Effect of algae on fouling and efficiency of UF membranes

Effect of algae on fouling and efficiency of UF membranes

DESALINATION Desalination 179 (2005) 203-214 ELSEVIER www.elsevie~corn/locate/desal Effect of algae on fouling and efficiency of UF membranes Bokso...

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DESALINATION Desalination 179 (2005) 203-214

ELSEVIER

www.elsevie~corn/locate/desal

Effect of algae on fouling and efficiency of UF membranes Boksoon Kwon, Noeon Park, Jaeweon Cho* Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 10ryong-dong, Buk-gu, Gwangju 500-712, Korea Tel. +82 (62) 970-2443; Fax +82 (62) 970-2434; email: [email protected] Received 30 September 2004; accepted 22 November 2004

Abstract

For UF/MF and NF membrane process for the drinking water treatment, algae have been known to directly or indirectly affect membrane filtration. During algae bloom, transmembrane pressure seriously increases or flux significantly decreases. Algae can also produce taste and odor causing compounds such as 2-methyl isoborneol (2MIB) and geosmin as well as toxic materials. In this study, microcystis aeruginosa was evaluated as a representative algae with an UF membrane in terms of fouling mechanisms by algae. The size and surface charge ofmicrocystis were 2.8 ~tm and -12.35 mV, respectively, as measured by electrophoresis light scattering method. 10 ~tm silica was used to serve as a secondary membrane which was expected to remove smaller particles (i.e., here microcystis) before they reach the membrane surface. However, silica did not effectively act as the secondary membrane for microcystis contrarily to expectation. Combination of microcystis and natural organic matter (NOM) provided higher flux decline than either of microcystis and NOM filtration with the UF (EW) membrane. Concentration polarization (CP) thickness of microcystis on the EW membrane was calculated by both convection-diffusionelectrophoretic migration (CDE) and convection-diffusion (CD) models with consideration of the negative charge of membrane and algae. The CP thickness estimated by the CDE model was higher than that by the CD model. And, microcystis fouled on the membrane surface could be more effectively cleaned by 100 ppm NaOCI solution than 0.025 N NaOH solution. Keywords: Algae; Microcystis; UF membrane; Secondary membrane; Convection-diffusion-electrophoretic migration (CDE); Convection--diffusion (CD) model

*Corresponding author.

Presented at the conference on Membranes in Drinking and Industrial WaterProduction, L 'Aquila, Italy, 15-17 November 2004. Organized by the European Desalination Society. 0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.11.068

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B. Kwon et al. / Desalination 179 (2005) 203-214

1. Introduction

Most studies to date with algae have been mainly concentrated on taste and odor, or by-products (e.g., algogenic organic matter) produced by algae, whereas the effects of algae itself on membranes have not been rigorously considered. Lots of algae together with organic matter deposited on the membrane can be easily observed by the scanning electron microscopy (SEM) image of the membrane surface (Fig. 1). When either water source or water treated by either sedimentation or sand filtration was directly fed into either MF or UF membrane, algae which were not removed by those treatment processes and contained in the feed waters, have high potential to affect the membrane process, especially at the period of algae bloom, causing significant flux decline and transmembrane pressure increase. Therefore, it is necessary to investigate the effects of algae on a membrane process. 2. Related theories

2.1. Secondary membrane

The secondary membrane is the cake layer

formed by larger particles which can protect the approach of smaller particles toward the membrane surface and provide a lower resistance. Thus, the secondary membrane may result in the increase in the permeate [1] and increase the removal efficiency of dissolved species (e.g., NOM). This study employs standard filtration theory for cake growth [2] with both algae and silica during cross-flow ultrafiltration. The total resistance to filtrate flow is fiat, =

(1)

where AP is the transmembrane pressure and p is the solution viscosity. Total resistance is R~.= Ro + R c. The cake resistance, Re, is: R~ = 13Gyy

(2)

where CP is the feed concentration of the particles, Vy is the volume of permeate per unit membrane, and 13 is the specific cake resistance. The cake resistance increases with the cake mass transferred to the membrane surface and may be assumed constant. 2.2. Convection-diffusion (CD) and convectiondiffusion-electrophoretic migration (CDE) models

Convection-diffusion model [Eq. (3)] describes the concentration of a neutral solute in the concentration polarization layer. JC, - D dC' = j C p dx

Fig. !. SEM imageof the membranesurface; it was taken from the NF membranefed with water treated by sedimentation for 3 monthsfromApril to July, and algae representedin this figurewas identifiedas a diatom, Synedra.

