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Separation Science and Technology
ISSN: 0149-6395 (Print) 1520-5754 (Online) Journal homepage: https://www.tandfonline.com/loi/lsst20
Fouling of Reverse Osmosis and Ultrafiltration Membranes: A Critical Review
M. F. A. Goosen , S. S. Sablani , H. Al‐Hinai , S. Al‐Obeidani , R. Al‐Belushi & D. Jackson
To cite this article: M. F. A. Goosen , S. S. Sablani , H. Al‐Hinai , S. Al‐Obeidani , R. Al‐Belushi & D. Jackson (2005) Fouling of Reverse Osmosis and Ultrafiltration Membranes: A Critical Review, Separation Science and Technology, 39:10, 2261-2297, DOI: 10.1081/SS-120039343
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Fouling of Reverse Osmosis and
Ultrafiltration Membranes:
A Critical Review
M. F. A. Goosen, 1,* S. S. Sablani,
2 H. Al-Hinai,
1
S. Al-Obeidani, 1 R. Al-Belushi,
2 and D. Jackson
3
1 Department of Mechanical and Industrial Engineering and 2 Department of Bioresource and Agricultural Engineering,
Sultan Qaboos University, Al-Khod, Muscat, Oman 3 Department of Chemical Engineering, Imperial College, UK
ABSTRACT
Desalination by using reverse osmosis (RO) membranes has become very
popular for producing freshwater from brackish water and seawater.
Membrane lifetime and permeate flux, however, are primarily affected
by the phenomena of concentration polarization and fouling at the mem-
brane surface. The scope of the current paper was to critically review the
literature on the fouling phenomena in RO and ultrafiltration (UF) mem-
brane systems, the analytical techniques used to quantify fouling, preven-
tive methods, and membrane cleaning strategies. The paper also makes
2261
DOI: 10.1081/SS-120039343 0149-6395 (Print); 1520-5754 (Online)
Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com
*Correspondence: M. F. A. Goosen, School of Science and Technology, P.O. Box
3030, University of Turabo, Gurabo, Puerto Rico, 00778-3030, USA; E-mail:
SEPARATION SCIENCE AND TECHNOLOGY
Vol. 39, No. 10, pp. 2261–2297, 2004
specific recommendations on how scientists, engineers, and technical
staff can assist in improving the performance of these systems through
fundamental and applied research.
Key Words: Fouling; Desalination; Reverse osmosis; Ultrafiltration
membranes.
INTRODUCTION
A large proportion of the world’s population is experiencing water
stress. [1]
Arid regions in particular suffer from the constraining effects of
limited water resources. [2–4]
There is a growing awareness by scientists,
political leaders, and the general public that the best way to approach this
problem lies in a coordinated approach involving water management, water
purification, and water conservation. [5]
The two most successful commercial water purification techniques
involve thermal and membrane systems. Desalination by using reverse
osmosis (RO) membranes, in particular, has become very popular for produ-
cing freshwater from brackish water and seawater. The technique has low
capital and operating costs compared with other alternative processes such
as multistage flash. [6]
Ultrafiltration (UF) may be used before RO for feed-
water pretreatment. [7]
Membrane separation processes also are widely used
in biochemical processing, in industrial wastewater treatment, in food and
beverage production, and in pharmaceutical applications. [8]
Membrane lifetime and permeate (i.e., pure water) fluxes are primarily
affected by the phenomena of concentration polarization (i.e., solute buildup)
and fouling (e.g., microbial adhesion, gel-layer formation, and solute
adhesion) at the membrane surface (Fig. 1). [9]
Koltuniewicz and Noworyta, [10]
in a highly recommended paper, summarized the phenomena responsible for
limiting the permeate flux during cyclic operation (i.e., permeation followed
by cleaning). Concentration polarization, during the initial period of operation
within a cycle, is one of the primary reasons for flux decline, Ja, (Fig. 2).
Large-scale membrane systems operate in a cyclic mode, where a clean-in-
place operation alternates with the normal run. The figure shows a decrease
in the flux for pure water from cycle to cycle, Jo(t), due to fouling, the flux
decline within a cycle due to concentration polarization, J(tp), and the
average flux under steady-state concentration, Ja. The latter also decreasing
from cycle to cycle, suggests irreversible solute adsorption or fouling.
Accumulation of the solute retained on a membrane surface leads to increasing
permeate flow resistance at the membrane wall region.
One of the most serious forms of membrane fouling is bacterial adhesion
and growth. [11]
Once they form, biofilms can be very difficult to remove, either
Goosen et al.2262
through disinfection or chemical cleaning. This wastes energy, degrades salt
rejection, and leads to shortened membrane life. This is one area, for
example, where further research is required.
Liquids to be Treated
RO and UF membranes have been used for the treatment of a variety of
liquids, ranging from seawater, to waste water, to milk, and to yeast suspen-
sions (Table 1). Each liquid varies in composition and in the type and the
fraction of the solute(s) to be retained by the membrane. Complicating
factors include the presence of substances, such as, for example, oil in sea-
water and in wastewater. [12–15]
The presence of the oil normally necessitates
an additional pretreatment step, as well as further complicating the fouling
process. The presence of humic acids in surface water and wastewater also
needs special attention. [16,17]
The fouling phenomena, the preventive means
(i.e., pretreatment), and the frequency and the type of membrane cleaning
cycle are all dependent on the type of liquid being treated.
Figure 1. A schematic representation of concentration polarization and fouling at the
membrane surface.
Fouling of RO and UF Membranes 2263
Membrane Materials
Numerous polymer membranes have been developed for RO and UF appli-
cations (Table 2). The membrane materials range from polysulfone (PS) and
polyethersulfone (PES) to cellulose acetate and cellulose diacetate. [12,18–23]
Commercially available polyamide composite membranes for desalination
of seawater, for example, are available from a variety of companies in the
United States, Europe, and Japan. [24]
The exact chemical composition and
physical morphology of the membranes may vary from manufacturer to manu-
facturer. Since the liquids to be treated and the operating conditions also vary
from application to application, it becomes difficult to draw general conclu-
sions on which materials are the best to use to inhibit membrane fouling.
The specific choice of which membrane material to use will depend on the
process (e.g., type of liquid to be treated, operating conditions) and economic
factors (e.g., cost of replacement membranes, cost of cleaning chemicals).
The scope of the current paper was to critically review the literature on
the fouling phenomena in RO and UF membrane systems, the analytical
Figure 2. Diagram of typical flux-time dependency during cyclic operation in large-
scale UF systems. Adapted from Koltuniewicz and Noworyta. [10]
Goosen et al.2264
techniques used to quantify fouling, preventive means, and membrane clean-
ing methods. The paper also makes specific recommendations on how scien-
tists, engineers, and technical staff can assist in improving the performance of
RO and UF systems through fundamental and applied research.
MEMBRANE FOULING PHENOMENA
Attempts to analyze membrane fouling have shown that the main
mechanisms are adsorption of feed components, clogging of pores, chemical
interaction between solutes and membrane material, gel formation, and bac-
terial growth. Let us first consider bacterial growth on membranes. Microbio-
logical fouling of RO membranes is one of the main factors in flux decline and
loss of salt rejection [25–29]
(Table 3).
Microbiological Fouling
Understanding the mechanism of bacterial attachment may assist in the
development of antifouling technologies for membrane systems. Bacterial
fouling of a surface (i.e., formation of a biofilm) can be divided into three
Table 1. Examples of liquids treated by RO and UF.
