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Tubular Membrane Filtration with A Side Stream and itsIntermittent Backwash OperationRome-Ming Wu a b & Yu-Ju Lin aa Department of Chemical and Materials Engineering , Tamkang University , New Taipei City ,Taiwanb Energy and Opto-Electronic Materials Research Center , Tamkang University , New TaipeiCity , TaiwanAccepted author version posted online: 25 Apr 2012.Published online: 07 Aug 2012.

To cite this article: Rome-Ming Wu & Yu-Ju Lin (2012) Tubular Membrane Filtration with A Side Stream and its IntermittentBackwash Operation, Separation Science and Technology, 47:12, 1689-1697, DOI: 10.1080/01496395.2012.659534

To link to this article: http://dx.doi.org/10.1080/01496395.2012.659534

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Tubular Membrane Filtration with A Side Stream and itsIntermittent Backwash Operation

Rome-Ming Wu1,2 and Yu-Ju Lin11Department of Chemical and Materials Engineering, Tamkang University, New Taipei City, Taiwan2Energy and Opto-Electronic Materials Research Center, Tamkang University, New Taipei City,Taiwan

The tubular membrane filtration system is widely applied to solid-liquid separation processes. Any improvements to the filtrationmodule would increase separation efficiency, thus reducing operatingcosts. In this experiment, PMMA powder with an average particlediameter of 0.8lm was filtered by a ceramic tubular membrane withan average pore size of 0.2lm, and the impacts of the operating vari-ables, such as suspension concentration, the filtration pressure, andthe crossflow velocity on the permeate flux were discussed. In orderto understand the increased permeate flux, the proposed module iscomparable to the tubular membrane filtration module, but with anadditional side stream under the same filtration mass flow rate. Inaddition, variations of shear force on the membrane surface are ana-lyzed by CFD simulation, and the influence of backwash operationson the permeate flux is discussed. The results show that the sidestream membrane filtration increased the shear force on the mem-brane surface, reduced fouling on the membrane surface, andincreased the permeate flux. Furthermore, a backwash operation witha side stream flow channel could effectively clean the particlesdeposited in the module, thus, increasing the permeate flux.

Keywords backwash; CFD; filtration; membrane; side stream

INTRODUCTION

Membrane filtration is widely used in many industries forthe separation of fine particles from liquids (1–3). In mem-brane filtration, the resistance derived from particle depo-sition has a significant impact on the effect of filtration,therefore, methods to slow the generation of filtration cakesand reduce filtration resistance to effectively improve thepermeate flux is an important topic in membrane filtration.The main causes for the rise of membrane resistance are theconcentration polarization and fouling (4). Several authorsstudied crossflow membrane ultrafiltration in terms of foul-ing resistances (5–7). Ahn et al. (8) found that concentrationpolarization could be reduced by reducing the pH value.Schwingea et al. (9) created turbulent flows at different

crossflow velocities, which assists in reducing concentrationpolarization and saves 60% of the costs. Field et al. (10)indicated that the fouling mechanism varied according tothe different species in the suspension: for larger particles,as long as the permeate flux was maintained below a certainlimit, then fouling would not occur. Since concentrationpolarization and fouling directly influence the effects offiltration, many improvement methods have been proposedto overcoming these problems. Consequently, many scho-lars have researched and analyzed changes in fluid dynamics(11–15).

In addition to fluid dynamic state, the influence of oper-ating variables on the permeate flux have been discussed bymany scholars. Xu et al. (16) experimented with aluminumparticles and showed that when the ratio of particle size tomembrane pore size was less than 2.4, the permeate fluxincreased linearly when the crossflow velocity increased.The permeate flux can also be increased by an intermittentbackwash. Ma et al. (17) combined an impulse reverse washwith a surface modification, and obtained a better permeateflux at a low suspension concentration within a shortperiod. Kuberkar and Davis (18) compared the crossflowfiltration with backwash, results indicated that crossflowfiltration could reduce the production rate of an externalcake, and reverse wash had better treatment effects oninternal fouling.

