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Separation Science and Technology
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Effect of Ultrasound on Membrane Filtration and
Cleaning Operations
Kok-Kwang Ng
Lin
a
a
, Chien-Ju Wu
, Pui-Kwan Andy Hong
b
a
, Hsiu-Lan Yang
, Chung-Hsin Wu
c
a
, Sri Chandana Panchangam
& Cheng-Fang Lin
a
, Yen-Ching
a
a
Graduat e Inst it ut e of Environment al Engineering, Nat ional Taiwan Universit y, Taipei,
Taiwan
b
Depart ment of Civil and Environment al Engineering, Universit y of Ut ah, Salt Lake Cit y, UT,
USA
c
Depart ment of Chemical and Mat erials Engineering, Nat ional Kaohsiung Universit y of
Applied Sciences, Kaohsiung, Taiwan
Accept ed aut hor version post ed online: 29 May 2012.
To cite this article: Kok-Kwang Ng , Chien-Ju Wu , Hsiu-Lan Yang , Sri Chandana Panchangam , Yen-Ching Lin , Pui-Kwan
Andy Hong , Chung-Hsin Wu & Cheng-Fang Lin (2012): Ef f ect of Ult rasound on Membrane Filt rat ion and Cleaning Operat ions,
Separat ion Science and Technology, 48: 2, 215-222
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Separation Science and Technology, 48: 215–222, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0149-6395 print=1520-5754 online
DOI: 10.1080/01496395.2012.682289
Effect of Ultrasound on Membrane Filtration and Cleaning
Operations
Kok-Kwang Ng,1 Chien-Ju Wu,1 Hsiu-Lan Yang,1 Sri Chandana Panchangam,1
Yen-Ching Lin,1 Pui-Kwan Andy Hong,2 Chung-Hsin Wu,3 and Cheng-Fang Lin1
1
Graduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan
Department of Civil and Environmental Engineering, University of Utah, Salt Lake City, UT, USA
3
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied
Sciences, Kaohsiung, Taiwan
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2
The impact of ultrasound (US) on membrane filtration and
cleaning were studied and compared at various operating
parameters of nominal pore sizes of 10 and 100 kDa membrane,
trans-membrane pressure (TMP) of 100 and 140 kPa, and US frequencies of 20 kHz and 40 kHz. An average of 15%–20% increase
of permeability was observed when US (20 kHz) was applied to
assist membrane filtration on 10 kDa membrane and 100 kPa transmembrane pressure (TMP). However, an insignificant improvement
was observed in the case of larger pore size membrane at higher
TMP (140 kPa). US also augmented the membrane cleaning process effectively. Lower frequency 20 kHz US exhibited a higher flux
recovery (>90%) than the high frequency 40 kHz (59%) using the
10 kDa pore size membrane with US-assisted membrane cleaning.
Important factors influencing optimization of US effectiveness lie
heavily on its configuration and operation. The experimental results
as supported with SEM images demonstrate that US-assisted filtration and cleaning are most effective when membrane pore size,
US frequency, and TMP are lower.
Keywords flux; fouling; membrane cleaning; membrane process;
ultrasound
INTRODUCTION
Because of the effectiveness of membrane filtration in
removing suspended particles and health concerned contaminants, it has been widely used to treat drinking water,
industrial wastewaters, and subsequently for reclamation
purposes. In treatment of drinking water, membrane processes play a vital role which complements traditional
water treatment processes in order to meet increasingly
stringent drinking water standards and secure public confidence (1). In treatment of industrial water, membranes can
also be configured in conjunction with other techniques to
achieve various levels of water quality requirements needed
Received 14 December 2011; accepted 1 April 2012.
Address correspondence to Cheng-Fang Lin, Graduate
Institute of Environmental Engineering, National Taiwan
University, 71 Chou-Shan Rd., Taipei 106, Taiwan. Tel.: þ8862-3366-7427; Fax: þ886-2-2392-8830. E-mail: cflin@ntu.edu.tw
by different industries. Moreover, as clean and easily
accessible water sources became scarcer, membrane processes have proven to be a very reliable technology for
water reuse in many parts of the world. Because of their
exhibited qualification membrane processes have become
indispensable as they:
1. produce effluents of high quality,
2. have small footprints and use indoor space that is
immune to harsh weather conditions,
3. are resilient and easy to operate and maintain,
4. allow expansion of existing facilities, and
5. reduce chemical uses and sludge production (2,3).