Liquid References
Waste water (e.g., paper-mill
effluent, municipal water, water
containing polysaccharides, and
amino sugars)
Chapman et al., [60]
Li et al., [23]
Dal-Cin et al., [52]
Ghayeni et al., [25]
Jarusutthirak et al. [41]
Surface water (humic acids) Nystrom et al., [16]
Domany et al. [34]
Water-in-oil emulsions Scott et al., [15]
Pope et al., [14]
Benito et al., [13]
Lindau and Jonsson [12]
Skimmed milk Rabiller-Baudry et al., [22]
Mohammadi et al. [81]
Seawater Glueckestern et al., [58]
Sablani et al., [7]
Wilf and Klinko [56]
Drinking water Han et al. [62]
Yeast suspensions Mores and Davis [82]
Water-containing proteins Schafer et al. [35]
Water-containing organic colloids Kabsch-Korbutowicz et al. [17]
Fouling of RO and UF Membranes 2265
phases: transport of the organisms to the surface, attachment to the substratum,
and growth at the surface. Fleming et al. [30]
have shown that it takes
about 3 days to completely cover a RO membrane with a biofilm. Ghayeni
et al. [25,26]
studied initial adhesion of sewage bacteria belonging to the genus
Pseudomonas to RO membranes. It was found that bacteria would sometimes
aggregate upon adhering. While minimal bacterial attachment occurred in a
very low ionic-strength solution, significantly higher numbers of attached
microbes occurred when using salt concentrations corresponding to waste
water.
In work similar to that of Ghayeni et al., [25,26]
Flemming and Schaule [20]
demonstrated that after a few minutes of contact between a membrane and raw
water, the first irreversible attachment of cells occurs. Pseudomonas was
identified as a fast adhering species out of a tap water microflora. If nonstar-
ving cells were used (i.e., sufficient nutrients and dissolved oxygen in the raw
water), the adhesion process improved with an increase in the number of cells
in suspension. When starving cells were used, incomplete coverage of the
surface occurred. This is similar to the surface aggregate formations observed
for membranes by Ghayeni et al. [25,26]
Flemming and Schaule [20]
also detected
a biological affinity of different membrane materials toward bacteria. Poly-
etherurea, for example, had a significantly lower biological affinity than
polyamide, PS, and PES. These results suggest that membrane manufacturers
should stay away from polyamide and PS materials, at least for wastewater
treatment applications.
Table 2. Examples of commercially available membrane materials.
Membrane material References
Polyamide Flemming and Schaule, [20]
Belfer et al., [24]
Jenkins and Tanner, [72]
Polyamide–urea Belfer et al. [24]
Polysulfone Flemming and Schaule, [20]
Rabiller-
Baudry et al., [22]
Li et al., [23]
Lindau
and Jonsson, [12]
Tran-Ha and Wiley, [80]
Mohammadi et al. [81]
Polyethersulfone Flemming and Schaule, [20]
Mohammadi
et al. [81]
Polyetherurea Flemming and Schaule [20]
Cellulose acetate/diacetate Ridgway et al., [18,19] Amerlaan et al.[21]
Regenerated cellulose Kabsch-Korbutowicz et al. [17]
Polyvinyl alcohol
derivative
Ghayeni et al. [25]
Goosen et al.2266
Table 3. Summary of membrane fouling studies reported in the literature.
Fouling studies References
Membrane fouling phenomena
Microbial cell attachment Flemming et al., [30]
Ghayeni et al., [25]
Flemming and Schaule, [20]
††Ridgway et al., [28,31]
Ridgway [33]
Humic acids and morphology of
fouling layer
Nystrom et al., [16]
Schafer et al., [35]
Khatib et al., [36]
Kabsch-Korbutowicz
et al., [17]
†Tu et al., [37]
Domany et al., [34]
††Ridgway [31]
Inorganics Sahachaiyunta et al. [38]
Proteins and colloids Yiantsios and Karabelas, [39]
Jarusutthirak
et al., [41]
Schafer et al., [35]
Bacchin et al. [40]
Reversible adsorbed layer ††Nikolova and Islam, [29]
††Koltuniewicz and Noworyta [10]
Transition from reversible to
irreversible fouling
††Chen et al. [42]
Variation in gel-layer
thickness
††Denisov [54]
Pore blockage and cake
formation
Zydney and Ho [27]
Analytical descriptions
Fouling-layer morphology and
growth
Riedl et al., [43]
Scott et al. [15]
Adhesion kinetics Ridgway et al. [18,19]
Hydrodynamics Altena and Belfort, [44]
†Drew et al., [45]
Cherkasov et al. [32]
Passage of bacteria through
membrane
†Ghayeni et al. [46]
Analysis of deposits: ATR,
FTIR, measuring fouling in
real time
Lindau and Jonsson, [12]
Howe et al., [47]
Rabiller-Baudry et al., [22]
Chan et al., [48]
Bowen et al., [49]
††Li et al. [23]
Measuring concentration
polarization
†Gownan and Ethier, [50,51]
Pope et al. [14]
Mathematical modeling of flux
decline
Dal-Cin et al., [52]
†† Koltuniewicz and Noworyta [10]
Preventive means and cleaning
methods
Feedwater pretreatment
Microfiltration and
ultrafiltration
Wilf and Klinko, [56]
Glueckstern and Priel, [58]
Ghayeni et al., [25]
†Ghayeni et al., [46]
Karakulski et al., [59]
Chapman et al. [60]
(continued)
Fouling of RO and UF Membranes 2267
In a similar but more thorough study than that performed by Ghayeni
et al., [25]
and Ridgway et al. [28,31]
in two excellent papers reported on the
biofouling of RO membranes with wastewater. Cellulose diacetate mem-
branes became uniformly coated with a fouling layer that was primarily
organic in composition. Calcium, phosphorous, sulfur, and chlorine were the
major inorganic constituents detected. Protein and carbohydrate represented
as much as 30% and 17%, respectively, of the dry weight of the biofilm. Elec-
tronmicroscopy revealed that the biofilm on the feed-water side surface of the
Table 3. Continued.
Fouling studies References
Coagulation and
flocculation
Nguyen and Ripperger, [61]
Han et al., [62]
Choksuchart et al., [63]
Park et al., [64]
Guigui et al., [65]
††Lopez-Ramirez et al., [66]
Benito et al., [13]
Shaalan [67]
Spacers ††Schwinge et al., [69]
Geraldes et al., [68]
Sablani et al., [7]
Li et al., [23,69]
Lipnizki and Jonsson [71]
Corrugated membranes Lindau and Jonsson, [12]
Scott et al. [15]
Surface chemistry †Jenkins and Tanner, [72]
Flemming and Schaule, [20]
Ridgway et al., [19]
Belfer et al. [24]
Hydrophobic and hydrophilic
membranes
Kabsch-Korbutowicz et al., [17]
†Tu et al., [37]
Cherkasov et al. [32]
Control of operating parameters
(Critical flux)
†Song, [79]
††Chen et al., [42]
††Koltuniewicz and Noworyta, [10]
Madireddi et al., [74]
Mallubhotla and Belfort, [77]
Avlonitis et al., [75]
Goosen et al., [3]
Jackson et al. [78]
Rinsing water quality Tran-Ha and Wiley, [80]
†Lindau and Jonsson [12]
Cleaning agents Mohammadi et al. [81]
Back pulsing Mores and Davis [82]
Membrane wear and degradation Roth et al., [83]
†Amerlaan et al., [21]
Ridgway et al. [19]
Economic aspects Glueckstern et al., [57]
Brehant et al. [84]
Note: Specific papers are (†) recommended and (††) highly recommended.
Goosen et al.2268
membrane was 10 to 20-mm thick and was composed of several layers of com-
pacted bacterial cells, many of which were partially or completely autolyzed.