In recent years of improved computer calculationprogresses, numerous models have been suggested withnumerical solutions to predict the impact of changes onthe operational factors of permeate flux. Lagana et al.(19) developed a model tubular membrane, with a relativelylarge fiber diameter, to evaluate membrane morphology,such as the thickness, the elastic modules, and the poreradius distribution on permeate flux. Kim and Zydney(20) treated the membrane as a uniformly porous boundaryand predicted particle trajectories. A tool Membrane Foul-ing Simulator (MFS) was developed by Vrouwenvelder et al.(21) for the validation of membrane fouling, while otherinvestigators showed that computational fluid dynamicshas been successfully applied on several processes, such as

Received 31 July 2011; accepted 17 January 2012.Address correspondence to Rome-Ming Wu, Department

of Chemical and Materials Engineering, Tamkang University,New Taipei City, Taiwan. Tel.: 886-2-2621-5656; Fax: 886-2-2620-9887. E-mail: romeman@mail.tku.edu.tw

Separation Science and Technology, 47: 1689–1697, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0149-6395 print=1520-5754 online

DOI: 10.1080/01496395.2012.659534

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microfiltration (22–24), porous medium (25), and hydro-cyclone separation (26–27).

This study uses different crossflow velocities (mass flowrates) and transmembrane pressure (TMP) for tubular mem-brane filtration experiments, then discusses the declined per-meate flux, as well as changes in filter membrane resistanceunder different operating conditions. An additional sidestream is introduced into the tubular membrane filtrationmodule in order to increase shear force on the membranesurface. The increased permeate flux by backwash opera-tions, both with and without a side stream flow channel,are observed. According to the experimental parameters offiltration=backwash, CFD is used in simulation to calculatethe shear force for the membrane surface. The numericalsolutions are carried out using commercial CFD codeFLUENT 6.1.

MATERIALS AND EXPERIMENTS

Particles and Tubular Membrane

The polymethylmethacrylate (PMMA) spherical parti-cles manufactured by the Soken Chemical & EngineeringCo. of Japan were used as fine particulate samples in experi-ments, which have a narrow size distribution and had amean diameter of 0.8 mm. The true density of the particleswas 1190 kg=m3, and when dispersed in de-ionized watertheir zeta potential was -32mV. The particle coagulationin the suspension could be ignored due to a high zeta poten-tial. The pH value of the suspension was maintained at 7 inthis study.

The suspension concentrations and pressure drops are0.1, 0.3, and 0.5wt%, and 25, 50, and 75 kPa, respectively.The inlet velocities are 0.1, 0.2, and 0.4m=s correspondingto mass flow rates of 8.5, 17, and 34 g=s in this study, result-ing in Reynolds numbers of about 360� 1440. The ceramicmembrane material was zirconia, with average pore size of0.2 mm, length of 0.6m, and a total surface area of 0.011m2.

Basic Structure of Tubular Membrane Filtration

The tubular membrane filtration unit was employed inthe experiment adopted for outside-in filter type. Theschematic diagram of this unit is as shown in Fig. 1a. Theinnermost layer of the thimble is the tube flow with a0.6 cm diameter; the outermost layer is the shell flow witha 1.4 cm diameter, the tubular membrane is 0.2 cm thick;and the overall length is 60 cm. Figure 1b shows theschematic diagram of the tubular membrane filtration witha side stream, where the only difference from Fig. 1a is thatthe 0.6 cm diameter circular tube side stream inlet is made atthe shell side, which is 15 cm from the feed side.

Figure 2 is the flow chart of the filtration experiment, inwhich a transmission pump (PP-1) (Masterflex1 model7518-00) was switched on, and the suspension fluid wasconveyed to the filtration chamber through the circulation

system. The feed rate of the module was controlled byadjusting the rotation rate of the pump (PP-1). The concen-trated solution flew into the storage tank (B-1) through aneedle valve (V-2) and a pressure gauge (P-2); the needlevalve controls the filtration pressure difference. The filtratedefecated by a filter membrane of a filter cell was collectedby a receiver (B-2), and was then automatically weighed andrecorded by an electronic balance (W-1) connected to thePC (C-1), and repeats this process until the permeate fluxbecomes stabilized.