While incorporation of membrane filtration in water
treatment processes offers the above advantages, reducing
membrane fouling and flux decline has been a vital challenge in membrane applications that can impact water production efficiency, membrane life, and operating costs (4).
The mechanisms involved in membrane fouling of ultrafiltration (UF) and microfiltration (MF) are pore blocking
(complete blocking), direct adsorption (standard blocking),
long term adsorption (intermediate blocking), boundary
layer resistance (cake filtration), and concentration polarization layer that leads to reversible or irreversible fouling
conditions (5–7). Reversible fouling blockage may be
cleaned relatively easily by physical means such as backwashing or air stripping. However, cleaning for irreversible
fouling usually requires chemicals such as a strong acid,
base, enzyme, or surfactant. The results of chemical cleaning are often less than satisfactory. In addition, the membrane surface can be gradually degraded by successive
chemical cleanings thereby reducing their lifespan. Moreover chemical cleaning also generates chemical wastes with
associated disposal problems (4).
Incorporation of ultrasound (US) irradiation for membrane cleaning is an interesting option that can improve
membrane operation (4,8–12). The principle in using US
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216
K. K. NG ET AL.
during membrane filtration relies primarily upon with the
cavitation effect of US. When the ultrasonic device emits
a sufficiently large supersonic wave to overcome the cohesive forces within water, water undergoes constant pressure
extrusion and creates a lot of micro-scale bubbles around
the membrane surface. As these bubbles form and burst
under different pressures, they release energy and cause
vibration that loosens particles on the membrane surface
that can reduce membrane fouling without compromising
treatment efficiency (4). Sabri et al. (13) studied intermittent US at 45 kHz during filtration of pulp and paper mill
wastewaters and they reported up to 400% flux enhancement. Kobayashi et al. (9) found stronger permeation
enhancement by US at low frequency (28 kHz) than at high
frequency (45 or 100 kHz). Kobayashi et al. (9) and
Kokugan et al. (14) found that US enhanced permeation
by increasing mass transfer coefficient of water across
microfiltration ceramic membrane as well as ultrafiltration
organic membrane. US irradiation studies identified the
prevention of concentration polarization and membrane
fouling due to cake layer stripping, providing a less compressible fouling cake, and decreasing solute concentration
nears the membrane surface (4,9,15).
Lamminen et al. (4) categorized US cleaning mechanisms into four distinct pathways:
1.
2.
3.
4.
acoustic streaming,
microstreaming,
microstreamers, and
micro-jets.
Only acoustic streaming does not involve the collapse of
cavitation bubbles. The ultrasonic waves cause bulk water
movement to and from the membrane surface that generates forces to push and pull the attached particles away
from the surface. Microstreaming occurs when bubbles
oscillate in size and create significant shear forces with an
effective range of bubble diameter. The microstreamers
phenomenon is defined as the cavitation bubbles travelling
in ribbon-like paths as these bubbles travel vigorously
around the membrane surface. They tend to coalesce and
scour away particles while translating to antinodes. Microjets occur when two asymmetrical cavitation bubbles
collapse at the same time or just one bubble collapses near
a solid interface and creates a strong jet of water that
scours particles away from the membrane surface.
Numerous researchers have demonstrated the effectiveness of US-associated membrane cleaning with the purpose
of increasing permeate flux and extending the membrane’s
lifespan. Li et al. (16) reported that US with forward flushing recovered flux on a 0.2 mm nylon membrane fouled by a
Kraft paper mill effluent. Matsumoto et al. (17) claimed
that US-assisted backwashing cleaned fouled membrane
more thoroughly than conventional backwashing. The use
of US frequency from 28 kHz to 1 MHz for membrane
cleaning was studied recently (4,9,18,19) and the results
showed benefits of US-assisted cleaning (20). Several
researchers (4,9,18,19) agreed that lower US frequencies
exhibited higher flux recoveries than higher frequencies
during US-assisted cleaning. Kobayashi et al. (19) observed
virtually no cleaning effect at 100 kHz when compared to
lower frequencies at 45 and 28 kHz. Using the MF membrane, Wakeman and Tarleton (21) concluded that US at
23 kHz offered not only better cleaning efficiency but also
less energy consumption than at 38 kHz. In addition,
Lozier and Sierka (22) combined US with ozone to reduce
suspended solids on the membrane surface. US has been
successfully combined with membrane backwashing in
the laboratory. However, it has not been widely commercialized because of membrane damages in several occasions
(13,23,24). Nevertheless, Lamminen et al. (4) and
Muthukumaran et al. (15) stated that US-assisted cleaning
did not affect membrane integrity. US irradiation can be
applied wither in intermittent mode or continuous mode.
