Journal of Membrane Science 427 (2013) 320–325

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Journal of Membrane Science

0376-73

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Flow analysis and fouling on the patterned membrane surface

Young Ki Lee, Young-June Won, Jae Hyun Yoo, Kyung Hyun Ahn n, Chung-Hak Lee

School of Chemical and Biological Engineering, Institute of Chemical Process, Seoul National University, Seoul 151-744, South Korea

a r t i c l e i n f o

Article history:

Received 3 September 2012

Received in revised form

3 October 2012

Accepted 6 October 2012Available online 17 October 2012

Keywords:

Membrane fouling

Computational fluid dynamics (CFD)

Vortex

Wall shear stress

88/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.memsci.2012.10.010

esponding author. Tel.: þ82 2 880 8322; fax:

ail address: ahnnet@snu.ac.kr (K.H. Ahn).

a b s t r a c t

One of the key issues in membrane technology is to avoid fouling which is formed on the membrane

surface. To get over this trouble, the use of a patterned surface was suggested as a way to reduce

fouling. In this report, both the experimental and numerical studies were performed with a prism

patterned membrane system. In our experiments, we observe the local distribution of microbials; more

fouling in the lower part of the pattern and less fouling in the upper region. In the simulation, the

vortex develops in the lower part of the pattern, in which the aggregates of solid components in the

sludge can be easily formed, leading to a favorable fouling formation. It is also observed that the local

wall shear stress is higher in the upper region of the pattern compared to the lower region where the

vortex is formed. High shear stress facilitates detachment and prohibits attachment of solid

components on the membrane surface. The simulation result (vortex formation in the lower region

of the pattern and higher stress in the upper region) is in accordance with the experimental results on

the distribution of microbials in the patterned membrane system (more fouling in the lower part of the

pattern, and less fouling in the upper region). This indicates the importance of flow characteristics as

well as the stress distribution to reduce the fouling in the patterned membrane system. In addition, it is

emphasized that the pattern should be designed efficiently by considering the local flow and stress

distribution in the cross flow membrane system.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The application of membrane technology to obtain highlypurified water has drawn more attention in recent years [1,2].Membrane systems such as reverse osmosis (RO), nanofiltration(NF) and microfiltration (MF) are widely used because they cansimplify the treatment process and reduce the scale of thetreatment facility. However, membrane fouling seems to beunavoidable in these applications. Membrane fouling induces anincrease in the transmembrane pressure, thus increases theoperation costs of the plant. Therefore, a membrane modificationthat can reduce fouling is an important issue in preparing themembranes [3–5].

Recently, micro-patterning was suggested as an approach toreduce fouling on the membrane surface [6,7]. A reduction offouling on the patterned membrane surface was observed. Thisphenomenon might be strongly related with the flow behaviornear the membrane surface. The research on the flow behaviorinside the membrane module and near the membrane surfacewas limited to the experimental domain, but the demand forcomputational fluid dynamics (CFD) has increased to understand

ll rights reserved.

þ82 2 880 1580.

the flow behavior in the membrane system, which eventuallysaves time and cost, leading to efficient design.

The CFD studies on the membrane system have been per-formed by previous researchers. Wiley et al. [8] studied thepressure driven membrane process using the finite volumemethod (FVM). They developed CFD models to investigate theconcentration polarization near the membrane wall with thepermeable wall flux, and they compared the CFD results withanalytical and numerical models suggested by Brian [9] and Gillet al. [10]. From this study, they emphasized the importance offine mesh near the wall and adaptation of high order numericalschemes to get accurate solutions regarding concentration polar-ization. Ma and Song [11] developed a finite element model tostudy concentration polarization in spacer-filled channel flow. Intheir study, the alleviation of the concentration polarization andthe enhancement of permeate flux in various spacer configura-tions were investigated. They suggested the zigzag configurationas the most effective one to alleviate concentration polarizationand to enhance the permeate flux. Picioreanu et al. [12] andVrouwenvelder et al. [13] described bio-fouling near the feedsurface using the CFD technique. In the feed channel spacer,whose size was equivalent to the real spacer, a simulation wasperformed and the concentration of soluble substrate around thespacer was considered by incorporating with the discrete cellularautomat biofilm model. The pressure change around the spacer as

Table 1Characteristics of the membranes used in this study; prism patterned and flat [15].

