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Analysis of local fouling in a pilot-scale submerged hollow-fiber membrane system for drinking water treatment by membrane autopsy
Separation and Purification Technology 95 (2012) 227–234
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Analysis of local fouling in a pilot-scale submerged hollow-fiber membrane system for drinking water treatment by membrane autopsy Mooseok Lee a, Jeonghwan Kim b,⇑ a b
ECO Research Institute, KOLON Central Research Park Mabuk-dong, Giheung-ku, Yongin-city, Kyungi-do 446-797, Republic of Korea Department of Environmental Engineering, INHA University Nam-gu, Yonghyun-dong 253, Incheon 407-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 January 2012 Received in revised form 15 April 2012 Accepted 17 April 2012 Available online 9 May 2012 Keywords: Submerged hollow-fiber membrane Local fouling Resistance-in-series model Local pressure Local flux
a b s t r a c t An analysis of local fouling in a pilot-scale submerged hollow-fiber membrane water treatment system for drinking water production is described in this paper. Membrane fouling was observed by using microscopic techniques with membrane specimen taken at different longitudinal and horizontal positions of a hollow-fiber membrane module. The resistance-in-series model was used to quantify local fouling resistances of membrane samples. Our results show that the recovery of the permeate flux of the hollow-fiber membrane located near the source of aeration was the highest after performing chemical cleaning. The irreversible fouling resistance was the largest for membrane samples taken near the open ends of the fibers, where the local pressure is expected to be highest. These axial features of irreversible fouling resistance were more pronounced at lower suspended solids concentration or higher set-point flux. However, the local aspect of reversible fouling was relatively less pronounced than the one of irreversible fouling. The results obtained from membrane autopsy were explained well by the local fouling phenomenon driven by the profile of local pressure along the fiber length and transient behavior of this profile during membrane operation. In addition, it was concluded that the individual membrane module experiencing more aeration would have a more localized pattern of membrane fouling, suggesting that the local fouling is more significant under relatively less demanding fouling conditions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Low-pressure driven membrane processes such as microfiltration (MF) or ultrafiltration (UF) are configured generally as a module that consists of a bundle of hollow-fibers that are submerged directly into a tank. Although the submerged, hollow-fiber membrane modules have been widely combined with biological processes for wastewater treatment, their installations have also increased for water treatment applications [1–3]. Toward this end, the MF or UF has been considered to be an alternative to coagulation, sedimentation and granular media filtration because it can produce higher quality of product water (permeate) while providing smaller footprint as requiring smaller reactor size [4–6]. Although submerged membrane technologies have changed the water treatment in a relatively short time, membrane fouling still remains one of the major obstacles in their implementations. Membrane fouling is an inevitable phenomenon caused by the depositions of foulant materials on membrane surface and/or
⇑ Corresponding author. Tel.: +82 32 860 7502; fax: +82 32 865 1425. E-mail address: [email protected] (J. Kim). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.04.017
adsorption of them within membrane pores. In order to reduce membrane fouling, external function such as aeration can be supplied continuously or semi-continuously from the bottom of membrane reactor. Foulant materials by the contact of membrane surface on the feed side are dislodged with rising air bubbles due to physical scrubbing effect and bulk shear rate along the fiber [7,8]. Although membrane fouling varies depending upon various operational conditions, one of important characteristics in submerged, hollow-fiber membrane system is the variation in suction pressure along the fiber length which is caused by the application in a negative pressure at the open ends of fibers [9]. It has been generally accepted that the local flux can be highest near the suction source of fibers while it becomes lowest near the opposite ends of fibers due to internal pressure drop in fiber lumen [4,10]. The variation of local flux can introduce axial features of membrane fouling and the local fouling near some region of the membrane where the local flux is higher than the set-point flux which is length-averaged value is not overlooked because the local fouling deteriorates membrane performance significantly. The local fouling can occur even if the membrane is operated even under critical flux where no fouling is expected [6]. Kim and DiGiano performed morphological examinations of local fouling at different longitudinal positions of hollow-fiber membrane
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from the laboratory-scale, submerged, hollow-fiber membrane system treating model fluorescence latex particles [6]. Fouling profiles were confirmed that deposition of particles was much more evident near the open ends of fibers than near the middle, and the closed ends of the fibers under the critical or sub-critical flux conditions. This non-uniform pattern of particle deposition along the fiber length became more significant during the initial period of membrane filtration. Since particles can be deposited preferentially near the open ends of the fibers where the local flux is greatest, this region can produce a more compact particle fouling against which aeration is not very effective [11,12]. Several attempts have been made to develop mathematical models in order to predict the behavior of local fouling in submerged, hollow-fiber membrane system. The local fouling was predicted by incorporating with the local flux distribution along the fiber length in clean water system. Chang and Fane found that the extent of local flux was higher for longer fiber length, smaller inner radius of hollow-fiber membrane or smaller membrane resistance [4]. Significant fraction of the local flux was higher than the measured flux from the open ends of fiber. In fouling system, however, initial distribution of local flux should become more uniform with filtration time as the local flux at some membrane region where it is higher than the set-point flux must drop due to fouling resistance at the region [10,13]. The necessary response is for the local flux to increase in less-fouled region to maintain constant flux along the fiber length. Although there has been growing interests in analyzing local fouling from submerged, hollow-fiber membrane system, most of studies have focused on the observations using only laboratoryscale, experimental set-up. In addition, current mathematical models considering the local fouling have not evolved yet far enough to compare practical observations with larger-scale performance of submerged membrane system. Therefore, results from the pilotscale operation with submerged hollow-fiber membrane system are necessary for the models to be improved. From the best of our knowledge, analysis of local fouling from the pilot- or full-scale design of submerged, hollow-fiber membrane system has not been performed yet. In this study, we analyzed local fouling from the pilot-scale operation with submerged, hollow-fiber membrane system treating the effluent from coagulation basin. Local fouling resistance was quantified with respect to the different positions of hollow fiber length and stack positions for the individual membrane module installed in submerged membrane reactor. Fouling analysis was performed by membrane autopsy works to better understand local fouling phenomena and obtain better insights for the practical design of submerged, hollow-fiber membrane system for water treatment applications.
