Fundamental Principles of Ultrafiltration.pdf

7/21/2019 Fundamental Principles of Ultrafiltration.pdf

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the membrane. For example, a membrane that is

cleaned before use has quite a different performance

than that of one that has not been cleaned.

Real-life performance

It may be argued that the real-life performance

of a membrane can only be obtained by industrial,

long-term, evaluations of the membrane. Theoretical

models would thus be merely of scientific interest.

We agree that experimental investigations are impor-

tant, but we are also convinced that theoretical

modelling of the ultrafiltration process is necessary if

we are to understand, and predict, the performance

of UF membranes.

The goal is, of course, to be able to design and

optimize systems where the ultrafiltration equipment

is an essential, but unnoticed, part of the system, as

for example in electrodeposition systems. In order to

reach this goal we need theoretical models to explain

the (too often unpredictable) behaviour of ultrafil-

tration membranes.

Influence of pressure

Fluid flow through porous media is usually de-

scribed by the well-known Darcy equation 91

KAP

J=-

p AL

(1)

where J is the water flux, K is the specific permeabil-

ity of the medium, AP is the pressure difference

across the medium, p is the fluid viscosity and AL is

the thickness of the medium.

It is, however, difficult to measure the effective

thickness of an asymmetric membrane. The thickness

and the specific permeability of the membrane are

therefore usually combined to form a medium-spe-

cific parameter, the hydraulic resistance of the mem-

brane, R, = AL /K.

In ultrafiltration, solutes and/or particles are re-

tained by the porous medium, the membrane. This

makes the introduction of more involved relations

than eqn. (1) necessary.

Solute is transmitted by convection towards the

membrane as soon as the transport of solvent

through the membrane commences, and the concen-

tration is increased on the feed side of the membrane

as solute is retained. The osmotic pressure difference

Ail across the membrane can then become substan-

tial. The driving force of the fluid is in this case given

by AP - c AD, according to the three-parameter

model of Kedem and Katchalsky [ 10, 111. The reflec-

tion coefficient 0 indicates the degree of permselectiv-

ity of the membrane. When CJ = 1 the solute is totally

retained and when 0 = 0 it is totally permeable.

The resistance of the accumulated solute at the

membrane surface is sometimes represented as a

hydraulic resistance R,. If we introduce hydraulic

resistances instead of permeability in Darcys equa-

tion and take the osmotic pressure of the solute into

Fig. 1. Alternative models of concentration polarization: a) the

osmotic pressure model, b) the gel layer model and c) the

resistance in series model. The concentrations c,, cb, cP, and ca

denote the concentrations at the membrane wall, in the bulk

solution, the permeate, and the gel, respectively. The hydraulic

resistances are: the membrane resistance R,,,, the resistance of the

gel layer

R

and the resistance of the solute

R .

consideration, the ultrafiltration flux may be de-

scribed by the generalized equation

AP--AII

J= /4%+%)

The theoretical models that will be treated in this

review can all be related to eqn. (2). These models

are the osmotic pressure model, the gel layer model

and the resistance in series model. Figure 1 shows

concentration polarization as represented by the

different models.

Theoretical models

In the

osmotic pressure model

the solute hydraulic

resistance R, is substituted by a continuous, steep,

concentration gradient at the membrane, resulting in

a substantial osmotic pressure.

In the gel layer model it is assumed that the solute

concentration at the membrane surface reaches a

limiting value, the gel concentration cg.

In the resistance in seri es model the resistance to

flow is represented by a hydraulic resistance of the

solute,

R,.

The osmotic pressure AII is neglected.

The osmotic pr essure model

At typical UF feed concentrations the osmotic

pressure of macrosolutes is negligible. Osmotic

effects are therefore frequently ignored. Very high

concentrations at the membrane wall and subse-

quently substantial osmotic pressures have, however,

been demonstrated for dextran and whey protein

solutions, for example [ 121.

The concentration at the membrane surface de-

pends on the ratio between the convective transport

of material to the membrane and the diffusion of

material back to the bulk stream. During normal

ultrafiltration conditions the solute concentration at

the membrane wall, c,,

may rise rapidly from the

bulk value, cb (see Fig. l(a)). The fluid flow through

the boundary layer adjacent to the membrane is

frequently described by the film model as


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where c,, is the concentration in the permeate and k

is the mass transfer coefficient.

Taking the osmotic pressure at the membrane

wall into account, Wijmans et

al. [

131 have derived a

relation between pressure and flux. Their calculated

results have been found to be in good accordance

with experimental results [ 141.

Wijmans et

al. [

131 used the following relationship

between the osmotic pressure and the concentration

at the membrane wall:

I

lT, =

ac,

(4)

where a and II are solution-dependent constants.

When the solute is totally retained (d = 1 and

cp = 0), and the hydraulic resistance of the solute,

R,,

is

neglected, combination of eqns. (2)-(4) gives the

following expression relating flux and applied pres-

sure [13]:

J = AP -

ac,, exp(nJ/k)

11&

(5)

A limitation of eqn. (5) is the availability of osmotic

pressure data for solutions containing high concen-

trations of macromolecules.

The pure water permeability (PWP) of ultrafiltra-

tion membranes is given as the pure water flux

(PWF) per unit pressure difference:

PWP = (PWFIAP),, = = l/(pR,)

(6)

The flux as a function of operating pressure has been

calculated for three membranes with varying pure

water fluxes. The results are given in Fig. 2.

