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Journal of Membrane Science 303 (2007) 428

Review article

Fouling strategies and the cleaning system of NF membranesand factors affecting cleaning efficiency

Ahmed Al-Amoudi a,b, Robert W. Lovitt a,

a Centre for Complex Fluids Processing, Multidisciplinary Nanotechnology Centre, School of Engineering,

University of Wales, Swansea SA2 8PP, UKb Saline Water Conversion Corporation (SWCC), Saline Water Desalination Research Institute, Saudi Arabia

Received 17 January 2007; received in revised form 25 May 2007; accepted 6 June 2007

Available online 14 June 2007

Abstract

Nanofiltration membranes play an important role in the desalination of brackish and seawater as well as membrane mediated waste water

reclamation and other industrial separations. Fouling of nanofiltration (NF) membranes is typically caused by inorganic and organic materials

present in water that adhere to the surface and pores of the membrane and results in deterioration of performance (reduced membrane flux) with a

consequent increase in costs of energy and membrane replacement.

Natural organic matter (NOM) fouling of NF membranes involves interrelationship between physical and chemical interactions and is described

in this review. Inorganic fouling due to scale formation of sparingly soluble inorganic salts occurs whenever the ionic salt concentration stream

exceeds the equilibrium solubility. Scale formation takes place by homogenous or heterogeneous crystallization mechanisms. Biofilm formation

also becomes an issue when its thickness and surface coverage reduces permeability.

There are two strategies that are usually employed to minimize the effect of fouling. The first group includes minimizing of fouling by using

adequate feed pretreatment, membrane treatment and membrane modification. The second group involves membrane remediation by chemical

cleaning which is carried out to restore membrane fluxes.

A large number of chemical cleaning agents are commercially available, and the commonly used ones fall into six categories: alkalis, acids,

metal chelating agents, surfactants, oxidation agents and enzymes. In general, these cleaning agents do improve the membrane flux to certainextent. Combination of these chemical agents has also been tried in order to improve the flux restoration. Even though, many of these cleaning

agents can restore the flux over 100% (enhanced flux), they can also impair the selectivity of the membrane reducing of the product water quality.

There are many traditional assessment methods for cleaning and at present these are being supplemented by methods using modern surface

analysis techniques. These are being now rapidly developed to give a more precise assessment and a better understanding of cleaning processes.

Generally, cleaning is assessed by flux, zeta potential measurement, atomic force microscope (AFM) and Fourier transforms infrared technique

(FTIR). Atomic force microscope and related techniques are particularly employed in order to evaluate the cleaning efficiency and other surface

phenomena.

There are several factors that can affect the chemical cleaning process which include temperature, pH, concentration of the cleaning chemicals,

contact time between the chemical solution and the membrane and the operation conditions such as cross-flow velocity and pressure. The role of

temperature and pH in cleaning are membrane dependent. These factors play very important role in flux recovery. A critical review of these factors

is also presented.

It appears from the literature that only very few papers on cleaning of NF membrane to regenerate membrane performance have been published

up to date, and there is an urgent need for extensive research work to investigate fouling mechanisms in order to obtain fundamental understanding

of fouling to provide more feasible, cost-effective cleaning and performance restoration procedures. This also provides further strategies for the

avoidance of fouling through better pretreatment and more appropriate membrane fabrication and modification.

2007 Elsevier B.V. All rights reserved.

Keywords: Cleaning agents; Nanofiltration membrane; Cleaning efficiency; Fouling

Corresponding author. Tel.: +44 1792 295709.

E-mail addresses: [email protected],[email protected]

(A. Al-Amoudi), [email protected](R.W. Lovitt).

0376-7388/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.memsci.2007.06.002
mailto:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_9/dx.doi.org/10.1016/j.memsci.2007.06.002http://localhost/var/www/apps/conversion/tmp/scratch_9/dx.doi.org/10.1016/j.memsci.2007.06.002mailto:[email protected]:[email protected]:[email protected]


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A. Al-Amoudi, R.W. Lovitt / Journal of Membrane Science 303 (2007) 428 5

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2. Fundamentals of NF separation and selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3. Fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1.1. Inorganic fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1.2. Organic fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.3. Biofouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2. Operational aspect of NF and fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3. Primary location for specific types of fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.4. Fouling minimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4.1. Coagulation followed by filtrationsedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4.2. Scale inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4.3. Membrane prefiltration and membrane modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4.4. Sonication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4. Membrane cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1. Remediation of the membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.2. General considerations and costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3. Assessment of cleaning agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.3.1. Type of cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.3.2. Cleaning mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.3.3. The impact of cleaning on NF permeate quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.4. Methods of assessing the cleaning effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.4.1. Flux measurement (non-destructive) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.4.2. Atomic force microscopy (AFM) (destructive method) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.4.3. Fourier transform infrared technique (FTIR) (destructive method). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.4.4. Zeta potential measurements (non-destructive) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.5. Factors affecting chemical cleaning efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.5.1. Effect of cleaning solution pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.5.2. Effect of ionic strength of the cleaning solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.5.3. Effect of cleaning solution concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.5.4. Effect of cross-flow velocity (hydrodynamic shear) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.5.5. Cleaning duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.5.6. Cleaning frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.5.7. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.5.8. Effect of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5. Discussion and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1. Introduction