2. Experimental 2.1. Pilot-scale operation of submerged membrane system Schematic diagram of the pilot-scale, submerged hollow-fiber membrane system for water treatment applied for this study is shown in Fig. 1. A pilot-scale, submerged membrane system was installed and operated for 3 months at the Kuei Water Treatment Plant. The feed water of Han River was pumped first to the coagulation tank at a rate of approximately 550 m3/day and polyaluminum chlorides (PACs) as a coagulant (17 vol.%) was injected to the feed water in order to remove particulate matters. The effluent from the coagulation tank was piped to the submerged, hollow-fiber membrane system subsequently. Microfiltration (MF) hollow-fiber membrane which is composed of polyvinylidenefluoride (PVDF) materials with braid reinforcement was used and fil-
tered in an outside-in flow pattern (KOLON Industries, Inc., Korea). Nominal pore size of the hollow-fiber membrane tested in this study was 0.1 lm. Inside-diameter and outside-diameter of the hollow fiber membrane was 0.8 mm and 2.0 mm, respectively. Membrane tank was configured as a two-stage reactor in order to achieve system recovery up to 99%. Concentrate from the MF membrane module submerged in the stage 1 of membrane reactor was flowed for the inlet of stage 2 of it (Fig. 1). As shown in Fig. 2, three membrane modules were stacked vertically and hollow-fiber membranes for each stack were oriented horizontally. Total effective length of hollow-fiber MF membrane for each module was 1 m and surface area of membrane per unit module was 20 m2. Permeate water was obtained from the open ends of fibers by applying suction pressure into them. The set-point flux applied for membrane modules in stage 1 and 2 of the reactor was 40 and 20 L/ m2/h, respectively. In order to control membrane fouling, aeration was applied from the bottom of reactor under the 0.2 Nm3/m2/h as normalized air flow rate. Backwashing was performed using membrane permeate for 30 s every 15 min of filtration. 2.2. Local fouling observations In order to observe and analyze local fouling with respect to the different positions of hollow fiber length and vertical positions of membrane modules, at the end of 3 months of operation, was membrane specimen taken from membrane module. In order to investigate the local fouling at different positions of fiber length, the membrane sample was cut every 0.2 m from the open ends of fiber at each membrane module. As described in Fig. 2, sample points for taking membrane specimen from different positions of fiber length are indicated as P1, P2 and P3. In order to observe the effect of position of membrane module on the local fouling, all membrane samples were taken from P2 at different vertical positions of membrane module which are indicated as S1, S2 and S3 (see Fig. 2). From Fig. 2, the S1 shows the vertical position of membrane module stacked closest to the source of aeration while the S3 is the module stacked most away from it. 2.3. Analysis of local fouling resistance Fouling of membrane specimen taken at different positions of hollow fiber length and stack positions was characterized applying resistance-in-series model. In this study, a simple dead-end filtration module was prepared for measuring permeability of used membrane and chemically-cleaned membrane (0.0012 m2 of surface area). Two hollow-fiber membrane samples were installed in a cylindrical filtration module which is connected to the pressure vessel containing DI water. The pressure vessel was pressurized by compressed nitrogen gas under 50 kPa to provide DI water into the module to measure the permeability of membrane samples. In order to obtain intrinsic properties of fouling resistance with minimization of the fiber length effect, the filtration was performed as outside-in mode using short hollow-fiber membrane samples having 10 cm length. The accumulated volume of permeate from the open ends of fibers was measured at a given filtration time to estimate membrane permeability. To measure flow rates of fouled membranes, variation of foulant on membranes was minimized by storing the samples in sterilized sample bottles and the permeate flow rate was measured within a day after sampling. For measuring the flow rates of chemically-cleaned membranes, membrane samples were cleaned immediately by chemical cleaning agents after taking them out from membrane modules. In order to estimate fouling resistances of membrane samples, Darcy’s equation was applied to estimate the total fouling resistances shown in Eq. (1). Permeate flux measured by dead-end fil-
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Fig. 1. Schematic diagram of integrated coagulation-submerged microfiltration hollow-fiber water treatment system.
per unit membrane area (J, L/m2/h) is proportional to the transmembrane pressure (DP, Pa) and inversely proportional to the viscosity of water (l, kg/m/s) and total resistances (Rtotal, m1).
J¼
DP
ð1Þ
lRtotal
Rtotal ¼ Rm þ Rre þ Rir
ð2Þ 1
where Rm is membrane resistance (m ), Rre is reversible resistance (m1) and Rir is irreversible resistance (m1). To determine each resistance, the Rm was obtained first by filtering deionized (DI) water with bare membrane. In this study, reversible resistance (Rre) was defined as the fouling resistance which can be removed by chemical cleaning. At the end of membrane operation, total resistance (Rtotal) was estimated by measuring permeate flux of fouled membrane with DI water. The reversible fouling resistance (Rre) should be caused mostly by the cake layer deposited on membrane surface and weak adsorption of foulant materials into membrane pores which can be removed by chemical cleaning. Given this condition, the remaining resistances including Rm and Rir could be obtained by measuring the permeate flux of the chemically-cleaned membrane using DI water. Since the Rm is already known, the Rir value can be calculated using Eq. (2). Chemical cleaning of fouled membrane was performed by sonicating the membrane samples with the cleaning agents of 2000 ppm NaOCl solution followed by 2500 ppm citric acid solution. Each chemical cleaning was performed for two hours with the fouled membrane samples taken. 2.4. Microscopic analysis of local fouling Fig. 2. Schematics of submerged membrane module describing sampling points to observe local characteristics of membrane fouling with respect to the axial positions along the fiber length (P1, P2 and P3) and vertical positions of membrane module(S1, S2 and S3).
tration test described above was divided by transmembrane pressure to calculate the membrane permeability (J/DP). In Darcy’s equation, the rate at which water passes through the membrane
In order to characterize foulant materials from each membrane specimen taken at different sampling locations, membrane autopsy works were performed using various microscopic techniques. Membrane specimen was placed on a sample mount and examined by scanning electron microscopy (SEM) to observe surface deposition of foulant on membrane. The membrane taken along the fiber
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length was mounted with double-side tape on aluminum stubs and coated with a thin layer of conductive metal (Au) by applying high vacuum sputter coating unit. Secondary electron (SE) images were also taken at highest magnification (120) from the SEM images. The targeted energy dispersive X-ray analysis (T-EDXA Model: Noran EDX System) uses the SE images to determine the inorganic elemental composition of the foulant materials on membrane samples. The wavelength of the X-rays produced was used to identify the presences and relative amounts of the chemical elements in the foulants on the membranes. A small portion of foulants was also removed from the membrane sample by rubber spatula carefully for Fourier Transform Infrared Analysis (FTIR Model: Nicolet/Thermo Electron Avatar 360) to measure organic fraction of foulant. 3. Results and discussion 3.1. Characteristics of feed water The integrated membrane system was fed with the raw water from the Han River. As mentioned above, the feed water for the submerged MF hollow-fiber module installed in the stage 1 of membrane reactor was pretreated by adding polyaluminum chloride. As shown in Table 1, the average DOCs in stages 1 and 2 of membrane reactor were 2.3 and 3.2 mg/L, respectively during operational period. The value of UV254 of the feed water in the stage 1 of membrane reactor was smaller than the one observed in the stage 2 of it (0.021 vs. 0.035 cm1). The average TSS concentrations in stage 1 and 2 of membrane reactor were measured to be 12.6 and
Table 1 Summary of water qualities of feed water for stage 1 and stage 2 of submerged membrane reactor. UV254 (cm1)
DOC (mg/L)
Alkalinity (mg/L)
SS (mg/L)
Stage 1 Min. Max. Avg.
0.012 0.036 0.021
1.23 4.19 2.34
6.5 45.6 30.4
1.5 35.4 12.6
Stage 2 Min. Max. Avg.