(3)

where C t is the concentration of solute, C is the concentration of solute in permeate, D is tPhediffusion coefficient of the solute calculated by the the approximate relationship for shear-induced diffusion coefficient ( D, = O.03:la 2 ) which was used by Zydney and Colton [3]. x is the distance from the bulk to the membrane. Here CP becomes

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B. Kwon et al. /Desalination 179 (2005) 203-214

zero as microcystis cannot penetrate the membrane pores. The convection-diffusion--electrophoretic migration model employs the extended NernstPlank Eq. (4) and this can be rewritten as Eq. (5) by using the Nernst-Einstein and the StokesEinstein relationships (g = zFD/RT):

_•

JC i = D

JC, - D dC, dr

d~g = .]Cp - ktiCi dr

(4)

z, FD R T C, = YCp

(5)

where tg is the electric potential of the charged surface, ~ti is the elelctrophoretic mobility of the charged solute, z, is the valence of ion i, F is the Faraday constant (96,500 C mol-1), R = 8.31 J mol-lK-I, and Tis the absolute temperature. Eq. (6) was used for calculation of the thickness of the concentration polarization layer formed with microcystis. These derivation procedures were well described by Rabiller-Baudry et al. [4].

-

Jd

)

(6)

here the Cw/Ch was first assumed then the CP thickness could be calculated. 2.3. DLVO and extended DL VO (XDL VO) interaction

For explaining the chemical interactions between particles and membrane with respect to fouling, Derj aguin-Landau-Verwey-Overbeek (DLVO) theory has been used; the double layer theory, DLVO, can be used to explain the relation of the interaction at a relatively large distance. However, the same trials at shorter distances (<20A = 2 nm) have not been successful [5]. The DLVO theory considers two types of interactions: van der Waals (LW) and electrostatic (EL) double layer interactions. In addition, The XDLVO theory considers acid-base (AB) interaction [6]. These

are written as uraDLVO lp

uXDLVO ralp

LW

EL

(7)

~---U mlp -}- U mlp

LW ..}.T[EL = g mlp

AB

V mlp "~ U mlp

(8)

where Uply° and UxDLv°are the total interaction energies between the membrane and particle immersed in water, respectively. The subscripts m, l, p represent membrane, liquid, particle, respectively.

3. Materials and methods 3.1. Membrane and algae

EW ultrafiltration membrane (GE Osmonics, US) was used for this study. The membrane was soaked in deionized water (DI) for one day and stabilized by pressurized filtration with DI for 24 h prior to actual filtration tests. A bench-scale crossflow filtration system accommodating flat-sheet type membrane (width of 6 cm, length of 9.6 cm, thickness of 0.5 mm) was used in a recycle mode. The initial permeate flux of 6.6× 104 m/s was used for UF filtration and with the cross-flow velocity of0.14 m/s at 25a:1°(2 (the corresponding Reynolds number is 155). Microcystis aeruginosa (NIER, Korea), a blue green algae, was selected. The microcystis exhibited an exponential growth, algal bloom causing dominant species in most rivers in Korea. In addition, microcystis is known to release potent toxic metabolite microcystin. This blue green algae release great quantities of mucous material to the outer surface of the cell and then the mucous materials form a layer structure thickly surround cell. This could be observed in Fig. 2a. The microcystis was revealed to flocculate, as shown in Fig. 2, thus it is difficult to count that. 3.2. Zeta potential o f membrane and algae and

size

The surface charge of the UF membrane and microcysitis was determined by the electro-

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B. Kwon et al. / Desalination 179 (2005) 203-214

(b)

(a)

Fig 2. Microcystis image taken by (a) FE-SEM and (b) confocal microscopy. phoretic method [7] with ELS-8000 (Otsuka Electronics, Japan). The electrolyte solution was 10 mM KC1 and membranes were cleaned with pure water for one day prior to measurements. The zeta potential of the membrane was measured with varying pH ranging from 2 to 12. The size of microcystis was measured by the light scattering method with the same apparatus mentioned above. These results are listed in Table 1. 3.3. Contact angle m e a s u r e m e n t