Water scarcity is a major political and economic problem in

the many parts of the world especially in the arid regions such as

the Middle East, Southern Europe, North and mid Africa, Aus-

tralia and many states of America such as California, Florida,New Mexico, etc. The shortage in natural fresh water supply

for domestic purposes is most acute for the Arabian Penin-

sula countries Saudi Arabia, Kuwait, Bahrain, Qatar, United

Arab Emirates, Oman and Yemen where demand for water

increases annually at a rate of 3% or more [1]. In addition,

the rapid further reduction of subterranean aquifers, and the

increasing salinity of these non-renewable sources will con-

tinue to exacerbate the water shortage problems in many areas

of the world. Desalination techniques are capable of provid-

ing the solution[2].Desalination or Desalinization refers

to water treatment processes that remove salts from saline

water. Desalination has already become an acceptable solu-

tion for shortages in conventional water resources and has

acknowledged as sustainable and effective process by rep-

utable institutions such as the World Bank [3]. This can be

achieved either by thermal processes involving evaporation

or by membrane filtration involving separation of ions from

water.As water demand increases, environment and safety regula-

tions are becoming more stringent, greater research efforts have

been put into the improvement of membrane processes. During

the past decade a variety of water treatment membranes has been

developed[4].These membranes have been vastly improved in

the area of water flux, salt rejection, and especially in their abil-

ity to maintain high performance levels at substantially lower

operating pressures than their predecessors [5]. Despite these

improvements,a decline in membrane performance over a period

resultingfrom membrane fouling that leads to a decrease in water

flux across the membrane and increased salt passage through the

membrane[6,7].


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6 A. Al-Amoudi, R.W. Lovitt / Journal of Membrane Science 303 (2007) 428

The membrane can be considered the heart of a desalina-

tion plant where the cost of membrane unit is about 2025%

of the total capital cost[8].Consequently, it is very important

to be familiar with factors involved in reduction of membrane

performance and longevity, in particular membrane fouling.

The factors affecting NF separation that can also play an

important role in membrane fouling and cleaning are as follows:

Membrane properties such as surface roughness pore size dis-

tribution, membrane thickness, membrane charge type and

charge density.

The chemistry of the treated solution such as solute compo-

sition, the size, geometry and the charge of the components,

the concentration of ions, the pH and the fouling potential of

the solution and its interaction with membranes.

The operation design of the NF systems, theircapacity,

dimensions and flow.

The processes environmenttemperature and pressure.

This review focuses on the type of foulants that leads toflux reduction of fresh water obtained from desalination plant:

explanations of desalination processed and their mechanism

of fouling are also reviewed to give clear understanding of

the cleaning processes. The main objective of this review is

to address the key factors in maintaining and restoring the

plant performance and the factors that affect the cleaning

efficiency.

2. Fundamentals of NF separation and selectivity

Nanofiltration (NF) membranes are mainly utilized for

softening brackish waters. The separation characteristics ofnanofiltration (NF) stand between ultrafiltration (UF) and

reverse osmosis (RO) and the membrane selectivity has often

been attributed to the interchange of both molecular siev-

ing mechanisms characteristic of ultrafiltration and diffusion

mechanisms characteristics of RO. NF membranes are usu-

ally made of polyamide based Thin Film Composites (TFC),

which are relatively close to RO membranes in chemical

structure. However, a key distinguishing feature of RO mem-

branes is their higher rejection of both monovalent and

divalent ions, the NF membranes are typically character-

ized by lower rejection of monovalent ions, but maintaining

higher rejection of divalent ions and higher flux than that

of RO membranes. In general NF membranes have relativelyhigh charge and also pores in the order of about 1 nm [9].

Consequently both, charge effects and sieving mechanisms

influence the rejection behavior of solutes in NF membranes

[10].

Generally, the basic chemical structure of the synthetic poly-

mers used in the preparation of RO, UF and NF membranes are

almost same apart from the pore size of the membranes. There-

fore overlap of properties of RO and UF with NF in terms of

both transport phenomena and consequent fouling is common

in the area of water treatment. Consequently the cleaning pro-

cesses for the RO and UF are also similar for NF membrane.

Hence, this literature review sometimes considers UF and RO

membranes and their fouling or/and cleaning system in order to

elaborate the points where lack of information on NF fouling

and cleaning exists.

The forces of the interaction between the membrane sur-

face and particles in solution are important in understanding

the fouling phenomena. The normal basis for quantify-

ing particlesurface interaction is DLVO theory where the

particlesurface interactions in aqueous environments could be

predicted by the summation of van der Waals and electrostatic

double layer forces. The Fig. 1 is a schematic description of

the DLVO interaction profiles and the summation of these two

forces. There are several important features about this diagram:

Unlikethe doublelayer interaction,the van der Waals interaction

potential is largely insensitive to variations in pH and electrolyte

concentration.

Reducing the interaction between the particles and the mem-

brane as much possible can reduce the fouling phenomena. This

can be achieved when the critical values (flux and pressure)

arise as a balance between the hydrodynamic force driving

solute towards the pore and the electrostatic forces opposingthis motion. Critical flux stems from the concept that the higher

the flux the stronger is the drag force towards the membrane,

the stronger concentration polarization and the higher the com-

paction of particles. Critical flux is defined as the limiting flux

value below which a flux decline over time does not occur [11].

A number of parameters influenced this critical flux have been

discussed in detail and can be found elsewhere[12].It is main-

tained that if one operates below the critical flux the fouling can

be avoided or minimized.

Fouling is common to all types of membrane separation. The

type of fouling various from Microfiltration (MF) membrane

processes where hydrodynamic force can predominate to ROmembrane processes where hydrodynamic forces have minor

effects compared to the forces associated with particles and their

interaction with the membrane surface.

3. Fouling

3.1. Background

To devise effective cleaning strategies a thorough understand-

ing of membrane fouling and it causes is required and first part

of the review is therefore dedicated to the nature of fouling and

fouling processes membrane fouling is an extremely complex

phenomenon that has not been defined precisely. In general theterm is used to describe the undesirable formation of deposits

on membrane surfaces. This occurs when rejected particles are

not transported from the surface of the membrane back to the

bulk stream.