0.015 0.065 0.035
1.08 6.83 3.20
19.5 79.8 30.8
56 233 121
121 mg/L, respectively. Results indicated that the suspended solids concentration in stage 2 should be higher due to the concentrate stream from the MF membranes installed in stage 1. 3.2. Microscopic observations of membrane fouling at different stages of membrane tank The SEM images of membrane specimen taken at the end of 3month of pilot-scale operation are presented in Fig. 3. Inorganic fraction of foulant on membrane was also analyzed by performing SEM-Targeted Energy-Dispersive X-ray analysis (T-EDXA). The membrane module located at stack 2 (S2) and position 2 (P2) was selected to take the membrane sample in order to perform SEM observations on membrane surface (see Fig. 2). Results were also compared at different stages of membrane reactor. While solid conclusions may not be drawn about the relative extent of foulant accumulated on membrane surface at different stages of membrane tank, the foulant layer on the membrane taken from the stage 1 appears to be less than one taken from stage 2. This is clearly attributed to the higher TSS and organic concentration in bulk solution fed into stage 2, as shown in Table 1. Differences in the inorganic fouling were clear from the both stages of submerged membrane reactor. The T-EDXA showed that most dominant inorganic ions of foulant in stage 1 are aluminum and silicate while the silicate is more prevalent than aluminum in stage 2. The Al ions are very likely in stage 1 because of the addition of polyaluminum chloride (PAC) as a coagulant into the feed water (Fig. 1). The order of prevalence shifts for the membrane located at stage 2 to favor silicate, which can be explained by higher TSS concentration of bulk solution in the stage. The predominance of fluoride was detected from the SEM image indicating membrane materials composed of PVDF. In order to analyze organic fouling of membrane samples, the FTIR analysis was performed with fouled membranes taken from the stack 2 (S2) at two different stages of membrane reactor. For this test, the membrane samples were cut from the position 2 (P2) along fiber length. Foulant materials were scrapped from the membrane surface carefully using rubber spatula. Fig. 4 shows that the strong absorption peaks are detected near 1100 cm1, 1700 cm1 and 3400 cm1. The FTIR peak near 1100 cm1 is indicative of alcohol groups in carbohydrates possibly resulting from amino sugars and polysaccharides. The absorption band at
Fig. 3. SEM-EDXA results of membrane samples taken at two different stages of submerged MF hollow-fiber membrane reactor.
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Fig. 4. FTIR spectra for virgin membrane and foulant materials on membranes (membrane samples were taken at P2 and S2 in both stages).
1700 cm1 is characteristic of carboxylic group for the organic materials, which can induce membrane fouling caused by neutral constituent of natural organic matter (NOM) existing in surface water. The library search of the spectra suggested possible match with polysaccharides while this would need to be confirmed further by other quantitative analysis, such as mass spectroscopy.
Table 2 Permeabilities of membrane samples with vertical position of membrane modules (S1 and S3) and membrane stages (stage 1 and stage 2): Lpf: permeability of fouled membrane, Lpc: permeability of cleaned membrane by chemicals, Lpv: permeability of virgin membrane. Permeability
S1
S3
Stage1
Lpv (L/m2/h/Pa) 107 Lpf (L/m2/h/Pa) 107 Lpc (L/m2/h/Pa) 107
11.8 5.2 8.9
11.8 3.2 5.5
Stage2
Lpv (L/m2/h/Pa) 107 Lpf (L/m2/h/Pa) 107 Lpc (L/m2/h/Pa) 107
11.8 2.4 6.2
11.8 2.8 5.3
3.3. Relative permeability index In order to analyze local fouling, the permeate flux was measured with the membrane specimen taken at axial positions of fiber length and vertical positions of membrane modules. Membrane samples were then cleaned by performing chemical cleaning to estimate relative permeability index (RPI). The RPI was defined as the difference in the permeability obtained between cleaned membrane by chemical cleaning agents (Lpc) and fouled membrane (Lpf) divided by the permeability of fouled membrane, as shown in Eq. (3). Same approach to estimate the RPI value was attempted in other research [14]. All permeabilites were measured by using DI water as outside-in mode filtration under the constant pressure of 50 kPa. The permeability for each membrane sample are listed in Table 2.
RPI ¼
Lpc Lpf 100 ð%Þ Lpf
ð3Þ
Fig. 5 shows the RPI values obtained at different vertical positions of membrane modules or stack positions (S1 and S3) and stages of membrane reactors. For the stage 2 of membrane reactor, it was observed that the RPI value was much higher at S1 than S3
(160 vs. 90%), suggesting that the chemical cleaning should be more efficient for the membrane which is located near the source of aeration (S1). However, the differences in RPI values with respect to stack positions were relatively smaller in stage 1 than in stage 2 possibly due to lower SS concentration in stage 1. It has been reported that fouling reduction under aeration was more pronounced at higher solid concentration than at lower solid concentration in submerged membrane system [12,14]. Different RPI values of the membranes taken from two different membrane stages may also be related to the different natures of foulant materials between stage 1 and stage 2. As shown in Fig. 3, a significant portion of Al is observed in the membranes in stage 1, but the Si is more predominant in stage 2. The addition of PACs in stage 1 increases Al ions which can be associated with natural organic matter, resulting in more compact fouling layer which may not be removed by chemical cleaning easily.
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Fig. 5. Relative permeability index (RPI) of membranes by chemical cleaning at two vertical positions of membrane module (S1 and S3) and stages as described in Fig. 2.
3.4. Analysis of local fouling by resistance-in-series model As discussed above, Eq. (1) can be used to distinguish fouling resistances for each membrane specimen taken from different positions of hollow-fiber length to quantify local fouling resistances. Results by local fouling analysis for each membrane specimen are presented in Fig. 6. It shows that total resistances of fouled
Fig. 6. Combined effect of axial position of fiber length and stage of membrane reactor on fouling resistances.
membranes obtained from the stage 2 were higher than ones obtained from the stage 1 of submerged membrane tank. Higher total resistance is caused clearly by higher TSS concentration fed into the stage 2 from the stage 1 of membrane tank. Fig. 6 also shows that the reversible fouling resistance (Rre) has greatest portion in total fouling resistance (Rtotal) and this portion tends to be more pronounced for the membranes in stage 2. As each of the membrane sample taken along the fiber length was examined, it was interesting to note that irreversible fouling resistance (Rir) was much higher for the membrane taken at P1 where it is located near the suction source (or open ends of fibers) (see Fig. 2). Local fouling can be greatest near the open ends of fibers where the local flux is highest due to lumen suction pressure drop [4,8–10,6]. For the membrane in the stage 2 containing higher TSS concentration, however, the axial features of irreversible fouling resistance was not significant. The local fouling may be progressed toward the closed-ends of the fibers over filtration time since the local flux needs to be redistributed along the fiber to maintain a constant, length-averaged flux, which will be discussed in detail later. 3.5. Axial characteristics of membrane fouling In order to analyze local fouling in more depth, the extent of irreversible fouling resistance was analyzed with respect to the different positions of hollow-fiber membrane length. Fig. 7a shows that the membrane sample taken near the suction source (or open ends of fiber) provides the greatest portion for the irreversible fouling resistance. However, this non-uniform pattern of irreversible fouling resistance along the fiber length became more significant in S1 rather than S3, suggesting that local fouling be more signifi-
Fig. 7. The ratio of irreversible fouling resistance to total fouling resistance with respect to axial position along the fiber length and vertical position of submerged module comparing in Stage 1 (a) and Stage 2 (b).
M. Lee, J. Kim / Separation and Purification Technology 95 (2012) 227–234
cant as the membranes are located near the source of aeration. As the membrane is located apart from the source of aeration (S3), however, the axial features of membrane fouling become less pronounced due to higher potential of membrane fouling [8,10,6]. In the case of stage 2 of membrane reactor containing higher TSS concentration than stage 1, the axial features of irreversible fouling resistance was also observed to be less significant evidenced in Fig. 7b. Result indicates clearly that the local fouling depends upon the feed concentration strongly in membrane reactor. It could be confirmed from our membrane autopsy works that higher fouling potential results in less propensity of irreversible local fouling along the fiber length in submerged membrane system [12]. Higher localized pattern of membrane fouling in the stage 1 of membrane reactor can also be related to higher flux applied. From Fig. 7, it shows that the axial features of irreversible fouling resistance are more pronounced in stage 1 than stage 2. Several studies on developing mathematical models have predicted that the pressure distribution is more localized along the fiber as the permeate flux increases and this is consistent with our results [13]. As filtration time progresses, however, the non-uniform pattern of local flux should be changed to more uniform pattern due to self-adjustment of local flux to maintain constant flux which is length-averaged value. Conceptual diagram on this transient behavior of local fouling is described in Fig. 8.
