
4
Time (days)
100 M1E-On 80
M1E-Off
60 40 20
E
0 0
1
2
3
4
5
6
7
8
Time (days)
Fig. 4. TMP obtained during the exploratory test for nanofiber-membranes: (A) M1A, (B) M1B, (C) M1D, (D) M1C and (E) M1E.
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35
35
30
30
Critical Flux (l/m2.h)
Critical Flux (l/m 2.h)
Fig. 5. The effect of heat treatment on microstructure of nanofiber-membranes: (A) M1B and (B) M3C.
25 20 15 150°C
10
160°C
170°C
5
25 20 15 10s
10
30s
60s
M2E
M2F
5
A 0
B
0 M2A
M2B
M2C
M2D
Nanofiber-membrane samples
Nanofiber-membrane samples
Fig. 6. CFs of nanofiber-membranes treated (A) at different temperatures but at a similar exposure time of 30 s, (B) at different exposure times but at a similar temperature of 170 ◦ C.
melting points, as specified in Table 2. The effect of heat treatment on the nanofiber membrane structure is shown in Fig. 5. Upon heating, overlapping fibers tend to fuse together, and are thus expected to have improved integrity and mechanical strength. Furthermore, the heat-treated sheets can be used further as self-supported membranes without the need for a non-woven support [16]. Fig. 6 shows the effect of heat treatment on the CF. In general, a more pronounced heat treatment reduces the CF of the nanofiber membranes. The CF of M1D decreases to 14–25 l/m2 h after the heat treatment. There is no general trend observed on the effect of exposure temperature and exposure time. Two of the membranes (M2C and M2F) were tested in duplicate giving a relatively high standard deviation. The effect of heat treatment on a somewhat longer term is shown in Fig. 7. The final resistance after seven days of filtration is much lower than the one without the heat treatment shown in Fig. 4. Visual observation of the membranes after the test also
revealed that layered fouling had not occurred with the temperature treated nanofiber sheets. Fig. 7A also shows that M2A has a better performance than M2F, most certainly because M2A has a higher CF. The results thus confirm that by applying heat treatment, a better sheet integrity is obtained, facilitating the prevention of layered fouling onto the nanofiber-sheets. Apparently, the results show no significant changes in overall performance of the nanofiber membranes when heat-treated at different temperatures or for different duration. The exposure temperature of 170 ◦ C for 30 s was arbitrary chosen for the following experiments. 3.4. The effect of nanofiber diameter To investigate the effect of the nanofiber diameter, a series of nanofiber sheets was prepared from the different nanofiber diameters. The SEM image of nanofiber membrane with different nanofiber diameters is shown in Fig. 8. Fig. 8 clearly shows that 30
Trans membrane pressure (kPa)
Trans membrane pressure (kPa)
30
TMP (On)
24
TMP (Off)
18
12
6
A 0
TMP (On)
24
TMP (Off)
18
12
6
B 0
0
1
2
3
4
5
Time (Days)
6
7
8
0
1
2
3
4
5
6
Time (Days)
Fig. 7. The seven days filtration test for the heat treated nanofiber-membranes: (A) M2A and (B) M2F.
7
8
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187
40
40
35
35
Critical Flux (l/m 2.h)
Critical Flux (l/m2.h)
Fig. 8. The effect of fiber diameter on the nanofiber-membrane surface pore: (A) M3E and (B) M3C.
30 25 20 15 10 5
30 25 20 15 10 5 0
0 0
100
200
300
400
500
Fiber diameter (nm)
0
20
40
60
80
100
Area-weight (g/m2)
Fig. 9. The effect of nanofiber diameter on CF.
Fig. 11. The effect of area-weight.
the bigger nanofiber resulting in the bigger pore size of nanofiber sheet. For instance, based on image analysis using ImageJ, M3E has a pore size of 0.7 ± 1 m with surface porosity of 44.6% and M3C has a pore size of 0.25 ± 0.36 m with surface porosity of 45.7%. In other word, the bigger the fiber diameter corresponds to the bigger pore size of the membrane. Fig. 9 shows that different nanofiber membranes formed from the different nanofiber diameters did not really affect the CF. The CFs range from 30 to 34 l/m2 h. It is probably due to a similar overall porosity. The hydraulic performance is more pronounced to be affected from surface porosity or overall porosity. Therefore, no significant effect can be observed from using different fiber diameters. Indeed, the filtration duration for this CF measurement (3–4 h) is too short to obtain a distinct difference. A different result was found by Aussawasathien et al. [35]. As the thickness of the nanofiber increased, a lower flux was observed. This result is understandable since their membrane was obtained from non-heat treated
nanofibers causing them to act as a depth filter instead of screen filter. 3.5. The effect of area-weight The area-weight is mostly associated to material cost for nanofiber-sheet fabrication. A higher area-weight gives a thicker sheet. The microstructure of the nanofiber-membranes with different area-weight is shown in Fig. 10. Based on image analysis, M4C and M4D have a pore size of 0.36 ± 0.49 m and 0.35 ± 0.45 m with surface porosity of 47.8% and 44.1%, respectively. The change on area-weight thus only changes the sheet thickness without changing the pore properties. The influence of different area-weights of nanofibermembranes on their CF is shown in Fig. 11. Results show that the area-weight of nanofiber-sheet is not substantially affecting the CF. The change in area-weight from 10 to 100 g/m2 yields CFs in the
Fig. 10. The effect of area-weight on the nanofiber-membrane surface pore: (A) M4C and (B) M4D.
M.R. Bilad et al. / Journal of Membrane Science 380 (2011) 181–191
50
35 Nanofiber
30
PVDFT
PE (Kubota) PVDF (Lab-made)
25
PVDFL Nanofiber Flux
TMP (kPa)
Trans membrane pressure (kPa)
40
18
PE K
PVDF (Toray)
30
16 14 12
20
10
15
8 6
10
4 20
5
2 0
0 0
10
Operational flux (l/m 2.h)
188
2
4
6
8
10
12
14
16
18
Time (days) Fig. 13. The TMP profile of the long-term test for two commercial MBR membranes, a lab prepared reference membrane and a nanofiber-membrane.
