www.elsevier.com/locate/ultsonch

Ultrasonics Sonochemistry 12 (2005) 29–35

The optimisation of ultrasonic cleaning proceduresfor dairy fouled ultrafiltration membranes

Shobha Muthukumaran a, Sandra Kentish a, Sharan Lalchandani a,Muthupandian Ashokkumar b,*, RaymondMawson c, Geoff. W. Stevens a, Franz Grieser b

a Department of Chemical and Biomolecular Engineering, University of Melbourne, Parkville, Victoria 3010, Australiab Department of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia

c Food Science Australia, Private Bag 16, Werribee Victoria 3030, Australia

Received 8 February 2004; accepted 10 May 2004

Available online 23 July 2004

Abstract

Ultrafiltration (UF) of whey is a major membrane based process in the dairy industry. However, commercialization of this appli-

cation has been limited by membrane fouling, which has a detrimental influence on the permeation rate. There are a number of dif-

ferent chemical and physical cleaning methods currently used for cleaning a fouled membrane. It has been suggested that the

cleaning frequency and the severity of such cleaning procedures control the membrane lifetime. The development of an optimal

cleaning strategy should therefore have a direct implication on the process economics. Recently, the use of ultrasound has attracted

considerable interest as an alternative approach to the conventional methods. In the present study, we have studied the ultrasonic

cleaning of polysulfone ultrafiltration membranes fouled with dairy whey solutions. The effects of a number of cleaning process

parameters have been examined in the presence of ultrasound and results compared with the conventional operation. Experiments

were conducted using a small single sheet membrane unit that was immersed totally within an ultrasonic bath. Results show that

ultrasonic cleaning improves the cleaning efficiency under all experimental conditions. The ultrasonic effect is more significant in

the absence of surfactant, but is less influenced by temperature and transmembrane pressure. Our results suggest that the ultrasonic

energy acts primarily by increasing the turbulence within the cleaning solution.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Membrane cleaning; Ultrafiltration; Whey protein; Cavitation

1. Introduction

Membrane separation processes have major applica-

tion within the dairy industry, being widely used in the

processing of whey and milk products. One of the seri-

ous hurdles in the application of membrane technology

to the dairy industry is fouling of the membrane surface

and its pores with organic and inorganic components.

1350-4177/$ - see front matter � 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.ultsonch.2004.05.007

* Corresponding author. Address: Department of Chemical and

Biomolecular Engineering, University of Melbourne, Parkville, Victo-

ria 3010, Australia. Fax: +61 3 93475180.

E-mail address: masho@unimelb.edu.au (M. Ashokkumar).

Membranes used for ultrafiltration of whey or milk

are cleaned on a regular basis to ensure hygienic opera-tions and maintain membrane performance [1]. Unfor-

tunately, the users are often reliant on protocols

recommended by the membrane suppliers. However,

these may not necessarily be optimized for a particular

application. The use of non-optimal cleaning conditions

incurs unnecessary operational costs through the over

use of chemicals. Further, non-optimal conditions may

significantly reduce the lifetime of the membrane, whichin turn increases the replacement cost.

The development of an optimal membrane cleaning

strategy should lead to important process improvements

PG

Computer

Whey solution

Feed

Permeate balance

Gear pump

Ultrasonic bath

Membrane

Permeate

Back pressure valve

Retentate

Transducers

Computer

Whey solution

Feed

Permeate balance

Gear pump

Membrane

Permeate

Back pressure valve

Retentate

PG

Computer

Whey solution

Feed

Permeate balance

Gear pump

Membrane

Permeate

Back pressure valve

Retentate

Fig. 1. Experimental set-up for ultrasound-assisted use of polymeric

membranes in a cross-flow unit (PG represents the pressure gauge).

30 S. Muthukumaran et al. / Ultrasonics Sonochemistry 12 (2005) 29–35

including reduction in the loss of production time, op-

timized use of chemicals, increased lifetime of the mem-

branes, and improved permeate flux and quality control,

which bear direct implication on process economics.

However, little attention is often paid to such optimisa-

tion. Cleaning sequences have been almost identical fordifferent types of membranes and feed solutions.