233
3.6. Pressure gradient along the fiber Our observations of local fouling can be explained further by developing simple mathematical model to predict pressure gradient along the fiber in submerged hollow-fiber membrane system. A second-order differential equation to predict local pressure gradient along the fiber length can be described with respect to the local coordinate of fiber length as below [4,9]: 2
d pðxÞ 2
dx
k2 pðxÞ þ k2 pw ¼ 0
ð4Þ
where x is axial coordinate, p(x) is pressure in the fiber lumen at x coordinate and k is coefficient which is defined as 16=ðR3i Rm Þ, where Ri is fiber inner radius and Rm is intrinsic membrane resistance, and pw is hydraulic pressure. In the case of both open ends of fiber tested in this study, boundary conditions for Eq. (4) can be given as follows.
x ¼ 0;
pð0Þ ¼ p0
x ¼ L;
pðLÞ ¼ pL
ð5Þ
By applying these boundary conditions, the final solution of pressure as function of fiber length can be obtained as
pðxÞ ¼ C 1 ekx þ C 2 ekx þ pw
ð6Þ
Fig. 8. Conceptual diagrams on transient behavior of local fouling in submerged hollow-fiber water treatment system and chemical cleaning efficiency on local fouling resistance.
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Fig. 9. Local pressure profile along the hollow-fiber membrane length in submerged membrane water treatment system.
where
C 1 ¼ p0 pw C 2 C2 ¼
pL pw þ ðpw p0 Þekx 2 sinhðkLÞ
Assuming that the local flux is equal to radial component of velocity across the interface between the membrane and the porous medium, the local flux along the fiber can be obtained by using Darcy’s equation as shown in Eq. (7). To calculate imposed flux which is based upon the local flux profile along the fiber, the axial velocity in the hollow-fiber membrane can be expressed by neglecting inertial forces in Navier–Stokes equation, which is shown in Eqs. (8) and (9):
JðxÞ ¼
DP 1 ¼ ðP PðxÞÞ lRm lRm w
v ðx; rÞ ¼ v ðxÞ ¼
1 dPðxÞ þ qg ðR2i r 2 Þ 4l dx
R2i dPðxÞ 8l dx
ð7Þ
ð8Þ
ð9Þ
Results on the pressure profile along the fiber at three different imposed fluxes of 20, 30 and 40 L/m2/h are provided in Fig. 9. It shows that pressure along the fiber is highest near the source of suction pressure. The resulting pressure profiles along the fiber imply that deposition rate of fouling should be greater near the source of suction pressure than other regions. Fig. 9 also shows that the extent of pressure gradient can be greater for the higher imposed flux, suggesting that fouling should be more localized at higher imposed flux. This can support further our findings that the axial features of irreversible fouling resistance becomes more significant at stage 1 than stage 2 of membrane tank at which higher set-point flux is applied (40 vs. 20 L/m2/h). The pressure drop in the lumen causes highest flux at the outlet of the fiber, thus particles are deposited preferentially at this region. As filtration time progresses, the degree of non-uniformity of flux was reduced due to self-adjustment of local flux along the fiber length. 4. Conclusions Observations of local membrane fouling by membrane autopsy works from the pilot-scale operation of submerged, hollow-fiber
membrane reactor treating the effluent from coagulation basin showed that the local fouling varied depending upon axial positions of fiber length, vertical positions of membrane module and feed characteristics in the stage in which the membrane module is placed. It was found that irreversible fouling resistance after chemical cleaning with fouled membrane produced higher axial features of membrane fouling along the length of hollow-fiber membrane. Lower feed concentration of suspended solids or higher set-point flux applied into membrane module increased axial features of the membrane fouling in submerged hollow-fiber membrane system. As the position of membrane module was closer to the source of aeration, the axial features of irreversible fouling resistance became more significant. Fouling resistance have relatively uniform pattern for the hollow-fiber membranes located away from the source of aeration, suggesting that local fouling should be less important under higher fouling potential. However, there were no clear observations on the local characteristics with reversible fouling resistance. Our findings could be supported by the analysis of local pressure along the fiber showing that the local fouling should occur dominantly near the outlet of hollow-fiber membrane where local flux is highest. As fouling progresses significantly, this non-uniform pattern of fouling along the fiber becomes uniform due to the self-adjustment of local flux along fiber to maintain constant flux along the fiber. Acknowledgements This research was supported by a Grant (I2WATERTECH 04-1) from I2WaterTech of Eco-STAR project funded by Ministry of Environment, Korea. This research is supported by INHA University Research Grant. References [1] S. Chae, H. Yamamura, B. Choi, Y. Watanabe, Fouling characteristics of pressurized and submerged PVDF (polyvinylidene fluoride) microfiltration membranes in a pilot-scale drinking water treatment system under low and high turbidity conditions, Desalination 244 (2009) 215–226. [2] Y. Ye, L.N. Sim, B. Herulah, V. Chen, A.G. Fane, Effects of operating conditions on submerged hollow fiber membrane systems used as pre-treatment for seawater reverse osmosis, J. Membr. Sci. 365 (1–2) (2010) 78–88. [3] K.J. Hwang, C.S. Chan, F.F. Chen, A comparison of hydrodynamic methods for mitigating particle fouling in submerged membrane filtration, J. Chin. Inst. Chem. Eng. 39 (2008) 257–264. [4] S. Chang, A.G. Fane, The effect of fiber diameter on filtration and flux distribution-relevance to submerged hollow-fiber modules, J. Membr. Sci 184 (2001) 221–231. [5] K. Parameshwaran, C. Visvanathen, R.B. Aim, Membrane as solid/liquid separator and air diffuser in bioreactor, J. Environ. Eng. 125 (1999) 825–834. [6] J. Kim, F.A. DiGiano, Defining critical flux in submerged membranes: influence of length-distributed flux, J. Membr. Sci. 280 (2006) 752–761. [7] Z.F. Cui, S. Chang, A.G. Fane, The use of gas bubbling to enhance membrane processes, J. Membr. Sci. 221 (2003) 1–35. [8] A.G. Fane, S. Chang, E. Chardon, Submerged hollow fiber membrane module design options and operational considerations, Desalination 146 (2002) 231– 236. [9] J. Kim, F.A. DiGiano, Fouling models for low-pressure membrane systems, Sep. Purif. Technol. 68 (2009) 293–304. [10] S. Lee, P. Park, J. Kim, K. Yeon, C. Lee, Analysis of filtration characteristics in submerged microfiltration for drinking water treatment, Water Res. 42 (2008) 3109–3121. [11] H.P. Chu, X. Li, Membrane fouling in membrane bioreactor (MBR): sludge cake formation and fouling characteristics, Biotechnol. Bioeng. 90 (2004) 323–331. [12] J. Kim, F.A. DiGiano, Particle fouling in submerged microfiltration membranes: effects of hollow-fiber length and aeration rate, J. Water Supply Res. Technol. 55 (2006) 535–547. [13] T. Carroll, N.A. Booker, Axial features in the fouling of hollow fiber membranes, J. Membr. Sci. 168 (2000) 203–212. [14] S. Muthukumaran, K. Yang, A. Seuren, S. Kentish, M. Ashokkumar, G.W. Stevens, F. Grieser, The use of ultrasonic cleaning for ultrafiltration membranes in the dairy industry, Sep. Purif. Technol. 39 (2004) 99–107.