The bacteria were firmly attached to the membrane surface by an extensive
network of extracellular polymeric fibrils. They showed that mycobacteria
adhered to the cellulose acetate membrane surface 25-fold more effectively
than a wild-type strain of Escherichia coli. In a key finding, the ability of
Mycobacterium and E. coli to adhere to the membrane was correlated with
their relative surface hydrophobicities, as determined by their affinities for
n-hexadecane. [31]
The results suggested that hydrophobic interaction
between bacterial cell-surface components and the cellulose membrane
surface plays an important role in the initial stages of bacterial adhesion
and biofilm formation. A key question that arises is whether the importance
of this hydrophobic interaction between the cell and the membrane also
holds true for other polymers. This work is similar to that reported by
Cherkasov et al. [32]
on fouling resistance of hydrophilic and hydrophobic
membranes (Fig. 3). A later research study carried out by Ridgway [33]
con-
firmed that bacterial adhesion is regulated by the physicochemical nature of
both the bacterial cell and the polymer membrane surface. The chemical
composition of the feed water also was found to be critical.
Figure 3. Gel-layer formation on surface of an UF membrane made from (I)
hydrophobic and (II) hydrophilic material. C, solute concentration; C1 , C2 , C3,
1 adsorption layer, 2 gel-polarization layer, 3 membrane material. Adapted from
Cherkasov et al. [32]
Fouling of RO and UF Membranes 2269
Effect of Humic Acids
As organic matter, such as plants, degrades in the soil, a mixture of
complex macromolecules, called humic acids, is produced. These complex
molecules have polymeric phenolic structures with the ability to chelate
metals, especially iron. They give surface water a yellowish to brownish
color and often cause fouling problems in membrane filtration. [16,34]
The
fouling tendency of humic acids appears to be due to their ability to bind to
multivalent salts. Nystrom et al., [16]
for example, showed that humic acids
were most harmful in membranes that were positively charged (i.e., containing
alumina—Al, and silica—Si). Humic acids formed chelates with the metals
(i.e., multivalent ions) and could be seen as a gel-like layer on the filter
surface. It was recommended that humic acids be removed from the process
water before filtration by complexation (i.e., flocculation/coagulation; see section “Feed Water Pretreatment”).
Morphology of Humic Acid Fouling Layer
Schafer et al. [35]
studied the role of concentration polarization and solu-
tion chemistry on the morphology of the humic acid fouling layer. Irreversible
fouling occurred with all membranes at high calcium concentrations. Interest-
ingly, it was found that the hydrophobic fraction of the humic acids was depos-
ited preferentially on the membrane surface. This result is similar to the work
of Ridgway et al. [31]
who showed that the hydrophobic interaction between
a bacterial cell surface and a membrane surface plays a key role in biofilm
formation. Schafer et al. [35]
demonstrated that calcium-humate complexes
caused the highest flux decline due to their highly compactable floc-like struc-
tures. Deposition increased with pH due to precipitation of calcite and adsorp-
tion of humic acid complexes on top of this layer. Humic acids had the highest
concentration in the boundary layer. They also had the largest molecular
weight and, therefore, the smallest back-diffusion rate and the greatest ten-
dency toward precipitation. The formation of two layers, one on top of the
other, also was observed by Khatib et al. [36]
The formation of a Fe–Si gel
layer directly on the membrane surface was mainly responsible for the
fouling. Reducing the electrostatic repulsion between the ferric gel and the
membrane surface encouraged adhesion.
Fouling Resistance of Hydrophilic Membranes
Kabsch-Korbutowicz et al. [17]
demonstrated that the most hydrophilic
of the membranes tested (i.e., regenerated cellulose) had the lowest proneness
to fouling by organic colloids (i.e., humic acids). This is similar to the results
Goosen et al.2270
of Schafer et al. [35]
who showed that hydrophobic humic acid compounds
had the greatest tendency toward membrane fouling. In the work of
Kabsch-Korbutowicz et al., [17]
the best membrane displayed the highest per-
meability to humic acid solutions. The presence of mineral salts intensified
the fouling process.
What these studies tell us is that to reduce fouling due to humic acids, it is
best to use hydrophilic membranes, to have feed water with a low mineral salts
content (e.g., calcium), and to work at low pH. These conclusions are sup-
ported by the excellent work of Tu et al. [37]
who showed that membranes
with a higher negative surface charge and greater hydrophilicity were less
prone to fouling due to fewer interactions between the chemical groups in
the organic solute and the polar groups on the membrane surface.
Effect of Inorganics
Dynamic tests were conducted by Sahachaiyunta et al. [38]
to investigate
the effect of silica fouling of RO membranes in the presence of minute
amounts of various inorganic cations such as iron, manganese, nickel, and
barium that are present in industrial and mineral processing wastewaters.
Experimental results showed that the presence of iron greatly affected the
scale structure on the membrane surface when compared with the other
metal species.
Effect of Proteins and Colloid Stability
A dual-mode fouling process, similar to that observed for humic acids, [35]
was found for protein [(i.e., bovine serum albumin (BSA)] fouling of micro-
filtration (MF) membranes. Protein aggregates first formed on the membrane
surface, followed by native (i.e., nonaggregated) protein. The native protein
attached to an existing protein via the formation of intermolecular disulfide
linkages. The researchers successfully developed a mathematical model to
describe this dual-mode process.
Yiantsios and Karabelas, [39]
in a very interesting paper, found that apart
from particle size and concentration, colloid stability plays a major role in
RO and UF membrane fouling. Stable colloidal suspensions caused less
fouling. They demonstrated that standard fouling tests as well as most well-
known fouling models are inadequate. A key finding was that the use of
acid, which is a common practice to avoid scaling in desalination, might
promote colloidal fouling. Lowering the pH reduces the negative charge on
particles, causing aggregate formation that deposits on the membrane
surface. Colloidal fouling of membranes also has been modeled. [40]
Fouling of RO and UF Membranes 2271
Wastewater effluent organic matter was isolated into different fractions by
Jarusutthirak et al. [41]
Each isolate exhibited different characteristics in
fouling of nanofiltration (NF) and UF membranes. For example, the colloidal
fractions gave a high flux decline due to pore blockage, and hydrophobic inter-
actions were very important for hydrophobic membranes, causing a reduction
in permeate flux. In particular, polysaccharides and amino sugars were found
to play an important role in fouling.
Reversible Adsorbed Layer Resistance
Nikolova and Islam [29]
reported concentration polarization in the absence
of gel-layer formation by using a laboratory scale UF unit equipped with a
tubular membrane (Table 1). In a key study, they found that the decisive
factor in flux decline was the adsorption resistance. With the development
of a concentration polarization layer, the adsorbed layer resistance at the
membrane wall increased linearly as a function of the solute concentration
at the wall. They described the flux by the following relationship:
J ¼ DP ÿ DpðwÞ
mðRm þ kCwÞ ð1Þ
where DP is the hydraulic pressure difference across membrane, Cw is the con-
centration at the membrane surface, Dp (w) is the corresponding osmotic
pressure, Rm is the membrane resistance, kCw is the adsorbed layer resistance,
and m is the fluid viscosity. In a key finding, they showed that the adsorption
resistance was of the same order of magnitude as the membrane resistance.
Surprisingly, the osmotic pressure was negligible in comparison with the
applied transmembrane pressure. The significance of this study is that it
showed that the reversible adsorbed solute layer at the membrane surface is
the primary cause of flux decline and not the higher osmotic pressure at the
membrane surface. This is supported by the work of Koltuniewicz and
Noworyta [10]
(Fig. 2).
Transition from Reversible Adsorption to Irreversible Fouling
The solute adsorption described by Nikolova and Islam [29]
is reversible.
The transition between this type of adsorption and irreversible fouling is
crucial to determining the strategy for improved membrane performance
and for understanding the threshold values for which optimal flux and rejec-
tion can be maintained. In a very thorough study, Chen et al. [42]
reported on
Goosen et al.2272
the dynamic transition from concentration polarization to cake (i.e., gel layer)
formation for membrane filtration of colloidal silica. Once a critical flux, Jcrit,
was exceeded, the colloids in the polarized layer formed a consolidated cake
structure that was slow to depolarize and in which reduced the flux. This study
showed that by controlling the flux below Jcrit, a polarization layer may form
and solute adsorption may occur, but it is reversible and responds quickly to
any changes in convection. This paper is a very valuable source of information
for membrane plant operators. By operating just below Jcrit, they can maxi-
mize the flux while at the same time reduce the frequency of membrane
cleaning.