Initially, the flux increases linearly with pressure,

as can be seen in Fig. 2. As the flux increases further,

more and more material is transported to the mem-

brane wall and concentration polarization increases.

The flux levels off as a consequence of the increasing

concentration polarization (see point ( 1) under Gen-

eral trends).

250

f 200

g 150

x 100

50

0

0 0.2 0.4 0. 6 0. 8 1.0 1.2

PRESSURE (MPa)

Fig. 2. The effect of operating pressure on flux for three ultrafil-

tration membranes with varying permeabilities, calculated using

eqn. (5). The PWPs of the three membranes were 1000, 250 and

1C01m-2h- MPa- at T=2OC (this corresponds to

& = 3.6 X 102, 14.4 x 10 and 36.0 x 10tZm~, respectively).

Lines without symbols represent the pure water fluxes of the three

membranes and lines with symbols represent the flux of a

1.0 wt.% solution (c, = O.Ol), which has an osmotic pressure of

0.1 MPa at a concentration of 10 wt.% (a = 10 MPa and n = 2).

This applies fairly well to solutions of dextran [ 141. The value of

the mass transfer coefficient was 2 x 10m5 m SK.

The inf luence of the pure water j?ux

Compare, for example, the two membranes with

PWP = 1000 and 100 1 m- h- MPa- in Fig. 2. As

can be seen, the increase of the flux declines faster

for the high permeability membrane than for the low

permeability membrane. This explains why mem-

branes with an initially high flux exhibit greater flux

declines than low permeability membranes (see point

(2) under General trends).

The influence of concentration polarization is

small for membranes with low pure water fluxes.

They are, consequently, almost linearly dependent

on pressure, as stated in Darcys equation.

Membranes with high pure water fluxes (those

that look so promising in membrane tables ) suffer

from high osmotic pressures which are caused by

high concentrations at the membrane wall. Owing to

this high osmotic pressure, the flux of these mem-

branes is much lower than the pure water flux.

A reasonable question is, of course, if we can ever

reach the level of the pure water flux of the high

permeability membranes. One way of accomplishing

this is by increasing the mass transfer coefficient

k.

The influence of other parameters may be obtained

from the derivative of eqn. (5). The derivative of this

equation is

[ 131

dJ=L(l +A %)-

d AP pR,

The order of magnitude of the deviation from the

pure water flux is given by the second term in eqn.

(7), AfI

n/pR,k.

The product flux is affected by, for

example, the permeability of the membrane (the

effect of

R,

is demonstrated in Fig. 2), the tempera-

ture of the solution (which affects p), the osmotic

pressure (AD and n) and the cross-flow velocity

(affects

k).

These parameters are discussed further

later in this paper.

The gel layer model

The pressure independence of flux when the oper-

ating pressure is increased was originally explained

by Blatt

et al. [

151 as being due to the formation of

a gel layer at the membrane surface. They stated that

a limiting flux value is reached when the concentra-

tion of solute in the boundary layer reaches the gel

concentration cg. The flux-limiting value for a totally

retained solute (cr = 0) at gel layer conditions is

given by eqn. (3) as

J=kln 2

0

(8)

In the gel layer model the osmotic pressure is

assumed to be zero. The fluid flow is then described

by

J=

AP

14Rm + Rs)

(9)

When the concentration at the wall has not

yet reached the gel concentration, the polarized


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boundary layer offers a resistance to the flux,H&,

an d R, = R, ,

.

Under gel layer conditions the resis-

tance of the gel layer represents the resistance of the

solute, R, = R ,. When both concentration and gel

polarization occur the resistance of the solute is

R,= R,+Rb, [15].

The gel layer model predicts the flux to be inde-

pendent of operating pressure. An increased pressure

merely results in a thicker gel layer (larger &),

which retards the flux to its original value.

The gel layer model has been frequently used to

correlate experimental limiting fluxes [ 16- 181. The

gel concentration may be obtained by extrapolation

of a plot of J versus In cb. It has, however, been

shown that the information obtained on gel concen-

trations is not reliable. For identical solutions differ-

ent authors have found widely varying values at cg

[191.

Experimental evidence [ 151 indicates that the vis-

cous solution in the boundary layer approaches a

close-packed configuration of low hydraulic perme-

ability. It has been argued, however, that it is only

materials such as agar, pectin, gelatin and some

proteins that might denaturate at the membrane

surface to give true gels [ 141. It has also been shown

that feed solutions of various macrosolutes with

concentration cb = cg did not give zero flux [20].

Based on their analysis, Wijmans et a l . [ 131 con-

cluded that osmotic pressure limitations would be

expected in the ultrafiltration of macrosolutes with

molecular weights of 10 000- 100 000, and gel layer

limitations would be expected for larger molecules.

Th e r es is tan ce n ser ies mod el

The concentrated solution of macromolecules at

the membrane wall provides a physical barrier to

solvent and solute transport. The resistance of this

boundary layer may be treated as one or several

resistances in series with the membrane, as already

discussed in the gel layer model.

The resistance in series model has been shown to

apply for dextran solutions [21]. The resistance of

the solute in this investigation was estimated from

independently measured sedimentation coefficients.

The results were also in reasonable agreement with

results obtained from the osmotic pressure model.

The resistance in series model predicts a pressure-

independent flux, as does the gel layer model. That is

to say, an increase in the pressure results in a thicker

gel layer and an increased hydraulic resistance.