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Analysis of local fouling in a pilot-scale submerged hollow-fiber membrane system for drinking water treatment by membrane autopsy Mooseok Lee a, Jeonghwan Kim b,⇑ a b
ECO Research Institute, KOLON Central Research Park Mabuk-dong, Giheung-ku, Yongin-city, Kyungi-do 446-797, Republic of Korea Department of Environmental Engineering, INHA University Nam-gu, Yonghyun-dong 253, Incheon 407-751, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 January 2012 Received in revised form 15 April 2012 Accepted 17 April 2012 Available online 9 May 2012 Keywords: Submerged hollow-fiber membrane Local fouling Resistance-in-series model Local pressure Local flux
a b s t r a c t An analysis of local fouling in a pilot-scale submerged hollow-fiber membrane water treatment system for drinking water production is described in this paper. Membrane fouling was observed by using microscopic techniques with membrane specimen taken at different longitudinal and horizontal positions of a hollow-fiber membrane module. The resistance-in-series model was used to quantify local fouling resistances of membrane samples. Our results show that the recovery of the permeate flux of the hollow-fiber membrane located near the source of aeration was the highest after performing chemical cleaning. The irreversible fouling resistance was the largest for membrane samples taken near the open ends of the fibers, where the local pressure is expected to be highest. These axial features of irreversible fouling resistance were more pronounced at lower suspended solids concentration or higher set-point flux. However, the local aspect of reversible fouling was relatively less pronounced than the one of irreversible fouling. The results obtained from membrane autopsy were explained well by the local fouling phenomenon driven by the profile of local pressure along the fiber length and transient behavior of this profile during membrane operation. In addition, it was concluded that the individual membrane module experiencing more aeration would have a more localized pattern of membrane fouling, suggesting that the local fouling is more significant under relatively less demanding fouling conditions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Low-pressure driven membrane processes such as microfiltration (MF) or ultrafiltration (UF) are configured generally as a module that consists of a bundle of hollow-fibers that are submerged directly into a tank. Although the submerged, hollow-fiber membrane modules have been widely combined with biological processes for wastewater treatment, their installations have also increased for water treatment applications [1–3]. Toward this end, the MF or UF has been considered to be an alternative to coagulation, sedimentation and granular media filtration because it can produce higher quality of product water (permeate) while providing smaller footprint as requiring smaller reactor size [4–6]. Although submerged membrane technologies have changed the water treatment in a relatively short time, membrane fouling still remains one of the major obstacles in their implementations. Membrane fouling is an inevitable phenomenon caused by the depositions of foulant materials on membrane surface and/or
⇑ Corresponding author. Tel.: +82 32 860 7502; fax: +82 32 865 1425. E-mail address: [email protected] (J. Kim). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.04.017
adsorption of them within membrane pores. In order to reduce membrane fouling, external function such as aeration can be supplied continuously or semi-continuously from the bottom of membrane reactor. Foulant materials by the contact of membrane surface on the feed side are dislodged with rising air bubbles due to physical scrubbing effect and bulk shear rate along the fiber [7,8]. Although membrane fouling varies depending upon various operational conditions, one of important characteristics in submerged, hollow-fiber membrane system is the variation in suction pressure along the fiber length which is caused by the application in a negative pressure at the open ends of fibers [9]. It has been generally accepted that the local flux can be highest near the suction source of fibers while it becomes lowest near the opposite ends of fibers due to internal pressure drop in fiber lumen [4,10]. The variation of local flux can introduce axial features of membrane fouling and the local fouling near some region of the membrane where the local flux is higher than the set-point flux which is length-averaged value is not overlooked because the local fouling deteriorates membrane performance significantly. The local fouling can occur even if the membrane is operated even under critical flux where no fouling is expected [6]. Kim and DiGiano performed morphological examinations of local fouling at different longitudinal positions of hollow-fiber membrane
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from the laboratory-scale, submerged, hollow-fiber membrane system treating model fluorescence latex particles [6]. Fouling profiles were confirmed that deposition of particles was much more evident near the open ends of fibers than near the middle, and the closed ends of the fibers under the critical or sub-critical flux conditions. This non-uniform pattern of particle deposition along the fiber length became more significant during the initial period of membrane filtration. Since particles can be deposited preferentially near the open ends of the fibers where the local flux is greatest, this region can produce a more compact particle fouling against which aeration is not very effective [11,12]. Several attempts have been made to develop mathematical models in order to predict the behavior of local fouling in submerged, hollow-fiber membrane system. The local fouling was predicted by incorporating with the local flux distribution along the fiber length in clean water system. Chang and Fane found that the extent of local flux was higher for longer fiber length, smaller inner radius of hollow-fiber membrane or smaller membrane resistance [4]. Significant fraction of the local flux was higher than the measured flux from the open ends of fiber. In fouling system, however, initial distribution of local flux should become more uniform with filtration time as the local flux at some membrane region where it is higher than the set-point flux must drop due to fouling resistance at the region [10,13]. The necessary response is for the local flux to increase in less-fouled region to maintain constant flux along the fiber length. Although there has been growing interests in analyzing local fouling from submerged, hollow-fiber membrane system, most of studies have focused on the observations using only laboratoryscale, experimental set-up. In addition, current mathematical models considering the local fouling have not evolved yet far enough to compare practical observations with larger-scale performance of submerged membrane system. Therefore, results from the pilotscale operation with submerged hollow-fiber membrane system are necessary for the models to be improved. From the best of our knowledge, analysis of local fouling from the pilot- or full-scale design of submerged, hollow-fiber membrane system has not been performed yet. In this study, we analyzed local fouling from the pilot-scale operation with submerged, hollow-fiber membrane system treating the effluent from coagulation basin. Local fouling resistance was quantified with respect to the different positions of hollow fiber length and stack positions for the individual membrane module installed in submerged membrane reactor. Fouling analysis was performed by membrane autopsy works to better understand local fouling phenomena and obtain better insights for the practical design of submerged, hollow-fiber membrane system for water treatment applications.