The foulants are typically colloidal materials of one sort or

another and these properties and interaction with the membrane

dominate fouling/cleaning processes. Colloids are defined as

fine suspended particles in the size range of a few nanometres to

a few micrometers. Examples of common colloids sized foulant

include inorganic (clays, silica salt, and metal oxides), organic

(aggregated natural and synthetic organic), biological (bacteria

and other micro-organism)[7,1318].Champlin[19]reported


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Fig. 1. Schematic energy versus distance profiles of DLVO interaction profiles (a) Surfaces repel strongly; small colloidal particles remain stable. (b) Surface

come into stable equilibrium at secondary minimum if it is deep enough; colloids remain kinetically stable. (c) Surfaces come into secondary minimum; colloids

coagulate slowly. (d) The critical coagulation concentration. Surfaces may remain in secondary minimum or adhere; colloids coagulate rapidly. (e) Surfaces and

colloids coalesce rapidly.[10].

that removing of the particles size of down to 1 m may not be

sufficient to avoid fouling in many cases. Not only do MF and

UF process sometimes fail to remove all colloids below a few

hundred nm in diameter but also conventional processes used to

pre-treat NF feed water fail to remove sub-micron colloids[10].

The high concentration of the rejected ions in the membrane

surface could encourage aggregation of dissolved matter in to

colloidal sized particles. More to the point, the influenced of

salt retention and concentration polarization in the vicinity of

the membrane surface screens electrostatic particlemembrane

and particleparticle interactions allowing colloids to foul themembrane.

The sites fouling of membrane can be divided into exter-

nal surface fouling (build-up of a cake/gel-like layer on the

upstream face of a membrane) and pore blocking fouling [20].

In a dead-end filtration system, the latter is divided into three

types:completepore blocking (blocking a pore by a particle with

approximately thesame as thepore size),incomplete pore block-

ing (intermediate fouling) and standard pore blocking (gradual

pore narrowing and constriction by particle that is much smaller

than the pore size) [21].The flux decline of NF membrane is

mainly attributed to the pore blocking and it is observed in

dead-end and the cross-flow filtration systems.

Vrouwenvelder and Kooij[22]showed that diagnosis of the

type/cause of fouling is an essential first step aiming at control-

ling fouling. Autopsy gives conclusive information and further

understanding about the types and extent of fouling in the mem-

brane filtration plant and provides specific ways for reduction

and control of fouling. The tools which have been developed

for diagnosis, prediction, reduction, and control of fouling have

proven theirvalue in controllingfouling in practice.An overview

of the tools is shown inTable 1[23].

A setof coherent tools hasbeendeveloped for(i) Determining

the fouling potential of the feed water. (ii) Analyzing the foulingof NF and RO membranes. The tools presented can be used to

(a) assess the cause of fouling, (b) further define criteria for

feed water to predict and minimize the risk of fouling and (c)

evaluate cleaning strategies. Appropriate use of these tools can

provide strategies for cleaning to reduce the operational costs of

membrane plants (Table 1)[23].

When fouling takes place on themembranesurfaces itscauses

flux decline leading to an increase in production cost due to

increased energy demand, chemical cleaning, reduction in mem-

brane life expectancy and additional labor for maintenance.

The types of NF Fouling can be classified on the basis of

fouling material into three types[2225]:


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Table1

overviewoftoolsavailablefordeterminingthe

foulingpotentialoffeedwaterandfoulingdiagnosisofNFandormembranesusedinwatertreatmentadopted[90]

Tools

Integrateddiagnosis(autopsy)

Biofilmmonitorofbiofou

ling

formationrate

Biofoulingformationrate

MFI-UF

ScaleGu

ard

Foulingdiagnosis

Biofouling,

inorganic,co

mpoundsand

particles

Biofouling

Biofouling

Particulate

Scaling

Method

SEM,

EDX,

XRD,

FTIR,

AFM

XPS,

Zetapotential,

Contactangle,

NMRand

Chemicalanalysisofthe

foulantby

TOC,

ICP,H

PLCetc.

Assimilableorganiccarbo

n(AOC),

cylinder(glass)surface

SpecificOxygenConsumption

Rate(SOCR)

Deadendequipment

Continuouson-linemonitor-

Brinesta

gemodulewithasingle

spiralwoundmembraneelement

Comment

Comparisonoffouledan

dunfouled

systemisolatethediffere

ncesboth

chemicalandphysicalpr

opertiesofthe

membrane.Thisconfirmedwithwater

treatmentpropertiesprov

idelikely

causesoffouling

Predictiveandprevention

of

Biofoulingbydeterminingthe

(growth)potentialofwate

r

Non-destructivemethodfor

determiningactivebiomassin

membranesystems

Particulatefouling

potentialofwater

Optimizingrecovery,aciddose

andanti-scalantdose

1. inorganic fouling due to deposition on membrane surface of

inorganic scales (mainly BaSO4, CaSO4CaCO3),

2. organic fouling due to natural organic material (NOM) found

in the process stream (humic acids, protein and carbohy-

drate), and

3. biofouling due to microbial attachment to membrane sur-

face followed thereafter by their growth and multiplication

in presence of adequate supply of nutrients in the pretreated

feed or nutrients that deposited on membrane surfaces.

3.1.1. Inorganic foulingScale formation at the membrane surface is serious problem

and resulting from the increased concentration of one or more

species beyond their solubility limits and their ultimate precip-

itation onto the membranes[26]. In order to avoid scaling, it

is very important to operate NF systems at conditions lower

than the critical solubility limits, unless the water chemistry and

physical conditions are adjusted to prevent the type of precipita-

tion. Currently, due to the complexity of the problem, there is no

reliable way to predict the limiting concentration level at whichthere is no a risk of scale formation with a given membrane sys-

tem and treated water. Similarly, specific antiscalant treatments

are hard to define with confidence[27].Schafer et al.[10],have

reported that scaling (scale formation) or precipitation fouling,

occurs in a membrane process whenever the ionic product of

a sparingly soluble salt in the concentration stream exceeds its

equilibrium solubility product.

