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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:

mgoosen@suagm.edu.

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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Fouling of RO and UF Membranes 2297

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