0 0
5
10
15
20
25
30
35
2
Flux (l/m .h) Fig. 12. The flux stepping profiles for the CF determination of membranes used later in the long-term test (the arrows indicate the CF values).
range of 30–36 l/m2 h. In the traditional microfiltration processes, the performance is most often affected by the membrane pores (size, geometry, distribution) [1]. Increasing the thickness of a membrane, as in this case, does not affect the pores, as shown in Fig. 11. A similar behavior occurs for the nanofiber-membranes tested in this study. This result implies that for economic reasons the area-weight of 10 g/m2 is sufficient to meet a good filtration performance. 3.6. Comparative performance with conventional MBR membranes The nanofiber membrane M4B was used for this comparative performance test. Before use in longer-term test, the CFs of all membranes were measured. Fig. 12 shows the flux stepping plot for the CF determination and the summary of the CF, respectively. The nanofiber and the PEK membranes show higher CFs compared to PVDFT and PVDFL . This is almost certainly due to the higher surface porosity and the bigger pore size of both membranes. A similar result was also found by van der Marel et al. [29]. Further observations of the TMP profile on flux for nanofiber and PEK membranes showed that at a higher flux, the TMP of a nanofiber membrane tends to increase faster. This is an initial indication that the nanofiber-membrane has a relatively higher fouling propensity. A more decent TMP profile is expected from a longer-term test. The long-term test was performed by installing four modules in parallel. The initial flux was chosen as being sub-critical to the lowest CF among the tested membranes to avoid an immediate TMP jump. Whenever no significant increment of TMP could be observed or when the TMP build up was very low, the flux was increased to higher level. In this test, the fluxes were periodically increased by 1 l/m2 h within typically 1–2 days. The modules that had reached
a TMP of 20 kPa were taken out of the HT-MBR. The TMP of 20 kPa was chosen as the critical TMP since, as commonly used in largescale applications. Due to technical limitations, the TMP data were only recorded 2–3 times per day. Fig. 13 shows the TMP profile of the long-term test for four different membranes. In general, the TMP increment consists of two main stages: a slow TMP rise followed by a TMP jump [3]. At the first stage, no visual TMP increments were observed for any membrane, except for PVDFL . The TMP increased gradually after the flux was increased to 9 l/m2 h for PVDFT , and exponentially for PVDFL . The PVDFL reached its critical TMP within three days of operation. A gradual increment of TMP was observed on nanofiber-membrane and PVDFT in the fluxes rage of 11–14 l/m2 h until it reached the critical value after 16 days of operation. There was no significant TMP increment on PEK until it suddenly jumped after the 16th day of operation. This occurred at a flux of 15 l/m2 h. The PEK membrane clearly showed the best performance followed by the nanofibermembrane, PVDFT and PVDFL based on their TMP increment during the slow TMP built-up stage. However, when compared based on critical time to reach the critical TMP, no significant difference between the PEK , PVDFT or PVDFL membranes were observed. At the end of the test, the membranes were visually observed, as shown in Fig. 14. The cake layer formation on the membrane surfaces occurred for all tested membranes. The cake layer mainly covered a small part of the membrane surface, especially at the edges of the module. This is obviously the module part that experiences least scouring. Similar behavior was reported by Cho and Fane [34]. It is clearly seen that both PVDF membranes have a browner colour. This corresponds to the adsorption of colorant substances from the feed. The sludge intrusion on the nanofiber-membrane indicates that the heat treatment still needs further optimization to minimize this effect. 4. Discussions The experiments described in this paper focused on assessing and optimizing the nanofiber-membranes as a potential substitute
Fig. 14. Visual appearance of fouled membranes: (A) PVDFL , (B) PEK , (c) nanofiber-membrane and (D) PVDFT .
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for traditional phase inversed membranes in MBRs application. The results clearly demonstrate the impact of heat treatment on the nanofiber sheet to improve their performance. The SEM images of nanofiber membranes given in Fig. 2D, 5, 8 and 10 show the unique properties of the sheets. Nanofiber sheets are very permeable due to their higher porosity, interconnected pore structures and high surface to volume ratio that improves the surface cohesion [35,36]. For a similar material and pore size, Kraur et el. [15] found that a nanofiber-membrane was 1.5–2 times more permeable than a traditional membrane. The surface porosity of nanofiber membranes in this study were almost 50% with relatively high distribution. Those properties are almost impossible to exhibit by traditional membrane due to the difficulty to decouple the pore size and surface porosity. Only a very high pore size membrane can only achieve those values. Membrane with high pore size and/or surface porosity is recently revealed as the most influential parameter for fouling in MBRs. They experience lower local flux through the pores and a smaller retention of feed water constituents [29]. Therefore, much lower fouling was initially expected by nanofiber membrane, thus offer their possibility and competitiveness as the substitute to traditional membrane in MBR application. The advantage of high surface porosity on nanofiber is apparently not strongly beneficial in MBRs process. This represented by only comparable CF to traditional membranes. This is most probably because they have relatively high pore distribution. This is obvious from the surface SEM images and images analysis, as well as from CWP (Fig. 1). Consequently, There exist relatively much bigger pores than average pore size. A quick deposition or even penetration of solid onto the membrane surface is expected through these pores. This is more severe as the result from a wide range of solid sizes in activated sludge. Free bacterial or even smaller flocks can enter and penetrate into the internal membrane. Furthermore, in most lab-scale operation, higher energy input is given to the system, leads to much smaller flocs and thus denser cake layer and eventually worse filterability [37]. The performance of fresh (non-heat treated) nanofiber membrane in exploratory long-term test was also poor. Due to air scouring and particles scratching, the nanofiber-membrane might become leaky and sludge can then enter the nanofiber-sheet layers as illustrated in Fig. 15A and B. The particles tend to accumulate within the pores, especially due to the nature of the sheet that build-up from multiple nano-sheets as illustrated in Fig. 15A. This phenomenon is obvious for the filtration with a very open filter surface structure. Fatarella et al. [11] observed a very severe fouling in textile bioreactor. Due to relatively open structure of the textile filter, the small flocks not only form dense cake but also penetrate inside the filter media resulting pore clogging. The layered fouling was also found by Gopal et al. [14] for the filtration of sub-micron particles, which have diameters similar or smaller than the pore size of the nanofiber-membrane. Apparently, membrane with high surface porosity is beneficial only to some extent, but detrimental beyond that value. The optimization of pore structure, pore size and surface porosity of nanofiber membrane for MBRs application are still required. The results from Section 3.3 suggest the effectiveness of heat treatment to prevent the layered fouling. However, a reduction of intrinsic membrane properties is also observed upon heat treating the membranes: interconnecting the nanofibers eventually reduces their surface area and the membrane porosity. This is obvious when comparing the CF data from Figs. 6 and 3. The heat treatment might thus well improve the membrane integrity, but not the pore structure. Hence, even though this treatment does not allow full exploitation of the high surface porosity of the nanofiber sheet it significantly improves the long-term filtration performance of nanofiber membrane.
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Fig. 15. Comparison of membrane structure: (A) nanofiber membrane, (B) the illustration of sludge entrapment in a nanofiber membrane and (C) the traditional phase inversed membrane.