ANALYTICAL DESCRIPTIONS
Measuring Fouling-Layer Morphology and Growth
The physical structure of the membrane surface (i.e., surface roughness)
can influence the morphology of the fouling layer. Riedl et al. [43]
used an
atomic force microscopy (AFM) technique to measure membrane surface
roughness and scanning electron microscopy (SEM) to assess the fouling
layer. It was shown that smooth membranes produced a dense surface
fouling layer, whereas, this same layer or biofilm on rough membranes was
much more open. The primary conclusion of a study by Riedl et al. was
that fluxes through rough membranes are less affected by fouling formation
than fluxes through smooth membranes. In a related study with a water-in-
oil emulsion, Scott et al. [15]
found that the use of corrugated membranes
enhanced the flux in a more efficient way by promoting turbulence near the
wall region, resulting in mixing of the boundary layer and, hence, reducing
fouling.
Cell Adhesion Kinetics
The kinetics of adhesion of Mycobacterium to cellulose diacetate RO
membranes have been described. [19]
Adhesion of the cells to the membrane
surface occurred within 1–2 hr and exhibited saturation-type kinetics that
conformed closely to the Langmuir adsorption isotherm, a mathematical
expression describing the partitioning of substances between a solution and
a solid–liquid interface. This suggested that cellulose diacetate membrane
surfaces may possess a finite number of available binding sites to which the
mycobacteria can adhere. Treatment of the attached mycobacteria with differ-
ent enzymes suggested that cell-surface polypeptides, 4- or a-1.6 linked
Fouling of RO and UF Membranes 2273
glucan polymers, and carboxyl ester bond-containing substances (possibly
peptiglycolipids) may be involved in the adhesion process. The exact molecu-
lar mechanisms of adhesion, however, have not as yet been clearly defined.
Nor have all the specific macromolecular cell-surface ligands that mediate
the attachment been identified. This is one area where further research is
needed.
Hydrodynamic Studies of Microbial Adhesion
Fundamental studies of the membrane fouling process based on the move-
ment of rigid neutrally buoyant spherical particles (i.e., a model bacterial
foulant) toward a membrane surface were performed by Altena and
Belfort [44]
and Drew et al. [45]
Their studies were an attempt to give clearer
insight into the hydrodynamics behind the mechanism of microbial adhesion
in RO systems. Under typical laminar flow conditions, particles with a radius
smaller than 1 mm were captured by a porous membrane surface (i.e., the
microbial adhesion step), resulting in cake formation. Due to convective
flow into the membrane wall, particles moved laterally toward the membrane.
The particle concentration near the membrane surface increased significantly
over that in the bulk solution and resulted in a fouling layer. In their cross-flow
membrane filtration experiments there appeared to be two major causes for
lateral migration: a drag force exerted by the fluid on the particle due to the
convective flow into the membrane wall (i.e., wall suction effect or permeation
drag force) that carried particles toward the membrane and an inertial lift force
that carried particles near the membrane away from the porous wall. For small
particles (,1 mm) the permeation drag force dominated. An expression was
developed from first principles to predict conditions under which a membrane
module exposed to dilute suspensions of spherical particles will not foul.
While these researchers did not work directly with microbial cells, their
hydrodynamic studies do provide useful information on how the particle
size and fluid flow affects microbial adhesion.
Passage of Bacteria Through MF Membranes
In a recommended paper, Ghayeni et al. [46]
studied the passage of bacteria
(0.5-m diameter) through MF membranes in wastewater applications. Total
and viable cell counts were measured microscopically by using two stains con-
sisting of a bright blue DNA fluorochrome 4,6-diamidino-2-phenylindole
(DAPI) and a red fluorescent flourochrome 5-cyano-2,3-ditolyl tetrazolium
chloride (CTC), respectively. Membranes with pore sizes smaller than
Goosen et al.2274
0.2 mm still transmitted secondary effluent cells. This is an interesting study
which showed that based on total cell counts (DAPI) up to 1% of the bacteria
in the feed can pass to the permeate side. While a significant portion of the
cells (e.g., 50%) in the permeate showed biological (CTC) activity, none of
the cells were able to reproduce (i.e., culture on agar or in suspension). This
is a good quantitative method for measuring cell injury. We can speculate
that smaller cells, or membranes with larger pores, would allow for the
passage of viable bacteria that would be able to reproduce. This could occur
at some critical cell/pore ratio (Fig. 4).
Analysis of Deposits on Membrane Surface
Deposits on a membrane surface, before and after cleaning, can be
analyzed by using SEM in combination with energy dispersive x-ray (EDX)
combined with a microanalysis system permitting quantitative determination
of elements. [12]
Attenuated Total Reflection and Fourier Transform
Infrared Spectroscopy
Attenuated total reflection (ATR) and Fourier transform infrared (FTIR)
spectroscopy can provide insight into the chemical nature of deposits on
Figure 4. Passage of bacterial cells through membrane pores. Cell damage occurs at
critical pore radius/cell radius ratio.
Fouling of RO and UF Membranes 2275
membranes. [47]
The spectra of the foulants can be easily distinguished from
the spectra of the membrane material. ATR–FTIR spectroscopy also can
indicate the presence of inorganic foulants as well as the ratio of inorganic
to organic foulants.
The surface deposits on UF PES membranes fouled by skimmed milk
have been studied by using ATR–FTIR spectroscopy to detect the functional
groups of the fouling species. [22]
Two types of fouling conditions were
assessed: static conditions as performed in a beaker and dynamic conditions
as performed on a UF loop with applied pressure. For static conditions, all
milk components adsorbed onto the PES surface. Some milk components
(lactose and salts) were eliminated by water rinsing, whereas, proteins were
only partially removed by chemical cleaning at basic pH. For dynamic
conditions, the cleanliness of the membrane was evaluated through two
criteria: hydraulic (i.e., recovery of initial flux) and chemical (i.e., no more
contaminants detected). The hydraulic cleanliness of the membrane was
achieved, whereas, the membrane initial surface state was not restored.
Also, ATR–FTIR spectroscopy is a useful tool for evaluating other fouling
species such as oil and humic acids.
Identification of specific species deposited onto membrane surfaces also
can be carried out by using matrix assisted laser desorption ionization mass
spectroscopy (MALDI-MS). Chan et al. [48]
used this technique to differentiate
between desorption of proteins from the membrane surface, from inside pores
and from the membrane substrate. It was shown that the technique is a power-
ful tool for distinguishing between different proteins in fouling deposits. It has
the potential for quantitative measurement of protein fouling on membrane
surfaces.
Measuring Fouling in Real Time
AFM has proved to be a rapid method for assessing membrane–solute
interactions (fouling) of membranes under process conditions. [49]
Given the
good agreement between the correlations when using AFM and the operating
performance, it should be possible, in the future, to use these techniques to
allow prior assessment of the fouling propensity of process streams.
Nondestructive, real-time observation techniques to detect and to monitor
fouling during liquid separation processes are of great importance in the devel-
opment of strategies to improve operating conditions. In a recommended
paper by Li et al., [23]
ultrasonic time-domain reflectometry (UTDR) was
used to measure organic fouling, in real time, during UF with PS membranes.
The feed solution was a paper-mill effluent, which contained breakdown
products of lignin or lignosulphonate, from a wastewater treatment plant.
Experimental results showed that the ultrasonic signal response can be used
Goosen et al.2276
to monitor fouling-layer formation and growth on the membrane in real time.
Traditional flux measurements and analysis of the membrane surface by
microscopy corroborated the UTDR results. Furthermore, the differential
signal developed indicated the state and progress of the fouling layer and
gave warning of advanced fouling during operation. This is a useful paper.