Tubular Membrane Filtration with AdditionalSide Stream

The operational flow of the proposed model approxi-mates that of a tubular membrane filtration system, and

FIG. 1. Tubular membrane filtration unit (a) filtration (b) filtration with

a side stream. (Color figure available online)

FIG. 2. Experimental setup of the membrane separator system. (Color

figure available online)

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differs only in that the rotation rate of the pump (PP-1)must be adjusted in order to control the feed rate. Mean-while the needle valves, (V-2) and (V-3), should be adjustedin order to control the ratio of the main stream flow to theside stream flow of the filter cell. The feed concentration inthe experiment is set as 0.3wt%, the operating pressuredifference is 50 kPa, and the ratio of main stream (MS) toside stream (SS) is as shown in Table 1:

Tubular Membrane Filtration of Intermittent BackwashOperation (Case-1)

The tubular membrane filtration of the intermittentbackwash operation (Case-1) is constructed by adding atiming controller (CON-1) into the basic structure of theoriginal tubular membrane filtration system, which is addedin order to control the electromagnetic valve (CV-1 & CV-2)(Fig. 3), allowing filtration to be temporarily paused and theclean water to flow back into the membrane tube for back-wash operations. Figure 4a shows the structural diagram ofthe backwash module (Case-1), where this backwash modehas the fresh water entering from the filtrate side, passesthrough a thin film by pressure difference, washes off theparticles deposited on the membrane surface, which arethen removed through the dense discharge end. The oper-ational processes are similar to those of tubular membranefiltration, with the exception that timing control must be setprior to the experiment, which in this experiment is set asfollows, a backwash procedure of 30 s occurs after each

600 s of filtration, and repeats this operation until aconstant permeate flux is reached.

Basic Structure of Tubular Membrane Filtration ofIntermittent Backwash Operation (Case-2)

The structure of the tubular membrane filtration of anintermittent backwash operation (Case-2) is almost ident-ical to that of a tubular membrane filtration of intermittentbackwash operation (Case-1); the only difference is that(Case-2) adopts the shell of a tubular membrane filtrationwith an additional side stream, meaning there is anadditional side wash outlet at 15 cm of shell side (Fig. 4b).The clean water enters from the filtrate side, passes througha thin film by pressure difference washes off the particlesdeposited on the membrane surface, which are thenremoved through the dense discharge end and the sidestream.

As for the operational process, the timing controllermust be set prior to the experiment, and the needle valve(V-2) must be turned on regularly during the experimentin order to discharge the backwashed suspension solutionthrough the wash side outlet. The timing controller of thisexperiment is set as follows, backwash procedures for 30 safter 600 s of filtration, repeat the operation; turn on theneedle valve for the backwash operation, and turn off theneedle valve for filtration.

NUMERICAL METHODS

Geometry and Mesh

The governing equations andmeshes of the tubular mem-brane system used for CFD calculations in this study is pre-sented in Fig. 5. Grid independence studies are conductedwith mesh sizes of 100,000, 200,000, 280,000, 380,000, and500,000. It is observed that the obtained numerical resultsbecame independent of the total number of computationalcells beyond 280,000. During the remainder of this study,

TABLE 1Mass flow rate of influent stream

Filtration Type MS SS

Single inlet filtration 17 g=s NAFiltration with side stream (two inlets) 8.5 g=s 8.5 g=sFiltration with side stream (two inlets) 17 g=s 8.5 g=sFiltration with side stream (two inlets) 17 g=s 17 g=sSingle inlet filtration 34 g=s NA

FIG. 3. Experimental setup of the intermittent backwash system. (Color

figure available online)

FIG. 4. Flow direction during filtration=backwash (a) Case-1 (b) Case-2.

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the total number of computational cells used to discretizethe entire geometry is approximately 280,000, which isdeemed optimal for reasonably accurate numerical simula-tion results that consider the computational time required.The numbers of mesh grid of the innermost filter cell, theintermediate membrane, and the outermost flow field withinthe thimble are 26,000, 127,000, and 127,000, respectively.Simulations were carried out for approximately 20,000times, in which the pressure and momentum equations aresolved with tolerance residuals of 10�4.