Using intermittent US application is likely to be less energy
intensive than continuous US irradiation and pulsed US
has been discussed to be effective at removing Bovine
serum albumin (BSA) fouling layers from a cross-flow
MF (25). However, Simon et al. (26) indicated that the
use of continuous application was more effective than
intermittent application when applied to polymer solutions. Clearly, more studies are required to resolve the
conflicting results.
Owing to the advantages offered by US-assisted membrane filtration and cleaning in the present study, optimal
US-assisted cross-flow membrane operation conditions
were conducted. Both the filtration and cleaning processes
were evaluated with the aid of intermittent (10 sec every
10 min of operation) ultrasonic irradiation. Hypotheses of
membrane fouling at two different trans-membrane pressures (TMPs) 100 and 140 kPa and membrane pore sizes
(10 and 100 kDa) were discussed. Regular backwashing
and US-assisted backwashing were performed. The overall
objective of this research was to reduce membrane fouling
and enhance membrane cleaning by exploring
1. the ability of US to clean fouling associated with two
specific membrane pore sizes (10 and 100 kDa) in a
flat-sheet cell,
2. the ability of US to clean the membrane under two
different trans-membrane pressures and
3. two different US frequencies in cleaning membrane
fouling.
MATERIALS AND METHODS
Feed Water
Humic acid (sodium salt) from Aldrich was used to prepare a stock feed solution of 9 mg L1 by dissolving 1 g of
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ULTRASOUND IMPACT ON MEMBRANE FILTRATION AND CLEANING
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humic acid in 1 L deionized water, which then was filtered
through a 0.45 mm membrane to remove particles from the
solution. A calibration curve of 0 to 10 mg L1 was prepared from a working solution of 100 mg L1 potassium
hydrogen phthalate (C8H5KO4). Humic acid (feed solution) was analyzed for its total organic carbon (TOC)
concentration by using a LACHAT (Model IL-550 TOCTN) and a concentration of 9.02 mg L1 was recorded with
calibration curve. 1 N of sodium hydroxide (NaOH) and
1 N of hydrochloric acid (HCl) were used to adjust the
pH of the solution to 7.2. Feed solutions were supplemented with sodium chlorite (NaCl) to maintain conductivity
near 300 mS cm1. The stock humic acid solution and feed
solutions of 9 mg L1 were stored at 4 C for later use.
Ultrafiltration (UF) Membrane
Negatively charged Pall-polyethersulfone (PES) flat
sheet ultrafiltration membranes (effective filtration area ¼
25 cm2; pore size ¼ 10 kDa and 100 kDa) were used in this
study at constant trans-membrane pressures (TMPs) of
100 kPa and 140 kPa. Membranes of two different pore
sizes were compared in US-assisted operations. Prior to
experiment, the membrane was washed with methyl alcohol
(CH3OH, anhydrous) purchased from Macron Chemicals
for 5 min and rinsed with distilled water for 8 h. The PES
flat sheet membrane was then operated with distilled water
for 2 h until a near-constant flux was reached to eliminate
pre-compaction at the same pressure. The membranes were
operated for 3 h with and without US operation and were
analyzed for fouling. All fouled membranes were cleaned
by forward flushing, backwashing, and US-assisted
backwashing.
Ultrasound Module
The US device consisted of a generator, frequency converter, amplifier, fixed stand and transducer manufactured
by Bensonic (BW-2015P). During the US-assisted filtration
experiment, the power intensity was measured at 105 W
with 20 kHz. The US-assisted filtration setup is shown in
Fig. 1. The US transducer was kept at 3.2 cm directly above
membrane surface. The feed solution was pre-warmed to
room temperature (25 1 C) before each membrane test.