Prism patterned membrane Flat membrane

Thickness of membrane (mm) About 200 About 200

Mean pore size (mm) 0.9170.22 0.8570.14

Pure water flux at 25 kPa (LMH) 430–450 360–390

Y.K. Lee et al. / Journal of Membrane Science 427 (2013) 320–325 321

well as the growth of the biomass was successfully described. Leeand Wu [14] reported the effect of shear force on the membranesurface through the CFD study. Even though the analysis waslimited to a pure liquid system, they could find the relationshipbetween the cake formation and shear force on the membranesurface. In addition, they demonstrated that the hydrodynamicshear force analysis on the membrane surface can be a useful toolto reduce the fouling in the cross-flow microfiltration.

In this study, we focused on the flow around the patternedmembrane surface to explain the experimental observation on thepatterned membrane, in which the attachment of microbials onthe membrane surface was reduced with different local distribu-tion of microbials on the pattern membrane surface. Most of theprevious studies focused on the flow around the spacer or flow insimple geometries, but they are not appropriate to explain theeffect of the pattern on the membrane surface because the flowpattern is strongly dependent on structural characteristics of theflow domain. Furthermore, they did not consider flow character-istics such as local shear stress distribution and vortex formation.In this sense, this report may be the first to relate heterogeneousfouling which was experimentally observed on the patternedmembrane surface with local flow characteristics which wasstudied by numerical simulation. The paper is organized asfollows. The experimental setup and membrane characteristicsare introduced in Section 2, and the CFD outline is brieflydescribed in Section 3. The experimental results to which oursimulation is targeted are introduced in Section 4.1 and thesimulation results are provided in Section 4.2, where the devel-opment of vortices near the pattern and the local stress distribu-tion is discussed as an origin to the reduction of membranefouling. Finally, conclusions will be drawn in Section 5.

Fig. 2. Schematic diagram of the crossflow microfiltration (MF) system. The

crossflow system was operated by the total recycle mode and linear velocity of

the activated sludge was controlled by a valve.

2. Experimental

2.1. Membrane preparation

In this study, we adopted novel fabrication technique toprepare the patterned membrane surface [15]. Using a soft-lithographic technique, the prism pattern could be fabricated. Inorder to relieve the dense layer on the opposite side of thepatterned surface, a new modified phase inversion process wasalso applied. A detail preparation process is described as follows:PVDF (poly(vinylidene fluoride)) was dissolved in DMF (dimethyl-formamide) at 60 1C for 6 h, and acetone was then added at roomtemperature. The mixed solution of PVDF, DMF and acetone wasstirred overnight for complete mixing. After that, the solution waspoured on the patterned PDMS (poly(dimethyl siloxane)) replicamold and uniformly doped with the casting knife. Immediately

Fig. 1. Cross-sectional SEM images of patterned membrane and flat sheet membrane: (

immersion precipitation process with the same polymer solution (solution 3; PVDF 1.5

after the casting, the fabric was placed on top of the castingsolution (the nascent membrane). The nascent membrane,together with the fabric and PDMS replica, was dipped into theprecipitation bath and coagulated. The patterned membrane withthe fabric as a support layer was then released from the PDMSreplica mold and stored in a bath filled with de-ionized water. Theflat membrane which was prepared by the same method wasused as a control group.

2.2. Characteristics of membrane

Fig. 1 shows the cross-sectional SEM images of patternedmembrane and flat sheet membrane [15]. The characteristics arelisted in Table 1. The pore size of the membrane was measured bycapillary flow porometer (CFP-1500AE, PMI, USA). The topographyof the membrane was observed by a Scanning Electron Microscope(SEM) (JSM-6701F, Jeol, USA). The pure water flux of the mem-brane was measured by MF cross flow system and schematicdiagram of the system was illustrated in Fig. 2. The fouling test wasperformed using the same crossflow microfiltration (MF) which is

a) prism type and (b) flat sheet type. All membranes were prepared by a modified

g DMF 4.5 g Acetone 4 g) [15].

Fig. 3. The geometry and mesh configuration: (a) plane (number of elements:

6154) and (b) prism (number of elements: 19,459). Arrows denote the velocity

profile at the inlet boundary.

