ELSEVIER Desalination 118 (1998) 175-188

DESALINATION

Intermittent crossflushing of hollow fiber ultrafiltration systems

Maria Kennedy a*, Soo-Myung Kim a, Isaac Mutenyo a, Leute Broens b, Jan Schippers a,c alnternational Institute for Infrastructure, Hydraulic & Environmental Engineering, Westvest 7, 2601 DA Delft,

The Netherlands; bNorit Membrane Filtrations, Opaalstraat 22, 7554 TS Hengelo, The Netherlands; and cKIWA Research & Consultancy, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands

Received 6 July 1998; accepted 10 July 1998

Abstract

The successful operation of dead-end ultrafiltration systems depends on the effectiveness of cleaning (hydraulic and/or chemical cleaning). In this study, backwashing conditions such as pressure and duration were studied in order to maximize the net flux per cycle. Monitoring the backwash flux and turbidity of the backwash water showed that the efficiency of backwashing was more dependent on backwashing time than pressure. The effectiveness of backwashing was significantly improved by the use of crossflushing. In general, 99.9__.1% flux restoration could be achieved when backwashing was preceded by crossflushing while 94.5+_1% could be achieved by backwashing alone. Crossflushing was more effective when the crossflush velocity in the fiber was in the turbulent regime (v ~ 1.6 m/s). Crossflushing and pressure pulsing can be employed to increase the efficiency of dead-end, hollow fiber ultrafiltration systems because the frequency of backwashing and chemical cleaning, permeate consumption and system down time are reduced.

Keywords: Dead-end; Ultrafiltration; Backwashing; Flux; Crossflushing

1. Introduction

The principal operational costs in crossflow microfiltration and ultrafiltration processes are energy, which is required to create high crossflow velocities (ca. 2 m/s) and membrane replacement [1]. Although high crossf low ve loc i ty enhances the pe r fo rmance of microfiltration and ultrafiltration, the high energy requirement associated with crossflow prevents the breakthrough of these processes in large scale water treatment applications [2].

*Corresponding author

Dead-end ultrafiltration is emerging as an important process in the product ion of potable water, industrial water and water recycling applications as energy costs are ca. 10 times lower than crossflow ultrafiltration processes [2]. Fouling is an inherent part of dead-end filtration as all particles, colloids, mac romolecu l e s , etc smal ler than the molecular weight cut-off (MWCO) of the membrane accumulate on the membrane surface and/or in the pores. The cake deposit grows with time of filtration and in the

Presented at the Conference on Membranes in Drinking and lndustrial Water Production, Amsterdam, September 21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0011-9164(98)00121-0

176 M. Kennedy et al. /Desalination 118 (1998) 175-188

T4

I

Pbw

{1)v,

Plead

Fig. 1. Laboratory scale UF system.

v4

Module

V 5

@

m V 3

absence of high crossvelocity, can be removed only by appropriate cleaning [3-5].

Hydraulic cleaning (backwashing) is known to be effective in controlling fouling in hollow fibre membranes in many fields [6-7]. However, backwashing has the limitation of consuming permeate and increasing down time, both of which reduce the overall system efficiency. Moreover, over long periods of operation, membrane fouling is generally not totally reversible by backwashing and as the number of filtration cycles increase the degree of irreversible fouling tends to increase. In order to remove adsorbed material from the membrane and restore the initial flux of the system, chemical cleaning of the membrane is required. Chemical cleaning is typically cumbersome and costly in terms of chemicals, labor and energy [8]. There is also concern that chemicals used in cleaning may react with the membrane, shortening the lifetime and

increasing the frequency of membrane replacement [9], which is one of the major costs in ultrafiltration.

The successful operation of dead-end ultrafiltration systems depends on the effectiveness of cleaning (hydraulic and/or chemica l c leaning) . Improving the

performance of backwashing and reducing the frequency of chemical cleaning will result in increased system efficiency and reduced operational costs.

Backwashing conditions such as pressure and duration are usually obtained by trial and error, because the complexity of fouling has made it difficult to define equations characterizing its operation. In this study, backwashing conditions such as pressure and duration were studied in order to maximize the net flux per cycle. In addition, crossflushing (fast/forward flushing) and pressure pulsing were employed to improve the efficiency of cleaning.