Measurement of Concentration Polarization
In other recommended papers, Gowman and Ethier [50,51]
developed an
automated laser-based refractometric technique to measure the solute concen-
tration gradient during dead-end filtration of a biopolymer solution. This is
a good paper that attempts to reconcile theory with experimental data. The
refractometric technique may be useful to other researchers working on
quantification of membrane fouling.
A nuclear magnetic resonance technique was used by Pope et al. [14]
to
quantitatively measure the concentration polarization layer thickness during
cross-flow filtration of an oil–water emulsion. The technique, which measured
layer thickness by using chemical shift selective microimaging, may be useful
in studying other membrane fouling situations that occur in food processing
and desalination. This method will help to clarify the relative quantitative con-
tributions to flux decline of the adsorbed layer resistance and the concentration
polarization layer gradient and thickness. It can help to explain the flux
declines due to different resistances, as shown in Fig. 2.
Mathematical Models for Flux Decline and
Relative Contributions
Dal-Cin et al. [52]
developed a series resistance model to quantify the rela-
tive contributions of adsorption, pore plugging, and concentration polarization
to flux decline during UF of a pulp mill effluent. They proposed a relative flux
loss ratio as an alternative measure to the conventional resistance model that
was found to be a misleading indicator of the flux loss. By using experimental
and simulated flux data, the series resistance model was shown to underpredict
fouling due to adsorption and to overpredict concentration polarization. This
appears to be a disadvantage and would make the model of limited use
in its current form. As mentioned in the Introduction, Koltuniewicz and
Noworyta [10]
modeled the flux decline as a result of the development of a con-
centration polarization layer based on the surface renewal theory developed by
Danckwerts. [53]
This is a highly recommended paper. The surface renewal
model is more realistic than the commonly used film model, because mass
Fouling of RO and UF Membranes 2277
transfer at the membrane boundary layer is random in nature due to membrane
roughness. Specifically, the membrane is not covered by a uniform concen-
tration polarization layer, as was assumed in the film model, but rather by a
mosaic of small surface elements with different ages and, therefore, different
permeate flow resistance. Any element can be swept away randomly by a
hydrodynamic impulse and then a new element starts building up a layer of
retained solute at the same place on the membrane surface. They showed
that the decrease in flux with respect to time, J(tp), due to the development
of the concentration polarization layer is given by the following equation,
which also takes into account the rate of membrane surface renewal, s
(area/unit time):
�JJðtpÞ ¼ ðJo ÿ J �Þ
s
s þ A
1 ÿ eÿðsþAÞtp
1 ÿ eÿstp þ J� ð2Þ
where A is rate of loss of membrane surface area as a function of time, Jo is the
initial value of the flux, J� is the flux observed after infinite time, and tp is the
time of permeation.
s ¼ A Jlim ÿ J
�
Jo ÿ Jlim ð3Þ
where Jlim is the limiting flux, which is similar to critical flux, Jcrit. The former
can be obtained from literature data. The average flux under steady-state
conditions, Ja, can be calculated directly from Eq. (6) as a limit:
�JJa ¼ lim tp!1
�JJðtpÞ ¼ ðJo ÿ J �Þ
s
A þ s þ J� ð4Þ
In support of this model, calculated values of flux by using Eqs. (2) and
(3) agreed well with experimental data. The two equations describe a permea-
tion cycle of duration, tp, as shown in Fig. 2. This is a highly recommended
paper for those who are operating large-scale continuous UF plants and to a
certain extent RO plants. The model developed describes not only the
dynamic behavior of a plant but it also allows for optimization of operating
conditions (i.e., permeation time, cleaning time, cleaning strategy).
Variation in Gel-Layer Thickness along Flow Channel
It often is assumed that the thickness of the gel layer and the concentration
of the solute are uniform over the membrane surface. However, these assump-
tions are only valid for systems where the hydrodynamic conditions of the
solution flow near the membrane provide equal accessibility of solute to the
Goosen et al.2278
entire membrane surface. [54]
This is not true in the case of cross-flow filtration.
One can, thus, expect that the gel-layer thickness and/or the surface concen- tration of the solute will vary with distance from the channel entrance. As a
consequence, the local permeate flux will also vary with longitudinal position.
In a highly recommended article, Denisov [54]
presented a mathematically
rigorous theory of concentration polarization in cross-flow UF, which takes
into account the nonuniformity of the local permeate membrane flux. He
derived equations describing the pressure/flux curve. In the case of the gel-layer model, the theory led to a simple analytical
formula for a limiting or critical flux, Jlim. The flux turned out to be pro-
portional to the cube root of the ratio of the gel concentration to the feed solu-
tion concentration, rather than to the logarithm of this ratio, as the simplified
Michaels–Blatt theory predicted:
Jlim ¼ 3
2
� �2=3
KPg ¼ 1:31 Cg
Co
� �
m1=3D2=3U1=3o L1=3h1=3
ð5Þ
where
Pg ¼ CgmD
2Uo
CoK 3Lh
� �
ð6Þ
where K is hydraulic permeability of membrane to pure solvent (m 3/N sec),
Cg is the gel concentration (kmol/m 3 ), Co is the solute concentration in
feed solution (kmol/m3), m is the channel parameter, D is the solute diffusion coefficient (m
2/sec), Uo is the longitudal component of fluid velocity averaged over the channel cross section (m/sec), L is the channel length (m), h is the transversal dimension of the channel (m).
In the case of the osmotic pressure model, the rigorous theory allowed the
conclusion that at high applied transmembrane pressure, the permeate flux
increased as a cube root of the pressure, so that the limiting flux was never
reached:
J � 3
2
� �2=3
KP 1=3
P 2=3 o � 1:31
P
RTCo
� �1=3 m1=3D2=3U1=3o
L1=3h1=3 ð7Þ
where
Po ¼ mD2Uo
RTCoK 3Lh
� �
ð8Þ
where J is the average flux over the channel (m/sec), P is the transmembrane pressure (N/m2), R is the gas constant (J/kmol K), T is the temperature (K). However, one minor weakness of the study was that the analysis ignored
Fouling of RO and UF Membranes 2279
the concentration dependence of the viscosity and the partial transmission of
the solute through the membrane.
Pore Blockage and Cake Formation
To understand the effect of membrane fouling on system capacity, the
Vmax test is often used to accelerate testing. This test assumes that fouling
occurs by uniform constriction of the cylindrical membrane pores. This
does not happen in practice. Zydney and Ho [27]
examined the validity of the
Vmax model and compared the results with predictions from a new model
that accounts for fouling due to both pore blockage and cake formation. It
was found that the Vmax analysis significantly overestimates the system
capacity for proteins that foul primarily by pore blockage, but it underesti-
mates the capacity for compounds that foul primarily by cake formation. In
contrast, the pore blockage–cake filtration model provides a much better
description of membrane fouling, leading to more accurate sizing and scale-
up of normal flow filtration devices. Cake formation, shear forces, and other
mathematical aspects, and the kinetics of the boundary layer are also described
in an early study by Hermia. [55]
PREVENTIVE MEANS AND CLEANING METHODS
Feed Water Pretreatment
MF and UF
RO seawater systems that operate on surface feed water normally require
an extensive pretreatment process to control membrane fouling. In recent
years, new effective water microfiltration technologies have been introduced
commercially. Wilf and Klinko [56]
and Glueckstern et al. [57]
noted that
these developments can improve the quality of surface seawater feed to a
level comparable with or better than the water quality from well-water
sources. The utilization of capillary UF as a pretreatment step enabled oper-
ation of the RO system at a high recovery (15%) and permeate flux rate. In
a similar study using MF and UF as seawater pretreatment steps for RO,
Glueckstern and Priel [58]
showed that such technology can dramatically
improve the quality of the feed water. This is especially important if cooling
water from existing power stations is used as feed water for desalination
plants.