The contact angles of both membrane and microcystis were measured with three different probe liquids with known surface tension values

to determine the surface energy properties. Two liquids are relatively polar (i.e., deionized water and formamide) and the other is relatively apolar (i.e., iiodomethane). A goniometer (NRL, Rame Hart, Mountain Lakes, N J) with an image software (DROP) were used for measurements of the contact angle using the sessile drop method. Microcystis was deposited on the surface of an RO membrane (RF membrane, Saehan, Korea) through filtration using a dead-end cell (Amicon cell, Millipore) with stirring and being pressurized at approximately 50 psi (= 345 kPa). Then, it was dried by freeze-dryer for approximately 2 h for further contact angle measurements.

Table ! Characteristics of the EW membrane and microcystis EW membrane

Microcystis aeruginosa

Material Polysulfone UF Algal species

Pore size* (MWCO) 60,000 Size (nm)

Blue-green algae or 2,765 cyanobacteria ' by manufacturer **shear-induced hydrodynamic diffusivity

Zeta potential @ pH 7(mV) -10.83+0.52 Diffusion constant** (cm2/s) 9.56x 10-7

Permeability (L/d.m2.kPa) 13.0-~1.3 Zeta potential @ pH 7 (mV) -12.35±2.8

Membraneresistance, R,, (10I° cm-1) 7.60-~0.85 Mobility @ pH 7 (cm2/Vs) - 1 . 0 3 5 x 1 0 -~

207

B. Kwon et al. / Desalination 179 (2005) 203-214 4. Results and discussion 4.1. Calibration o f microcystis concentration

The concentration of microcystis was calibrated with UV absorbance and cell density to maintain the same concentration for all of the u Itrafiltration experiments. For the determination of a suitable wavelength that can detect microcystis, UV absorbance was scanned with wavelengths ranging 200-800 nm with a different microcystis concentrations (i.e., cell density). Consequently, a wavelength of 683 nm was determined. The cell density [counted using Haemacytometer (Superior, Germany)] of microcystis was calibrated with UV absorbance measured at 683 nm shown in Fig. 3. The cell density used for UF membrane filtration was 104cells/mL. 4. 2. The surface charge

To demonstrate the effect of electrostatic interaction between the EW membrane and microcystis, on which their surface zeta potentials were measured, microcystis incubated at 25°C was collected as pure cells by repeating of centrifuge and mixing with phosphate buffer solution. The pH of a solution containing microcystis and polystyrenelatex particles (200-204 nm) as a monitor

particle for measurement of zeta potential of membrane, respectively, was adjusted to be pH 2-12 with 0.1 N HC! and 0.1 N NaOH. The isoelectric points (i.e.p) were found to be at pH 4 and 5 for microcysitis and the EW membrane, respectively. The zeta potentials at pH 7 used for membrane filtration were -12.35 mV and -10.83 mV for microcystis and EW membrane, respectively (Fig. 4). These charges were applied to CDE model, DLVO and XDLVO theories. 4.3. Filtration o f sil&a and microcystis

The flux decline with microcystis (with average size of 2.8 ~tm) expected to be recovered if silica particles (with average size of 10 ~tm) were filtered together with microcystis because silica may act as the secondary membrane. The flux of a mixture ofmicrocystis and silica shows a slightly higher flux decline than microcystis only different from expectation (i.e., the secondary membrane hypothesis) (Fig. 5). Thus, silica did not act as the secondary membrane for microcystis. The concentration of silica used for this experiment was 50 mg/L; if the concentration of silica decreased or increased, the flux by microcystis might be changed. Through filtration experiments, the flux vs.

7e+5

- -I I--I

3

8

.............

Initial 2 times dilution 3 times dilution 4 times dilution 6 tirnes dilution 10 times dilution ,O0,m

di,

6e+5 5¢+5

~

o.

°2 ..o

-- 0.9691

4¢+5 3e+5

< >

=

2e+5

l 16.+5

0

....Z i , : f = = 200

(a)

300

400

500

600

700

0.0

800

Wavelength(nm)

(b)

0.2

0.4

0.6

0.$

1.0

1.2

1.4

1.6

U V a b s o r b a n e e (1/era)

Fig. 3. (a) UV absorbanceof microcystiswith wavelengthand (b) calibrationof microcystisdensityand UV absorbance at 283 nm. Author: Please note that Fig. 3a will be printed in black-and-white.