Stoner et al. [2] have found that the cleaning efficiency

of a polysulfone ultrafiltration membrane fouled with

whey was increased when prefiltered through a 0.45-lmmicrofiltration membrane. This was due to the removal

of large aggregates of protein. The effect of membrane

hydrophilicity, pH and ionic strength on cleaning was

studied by Kim and Fane [3] in Bovine Serum Albuminfouling of retentive membranes. Tran-Ha and Wiley [4]

reported that the cleaning efficiency was improved at

higher ionic strengths, as the changes in ionic strength

appeared to produce changes in the conformation of

bound foulant either through charge interaction effects

or through binding. Enzymatic cleaning has been found

useful for the removal of whey protein solutions from

inorganic membranes [5] and totally retentive polysulf-one membrane [6]. Matzinos and Alvarez [7] studied

the kinetics of rinsing and cleaning of an ultrafiltration

inorganic membrane fouled with whey proteins. It was

observed that the amount of protein removed during

rinsing was quite significant, as compared to the total

amount removed during rinsing and cleaning.

As reported by Deqian [8], ultrasound is one of the

most effective cleaning technologies for reverse osmosisand ultrafiltration membranes. Recently we have re-

ported that ultrasonic cleaning is an alternate tool spe-

cifically for cleaning dairy fouled ultrafiltration

membranes [9]. Ultrasound has been widely used as a

method for cleaning materials because of the physical ef-

fects of the cavitation phenomenon. Ultrasonic waves

provide a vigorous mixing within the whole volume of

the system. At a macroscopic scale, strong convectivecurrents known as acoustic streaming increase turbu-

lence. At a microscopic scale, the physical effects associ-

ated with the implosive collapse of cavitation bubbles

help to generate micromixing in a liquid. The asymmet-

ric violent collapse of these bubbles near a solid surface

leads to formation of liquid microjets, which enhances

the cleaning process [10].

Li et al. [11] recently reported that ultrasound associ-ated cleaning was useful for nylon microfiltration mem-

branes fouled by Kraft paper mill effluent. Kobayashi

et al. [12] have shown that the ultrasonic technique

was very effective to remove the fouling of ultrafiltration

and microfiltration membranes, which were used to

treat peptone and milk solutions, respectively. Our pre-

vious work [9] has shown that the use of surfactants

in combination with the ultrasound has a synergisticeffect, leading to a substantial improvement in the flux

recovery.

The purpose of the present study is to investigate the

effect of cleaning parameters such as, pH, surfactant

concentration, transmembrane pressure and tempera-

ture when used in combination with ultrasound. Prelim-

inary data involving some of these parameters were

shown in our previous report [9]. Further experimentaldata incorporating a range of concentrations has been

provided in this report. We consider specifically the

cleaning of whey fouled ultrafiltration membranes, with

a view to developing an optimal cleaning regime.

2. Experimental procedures

2.1. Experimental set up

The experimental set-up used for this study is shown

in Fig. 1. Flat sheet polymeric ultrafiltration membranes

with 30,000 MWCO and an effective membrane area of

30 cm2 were used in a Minitan S unit (Millipore, Inc.).

The membrane (15 cm·11 cm) was placed betweentwo acrylic manifolds of thickness 2.3 cm, which werein turn held in place by stainless steel plates of 1.1 cm

thickness.

Perforated silicone separators of approximately 1 mm

thickness were placed between the membrane and each

manifold in order to create a series of linear flow chan-

nels. A gear pump, operating at 0.5–1.0 litre/min was

used to pump the feed solution through the cross-flow

ultrafiltration unit. The permeate flux was measuredby an electronic balance, connected to a PC. During

the experiments the retentate was recycled to the feed

tank. An ultrasonic bath (Ultrasonics, Australia, Model

FXP14DH) of size (29.5 cm·24 cm·20 cm) with a fre-quency of 50 kHz and a nominal power of 300 W was

employed in this study. The membrane unit was com-

pletely immersed in 5000 ml of water contained in the

ultrasonic bath and kept 3 cm above the bottom ofthe ultrasonic bath throughout the entire experimental

S. Muthukumaran et al. / Ultrasonics Sonochemistry 12 (2005) 29–35 31

period, but with sonication only turned on as needed.