A relatively high standard deviation was found for the CFs of M2C, M2E, M3D and M4B, which were prepared from a similar material and with comparable treatment conditions. This is most probably due to the differences in the original sheets made at lab scale, and can be attributed to an inhomogenous area-weight and distribution of nanofibers over the full sheet. In addition, it has to be noticed that this deviation may also be due to the change in the feed properties, which was not monitored specifically over the testing period. Therefore, a direct comparison with the results in Section 3.2 is not completely relevant. Within the optimization study, minimum impact was observed from fiber diameter and sheet thickness. The fiber diameter is obviously important to design the pore size that determines the degree of solid retention. An optimum value needs to be obtained to maintain sufficient filtration productivity and effluent quality. Optimization of sheet thickness is also key to reduce material cost. Lamination of the nanofiber on the non-woven support to form an integrated nanofiber-support is necessary to facilitate the module preparation process. On the other hand, a thick sheet can also be used as self-supporting membrane, eliminating the cost for a non-woven support. The comparative performance test showed that the heat-treated nanofiber membrane exhibits a better performance than PVDFT and PVDFK , but worse than PEK . The TMP build up is also identical to the profile of traditional membranes in submerged MBRs. The gradual rise of TMP for all tested membranes can be explained by irreversible fouling. Adsorption, pore blocking and deposition of macromolecules and solids normally take place during this stage. In the case of nanofiber membranes, some sludge intrusion into the sheet layers, as shown in Fig. 14, could not yet be fully prevented via the applied heat treatment. Further optimization of the membrane synthesis process is thus still required. 5. Conclusions The overall results show the feasibility of nanofiber-membranes to be used as a substitute for traditional phase inversion based membranes in MBRs. The nanofiber-sheets have a unique architecture, high surface porosity and permeability. Nanofiber membranes
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prepared from the more hydrophilic PA6 gave the highest CWP and CF, while a heat treatment of the nanofiber-sheets was required to prevent the formation of “layered fouling”. The fiber diameter and area-weight of the nanofiber-sheets are not significantly affecting the membrane performance. The comparative performance in a parallel test with the traditional membranes showed a comparable performance, with respect to CF and critical time. The overall performance of the optimized nanofiber membrane was better than the one of other related membranes reported earlier [12,21]. However, further improvement on the nanofiber membrane, especially to find the optimum pore size, structure, surface porosity as well as heat treatment is essential to maximize its intrinsic properties. Development of the membranes either as an integrated supportnanofiber sheet or as a self-supporting membrane is also necessary. Acknowledgements The authors thank K.U. Leuven for support in the frame of the CECAT excellence, GOA, FWO (G.0808.10N) and IDO financing, and the Flemish Government for the Methusalem funding and the Federal Government for an IAP grant. The authors also thank Kubota and Toray Membrane Europe, for providing the A4 size membrane element samples.
Nomenclature A CF h HT J L MBR min MLSS nm PA PAN PE PEK PES PSFL PVDF PVDFT PSF SEM SVI t TBR TMP V
effective filtration area (m2 ) critical flux (l/m2 h) hours high-throughput flux (l/m2 h) permeability (l/m2 h bar) membrane bioreactor minute mixed liquor suspended solid (g/l) nanometer polyamide polyacrylonitrile polyethylene polyethylene from Kubota polyethersulfone lab-made polysulfone polyvinylidene fluoride polyvinylidene fluoride from Toray polysulfone scanning electron microscopy sludge volume index (l/g) filtration time (min or h) textile bioreactors trans membrane pressure (bar, kPa) volume (l)
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Assessment and optimization of electrospun nanofiber-membranes in a membrane bioreactor (MBR) M.R. Bilad a , P. Westbroek b , Ivo F.J. Vankelecom a,∗ a b
Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, Box 2461, 3001 Leuven, Belgium Gent University, Department of Textiles, Technologiepark 907, B-9052 Gent, Belgium
a r t i c l e
i n f o
Article history: Received 7 March 2011 Received in revised form 5 June 2011 Accepted 1 July 2011 Available online 7 July 2011 Keywords: Membrane bioreactor Membrane fouling High-throughput Nanofiber Electrospinning Non-woven Electrospun
a b s t r a c t The feasibility and optimization of low-cost nanofiber-membranes as potential MBR-membranes is studied. The nanofiber-membranes have a unique surface architecture, a high surface porosity and permeability and adjustable pore sizes. The materials, their heat treatment as well as the diameters and area-weights of nanofiber sheets were optimized. The comparative performance of a nanofibermembrane to lab-made polysulfone (PSFL ) and commercial polyvinylidene fluoride (PVDFT ) and polyethylene (PEK ) membranes was performed. The critical flux (CF) and trans membrane pressure (TMP) profiles were used as evaluation parameters. A heat treatment was efficient to prevent layered fouling on the nanofiber-sheets. The electrospun nanofiber-membranes showed performances comparable to those of the tested commercial membranes at short and long term. Further developments on nanofibermembranes are still required to further improve their performance and enhance their competitiveness in MBRs applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Membrane bioreactors (MBRs) are becoming more popular for wastewater treatment, especially when an excellent effluent quality is required [1]. However, membrane fouling and high membrane investment costs remain major obstacles [2]. Membrane bioreactors can only compete with, e.g. activated sludge system, by applying a low cost filter to substitute the traditional membrane and tackle the fouling problem by mean of low cost technology [3]. In MBRs, membranes principally act as a selective barrier retaining suspended solids and allowing the treated water to pass through it. Low cost filters such as non-wovens, meshes, filter cloths or nanofiber have sometimes found to be suitable to substitute the function of membranes in MBRs [3]. Recent studies proved the feasibility of non-wovens to substitute the traditional membranes in the lab scale or the pilot-scale textile bioreactors where no significant difference in effluent quality was found compared to MBRs [4–7]. However, the major problem limiting the application of nonwovens is the acute fouling due to their rough surface and a rather large pore sizes and wide pore size distributions [8–10]. The sludge flock can then be entrapped in the voids inside the fiber matrix
∗ Corresponding author. Tel.: +32 16 321594; fax: +32 16 321998. E-mail address: [email protected] (I.F.J. Vankelecom). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.07.003