An advantage with the resistance in series model

is that it makes it possible to distinguish between th0

influence of different flux decline phenomena.

The resistance to flow may be accounted for by a

number of resistances: the resistance R, of the mem-

brane, the boundary layer resistance R,,, the gel

layer resistance R,,

and the adsorbed layer resistance

R,. Equation (2) may then be written as

AP

J =p(R,+R,+R,+Rh , )

(10)

01

0 a

b

1000

2000 3000

4000

ROTATION SPEED (rpm)

Fig. 3. Determination of the influence of the membrane resis-

tance, boundary layer resistance, gel layer resistance and adsorbed

layer resistance for 1.0 wt.% BSA at 0.5 MPa. The numbers

denote the order in which the measurements were performed. The

rotary module is described in detail in ref. 22.

The influence of different resistances on flux has

been investigated for bovine serum albumin (BSA) in

a rotary module [22]. Different flow resistance re-

gions were distinguished by varying the rotation

speed. Figure 3 shows the experimental results.

First, the membrane resistance & was deter-

mined by ultrafiltration of pure water at high rota-

tion velocity. From the pure water flux the

membrane resistance could be calculated as, in this

case, R, = R , = R b, = 0.

The pure water was then replaced by a 1.0% BSA

solution. The flux was measured at decreasing

rotation velocities. Concentration polarization and

gel layer formation were assumed to be zero

(R , = R,, = 0) as long as the flux remained constant.

The flux began to decrease when the velocity was

decreased to about 2400 rev min- as can be seen in

Fig. 3. As long as it was possible to restore the initial

flux value by increasing the rotation velocity to the

original value, the influence of the gel layer was

assumed to be zero, i.e.

R, = 0.

Down to about 1000 rev min- the original per-

meate flux was almost completely restored when the

velocity was increased. When it was no longer possi-

ble to attain the original flux values when the veloc-

ity was increased, it was deduced that an irreversible

phenomenon had taken place, namely gel layer for-

mation.

Compac t i o n o f memb r ane an d gel l a yer

The theoretical models described above all predict

a limiting flux value at increasing pressure. They do

not, however, explain why the flux may decrease

when pressure is increased. This phenomenon may,

however, be explained as being the result of com-

paction of the membrane and/or the gel layer.

The membrane pure water permeability K has

been related to pressure for reverse osmosis cellulose

acetate membranes by [23 ]

K= exp( -uP)

(11)

where K,, is the specific membrane permeability at

zero pressure and LX s a measure of the susceptibility

of the membrane to compaction.

It has been questioned whether the porosity of

ultrafiltration membranes is influenced to any appre-


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250

200

150

100

50

0

0 5 10

15

20

CONCENTRATION IN BULK (w- )

Fig. 6. The influence of bulk concentration on flux at two operat-

ing pressures, 0.5 and 1.0 MPa. The values of the parameters in

eon. (5) were: PWP-2501m-2h-MPa-1 at T=2OC

(R, = 1.4 x lOI rn-), k = 2 x 10e5 m s-, (I = 10 MPa and

n = 2.

trends). The film model (eqn. (3)) predicts, for exam-

ple, that the flux J varies proportionally with the

logarithm of the bulk concentration ct,, and the gel

layer model predicts that the intercept of a plot of J

versus In ct, corresponds to the gel concentration cg.

The variation of flux with concentration, as pre-

dicted by the osmotic pressure model (eqn. (S)), is

shown in Fig. 6.

The flux is, as expected, higher at the high operat-

ing pressure. But, although the pure water flux (at

c,, = 0 in Fig. 6) is doubled when the pressure is

doubled, this considerable difference between the two

flux curves vanishes as the concentration increases.

The two flux curves in Fig. 6 approach zero at the

bulk concentration at which the osmotic pressure

equals the operating pressure. It is thus possible to

increase the final concentration of the solution by

increasing the operating pressure.

The osmotic-pressure-concentration relationship

of the solution not only determines the final retentate

150

125

100

E-

2 75

=

s

k

50

25

CONCENTRATION IN BULK (W- )

Fig. 7. The influence of bulk concentration on flux for two solutes

with different osmotic pressure-concentration relationships. The

high osmotic pressure solute (a = 23 and n = 2) corresponds to a

solution of PEG. The low osmotic pressure solute (a = 10

and n = 2) corresponds to a solution of dextran. The values of

other parameters in eon. (5) were: PWP = 250 1 m mz h

MPa

at

T = 20 C

R, =

1.4 x 10s mm), AP = 0.5 MPa and

k =

2 x 10VSm s-r.

200

150

100

50

0

CONCENTRATION IN BULK (W-/o)

Fig. 8. The influence of bulk concentration on flux for two

membranes with different pure water permeabihties, 1000 and

250 1 rnpz hh MPa- at T=20C (&=3.6x IO* and

1.4 x IOr m-, respectively). The values of the parameters in eqn.

(5) were: AP = 0.5 MPa,

k = x

IO-srn s-r, a = 10 MPa and

n = 2.

concentration. It also determines the magnitude of

the ultrafiltration flux. In Fig. 7 the flux of two

solutions with different osmotic-pressure-concentra-

tion relationships is shown. The high osmotic-pres-

sure-concentration relationship applies fairly well to

a solution of polyethylene glycol (PEG) and the low

osmotic-pressure-concentration relationship applies

to a solution of dextran [14]. As shown in the

Figure, the flux of the solute with the higher osmotic

pressure (PEG) was not only lower, but also de-

creased faster.

