Effects of milk minerals and proteins on fouling and cleaning of polyamide
reverse osmosis membranes
Xiao Wei Tew & Ken R Morison *
Department of Chemical and Process Engineering, University of Canterbury, Private Bag
4800, Christchurch, New Zealand
ABSTRACT
Reverse osmosis is used for the concentration of milk to reduce transportation costs from
farms, and also within dairy factories to concentrate dairy liquids. A SEPA flat sheet reverse
osmosis apparatus with a polyamide membrane was used to measure the flux, rejection,
fouling and cleaning of components of milk, both as model solutions and also as whole or
skimmed milk at up to 24 bar transmembrane pressure (TMP).
Both whole milk and skimmed milk were found to have similar fluxes with a maximum at
about 14-16 bar TMP. At higher pressures the flux decreased below the maximum. The flux of
a solution of twice the concentration of SMUF caused significant flux reduction with a
maximum flux at 16 bar TMP. Neither the addition of lactose nor whey protein isolate to
normal SMUF caused additional flux reduction. Skimmed milk with enhanced and depleted
mineral and/or protein contents were also filtered. It was found that altering the mineral
content either up or down increased the fouling resistance for filtration, whereas both
changes in protein content reduced the amount of fouling. The fouling was found to be greater
at higher TMP values. It is thought that high mineral contents led to mineral precipitation but
low mineral contents cause casein micelle disintegration. The membranes were cleaned using
a cycle of 10 minutes each of 0.5% NaOH, 0.8% HNO3, 0.5% NaOH with water rinses before
each step. After 12 bar TMP filtration only one cleaning cycle was required but after 24 bar
filtration, two cycles were required.
INTRODUCTION
Reverse osmosis is used to concentrate raw milk to reduce transportation costs from farms,
and also within dairy factories to concentrate dairy liquids (Grandison and Lewis, 1996). In
the process only water and very low concentrations of salts pass through the membrane.
Reverse osmosis membranes in dairy applications have been found to foul for unknown
reasons, and premature replacement of membranes has been required. There are many
components, including salts, proteins, and fat that could potentially foul membranes, and
some of these accumulate at the membrane causing increases in the effective osmotic pressure
and/or resistance to flux.
Membrane fouling has been defined as the attachment, accumulation, or adsorption of
foulants onto membrane surfaces and subsequently deteriorating the performance of the
membranes over time (Song and Tay 2010). The performance of reverse osmosis membranes
in dairy and food processing is dominated by interactions between membranes and
components of milk or cleaning chemicals. The exact interactions between solutes and the
membranes are not clearly understood yet. Food components such as proteins can adsorb onto
the membrane surface and require chemical cleaning for their removal. Salts, proteins, and fat
in milk accumulate at the membrane causing increases in the effective osmotic pressure
and/or resistance to flux. Once the solubility limit of solutes is exceeded, a deposited layer can
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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form on the membrane reducing its permeability and at the same time inhibiting the back
diffusion of salts to the bulk solution (Hoek and Elimelech, 2003). Acids such as nitric,
phosphoric and citric, are generally used to clean inorganic precipitates from membranes
while high pH cleaners, primarily NaOH, are employed for the removal of biofilms, proteins
and other organic fouling (D'Souza and Mawson, 2005).
Protein, in particular casein, is the main component found in the deposit that forms on
membrane surfaces during reverse osmosis of milk solutions (Skudder et al., 1977). The role
of casein in establishing the resistance to permeance due to deposit formation is indicated by
the higher flux of whey than milk during reverse osmosis (Hiddink et al. 1980; Morales et al.
1990) and the similar characteristic shape for permeation curve of skim milk and pure milk
protein solutions (Kulozik and Kessler 1988A). For solutions without casein, calciumphosphate precipitation appeared to cause strong fouling on the membrane (Hiddink et al.,
1980).