M. Kennedy et al. / Desalination 118 (1998) 175-188

Table 1 Operating conditions for backwash pressure and duration tests

1 7 7

Parameter Operating conditions Pb/Pf= 1 Pb/Pf= 2.5 Pb/Pf= 4 Pb/Pf = 8

Filtration Pressure (bar) 0.2 0.2 0.2 0.2 Duration 20% flux loss 20% flux loss 20% flux loss 20% flux loss

Backwashing Pressure (bar) 0.2 0.5 0.8 1.6 Duration (min) 0.5 / 1.0 / 2.0 0.5 / 1.0 / 2.0 0.5 / 1.0 / 2.0 0.5 / 1.0 / 2.0

lOO . 19o

n~ s5

8o

6o

4o i [

o

1 2.5 4 8

pressure R~lo (Pb/Pf)

i i t=0.5min mmt=- lmin t o N I t=-2rnin x t=0.Sn~ ,,,~_ t=lnin - . l l - -~-2nJn

Fig. 2. Effect of BW pressure on flux restoration and water consumption.

2. Experimental

A laboratory scale ultrafiltration unit (Fig. 1) was used in this study. Canal water was used as feed water. The turbidity of the canal water ranged from 7-20 FTU, and was diluted with tap water to maintain a constant t u rb id i ty of 11±2 FTU. The TOC concentration was ca 9±2 ppm. Temperature was kept constant at 20±1°C. Canal water was collected in tank Tl and continuously stirred to allow thorough mixing. Polyethersulphone (PES) hollow fibre membrane modules with a MWCO of 50,000 Da (supplied by X-FLOW) were used in this study. The hollow fibers had an internal diameter of 1.5 mm. Each module had ca. 50 fibres with a total surface area of

0.1 m 2. The module length was 50 cm. Fil trat ion was pe r fo rmed at constant

transmembrane pressure with valves V] and V 5 open while V2, V3 and V 4 were closed. B a c k w a s h i n g was p e r f o r m e d wi th demineralised water (tank T4) with valves V6, V 4 and V3 open while V1, V2 and V5 are closed. Crossflushing was performed with valve V6, V2 and V3 open while V1, V4 and V 5 were closed.

3. Results and discussion

3.1. The effect of backwash pressure and duration

Flux restoration as a function of the appl ied b a c k w a s h (BW) pressure for backwash durations of 0 .5 -2 .0 min is presented in Fig. 2. The backwash pressure is presented as a ratio (PgP\f) of the backwash pressure (Pb) to the filtration pressure (Pf). Backwashing was carried out when 20% loss of the initial f lux was reached. The conditions of these tests are summarized in Table 1.

It can be observed from Fig. 2 (line graph) that increasing the backwash to filtration pressure ratio above 2.5 did not result in any significant increase in flux restoration within the range of backwash pressures tested (0.2- 1.6 bar), which suggests that the application of very high backwash pressures (8 times the filtration pressure) cannot restore irreversible flux decline due to adsorption etc.

178

100

M. K e n n e d y et al. / Desa l ina t ion 118 (1998) 1 7 5 - 1 8 8

3 o I J -

t -

-I-

+

+ | • -I-

"1-

-I-

I , I

0 30 60

Fig. 3. Turbidity variation during BW.

\

1.6bar -.e=-O.8bar f 0.5bar

\ +

\ \

' I '

9O "lime (sec)

+

120 150 180

A pressure of 0.5 bar (Pb/Pf = 2.5) was considered as the minimum backwash pressure required to effectively remove particles accumulated on the membrane. These results correspond to literature [10] and recommendations from membrane suppliers that backwash pressure should at least be twice as high as the filtration pressure. Increasing the BW time from 0.5 to 1 minute had almost negligible effect on flux restoration. However, when the BW time was

increased to 2 minutes, a significant effect on flux restoration (>97 %) was observed at all BW pressures tested (0.2-1.6 bar). If sufficient time for the removal of particles accumulated in the membrane is allowed, high flux restoration can be achieved (Fig. 2), suggesting that the per formance of backwashing may be more dependent on BW duration than pressure.