When the concentration in the feed increases, the

flux becomes less and less affected by the PWP of the

membrane, as can be seen in Fig. 8. Thus, the flux at

the end of the concentration operation cannot be

increased by the introduction of a membrane with a

higher pure water flux.

Wijmans et al. [ 131 have rearranged the osmotic

pressure model relationship, eqn. (5), to give

dJ

k

-= _

d In ct, 1 + &, k/AII n

(16)

Equation ( 16) predicts a slope approaching -k for

the J versus In c,, plot at high bulk concentrations

(high AII values). This is also predicted by the gel

layer model.

The mass trader coefficient

The flux is thus very sensitive to operational

parameters that may affect the mass transfer co-

efficient

k. The

mass transfer coefficient may be

obtained from correlations of the form

where

d,,

is the hydraulic diameter of the flow chan-

nel,

D

the diffusivity, and Sh, Re and Sc are the

Sherwood, Reynolds and Schmidt numbers. The

constants

A, a, b

and c vary with flow conditions.

The mass transfer coefficient has been shown to

have a slight tendency to decrease with increasing

pressure and bulk concentration [ 121. The decrease

in flux with increasing pressure at elevated pressures


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73

was suggested earlier in this paper to be the result of

the compaction of the membrane and gel layer at

high pressures. The decrease in the mass transfer

coefficient with increasing pressure has been put

forward as another explanation of this phenomenon

[141.

The diffusivity of macrosolutes is usually depen-

dent on concentration. Owing to the concentration

polarization, the concentration at the membrane sur-

face may differ significantly from the concentration

in the bulk, This may, of course, modify the effective

mass transfer coefficient. The diffusivity is also in-

creased by an increase in the temperature. However,

the operational parameter that has the largest impact

on the mass transfer coefficient is the cross-flow

velocity.

I nfl uence of the cross ow velocity

In both the osmotic pressure and the gel layer

models it is presumed that the mass transfer co-

efficient is proportional to un, where u is the cross-

flow velocity and a is the Reynolds number exponent

in eqn. ( 17). The value of the exponent has been

much debated. A thorough review of different mass

transfer coefficients, together with their adaption to

ultrafiltration, is given in ref. 25.

In Figs. 9 and 10 a value of II = 0.75 according to

the Chilton-Colburn model has been used. The in-

fluence of the cross-flow velocity on the flux for two

membranes of differing pure water flux (Fig. 9) and

at varying bulk concentrations (Fig. 10) has been

calculated from eqn. (5).

The influence of the cross-flow velocity increases

as the permeability of the membrane increases, as

can be seen in Fig. 9. This is easily understood as we

know that the concentration polarization is more

pronounced for membranes with higher pure water

fluxes.

It is, perhaps, not that easy to anticipate how the

cross-flow velocity influences the flux at various con-

centrations. As a rule, we can say that flux becomes

independent of cross-flow velocity at a much lower

velocity for low concentration solutions. In Fig. 10

it can be seen, for example, that the solution with

2 5 0

I

CROSS FLOW VELOCITY (m/s)

Fig. 9. The influence of the cross-flow velocity on flux for two

membranes with different pure water permeabilities, 500 and

250 1 m-* h- MPa-

at T=20C (R,,,=7.2x IO* and

1.4 x IO m-l). The values of the other parameters in eqn. (5)

were: AP = 0.5 MPa,

cb = 1.0 wt.%, k = 2 x 10-s m s-

at

U =4ms-1,

cr = 10 MPa and n = 2.

01

8

1

0

2

4

6

8

1

CROSS FLOW VELOCITY (m/s)

Fig. 10. The influence of the cross-flow velocity on flux at three

different bulk concentrations, 0.1, 1.0 and 5.0 wt.%. The values

of the parameters in eqn. (5) were: PWP = 500

1mm*

hh

MPa-

at T = 20 C (R, = 7.2 x 10 II-),

AP = 0.5 MPa,

k=2x10-5ms-atu=4ms-.a=10MPaandn=2.

ci, = 0.1 wt.% has reached its maximum flux at a

velocity of approximately 4 m s-l, while the flux of

the 5.0 wt.%

solution is still increasing at

u =8ms-.

If a gel layer is formed, the flux may become

independent of cross-flow velocity at low velocities,

even for high concentration solutions, because once

a gel layer has been formed it is not possible to

recover the flux by simply increasing the velocity

(this was illustrated in Fig. 3). Thus, the velocity

ought to be kept, if possible, at such a level that gel

formation is avoided. If the cross-flow velocity is

decreased temporarily during operation, the forma-

tion of a gel layer may be avoided if the flux also is

decreased, for example, by temporarily lowering the

operating pressure.

Rotating modules

The ultrafiltration flux of a membrane is con-

trolled by the rate at which the retained solutes can

be transferred from the membrane wall back into the

bulk fluid. Thus, operational variables that aid back-

transport from the membrane will directly increase

the flux.

The shear rate at the membrane wall is, as has

been clear from the above, the major depolarizing

parameter. This fact initiated the development of

rotating modules [26,27]. However, the commercial

success of rotating modules has been limited (due to

high investment costs).

Positive results from tests with a modified plate

and frame rotary module, the ABB CROT filter,

have been reported recently. In pilot-plant equip-

ment, bleaching effluent from a Swedish pulp mill

has been treated successfully over a one-year period.

A larger pilot plant (membrane area 200 m2) has

recently been installed at the same mill.