The formation of a deposited layer was shown to begin immediately after the concentration
process started (Kulozik and Kessler 1990A). The deposit formation for reverse osmosis with
skim milk is affected by the stability of casein micelle structure (Kulozik 1998). The
deposited layer behaves like a membrane whose selectivity depends on the solubility and
mobility of the dissolved substances inside it (Kulozik and Kessler 1990B). The presence of
inorganic ions reduces the thickness of the electric double layer around the casein particles
and brings them close together forming a more compacted deposited layer (Kulozik and
Kessler 1988A, 1988B). The fouling resistance appeared to increase with the soluble calcium
and phosphate fractions in skim milk and particularly with the free calcium (Ca2+) amount
(Bouzid et al. 2012). However, removal of total calcium, both diffusible and the calcium
bound in the casein micelles, in skim milk by means of ion exchange resulted in a significant
reduction of the permeance through tubular polyamide reverse osmosis membranes. Removal
of calcium by ion exchange led to the disintegration of the micelle structure, causing a very
dense deposited protein layer consisting of casein submicelles and protein molecules. Flow
resistance exerted by the deposited protein layer for diafiltered skim milk was less than ion
exchanged skim milk with similar calcium content. Diafiltration only able to remove soluble
calcium from skim milk and the casein micelle structure stabilized by calcium phosphate
bridges remained unchanged. A loosely packed deposit structure is formed during reverse
osmosis of diafiltered skim milk as no soluble calcium was available to form links between
casein particles in the deposited layer (Kulozik 1998).
Membrane flux, J, is defined as the mass flow rate of permeate passing through a unit area of
membrane (kg m-2 s-1). Membrane permeance is defined as the flux divided by transmembrane
pressure (TMP) (kg m-2 bar-1 s-1) and this is very useful when comparing performance at
different pressures. The water flux or permeance can be normalized relative to the water flux
or permeance for a pre-treated membrane. Flux decline can be attributed to the effects of
osmotic pressure, irreversible fouling and other reversible phenomena such as concentration
polarization and gel formation (Bouzid et al., 2012). The governing equation used in this
study is:
∆𝑃 − ∆𝜋
𝐽=
(1)
𝜇(𝑅𝑚 + 𝑅𝑓𝑙𝑢𝑠ℎ𝑎𝑏𝑙𝑒 + 𝑅𝑛𝑜𝑛−𝑓𝑙𝑢𝑠ℎ𝑎𝑏𝑙𝑒 )
Here ∆𝑃 is the effective TMP, bar; ∆𝜋 is the osmotic pressure difference between feed and
permeate, bar; 𝜇 is the dynamic viscosity of permeating fluid, Pa.s; 𝑅 is the resistance,
bar m3 s2 kg-2. The subscript m refers to a clean pre-treated membrane, flushable refers to
resistance during milk filtration that can be removed by a water rinse and non-flushable refers
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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to resistance that remains after a water rinse. The amount of non-flushable resistance
indicates the extent to which components have precipitated or cross-linked to form a stable
matrix.
The main objective of this study was to improve the understanding of the mechanisms by
which the components of processed liquids and chemical cleaning agents interact with thinfilm composite polyamide reverse osmosis membranes, so as to be able to reduce membrane
fouling and enhance cleaning effectiveness.
METHODS
Apparatus and operating conditions
A SEPA flat sheet membrane system (Sepa CF, Osmonics, USA), with a filtration area of 134
cm2 and with a feed spacer was used. The system was operated at up to 25 bar TMP using a
Hydra-Cell diaphragm pump (Wanner Engineering, USA) with a pulsation dampener that
reduced the pressure range to ± 10% of the operating pressure. The cross flow velocity was
0.2 m s-1. The feed solution was maintained at 30 °C by immersing the feed container in a
water bath. The dynamics of start-up caused temperature variations of ±4.0 °C but the flux
was corrected inversely proportionally to the viscosity of water at the feed temperature. The
feed temperature, mass of permeate and applied TMP were recorded by a data logging PC.
The DOW Filmtec FT30 polyamide thin-film composite membranes (DOW
HYPERSHELLTM RO-8038/48 element) were cut from a spiral module and kept moist in
sealed bags at about 4 °C. Permeate and retentate were recycled into the feed. Conductivity
and pH were used to determine concentrations for rejection. The rejection coefficient for
minerals was calculated using the electrical conductivity of the bulk feed and permeate.
Methods
All concentrations in this paper are given as percent by mass unless otherwise noted. The
water used in all experiments was supplied from a Millipore water purification system (Elix-5,
Millipore, USA). The membrane was thoroughly rinsed with water and left to soak in water
for 1 hour to ensure complete hydration. The immersed membrane was rinsed with water
again before placement in the rig. The membrane was pre-treated with 0.8% HNO3, water,
0.5% NaOH and water. The solutions were circulated for 10 minutes with no applied TMP.