The term membrane scaling is commonly used when the

precipitate formed is a hard scale on the surface of the mem-

brane. Scaling usually refers to the formation of deposits of

inverse-solubility salts such as CaCO3, CaSO4xH2O, silica,

and calcium phosphate. Inorganic scale formation can even leadto physical damage of the NF membrane, and it is difficult to

restore NF membrane performance dueto thedifficultiesof scale

removal and irreversible membrane pore plugging [10]. The

greatest scaling potential species in NF membrane are CaCO 3,

CaSO42H2O andsilica, while theotherpotential scaling species

are BaSO4, SrSO4, Ca(PO4)2, ferric and aluminium hydroxides

[26,28]. Calcium sulphate precipitates in six different phases,

dihydrate (so-called gypsum), two hemihydrates and three anhy-

drites, although at ambient temperatures (about 20 C), gypsum

is the most common. The other phases are the product of gyp-

sum dehydration at relatively higher temperature, whereas the

calcium carbonate precipitates in three phases: calcite, valerite

and aragonite.The most common crystal of calcium carbonate is calcite.

It is widely accepted that the crystallization (precipitation) of

salts that takes place on the NF membrane surface requires at

least two stages, a nucleation stage and a crystal growth from

supersaturated solution. It is important to be familiar with the

mechanism of scale formation in order to avoid flux reduction

through membrane. Gilron and Hasson[27,2931]considered

that the flux decline was due to the blockage of the membrane

surface by lateral growth of the deposits on the membrane (het-

erogeneouscrystallization(two phase)) whereas Pervov [2931]

reported that the flux decline was due to the crystal formation

that took place in thebulksolutionfollowedby crystal deposition


6/25


Fig. 2. Scale formation mechanisms in NF membrane (a) show homogenous precipitation in the liquid phase while (b) show heterogeneous precipitation between

liquid phase and solid surface phase and the factor effect the crystallizations [28].

on the surface of the membrane (homogenous crystallization).

Clearly, this process will be a mixture of these two extremes and

will be affected by membrane morphology and process condi-

tions.Fig. 2represents homogeneous and heterogeneous modes

of crystallization[30].Her et al.[8,32],reported that if the sur-

face of the solid substrate matches well with the crystal and

the interfacial energy between the two solids is smaller than

the interfacial energy between the crystal and the solution, then

nucleation may take place at a lower saturation ratio on a solidsubstrate surface (heterogeneous crystallization) rather than in

the solution (homogenous crystallization) [8,32]. When the bulk

phase becomes supersaturated due to the increasing of concen-

tration polarisation layer, it is possible that both mechanisms of

crystallization simultaneously occur in NF system[30].

Aluminium oxide, inorganic salts, clays, sand and biologi-

cal surfaces can also act as suitable substrates for crystallization

[33]. Dydo et al. [8] have reported that most researchers indicate

that the gypsum scale precipitates as a bulk phase precipitation

process (homogenous crystallization) rather than on membrane

surface (heterogeneous crystallization). Lee et al. [30] has

also demonstrated that the homogeneous crystallization in the

retentate is a more important mechanism than heterogeneous

crystallization of the membrane fouling and flux decline [30].

Hasson et al.[27]have reported that the effect of CaSO4scaling

in RO and NF membranes on the flux decline was a function of

the super-saturation level on membrane surface and in the bulk.

Various physical and chemical parameters that affect the

crystallization process within a membrane system and include

temperature [34,35], pH [36], flow velocity, permeation rate

[37],types of pretreatment[38],salt concentration and concen-tration polarization[30,3942],membrane type, materials[37]

and metal ions[43].In addition to these parameters, NOM has

also been considered to affect various forms of scaling [44].

These factors also have been summarized as to whether they

increase or decrease the scaling, in Table 2and more informa-

tion can be found elsewhere [45]. It can be drawn from this

table that there are several factors either alone or combined with

each other play an important role in crystallization and subse-

quent cake formation. However, this is further complicated by

the nature of mixed solutions of how these alter substantially

the solubility product, strength and morphology of precipitates

from those in pure state. When the structure of the precipitate is


7/25


Table 2

scaling factors[38]

Value Crystallization Cause

Ionic strength High Increased Solubility and supersaturating

CP High Increased Solubility and supersaturating

Co-precipitation Presence Increased Changing structure of the precipitate

pH Higher Increased Solubility decreased

Pressure Higher Increased Increasing CP and Osmotic pressure at membrane surface(solubility and supersaturating)

Velocity (flow rate) Higher Decreased Higher wall shear rate

Temperature Higher Increased Solubility decreased

Surface morphology Higher Increased Valley blocking

altered so are the intramolecular forces holding the precipitate

together[40].

3.1.2. Organic fouling

In general, NF membrane are used in water treatment as

alternative processes for the removal of natural organic mat-

ter NOM that cause contamination, taints and color and arevehicles for other materials that bind to these substances [46].

Organic fouling could cause either reversible or irreversible

flux decline. The reversible flux decline, due to NOM fouling,

can be restored partially or fully by chemical cleaning [24].