The temperature after the sonication was measured slightly
increased to 26 C. Therefore, the influence of temperature
fluctuation on the effect of US-assisted filtration and the
feed water properties are considered to be negligible. After
30 min of filtration, US irradiation was intermittently
switched on for 10 s at every 10 min operation. Filtration
tests were performed in cross-flow mode with a cross-flow
velocity of 0.13 ms1 and fresh membrane was used for
each analysis. The feed solution passed through the membrane installed on the membrane system and permeate
was collected on the outside of the membrane system. Flux
was determined by weighing permeate with an electronic
FIG. 1. Schematic diagram of the cross-flow experimental setup for USassisted membrane operation controlled with a PC-PLC (Personal
Computed-Programmable Logic Controller). (Color figure available
online)
balance at timed intervals. All permeation and cleaning
experiments were monitored and controlled by a computer
with a programmable logic controller (PLC) and operating
software Gensis 32 automation suite. The six continuous
operating steps include:
1.
2.
3.
4.
5.
6.
humic acid solution filtration,
DI water forward flushing,
DI water backwashing,
DI water filtration,
chemical cleaning (soaking), and
DI water flushing. This semi-automatic system was operated consistently to minimize experimental variations.
The normalized flux was calculated based on initial flux
permeation as follows:
Normalized Flux ¼
Jf
100%
JI
ð1Þ
where
Jf ¼ flux measured at the point of operation
JI ¼ initial permeation flux
Ultrasound-Assisted Membrane Cleaning
The cleaning processes include regular and US-assisted
backwashing. The membrane was operated for 8 h without
US operation before the cleaning experiments. After filtration, the fouled membrane was subjected to forward
218
K. K. NG ET AL.
TABLE 1
Specifications of FEG-SEM
Content
SEM specification
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Manufacturer
Acceleration Voltage
Magnification
SEI resolution
Functions
Stand holder
JEOL, JSM-6330 F
0.5 kv 30 kv
X 10 X 500 k
1.5 nm (at 15 kv), 5.0 nm (at 1 kv)
Auto focus, lightening, contrast
X: 50 mm, Y: 70 mm,
Z: 4,8,15,25,39 mm
flushing with DI water for 30 min. The cleaning processes
were regular backwashing and US-assisted backwashing.
The US-assisted backwashing was operated with ultrasound at 20 or 40 kHz intermittently on for 10 s at every
10 min operation with a fixed power intensity of 105 W.
During US-assisted cleaning, permeate flux was recorded
every 3 min. The flux recovery rate was calculated with
the following equation:
Flux recovery ¼
Jc
100%
Jo
ð2Þ
Where:
Jc ¼ final flux after cleaning process
Jo ¼ initial flux with DI water
Scanning Electron Microscopy (SEM) Analysis
The fouled membranes were sampled for morphological
observation using a JEOL JSM-6330 F scanning electron
microscopy (FEG-SEM). The scanning electron microscopy (SEM) device consisted of an acceleration voltage,
stand holder with (X: 50 mm, Y: 70 mm, Z: 4, 8, 15, 25,
39 mm), and resolution 1.5 nm (at 15 kv), 5.0 nm (at 1 kv).
The SEM was operated at 20 keV and with a magnification
of 3,000X. The images of the membrane surfaces before
and after US-assisted filtration and cleaning were taken
for analysis. The methods were similar to those described
previously by Lin et al. (27). The detailed SEM analysis
is listed in Table 1.
RESULTS AND DISCUSSION
Impact of US-Assistance on Membrane Filtration at
Different TMPs
TMP is a primary factor that affects permeation flux
and membrane fouling. The cross-flow UF (100 kDa) permeate fluxes at TMPs 100 and 140 kPa with and without
intermittent US irradiation at 20 kHz after 30 min of filtration were evaluated. The results are presented in Fig. 2.
Both TMP experiments lasted 180 min and the permeation
fluxes were recorded every 3 min. At TMP of 100 and
FIG. 2. Comparison of permeation fluxes over time at different TMP
and MWCO of (a) 100 kPa with 100 kDa membrane pore size, (b)
140 kPa with 100 kDa membrane pore size and (c) 100 kPa with 10 kDa
membrane pore size, influent DOC ¼ 9.0 mg L1, US frequency ¼ 20 kHz,
kHz, filtration time ¼ 180 min, cross flow velocity ¼ 0.13 ms1, US
irradiation for 10 s every 10 min starting at 30 min of filtration.