2. Experimental 2.1. Pilot-scale operation of submerged membrane system Schematic diagram of the pilot-scale, submerged hollow-fiber membrane system for water treatment applied for this study is shown in Fig. 1. A pilot-scale, submerged membrane system was installed and operated for 3 months at the Kuei Water Treatment Plant. The feed water of Han River was pumped first to the coagulation tank at a rate of approximately 550 m3/day and polyaluminum chlorides (PACs) as a coagulant (17 vol.%) was injected to the feed water in order to remove particulate matters. The effluent from the coagulation tank was piped to the submerged, hollow-fiber membrane system subsequently. Microfiltration (MF) hollow-fiber membrane which is composed of polyvinylidenefluoride (PVDF) materials with braid reinforcement was used and fil-
tered in an outside-in flow pattern (KOLON Industries, Inc., Korea). Nominal pore size of the hollow-fiber membrane tested in this study was 0.1 lm. Inside-diameter and outside-diameter of the hollow fiber membrane was 0.8 mm and 2.0 mm, respectively. Membrane tank was configured as a two-stage reactor in order to achieve system recovery up to 99%. Concentrate from the MF membrane module submerged in the stage 1 of membrane reactor was flowed for the inlet of stage 2 of it (Fig. 1). As shown in Fig. 2, three membrane modules were stacked vertically and hollow-fiber membranes for each stack were oriented horizontally. Total effective length of hollow-fiber MF membrane for each module was 1 m and surface area of membrane per unit module was 20 m2. Permeate water was obtained from the open ends of fibers by applying suction pressure into them. The set-point flux applied for membrane modules in stage 1 and 2 of the reactor was 40 and 20 L/ m2/h, respectively. In order to control membrane fouling, aeration was applied from the bottom of reactor under the 0.2 Nm3/m2/h as normalized air flow rate. Backwashing was performed using membrane permeate for 30 s every 15 min of filtration. 2.2. Local fouling observations In order to observe and analyze local fouling with respect to the different positions of hollow fiber length and vertical positions of membrane modules, at the end of 3 months of operation, was membrane specimen taken from membrane module. In order to investigate the local fouling at different positions of fiber length, the membrane sample was cut every 0.2 m from the open ends of fiber at each membrane module. As described in Fig. 2, sample points for taking membrane specimen from different positions of fiber length are indicated as P1, P2 and P3. In order to observe the effect of position of membrane module on the local fouling, all membrane samples were taken from P2 at different vertical positions of membrane module which are indicated as S1, S2 and S3 (see Fig. 2). From Fig. 2, the S1 shows the vertical position of membrane module stacked closest to the source of aeration while the S3 is the module stacked most away from it. 2.3. Analysis of local fouling resistance Fouling of membrane specimen taken at different positions of hollow fiber length and stack positions was characterized applying resistance-in-series model. In this study, a simple dead-end filtration module was prepared for measuring permeability of used membrane and chemically-cleaned membrane (0.0012 m2 of surface area). Two hollow-fiber membrane samples were installed in a cylindrical filtration module which is connected to the pressure vessel containing DI water. The pressure vessel was pressurized by compressed nitrogen gas under 50 kPa to provide DI water into the module to measure the permeability of membrane samples. In order to obtain intrinsic properties of fouling resistance with minimization of the fiber length effect, the filtration was performed as outside-in mode using short hollow-fiber membrane samples having 10 cm length. The accumulated volume of permeate from the open ends of fibers was measured at a given filtration time to estimate membrane permeability. To measure flow rates of fouled membranes, variation of foulant on membranes was minimized by storing the samples in sterilized sample bottles and the permeate flow rate was measured within a day after sampling. For measuring the flow rates of chemically-cleaned membranes, membrane samples were cleaned immediately by chemical cleaning agents after taking them out from membrane modules. In order to estimate fouling resistances of membrane samples, Darcy’s equation was applied to estimate the total fouling resistances shown in Eq. (1). Permeate flux measured by dead-end fil-
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Fig. 1. Schematic diagram of integrated coagulation-submerged microfiltration hollow-fiber water treatment system.
per unit membrane area (J, L/m2/h) is proportional to the transmembrane pressure (DP, Pa) and inversely proportional to the viscosity of water (l, kg/m/s) and total resistances (Rtotal, m1).
J¼
DP
ð1Þ
lRtotal
Rtotal ¼ Rm þ Rre þ Rir
ð2Þ 1
where Rm is membrane resistance (m ), Rre is reversible resistance (m1) and Rir is irreversible resistance (m1). To determine each resistance, the Rm was obtained first by filtering deionized (DI) water with bare membrane. In this study, reversible resistance (Rre) was defined as the fouling resistance which can be removed by chemical cleaning. At the end of membrane operation, total resistance (Rtotal) was estimated by measuring permeate flux of fouled membrane with DI water. The reversible fouling resistance (Rre) should be caused mostly by the cake layer deposited on membrane surface and weak adsorption of foulant materials into membrane pores which can be removed by chemical cleaning. Given this condition, the remaining resistances including Rm and Rir could be obtained by measuring the permeate flux of the chemically-cleaned membrane using DI water. Since the Rm is already known, the Rir value can be calculated using Eq. (2). Chemical cleaning of fouled membrane was performed by sonicating the membrane samples with the cleaning agents of 2000 ppm NaOCl solution followed by 2500 ppm citric acid solution. Each chemical cleaning was performed for two hours with the fouled membrane samples taken. 2.4. Microscopic analysis of local fouling Fig. 2. Schematics of submerged membrane module describing sampling points to observe local characteristics of membrane fouling with respect to the axial positions along the fiber length (P1, P2 and P3) and vertical positions of membrane module(S1, S2 and S3).
tration test described above was divided by transmembrane pressure to calculate the membrane permeability (J/DP). In Darcy’s equation, the rate at which water passes through the membrane
In order to characterize foulant materials from each membrane specimen taken at different sampling locations, membrane autopsy works were performed using various microscopic techniques. Membrane specimen was placed on a sample mount and examined by scanning electron microscopy (SEM) to observe surface deposition of foulant on membrane. The membrane taken along the fiber
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length was mounted with double-side tape on aluminum stubs and coated with a thin layer of conductive metal (Au) by applying high vacuum sputter coating unit. Secondary electron (SE) images were also taken at highest magnification (120) from the SEM images. The targeted energy dispersive X-ray analysis (T-EDXA Model: Noran EDX System) uses the SE images to determine the inorganic elemental composition of the foulant materials on membrane samples. The wavelength of the X-rays produced was used to identify the presences and relative amounts of the chemical elements in the foulants on the membranes. A small portion of foulants was also removed from the membrane sample by rubber spatula carefully for Fourier Transform Infrared Analysis (FTIR Model: Nicolet/Thermo Electron Avatar 360) to measure organic fraction of foulant. 3. Results and discussion 3.1. Characteristics of feed water The integrated membrane system was fed with the raw water from the Han River. As mentioned above, the feed water for the submerged MF hollow-fiber module installed in the stage 1 of membrane reactor was pretreated by adding polyaluminum chloride. As shown in Table 1, the average DOCs in stages 1 and 2 of membrane reactor were 2.3 and 3.2 mg/L, respectively during operational period. The value of UV254 of the feed water in the stage 1 of membrane reactor was smaller than the one observed in the stage 2 of it (0.021 vs. 0.035 cm1). The average TSS concentrations in stage 1 and 2 of membrane reactor were measured to be 12.6 and
Table 1 Summary of water qualities of feed water for stage 1 and stage 2 of submerged membrane reactor. UV254 (cm1)
DOC (mg/L)
Alkalinity (mg/L)
SS (mg/L)
Stage 1 Min. Max. Avg.
0.012 0.036 0.021
1.23 4.19 2.34
6.5 45.6 30.4
1.5 35.4 12.6
Stage 2 Min. Max. Avg.