After each solution the system was rinsed thoroughly with at least 3 L of water, each litre for
5 minutes, until the electrical conductivity dropped below 25 μS cm-1. The water fluxes
before and after pre-treatments were measured and are termed initial water flux and pretreated water flux respectively. The flux after pre-treatment was used as the reference flux.
For fixed TMP runs at 12 or 24 bar, the filtration step was for 3 or 5 hours. For increasing
TMP runs, the TMP was increased stepwise by 2 bar or 4 bar from 8 bar up to 24 bar. The
flux was measured for about 30 minutes or until a steady permeance was achieved depending
on the TMP. The water flux, corresponding to non-flushable fouling, was measured after
system rinsing. The feed solutions used in this study are listed in Table 1.
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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Table 1 Preparation method for feed solutions.
Feed solution
Skimmed milk
Whole milk
Simulated milk
ultrafiltrate (SMUF)
Concentrated SMUF
SMUF + lactose
SMUF + lactose + WPI
Mineral enhanced
skimmed milk
Mineral depleted
skimmed milk
Protein enhanced
skimmed milk
Protein depleted
skimmed milk
Description
Commercially available Trim milk from Meadow Fresh, Auckland, NZ, 4 g L-1 fat, 37
g L-1 protein.
Commercially available from Klondyke Fresh Limited, Christchurch, NZ, 34 g L-1 fat,
33 g L-1 protein.
Salts were dissolved in the amount and order specified in Table 1 of Jenness and
Koops (1962). The pH was adjusted to 6.6.
Solution was prepared with similar method as described for normal SMUF but with
twice the salt quantity.
A solution 55.36 g L–1 lactose monohydrate (52.59 g L–1 lactose) was used instead of
water to prepare SMUF.
A solution was prepared by dissolving 6.24 g of whey protein isolate (WPI)
(Balance, Vitaco Health Ltd., Auckland, NZ) for 0.6% or 34.32 g of WPI for 3.3% in
1 L of SMUF and lactose solution.
A solution was prepared by dissolving 25% of the amount of SMUF salts into 1 L of
commercial Trim milk.
A solution was prepared by removing 250 mL milk ultrafiltrate from 1 L Trim milk
by ultrafiltration with a PES 10 membrane (Synder Filtration, USA) at 30 °C, TMP of
4 bar and cross flow velocity of 0.2 m s-1. 250 mL of 52.59 g L-1 lactose solution was
added into the ultrafiltered milk to make it up to 1 L.
A solution was prepared by removing 250 mL milk ultrafiltrate from 1 L Trim milk
by ultrafiltration at the same conditions as the previous sample. Protein was enhanced
by 33.3%.
A solution was prepared by diluting the commercial Trim milk with 250 mL SMUF
and lactose solution. Protein was depleted by 20%.
After filtration the membrane was cleaned with a standard cleaning cycle consisting of 0.5%
NaOH, water, 0.8% HNO3, water, 0.5% NaOH and water. The concentrations were based on
previous work in our laboratory, but are not confirmed as being optimal. Each cleaning
chemical was circulated in the system for 10 minutes with no applied TMP to minimize the
production of permeate during cleaning and reduce the convective redeposition of foulant
back onto the membrane surface making it more adhesive (D'Souza and Mawson, 2005). The
cleaning cycle was repeated two to three times with the same concentration and order. The
water flux was measured after each rinse and the permeance ratio, a ratio between water
permeance after each chemical exposure and pre-treatment permeance, was used to indicate
cleaning effectiveness. The sequence of cleaning cycle, concentration of cleaning chemical or
cleaning duration was altered in some studies to investigate the interaction of cleaning
chemical with foulants as well as with thin-film composite polyamide reverse osmosis
membranes.
RESULTS
Figure 1 shows the fluxes for reverse osmosis of milk solutions, both whole and skimmed
milk, as well as its model solutions simulating different components in milk at increasing
TMPs. As revealed by the figure, both whole milk and skimmed milk were found to have
similar behaviour with a maximum flux at 14 – 16 bar. The characteristic curve of milk is
consistent with the work of Rabiller-Baudry et al. (2009) and Bouzid et al. (2012) who
utilized the concept of limiting and critical fluxes to study the effect of feed pH. Similar
behaviour for both whole milk and skimmed milk suggested that fat does not play a
significant role in membrane fouling. The flux of simulated milk ultrafiltrate (SMUF) was
consistent with its osmotic pressure with a slight deviation from the straight line at high TMP,
while concentrated SMUF caused a significant flux reduction with maximum flux at 16 bar
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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TMP. Neither the addition of lactose or whey protein isolate to normal SMUF caused large
additional flux reduction.