140 kPa, the initial permeate fluxes were monitored at
0.54 mL min1 cm2 and 0.56 mL min1 cm2, respectively.
The final permeation of 100 kPa without US was recorded
at 0.211 mL min1 cm2, while it was measured at
0.3 mL min1 cm2 with the assistance of US which
accounted to 13% of permeation flux improvement. On
the other hand, 0.12 (without US) and 0.14 mL min1
cm2 (with US) were observed at the end of membrane
filtration with 140 kPa.
Figure 2a demonstrates that when US irradiation is
applied during the membrane filtration (after 30 min of
operation) on 100 kDa pore size membrane at constant
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ULTRASOUND IMPACT ON MEMBRANE FILTRATION AND CLEANING
100 kPa TMP, a consistent 15%–20% higher normalized
flux was recorded for the subsequent 150 min of operation.
This improvement can be attributed to US irradiation of
water that forms streams of micro-scale air bubbles around
the membrane surface. These streams of air bubbles continually fluctuate around the membrane surface in a way
of simulating the air-stripping phenomenon that scours
particulates off the surface thus enhancing membrane
permeation (4). Contrarily, the effect of US was barely
noticeable (<1%) at TMP of 140 kPa (Fig. 2b). These
experimental results support the hypothesis of US-assisted
membrane filtration is more effective at lower TMP. This
finding agrees well with results of Chen et al. (11). Higher
filtration pressure would increase compressive force and
decrease the quantity of cavitation bubbles formed. This
can be understood in light of the fact that high filtration
pressure increases liquid solution cohesiveness that hinders
cavitation formation and therefore reduces the quantity of
cavitation bubbles and their impact on permeability.
Impact of US-Assisted Filtration on Membranes of
Different Pore Sizes
Molecular weight cut-off (MWCO), as the term suggests, is an indication of the lowest molecular weight of
solute that is retained by the membrane by 90%. In many
instances, MWCO is a good assessment of how well a particular compound will be rejected during filtration based on
its molecular weight. The effects of MWCO on membrane
are strongly related to the feed solution characteristics and
the particle size distribution. However, in the comprehensive literature review, Le-Clech et al. (28) found many
contradicting conclusions on MWCO and particle size distribution. Therefore, in the present work, membranes of
pore sizes of both 10 kDa and 100 kDa were examined
for US-assisted permeation tests at TMP of 100 kPa and
cross-flow velocity of 0.13 m s1. As indicated in Fig. 2a
and Fig. 2c, the initial permeation flux (100% normalized
flux) for 100 kDa was recorded at 0.54 mL min1 cm2
and for 10 kDa at 0.33 mL min1 cm2. The permeate flux
declined faster with the 10 kDa membrane during filtration
and particularly severely in the first 60 min. He et al. (29)
tested an anaerobic MBR at constant TMP and found that
the smallest MWCO (20 kDa) showed the largest permeability loss within the first 15 min of filtration when compared to membranes of 30, 50, and 70 kDa. Membranes
with smaller pores reject a wider range of materials and
are expected to show higher flux resistance as compared
to membranes with large pores. After 180 min of filtration
without US, permeation of 10 kDa membrane dropped to
approximately 22% of its initial flux, while the permeation
of 100 kDa membrane dropped to 38.9% of its initial value
as shown in Figs. 2c and 2a, respectively.
With the assistance of US, the 10 kDa membrane exhibited stronger improvement in performance than that of
219
100 kDa membrane. As indicated in Figs. 2a and 2c, ultrasonic irradiation was started after 30 min of filtration and
the flux was immediately improved for the 10 kDa membrane but not as noticeably for the 100 kDa membrane.
A consistent 20% flux enhancement was recorded throughout filtration with the 10 kDa membrane. At the end of the
permeation experiment, ultrasonic irradiation was able to
restore the permeation flux from 22% to 50% of the initial
flux for the 10 kDa membrane but negligible permeation
improvement (<5%) for the 100 kDa membrane.