0.015 0.065 0.035
1.08 6.83 3.20
19.5 79.8 30.8
56 233 121
121 mg/L, respectively. Results indicated that the suspended solids concentration in stage 2 should be higher due to the concentrate stream from the MF membranes installed in stage 1. 3.2. Microscopic observations of membrane fouling at different stages of membrane tank The SEM images of membrane specimen taken at the end of 3month of pilot-scale operation are presented in Fig. 3. Inorganic fraction of foulant on membrane was also analyzed by performing SEM-Targeted Energy-Dispersive X-ray analysis (T-EDXA). The membrane module located at stack 2 (S2) and position 2 (P2) was selected to take the membrane sample in order to perform SEM observations on membrane surface (see Fig. 2). Results were also compared at different stages of membrane reactor. While solid conclusions may not be drawn about the relative extent of foulant accumulated on membrane surface at different stages of membrane tank, the foulant layer on the membrane taken from the stage 1 appears to be less than one taken from stage 2. This is clearly attributed to the higher TSS and organic concentration in bulk solution fed into stage 2, as shown in Table 1. Differences in the inorganic fouling were clear from the both stages of submerged membrane reactor. The T-EDXA showed that most dominant inorganic ions of foulant in stage 1 are aluminum and silicate while the silicate is more prevalent than aluminum in stage 2. The Al ions are very likely in stage 1 because of the addition of polyaluminum chloride (PAC) as a coagulant into the feed water (Fig. 1). The order of prevalence shifts for the membrane located at stage 2 to favor silicate, which can be explained by higher TSS concentration of bulk solution in the stage. The predominance of fluoride was detected from the SEM image indicating membrane materials composed of PVDF. In order to analyze organic fouling of membrane samples, the FTIR analysis was performed with fouled membranes taken from the stack 2 (S2) at two different stages of membrane reactor. For this test, the membrane samples were cut from the position 2 (P2) along fiber length. Foulant materials were scrapped from the membrane surface carefully using rubber spatula. Fig. 4 shows that the strong absorption peaks are detected near 1100 cm1, 1700 cm1 and 3400 cm1. The FTIR peak near 1100 cm1 is indicative of alcohol groups in carbohydrates possibly resulting from amino sugars and polysaccharides. The absorption band at
Fig. 3. SEM-EDXA results of membrane samples taken at two different stages of submerged MF hollow-fiber membrane reactor.
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Fig. 4. FTIR spectra for virgin membrane and foulant materials on membranes (membrane samples were taken at P2 and S2 in both stages).
1700 cm1 is characteristic of carboxylic group for the organic materials, which can induce membrane fouling caused by neutral constituent of natural organic matter (NOM) existing in surface water. The library search of the spectra suggested possible match with polysaccharides while this would need to be confirmed further by other quantitative analysis, such as mass spectroscopy.
Table 2 Permeabilities of membrane samples with vertical position of membrane modules (S1 and S3) and membrane stages (stage 1 and stage 2): Lpf: permeability of fouled membrane, Lpc: permeability of cleaned membrane by chemicals, Lpv: permeability of virgin membrane. Permeability
S1
S3
Stage1
Lpv (L/m2/h/Pa) 107 Lpf (L/m2/h/Pa) 107 Lpc (L/m2/h/Pa) 107
11.8 5.2 8.9
11.8 3.2 5.5
Stage2
Lpv (L/m2/h/Pa) 107 Lpf (L/m2/h/Pa) 107 Lpc (L/m2/h/Pa) 107
11.8 2.4 6.2
11.8 2.8 5.3
3.3. Relative permeability index In order to analyze local fouling, the permeate flux was measured with the membrane specimen taken at axial positions of fiber length and vertical positions of membrane modules. Membrane samples were then cleaned by performing chemical cleaning to estimate relative permeability index (RPI). The RPI was defined as the difference in the permeability obtained between cleaned membrane by chemical cleaning agents (Lpc) and fouled membrane (Lpf) divided by the permeability of fouled membrane, as shown in Eq. (3). Same approach to estimate the RPI value was attempted in other research [14]. All permeabilites were measured by using DI water as outside-in mode filtration under the constant pressure of 50 kPa. The permeability for each membrane sample are listed in Table 2.
RPI ¼
Lpc Lpf 100 ð%Þ Lpf
ð3Þ
Fig. 5 shows the RPI values obtained at different vertical positions of membrane modules or stack positions (S1 and S3) and stages of membrane reactors. For the stage 2 of membrane reactor, it was observed that the RPI value was much higher at S1 than S3
(160 vs. 90%), suggesting that the chemical cleaning should be more efficient for the membrane which is located near the source of aeration (S1). However, the differences in RPI values with respect to stack positions were relatively smaller in stage 1 than in stage 2 possibly due to lower SS concentration in stage 1. It has been reported that fouling reduction under aeration was more pronounced at higher solid concentration than at lower solid concentration in submerged membrane system [12,14]. Different RPI values of the membranes taken from two different membrane stages may also be related to the different natures of foulant materials between stage 1 and stage 2. As shown in Fig. 3, a significant portion of Al is observed in the membranes in stage 1, but the Si is more predominant in stage 2. The addition of PACs in stage 1 increases Al ions which can be associated with natural organic matter, resulting in more compact fouling layer which may not be removed by chemical cleaning easily.
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Fig. 5. Relative permeability index (RPI) of membranes by chemical cleaning at two vertical positions of membrane module (S1 and S3) and stages as described in Fig. 2.
3.4. Analysis of local fouling by resistance-in-series model As discussed above, Eq. (1) can be used to distinguish fouling resistances for each membrane specimen taken from different positions of hollow-fiber length to quantify local fouling resistances. Results by local fouling analysis for each membrane specimen are presented in Fig. 6. It shows that total resistances of fouled
Fig. 6. Combined effect of axial position of fiber length and stage of membrane reactor on fouling resistances.
membranes obtained from the stage 2 were higher than ones obtained from the stage 1 of submerged membrane tank. Higher total resistance is caused clearly by higher TSS concentration fed into the stage 2 from the stage 1 of membrane tank. Fig. 6 also shows that the reversible fouling resistance (Rre) has greatest portion in total fouling resistance (Rtotal) and this portion tends to be more pronounced for the membranes in stage 2. As each of the membrane sample taken along the fiber length was examined, it was interesting to note that irreversible fouling resistance (Rir) was much higher for the membrane taken at P1 where it is located near the suction source (or open ends of fibers) (see Fig. 2). Local fouling can be greatest near the open ends of fibers where the local flux is highest due to lumen suction pressure drop [4,8–10,6]. For the membrane in the stage 2 containing higher TSS concentration, however, the axial features of irreversible fouling resistance was not significant. The local fouling may be progressed toward the closed-ends of the fibers over filtration time since the local flux needs to be redistributed along the fiber to maintain a constant, length-averaged flux, which will be discussed in detail later. 3.5. Axial characteristics of membrane fouling In order to analyze local fouling in more depth, the extent of irreversible fouling resistance was analyzed with respect to the different positions of hollow-fiber membrane length. Fig. 7a shows that the membrane sample taken near the suction source (or open ends of fiber) provides the greatest portion for the irreversible fouling resistance. However, this non-uniform pattern of irreversible fouling resistance along the fiber length became more significant in S1 rather than S3, suggesting that local fouling be more signifi-
Fig. 7. The ratio of irreversible fouling resistance to total fouling resistance with respect to axial position along the fiber length and vertical position of submerged module comparing in Stage 1 (a) and Stage 2 (b).
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cant as the membranes are located near the source of aeration. As the membrane is located apart from the source of aeration (S3), however, the axial features of membrane fouling become less pronounced due to higher potential of membrane fouling [8,10,6]. In the case of stage 2 of membrane reactor containing higher TSS concentration than stage 1, the axial features of irreversible fouling resistance was also observed to be less significant evidenced in Fig. 7b. Result indicates clearly that the local fouling depends upon the feed concentration strongly in membrane reactor. It could be confirmed from our membrane autopsy works that higher fouling potential results in less propensity of irreversible local fouling along the fiber length in submerged membrane system [12]. Higher localized pattern of membrane fouling in the stage 1 of membrane reactor can also be related to higher flux applied. From Fig. 7, it shows that the axial features of irreversible fouling resistance are more pronounced in stage 1 than stage 2. Several studies on developing mathematical models have predicted that the pressure distribution is more localized along the fiber as the permeate flux increases and this is consistent with our results [13]. As filtration time progresses, however, the non-uniform pattern of local flux should be changed to more uniform pattern due to self-adjustment of local flux to maintain constant flux which is length-averaged value. Conceptual diagram on this transient behavior of local fouling is described in Fig. 8.