During ultrafiltration of oil emulsions in a rotary

module it was found that the flux could be increased

further if the roughness of the module wall was

increased [28]. The increase in the wall roughness

was accomplished by the introduction of a net. The

increase in the flux was ascribed to an increase in the

mass transfer coefficient due to the presence of the

net, acting as a static convection promoter.


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74

It has also been shown that the mass transfer

coefficient can be increased, not only by increasing

the wall roughness, but also by using corrugated

membranes [29]. It was found that corrugations in-

creased the mass transfer more effectively than an

increase in the cross-flow velocity. These tests were,

however, performed with reverse osmosis mem-

branes, but a similar type of effect of membrane

surface roughness has been demonstrated for ultrafil-

tration membranes [301.

I nfl uence of suspended soli ds

The presence of suspended solids may enhance the

flux. There are several examples of this phenomenon

[31]. In such cases of flux enhancement it is thought

that the solids intermittently disturb the polarized

layer, sweeping away deposited solutes [32, 331.

Influence of time

Filtration theory predicts that flux decreases as

t I* under unstirred conditions [34]. This is also

predicted by the gel layer [35] and osmotic pressure

models [361.

The influence of time under cross-flow conditions

is more complicated. The film mode1 predicts that a

rapid drop in flux occurs as the boundary layer is

built up. Several workers have studied the initial

polarization period experimentally. Polarization time

varied from about 5 to 50 s [ 181. The flux at the end

of the initial polarization period should be the limit-

ing flux predicted by eqn. (8).

The gel layer model, as well as the osmotic pres-

sure model, predicts that, after the initial polariza-

tion period, flux remains constant with time.

However, a gradual, long-term decay in flux is noted

in many applications. This flux decline is due to

fouling. The causes of fouling are very disparate. It is

thus difficult to derive a general theoretical expres-

sion for the flux-time relationship.

A simple relation between flux and time is the

empirical relationship

J = Jot

(18)

where JO is the initial flux and n is an exponent less

than zero. Equation (18) has been much used to

predict flux decline in reverse osmosis systems [37].

Equation ( 18) gives, however, no information about

the influence of the operational parameters. Nor is

the influence of the properties of the solution taken

into account.

In the remaining part of this paper we will discuss

the influence of the nature of the solutes, the mem-

brane material, pH and ionic strength on fouling.

We will also discuss how fouling can be controlled.

Fouling

It is very risky to make general statements about

the influence of different parameters on fouling. The

(a)

:,:,:>

D

\I

\\\

,.;


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Characteristic pore sizes were determined for

clean and fouled membranes by Hanemaaijer et al.

[45]. The pore size changes caused by solute adsorp-

tion were derived from saccharide retention data. It

was shown that for a hydrophilic low protein ad-

sorbing membrane, the calculated pore size was not

affected by protein adsorption, whereas the pore size

of hydrophobic membranes was reduced.

Adsorption of one of the solutes in a mixture may

thus also affect the retention of other compounds.

Solutes that are not retained by the membrane when

tested alone may very well be retained due to solute-

membrane and solute-solute interactions of other

compounds in the solution. It is thus advisable to

confirm the expected retention characteristics of a

membrane by tests with real solutions and not base

predictions of membrane performance on tests with

idealized solutions. But even predictions based on

tests with real solutions may be unreliable as the

composition of many process streams may occasion-

ally differ.

I nfl uence of membrane materi al

It is commonly recognized that hydrophobic

membranes have a larger fouling tendency than hy-

drophilic membranes. Even the pure water flux may

be affected by the nature of the membrane material

as absolutely pure water is very hard to find (a

well-known problem for anyone who works with

membranes).

Investigations

[

3 1,46,47] have shown that the

pure water flux of hydrophobic membranes decreases

with time, while the pure water flux of hydrophilic

membranes is less time dependent. This phenomenon

has been attributed to contamination of the pure

water by bacteria and trace colloids [31] and to trace

amounts of waste products of micro-organisms [46]

present in the pure water.

I nf ruence of membrane pore size

Membrane performance cannot, however, be re-

lated simply to the membrane material. For example,

the flux decline of three membranes, all made of

polysulphone, was noticeably different when the pH

of a BSA solution was altered [48].

The same disparate behaviour was found for three

membranes with varying pore sizes used for the

treatment of caustic bleach plant effluent [49]. The

effect on the flux of the three membranes when the

pH was lowered differed considerably, as can be seen

in Fig. 12.

At the higher pH the flux of all three membranes

was almost identical, while when the pH was lowered

to below the isoelectric pH of the solution, the

magnitude of the flux followed the cut-off of the

membranes, that is, the densest membrane experi-

enced the largest flux decline, while the flux of the

membrane with the largest pores was unchanged.

It was thus the membrane with the smallest pores

that exhibited the greatest flux decline in this case.

-0 20 40 80 100

a)

TIME

(h)

E 200

2 150

zj 100

ii 50

0

0

20 40

TIME (:;

80 100

b)

Fig. 12. Flux of three membranes with varying cut-off at different

values of pH: a) pH 11 and b) pH 4.6. Two membranes were

made of polysulphone, PU120 with a cut-off of 20 000 and PU608

with a cut-off of 8000, and one was made of polyethersulphone,

ES404 with a cut-off of 4000. All three membranes were manufac-

tured by PC1 Membrane Systems. The operating pressure was

0.8 MPa and the temperature 55 C. Data from ref. 49.)

Usually, however, the relative flux decline is greatest

for the membrane with the highest pure water flux

(which is often the membrane with the largest pores).