and block. Such blockage is very difficult to be removed by shear stresses from the feed side, as applied currently in MBRs. To minimize the fouling, application of nanofiber seems to be more effective. Nanofiber-membranes have become an interesting option because of their acceptable selectivity, high permeability and low production cost. Fatarella et al. [11] estimated the production cost of nanofiber-membrane at 5 D /m2 . Most of the cost (75%) is from non-woven support. Application of cheaper supports indeed will further reduce it. This value is much lower than the 14–50 D /m2 production cost of traditional membranes [1,11,12]. A nanofiber is a fiber having a diameter smaller than one micron, and is commonly produced via electrospinning [13]. It can be used to produce many types of membrane for microfiltration [14–16], ultrafiltration [17], nanofiltration [18,19] and membrane distillation [20]. To form the nanofiber-membranes, nanofibers are used as a coating material onto non-wovens [11], as a sheet that is later fixed onto a non-woven at both feed and permeate sides [21], as a blending material together with micron fiber [22] or after heattreatment as a self-supporting membrane [16]. Fatarella et al. [11] coated a nanofiber onto a non-woven and combined a plasma treatment to modify the surface properties. Initial testing showed that this nanofiber-membrane had a CF of 35 l/m2 h, comparable to the traditional polyvinylidene fluoride (PVDF) membranes. Bjorge et al. [12] fixed a thin nanofiber layer on a non-woven support to form a nanofiber-membrane. They also revealed the feasibility of the nanofiber-membranes in
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water filtration: for pathogens removal, as a stand-alone filter and as a substitute for traditional membranes in an MBR. More recently, Daels et al. [21] reported the application of the nanofibermembrane to substitute the membrane in a bench-scale activated sludge MBR and a trickling filter MBR. The previous studies mainly focused on the applicability and feasibility of nanofiber-membranes as a low cost filter in MBRs in the preliminary stages. Bjorge et al. [12] applied nanofiber membranes directly as a stand-alone filter without addition of any support. The nanofiber faced an acute problem due to layered clogging. Further improvement on membrane by sandwiching it in-between two non-woven has been reported by Daels et al. [21]. This induced a loss of productivity in term of flux from ±21 to ±4 l/m2 h in a few days. Furthermore, their testing parameters, such as an MLSS of 1–3.9 g/l, are also far below the ones normally used in MBRs. Further optimization of these nanofiber-membranes is thus still required to achieve better performances, together with a more detailed study to investigate their competitiveness. In the present study, the preparation of electrospun sheets was optimized. The membranes were assessed for their feasibility as a substitute for traditional membranes in a lab-scale MBR. The nanofiber sheet was improved by adding a non-woven as a support and by introducing a heat treatment on the sheet surface at the feed side to prevent layered clogging. The study was performed in a lab-scale high-throughput MBR (HT-MBR) [23]. The CF and the TMP profile as evaluation parameters. The effect of nanofiber material, heat treatment on the nanofiber-sheets, fiber diameter and area-weight on the CF were studied. At the end, the performance of the optimum obtained nanofiber-membrane was compared to a lab-made membrane and two commercial membranes. 2. Materials and methods 2.1. Activated sludge and synthetic wastewater The activated sludge used to inoculate the lab-scale HT-MBR was obtained from a pilot-scale MBR treating molasses wastewater in the Waterleau wastewater laboratory (Wespelaar, Belgium). The acclimatization and biological performance of the sludge was discussed earlier [24]. For this study, the activated sludge was maintained by applying a fed-batch operation. To prevent the accumulation of slowly biodegradable substances, a part of the liquid in the reactor was discharged every week. The activated sludge was settled and a part of the supernatant was withdrawn and replaced with tap water. During the test, the mixed liquor suspended solid (MLSS) concentrations were kept at 10–12 g/l by partially withdrawing the sludge and the sludge volume index (SVI) was in the range of 55–75 l/g, measured according to the standard method [25]. This activated sludge was used as a feed for both CF and long-term filtration test and was performed batch-by-batch over a year along with the membrane optimization process. The feed solutions, further referred to as ‘synthetic wastewater’, were prepared by diluting 0.45 ml/l of molasses stock solution. The diluted molasses solution was chosen as feed wastewater because it does not require pre-fine screening, it has a good COD/N ratio and contains trace elements [26]. The characteristics of the feed solution are given in Table 1. All were measured using a HACH-Lange cuvette. 2.2. Membrane preparation, characterization and module potting Nanofiber-membranes were formed from nanofiber-sheets, which were fixed by glueing the edges onto polypropylene nonwoven supports (Novatexx 2471, kindly supplied by Freudenberg, Germany), so that only the central part acts as the active separation layer. The nanofiber-sheets were prepared at the Department
Table 1 Characteristics of the synthetic wastewater. Parameters Chemical oxygen demand (COD) Total Nitrogen (TN) Total phosphorous (TP) Ammonium-N (NH4 -N) Nitrate-N (NO3 -N) Orthophosphate-P (PO4 -P)
Average concentration (unit) 433 (mg/l) 11.3 (mg/l) 1.8 (mg/l) 1.8 (mg/l) 1.5 (mg/l) 0.7 (mg/l)
of Textiles University of Gent, Belgium. Bjorge et al. [12] and Daels et al. [21] explain the production and functionalization of these sheets in detail. Three polyamide polymers were used to prepare them, namely Polyamide 6 (PA6) with repeating unit C6 H11 ON, Polyamide 66 (PA66) with repeating unit C12 H22 O2 N2 , and Polyamide 69 (PA69) with repeating unit of C15 H28 O2 N. PA6 and PA66 were also prepared in the presence of tetrabuthyl ammonium chloride ([CH3 (CH2 )3 ]4 NCl) as an additive to further improve the hydrophilicty. Four batches of nanofiber-sheets with different properties were used in this study, and their specifications are summarized in Table 2. The term “area-weight” (g/m2 ) is used to quantify the amount of polymer used to prepare one square meter of nanofiber sheet. For comparative performance test, three different traditional flat sheet membranes were used; a lab-made polysulfone (PSFL ), a commercial Polyvinylidene Fluoride (PVDFT ) and a commercial polyethylene (PEK ). The PSFL was prepared from a 10% PSF (BASFUltrason)/N-Methyl-2-pyrrolidone (Acros Organics) solution by phase inversion [27]. The solution was cast on a polypropylene non-woven support (Novatexx 2471, was also kindly supplied by Freudenberg, Germany) and then brought into contact with a nonsolvent (water). In the non-solvent bath, the polymer solidifies to form a thin porous membrane. The PVDFT and PEK were purchased from membrane suppliers, Toray and Kubota respectively. Prior to use, all membranes were potted to form modules with an effective membrane area of 0.016 m2 . A flat sheet membrane was fixed to a PVC frame by glueing the edges together using two components epoxy glue (UHU-Plus endfest 300, Germany). The membrane sheet was folded to form a small envelope. Both membrane sides were separated by two sheets of spacer (Agfa, Belgium) in the interior of the module. Each spacer has a thickness of 2 mm giving the total thickness of the module ±5 mm. Permeate was sucked from the module interior through the permeate line. More detailed information about the module potting is available in Bilad et al. [23]. 