Whereas the irreversible flux decline can not be restored at

all even by rigorous chemical cleaning is applied to remove

NOM [47]. Membrane fouling in the presence of NOM can

be influenced by: membrane characteristics[25,4852],includ-

ing surface structure as well as surface chemical properties,

chemistry of feed solution including ionic strength[51,53],pH

[48,50,51,5458]; the concentration of monovalent ions and

divalentions [50,51,54,59,60]; the propertiesof NOM,includingmolecular weight and polarity [25,49,52,61,62]; the hydrody-

namics and the operating conditions at the membrane surface

including permeate flux [25,51,6365], pressure [47,50,66],

concentration polarization[50], and the mass transfer proper-

ties of the fluid boundary layer. These factors either increased

or decrease the fouling rate have been summarized inTable 3

and more information can be found elsewhere[45].As it can be

seen theTable 3that the chemical (Ionic strength, NOM frac-

tion, etc.) and physical parameters, such as pressure, velocity,

and permeate flux, play a major role in NOM fouling at NF

membrane surface.

Humic substances in aquatic environments are considered

to be the major fraction of NOM, are refractory anionic

macromolecules of low to moderate molecular weight. Humic

substance contains both aromatic as well as aliphatic compo-

nents with primarily carboxylic (carboxylic functional groupsaccount for 6090% of all functional groups) and phenolic

functional groups [67]. As a result, humic substances gen-

erally are negatively charged in the pH range of natural

waters [18]. Nilson and DiGiano found that only the large

molecular weight fraction of NOM contributed to the layer

formation. In addition, while studying the effect of NOM prop-

erties on fouling of NF membranes, they fractionated NOM

into hydrophilic and hydrophobic components. They found

that the hydrophobic fraction was the major factor causing

permeate flux decline while the hydrophilic fraction had rel-

atively small effect [68]. The hydrophobic fraction of NOM

tends to adsorb more than hydrophilic fraction of NOM to themembrane surface. The hydrophobicity of the NOM increases

with increasing molecular weight [18,68]. Jucker and Clark

have also observed the same trend [69]. The fouling effect of

divalent ions on high molecular weight of NOM was more pro-

nounced than with low molecular weight of NOM. Braeken

[70] et al. have reported that hydrophobicity and molecular

size play an important role in retention of dissolved organic

compounds. Hydrophobicity is the most important parame-

ter determining the retention of molecules with a molecular

Table 3

Natural organic matter fouling factors[38]

Value NOM fouling Rate Cause

Ionic strength concentration Increased Increased Electrostatic repulsion

pH High pH Increased Hydrophobic forces

Low pH Increased Electrostatic repulsion

Divalent cations Presence Increased Electrostatic repulsion and bridging between NOM and

membrane surface

NOM fraction Hydrophobic Increased Hydrophobicity

Hydrophilic Decreased

Molecule or membrane Charge High charge Increase Electrostatic repulsion

CP High Increased

Surface morphology Higher Increased Valley blocking

Permeate flux (High recovery) Higher Increased Hydrophobicity

Pressure Higher Increased Compaction


8/25


Fig. 3. Conceptual sketch of the swollen membrane matrix for different ionic

environments (a) thick electrical double layer at high pH and low ionic strength

and (b) thin electrical double layer at high ionic strength and low pH)[51].

weight below the molecular weight cut off (MWCO) of the

membrane, presumably binding on or in the membrane sur-

face.

The apparent membrane surface structured in the solution is

a function of pH and ionic strength. Fig. 3shows the potential

impact of high and low ionic strength on membrane structure. At

high ionic strength, the membrane pore size was found to exhibit

larger pore size compared at low ionic strength. The forces that

control secondary and tertiary structure of NOM are also altered

with increasing salt concentration, and results the slow restruc-

turing or transition of the NOM particles [57,71].For example,the NOM particles can stretch to more linear chains at low

concentrations, low ionic strengths and at neutral pH because

of higher intermolecular repulsion. Whereas a rigid, compact,

spherocolloidal macromolecule is found at high ionic strength,

low pH and high solution concentration when intramolecular

charge shielding acts to neutralize functional groups (Fig. 4)

[53].

The pH also has a major effect on the fouling behavior of

humic acids. Typically humic acid contains carboxylic acid

groups that lose their charge at acidic pH. At low pH (


9/25


Fig. 5. numerically calculated isopotential lines at the entrance to a membrane pore of diameter 0.1 I.tm. (a) In 101 M solution, (b) in104 M solution. The pore size

distribution obtained from AFM images at various concentrations. The line BC is the front of the membrane, CD is the internal pore wall, FE is the axis of symmetry

along the centre of the pore (due to symmetry, only a half section is shown), AB and AF are the natural boundaries in the solution and DE is a natural boundary in

the pore. The front surface and pore wall of the membrane have a normalised potential of 1.0.[60].

required to figure out the mechanisms of NOM fouling dur-

ing driven-pressure membrane applications. A systematic and

comprehensive study is still needed in order to identify key

parameters that could be used effectively for the prediction of

NOM fouling in order to maintain the flux. In addition to solute

characteristics, a comprehensive understanding of membrane

properties is also needed to predict solutemembrane interac-

tions and eventually NOM fouling. Clearly there is scope to

Fig. 6. Schematic description of the effect of solution chemistry on the conformation of NOM macromolecules in the solution and on the membrane surface and

the resulting effect on membrane permeate flux. The NOM fouling described in the diagram is applicable for permeation rates above the critical flux. The difference

between the two chemical conditions shown becomes less clear at very high permeate flux. At low permeate flux (below the critical flux), no significant fouling is

observed for both conditions[22].


10/25


improve processes and membrane by understanding the nature

NOMmembrane interaction.