Matsumoto et al. (17) investigated the effect of pore size
on ceramic membranes in US-assisted filtration. Their
results consistently showed flux enhancement in the case
of small pore size (0.2 mm) rather than in larger pore sizes
(0.8 mm and 1.5 mm). This phenomenon led to a hypothesis
that the primary fouling mechanism for the 10 kDa
membrane was the gel layer formation and the major
mechanism for the membrane of 100 kDa membrane was
inner pore blocking.
Lin et al. (30) observed that the fraction of humic materials with MWCO <1 kDa prepared from Aldrich stock is
less than 25% of the total dissolved organic carbon in the
feed solution. They hypothesized that the primary fouling
mechanism on the 10 kDa membrane was possibly contributed by gel layer formation with very little inner pore
adsorption or clogging (Fig. 3a). On the other hand, a
much larger portion of the humic materials was more likely
to enter the pore channels of the 100 kDa membrane, which
would subsequently aggregate and stick together resulting
in a pore blocking mechanism (Fig. 3b). Katsoufidou
et al. (31) supported this hypothesis and stated that irreversible fouling was caused by internal pore adsorption
that led to progressive deterioration of membrane performance. US irradiation in water produces acoustic streaming, microstreaming, microstreamers, and micro-jets that
loosen only the accumulated particulates on surface of
membranes of small pores. It does little to membranes of
large pores in which constriction within open channels
occur (32). Therefore, US offered no permeation enhancement for the membrane of 100 kDa but great enhancement
FIG. 3. Schematic of Fouling mechanisms of UF membrane of different
pore sizes.
220
K. K. NG ET AL.
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for membranes with pore size smaller than the fouling
particles in feed solution.
During the course of this experiment, no evident
damages by the intermittent US irradiation was observed
since the permeate flux of membranes did not increase during filtration, even though Masselin et al. (23) indicated
that the PES membranes could be damaged after about
5 min exposure at 47 kHz US irradiation. On the other
hand, Cai et al. (33) reported no damage was found on
the membrane surface after US irradiation from SEM
images taken and recommended the use of US irradiation
intermittently because it is more convenient and economical to remain as less energy dependent as possible.
Ultrasound-Assisted Enhanced Membrane Cleaning
Process
Volumetric flux of PES membranes with pore sizes
10 kDa and 100 kDa were recorded for 480 min at feed solution concentration of 9 mg L1. The fouled membranes
were subjected to 30 min of forward flushing and backwashing. Conventional cleaning was compared with USassisted cleaning at 20 kHz and 40 kHz. As seen in Fig. 4,
initial permeation fluxes of DI water were recorded at
0.68 and 2.14 mL min1 cm2 for membranes with pore
sizes of 10 and 100 kDa, respectively. Once filtration
FIG. 4. US-assisted cleaning of membranes of (a) 10 kDa and (b) 100 kDa
for flux recovery, influent humic acid concentration ¼ 9.0 mg L1, membrane MWCO ¼ 100 kDa, TMP ¼ 100 kPa, cross flow velocity ¼ 0.13 ms1.
began, the fluxes dropped sharply to 0.11 and 0.24 mL
min1 cm2, respectively. During conventional cleaning,
the forward flushing and backwashing achieved 28% and
41% of flux recovery for the membrane of 10 kDa, and
19% and 24% of flux recovery for the membrane of
100 kDa. These flux recoveries were due to the removal
of foulants on the membrane surface and in the membrane
pores by water pressure (34,35).
A more significant improvement in flux recovery was
observed with the assistance of US during membrane
cleaning. Dramatic improvements were observed, namely,
more than 90% (0.67 mL min1 cm2) of flux recovery with
US at 20 kHz and 59% (0.42 mL min1 cm2) of flux recovery with US at 40 kHz when forward flushing and backwashing were performed on the membrane of 10 kDa
with US. For the membrane of 100 kDa, flux recovery
was restored to 81% of the initial flux (1.75 mL min1
cm2) with US at 20 kHz and to 69% of the initial flux
(1.49 mL min1 cm2) with US at 40 kHz. The results
demonstrated the effectiveness of US in enhancing membrane cleaning. As US irradiation forms micro-scales of
cavitation in water, energy released from collapses of cavitation bubbles can produce approximately 110 ms1 of
micro jet stream that assists in cleavages of large particles
and detachment or erosion of smaller particles adsorbed
on the membrane surface (36), thus enhancing the effectiveness of membrane cleaning. Lamminen et al. (4) agreed
that the detachment of particles from membrane pores
was primarily due to the cavitation effects, while acoustic
streaming provided a means to transport the loosened
particles away from membrane surface.