2.3. Experimental setup The permeability of nanofiber-membranes was measured in the high-throughput (HT) filtration apparatus for pressure driven processes [28], while the CF test was performed in the labscale HT-MBR [23]. All HT-equipment was purchased from HTML (www.HTML-membrane.be, Belgium). The HT-MBR set-up has a working volume of 18.6 L and is equipped with a coarse and fine air bubble aeration mechanism for membrane and biological aeration respectively. To allow simultaneous filtration, the modules are fixed into two parallel holders, each consisting of three modules. This setup allows a correct comparison of membrane performances in MBR by applying an identical operating and mixed liquor conditions for all tested membranes. Parallel operation is essential to avoid discrepancies on the feed, due to the dynamic behavior of the activated sludge over the testing period. To ensure continuous and homogeneously distributed membrane aeration, each module holder was equipped with an individual, interconnected and adjustable aeration device, which was located beneath the modules. Inside the reactor, each permeate line was connected
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Table 2 The properties of the nanofiber-sheets applied. Sample batch
Sample code
Material
Heat treatment ◦
Temperature ( C) PA66 PA6 PA69 PA6 + additive PA66 + additive
Nanofiber diamater (nm)
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to an individual line, an individual vacuum gauge and passed to a separate channel in the multi-channel peristaltic pump (WatsonMarlow 205U 16 Channel Pump, UK) using isoprene manifold tubes (Watson-Marlow, UK). The filtration flux could be adjusted by changing the rotational speed of the pump, changing the membrane area or by changing the diameter of the manifold pump tubing. 2.4. Analytical methods 2.4.1. Membrane characterization The microstructures of all fresh membranes that were used in the comparative performance test were observed with scanning electron microscopy (SEM, Philips SEM XL30 FEG with Edax dx4i system). The properties of the membranes, surface pore size and porosity, were identified with an image processing software, imageJ (NIH, USA) [29]. The performance of the long-term tests was evaluated based on the TMP profile. The flux (J) and the permeability (L) of the membrane were calculated by using Eqs. (1) and (2), respectively. J=
V At
L=
J TMP
(l/m2 h) (l/m2 h bar)
(1)
flux at which the TMP proportionality to flux ceases to exist [31]. All CF measurements were performed using fresh membranes after pre-treatment by soaking for 3 h in 30% ethanol/water solution and compaction by filtration with clean water at a flux of 90 l/m2 h for 3 h. 2.4.3. Long-term experiment For the exploratory tests in Sections 3.2 and 3.3, the filtration was performed in a cycle of 8 min filtration at 20 l/m2 h followed by a 2 min relaxation, resulting in a net flux of 16 l/m2 h. The relaxation was applied by simply stopping the filtration process. The cycle duration was arbitrarily chosen as being a standard for fullscale submerged flat sheet MBRs. For the comparative performance test in Section 3.6, the filtration at incremental fluxes without relaxation was applied. The filtration fluxes were gradually step-wise increased to speed up the fouling process, to perform a so-called extended flux step experiment. The initial flux was 8 l/m2 h, well below all CFs of the tested membranes. This flux was and step-wise increased to 16 l/m2 h. During the test, all permeates were recycled into the MBR to maintain the sludge concentration. The total recycle procedure was also applied for the CF tests.
(2)
where V is volume (l), t time (h), A effective filtration area (m2 ) and TMP trans-membrane pressure (bar) or (kPa, as presented in the figures). 2.4.2. Critical flux Critical flux is an important concept in MBRs that provides useful information for an operational point of view. In many cases, CF is also used as a parameter for a direct comparison of membrane performances in a short-term experiment [29]. In this study, CF was measured using the stepwise method suggested by Le-Clech et al. [30]. The applied initial flux, step height and step duration were 2 l/m2 h, 2 l/m2 h and 5 min, respectively. To determine the CF, the final TMP values of each step are plotted against the fluxes. Below the CF, a linear relationship exists between the TMP increment and the imposed fluxes. The CF was determined to be the minimum
Clean water permeability (l/m 2.h.bar)
PA6
–
–
14000 12000 10000 8000 6000 4000 2000 0 M1A
M1B
M1C
M1D
Nanofiber sheets Fig. 1. The CWP of nanofiber-sheets.
M1E
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Fig. 2. The microstructure of the membranes used for the comparative performance test: (A) PVDFT , (B) PEK , (C) PVDFL and (D) nanofiber-sheet.
2.5. Membrane cleaning
45 + additive
40
3. Results
35
Critical Flux (l/m 2.h)
After CF measurements, the membranes were physically cleaned prior to use for the long-term tests. First, the flat sheet module was removed from the bioreactor tank. A physical cleaning was applied with tap water for 10 min to remove cake layers from the membrane surfaces.
30 25 20 15 10
3.1. Membrane characterization Fig. 1 shows that the clean water permeabilities (CWPs) of nanofiber-sheets range from 5300 to 9200 l/m2 h bar with relatively high deviation. The CWPs obtained for all nanofiber-sheets are much higher than those of the commercial membranes currently used in MBRs. The properties and CWP of the typical commercial membranes in MBRs were given in detail by Judd [1]. Fig. 2 shows that the nanofiber-sheet has a unique structure compared to traditional membranes. The pores are formed from the interspaces between cross-linked nanofibers. The pore size of the nanofibersheets used in this study, more towards the upper side, is in the range of the pore sizes normally used in MBRs. They are in the magnitude of ultra- to micro-filtration. Table 3 summarizes the properties of membranes and nanofiber-sheets used for the long-term experiment in Section 3.6. The nanofiber-sheet has a significantly higher surface porosity, almost twice that of PEK and PVDFT membranes. 3.2. The effect of nanofiber material Fig. 3 shows that the CFs are in the range of 25–40 l/m2 h. These values are comparable to commercial membranes [32]. PA6 (M1A)
5 0 M1A
M1B
M1C
M1D
M1E
Nanofiber-membrane samples
Fig. 3. The CF of nanofiber-membranes with different nanofiber materials.