Y.K. Lee et al. / Journal of Membrane Science 427 (2013) 320–325322

shown in Fig. 2. The crossflow MF (microfiltration) system consistsof two cross flow test cells; pressure gauge and pump. Thedimension of the test cell was: width, 22 mm; channel length,64 mm; channel height, 2 mm. The effective surface area of themembrane was 14.1 cm2. Activated sludge was taken from a wastewater treatment plant (Sihwa, Korea). The crossflow MF systemwas operated under the total recycle mode and the workingvolume of activated sludge was maintained as 2 L. After runningthe crossflow MF system for 2 h, the membrane was released fromthe module and stained and dyed for observing by CLSM. For CLSMobservation, the membrane was stained with 100 mL concanavalinA (ConA, 1 mg/mL, Molecular Probes, Eugene, US; ex¼568 nm;em¼600/50 nm) conjugated to tetramethylrhodamine isothiocya-nate (TRITC). The microbials were stained by same amount of SYTO9 (Molecular Probes, Eugene, US; excitation (ex)¼488 nm; emis-sion (em)¼515/30 nm) for 20 min in the dark. Acquired imagesusing the CLSM were visualized and reconstructed to the3D view by the IMARIS software (ver. 4.1.3, Bitplane AG, Zurich,Switzerland).

Fig. 4. Mesh convergence: overall wall shear stress vs. maximum velocity.

3. Numerical method

3.1. Governing equations

2D simulation was performed with the steady state incom-pressible Newtonian fluid. The governing equations are theNavier–stokes equation, Eq. (1) and continuity equation, Eq. (2).

r urð Þu¼r½�pIþZ ruþ ruð ÞT

� �� ð1Þ

ru¼ 0 ð2Þ

where u is the velocity vector of the fluid, p is the pressure, r isthe density of the fluid, and Z is the viscosity of the fluid. Eventhough the sludge is a complex fluid containing poly-dispersedsolid components, we assumed it to be a Newtonian fluid to simplifythe calculation (density: 1000 kg/m3, viscosity: 0.001 kg/m/s). Thefinite element method (FEM) was employed to discretize the govern-ing equation, and commercial software COMSOL Multiphysics (COM-SOL 3.2, Comsol Inc., USA) was used for the calculation.

3.2. Geometry and boundary conditions

Flat and prism patterned geometry were used in this study. Thegeometry was setup as similar in size with the membrane inexperiments. The overall domain size was set as 6000 mm�2000 mm(width�height) for both geometries, and the prism pattern was setas 400 mm�200 mm (width�height). The mesh consisted of trian-gular elements as shown in Fig. 3. We tried to find optimized numberof elements (NOE) by checking the mesh convergence. We comparedthe wall shear stress as defined by Eq. (5) for various NOE. 3344,6154, and 14,170 elements were tested for the flat geometry, and8624, 19,459, and 66,602 elements were tested for the prism.As shown in Fig. 4, mesh dependency was not strong for the overallwall shear stress. Though the details of the velocity profile mayrequire fine mesh, it seems to be adequate enough to use relativelycoarse mesh (except close to the pattern) for our purposes. Wedetermined the optimized NOE as 6154 for the flat geometry and19,459 for the prism patterned geometry.

The inlet boundary condition was imposed in the form ofEq. (3) under the fully developed flow assumption, where D is theheight of the channel, y is the coordinate, and umax is themaximum inlet velocity. In this study, the flow condition waslimited to the laminar region. Maximum inlet velocities were0.01, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 m/s, which correspond to theReynolds number (Eq. (4)) 50, 100, 200, 400, 600, 800, and 1000,

respectively. They were all in the laminar flow region.

uxðyÞ ¼ 4umaxy D�yð Þ=D2ð3Þ

Re¼rumaxD

Zð4Þ

At the outlet of the channel, the Dirichlet type boundarycondition (p¼ 0) was imposed. At the upper wall of the channeland membrane surface, no-slip boundary condition (u¼ 0) wasadopted under the assumption of little permeate flux.

4. Results and discussion

4.1. Results of crossflow MF system with activated sludge

The confocal microscopy images of the membrane surface takenfrom the experiments are shown in Fig. 5, in which green pixeldenotes attached microbials and red pixel denotes membrane sur-face. As shown in Fig. 5(a), the membrane surface was fully coveredwith microbials. It means that biofouling occurred all over the flatmembrane surface. On the other hand, green pixels were dramati-cally reduced on the prism patterned membrane surface as shown inFig. 5(b). In particular, the deposition of microbials was mostlylocalized to the valley region between the prism patterns. In thiscase, even though biofouling was not removed completely and stilloccurred in some areas, it might be helpful to extend the lifespan ofthe membrane. We expect this observation is strongly related withthe flow around the membrane surface, so we adopted the CFD

Y.K. Lee et al. / Journal of Membrane Science 427 (2013) 320–325 323

technique to explain why there was less microbials on the apex ofprism-patterned membrane surface.