Figure 1. Effect of feed composition on fluxes during runs with increasing TMP.
The permeance (flux/TMP) as a function of time for selected runs is shown in Figure 2. The
raw data, with oscillation of about ±10% or less, which was caused by the use of an electronic
balance for flow measurement, was smoothed by taking the average of each 10 successive
permeance values. The recorded data points for the first 10 minutes were retained to identify
the initial changes in permeance. After the initial drop, the permeance of whole milk at both
TMPs decreased slightly with time for the first 3 hours before it reached its steady state. The
steady state permeance for 12 bar and 24 bar after 5 hours filtration were 0.203 g s-1 bar-1 m-2
and 0.083 g s-1 bar-1 m-2 respectively. The reduction of permeance with time was higher for
runs at 24 bar than at 12 bar.
The permeance values of skimmed milk for constant pressure runs at 24 bar TMP exhibited
behaviour similar to whole milk. Concentrated SMUF had a much higher permeance at the
start of the run as compared to whole milk and skimmed milk. Unlike milk, the permeance
dropped dramatically with time during the first hour of the run indicating the development of
mineral precipitate and concentration polarization.
Permeance (g s-1 bar-1 m-2)
0.8
Whole milk - 12 bar
Whole milk - 24 bar
Trim milk - 24 bar
Concentrated SMUF - 24 bar
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Time (hours)
Figure 2. Permeance as a function of time for runs with whole milk, skimmed milk and
concentrated SMUF at 12 bar or 24 bar TMP. Data points shown after the first 10
minutes are the average of each 10 successive values.
Membrane and fouling resistance
To compare the amount of fouling caused by the various feed liquids, resistances of the
membrane, flushable fouling and non-flushable fouling were calculated using Equation 1 and
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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are presented in Table 2. Generally, flushable resistance increased with increasing nonflushable resistance. The resistance corresponding to flushable and non-flushable fouling for
runs with whole milk at 24 bar TMP was 5 – 6 times higher than at 12 bar. The flushable and
non-flushable resistance for SMUF and mixtures of SMUF, lactose and whey protein isolate
were relatively low compared with skimmed milk and whole milk. This indicates that casein,
probably with calcium phosphate, is the major contributor of resistance formation in milk.
Concentrated SMUF increased the flushable resistance by a factor of 10 and the non-flushable
resistance by a factor of 4 as compared to normal SMUF. This was probably because the
higher concentration increased the amount of precipitation, and any precipitation that did
occur increased concentration polarization and hence precipitation. Both enhancement and
reduction of protein concentration in skimmed milk reduced flushable resistance, but it is
notable that both mineral enhancement and depletion increase the flushable resistance.
Table 2 Membrane, flushable and non-flushable resistance.
Calculated osmotic
pressure difference
bar
SMUF
2.97
SMUF and lactose
6.71
SMUF, lactose + 3.3% WPI
6.86
Concentrated SMUF
5.97
Whole milk (12 bar)
7.10
Whole milk (24 bar)
7.10
Skimmed milk (24 bar)
7.10
Mineral enhanced skimmed milk
7.89
Mineral depleted skimmed milk
6.30
Protein enhanced skimmed milk
7.20
Protein depleted skimmed milk
7.04
Note: ± indicates the range of measurement over 6 runs.
Feed solutions
Membrane
resistance
1010 m2 kg-1
11.7
11.0
10.9
10.6
12.2
11.7
10.9 ± 0.4
10.9
10.3
11.7
10.6
Flushable
resistance
1010 m2 kg-1
3.5
5.8
8.2
35
10.8
81
68 ± 6
78
73
66
64
Non-flushable
resistance
1010 m2 kg-1
1.5
1.5
1.9
6.0
2.1
14
13 ± 1
16
12
11
11
Chemical cleaning
Before testing the cleaning of membranes fouled with milk, a virgin membrane was subjected
to pre-treatment, water fluxing for 1 hour and 4 cycles of cleaning. The permeance values of
all water flux tests are shown relative to the pre-treated permeance in Figure 3. Each cycle of
cleaning was found to have an additive effect on membrane permeability. It can be seen that
lower chemical concentrations produced less of an effect. The permeance ratios obtained from
these runs can be used as a control for cleaning efficiency after milk fouling.