208

B. Kwon et aL /Desalination 179 (2005) 203-214

(a)

(b)

pH 30

2

3

4

5

6

7

8

9

I0

11

i

i

f

i

i

i

i

t

i

i

60

[ --o-- Microcystis ]

2

3

4

5

6

7

8

9

10

11

i

i

i

i

i

i

i

i

i

i

> E

10 0

12

I - - ° - - EW membrane

40'

20

20 0 -20

o e~ 0~

pH

12

t~ e~

-10

-40 -60

-20 -80

-30

-100

Fig. 4. Zeta potential of microcystis and EW membrane with increasing pH.

0.008

Table 2 The initial specific cake resistances determined from the slope of the tangents to the R r vs. CVr curves

0.006'

Cp(mg/cm3) 13o(1014era/g) "~ 0.004 o

Microcystis 2.3 Silica (10 ftm) 50.0 Silica + microcystis 52.3

2 A Silica (~,) Mieroeyslis ;;i' Silica+Mi6Toeystis 0.000 0

50

100

150

200

250

300

Time (min)

Fig. 5. Flux decline of silica (10 I.tm)only, microcystis only, and a mixture of silica and microcystis for the EW membrane. time and total resistance vs. mass of particles rejected per unit membrane area are represented in Fig. 6. The total resistance was represented in the increasing order of silica, microcystis, and a mixture of microcystis and silica. As shown in Fig. 6, the resistance curve is linear in time regions below 1000 s for three filtration cases. The initial specific cake resistances [130= ( R r - Ro)/(C I V/)] could be obtained in these regions. B u t t h e resistance curve of silica filtration and mieroeystis filtration were nearly linear during filtration

1.65+0.44 O.1l+0.21 I. 14±0.12

differently from the case of a mixture of silica and microcystis. This may result from that the cake formed by either silica or microcystis becomes gradually compact without being provided with more particle mass. This phenomena could be demonstrated from the fact that the color of feed solution becomes clear over time. The case of a mixture of microcystis and silica also showed the same phenomena, but the tendency of compaction may be somewhat different. The initial specific cake resistance ofmicrocystis exhibited a relatively higher value even with less concentration (Table 2). 4. 4. F i l t r a t i o n o f N O M a n d m i c r o e y s t i s

Similarly to silica, microcystis was hypothesized to be the secondary membrane for NOM (obtained from Dongbok lake in Korea; an average molecular weight of 1050 g/tool) filtration by the

B. Kwon et al. /Desalination 179 (2005) 203-214 CpVf

0.001

0.000

(g/cm2)

0.002 '

0.003 '

0 OO6

0.004 0.6 0,5 04

0.004 0.3 = 0,2 0.002

0.1

(a) 0.0 2000 4000 6000 8000 10000 12000 14000 16000 18000

0000

Time (see)

C o Y f (g/cm ~)

0.000

O.OOI i

0.002 i

0.003 i

0.004 i

°°°°I

0.005 0.3

209

EW membrane. In the aspect of the flux, the microcystis cake provided a much higher flux decline than either microcystis only or NOM only filtration (Fig. 7). This explains why the flux decline from when either NOM or algae individually filtered by membrane rarely provided a high flux decline but filtration with both NOM and algae resulted in a higher flux decline (i.e., organic and/ or biofouling). Thus, algae seems to have an adverse effect on the membrane process. While microcystis could capture NOM that was supposed to be completely transmitted into the EW membrane pores, which resulted from the efficient removal of NOM (Fig. 8). This might be from mucous materials which microcystis have around their cell wall. It seems that microcystis acts as the secondary membrane with respect not to flux but to NOM removal.