I nfl uence of surfactants

Surface chemical phenomena play an important

role in the fouling of membranes. It is well known,

for example, that hydrophobic solutes (e.g. BSA) are

more readily adsorbed onto the membrane surface

than hydrophilic solutes (e.g. dextran).

Antifoams used in fermentation can cause consid-

erable fouling of membranes during downstream

processing [50-541. The membranes in some com-

mercial membrane plants have even been destroyed

when an antifoam agent has been exchanged. In such

cases irreversible adsorption of the new antifoam

agent has caused a nearly zero flux and made it

necessary to replace all the membranes.

Despite some frightening examples of the influ-

ence of surfactants, as a class of materials they do

not lead to fouling problems. It has been shown, for

example, that the flux of ultrafiltration membranes

can be enhanced if the membranes are pretreated

with non-ionic surfactants before use [55]. There is

also evidence that cleaning, due to the surfactants in

the cleaning solution, may not be considered merely

as a cleaning operation, but also as an in situ surface

modification [49].

Some surfactants, however, may interact with the

membrane and cause an irreversible flux decline.

Many parameters may affect the flux decline of


10/15

76

surfactant solutions, for example, the membrane ma-

terial, the pore size of the membrane and, of course,

the nature of the surfactant.

Surf actants causing fou li ng problems

The flux of antifoams with cloud points has been

shown to be virtually zero for polysulphone mem-

branes at, and above, the cloud-point temperature

[54]. When the temperature was lowered the flux

returned approximately to the original level. No flux

decrease was observed for a hydrophilic membrane.

This type of flux decline was not observed for non-

cloud-point antifoams.

Irreversible fouling is often observed when

cationic polymers and surfactants are in contact with

negatively charged membranes. Cationic surfactants

do not always

cause irreversible flux declines, how-

ever. The initial flux of cellulose acetate membranes

was restored when the membranes were flushed with

pure water after ultrafiltration of the cationic surfac-

tant hexadecyl trimethylammonium bromide [47]. A

slight, irreversible flux decline was observed for

membranes of polysulphone and polyvinyldifluoride

when exposed to the same surfactant.

Critical

micelle concentration

The flux decline on the addition of a surfactant is

usually very fast, within minutes, and even small

amounts of surfactant are enough to cause a consid-

erable drop in flux. At the critical micelle concentra-

tion (CMC) micelles are formed and the retention

then often increases as the micelles are too large to

pass freely through the membrane pores.

At CMC the flux often stabilizes at a constant

level, as can be seen in Fig. 13. The Figure shows the

influence of a non-ionic surfactant on the flux of a

600

506

406

300

266

160

0

100

200 300

400 500

TIME

(h)

Fig. 13. Variation of flux during addition of the non-ionic surfac-

tant Triton X-100. The polysulphone membrane, DDS GRIO, has

a nominal cut-off of 500 000. The line with the open circles shows

the Aux decline of a membrane that has only been conditioned

with pure water. The line with the solid dots shows the flux decline

of the same membrane, but after cleaning of the membrane with

a commercial alkaline cleaning agent, Ultrasil 10 from Henkel.

Surfactant was added stepwise, the concentration 0.1 CMC, for

example, indicates that the concentration was 10 of the critical

micelle concentration. The operating pressure was 0.1 MPa and

the temperature 30 C.

polysulphone membrane, before and after cleaning

of the membrane. After cleaning, the pure water

flux of the membrane increased from 140 to

540 1m- h-.

The tlux increase after cleaning, shown in Fig. 13,

makes it easy to understand why cleaning is such a

vital part of membrane operations. This will, how-

ever, be treated in more detail in a following section.

We will now take a look at two other properties that

may influence the fouling tendency of solutes,

namely the solution pH and the ionic strength.

Influence

of

pH and ionic strength

The solution pH and ionic strength affect the

charge, stability and tendency to aggregate of the

solute molecules and this influences, of course, the

membrane performance.

A reduction in the flux at the isoelectric pH has

been found for many solutes (BSA [20,31,56] and

whey [57,58], for example). At the isoelectric pH the

net charge is zero. Thus, a solute which under nor-

mal conditions is not adsorbed onto the membrane

due to electrostatic repulsion (Fig. 14(a)) is no

longer repelled at the isoelectric pH (Fig. 14(b)).

Alteration of pH does not only influence the

solute-membrane interaction, but also the solute-

solute interaction. If the electrostatic repulsion be-

tween solute molecules ceases this may result in a

more closely packed and denser gel layer. The same

result may be obtained if salt is added to the solu-

tion.

It has been shown that the addition of salt to a

starch solution results in a drastic decrease in flux

[59]. It was suggested that the cations shielded the

charges of ionized carboxyl groups on the oxidized

starch chain. This shielding caused contraction of the

starch molecules which resulted in lower viscosity

and stability of starch in the solution. Densification

of the starch gel increased the hydraulic resistance of

the gel layer and the flux was thus decreased. No

interactions were observed for completely un-ionized

pearl starch.

t hydrophob ic t ll

(b)

Fig. 14. (a) A charged solute is repelled by a membrane of the

same charge. (b) At the isoelectric pH the solute is uncharged and

may be adsorbed onto the membrane.