3.1.3. Biofouling

Biofouling is a term used to describe all instances of foul-

ing where biologically active organisms are involved[74].This

is distinct from NOM fouling caused by contaminated organic

matter that may be derived from biological systems. Membrane

biofouling is caused by bacteria and to a lesser degree, fungi and

other eukaryote microorganisms[75].Biofouling is a dynamic

process of microbialcolonization andgrowth,which result in the

formation of microbial biofilms. Biofilm formation invariably

precedes biofouling,which becomes an issue onlywhenbiofilms

reach thickness and surface coverage that may cause problems

such as declined normalized flux and/or increase in normal-

ized pressure drops during NF or RO operation[22,76].Many

products from biofilms have been shown to enhance inorganic

precipitation through enhanced nucleation and crystallization

kinetics, e.g. carbonate and silicates. Biofouling can be con-

trolled by (1) removal of degradable components from the feedwater, (2), ensuring the relative purity of the chemicals dosed

and (3) performing effective cleaning procedures. Also, it has

been reported that cleaning procedures applied when fouling is

not a problem might delay biofilm formation[77].The surface

of the membrane offers good site for microbial colonization as

it concentrates nutrients for growth.

3.2. Operational aspect of NF and fouling

From the mechanisms of fouling process above, many oper-

ating procedures have a direct impact on fouling of membranes.

This section reviews the effect of membrane process design onfouling. In most cases flux rate is considered as a key design

parameters for membrane system and reflect membrane produc-

tivity. The two factors that lead to deterioration the flux rate

are fouling and concentration polarization. In order to over-

come these shortcomingsthe membrane array is to be introduced

[21,67,7881].An appropriate membrane array was considered

in designing membrane treatment system in order to reduce the

effects of both concentration polarization and to minimize the

membrane fouling. Typically, membrane systems use multiple

parallel modules so that the plant performance in terms of the

product quality and recovery remain identical for a single mod-

ule (Straight brine stages; typically a single module contains

six elements). In the tapered systems (Tappered brine stages;membrane array design 2:1, 3:2:1, 4:2:1), the feed stream is

passed through the first module (or parallel set of modules) and

is divided two streams. These streams are the product and the

reject stream, the reject stream from first module (or parallel set

of modules) is passed through as feed to the second module (or

set of modules). Here the velocities are boosted at each stage by

decreasing the number of modules in parallel. Thus it is possible

to obtain a high recovery while still avoiding the worst effects

of fouling and concentration polarization (Fig. 7).

The membrane arrangements are designed with the aim of

minimizing fouling and reducing concentration polarization by

increasing number of stages and reducing the number of ele-

Fig. 7. Membrane arraysstraight and tapered brine stage (a) straight brine

stage one pressure vessel contained six element in series), (b) tapered brine

staging in the ratio of 2:1 each presser vessel have six membrane elements),

(c) tapered brine staging in the ratio of 3:2:1 each presser vessel contained four

membrane elements) and (d) tapered brine staging in the ratio of 4:2:1 each

pressure vessel contained four elements).

ments per stage in order to maintain the same or high recovery

will involve the following constraints[10,82]:

1. The flow rate should not exceed the maximum flow rate per

element, qmax, to avoidlargeaxial pressure drops which could

cause membrane element damage such as telescoping.

2. There is a lower limit on the flow rate per element,qmin, in

order to control concentration polarization and scaling.

3. There isa maximum recovery for eachstage aswellas overall

maximum recovery in order to minimize the fouling.

3.3. Primary location for specific types of fouling

When reviewing the major causes of NF membrane fouling

and associated mechanisms, it is very important to understand

where the fouling takes place in membrane system in order to

arrange the module and optimize fluid handling (see section

above). Typically the fouling typically takes place either in the

lead element (first element in the pressure vessel) where parti-

cles became entrapped on the surface or in the end element (last

element in the same pressure vessel) where salts are highly con-

centrated). Usually organic and metal oxide fouling take place in

the first stage of lead element, metal oxide and colloids deposit

early in the process as drag forces are relatively high. However,

organic fouling usually occurs heavily in the feed side of the


11/25


Table 4

where fouling occurs first, adapted from hydranautics technical service bulletin

tsp[82]

Type of foulant Most susceptible stage of NF/RO

Scaling/silica Last membrane in last stage

Metal oxides First membranes of first stage

Colloids First membranes of first stage

Organic First membranes of first stageBiofouling (rapid) First membranes of first stage

Biofouling (slow) Throughout the whole installation

module. While biofouling can be found throughout the filtration

stages, however, rapid biofouling was found mostly in the feed

side as a result of particle and nutrient attachment [83].

In general, scaling and silica fouling take place in the brine

side membrane elements when the concentration of inorganic

salts exceeds the solubility limit. The types of the foulants and

where they usually cause fouling in typical NF/RO systems are

summarized inTable 4[83].

There are two strategies in order to minimize the effect of

fouling, and these can be classified into two major groups min-

imization and remediation. Both of the strategies are practiced

in membrane process industries.

3.4. Fouling minimization

It is possible to avoid or control fouling to certain extent by

using adequate pretreatment such as coagulation/precipitation,

or slow sand filtration and membrane surface modification.

3.4.1. Coagulation followed by filtrationsedimentationConventional coagulation filtration pretreatment was

designed to remove most of the potential foulant materials

from the pretreated feed by prefiltration and more rarely by

sedimentation. The degree of the pretreatment, however, is

dependent on the raw water quality, particularly its content of

organic (including biological) and inorganic suspended matter.

Coagulant and coagulant aids can be added in a pretreatment to

increase separation efficiency. Several studies were have been

carried out on the pretreatment side in order to remove the

foulant materials by optimizing the operational conditions of

the pretreatment process such as flow rate, backwash frequency,

pH, etc. [84,85]. Howe and Clark [86], reported studies that

were focused on the effect of coagulation on the foulingby dissolved and particulate colloidal matter. Tests with and

without prefiltration were able to provide a comparison between

the effect of particulate versus dissolved and colloidal matter.