• 0,2

4.5. CP thickness of microcystis (CDE vs. CD models)

0.004

0.1

~

0.002

(b) 0.000 / 0

0.0

2000

4000

6000 sooo Ioooo 12000 14o0o 16ooo Time(sec)

Due to the effect of charge interactions on rejection ofmicrocystis by the EW membrane, both CDE model and CD model can be applied for the ultrafiltration of microcystis. Concentration polarization thickness of microcystis was cal-

CpVf(g/cm2) 0.00000 0.007 O006

000003 '

000006 0.00009 0.00012 '

'

'

000015 '

000018 0.5

~ 0.4

o.oo

0.004 ~e 0.003

03=

~4

0.2

"-'3

_=

0.002 0.1 0.001

0000

5 ¸

2 A

(c) O0 2000 4000 6000 8000 10000 12000 14000 16000 18000 T i m e (see)

Mieroeystis

,, NOM i'~Si M i e r o e y s t i s + N O M

0

50

100

150

200

250

300

Time (rain) Fig. 6. Flux vs. time and total resistance vs. rejected particles per membrane surface area for filtration of(a) silica

only, (b) microcystis, and (c) a mixture of silica and microcystis.

Fig. 7. Flux decline of NOM only, microcystis only, and a mixture of N O M and microcystis for the EW m e m brane.

210

B. Kwon et al. / Desalination 179 (2005.) 203-214 30 25

I

~ NOM Only t................I Microcystis+NOM

23.0

23.7

22.4

21.5

20 "d ~- 15

6.8

6.5

60

180

240

300

Time (min)

Fig. 8. Removal efficiencies of NOM obtained from illtrations of NOM only, a mixture of NOM (4 mg/L) and microcystis.

(a)

9.E-04 8.E-04

•c ~

5.E-04

(b)

<+

y : 0.001e -°.°zArx

E" 7.E-04 (.I ~" 6.E-04 14

culated by the two models assuming C / C h is 4. Fig. 9 shows that the CP thicknesses o f microcystis were 2.4-8.9 p,m and 1.3-4.8 p.m for CDE model and CD model, respectively. Two models exhibited that CP thickness decrease with increasing pressure due to the cake layer compaction. The CP thickness considering electrostatic repulsions occurring between the charged EW membrane and charged microcystis was higher than that estimated by the CD model. Also, the CP thickness calculated by the CDE model and CD model altered with changing C / Q (Fig. 10). When both solutes and the membrane surface have charge, the electrostatic interactions highly affect the CP thickness.

Rz

y : 0.0005e "0-°~7x 4.E-04

: 0.8539 =

5.E-04 4.E4N

Q t..

;S 3.E-04 ~) 2.E-04 I .E-04

5

R 2 : 0.8539

3.E-04

2,E-04

+ ............+..........+

el

o

0.E+0O 10

20

30

, 40

, 50

1.E-04 0.E+00

60

70

, 20

10

Applied pressure (psi)

, 30

, 40

50

60

70

Applied pressure (psi)

Fig. 9. Concentration polarization (CP) thickness of microcystis determined by (a) CDE model and (b) CD model on the EW membrane.

Ca)

+.,=..o4

(b)

s.,=.o4"I" .

8.E-04

4.E-04

7.E-04 A

E 6.E-04 ~ 3.E-04

5.E-04

It o g: Jg o

.~ 4.E4N

£

a. 3 . E 4 ) 4 2.E.~

2.E-04

G.

0

'

1 .E-04

1.E.04,

0.E+00. ~ 10

20

30 40 SO Applied pressure (psi)

60

0.E+00 -110

20

30 40 50 Applied pressure (psi)

60

Fig. 10. The change of CP thickness of microcystis by application of (a) CDE model and (b) CD model with assumption of the ratio ofC mand Ca.

B. Kwon et aL / Desalination 179 (2005) 203-214 4. 6. Interaction energy o f microcystis and membrane (DLVO vs. XDLVO)

211

provides information on relative hydrophobicity or hydrophilicity between the membrane and microcystis. A positive value means hydrophilic surface and a negative value means hydrophobic. Thus, the EW membrane and microcystis have a hydrophilic surface (Table 3). Based on the contact angle and zeta potential measurements, the DLVO and XDLVO theories were introduced to find the interactions between the EW membrane and microcystis. The DLVO and XDLVO predictions represent a high repulsion between the EW

The contact angles o f microcystis and EW membrane were measured to examine the interaction energy between them. The EW membrane was relatively hydrophilic as it showed high AB surface free energy, as compared with its LW component. The reason is explained by Brant et. al. [8]. The EW membrane has a relatively high electron donor monopolarity (T) and relatively low electron acceptor components (y+). AG~2~