Water consumption, expressed as a percentage of the water produced is

M. Kennedy et al. /Desalination 118 (1998) 175-188 179

70

t -

~E

X

t'7

Z

60

50

40

30

20

"-Z

- - I I - - t=0.5rnin ~ t=1.0min . - . . . - t=2.0min

q I i I

1

Fig. 4. Effect of BW pressure on net flux.

2.5 Pressure Ratio (Pb / Pf)

4 8

represented by the bar graph in Fig. 2. Increas ing the backwash ing pressure increased the water consumption which reduced the system recovery. Therefore, the optimum backwashing pressure and duration should be determined considering both the degree of flux restoration and the system recovery.

Fig. 3 shows the variation in turbidity during backwashing at pressures of 0.5-1.6 bar. This graph partially explains the stronger influence of backwashing time on flux restoration. The turbidity of the backwash water was monitored every 10 seconds and decreased rapidly during the first 80 sec, 100 sec and 150 sec at 1.6 bar, 0.8 bar and 0.5

bar, respectively. The time required to reach minimum turbidity depended on the pressure applied, and corresponded to the point in time when most particles were removed from the fibres. A minimum turbidity was reached earlier at high BW pressures (i.e., 80 sec at 1.6 bar as opposed to 150 sec at 0.5 bar). However, even after the minimum turbidity was reached, a further increase was observed for all pressures. This phenomenon is attributed to the fact that the majority of particles in the cake layer were removed first, while particles lodged inside the membrane pores appeared later and caused a slight increase in turbidity.

1 80 M. Kennedy et al. / Desalination 118 (1998) 175-188

3.2. Optimizing flux restoration and recovery

In order to determine the optimal backwashing conditions in terms of pressure and duration, the net flux per cycle (Jnet) was calculated using Eq. (1). Net flux can serve as a tool to determine the optimal backwashing conditions because it reflects the operation efficiency by taking into account water production and consumption including the time spent for backwashing.

time whereas the flux during filtration decreases if the system is not operated at constant flux, as was the case in this study. Theoretically, during backwashing, the cake accumulated on the membrane is instantly lifted off the membrane and then swept out of the channel by axial flow [11] due to the applied backwash pressure. As a result, an increase in flux is expected during backwashing as the cake resistance and thickness decrease.

(Vf - V b) (1) J = A p (2) Jnet - (tf + t b) A m l't (Rm + Rc)

where Vf = Volume of water produced during

filtration Vb = Volume of water consumed

backwashing tf = Filtration time tb = Backwashing time

A m = Membrane surface area.

for

From Fig. 4 it can be observed that, for all BW pressures and durations tested, the net flux increased up to a pressure ratio of 2.5, then decreased even though flux restoration increased. This phenomenon may be explained by the fact that above a pressure ratio of 2.5, the flux decline which was recovered by backwashing was small compared to the quantity of water consumed to realize the flux restoration. The optimal backwashing conditions for this water type, in terms of flux restoration and operation efficiency can be determined from the maximum in Fig. 4, i.e. BW at 0.5 bar (Pb/Pf = 2.5) for 0.5 minutes.

3.3. Flux variation during backwashing

R c = a M = t:t Am J (3)

Assuming that only cake filtration takes place during ultrafiltration, then flux change can be expressed by the well-known resistance in series model from conventional dead-end filtration as in Eq. (2) [12] where J = Filtrate flux R m = Membrane resistance Rc = Cake resistance AP = Transmembrane pressure /.t = Fluid viscosity V = Filtrate volume a = Specific resistance Am = Membrane surface area.

This relationship can also be used to predict the change in flux during backwashing. The cake resistance and mass balance in one cycle (i .e. between two backwashes) can be described by Eq. (3, 4 and 5), where Ma = Mass accumulated during filtration Mr = Mass being removed during back-

washing.