4.2. Flow characteristics near the membrane surface

We performed CFD analysis to investigate the flow character-istics for two geometries; flat and patterned membranes. First, wefocused on the flow separation around the prism pattern. In Fig. 6,

Fig. 5. Confocal microscopy images of membrane surfaces (red) and microbials

(green) in experiment: (a) flat membrane and (b) prism patterned membrane.

Microbials were observed less on the upper region of the prism pattern.

(For interpretation of the references to color in this figure legend, the reader is

referred to the web version of this article.)

Fig. 6. Streamlines in the region of x-coordinate from 1800 mm to 3000 mm. (a) umax¼

and (f) umax¼0.5 m/s. Vortex region was expanded with the increase of maximum inle

the streamlines are shown in the patterned membrane (in theregion of x-coordinate from 1800 mm to 3000 mm). As shown inthe figure, the vortex was observed in valley area between the prismpatterns, and it’s streaming to the clockwise direction. The vortexsize was different depending on the inlet velocity. At low inletvelocity, the vortex development was limited to the lower regionof the valley, while it was expanded to the upper region with theincrease of inlet velocity; that is, the range of flow separation isdetermined by the inlet velocity. The inlet velocity also affectsvelocity profile in vortex region. As shown in Fig. 7, the verticalvelocity at the height of 100 mm from bottom shows a periodicalfluctuation with positive and negative values, it means that thevelocity profile in vortex region is strongly dependent on theinlet velocity as much as vortex development itself. The minimumvelocity in the vortex was as small as 0.002% of the main streamvelocity just outside of the vortex when umax¼0.1 m/s, so theflow in the vortex is almost stagnant. This flow characteristic hasa unique effect on fouling. It is a good condition for floatingparticles in the sludge to form aggregates. As the biological solidingredients are attractive in nature, they can easily form largeaggregates in this flow environment. This seems to be one of thereasons why the fouling was dominant in the valley region inFig. 5. The flow separation did not appear in the laminar flow witha flat geometry. So it is clear that the dramatic change in the localflow characteristics was induced by the patterned geometry.

In addition to flow separation, it is also important to considerthe shear stress because it can either disturb the attachment ofparticles on the surface or facilitate detachment of the particlesfrom the surface. For this purpose, we plotted the contour of shear

0.01 m/s, (b) umax¼0.1 m/s, (c) umax¼0.2 m/s, (d) umax¼0.3 m/s, (e) umax¼0.4 m/s

t velocity.

Fig. 7. Vertical velocity at y¼100 mm (the height of the prism pattern¼200 mm).

Fig. 9. Overall wall shear stress on the membrane surface. The diamond denotes

the flat membrane and the circle denotes the prism patterned membrane. A higher

wall shear stress was measured for the flat membrane.

Y.K. Lee et al. / Journal of Membrane Science 427 (2013) 320–325324

stress which gives information on the distribution of local shearstresses. As shown in Fig. 8, the shear stress was uniformlydistributed on the flat membrane; higher stress on the wall andlower stress at the center was observed. On the other hand, theshear stress was non-uniform around the prism, and very highshear stress developed near the top of the prism pattern. Thestress was very low not only at the center but also in the valleyregion with the stagnant flow. Low shear stress was measured inthe vortex region, while high shear stress was measured in theouter region of the vortex. Based on these results, we estimatedthe importance of flow characteristics in the local region, andquantitatively analyzed it near the membrane wall by measuringthe overall and local wall shear stresses.

4.3. Overall wall shear stress on the membrane surface

We examined the overall wall shear stress for both the flat andprism patterned membranes. The overall wall shear stress wasdefined as a line integral, Eq. (5) over the length of the membranesurface.