Table 3 Contact angles and surface energy parameters (mJ/m2) of EW membrane and microcystis Contact angle (o) DI EW membrane 60.2a:0.6 Microcystis 2.31±0. i Surface energy parameters (mJ/m2) LW ~/+ 7 EWmembrane 0.12 1.5 Microcystis 38.8 1.6

Formamide

Diiodomethane

134.1±0.3 4.6±0.02

133.6±0.3 7.3+0.4 AB 7 30.2 18.9

Y152.7 54.5

yTOT

AGIel 74.43 30.45

30.3 57.7

LW: Lifshitz-van der Waais, +: electron acceptor, -: electron donor, AB: acid-base, TOT: total, AG: free energy, subscripts: 1 solid, 2 liquid. Microcvstis

(a)

............DLVO XDLVO

800

(b) [..

\

--,,+o

\

~

600

//o~/7.4nm°

i

//

e!

\

~g

400

"-,,+\ .............................................

0

-200

200

-400

N++,~,,,.~+,,+ ...... , , ++°"~,.+.,:...:,.+.+.+,~,, .

I .E-.09

.

.

.

.

.

.

.

.

1.E..08

Disllnce (m)

] 200

~ i :+'*2.3nm

\\+

~L._L.~ 1- 600 t 400

0

I.E-07

.

1.E-09

.

.

.

.

.

.

.

i

I.E-08 Distance (m)

........

-600 1.E-07

Fig. 11. DLVO and XDLVO interaction energy profiles for (a) microcystis and EW membrane and (b) the cohesive energy between microcystis and microcystis.

212

B. Kwon et al. / Desalination 179 (2005) 203-214

membrane and microcystis due to electrostatic repulsion. In addition, the repulsion value of the XDLVO is higher than that of the DLVO in a distance range below 7.6 nm. The interaction energy among microcystis themselves was also evaluated; the DLVO prediction exhibits repulsion but the XDLVO exhibita attraction at distances lower than 2.3 nm (Fig. 11). The fouling mechanisms ofmicrocystis on the EW membrane surface may be predominantly controlled by interaction between microcystes.

Stabilization

24hrs, w/DI

Clean membrane l Fouling Microcystis 104cells/mL, pH 7, 25~,

~'[ Fouledmembrane I Cleaning DI, NaOH, NaOCI, 30rain

~-{ Cleanedmembrane] Fig. 12. Experimental procedures: stabilization, filtration and cleaning.

4. 7. Cleaning of microcystis The cleaning efficiency of the UF membrane fouled by microcystis was investigated through an experimental procedure as shown in Fig. 12. Used chemicals were deionized water (i.e., reference), 0.025 N NaOH (pH 12.4), 100 ppm NaOC! solution. Cleaning was conducted without pressure and at a cross-flow velocity of 3.3 x 10-sm/s. The results of fouling experiment are shown in Fig. 13. The pure water fluxes of clean, fouled, and cleaned membranes were recorded and compared as shown in Fig. 14. After cleaning with deionized water, the flux was not efficiently recovered. The cleaning by NaOH for 30 min exhibited less recovery than NaOCI. Meanwhile, NaOCI provided higher cleaning efficiency for membranes fouled by microcystis thus NaOC1 seems more effective than NaOH; it is probably due to that NaOCI may act as a swelling agent and have an ability to break the binding between the foulants and the mem-

5

"-" 3 2

50

100

150

200

250

Fig. 13. Flux decline of microcystis with the EW membrane.

brane [9]. Those comparisons were expressed as numerical values (listed in Table 4) as calculated by (Ri - Rc)x I O0/(Ry- Rm): here Rs is membrane hydraulic resistance after fouling, R is the mem-

Table 4 Cleaning efficiency (%) with various chemicals Pressure (kPa)

Pure water (30 min)

NaOH (30 min)

NaOCI (30 min)

NaOH (90 min)

120 147 216 285 353

21.4 10.3 11.0 9.8 11.7

69.6 60.0 50.2 37.8 21.2

88.6 84.2 84.0 61.9 45.6

116.2 106.4 76.9 63.8 55.0

°lkPa = 0.145psi

300

Time (rain)

B. Kwon et al. / Desalination 179 (2005) 203-214

14

I " O ' - Clean membrane J i --0"- Fouled membrane [ " 0 - - Cleaned mem~erte

12 10

---O--- Clean membrane --O-- Fouled membrane

12 /

213

/ ' 3 / /

~

0"/

~

s

~ 6 ~

~

4

4 2

2 0 50

loo

150

200

250

300

350

100

400

15(1

2(10

25(1

300

35(1

4(1(1

Pressure (kPa)

Pressure (kPa)

(a) Pure water cleaning

(b) NaOH cleaning (30 min)

12

10 ¸ -0-

Clean memebrae

1(1

8

8

6

6

!

