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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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