Goosen et al.2280
Municipal wastewater is one of the most reliable sources of water since its
volume varies little through the year. The reuse of such water requires treat-
ment to an acceptable quality level that satisfies regulatory guidelines.
Ghayani et al. [25]
used hollow fiber MF as a pretreatment for wastewater for
RO in the production of high-quality water. Organisms present in MF-
treated secondary effluent were able to attach to RO membranes and prolifer-
ate to form a biofilm. Total cell counts in this treated effluent (i.e., permeate
from the MF unit) were several orders of magnitude higher than viable cell
counts. This was confirmed in a later study. [46]
What these results indicate
is that MF membranes will not be totally effective in removal of bacteria
from the feed water stream. The result showed that most cells were severely
damaged by passage through the membrane (Fig. 4). However, we can specu-
late that this damaging effect may be cell-strain specific and/or dependent on the cell/pore diameter. Other types of microbial cells may survive the passage through the MF polymer membrane, resulting in possible fouling of the RO
membrane farther upstream.
Ultrafiltration membranes also may be used to improve the quality of
treated, potable water by removing suspended solids and colloids. [59]
Coagulation and Flocculation
Studies have looked at flocculation and its effects on membrane fouling
from a range of different angles. In a study by Chapman et al., [60]
a flocculator
was used to remove suspended solids, organics, and phosphorus from waste-
water. The flocculator produced uniform microflocs, which were removed by
cross-flow MF. Flocculated particles can form a highly porous filtration cake
on a membrane surface. This will help inhibit fouling on the membrane by pre-
venting the deposition of particles and, therefore, reducing the number of
membrane cleaning cycles. [61]
Arsenic removal from drinking water is a major problem in many parts of
the world. Han et al. [62]
investigated arsenic removal by flocculation and MF.
Ferric chloride and ferric sulfate were used as flocculents. The results showed
that flocculation before MF led to significant arsenic removal in the permeate.
Furthermore, the addition of small amounts of cationic polymeric flocculants
resulted in significantly improved permeate fluxes during MF.
Coagulation, to remove turbidity from water by the addition of cationic
compounds, is another commonly used method. The usefulness of coagulation
as a pretreatment to remove microparticles in aqueous suspension before
a membrane filtration was shown by Choksuchart et al. [63]
There are several
types of coagulation systems. Comparisons were made by Park et al. [64]
between coagulation with only rapid mixing in a separate tank (i.e., ordinary
Fouling of RO and UF Membranes 2281
coagulation) and coagulation with no mixing tank (i.e., in-line coagulation)
before a UF process. The former was superior. An in-line coagulation
(without settling) UF process also was ussed by Guigui et al. [65]
Floc cake
resistance was found to be lower than resistance due to the unsettled floc
and the uncoagulated organics. A reduction in coagulant dose induced an
increase in the mass transfer resistance. This study supported the results of
Nguyen and Ripperger [61]
who found that the flocculant cake was very porous.
Combining flocculation and coagulation in a pretreatment process also
has been studied. In an key paper by Lopez-Ramirez et al., [66]
the secondary
effluent from an activated sludge unit was pretreated, before RO, with three
levels: intense (coagulation–flocculation with ferric chloride and polyelectro-
lite and high pH sedimentation), moderate (coagulation–flocculation with
ferric chloride and polyelectrolite and sedimentation), and minimum (only
sedimentation). The optimum for membrane protection, in terms of cal-
cium, conductivity, and bicarbonates reduction, was the intense treatment.
Membrane performance varied with pretreatment but not reclaimed water
quality. The study recommended intense pretreatment to protect the
membrane.
A modular pilot-size plant involving coagulation/flocculation, centrifu- gation, UF, and sorption processes was designed and constructed by Benito
et al. [13]
for the treatment of oily wastewaters. Different treatments were con-
sidered, depending on the nature of the oily waste emulsion. The main advan-
tage of the plant was its versatility by allowing combinations of different
treatments to be used for the most economic and safest treatment scheme
for a given wastewater.
Empirical equations developed by Shaalan [67]
predict the impact of water
contaminants on flux decline. These formulas enable decision making con-
cerning a suitable water pretreatment scheme and also selection of the most
appropriate cleaning cycle.
Effects of Spacers on Permeate Flux and Fouling
Influence of Spacer Geometry on Boundary Layer Disruption
Sablani et al. [7]
studied the influence of spacer thickness in spiral-wound
membrane units on permeate flow and its salinity. Membrane parameters also
were estimated by using an analytical osmotic pressure model for high salinity
applications. The effects of spacer thickness on permeate flux showed that the
observed flux decreases by up to 50% in going from a spacer thickness of
0.1168–0.0508 cm. The authors commented that the different geometry/ configuration of the spacer influenced turbulence at the membrane surface
Goosen et al.2282
and that, in turn, affected concentration polarization. This suggested less tur-
bulence with the smaller spacer thickness and is opposite to what is normally
expected. A membrane module with an intermediate spacer thickness of
0.0711 cm was found to be the best economically since it gave the highest
water production rate (L/h). Geraldes et al.
[68] assessed the effect of a ladder-type spacer configuration
in NF spiral-wound modules on concentration boundary layer disruption. The
results showed that the average concentration polarization for the membrane
wall was independent of the distance to the channel inlet, while for the mem-
brane wall without adjacent filaments, the average concentration polarization
increased with the channel length. This was due to the fact that in the first case
the transverse filaments periodically disrupted the concentration boundary
layer, while, in the second case, the concentration boundary layer grew con-
tinuously along the channel length. The experimental results of the apparent
rejection coefficients were compared with model predictions, the agreement
being good. Their results clearly established how crucial the spacers configur-
ation is in the optimization of the spiral-wound module efficiency.
Computational Fluid Dynamics of Flow in Spacer-Filled Channels
The unexpected results of Sablani et al. [7]
(i.e., less turbulence with
smaller spacer thickness) may be best explained by an excellent paper by
Schwinge et al. [69]
The latter used computational fluid dynamics (CFD) in a
study of unsteady flow in narrow spacer-filled channels for spiral-wound
membrane modules. The flow patterns were visualized for different filament
configurations incorporating variations in mesh length and filament diameter,
and for channel Reynolds numbers, Rech, up to 1000. The simulated flow
patterns revealed the dependence of the formation of recirculation regions
on the filament configuration, mesh length, filament diameter, and the
Reynolds number. When the channel Reynolds number was increased above
300, the flow became super critical, showing time-dependent movements
for a filament located in the center of a narrow channel; and, when the
channel Reynolds number was increased above 500, the flow became super-
critical for a filament adjacent to the membrane wall. For multiple filament
configurations, flow transition can occur at channel Reynolds numbers as
low as 80 for the submerged spacer at a very small mesh length [mesh
length/channel height (Lm/hch) ¼ 1] and at a slightly larger Reynolds number at a larger mesh length (Lm/hch ¼ 4). The transition occurred above Rech of 300 for a cavity spacer and above Rech of 400 for a zigzag spacer.
We can speculate that the conclusions of Sablani et al., [7]
less turbulence
with smaller spacer thickness, was due to fewer recirculating regions as a
result of smaller mesh length and filament diameter.
Fouling of RO and UF Membranes 2283
The CFD simulations were used by Li et al. [70]
to determine mass transfer
coefficients and power consumption in channels filled with nonwoven net
spacers. The geometric parameters of a nonwoven spacer were found to
have a great influence on the performance of a spacer in terms of mass transfer
enhancement and power consumption. The results from the CFD simulations
indicated that an optimal spacer geometry exists. Lipnizki and Jonsson [71]
also
studied mass transfer in membrane modules. Their experiments were used to
calculate the energy consumption vs. the mass transfer coefficient for different
spacers.