The variation in the flux during backwashing is of great interest in several respects. The backwashing efficiency over time can be predicted if the flux during backwashing is monitored. It is assumed that the flux during backwashing increases over

Re = dt cycle

( - - - ~ ) = M a - M r ( 5 ) cycle

M. Kennedy et al. / Desalination 118 (1998) 175-188 181

At the beginning of backwashing, the mass removed (Mr) should be zero and at end of backwashing, Mr will be equal to M a, if it is assumed that all particles in the cake layer are completely removed by backwashing, and the resulting cake resistance will approach zero. Therefore, it is of great importance to know how the flux varies during backwashing because a "flux plateau" may exist [13] where any further increase in the backwashing time does not result in an increase in flux.

The initial clean water backwash flux (CBF) at different pressures was measured in order to estimate the maximum possible flux restoration which could be achieved by backwashing. The flux restoration during backwashing was calculated by comparing the flux during backwashing to the initial clean water backwash flux (CBF). The backwash water was collected every 10 seconds and its turbidity measured. The operation conditions for the filtration were held constant, i.e., 0.2 bar and 20% flux loss while the backwashing pressure were varied from 1.6-0.2 bar.

The flux restoration and turbidity variation over time at a backwash pressure of 1.6 bar are presented in Fig. 5. It is of interest to see that the flux during backwashing decreased unexpectedly in the first 30 seconds and there after increased gradually to a constant level (__.96%). This phenomenon may be due to the fact that part of the cake layer lifted off the membrane surface (particularly at the inlet of the hollow fiber) accumulated at the exit of the fiber, which reduced the effective fiber diameter , resul t ing in increased resistance until the majority of the particles were removed by backwashing.

The measurement of turbidity during backwash ing (Fig. 5) suppor ts this explanation. The majority of turbidity was removed in the first 30 seconds, which corresponds to the point when the flux increased. Therefore, this time can be considered as the min im um effect ive backwashing time to effectively remove the cake layer at this backwashing pressure.

100

. . . .

× 2O \

X "X :X :X :X :X :X :X :X :X :X :X :X :X :X :X 0

0 1 2 3

Time during B W (min)

- - x - Turbidity zl Restoration

Fig. 5. Variation in BW flux and turbidity.

100

90 o 10 Q o:

85 x u.

80

3.4. Methods to improve backwashing efficiency

For good flux restoration, high backwash pressures and long BW durations are required, however, the consequence is reduced system recovery and net flux. On the other hand, if backwashing is performed at lower pressures or for short durations, the flux decline is rapid and the chemical cleaning frequency increases. Repeated chemical cleaning should be limited because it may reduce the membrane lifetime, increase the m e m b r a n e r ep l acemen t frequency and system costs. It is therefore of great interest to reduce the frequency of chemica l c leaning by enhanc ing the performance of backwashing. Two different methods were tested for this purpose: crossflushing and pressure pulsing.

3.5. The effect of crossflushing on flux restoration

Crossflushing was tested to improve the efficiency of backwashing by applying a water flush along the fiber from the feed side to retentate side with the permeate closed. In order to determine the optimal crossflushing conditions, a range of crossflush velocities

100

DO

ra !

182 M. Kennedy et al. /Desalination 118 (1998) 175-188

lid 0 . 2 3 @.11 I.B f-$

¢F Velocity (mle~)

Fig. 6. Effect of CF velocity on flux restoration.

100

eO

_g so £ E To M J u_

6O

U 1 3 10 BW(30 sec.)

CF duration (eec)

Fig. 7. Effect of CF duration on flux restoration.

(0.4-0.8 m/sec) and durations (1 to 10 sec) were tested and compared with backwashing at 0.5 bar for 30 sec.

3.6. The effect of CF velocity

Increasing the flow velocity above 1.6 rn/sec (Fig. 6) did not result in any further increase in flux restoration. Employing a crossflush velocity above 1.6 m/s, CF = 0.5 bar) gave almost the same flux restoration as BW at 0.5 bar (vf = 0.23 m/s) for 30 seconds. On the other hand, poor flux restoration was observed at a low crossflush velocity of ca 0.8 m/s where the flow was laminar (R e = 900). A transition from laminar to turbulent flow occurs at a velocity of ca 1.6 m/s, above which no further significant flux restoration was observed. This may imply that once turbulent flow is established, further increase of velocity through the fibres may not cause any significant flux restoration. These results support the idea that cake filtration is the main reason for flux decline under these test conditions. Thus, CF alone was capable of removing most of the particles from the membrane fiber when a minimum cross flush velocity of ca. 1.6 m/s was applied. The Reynolds number (Re) at crossflush velocities of 0.8, 1.6 and 1.8 rn/s are ca 900, 2300 and 2700, respectively.