Governing Equation and Boundary Conditions

For Newtonian fluid flow in the flow channel, the gov-erning equation is the steady-state Navier-Stokes equation,which can be stated as follows:

~uu�f � r� �

~uu�f þ EurP� ¼ 1

Rer2~uu�f ð1Þ

where ~uuf denotes the fluid velocity, Eu is Euler number(Eu¼Po=qV

2), and the first and the second terms of theleft-hand-side of Eq. (1) correspond to the inertial andpressure effect, respectively; while the right-hand-side,denotes the viscous effect.

The governing equation for the fluid velocity ~uup withinthe porous membrane, and taking into account the viscouseffect, is stated as follows:

Re~uu�2p ¼ �b2~uu�p � EuRerP� ð2Þ

where b is dimensionless permeability (b¼ df=2k0.5). The

two terms in the right-hand-side of Eq. (2) are Darcy’slaws. The left-hand-side is attributed to the momentumsink, which is a modification version of Darcy’s law.

The boundary conditions are as follows:

~uuf ¼ v; @ inletðsÞ ð3aÞ

~uuf ¼ vrecycle; @ reject outlet ð3bÞ

P ¼ 0; @ permeate outlet ð3cÞ

~uup ¼~uuf ; @ membrane0s surface ð3dÞ

r~uup ¼ r~uuf ; @ membrane0s surface ð3eÞ

Equation (3a) states that the cross-flow velocity (mainstream and side stream) moves at a constant speed, whileEq. (3b) states that according to experiments, the recyclevelocity is vrecycle. Equation (3c) describes the outlet press-ure as atmospheric pressure. Equations (3d) and (3e) arethe continuation conditions of fluid velocity and shearstress across the surface of the membrane.

The computational fluid dynamics program FLUENT6.1 (Fluent Inc., USA) solved the governing Eqs. (1)and (2) together with the associated boundary conditionsEqs. (3a)-(3e), using constructed mesh volumes.

RESULTS AND DISCUSSIONS

Permeate Flux and Overall Resistance

The experimental results of Fig. 6 show permeate fluxversus time. The operational parameters, including 3

FIG. 5. Governing equations and meshes of the tubular membrane.

(Color figure available online)

FIG. 6. Plots of filtrate q versus time t (a) DP¼ 50 kPa, V¼ 0.2m=s

(b) C¼ 0.3wt%, V¼ 0.2m=s (c) DP¼ 50 kPa, C¼ 0.3wt%. (Color figure

available online)

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different TMPs of 25, 50, and 75 kPa; 3 different suspensionconcentrations of 0.1, 0.2, and 0.3wt%; and 3 differentcross-flow velocities of 0.1, 0.2, and 0.4m=s. The permeateflux decays quickly at approximately 2000s and approachesa constant value. From Fig. 6a, at a TMP of 50 kPa and across-flow velocity of 0.2m=s, the lower the suspension con-centration, the less particles deposited on the membranesurface, and hence, the higher the constant permeate flux.From Fig. 6b, at a cross-flow velocity of 0.2m=s and a sus-pension concentration of 0.3wt%, owing to a driving forcetoward the membrane surface, the higher the pressure dropthe higher the constant permeate flux. From Fig. 6c, at apressure drop of 50 kPa and a suspension concentrationof 0.3wt%, the higher the cross-flow velocity, the higherthe constant permeate flux. The high cross-flow velocityoffers high hydrodynamic shear force, which retards thefouling rate of the membrane surface. As a consequence,an increase of cross-flow velocity has a marked beneficialeffect on reducing fouling (28). A mix of a high cross-flowvelocity, a high pressure drop, and a low particle concen-tration will lead to a high permeate flux.