MF by Using Corrugated Membranes
In a study with an oil-in-water emulsion Scott et al. [15]
compared fluxes
and fouling between flat membranes and corrugated membranes. Membrane
fouling was found to consist of two distinct stages: initial pore blocking
followed by cake layer formation. They found that the use of corrugated mem-
branes enhanced the flux in a more efficient way by promoting turbulence near
the wall region, similar to spacers, resulting in mixing of the boundary layer
and, hence, reducing the concentration polarization.
Membrane Surface Modification
A fouling-resistant RO membrane that reduces microbial adhesion was
reported by Jenkins and Tanner. [72]
In this interesting study that confirmed
the results of Flemming and Schaule, [20]
they compared two types of thin-
film composite membranes with different chemistries. One type was classified
as a polyamide, the other used a new chemistry that formed a polyamide–urea
barrier (i.e., surface) layer. The latter composite membrane proved superior in
RO operation similar to that of the polyetherurea membrane of Flemming and
Schaule, [20]
including rejection of certain dissolved species and fouling resist-
ance. These results suggest that the presence of urea groups in the membrane
reduces microbial adhesion, perhaps through charge repulsion. The results of
work by Ridgway [19]
on the kinetics of adhesion of Mycobacterium to cellu-
lose diacetate RO membranes has similar implications. Scientists should,
therefore, be able to minimize microbial adhesion by controlling the surface
chemistry of polymer membranes through, for example, the inclusion of
urea groups.
Belfer [24]
described a simple method for surface modification of commer-
cial composite polyamide RO membranes. The procedure involved radial
grafting with a redox system consisting of potassium persulfate/sodium
Goosen et al.2284
metabisulfite. The ATR–FTIR spectroscopy provided valuable information
about the degree of grafting and the microstructure of the grafted chain on
the membrane surface. Both acrylic and sulfo-acidic monomers and neutral
monomers such as polyethylene glycol methacrylate were used to demonstrate
the wide possibilities of the method in terms of grafting of different monomers
and initiators. It was shown that some of the modified membranes conserved
their previous operating characteristics, flux, or rejection, but exhibited a
higher resistance to humic acid. Additional work needs to be done to find
out what happens to the fouling resistance of such membranes over the long
term (i.e., after initial biofilm formation).
Chemical modification of a membrane surface can be used in combination
with spacers and periodic applications of bioacids. [73]
The paper by Redondo,
however, is short on specifics (e.g., details of chemical modification of aro-
matic polyamides membrane surface) and, therefore, is not very useful to
those looking for insights into membrane fouling
Fouling Resistance of Hydrophilic and
Hydrophobic Membranes
Kabsch-Korbutowicz et al. [17]
demonstrated that the most hydrophilic of
the membranes tested (i.e., regenerated cellulose) had the lowest proneness to
fouling by organic colloids (i.e., humic acids). These conclusions were further
supported by the thorough work of Tu et al. [37]
who showed that membranes
with a higher negative surface charge and greater hydrophilicity were less
prone to fouling due to fewer interactions between the chemical groups in
the organic solute and the polar groups on the membrane surface. Cherkasov
et al. [32]
presented an analysis of membrane selectivity from the standpoint of
concentration polarization and adsorption phenomena. The results of their
study also showed that hydrophobic membranes attracted a thicker irreversible
adsorption layer than hydrophilic membranes. The layer thickness was deter-
mined by the intensity of concentration polarization (Fig. 3). This may be due
to the stronger attraction of water to hydrophilic membranes.
System Design and Control of Operating Parameters
Predicting Membrane Performance
A comprehensive difference model was developed by Madireddi et al. [74]
to predict membrane fouling in commercial spiral-wound membranes with
Fouling of RO and UF Membranes 2285
various spacers. This is a useful paper for experimental studies on the effect of
flow-channel thickness on flux and fouling.
Avlonitis et al. [75]
presented an analytical solution for the performance of
spiral-wound modules with seawater as the feed. In a key finding, they showed
that it was necessary to incorporate the concentration and pressure of the feed
into the correlation for the mass transfer coefficient. In a similar study,
Boudinar et al. [76]
developed the following relationship for calculating mass
transfer coefficients in channels equipped with a spacer:
k ¼ 0:753 K
2 ÿ K
� �1=2 DS
hB Sc
ÿ1=6 PehB
M
� �
ð9Þ
where Pe is Peclet number, K ¼ 0.5 and M ¼ 0.6 (cm).
Controlled centrifugal instabilities (called Dean vortices), resulting from
flow around a curved channel, were used by Mallubhotla and Belfort [77]
to
reduce both concentration polarization and the tendency toward membrane
fouling. These vortices enhanced back-migration through convective flow
away from the membrane–solution interface and allowed for increased mem-
brane permeation rates.
Temperature Effects
Goosen et al. [3]
showed that the polymer membrane can be very sensitive
to changes in the feed temperature. There was up to a 100% difference in the
permeate flux between feed temperatures of 308C and 408C. A more recent
study showed that the improved flux was due primarily, though not comple-
tely, to viscosity effects on the water. Reversible physical changes in the mem-
brane also may have occurred. [78]
Critical Flux
A key phase in membrane separation processes is the transition from
concentration polarization to fouling. This occurs at a critical flux. Song [79]
indicated that in most theories developed, the limiting or critical flux is
based on semi-empirical knowledge rather than being predicted from funda-
mental principles. To overcome this shortcoming, he developed a mechanistic
model, based on first principles, for predicting the limiting flux. Similar to the
critical flux results of Chen et al. [42]
and the limiting flux of Koltuniewicz
and Noworyta, [10]
Song showed that there is a critical pressure for a given
suspension. When the applied pressure is below the critical pressure, only a
concentration polarization layer exists over the membrane surface. A fouling
layer, however, will form between the polarization and the membrane surface
Goosen et al.2286
when the applied pressure exceeds the critical pressure. The limiting or critical
flux values predicted by the mechanistic model compared well with the
integral model for a low concentration feed. Operators of RO/UF plants/ units should, therefore, operate their systems just below the critical flux to
maximize productivity while minimizing membrane fouling.
Membrane Cleaning
Since feed water pretreatment helps to prevent biofouling, once a mem-
brane surface has been fouled, it must be cleaned. This will result in wear
and tear and eventual loss of membrane properties.
Rinsing Water Quality
Membranes used in the food industry for UF of milk or whey are cleaned
on a regular basis with water and various aqueous solutions to ensure hygienic
operation and to maintain membrane performance. Water quality, therefore, is
of special importance in the rinsing and cleaning process, because impurities
present in the water could affect cleaning efficiency and, in the long term,
could contribute to a reduction in performance and life of the membrane. [80]
Membrane manufacturers generally recommend the use of high-quality
water such as filtered and demineralized water. Installing and running
water-purification systems, however, is expensive. Alternatively, water-
treatment chemicals such as sequestering agents (e.g., ethylene diamine
tetra-acetic acid (EDTA), polyphosphates) can be added to low-quality
water to increase the solubility of metal ions such as calcium, magnesium
manganese, and iron. RO permeate also may be of suitable quality for use
in cleaning.
In a study by Tran-Ha and Wiley, [80]
it was shown that impurities, such as
particulate and dissolved salts present in the water, can affect the cleaning effi-
ciency of a PS UF membrane. The water used for cleaning was doped with a
known amount of specific ions (i.e., calcium, sodium, chloride, nitrate, and
sulfate). The presence of calcium in water, at the usual concentrations
found in tap water, did not greatly affect cleaning efficiency, chloride was
found to reduce it. Sodium, nitrate, and sulfate appeared to improve the flux
recovery during membrane cleaning. The cleaning efficiency also was
improved at higher ionic strengths. For further reading, a similar study by
Lindau and Jonsson [12]
is recommended. They assessed the influence of differ-
ent types of cleaning agents on a polysulfone UF membrane after treatment
of oily wastewater.