3. 7. The effect of CF duration

260

200

Z~'160 L

~100

80

" BW Period

10 100 Time (oec)

Fig. 8. Turbidity change during CF and BW.

The crossflush duration was varied between 1-10 sec, using a CF velocity of 1.6 m/s. It is apparent that crossflushing longer than 3 seconds did not improve flux restoration significantly (Fig. 7). The turbidity change during CF is presented in Fig. 8. Most turbidity was removed in less than 5 seconds, however, crossflushing longer than 5 seconds just increased water consumption and down time while no real improvement in flux restoration was achieved. A crossflush duration of 3-5 seconds was therefore optimal for these tests. After 10 seconds of crossflushing, the turbidity was almost completely removed (95%), however,

M. Kennedy et al. /Desalination 118 (1998) 175-188 183

applying a backwash after 10 seconds, increased the turbidity slightly. This phenomenon supports the results of previous experiments, indicating that majority of particles in the cake layer are removed by CF, and particles blocking the pores or lodged inside the pores are removed later by BW. These results confirm that CF can only remove particles on the surface of membrane, and should be combined with BW to provide effective cleaning.

A comparison of the velocity in the fibers during CF and BW shows that the velocity during CF is ca 4-8 times higher than during backwashing at the same pressure (Table 2), e.g., 0.23 m/s during BW and 1.6 m/s during

CF at same pressure of 0.5 bar. This phenomenon also explains why CF is an effective means of flux restoration even at very short durations (3-5 seconds).

100

g c- O

0 • 90 X

t l .

80 BW(0.5) B+C B/C/B C/B

Fig. 9. Flux restoration under different BW/CF combinations.

Table 2 Linear velocities in fibres during crossflushing and backwashing

Pressure (bar)

Linear velocity in fibres (m/s) during CF during BW

0.2 0.8 0.09 0.5 1.6 0.23 0.7 1.8 0.31 1.0 1.9 0.43

3.8. Optimizing crossflushing and back- washing

In order to find the most effective method of cleaning, several approaches were tested: (i) Simultaneous CF (v = 1.6 s; 5 s) and BW (0.5 bar; 30 s): ( CF+BW); (ii) CF (v = 1.6 s;

5 s) followed by BW (0.5 bar; 30 s): ( CF/BW); (iii) BW (0.5 bar; 15 s), then CF (v = 1.6 s; 5 s) followed by BW (0.5 bar; 15 s): (BW/CF/BW).

The flux restoration achieved with these

three scenarios is presented in Fig. 9. and compared with BW (0.5 bar for 30 see). Flux restoration was slightly lower (91.6_+5%) when CF and BW were performed simultaneously, compared to BW alone (93_+4.7%). This may be attributed to the fact that when CF and BW are performed simultaneously, the net force on the membrane fiber is reduced. Good flux restoration (99.9_+1.0%) was achieved when CF was followed by BW. This phenomenon may be attributed to the fact that canal water has a large proportion of particles greater than 3 gm, where as the average pore size of the 50,000 Da membrane used in this study was ca 15 nm. Therefore, the majority of particles accumulate on the membrane surface forming a cake, which can be easily swept out of the membrane by the shear force exerted during crossflushing. The remaining particles in the cake layer or within the pores can be removed easily by backwashing. The BW/CF/BW process was also effective (97.5_+3.8% flux restoration), but was difficult to operate. It should be noted that all the operations involving crossflushing had the same water recovery as BW alone.