In terms of different US frequencies, it was agreed that
lower US frequency offers better cleaning efficiency than
does high US frequency (4,9,19). The same phenomena
were observed in the current studies as well. This may be
attributed to the fact that higher US frequencies create
smaller cavitation bubbles, which generate weaker water
extrusion to loosen the particles from the membrane surface (18). Mason and Lorimer (37) stated that the production and intensity of cavitation in liquids increased
with decreasing US frequency. This is because the rarefaction cycles are too short to allow a bubble to grow to a
large size at high frequency and the effect of US-assisted
cleaning was more effective with lower US frequency.
SEM on Fouled Membrane Surfaces after US-Assisted
Cleaning
To validate the effectiveness of US-assisted cleaning,
SEM pictures were taken as shown in Fig 5. Surface conditions of the membranes of 10 kDa and 100 kDa prior to
the filtration are depicted in Fig. 5a and Fig. 5b. These
images reveal that after 480 min of filtration, both membranes were covered with clusters of particles as shown
in Fig. 5c and Fig. 5d. Backwashing was performed on
ULTRASOUND IMPACT ON MEMBRANE FILTRATION AND CLEANING
221
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US frequency (20 kHz) in Fig. 6a exhibited better foulant
removal on the membrane surface than higher US
frequency (40 kHz) in Fig. 6c. US irradiation has been
demonstrated to enhance membrane backwashing
although important factors on optimizing US effectiveness
rely heavily on its configuration and operation.
FIG. 5. Surfaces of membranes before filtration: (a) 10 kDa and (b)
100 kDa; and surfaces after 480 min of filtration: (c) 10 kDa and (d)
100 kDa. Magnification: 3,000X.
membrane of 10 kDa and 100 kDa as assisted by US at
20 kHz and 40 kHz. All SEM images were taken after 30
minutes of US-assisted backwashing. Figure 6a and
Fig. 6b showed the 10 kDa and 100 kDa membrane surface
conditions, respectively, after US-assisted cleaning at
20 kHz. US-assisted cleaning was more effective on membrane of 10 kDa than on membrane of 100 kDa. These
results correlate our permeation records that show higher
flux recovery for membrane of 10 kDa. Figure 6a and
Fig. 6c are images taken to compare the impact of US frequencies on membrane cleaning efficiency. Evidently, lower
CONCLUSION
Ultrasound-assisted membrane filtration=cleaning do
not alter the membrane’s intrinsic permeability. Instead it
enhances the permeate flux through decreasing solute concentration near the membrane. The technique is highly
influenced by several factors of feed solution properties,
operating pressure (TMP), US frequency, and membrane
pore size. This current research has demonstrated that different intermittent US configurations could significantly
impact the effectiveness of the technique in improving
membrane filtration and cleaning. Under the experimental
conditions, an increment of 15%–20% permeate flux was
observed when 20 kHz of US was applied to assist membrane permeation on 10 kDa pore size membrane with
100 kPa TMP. On the other hand, insignificant improvement was observed in larger pore size membrane
(100 kDa) at higher TMP (140 kPa). Lower frequency of
US (20 kHz) was found to clean the fouled membrane
efficiently and exhibited more than 80% flux recovery than
higher US frequency (40 kHz). Clearly, US-assisted filtration and cleaning are most effective when TMP, US
frequency, and membrane pore sizes are smaller. Our
experimental results suggest the potential of incorporating
ultrasound in membrane applications and incorporation of
intermittent US exhibits advantages by extending membrane filtration operating time without damaging the membrane surface and thereby exhibiting excellent flux recovery
while improving membrane cleaning.
ACKNOWLEDGEMENTS
The authors would like to thank the National Science
Council of the Republic of China (Contract No. NSC
96-2221-E-002-056-MY2) for financially supporting this
research.
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FIG. 6. Comparison of US-assisted cleaning on different pore size membranes and frequencies: (a)10 kDa membrane cleaned with US at 20 kHz,
(b) 100 kDa membrane cleaned with US at 20 kHz, (c) 10 kDa membrane
cleaned with US at 40 kHz. Magnification: 3,000X.
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