4

I ''O'- Clean membrane ] "-O'- Fouled membrane ] ~ Cleaned membrane ]

4 2

2

0

100

15(1

200

250

300

350

4(10

Pressure (kPa)

(c) NaOCI cleaning

~

100

r

150

200

25(1

300

r---'----------

350

400

Pressure (kPa)

(d) NaOH cleaning (90 min)

Fig. 14. Flux of deionized water of clean, fouled, and cleaned membranesat different pressures.

brane hydraulic resistance after cleaning, and hydraulic resistance of a virgin membrane. The cleaning efficiency of NaOH operated for 90 min was similar to that of NaOCl, but they are both inefficient. 5. Conclusion The fouling of microcystis on the EW membrane (MWCO, 60,000 Daltons) was evaluated in various ways: the secondary membrane con-

cept, CDE and CD models, and DLVO and XDLVO theories. First, silica with a higher size compared to microcystis did not act as the secondary membrane for microcystis, however, microcystis somewhat enhanced the rejection of NOM which was supposed to interact completely transmiring through the EW membrane without the secondary membrane. Second, the CP thickness ofmicrocystis calculated by the CDE model considering the charge between the membrane and algae was proved to be higher than that of the CD

214

B. Kwon et al. / Desalination 179 (2005) 203-214

model. Third, the DLVO and XDLVO theories provided the strong repulsion caused by the electrostatic charge between the EW membrane and microcystis. However, the XDLVO exhibited the attraction between microcystis at a shorter distance. Finally, the cleaning efficiency of NaOCI for membranes fouled by microcystis was higher than that o f NaOH.

[3] [4]

[5]

Acknowedgments This work was supported by a grant (4-I-2) from Sustainable Water Resources Research Center of 21 st Century Frontier Research Program through theWater Reuse Technology Center (WRTC) at GIST and also by the Center for Distributed Sensor Network (MDSN) at GIST.

References [!] V.T.Kuberkar and R.H. Davis, Modeling of fouling reduction by secondary membranes, J. Membr. Sci., 168 (2000) 243-258. [2] R.H. Davis and D.C. Grant, Theory for deadend microfiltration, in: W.S.W. Ho and K.K. Sirkar, Eds.,

[6]

[7]

[8] [9]

Membrane Handbook. Van Nostrand Reinhold, New York, 1992, pp. 461-479. A.L. Zydney and C.K. Colton, Chem. Eng. Commun., 47 (1986) 1-21. M. Rabiller-Baudry, B. Chaufer, P. Aimar, B. Bariou and D. Lucas, Application of a convection-diffusion-electrophoretic migration model to ultrafiltration of lysozyme at different pH values and ionic strengths, J. Membr. Sci., 179 (2000) 163-174. S. Ohki and H. Ohshima, Interaction and aggregation of lipid vesicles (DLVO theory versus modified DLVO theory), Colloids and Surface, B: Biointerfaces, 14 (1999) 27-45. J.A.Brant and A.E. Childress, Assessing short-range membrane-colloids interactions using surface energetics, J. Membr. Sci., 203 (2002) 257-273. Y. Shim, H.J. Lee, S. Lee, S.H. Moon and J. Cho, Effects of natural organic matter and ionic species on membrane surface charge, Environ. Sci. Technol., 28 (1994) 3864-3871. J.A. Brant and A.E. Childress, Colloidal adhesion to hydrophilic membrane surfaces, J. Membr. Sci., 241 (2004) 235-248. P. Matzinos and R. ,~lvarez, Effect of ionic strength on rinsing and alkaline cleaning of ultrafiltration inorganic membranes fouled with whey proteins, J. Membr. Sei., 208 (2002) 23-30.