Overall wall shear stress¼1

Loverall

Zoverall

sxydx

Loverall : overall length of the membrane surfaceð Þ ð5Þ

As shown in Fig. 9, the overall shear stress increased with themaximum velocity for both the flat and prism patterned geome-tries, and the overall wall shear stress in the flat case was higherthan the case of the prism patterned membrane. At first sight, itappeared that the prism pattern was not efficient for the preven-tion of fouling. However, the attachment of microbials waslocalized to the patterned surface in experiments. In particular,most of the microbials accumulated in the lower part of the valleybetween prism patterns. Shear stress followed a similar pattern asshown in Fig. 8; low shear stress in the lower part of the valley. Totake this localization into account, we measured the local wallshear stress on the patterned membrane surface.

4.4. Local wall shear stress on the membrane surface

Before we measure the local wall shear stress, the valley areawas divided into two regions depending on the height in theprism patterned membrane. As shown in Fig. 10, the region above

Fig. 8. Contour of shear stress for umax¼0.1 m/s: (a) flat membrane and (b) prism

patterned membrane.

a height of 180 mm was defined as upper region, and theremaining part was defined as lower region. Local wall shearstress was calculated in each region by a line integral:

Local wall shear stress¼1

Llocal

Zlocal

sxydx

Llocal : local length of the membrane surfaceð Þ ð6Þ

In Fig. 11, the local wall shear stresses in each region arecompared with that of flat membrane. The stress increasedlinearly with the inlet velocity. As shown in the figure, the localwall shear stress was higher in the upper region of the prism thaneither the lower region or flat membrane, and the local wall shearstress of the flat membrane was higher than that in the lowerregion of the prism patterned membrane. The overall wall shearstress discussed in Section 4.2 could not explain the localdistribution of microbials on the membrane surface as shown inFig. 5. On the other hand, the local wall shear stress matched theexperimental results fairly well: microbials were distributedmainly in the lower region of the pattern, while it was notattached to the upper region. It means that the local flowcharacteristics are important in determining the attachment ofthe sludges to the membrane surface. This result is in accordancewith the previous report in which the sludge or the particles weredetached from the membrane surface with an increase in wallshear stress [14], but their results were limited to the flatgeometry and they did not consider the effect of geometry. Fromthese observations, we can estimate that the local reduction ofmembrane fouling was induced by the local distribution of shearstresses on the membrane surface; high shear stress on the upperregion of the prism was particularly effective. It means that thepattern should be designed efficiently by considering the localflow and stress distribution in the cross flow membrane system.

In experiments, mass transfer induced by the flow separationaround the pattern also can be an important factor in reducingmembrane fouling. Flow separation disturbs the import of micro-bials into the valley on one hand, and it prevents the export of theremaining microbials out of the valley on the other. Even thoughthe balance may be sensitive, there may exist a critical flowcondition that divides the attachment and detachment of micro-bials on the patterned surface. It will be closely related with theinteractions between sludge components and between microbialand membrane surface. In this study, we did not considerindividual solid components and their interactions, but assumedthe liquid as a Newtonian fluid. The relationship between flowand mass transfer in the patterned membrane system will be

Fig. 10. Schematic diagram of analyzed regions on prism pattern surface.

Fig. 11. Local wall shear stress on the membrane surface as a function of

maximum inlet velocity. The circle denotes the results of the upper region in

the prism patterned membrane, the square denotes the lower region of prism

patterned membrane, and the diamond denotes the flat membrane. Higher local

wall shear stress was developed on the upper region of the prism patterned wall in

comparison to the flat wall.

Y.K. Lee et al. / Journal of Membrane Science 427 (2013) 320–325 325

studied as a next step with a new algorithm that includesparticles with different interactions.

5. Conclusions

We performed both experimental and CFD studies to explain theexperimental observation that the attachment of microbials wasreduced on the patterned membrane surface. A vortex was observedin the lower part of the valley around the patterned membrane, inwhich the aggregates of solid components in the sludge could beeasily formed, leading to favorable fouling formation. It was alsoobserved that the local wall shear stress was higher in the upperregion of the pattern and lower in the lower region where the vortexwas formed. This result (vortex formation in the lower region of thepattern and higher stress in the upper region) was in accordancewith the experimental observations on the distribution of microbialsin the patterned membrane system (more fouling in the lower partof the pattern, and less fouling in the upper region). From this study,we confirmed the importance of flow characteristics as well as thestress distribution to reduce the fouling in the patterned membrane

system. In addition, it should be noted that the pattern should bedesigned efficiently by considering the local flow and stress dis-tribution in the cross flow membrane system.

Acknowledgment

This work was supported by the National Research Foundationof Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0029083).

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