gives the highest CF, followed by PA66 and PA69. The order for the 3 polymer types reflects their hydrophilic character. By nature, nanofibers exhibit a very high contact angle when compared to a polymer film made from the same material. For instance, membranes and nanofibers prepared from PSF have contact angles of 70–80◦ and 140◦ respectively [14]. Despite the discrepancy on Table 3 The summary of the characteristics of the membranes used for the comparative performance test, as obtained from SEM image analysis using ImageJ. Parameter
PVDFT
PEK
PVDFL
Nanofiber-sheet
Pore size (m) Pore size from manufacturer (%) Surface porosity (%)
0.03 0.08 0.2
0.22 0.4 11
0.1 – 8.2
0.21 – 16.2
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due to defective potting. Surprisingly, a very constant and low TMP is found for M1A, which corresponds to the absence of fouling. After eight days of filtration, the samples were removed from the module holder for visual observation. Almost all membrane surfaces are covered with activated sludge, except for M1A. Closer investigation showed that the sludge is entrapped inside the nanofiber-sheets to form a sludge pocket. This way, the sheet acts as a depth filter, not a screen filter as expected. Daels et al. [21] also found a similar phenomenon in the lab-scale nanofiber-membrane bioreactor, and they defined this as “layered” fouling. This type of fouling is about impossible to be cleaned by air bubble scouring. The use of tap water to clean these membranes was thus not effective. This kind of fouling is not found in traditional membranes, since they consist of an integrated polymer matrix. To prevent this severe fouling of nanofiber based membranes, a heat treatment was applied to the nanofiber-sheet to improve their integrity and mechanical strength, as discussed in Section 3.3. The M1A membrane was visually found to be very clean. This explains its constant and low TMP shown in Fig. 6A. However, several open pores due to sheet scratching from particle movement could be observed on the sheet surfaces. In this condition, eventually this membrane should also experience the layered fouling. Without the treatment, the sheet layers are too weak to withstand the shear stress from air bubbles and particles scouring. 3.3. The effect of heat-treatment To improve the properties of nanofiber-membranes and especially their resistance to layered fouling, heat treatments were applied. Right after electrospinning, the nanofiber-sheets were heat-treated by exposing them to a temperature just below their
100
Trans membrane presure (kPa)
Trans membrane presure (kPa)
the effect of hydrophilicity of the membrane materials on fouling, recent studies are in an agreement that the hydrophilic materials are more favorable to a better fouling control [29]. Tetrabutyl ammonium chloride as an additive to the polymer did not give any advantages with respect to the CF. For both PA6 and PA66, CFs decrease in the presence of additives in the polymer/solvent spinning solutions. The CF is strongly depending on the applied operational parameters and the sludge conditions [33]. Therefore, it is very difficult to make a direct comparison between the CFs among the results obtained under different tests and setups. This is also the reason for the comparative performance test in parallel with the traditional membranes, discussed in Section 3.6. Nevertheless, this result gives an initial indication that the developed nanofiber-membranes have potential as a low cost filter in MBR applications. The CF of commercial membranes is in the range of 10–50 l/m2 h, as summarized by Tewari et al. [32]. To observe the feasibility of nanofiber-membrane to be applied in MBR, an exploratory long-term filtration was conducted. Five modules were placed in parallel at the module holder, and the filtration was performed continuously at a fixed flux of 20 l/m2 h for eight days. The ‘TMP-on’ and ‘TMP-off’ is given as the TMP measured just before the relaxation and filtration started, respectively. Fig. 4 shows the resistance profile of this exploratory test. The results show a severe fouling on M1B to M1E, which is indicated by a very high TMP of up to 50 kPa within the first three days of operation. This value is far beyond the critical TMP used for large scale MBR, which is normally 20 kPa. The relaxation is also found inefficient to restore the fouling, represented by a high TMP-off, very close to the TMP-on. For M1C, after three days of filtration, the TMP went down to zero. The only possible reason for this is the membrane leaking
M1A-On 80
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80 60 40 20
D
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Time (days)
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Time (days)
100 M1E-On 80
M1E-Off
60 40 20
E
0 0
1
2
3
4
5
6
7
8
Time (days)
Fig. 4. TMP obtained during the exploratory test for nanofiber-membranes: (A) M1A, (B) M1B, (C) M1D, (D) M1C and (E) M1E.
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35
35
30
30
Critical Flux (l/m2.h)
Critical Flux (l/m 2.h)
Fig. 5. The effect of heat treatment on microstructure of nanofiber-membranes: (A) M1B and (B) M3C.
25 20 15 150°C
10
160°C
170°C
5
25 20 15 10s
10
30s
60s
M2E
M2F
5
A 0
B
0 M2A
M2B
M2C
M2D
Nanofiber-membrane samples
Nanofiber-membrane samples
Fig. 6. CFs of nanofiber-membranes treated (A) at different temperatures but at a similar exposure time of 30 s, (B) at different exposure times but at a similar temperature of 170 ◦ C.
melting points, as specified in Table 2. The effect of heat treatment on the nanofiber membrane structure is shown in Fig. 5. Upon heating, overlapping fibers tend to fuse together, and are thus expected to have improved integrity and mechanical strength. Furthermore, the heat-treated sheets can be used further as self-supported membranes without the need for a non-woven support [16]. Fig. 6 shows the effect of heat treatment on the CF. In general, a more pronounced heat treatment reduces the CF of the nanofiber membranes. The CF of M1D decreases to 14–25 l/m2 h after the heat treatment. There is no general trend observed on the effect of exposure temperature and exposure time. Two of the membranes (M2C and M2F) were tested in duplicate giving a relatively high standard deviation. The effect of heat treatment on a somewhat longer term is shown in Fig. 7. The final resistance after seven days of filtration is much lower than the one without the heat treatment shown in Fig. 4. Visual observation of the membranes after the test also
revealed that layered fouling had not occurred with the temperature treated nanofiber sheets. Fig. 7A also shows that M2A has a better performance than M2F, most certainly because M2A has a higher CF. The results thus confirm that by applying heat treatment, a better sheet integrity is obtained, facilitating the prevention of layered fouling onto the nanofiber-sheets. Apparently, the results show no significant changes in overall performance of the nanofiber membranes when heat-treated at different temperatures or for different duration. The exposure temperature of 170 ◦ C for 30 s was arbitrary chosen for the following experiments. 3.4. The effect of nanofiber diameter To investigate the effect of the nanofiber diameter, a series of nanofiber sheets was prepared from the different nanofiber diameters. The SEM image of nanofiber membrane with different nanofiber diameters is shown in Fig. 8. Fig. 8 clearly shows that 30
Trans membrane pressure (kPa)
Trans membrane pressure (kPa)
30
TMP (On)
24
TMP (Off)
18
12
6
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TMP (On)
24
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0
1
2
3
4
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Time (Days)
6
7
8
0
1
2
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Fig. 7. The seven days filtration test for the heat treated nanofiber-membranes: (A) M2A and (B) M2F.
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187
40
40
35
35
Critical Flux (l/m 2.h)
Critical Flux (l/m2.h)
Fig. 8. The effect of fiber diameter on the nanofiber-membrane surface pore: (A) M3E and (B) M3C.