Similar results were reported by Madaeni and Mansourpanah (2004) for the cleaning of
reverse osmosis membrane fouled by whey with 0.1% NaOH (pH 12.4) who suggested that
alkaline damage was the cause. Sohrabi et al. (2011) suggested that it was caused by changes
in hydrophilicity, while Nilsson et al. (2008) claimed reversible membrane swelling is the
major reason for the changes in water permeability. In the current work membrane damage
was thought to be unlikely as a run with 15 hours of soaking at each chemical step showed no
deterioration in either permeability or NaCl rejection.
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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Figure 3. Permeance ratios during cleaning of a clean membrane.
The permeance ratios before and after cleaning for concentrated SMUF, modified and
unmodified milk solutions are listed in Table 3. Considering the second row for skimmed
milk filtered at 24 bar, it can be seem that the permeance while operating with milk was about
8% of the pre-treated water permeance. After a water rinse this increased to 45% showing that
much of the flux reduction is due to the temporary build-up of a concentration polarization
layer. After the first NaOH clean the permeance ratio increased to 85% showing that NaOH is
not sufficient to remove all the foulant. After the subsequent HNO3 and NaOH steps of
cleaning cycle 1, the permeance ratio increased to about 97%. This needs to be compared to
103% for cleaning of a clean membrane (first row) and it shows that cleaning is not complete.
Even after another cleaning cycle the permeance was not restored the levels obtained for other
milk solutions. A single standard cleaning cycle was sufficient to remove the deposit and
recover the water permeance for whole milk run at 12 bar, probably because the cake layer
formed at low TMP is loose and easier to remove. A similar observation was reported by
Kulozik and Kessler (1990B). For whole milk, skimmed milk and most modified milk
solutions that operated at 24 bar, two or three cleaning cycles were needed to restore the water
permeance back to the level expected for a clean membrane. The exception was protein
depleted skimmed milk, which required only one cleaning cycle.
Table 3 Effect of feed composition on the permeance ratio and cleaning performance.
Feed solutions
Fouled on
product
0.968
0.081 ± 0.002
0.199
0.077
Permeance ratio = Permeance/pre-treated permeance
After first
After cleaning After cleaning
Rinsed
NaOH
cycle 1
cycle 2
0.99
1.03
1.05
0.45 ± 0.005
0.97 ± 0.01
0.99 ± 0.01
0.85± 0.01
0.86
1.04
0.45
0.97
1.05
Water
Skimmed milk
Whole milk (12 bar)
Whole milk (24 bar)
Mineral enhanced
0.070
0.40
skimmed milk
Mineral depleted skimmed
0.080
0.46
milk
Protein enhanced
0.093
0.52
skimmed milk
Protein depleted skimmed
0.088
0.50
milk
Concentrated SMUF
0.155
0.64
Note: ± indicates the range of measurement over 2 runs
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
0.83
0.96
1.02
0.91
0.96
1.00
0.90
0.97
1.01
0.90
1.00
1.05
0.83
0.99
1.02
7
OVERALL DISCUSSION
Attempts have been made to correlate the flux reduction, resistances development and
cleaning efficiency for reverse osmosis run with milk model solutions, modified and
unmodified milk solution to establish the likely mechanism for fouling formation. The origins
of flux decline during reverse osmosis run at transmembrane pressure higher than its limiting
condition is in agreement with Bouzid et al. (2012) which include concentration polarization
that subsequently reduced the effective transmembrane pressure, as well as the cake layer
formation that increased hydraulic resistance of the filtration process. Both cake layer
formation and concentration polarization effects are inter-related. During reverse osmosis runs,
concentration polarization occurred where the feed components, both proteins and minerals,
are rejected by the polyamide thin-film composite membrane. Minerals and protein
concentration increases at membrane boundary layer and may reach their nucleation limits
and precipitate on the membrane surface. The formation of cake layer due to protein and
mineral precipitation not only imposed additional hydraulic resistance to permeation as
proposed by Fenton-May et al. (1972), Kulozik and Kessler (1988A) and Kulozik and Kessler
(1990A), but also hindered the back diffusion of mineral ions from the membrane surface to
the bulk solution and consequently increased the mineral concentration at the membrane
significantly. Cake enhanced concentration polarization leads to mineral precipitation and
even greater osmotic pressure which defined as cake enhanced osmotic pressure by Hoek and
Elimelech (2003). Meanwhile, the minerals might either block and/or attached to the
membrane.