In order to elaborate upon the relations betweenoperational parameters and overall filtration resistance,FLUENT is used to simulate this cross-flow filtrationsystem. Figure 7 depicts the overall resistance duringfiltration, and corresponds to Fig. 6. The lines and symbolsdenote results from CFD and experiments, respectively.There is a reasonable coincidence between them. Theoverall resistance Rt is as shown in Fig. 7, as the followingequation:

q ¼ DPlðRc þ Rp þ Rif þ RmÞ

ð4Þ

where Rc denotes cake resistance, Rp denotes concentrationpolarization layer resistance, Rif denotes membrane internalfouling resistance, and Rm denotes clean membrane resist-ance. Note that in Fig. 7 the above four kinds of resistancesis the overall resistance, that is, Roverall¼RcþRpþRifþRm,and Rm is equal to Roverall at t¼ 0, which is the starting pointof filtration. A highly concentrated suspension means ahigher overall resistance because of the greater numbers ofparticles being deposited as a cake (Fig. 7a). In Fig. 7b itcan be observed that the membrane fouling resistance, Rf,increases with TMP for a constant feed rate. Similar resultswere reported by Kwon et al. (29). Pressure effects are evi-dent between 50 to 75kPa as the overall resistance increasesfrom 18.3 to 25.1 (�1011m�1), which is almost a 37%increase (Fig. 7b). Owing to the high shear force inducedby the high cross-flow velocity, the overall resistance is low(Fig. 7c). The total filtration resistance increases as the sus-pension concentration and the filtration pressure differenceincreases; and the resistance decreases as the crossflowvelocity increases.

Filtration with Side Stream

Figure 8 shows the permeate flux q with=without aside-stream filtration relative to time t. Again, F representsthe mass flow rate of the influent stream of traditional fil-tration, and SS represents the mass flow rate of the sidestream. In this figure, the solid symbol represents the changechart of the permeate flux for traditional filtration (1 inletfiltration), while the open symbol represents the changechart of permeate flux of filtration with side stream (2 inletsfiltration). The points of steady permeate flux are comparedas follows. In the filtration of suspension at 17 g=s, thesteady permeate flux of the filtration with a side stream is1.63 (open circle), which is increased by 23% as comparedwith the steady permeate flux of 1.32 (solid circle) of tra-ditional filtration. In the filtration of suspension at 34 g=s,the steady permeate flux of the filtration with a side streamis 2.77 (open square), which is increased by 33% as com-pared with the steady permeate flux of 2.09 (solid square)of traditional filtration. In addition, the filtration effect ofmain stream 17 g=s with a side stream 8.5 g=s is similar tothat of 34 g=s (solid square), which indicates filtration witha side stream has better effect than traditional filtration.

FIG. 7. Plots of overall resistance Rt versus time t (a) DP¼ 50 kPa,

V¼ 0.2m=s (b) C¼ 0.3wt%, V¼ 0.2m=s (c) DP¼ 50 kPa, C¼ 0.3wt%.

(Color figure available online)

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Figure 9 depicts changes in total resistance Rt, as well astime under pressure of 50 kPa, suspension concentration of0.3wt%, and different side stream mass flow. It shows thatresistance is reduced under the same filtration load, whenthere is a side stream. More worthy of note are the solid=open square data. The total flow rate is 34 g=s in bothcases; however, if the total flow rate is split into a mainstream of 17 g=s and a side stream of 17 g=s, then totalresistance is decreased by approximately 20% under asteady filtrate flux. This demonstrates that the appropriateintroduction of a side stream can help to increase filtrateflux, providing the same pump acts as the driving forcefor conveying the suspension fluid.