30 25 20 15 10 5
30 25 20 15 10 5 0
0 0
100
200
300
400
500
Fiber diameter (nm)
0
20
40
60
80
100
Area-weight (g/m2)
Fig. 9. The effect of nanofiber diameter on CF.
Fig. 11. The effect of area-weight.
the bigger nanofiber resulting in the bigger pore size of nanofiber sheet. For instance, based on image analysis using ImageJ, M3E has a pore size of 0.7 ± 1 m with surface porosity of 44.6% and M3C has a pore size of 0.25 ± 0.36 m with surface porosity of 45.7%. In other word, the bigger the fiber diameter corresponds to the bigger pore size of the membrane. Fig. 9 shows that different nanofiber membranes formed from the different nanofiber diameters did not really affect the CF. The CFs range from 30 to 34 l/m2 h. It is probably due to a similar overall porosity. The hydraulic performance is more pronounced to be affected from surface porosity or overall porosity. Therefore, no significant effect can be observed from using different fiber diameters. Indeed, the filtration duration for this CF measurement (3–4 h) is too short to obtain a distinct difference. A different result was found by Aussawasathien et al. [35]. As the thickness of the nanofiber increased, a lower flux was observed. This result is understandable since their membrane was obtained from non-heat treated
nanofibers causing them to act as a depth filter instead of screen filter. 3.5. The effect of area-weight The area-weight is mostly associated to material cost for nanofiber-sheet fabrication. A higher area-weight gives a thicker sheet. The microstructure of the nanofiber-membranes with different area-weight is shown in Fig. 10. Based on image analysis, M4C and M4D have a pore size of 0.36 ± 0.49 m and 0.35 ± 0.45 m with surface porosity of 47.8% and 44.1%, respectively. The change on area-weight thus only changes the sheet thickness without changing the pore properties. The influence of different area-weights of nanofibermembranes on their CF is shown in Fig. 11. Results show that the area-weight of nanofiber-sheet is not substantially affecting the CF. The change in area-weight from 10 to 100 g/m2 yields CFs in the
Fig. 10. The effect of area-weight on the nanofiber-membrane surface pore: (A) M4C and (B) M4D.
M.R. Bilad et al. / Journal of Membrane Science 380 (2011) 181–191
50
35 Nanofiber
30
PVDFT
PE (Kubota) PVDF (Lab-made)
25
PVDFL Nanofiber Flux
TMP (kPa)
Trans membrane pressure (kPa)
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PVDF (Toray)
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2
4
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Time (days) Fig. 13. The TMP profile of the long-term test for two commercial MBR membranes, a lab prepared reference membrane and a nanofiber-membrane.
0 0
5
10
15
20
25
30
35
2
Flux (l/m .h) Fig. 12. The flux stepping profiles for the CF determination of membranes used later in the long-term test (the arrows indicate the CF values).
range of 30–36 l/m2 h. In the traditional microfiltration processes, the performance is most often affected by the membrane pores (size, geometry, distribution) [1]. Increasing the thickness of a membrane, as in this case, does not affect the pores, as shown in Fig. 11. A similar behavior occurs for the nanofiber-membranes tested in this study. This result implies that for economic reasons the area-weight of 10 g/m2 is sufficient to meet a good filtration performance. 3.6. Comparative performance with conventional MBR membranes The nanofiber membrane M4B was used for this comparative performance test. Before use in longer-term test, the CFs of all membranes were measured. Fig. 12 shows the flux stepping plot for the CF determination and the summary of the CF, respectively. The nanofiber and the PEK membranes show higher CFs compared to PVDFT and PVDFL . This is almost certainly due to the higher surface porosity and the bigger pore size of both membranes. A similar result was also found by van der Marel et al. [29]. Further observations of the TMP profile on flux for nanofiber and PEK membranes showed that at a higher flux, the TMP of a nanofiber membrane tends to increase faster. This is an initial indication that the nanofiber-membrane has a relatively higher fouling propensity. A more decent TMP profile is expected from a longer-term test. The long-term test was performed by installing four modules in parallel. The initial flux was chosen as being sub-critical to the lowest CF among the tested membranes to avoid an immediate TMP jump. Whenever no significant increment of TMP could be observed or when the TMP build up was very low, the flux was increased to higher level. In this test, the fluxes were periodically increased by 1 l/m2 h within typically 1–2 days. The modules that had reached
a TMP of 20 kPa were taken out of the HT-MBR. The TMP of 20 kPa was chosen as the critical TMP since, as commonly used in largescale applications. Due to technical limitations, the TMP data were only recorded 2–3 times per day. Fig. 13 shows the TMP profile of the long-term test for four different membranes. In general, the TMP increment consists of two main stages: a slow TMP rise followed by a TMP jump [3]. At the first stage, no visual TMP increments were observed for any membrane, except for PVDFL . The TMP increased gradually after the flux was increased to 9 l/m2 h for PVDFT , and exponentially for PVDFL . The PVDFL reached its critical TMP within three days of operation. A gradual increment of TMP was observed on nanofiber-membrane and PVDFT in the fluxes rage of 11–14 l/m2 h until it reached the critical value after 16 days of operation. There was no significant TMP increment on PEK until it suddenly jumped after the 16th day of operation. This occurred at a flux of 15 l/m2 h. The PEK membrane clearly showed the best performance followed by the nanofibermembrane, PVDFT and PVDFL based on their TMP increment during the slow TMP built-up stage. However, when compared based on critical time to reach the critical TMP, no significant difference between the PEK , PVDFT or PVDFL membranes were observed. At the end of the test, the membranes were visually observed, as shown in Fig. 14. The cake layer formation on the membrane surfaces occurred for all tested membranes. The cake layer mainly covered a small part of the membrane surface, especially at the edges of the module. This is obviously the module part that experiences least scouring. Similar behavior was reported by Cho and Fane [34]. It is clearly seen that both PVDF membranes have a browner colour. This corresponds to the adsorption of colorant substances from the feed. The sludge intrusion on the nanofiber-membrane indicates that the heat treatment still needs further optimization to minimize this effect. 4. Discussions The experiments described in this paper focused on assessing and optimizing the nanofiber-membranes as a potential substitute
Fig. 14. Visual appearance of fouled membranes: (A) PVDFL , (B) PEK , (c) nanofiber-membrane and (D) PVDFT .