233
3.6. Pressure gradient along the fiber Our observations of local fouling can be explained further by developing simple mathematical model to predict pressure gradient along the fiber in submerged hollow-fiber membrane system. A second-order differential equation to predict local pressure gradient along the fiber length can be described with respect to the local coordinate of fiber length as below [4,9]: 2
d pðxÞ 2
dx
k2 pðxÞ þ k2 pw ¼ 0
ð4Þ
where x is axial coordinate, p(x) is pressure in the fiber lumen at x coordinate and k is coefficient which is defined as 16=ðR3i Rm Þ, where Ri is fiber inner radius and Rm is intrinsic membrane resistance, and pw is hydraulic pressure. In the case of both open ends of fiber tested in this study, boundary conditions for Eq. (4) can be given as follows.
x ¼ 0;
pð0Þ ¼ p0
x ¼ L;
pðLÞ ¼ pL
ð5Þ
By applying these boundary conditions, the final solution of pressure as function of fiber length can be obtained as
pðxÞ ¼ C 1 ekx þ C 2 ekx þ pw
ð6Þ
Fig. 8. Conceptual diagrams on transient behavior of local fouling in submerged hollow-fiber water treatment system and chemical cleaning efficiency on local fouling resistance.
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Fig. 9. Local pressure profile along the hollow-fiber membrane length in submerged membrane water treatment system.
where
C 1 ¼ p0 pw C 2 C2 ¼
pL pw þ ðpw p0 Þekx 2 sinhðkLÞ
Assuming that the local flux is equal to radial component of velocity across the interface between the membrane and the porous medium, the local flux along the fiber can be obtained by using Darcy’s equation as shown in Eq. (7). To calculate imposed flux which is based upon the local flux profile along the fiber, the axial velocity in the hollow-fiber membrane can be expressed by neglecting inertial forces in Navier–Stokes equation, which is shown in Eqs. (8) and (9):
JðxÞ ¼
DP 1 ¼ ðP PðxÞÞ lRm lRm w
v ðx; rÞ ¼ v ðxÞ ¼
1 dPðxÞ þ qg ðR2i r 2 Þ 4l dx
R2i dPðxÞ 8l dx
ð7Þ
ð8Þ
ð9Þ
Results on the pressure profile along the fiber at three different imposed fluxes of 20, 30 and 40 L/m2/h are provided in Fig. 9. It shows that pressure along the fiber is highest near the source of suction pressure. The resulting pressure profiles along the fiber imply that deposition rate of fouling should be greater near the source of suction pressure than other regions. Fig. 9 also shows that the extent of pressure gradient can be greater for the higher imposed flux, suggesting that fouling should be more localized at higher imposed flux. This can support further our findings that the axial features of irreversible fouling resistance becomes more significant at stage 1 than stage 2 of membrane tank at which higher set-point flux is applied (40 vs. 20 L/m2/h). The pressure drop in the lumen causes highest flux at the outlet of the fiber, thus particles are deposited preferentially at this region. As filtration time progresses, the degree of non-uniformity of flux was reduced due to self-adjustment of local flux along the fiber length. 4. Conclusions Observations of local membrane fouling by membrane autopsy works from the pilot-scale operation of submerged, hollow-fiber
membrane reactor treating the effluent from coagulation basin showed that the local fouling varied depending upon axial positions of fiber length, vertical positions of membrane module and feed characteristics in the stage in which the membrane module is placed. It was found that irreversible fouling resistance after chemical cleaning with fouled membrane produced higher axial features of membrane fouling along the length of hollow-fiber membrane. Lower feed concentration of suspended solids or higher set-point flux applied into membrane module increased axial features of the membrane fouling in submerged hollow-fiber membrane system. As the position of membrane module was closer to the source of aeration, the axial features of irreversible fouling resistance became more significant. Fouling resistance have relatively uniform pattern for the hollow-fiber membranes located away from the source of aeration, suggesting that local fouling should be less important under higher fouling potential. However, there were no clear observations on the local characteristics with reversible fouling resistance. Our findings could be supported by the analysis of local pressure along the fiber showing that the local fouling should occur dominantly near the outlet of hollow-fiber membrane where local flux is highest. As fouling progresses significantly, this non-uniform pattern of fouling along the fiber becomes uniform due to the self-adjustment of local flux along fiber to maintain constant flux along the fiber. Acknowledgements This research was supported by a Grant (I2WATERTECH 04-1) from I2WaterTech of Eco-STAR project funded by Ministry of Environment, Korea. This research is supported by INHA University Research Grant. References [1] S. Chae, H. Yamamura, B. Choi, Y. Watanabe, Fouling characteristics of pressurized and submerged PVDF (polyvinylidene fluoride) microfiltration membranes in a pilot-scale drinking water treatment system under low and high turbidity conditions, Desalination 244 (2009) 215–226. [2] Y. Ye, L.N. Sim, B. Herulah, V. Chen, A.G. Fane, Effects of operating conditions on submerged hollow fiber membrane systems used as pre-treatment for seawater reverse osmosis, J. Membr. Sci. 365 (1–2) (2010) 78–88. [3] K.J. Hwang, C.S. Chan, F.F. Chen, A comparison of hydrodynamic methods for mitigating particle fouling in submerged membrane filtration, J. Chin. Inst. Chem. Eng. 39 (2008) 257–264. [4] S. Chang, A.G. Fane, The effect of fiber diameter on filtration and flux distribution-relevance to submerged hollow-fiber modules, J. Membr. Sci 184 (2001) 221–231. [5] K. Parameshwaran, C. Visvanathen, R.B. Aim, Membrane as solid/liquid separator and air diffuser in bioreactor, J. Environ. Eng. 125 (1999) 825–834. [6] J. Kim, F.A. DiGiano, Defining critical flux in submerged membranes: influence of length-distributed flux, J. Membr. Sci. 280 (2006) 752–761. [7] Z.F. Cui, S. Chang, A.G. Fane, The use of gas bubbling to enhance membrane processes, J. Membr. Sci. 221 (2003) 1–35. [8] A.G. Fane, S. Chang, E. Chardon, Submerged hollow fiber membrane module design options and operational considerations, Desalination 146 (2002) 231– 236. [9] J. Kim, F.A. DiGiano, Fouling models for low-pressure membrane systems, Sep. Purif. Technol. 68 (2009) 293–304. [10] S. Lee, P. Park, J. Kim, K. Yeon, C. Lee, Analysis of filtration characteristics in submerged microfiltration for drinking water treatment, Water Res. 42 (2008) 3109–3121. [11] H.P. Chu, X. Li, Membrane fouling in membrane bioreactor (MBR): sludge cake formation and fouling characteristics, Biotechnol. Bioeng. 90 (2004) 323–331. [12] J. Kim, F.A. DiGiano, Particle fouling in submerged microfiltration membranes: effects of hollow-fiber length and aeration rate, J. Water Supply Res. Technol. 55 (2006) 535–547. [13] T. Carroll, N.A. Booker, Axial features in the fouling of hollow fiber membranes, J. Membr. Sci. 168 (2000) 203–212. [14] S. Muthukumaran, K. Yang, A. Seuren, S. Kentish, M. Ashokkumar, G.W. Stevens, F. Grieser, The use of ultrasonic cleaning for ultrafiltration membranes in the dairy industry, Sep. Purif. Technol. 39 (2004) 99–107.