11/15

77

During ultrafiltration of kraft black liquor it was

found that large molecules, that were able to perme-

ate the membrane at a high ionic strength, were

almost completely retained at low ionic strength [60].

The influence of pH may be altered if salt is

present in the solution. This has been demonstrated

for BSA [LX]. In the absence of salt the flux was at its

minimum at the isoelectric pH. In the presence of

0.2 M NaCl the flux was lowest at pH 2 and in-

creased monotonically with increasing pH.

The zeta potenti al of the membrane

The pH may, however, affect not only the charge

of the solute, but also the net charge of the mem-

brane, the zeta potential. It has been shown [61] that

a polycarbonate membrane has an isoelectric point

at pH 4. Above this pH the membrane had a slight

negative surface charge, which increased in magni-

tude with increasing pH. The surface was found to

be fully charged above a pH of about 6. The surface

was neutral below a pH of 3. The same dependence

on pH has been shown for membranes of polysul-

phone and polyamide [62].

Most natural substances are negatively charged.

The maximum flux of a negatively charged mem-

brane ought then to be found at a high pH, where

the electrostatic repulsion is at its maximum. This

supposition has been confirmed in many investiga-

tions. For example, for bleach plant effluent [49] and

for BSA solutions with salt present [56].

The flux may, however, also be higher at a low

pH. This has been found, for example, during ultra-

filtration of soybean extract [63] and for cheddar

cheese whey [64]. Of course, not only the pH may

change the charge of the membrane. If solutes are

adsorbed on the membrane this may change not only

the charge, but also the hydrophobic/hydrophilic

properties of the membrane.

Pretreatment

The fouling ability of a solution may be dimin-

ished by suitable pretreatment. The pretreatment can

be mechanical, thermal or chemical. Modifications to

the feed solution include adjustment of pH, removal

of fibres, fines, etc. and heat treatment of the feed

solution.

Mechanical pretreatment is especially important

when using thin-channel membrane modules. The

danger of plugging the flow channels is greatest for

spiral, hollow-fibre and plate and frame modules.

Tubular modules also require some solution pre-

treatment. When treating effluents in the pulp and

paper industry a continuous sand filter is often used

as a prefilter for plate and frame modules [65], while

for tubular membranes a 1 mm screen is sufficient

t 351.

Centrifugation is often used, for instance to re-

move fines from whey and fibres from starch pro-

cessing waters. Another example is centrifugation of

blood for the fractionation of red blood cells from

blood serum proteins.

Thermal pretreatment is often necessary in order

to avoid undesirable changes, especially when pro-

cessing biological materials which are sensitive to

biodegradation, such as liquid foods, food waste

waters and fermentation broths. The pH of whey, for

instance, decreases rapidly without heat pretreat-

ment.

Calcium phosphate is known to cause a flux de-

cline during the ultrafiltration of milk and whey

when the temperature and pH are not carefully

controlled [39]. Milk and whey are therefore nor-

mally heated to 55-60 C and held at this tempera-

ture for up to half an hour before ultrafiltration at

50-55 C. As the solubility of calcium phosphate

decreases with increasing temperature the salt precip-

itates during the preheating operation. If the milk, or

whey, is not preheated, calcium phosphate may pre-

cipitate at the membrane surface and in the pores.

This phenomenon, known as scaling, can also occur

in the porous support. The deposited salt is very

difficult to remove. However, no significant flux de-

cline is observed when the salt is precipitated before

the ultrafiltration.

Chemical pretreatment,

can, in many cases, im-

prove flux. The reduction of flux of protein solutions

at the isoelectric pH has already been discussed in

this paper. This reduction is avoided if the pH is

adjusted before the protein solution is concentrated.

Cleaning

Cleaning and disinfection of the membrane plant

are very important operations, especially in food

processing. Fouling of the equipment, as well as the

presence and growth of micro-organisms, necessi-

tates regular cleaning and disinfection cycles. Mem-

branes used in food plants are generally cleaned at

least once a day, while those used for the treatment

of electrodeposition paints, or for the production of

desalinated water, need to be cleaned less frequently,

usually no more than twice a year.

A cleaning cycle generally includes the following

stages: removal of product from the system, followed

by rinsing the system with water; cleaning in one or

several steps, followed by rinsing the system with

water; disinfection of the system.

The product should be removed at the same tem-

perature as that used in the process. This is impor-

tant, for instance, when dealing with products which

tend to form gels at low temperatures.

Both retentate and permeate should be discharged

when the system is

rinsed.

Rinsing should continue

until both the retentate and permeate streams are

totally clear and neutral.

Cleaning solutions

A large number of

cleaning

recommendations

are reported in the literature. Cleaning studies are


12/15

usually trial and error investigations, whereas sys-

tematic studies are very rare. Temperature, time,

concentration and type of surface active agent are

important cleaning parameters that vary with fou-

lants and membrane material.

As a rule, mineral deposits are removed by acids

and proteins by alkaline solutions. For some prod-

ucts, it is necessary to use different chemicals in

succession to obtain a satisfactory cleaning result.

For milk, where protein deposits dominate, an alka-

line formulation is used first, sometimes followed by

acid treatment to remove mineral deposits. If poly-

sulphone membranes are used, a final alkaline clean-

ing is often carried out in order to improve the flux.

For whey, where mineral deposits dominate, acid

cleaning is often performed first, and is then fol-

lowed by alkaline cleaning.