184 M. Kennedy et al. /Desalination 118 (1998) 175-188

e |

so

~ ss

4o

~. 3o 26

2O

c.c c.c c.c *

< , T-6.Slhour >'< T 5.1hour , >]< T.4.2hourl > .

t 2 l 4 8 6 7 8 i 10 11 t'* 11 14 11 11 17 t8

Time Ihour) Fig. 10. Long-term operation of BW.

,0j C h e m i c a l

7o

"ou o

1 = SO ]< T"14.4 hour >1

0 1 2 $ 4 S 0 7 | t) 10 11 Time [hour)

Fig. 11. Long-term operation of CF/BW.

12 15 14 18 18 17 18

3.9. Evaluation of crossflushing in long term operation

The " long- te rm" e f f ec t i venes s of crossflushing was evaluated by comparing flux restoration over a period of 18 hours via backwashing (Fig. 10) and crossflushing followed by backwashing (Fig. 11). The backwashing conditions were kept constant in both tests (0.5 bar for 30 seconds) and crossflushing (Fig. 11 only) was performed at a velocity of 1.6 m/s for 5 seconds. Backwashing and chemical cleaning were

applied when the flux declined by ca 30% and 40%, respectively. The chemical cleaning frequency could be reduced by a factor of 2- 3 when a combination of CF and BW was applied (Fig. 11) compared to BW alone (Fig. 10). It is clear that CF/BW is also effective in restoring flux decline even in "long-term" operation, indicating that CF/BW may be more cost effective than BW because the system recovery is increased, down time is reduced and the frequency of chemical cleaning is also reduced which affects the membrane replacement costs.

M. Kennedy et al. /Desalination 118 (1998) 175-188 185

3.10. The effect of pressure pulsing on flux restoration

In the previous section, crossflushing followed by backwashing was more effective than backwashing alone in terms of flux restoration. Therefore, in this section, the effect of pressure pulsing, i.e., pulsing the feed water by closing the permeate valve periodically, on flux enhancement was investigated. Pressure pulsing was performed by closing the permeate valve and opening the module out-let every 5 minutes during filtration. A pulse duration of 5 seconds was conducted every 5 minutes. The pulse duration is defined as the amount of time the process is operating with a negative transmembrane pressure during a cycle [14]. The effect of pressure pulsing on flux enhancemen t can be expla ined by concentration polarization, which is the dominant resistance to solvent flux in ultrafiltration.

Concentration polarization, resulting from the build-up of a boundary layer of more highly concentrated solute at the membrane surface due to convective transport of both solvent and solute, lowers flux due to increased hydrodynamic resistance in the boundary layer [15]. According to literature, this phenomenon can be reversed by changing the operating conditions i.e., by decreasing or releasing pressure along the membrane. When the t ransmembrane pressure is released with the permeate closed, solute is swept away from the membrane surface into the bulk stream by a scouring effect of feed water resulting in a reduced boundary layer. Pressure pulsing is easy to operate because the pulsing pressure is the same as the TMP which means the process involves only periodic opening and closing the permeate and module outlet valves.

Figs. 12 and 13 show flux decline curves of backwashing only and backwashing with pressure pulsing, respectively. Backwashing was conducted ca every 20 minutes on average whereas it were required every ca 42 minutes when pressure pulsing was applied.

Introducing a pressure pulsing of 5 seconds every 5 minutes reduced the frequency of backwashing by ca. 60%, the amount of permeate consumed and the system down time.

In Table 3 a comparison of flux restoration and system recovery between two processes is presented. From the table, ca 91% of flux restoration could be achieved by pressure pulsing alone (wi thout cons ider ing backwashing). High flux restoration in pressure pulsing increased the system recovery but more raw water was used to produce the same permeate.

Table 3 Comparison of backwashing only and backwashing with pressure pulsing (BW/PP)

Parameter BW BW/PP

Flux restoration (%) 89 91" System recovery (%) 91 97 Time required to meet 20%

loss of flux (min) 18 41

*Flux restoration of pressure pulsing alone (without considering backwashing restoration).

It can be concluded from these tests that pressure pulsing is an efficient method to improve flux and control fouling because it is simple to operate, does not require much energy and does not consume permeate.