Fouling of RO and UF Membranes 2287
Cleaning Agents
The effect of different cleaning agents on the recovery of the fouled mem-
brane was studied by Mohammadi et al. [81]
Results showed that a combination
of sodium dodecyl sulfate and sodium hydroxide can be used as a cleaning
material to reach the optimum recovery of the PS membranes used in milk
concentration industries. Also, a mixture of sodium hypocholorite and
sodium hydroxide showed acceptable results, whereas washing with acidic
solutions was not effective.
Backpulsing
Mores and Davis, [82]
to view membrane surfaces at different times in
cross-flow MF, used direct visual observation (DVO) of yeast suspensions
with rapid backpulsing at varied backpulsing duration and pressure. The
DVO photograph showed that the membranes were more effectively
cleaned by longer backpulse durations and higher backpulse pressures.
However, trade-offs existed between longer and stronger backpulses and
permeate loss during the backpulse. Shorter, stronger backpulses resulted in
higher net fluxes than longer, weaker backpulses.
Membrane Wear and Degradation
Roth et al. [83]
proposed a method to determine the state of membrane wear
by analyzing sodium chloride stimulus–response experiments. The shape of
the distribution of sodium chloride in the permeate flow of the membrane
revealed the solute permeation mechanisms for used membranes. For new
membranes, the distribution of sodium chloride collected in the permeate
side, as well in the rejection side, was unimodal. For fouled membranes, they
noted the presence of several modes. The existence of a salt leakage peak, as
well as an earlier detection of salt for all the fouled membranes, gave evidence
of membrane structure modification. The intensive use of the membranes might
have created an enlargement of the pore sizes. Salt and solvent permeabilities
increased as well. While this is a difficult paper to follow, it may be of use to
those who want to develop new methods for measuring membrane degradation.
Amerlaan et al. [21]
reported on membrane degradation resulting in a pre-
mature loss of salt rejection by cellulose acetate membranes. Tests were
initiated to find a solution to the problem and to gain a better understanding
of the mechanisms involved. It was found that removal of all free chlorine
solved the problem. This was accomplished by injecting ammonia in the
feed water, presumably resulting in formation of ammonium chloride.
Membrane damage by chlorine was also reported by Ridgway et al. [18]
Goosen et al.2288
They studied membrane fouling at a wastewater treatment plant under low-
and high-chlorine conditions. High chlorine residuals damaged the membrane
structure and reduced mineral rejection capacity.
ECONOMIC ASPECTS
Scientists often forget that successful commercialization of a new tech-
nology is dependent on economic factors. Just because a novel separation
technique works in the laboratory, for example, it does not mean that it will
replace current methods. The new technique must, at minimum, be compar-
able in overall cost and, preferably, be lower in cost.
Field evaluation of a hybrid membrane system consisting of an UF mem-
brane pretreatment unit and a RO seawater unit was conducted by Glueckstern
et al. [57]
For comparison, a second pilot system consisting of conventional
pretreatment and an RO unit was operated in parallel. The conventional
pretreatment unit included in-line flocculation followed by media filtration.
The study showed that UF provided a very reliable pretreatment for the RO
system, independent of the raw-water-quality fluctuations. However, the
cost of membrane pretreatment was higher than conventional pretreatment.
This suggested that membrane pretreatment for RO desalting systems is
only economical for sites that require extensive conventional pretreatment
or where wide fluctuations in the raw-water quality are expected.
The competitiveness of UF pretreatment in comparison with conventional
pretreatment (i.e., coagulation and media filtration) was assessed by Brehant
et al. [84] by looking at the impact on RO hydraulic performances. The
study showed that UF provided permeate water with high and constant
quality resulting in a higher reliability of the RO process than with a conven-
tional pretreatment. The combination of UF with a precoagulation at low dose
helped in controlling UF membrane fouling. The authors concluded that the
combined effect of a higher recovery and a higher flux rate promised to signifi-
cantly reduce the RO plant costs. The conclusions reached where opposite of
those reported in the paper by Glueckstern et al. [57]
above, and demonstrate the
complexity of the overall economics of a membrane separation process.
CONCLUDING REMARKS
Experimental and modeling studies were assessed to give a more funda-
mental insight into the mechanism of the biofouling process, how to quantify
it, and how to reduce it. This review has shown that the fouling process is a
complex mechanism where the physicochemical properties of the membrane,
Fouling of RO and UF Membranes 2289
the type of cells, the quality of the feed water, the type of solute molecules,
and the operating conditions all play a role. The end result of most membrane
processes is a fouled surface that the operator will not be able to clean to its
original state. To reduce the tendency to irreversible fouling, it is essential
to operate the plant/unit below the critical flux. This must go hand in hand with reliable feed water pretreatment schemes.
What areas need further research? Studies are required on effective
removal of biofilms without damaging the membrane. Additional work needs
to be done to find out what happens to the fouling resistance of chemically
modified membranes over the long term (i.e., after initial biofilm formation).
Membrane resistance to humic acids is another area for further study. It is
also noteworthy that the molecular tools needed for exploring the biochemical
details of the microbial adhesion process to membranes are now available.
In closing, consider for a moment the entire water resources issue on a
global scale. Various aspects of the water problem need to be considered
not only by developing nations but also by developed countries. Water is
required for urban development, industrialization, and agriculture. An increase
in the world population results in an increase in water usage. We can stipulate
that in the future serious conflicts will arise not because of a lack of oil but
because of water shortages. A three-pronged approach, therefore, needs to
be taken by society; water needs to be effectively managed, it needs to be econ-
omically purified, and it needs to be conserved. As scientists and engineers
continue to improve the technical and economic efficiency of membrane
desalination systems, it is imperative that we do not lose sight of the bigger
water resources picture. It is a challenge that we should be well able to meet.
NOMENCLATURE
A Rate of loss of membrane surface area as function of time
(m 2/sec)
AFM Atomic force microscopy
ATR Attenuated total reflection
cb Bulk solute concentration (mole/cm 3 )
Cg Gel concentration (kmol/m 3 )
Co Solute concentration in feed solution (kmol/m 3 )
cp Permeate solute concentration (mole/cm 3 )
Cw Concentration at membrane surface (mole/cm 3 )
D Solute diffusion coefficient (m 2/sec)
FTIR Fourier transform infrared
h Transversal dimension of channel (m)
i Cycle number
Goosen et al.2290
J Solvent flux across membrane (m 3/m2 sec)
J� Flux at infinite time (m 3/m2 sec)
Ja Average flux under steady-state conditions (m 3/m2 sec)
Jai Solvent flux at time a and in cycle i (m 3/m2 sec)
Jcrit Limiting or critical flux (m 3/m2sec)
Jlim Limiting or critical flux (m 3/m2 sec)
Jo Solvent flux at beginning of cycle (m 3/m2 sec)
Js Solute flux (mole/cm 2 sec)
J(tp) Solvent flux as function of permeation time (m 3/m2 sec)
Jv Permeate flux (mole/cm 2 sec)
K Hydraulic permeability of membrane to pure solvent (m 3/N sec)
k Mass transfer coefficient
kCw Adsorbed layer resistance
L Channel length (m)
m Channel parameter
DP Hydraulic pressure difference across membrane (cm/sec) P Transmembrane pressure (N/m2) Pe Peclet number
RO Reverse osmosis
Rm Membrane resistance
R Gas constant (J/kmol K) Sc Schmidt number
T Temperature (K)
tp Permeation time (hr)
tc Cleaning time (hr)
UF Ultrafiltration
UTDR Ultrasonic time-domain reflectometry
Uo Longitudal component of fluid velocity averaged over channel
cross section (m/sec)
Greek Symbols
D(w) Osmotic pressure at membrane surface (cm/sec) m Fluid viscosity
t Membrane lifetime (y)
ACKNOWLEDGMENTS
We gratefully acknowledge the financial assistance of the Middle East
Desalination Research Center (MEDRC), and Sultan Qaboos University
through grant number IG/AGR/BIOR/02/04 to M. F. A. Goosen.
Fouling of RO and UF Membranes 2291
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