Simulation of Shear Force on Membrane Surface

As mentioned, shear force is an important factor affect-ing particles deposited on a membrane surface. Figure 10 isa diagram depicting the shear force on a membrane surfacealong the flow direction obtained through CFD calcula-tions. Again solid symbols depict shear force on a mem-brane surface with main stream flow rates of 8.5, 17, and34 g=s, respectively, but with no side stream. The total fil-tration channel is 0.6m. Due to the end effect, only the forceimpacts of 0.03–0.57m are shown. With a main stream flowrate of 17 g=s as an example, the 0.00012N at 0.03m isquickly reduced to 0.00006N at 0.01m; thereafter, the shearforce values of the membrane are fixed, meaning that, withmain stream filtration, the shear force experienced by thefront end is greater; and thereafter is maintained at a fixedvalue. When the flow rate is 34 g=s (solid square), the shearforce values of all points are all greater than the value at17 g=s; when the mass flow rate is at 17 g=s, the shear forcevalues of almost all points are greater than the value at8.5 g=s (solid triangle). Therefore, high crossflow velocity(mass flow rate) results in larger shear force than low cross-flow velocity. As a result, less particles are likely to depositon the membrane surface and generate higher permeate fluxvalues under high cross-flow velocity.

This study introduces a side stream at 0.15m flow direc-tion, as seen in Fig. 10, which shows that when the sidestream enters the flow channel, the shear force experiencedby the membrane surface increases, and aids in slowing theformation of filter cakes. For example, filtration with a sidestream increases about 12% of the shear force on the mem-brane surface at the same handling capacity of 34 g=s.According to the aforesaid experimental results, a tubularmembrane filtration system with side stream produces abetter effect because the shear force of the membrane

FIG. 9. Comparisons of overall resistance Rt versus time t between

filtration and filtration with side stream at DP¼ 50 kPa and C¼ 0.3wt%.

(Color figure available online)

FIG. 10. Comparisons of calculated shear force versus flow direction

between filtration and filtration with side stream at DP¼ 50 kPa and

C¼ 0.3wt%. (Color figure available online)

FIG. 8. Comparisons of filtrate q versus time t between filtration and

filtration with side stream at DP¼ 50 kPa and C¼ 0.3wt%. (Color figure

available online)

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surface increases accordingly. Hence, the particles areunlikely to deposit on the membrane surface, and the per-meate flux value increases. The above results show that,with identical energy loss (with a single pump as the drivingforce source), the introduction of a side stream can effec-tively increase membrane shear force (22), slowing theformation of filter cakes and reducing the total filter resist-ance, and thus, increases filtrate flux.

Filtration with Backwash

This study also conducts two kinds of intermittent back-wash filtration systems, namely, in the traditional manner,30 s backwash after 600 s filtration, and repeat the operation,and this is Case-1. The other is a tubular membrane moduleshell with a side stream flow channel, hence, one inlet andtwo outlets during backwash. The operation is repeatedfor 30 s backwash after 600 s filtration, this is Case-2.

Figure 11 shows the time-varying permeate flux at afiltration pressure difference of 50 kPa, suspension concen-tration of 0.3wt%, and influent stream of 17 g=s. Accordingto the figure, simple filtration has the lowest steady per-meate flux of about 1.32. The steady permeate flux of a fil-tration system with side stream is 1.63, which is increased byabout 23%. The intermittent filtration Case-1 has highersteady permeate flux of about 2.40. The intermittent fil-tration of a side stream inlet as the second outlet for back-wash (Case-2) has the highest steady permeate flux of about3.06. According to Fig. 11, an additional side stream inletlocated outside the tubular membrane module can increasethe permeate flux during filtration and remove particlesmore efficiently during backwash.

Figure 12 shows the time-varying membrane resistancecorresponding to Fig. 11, coincident with the aforesaid

results, simple filtration has the largest resistance, second isthe filtration with the side stream; while the intermittent back-wash filtration (Case-1) has less resistance, and the intermit-tent backwash filtration (Case-2) has theminimum resistance.

Backwash Simulation Analysis in Multiphase Flow Mode

In the simulation, this study also used the Eulerianmultiphase flow model to simulate tubular membrane fil-tration and backwash. Fang and Wu (23) successfully usedthe Eulerian multiphase flow mode in simulating the move-ment of particles. There still remain some problems inapplications of Eulerian multiphase flow modes to mem-brane filtering systems, namely, the particles of the dis-persed phase are regarded as a continuous phase;moreover, the interaction force among particles is not dis-cussed, and the size distribution cannot be effectivelydescribed. This study merely uses multiphase flow to showmovements of particles so as to explain why the effect ofbackwash Case-2 is better than that of Case-1.