All the experiments were carried out at the full nominal

power level of 300 W unless otherwise specified.

Whey solutions used as the foulants were reconsti-

tuted using deionised water and spray dried non-hygro-

scopic whey powder from Murray Goulburn Co-Op Co.Ltd and Bonlac. Ltd. A 6% w/w reconstituted whey

solution was prepared at 50 �C and then cooled downto room temperature. All experiments were carried out

at room temperature unless otherwise specified. Milli-

Q and/or distilled water was used in all experiments.

Calculated amounts of sodium hydroxide, water, and a

mixture of 1:1 nitric/orthophosphoric acid were used

to form solutions of varying pH.

2.2. Experimental procedure

Each experiment commenced with distilled waterbeing circulated through the system at a fixed flow rate

(550 ml/min) and applied pressure (55 kPa) for about

15–20 min to compress the membrane and to build up

a stable flow field. The clean water flux was recorded

after 30 min of water filtration. This initial flux was used

to calculate the clean membrane resistance Rm from

Darcy�s Law i.e.:

Rm ¼ DP=lJ ð1Þwhere DP is the transmembrane pressure (TMP), l is thepermeate viscosity and J is the permeate flux [7,13]. The

average initial water flux was found to be 5.1±0.8·10�5

m3 m�2 s�1 based on two standard deviations from the

mean over 68 results. The average clean membrane

resistance was thus calculated as 1.1·1012 m�1. Therewas no trend in this value with time, consistent with

our previous results that membrane integrity was unaf-

fected by long term use of ultrasonic irradiation [9].

The pure water was then replaced with freshly prepared

whey solution. This feed solution was circulated for 30

min at 550 ml/min and at a transmembrane pressure

of 55 kPa to foul the membrane. After fouling, the mem-

brane surface was rinsed with water for 10 min at 55 kPaand 550 ml/min. The rinsing removed reversible fouling

resulting from labile surface deposits and concentration

polarization. The permeate rate of water for the latter 5

min of this step was recorded and used to calculate Rr,

the resistance of the irreversible fouling deposit (Eq.

1). The flux after fouling was 6.4±0.9·10�6 m3m�2

s�1 based on two standard deviations from the mean

over 52 results.The fouled membrane was next cleaned for 10 min,

using a specific combination of pH, surfactant concen-

tration, transmembrane pressure, cross-flow velocity

and sonication power as experimental variables. During

this step the ultrasonic bath water was replaced as nec-

essary to maintain the appropriate operating tempera-

ture to within 20±2 �C.

After cleaning, the membrane surface was rinsed with

water again for 10 min at 55 kPa to remove cleaning

solutions and provide a consistent regime for flux calcu-

lation. The permeate rate of water was again recorded in

the latter 5 min of this cycle as a measure of the effi-

ciency of the previous cleaning step. This was convertedusing Eq. (1) to the cleaning resistance Rc. This mem-

brane resistance after the cleaning step ranged from

1.6·1013 to 2.1·1012 m�1.

In the final step the membrane initial water flux was

recovered by further cleaning. We circulated 0.1 M so-

dium hydroxide with 15 mM sodium dodecyl sulfate

(SDS) for 10–15 min. The membrane was then left to

soak in this solution for 30 min and then flushed usingMilli-Q and/or distilled water. When not in use, the

membrane was soaked in 0.25% w/w sodium meta bisul-

phite, which was replaced regularly.

The cleaning efficiency has been used as the criterion

to assess the cleaning process. This is defined using the

approach of Matzinos and Alvarez [7], i.e.,

Cleaning efficiency; CE ¼ Rr � RcRr � Rm

� 100: ð2Þ

We also consider the incremental increase in cleaning

efficiency resulting from the use of ultrasound as:

CEU=S � CECE

: ð3Þ

3. Results and discussion

3.1. Effect of pH

In our previous work [9], we have reported that an in-

crease in pH enhanced the flux improvement. However

an optimal pH range was not identified. We have ex-

tended this study to include a range of pH in order to

identify an optimal pH for the ultrasonic cleaning pro-

cess. Fig. 2 shows that the cleaning efficiency increases

with an increase in the solution pH. A maximum clean-ing efficiency occurs at pH 12 (0.4 wt% NaOH), which is

consistent with previously published results [1,14]. These

workers conclude that as the sodium hydroxide concen-

tration is increased, the fouling deposit becomes increa-

singly swollen. The maximum cleaning efficiency occurs

when the deposit is of maximum voidage. Further in-

creases in sodium hydroxide concentration lead to a

reduction in voidage. By comparing the effect of pHwith and without sonication, it can also be noted that

larger percentage gains in ultrasonic cleaning efficiency

occur when the pH has been adjusted for optimal effec-

tiveness, i.e., between 11.5 and 13.

It is of relevance to mention that the use of acid (low

pH) to clean the membrane results in only a small flux

recovery. Acid cleaning is used in dairy ultrafiltration

10

20

30

40

50

60

70

80

4 6 8 10 12 14 16

Without UltrasoundWith ultrasound

Cle

anin

g E

ffic

ienc

y (%

)

pH

Fig. 2. Effect of solution pH on the cleaning efficiency in the presence

and absence of ultrasound.

32 S. Muthukumaran et al. / Ultrasonics Sonochemistry 12 (2005) 29–35

to remove calcium deposits. As our experiments werecarried out for relatively short fouling times and with

low calcium solutions, acid cleaning (low pH) is ineffec-

tive. This result is in contrast to acid cleaning of inor-

ganic membranes fouled with WPC, where the

concentration of calcium ions played a major role in

membrane fouling [15]. However, our results suggest

that the ultrasonic influence is significant under acidic

conditions, and so the use of ultrasound during acidiccleaning should be investigated further.

3.2. Effect of SDS

In our previous work [9], we have shown that the use

of an anionic surfactant, sodium dodecyl sulfate (SDS)

enhanced the flux rates. However the optimal concentra-

tion of SDS was again not identified and hence similarexperiments were carried out in order to find out an

optimal concentration of the surfactant. Fig. 3 shows

the effect of SDS concentration on the cleaning effi-

ciency, which increases with increasing surfactant con-

centration up to the critical micelle concentration of

SDS (about 8 mM).

20

40

60

80

0 5 10 15

Without ultrasoundWith ultrasound

Cle

anin

g E

ffic

ienc

y (%

)

[SDS] (mM)

Fig. 3. Effect of SDS concentration on the cleaning efficiency in the

presence and absence of ultrasound; solution pH=6.5.

The overall cleaning efficiency is always higher when

ultrasound is used in combination with surfactants, indi-

cating as we have previously reported [9], that there is at

least some synergism in the use of these two cleaning

agents together. However, the effect of ultrasound is

proportionately greater in the absence of surfactant(141% improvement in cleaning efficiency versus 4.6%).

This suggests that the action of the two agents is similar

in some respects, as both act to disrupt linkages between

the proteins and the membrane surface. The effect of

SDS can be attributed to the cleaning strength of emul-

sifiers acting to solubilise proteins.

3.3. Effect of SDS and pH

The data shown in Figs. 2 and 3 has indicated that

the ultrasonic cleaning is effective at a solution pH of

about 12 and an SDS concentration of about 8 mM.

However, the SDS results (Fig. 3) were obtained at a

neutral solution pH of about 6.5. To determine if the

solution pH influences the effect of SDS, experiments

were conducted with variable SDS levels at a solutionpH of 12 (Fig. 4).

It can be noted that the ultrasonic irradiation again

has proportionately more effect at low surfactant con-

centrations. The ultrasonic incremental increase is 9%

when no surfactant is present versus 3% at 10 mM

SDS. In this case, increases in the SDS concentration be-

yond 12 mM result in a decline in cleaning efficiency.

This may be due to the excess cleaning agent contribut-ing to the fouling load in itself.