M.R. Bilad et al. / Journal of Membrane Science 380 (2011) 181–191
for traditional phase inversed membranes in MBRs application. The results clearly demonstrate the impact of heat treatment on the nanofiber sheet to improve their performance. The SEM images of nanofiber membranes given in Fig. 2D, 5, 8 and 10 show the unique properties of the sheets. Nanofiber sheets are very permeable due to their higher porosity, interconnected pore structures and high surface to volume ratio that improves the surface cohesion [35,36]. For a similar material and pore size, Kraur et el. [15] found that a nanofiber-membrane was 1.5–2 times more permeable than a traditional membrane. The surface porosity of nanofiber membranes in this study were almost 50% with relatively high distribution. Those properties are almost impossible to exhibit by traditional membrane due to the difficulty to decouple the pore size and surface porosity. Only a very high pore size membrane can only achieve those values. Membrane with high pore size and/or surface porosity is recently revealed as the most influential parameter for fouling in MBRs. They experience lower local flux through the pores and a smaller retention of feed water constituents [29]. Therefore, much lower fouling was initially expected by nanofiber membrane, thus offer their possibility and competitiveness as the substitute to traditional membrane in MBR application. The advantage of high surface porosity on nanofiber is apparently not strongly beneficial in MBRs process. This represented by only comparable CF to traditional membranes. This is most probably because they have relatively high pore distribution. This is obvious from the surface SEM images and images analysis, as well as from CWP (Fig. 1). Consequently, There exist relatively much bigger pores than average pore size. A quick deposition or even penetration of solid onto the membrane surface is expected through these pores. This is more severe as the result from a wide range of solid sizes in activated sludge. Free bacterial or even smaller flocks can enter and penetrate into the internal membrane. Furthermore, in most lab-scale operation, higher energy input is given to the system, leads to much smaller flocs and thus denser cake layer and eventually worse filterability [37]. The performance of fresh (non-heat treated) nanofiber membrane in exploratory long-term test was also poor. Due to air scouring and particles scratching, the nanofiber-membrane might become leaky and sludge can then enter the nanofiber-sheet layers as illustrated in Fig. 15A and B. The particles tend to accumulate within the pores, especially due to the nature of the sheet that build-up from multiple nano-sheets as illustrated in Fig. 15A. This phenomenon is obvious for the filtration with a very open filter surface structure. Fatarella et al. [11] observed a very severe fouling in textile bioreactor. Due to relatively open structure of the textile filter, the small flocks not only form dense cake but also penetrate inside the filter media resulting pore clogging. The layered fouling was also found by Gopal et al. [14] for the filtration of sub-micron particles, which have diameters similar or smaller than the pore size of the nanofiber-membrane. Apparently, membrane with high surface porosity is beneficial only to some extent, but detrimental beyond that value. The optimization of pore structure, pore size and surface porosity of nanofiber membrane for MBRs application are still required. The results from Section 3.3 suggest the effectiveness of heat treatment to prevent the layered fouling. However, a reduction of intrinsic membrane properties is also observed upon heat treating the membranes: interconnecting the nanofibers eventually reduces their surface area and the membrane porosity. This is obvious when comparing the CF data from Figs. 6 and 3. The heat treatment might thus well improve the membrane integrity, but not the pore structure. Hence, even though this treatment does not allow full exploitation of the high surface porosity of the nanofiber sheet it significantly improves the long-term filtration performance of nanofiber membrane.
189
Fig. 15. Comparison of membrane structure: (A) nanofiber membrane, (B) the illustration of sludge entrapment in a nanofiber membrane and (C) the traditional phase inversed membrane.
A relatively high standard deviation was found for the CFs of M2C, M2E, M3D and M4B, which were prepared from a similar material and with comparable treatment conditions. This is most probably due to the differences in the original sheets made at lab scale, and can be attributed to an inhomogenous area-weight and distribution of nanofibers over the full sheet. In addition, it has to be noticed that this deviation may also be due to the change in the feed properties, which was not monitored specifically over the testing period. Therefore, a direct comparison with the results in Section 3.2 is not completely relevant. Within the optimization study, minimum impact was observed from fiber diameter and sheet thickness. The fiber diameter is obviously important to design the pore size that determines the degree of solid retention. An optimum value needs to be obtained to maintain sufficient filtration productivity and effluent quality. Optimization of sheet thickness is also key to reduce material cost. Lamination of the nanofiber on the non-woven support to form an integrated nanofiber-support is necessary to facilitate the module preparation process. On the other hand, a thick sheet can also be used as self-supporting membrane, eliminating the cost for a non-woven support. The comparative performance test showed that the heat-treated nanofiber membrane exhibits a better performance than PVDFT and PVDFK , but worse than PEK . The TMP build up is also identical to the profile of traditional membranes in submerged MBRs. The gradual rise of TMP for all tested membranes can be explained by irreversible fouling. Adsorption, pore blocking and deposition of macromolecules and solids normally take place during this stage. In the case of nanofiber membranes, some sludge intrusion into the sheet layers, as shown in Fig. 14, could not yet be fully prevented via the applied heat treatment. Further optimization of the membrane synthesis process is thus still required. 5. Conclusions The overall results show the feasibility of nanofiber-membranes to be used as a substitute for traditional phase inversion based membranes in MBRs. The nanofiber-sheets have a unique architecture, high surface porosity and permeability. Nanofiber membranes
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prepared from the more hydrophilic PA6 gave the highest CWP and CF, while a heat treatment of the nanofiber-sheets was required to prevent the formation of “layered fouling”. The fiber diameter and area-weight of the nanofiber-sheets are not significantly affecting the membrane performance. The comparative performance in a parallel test with the traditional membranes showed a comparable performance, with respect to CF and critical time. The overall performance of the optimized nanofiber membrane was better than the one of other related membranes reported earlier [12,21]. However, further improvement on the nanofiber membrane, especially to find the optimum pore size, structure, surface porosity as well as heat treatment is essential to maximize its intrinsic properties. Development of the membranes either as an integrated supportnanofiber sheet or as a self-supporting membrane is also necessary. Acknowledgements The authors thank K.U. Leuven for support in the frame of the CECAT excellence, GOA, FWO (G.0808.10N) and IDO financing, and the Flemish Government for the Methusalem funding and the Federal Government for an IAP grant. The authors also thank Kubota and Toray Membrane Europe, for providing the A4 size membrane element samples.
Nomenclature A CF h HT J L MBR min MLSS nm PA PAN PE PEK PES PSFL PVDF PVDFT PSF SEM SVI t TBR TMP V
effective filtration area (m2 ) critical flux (l/m2 h) hours high-throughput flux (l/m2 h) permeability (l/m2 h bar) membrane bioreactor minute mixed liquor suspended solid (g/l) nanometer polyamide polyacrylonitrile polyethylene polyethylene from Kubota polyethersulfone lab-made polysulfone polyvinylidene fluoride polyvinylidene fluoride from Toray polysulfone scanning electron microscopy sludge volume index (l/g) filtration time (min or h) textile bioreactors trans membrane pressure (bar, kPa) volume (l)
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