4. Conclusions

In this study, flux restoration was significantly improved by crossflushing directly before backwashing, as the accumulated cake was rapidly removed by the high crossflush velocity in the fiber. Crossflushing was more effective when the crossflush velocity in the fiber was in the turbulent regime (v 1.6 m/s). The successful application of crossflushing is dependent on the type of fouling and the membrane pore size. Crossflushing is a good method of cake

8 0

M. Kennedy et al. /Desalination 118 (1998) 175-188

BW BW BW BW BW BW BW BW BW BW

70 o o

0

I ; 60 ¢,..

¢,i

";'so [ L

40

186

90

3 0 , , , , I ' ' ' ' [ ' ' ' ' I J ' ' ' I ' ' ' ' ~ ' ' ' ' I . . . .

0.0 0.5 1.0 1.5 2~0 2.5 3.0 3.5

T i m e ( h o u r )

Fig. 12. Effect of backwashing alone on flux restoration.

.° I e 6 B W B W B W B W

/ l O s e c ] I I I

~ 6 0

I so

3 , 5 , 1 0 s e e p u l s i n g d u r a t i o n , , 5 s s c

3 0 . . ~ . . . . . . . . . . , , i . . . . b . . . . i . . . .

0 .0 0 .6 t .0 t . 6 2 .0 2 .6

Time(hour)

Fig. 13. Effect of backwashing with pressure pulsing on flux restoration.

3 .0 3 . 8

removal in dead end systems, particularly if low MWCO membranes are employed as the probability of surface deposition is increased. However, it does not solve the problem of plugged pores and/or adsorption of organic

matter on the membrane surface, both of which contribute significantly to flux decline in practice. Cross f lush ing should be combined with backwashing and chemical cleaning to solve all fouling problems.

M. Kennedy et al. /Desalination 118 (1998) 175-188 187

5. Abbreviations and symbols

tX Specific cake resis tance (m/kg) /z Fluid viscosi ty (kg/ms) Rc Cake resis tance (m -1) R m M e m b r a n e res is tance (m -1) Vf Filtrate vo lume (1) A m M e m b r a n e area (m 2) J Filtrate flux (1/mZh) tf Filtration t ime (min) tb Backwash t ime (min) Vb V o l u m e of backwash (1) Pb Backwash pressure (bar) Mr Mass of cake r e m o v e d dur ing back-

washing Ma M a s s o f c a k e a c c u m u l a t e d du r i ng

fi l t rat ion CF Cross f lush ing BW Backwash ing T M P T r a n s m e m b r a n e pressure

References

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[2] I. Blume, D.M. Koenhen, and H.D.W. Roesnik, In: Proceedings of Euromembrane 1995, X-Flow B.V., Almelo, pp. 1-6.

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[4] M.R. Wiesner and S. Chellam, J. AWWA, 1 (1992) 88-95.

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[6] K. Matsumoto, M. Kawahara, H. Ohya, and J. Ferment. Technol., 66 (1988) 199-205.

[7] R. Ben Amar, B.B. Gupta, and M.Y. Jaffrin, J. Food Science, 55 (1990) 1620-1625.

[8] S.G. Redkar and R.H. Davis, AIChE J., 41 (1995) 501-508.

[9] G.F. Crozes, J.G. Jacangelo, C. Anselme, and J.M. Laine, J. Membr. Sci., 124 (1997) 63-76.

[10] J.A. Howel and S.M. Finnigan, In: Effective Industrial Membrane Processes - Benefits and Opportunities, M.K. Turner, Ed., Elsevier Applied Science, 1991.

[I1] S. Nakatsuka, I. Nakate, and T. Miyano, Desalination, 106 (1996) 55-61.

[12] A.G. Fane, J. Membr. Sci., 20 (1984) 249-259. [13] S.N. Cross, In: Effective Industrial Membrane

Processes - Benefits and Opportunities, M.K. Turner, Ed., Elsevier Applied Science, 1991, pp. 61-90.

[14] V.G.J. Rodgers and R.E. Sparks, J. Membr. Sci., 68 (1992) 149-168.

[15] M. Cheryan, Ultrafiltration Handbook. Technomic Publishing Co., 1986.