3.4. Effect of transmembrane pressure (TMP)

If acoustic cavitation is a factor in the present results

then changes to the TMP can be expected to have an ef-

fect on the ultrasonic cleaning efficiency. An increase in

TMP will decrease the number of cavitating bubbles asthe cavitation threshold is increased. Conversely, at

65

70

75

80

85

90

0 5 10 15

Without ultrasoundWith ultrasound

Cle

anin

g E

ffic

ienc

y (%

)

[SDS] (mM)

Fig. 4. Effect of SDS on the cleaning efficiency in the presence and

absence of ultrasound; solution pH=12.

S. Muthukumaran et al. / Ultrasonics Sonochemistry 12 (2005) 29–35 33

higher pressures, the effects of cavitational collapse are

more vigorous. Experiments were thus carried out at dif-

ferent TMPs in order to investigate if indeed the TMP

has any influence on the cleaning efficiency. Experiments

were carried out with 30 min of membrane fouling at

300 kPa and 10 min of membrane cleaning with waterat different TMPs. All water fluxes were measured at

55 kPa as per normal experimental procedure.

It can be seen from Fig. 5 that the cleaning efficiency

is higher at lower TMP both with and without ultra-

sound. This contrast with the results from cleaning

membranes fouled with beer, where they have reported

that a higher TMP had little effect on the cleaning out-

come [16]. However, most of the reviewed work reportsthat the low TMP is more efficient in removing depos-

ited layers from organic membranes [1,14,17].

Generally, the results show that TMP has only a mar-

ginal influence on the ultrasonic cleaning effect. This

suggests that the ultrasonics acts predominantly through

acoustic streaming and increased turbulence rather than

cavitation.

To confirm the extent of cavitation directly, we con-ducted a simple test with a piece of aluminum foil. When

the aluminum foil was placed directly in the bath while

operating at full nominal power, vigorous cavitation

was observed, as evidenced by a large number of holes

appearing in the foil. However, when the aluminum foil

was placed inside the membrane unit, which in turn was

45

50

55

60

65

70

50 100 150 200 250 300 350

Without ultrasoundWith ultrasound

Cle

anin

g E

ffic

ienc

y (%

)

TMP (kPa)

Fig. 5. Effect of TMP on the cleaning efficiency (fouling 30 min at 300

kPa and sonication 10 min at different TMPs; flux measured at 55

kPa).

Table 1

Effect of cross-flow rate on cleaning efficiency at 25 �C and 55 kPa

Cross-flow rate (ml/min) Reynolds number SDS and pH

550 450 SDS 0 mM, pH 12

970 740 SDS 0 mM, pH 12

550 450 SDS 10 mM, pH 12

970 740 SDS 10 mM, pH 12

placed in the bath, only three or four small pinholes

were observed. This result suggests that only a small

proportion of the ultrasonic power penetrates the mem-

brane holder and confirms that the ultrasonic cleaning

effect occurs predominantly through acoustic streaming

and increased turbulence rather than cavitation.

3.5. Effect of cross-flow rate and temperature

We have varied the cleaning solution flow rate from

550 to 970 ml/min to examine the effect of cross-flow

velocity (see Table 1). This corresponds to Reynolds

numbers of 450 and 740 (cross-flow velocities of

0.2–0.33 m/s), implying laminar flow throughout. Table1 shows that there is essentially no effect upon cleaning

efficiency within this flow range.

These results agree with the findings of other workers

[14,18,19] who reported that cleaning performance was

not a strong function of the surface shear rate or the

cross-flow velocity, even when flow moved from the la-

minar to turbulent regime [14]. However the total quan-

tity of deposits and their stability within the stagnantboundary layer of the fluid is known to be dependent

on the cross-flow velocity and fluid turbulent patterns

[16]. The use of membrane spacers to achieve turbulent

conditions may result in higher cleaning efficiencies.

The influence of the solution temperature on the

cleaning efficiency is shown in Table 2, in which two sets

of experiments of cleaning at 25 and 55 �C were per-formed at 970 ml/min. A higher cleaning efficiency is ob-tained at 55 �C, as expected from previously publishedresults [1,17,18]. In general an increased in temperature

is known to lead to an improvement of diffusion, in-

crease of solubility, and an increase of chemical splitting

of soil. While the Reynolds number increases with tem-

perature as a result of lower fluid viscosity, the present

results are still within the laminar flow range and hence,

this is not expected to be a major cause of the increase incleaning efficiency.