High mineral content leads to precipitation of minerals within a concentration polarization
layer. In the absence of casein, the concentration polarization developed during fluxing is
likely to be enough to encourage nucleation at the surface. Similarly, the minerals will either
block the membrane or form a physical cake which will further enhance concentration
polarization and precipitation. Calcium-phosphate precipitation was likely to be the main
cause for fouling formation for reverse osmosis run with solution consisting of SMUF, lactose
and whey protein isolate, but the low value of non-flushable fouling indicates that the
membrane was not blocked.
When casein is present, the casein micelles are likely to immediately form a cake layer which
will enhance the concentration polarization of minerals and hence precipitation. In milk
solution, casein either exists as molecules, submicelles or micelles. Significant amounts of
calcium phosphate are associated with casein to stabilize the casein micelles in solution
(Gaucheron 2005). There is an equilibrium between casein micelles and minerals as
Gaucheron (2005) concluded that small changes in the mineral composition will cause
association or dissociation of casein and salts which consequently induced significant changes
on the structure and stability of casein micelles.
In the experimental run with mineral enhanced skimmed milk more total fouling and nonflushable fouling was formed and this is possible due to the formation of a stronger casein gel.
This is in consistent with the findings of Mo et al. (2008) for bovine serum albumin and
Bouzid et al. (2012) for skim milk. The formation of denser cake layer and higher mineral
concentration in mineral enhanced skimmed milk further enhanced the concentration
polarization and subsequently encouraged the nucleation of minerals at the membrane surface
and membrane blocking.
When the mineral content of the milk was reduced fouling also increased. It is possible that
the reduction of minerals led to casein micelle dissociation causing the formation of a denser
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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cake layer that partly consists of casein submicelles and protein molecules. This is in
agreement with the works of Kulozik (1998) and Obermeyer et al. (1993). However, the cake
layer was not as stable as those formed by unmodified skimmed milk and was more flushable,
leaving less non-flushable fouling.
The protein enhanced and depleted skimmed milk gave similar fouling to skimmed milk runs,
indicating the chemical environment of the casein micelles is more important than the
concentration of casein.
It was clear that operation at 24 bar TMP, rather than 12 bar, not only reduces the operating
flux but considerably increases the difficulty of all stages of cleaning. It is unclear why the
flux at 24 bar dropped more than that at 12 bar. Normally higher pressures give higher fluxes
which increase the deposit formation rate and the pressure gradients. However in this case the
flux at 24 bar was lower than at 12 bar, so the deposit build-up should have been slower. The
pressures involved are at least 100 times less than those associated with changes in rheology
after high pressure processing (Devi et al., 2013) so static pressure is unlikely to be directly
involved. Several mechanisms could be involved:
1. The initial flux at higher pressure is higher than at lower pressure causing a short term
increase of minerals and casein in the cake layer.
2. At higher TMP more minerals are forced into the membrane thus increasing its
resistance.
3. At higher TMP the pressure fluctuations are slightly higher causing cake build-up.
4. At higher concentrations of casein and mineral in the cake, higher static pressures
cause greater cross-linking.
When considering the cleaning (Table 3) the most dramatic result is the difference in cleaning
for whole milk at 12 bar and 24 bar. After processing at 12 bar, there was little fouling
resistance and most was removed by water flushing. After flushing the resistance at 12 bar
was only 15% of the resistance at 24 bar. It seems very likely that a stronger “gel” was formed
at 24 bar as discussed in the previous paragraph.
CONCLUSIONS
Flux reduction of reverse osmosis is caused by several different mechanisms. Casein micelles
form a cake appears to increase the hydraulic resistance. This cake also contributes to
concentration polarization which increases osmotic pressure and reduces flux. The
polarization, with or without casein micelles, can cause minerals, such as calcium phosphate,
to precipitate further enhancing the cake and adding hydraulic resistance by coating the
membrane. Higher TMP caused significantly greater fouling.
The effect of cleaning chemicals on a clean membrane needs to be measured before cleaning
of fouled membranes can be evaluated. Both acid and caustic cause changes in permeability
of clean membranes. At least two cycles of acid/caustic/acid cleaning were required to
achieve a steady flux. The foulant produced at 24 bar TMP was much more difficult to clean
than that at 12 bar.
Minerals and Dairy Products Symposium, Auckland, NZ, 26-27 Feb 2014
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