Figure 13a shows the variance in the volume fraction ofparticles on the membrane surface during 0–30 s of back-wash procedures (Case-1), the left color map representsthe volume fraction of particles. According to the figure,the volume fraction of particles on the membrane surfacedecreases as the backwash time increases, therefore, back-wash operations help improve filtration effects. Figure 13bshows the variance in the volume fraction of particles on themembrane surface during 0–30 s of a backwash procedure(Case-2), which according to the figure has the same resultas the backwash mode (Case-1).

According to the backwash process of Case-1, the par-ticle volume fraction of the lower part of the membranetube is obviously the largest part of the tube, therefore,

FIG. 11. Comparisons of filtrate q versus time t under different opera-

tions at DP¼ 50 kPa, C¼ 0.3wt%, and F¼ 17 g=s. (Color figure available

online)

FIG. 12. Comparisons of overall resistance Rt versus time t under

different operations at DP¼ 50 kPa, C¼ 0.3wt%, and F¼ 17 g=s. (Color

figure available online)

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particles in the lower part of the tube are unlikely to washoff, thus, the number of particles deposited influencefiltration. In order to clear up the particle deposition inthe lower part of membrane tube, the tubular membrane fil-tration module with a side stream outlet is applied for thebackwash mode (Case-2). The volume fraction distributionof particles on the membrane surface of Case-1, after 30 sbackwash, is compared with that of Case-2, see Fig. 14,which shows the comparison of the lower part of membranetube after 30 s backwash. It is obvious that after backwashusing Case-2, the volume fraction of particles on the mem-brane surface is less than that of Case-1. This is becausethere is an additional side outlet in the structure of Case-2,which discharges particles through the reject end andthrough the side stream inlet end in advance.

Efficiency Evaluation of Filtration Policy Change

Figure 10 shows that the two filter types proposed in thisstudy can increase the permeate flux value. Table 2 showsthe efficiency evaluation of a filtration policy change; wherethe total filtration yield of a tubular membrane filtration isQ1, and the efficiency is defined as 100%; the total filtrationyield of a tubular membrane filtration with a side stream isQ2, and the efficiency is defined as Q2=t

Q1=t; the total filtration

yield of a tubular membrane filtration with intermittentbackwash operation (Case-1) is Q3, and the efficiency isQ3

tþtb

�Q1

t ; the total filtration yield of a tubular membrane

filtration of an intermittent backwash operation (Case-2)

is Q4, and the efficiency is Q4

tþtb

�Q1

t . Among which, t is the

filtration time, and tb is the backwash hour. Therefore,disregarding the energy for the operation of the pump, thetubular membrane filtration of an intermittent backwashoperation (Case-2) is the best filter type.

CONCLUSIONS

This study used an additional side stream to change thefluid dynamics state inside the filtration chamber, so as toincrease the filtration rate. The advantage of the additionalside stream is that the stable filtration rate can be increasedby about 30% under the same pressure difference drivingforce. Another advantage is that the additional side stream

FIG. 13. Particle volume fraction on membrane surface during 1-30 s of

backwash (a) Case-1 (b) Case-2. (Color figure available online)

FIG. 14. Comparison of solid volume fraction at lower section of

membrane module at backwash 30 s between Case-1 and Case-2. (Color

figure available online)

TABLE 2Efficiency evaluation of filtration policy change

Filtration Type Efficiency (%)

Tubular membrane filtration 100Tubular membrane filtration withside stream

120

Intermittent backwash Case-1 124Intermittent backwash Case-2 132

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as original second inlet becomes the second outlet in thebackwash operation, so that the particles can be washedout of the filtration chamber effectively. The overallfiltration efficiency can be increased by 32% by usingadditional side stream and backwash operation.

Further work could investigate the optimization of theside stream flow rate, the number of side streams introduced,as well as the speed and angle of the introduced side streams.

ACKNOWLEDGEMENTS

This study has been supported by the National ScienceCouncil of the Republic of China, Taiwan, under ContractNo. NSC 97-2221-032-030-MY3.

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