The resulting Table 2 shows that the effect of ultra-

sound is most pronounced when neither heat nor surfac-

tant is available to assist with cleaning. The use of

concurrent heat and/or surfactant reduces the ultrasonic

effect. This again suggests that the action of these three

agents are similar in some respects, as all act to dis-

rupt linkages between the proteins and the membrane

Cleaning efficiency (%) Incremental increase (%)

With US Without US

78 67 16

76 65 17

85 82 4

83 82 1

Table 2

Effect of temperature on cleaning efficiency at 970 ml/min retentate flow rate and 55 kPa

Temperature(�C) Reynolds number SDS and pH Cleaning efficiency (%) Incremental increase (%)

With US Without US

25 740 SDS 0 mM, pH 12 76 65 17

55 1300 SDS 0 mM, pH 12 86 82 5

25 740 SDS 10 mM, pH 12 83 82 1

55 1300 SDS 10 mM, pH 12 91 87 5

34 S. Muthukumaran et al. / Ultrasonics Sonochemistry 12 (2005) 29–35

surface. Increases in temperature do not consistently

lead to a reduction in the ultrasonic effect however,

which again suggests that cavitation is not a dominant

factor in the observed flux recovery. It is generally

accepted that the intensity of acoustic cavitation bubble

collapse will decreases with an increase in solution tem-

perature [20].

Table 2 shows that the cold process cleaning effi-ciency can be improved by up to 40% [(91�65)/65] byadding heat, SDS and ultrasonics.

3.6. Effect of ultrasonic power on the cleaning efficiency

The ultrasonic power transferred to the medium can

be calculated calorimetrically from the increase in tem-

perature of the liquid medium. A digital thermometerwas used to measure the increases in water temperature

over time. The power (P) delivered was then calculated

using the following equation,

P ¼ DTt

� �cpM ð4Þ

where, t is duration of sonication; cp heat capacity of

water (4.18·103 Jkg�1K�1); M the mass of water in

the ultrasonic bath (kg).

Power calibration of the ultrasonic bath was carried

out without the membrane unit in place, but with thebath filled to the same vertical height as during experi-

ments. While the bath was not enclosed, our calculations

show that the heat loss due to free convection was less

than 1 W. Fig. 6 shows the effect of ultrasonic power

10

20

30

40

50

60

0 10 20 30 40 50 60

Cle

anin

g E

ffic

ienc

y (%

)

Ultrasonic Power (Watts)

Fig. 6. Effect of ultrasonic power on the cleaning efficiency. 30 min

fouling at 55 kPa; 10 min sonication time; flux measured at 55 kPa.

on the cleaning efficiency. Experiments were carried

out with 30 min of membrane fouling at 55 kPa and

10 min of cleaning with water at different ultrasonic

power. The results shown in Fig. 6 indicate that the

cleaning efficiency increased linearly with ultrasonic

power, confirming our previous results [9].

This gives an indication that further increases in

ultrasonic power could lead to higher cleaning efficiency.

4. Conclusions

A systematic experimental study confirms previously

established results that whey fouled ultrafiltration mem-

branes can be ultrasonically cleaned. The optimal exper-

imental parameters for effective ultrasonic cleaning are:high temperatures, low transmembrane pressures, a

solution pH of 12 and a surfactant concentration close

to that of the critical micelle concentration.

The concurrent use of ultrasound enhances the clean-

ing process, improving the cleaning efficiency under all

experimental conditions, typically by 5–10%. The ultra-

sonic effect is only marginally influenced by temperature

and transmembrane pressure and grows linearly withultrasonic power. These results suggest that the ultrasonic

energy acts predominantly by increasing the turbulence

within the cleaning solution. However, the influence of

acoustic cavitation cannot be completely excluded.

Acknowledgment

SM is the recipient of an Australian Post-graduate

Award. Financial support for this project has been pro-

vided through a University of Melbourne Research and

Development Grants Scheme award and a University of

Melbourne-CSIRO Collaborative Research Support

Scheme award. This support is gratefully acknowledged.

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