Some care is recommended when selecting a

cleaning procedure since an injudicious choice could

be catastrophic. As an example, in ref. 67 it is shown

that pectine, present in fruit juices, presents difficult

fouling problems. Especially alkaline detergents, and

also unsuitable acid products, may lead to complete

clogging of the membranes.

In order to obtain a good mechanical cleaning

effect, the cross-flow rate is often higher and the

pressure lower during cleaning than during normal

operating conditions.

Cleaning solutions are often a mixture of chemi-

cals. Alkaline cleaning solutions usually contain

sodium hydroxide, phosphate, sequestering agents

and surface active agents. Examples of some cleaning

chemicals and the concentrations used are given in

Table 1.

The choice of surface active agent is very impor-

tant, since some types may be adsorbed onto the

membrane surface, resulting in a flux reduction. On

the other hand, with a suitable choice of surfactant

in the cleaning solution considerably enhanced fluxes

may be obtained. Thus, as stated previously, it has

been suggested [49] that cleaning may be considered

not merely as a cleaning operation, but also as an in

situ surface modification.

The influence of the surfactant on the flux was

demonstrated in an investigation where membranes

fouled during ultrafiltration of whey were cleaned

with different types of surfactants [68]. Considerably

improved whey fluxes were experienced after clean-

ing with a solution containing an anionic surfactant,

TABLE I. Examples of concentrations of chemicals used in

cleaning and disinfection solutions

Cleaning/disinfecting agent

Concentration (%)

Caustic soda 0.5-1.0

EDTA-Na, 0.5-1.0

Nitric acid 0.3-0.5

Chlorine 0.002-0.02

Hydrogen peroxide 0.1

Sodium bisulphite 0.25

for example. Another example of the positive effect

of cleaning is demonstrated in Fig. 13.

Disinfection

A diluted solution of hypochlorite, hydrogen per-

oxide or sodium bisulphite is often used for

disinfec-

tion.

It is important to clean and disinfect not only

the active side of the membrane, but also the perme-

ate side. This fact has to be considered especially for

reverse osmosis membranes, since these dense mem-

branes retain the cleaning and disinfecting agents.

Membrane plants are often run discontinuously.

When idle, the modules should be preserved in a

diluted disinfection solution, for example sodium

bisulphite, in order to prevent microbial growth in

the membrane and/or membrane support.

Means of facil itating cleaning

Generally, the effect of the cleaning operation is

checked by measuring the water flux after cleaning at

a defined pressure, temperature and cross-flow veloc-

ity. This is not a reliable measure, however. A high

water flux does not guarantee a good product flux,

whereas a low water flux indicates that the cleaning

is not sufficient. The product flux in the following

run is a better indication of whether the membranes

have been cleaned satisfactorily [69].

The cleaning operation is facilitated if the mem-

brane fouling is reduced as much as possible. Factors

which must then be considered are:

_

membrane properties (choose the most suitable

type of membrane and module for each application);

_

solution pretreatment (before ultrafiltration, re-

move salts that can cause severe scaling, for exam-

ple) ;

- flow velocity (try to keep the cross-flow velocity at

such a level that gel formation is avoided);

_

rinsing water quality (be aware of the quality of

the rinsing water).

The presence of iron, silica, calcium and other

inorganic ions in the rinsing water can cause precipi-

tation of salts which are difficult, or impossible, to

remove. The rinsing water quality required is as

follows [701:

Iron < 0.05 ppm

Manganese < 0.02 ppm

Silicate ( SiOz)


13/15

understood. A vast number of empirical investiga-

tions show that it is very hazardous to make general

statements, especially about the influence of different

parameters on fouling.

Experience indicates that if we are to be able

to describe the dynamics of ultrafiltration it will

be necessary to combine knowledge from different

areas. The theoretical models must combine knowl-

edge and experience of fluid mechanics, mass trans-

fer and surface chemistry phenomena.

Nomenclature

A*

A

a

a

c

cb

57

CP

:

4

4

J

JO

K

Kg

KO

k

L

AL

AL,

AP

PWF

PWP

n

n

R

I

4

RS

Re

rP

SP

S

Sh

t

u

0:

&

L-I

mv

d

proportionality constant in eqn. (13)

constant in eqn. ( 17)

solution-dependent constant in eqn. (4), Pa

constant in eqn. (17)



bulk concentration, wt.

gel concentration, wt.

permeate concentration, wt.

concentration at membrane wall, wt.

diffusivity, m2 s-

hydraulic diameter of flow channel, m

diameter of solute molecules, m

permeate flux, 1 m-* h-

initial flux, 1 m-* h-

specific permeability, mz

permeability of a gel layer, m*

specific permeability at zero pressure, m*

mass transfer coefficient, m s-l

length of flow channel, m

thickness, m

thickness of gel layer, m

pressure difference across membrane, Pa

pure water flux, 1 mm* h

pure water permeability, 1 m-*

hh

Pa-

solution-dependent constant in eqn. (4)

exponent in eqn. ( 18)

adsorbed layer hydraulic resistance, m ~

boundary layer hydraulic resistance, m-

gel layer hydraulic resistance, m -

membrane hydraulic resistance, m ~

solute hydraulic resistance, m-

Reynolds number

pore radius, m

membrane surface porosity

Schmidt number

Sherwood number

time, s

cross-flow velocity, m s-

compaction constant in eqn. (1 l), Paa

porosity of gel

viscosity, Pa s

osmotic pressure difference across mem-

brane, Pa

osmotic pressure at membrane wall, Pa

reflection coefficient

79

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