Usually, less than 20% of the fouling in their experiments

could be attributed to particulate matter. They concluded that

when the water was treated with coagulant, the fouling usually

decrease after prefiltration and suggested that the coagulated

particulate matter was able to form a dynamic layer material

on the membrane surface. Thus, the fine particles could remove

materials that would otherwise foul the membrane. When

the dynamic layer was eliminated by prefiltration the fouling

actually worsened [86]. Earlier work [87] on surface water

pretreatment reducing Ca2+ and Mg2+ to very low level was

achieved using a complex multi-stage process of coagulation

and flocculation using lime; mechanical bed filtration; weak

cation ion exchange and deep-cartridge filtration, were success-

ful in obtaining pretreated water with Ca2+ and Mg2+ under the

detection limit.

3.4.2. Scale inhibitors

Another approach is to avoid scale formation the addition

of scale inhibitors. It is obvious that scaling intensity depends

upon the chemical composition of feed water; therefore water

with a high scaling potential requires treatment using scale [30].

The chemical species, such as lime and soda or caustic soda are

added to hard water in order to remove or reduce the hardness

ions. Alkaline chemical additives are added to hard water to

raise the pH in order to convert bicarbonates to carbonates and

then calcium and magnesium are removed from water as CaCO3and Mg(OH)2prior to filtration. Zero hardness water can not be

achieved due to the limited solubility of CaCO3and Mg(OH)2.

3.4.3. Membrane prefiltration and membrane modification

The application of microfiltration (MF) as well as ultrafiltra-

tion (UF) as NF prefilters has emerged in the last decade as an

efficient method in pretreating surface water[88].Both UF as

well as MF membranes offer good physical barrier to colloids,

suspended particles as well as microbes. Both MF and UF mem-

brane canbe used ahead of desalination units andhavecapability

of filtering out particles in the ranges between 0.005 to 0.1

(UF) whereas 0.1 to 3 (MF)[63].

Attempts have been made to modify membrane surfaces in

order to make them less vulnerable to fouling. In some cases,

the surface roughness increases membrane fouling by increasingthe rate of attachment onto the membrane surface and hence the

membranes with a rough surface is more prone to fouling than

membrane with a smoother surfaces [50,51]. Colloidal inter-

actions are also important in fouling and charged components

tend to cause fouling because of electrostatic attractions between

charged components and the membrane (see above). Develop-

ment of membranes with lower surface charge or surface charge

similar to that of the foulant, with hydrophilic character may

help solve these specific problems[89].

3.4.4. Sonication

The effect of the particle concentration on the ultrasonic con-trol of themembranefouling was investigated by Chen et al.[90].

The basic principle of operation is that ultrasound removes par-

ticles from the surface by causing particle movement in or near

membrane. In this experimental work it was concluded that the

ultrasound reduced ceramic membrane fouling by silica parti-

cles during cross-flow filtration. At low particle concentrations,

there was a little membrane fouling in the presence of ultra-

sound. However, the permeate recovery of the ultrasound treated

membrane decreasedwithan increasedin particle concentration.

At low particle concentrations (lower than 0.8 g/L) the particle

concentration effect was more apparent when the membrane

was far away from the cavitation region. However, at higher


12/25


particle concentrations of greater than 0.8g/L the effect of par-

ticle concentration was more pronounced when the membrane

sonifacation power was close to the cavitation region.

Standard water treatments, carried out in order to solve

specific pretreatment problems, could lead to further fouling

problems. Walton[91],outlined some of these problems as fol-

lows:

The use of certain phosphate anti-scalants stimulates biolog-

ical activity in both the internal and external environments.

The use of organic biocides to control biological growth often

results in organic slimeformationand subsequent aftergrowth

activity and colloidal entrapment.

The use of flocculants to control particulate matter results in

colloidal iron or aluminium floc fouling, especially in associ-

ation with organic slimes.

The introduction of oxygen or oxidants into anaerobic sys-

tems results in iron and sulphur precipitation and potential

stimulation of iron hydroxide slime-production.

Addition of minerals acid anti-scalant can produce avail-able CO2 allowing biological growth, especially algae and

autotrophic bacteria.

The use of activated carbon for dechlorination and/or organic

removal results in an excellent substrate for bacterial growth

producing fouling byproduct; it can also absorb polyelec-

trolyte and organic antiscalants.

4. Membrane cleaning

4.1. Remediation of the membrane

Remediation is usually conducted by chemical cleaning fornearly all membrane processes and application. However, the

frequency of the chemical cleaning could range from a routine

daily process such as in whey processing to long term annual

processes such as in desalination plant according to occurrence

of fouling [89]. In general, much of thedecline in membrane per-

formance can be corrected by cleaning the membrane. Cleaning

can be defined as a process where material is relieved of a

substance, which is not an integral part of the material, [92].

Physical cleaning methods include for example: hydrodynamic

forward or reverse flushing, permeate back pressure, air spurge

and automatic sponge ball cleaning. These methods depend on

a mechanical treatment to dislodge and remove foulants from

the membrane surface. Application of these methods usuallyresults in a more complex control and design of equipment.

The physio-chemical cleaning methods use mechanical clean-

ing methods with the addition of chemical agents to enhance

cleaning effectiveness[93].

Adequate pretreatment and appropriate membrane selection

as mentioned above can slow the fouling rate, but the membrane

cleaning is an essential step in maintaining the performance of

the membrane process. The ideal cleaning processes should not

only be effective against several foulants, but gentle to the mem-

branes so as to maintain and restore their characteristics. The

optimal (the least membrane damage and maximal effective-

ness of cleaning) choice of the cleaning agent is a function of

membrane material as well as foulants. Fu et al. [94]noticed

that two NF membranes with different properties (TS 80 and

NT47450), fed by the same feed water, required different clean-

ing processes. The results of cleaning procedures are sometimes

very difficult to determine using only flux recovery data. It is of

interest to know in what way the cleaning agent interacts with

the membrane and whether it actually modifies the membrane

surface structure and chemistryin such a way that fouling is pre-

vented. It has been noticed that cleaning often increases the flux

of the virgin membrane[95].The chemical reactions between

the chemical agents and the foulant takes place either by chang-

ing the morphology of the foulant or by altering the surface

chemistry of fouling layer in order to remove the foulants from

the membrane surfaces[96].Kosutic and Kunst[97],concluded

that an irreversible change in the porous structure of NF mem-

brane was observed asa resultof thechemicalcleaning. Cleaning

maymake thepore surfaces more hydrophilic andcharged by the

adsorption of the chemical agent [98]. Chemical cleaning proce-

dures and commercial membrane cleaning products are almost

specified by membrane manufacturers[96,99].

4.2. General considerations and costs

In any membrane processes, the need for proper and periodi-

cal cleaning is essential regardless of the type feed be; seawater,

brackish water, wastewater or industrial water. The objective of

the cleaningprocessesis to restore membrane performancewhen

it falls below the expected permeate yield typically by about

10%, or feed pressure increase by about 10% and/or differential

pressure increase by 1550%[24].Membrane replacement is a

necessary part of the plant operation that is needed to main-

tain the quality of the product water to the protocol agreedwith membrane manufacturers as well as to meet the design

productivity when the cleaning processes fail to restore the

declined flux [100]. Usually about 10% of the membrane is

annually replaced in order to maintain the targeted product qual-

ity as well as quantity. It has been reported that the cost of the

membrane replacement is about 23% of product water cost

from Jeddah SWRO Plant at power cost $ 0.1 kWh1 (water

cost = 1.473 $/m3) [101]. Although, there are a number of clean-

ing techniques such as physical or chemical or combination of

both, only the chemical cleaning methods are widely used by

NF and RO industries for membrane cleaning and regeneration.

The complexity and detailed understanding of cleaning pro-

cesses has not yet been addressed by many researchers andis needed for a clear knowledge of these processes. Although

cleaning is intended to restore the flux, it often deteriorates prod-

uct quality and increases the cleaning frequency affecting plant

availability. For example, cleaning processes sometimes takes

12 days to complete in large plants [24]. Desalination plant

availability is usually designed to be in the range of 9097%

and varies according to the type of water being treated. How-

ever, this percentage can be reduced if the cleaning frequency

is increased, but the costs routine of plant maintenance, the

additional manpower utilization and energy consumed during

cleaning processes can increase the overall cost of water pro-

duction. In general, the chemical consumption of the plant per


13/25


year is about 0.31% of total water treatment cost, neverthe-

less, the chemical consumption of the cleaning process per year

is much higher than the annual chemical consumption for the

overall ROprocess (conditioning etc.,) [102]. These general cost

figures exclude the additional facilities, manpower and energy

consumed for cleaning.

It is well recognized that the energy cost of the plant is about

5060% from the total water cost[101103].Moch[102]stated

that power, itself, can be a half to three quarters of the opera-

tional and maintenance costs. In general the cleaning process

increases the overall system energy efficiency, regardless of the

energy consumed during the cleaning. For example by reducing

the net driving pressure will be reduced after cleaning by about

1030% which is quiet considerable energy saving, especially

during the plant operation[24].

4.3. Assessment of cleaning agents

4.3.1. Type of cleaning

A large number of chemical cleaningagents are commerciallyavailable, and commonly used ones fall into six categories: alka-

lis, acids, metal chelating agents, surfactants, oxidizing agents

and enzymes [104,105]. Commercial cleaning products are usu-

ally mixture of these chemicals but the actual composition is

often not clearly specified. Table 5 [106]shows the chemical

cleaning agentrecommended by various membrane manufactur-

ers. The table gives the details of chemicals and its concentration

to be used for different type foulants. The choice of the pre-

ferred cleaning product depends on feed characteristics. For

example, acid cleaning is suitable for the removal of precipi-

tated salts, such as CaCO3, while alkaline cleaning is used to

remove adsorbed organics[89].When surfactant is introducedon the membrane surface in order to restore the membrane flux,

surfactant adsorption is possible from hydrophobic interactions

between the hydrophobic portion of the membrane surface and

hydrophobic tails of the surfactant. In spite of increased electro-

static repulsion between the negatively charged membrane and

anionic surfactant, even at a low surfactant concentration may

adsorb on to a negative-charged surface due to an ion exchange

mechanism leading to a higher concentration of surfactant near

to membrane surface compared to bulk solution, which may

induce a micellization process at the membrane solution inter-

face[107]. On the other hand, cationic surfactant could lead

to a reduction in membrane permeability owing to membrane

modification with a cationic surfactant[108,92].As mentioned earlier, NF membranes are extremely vulner-

able to natural organic matter (NOM) fouling, especially in

the presence of divalent cations [109,50,51]. Characterization

of NOM-fouled membranes by contact angle, zeta potential,

and attenuated total reflection-fourier transform infrared (ATR-

FTIR) spectroscopy as well as molecular weight distribution

measurements demonstrated that colloidal material with hetero-

geneous characteristics with variable area of hydrophobicity and

charge membrane[104].These materials are typified by humic

acids, fulvic acids, proteins and peptides. Typically, NOM tend

to have higher hydrophobic fraction of about 75% compared

to about 20% of hydrophilic fraction in water such as Orange

county ground water. While Horsetooth reservoir surface water

(HT-SW) found to have high fraction of hydrophilic NOM of

about 65% compared to about 27% of hydrophobic NOM frac-

tion. Lee reported that a caustic solution was more effective than

citric acid for fouled membrane with the hydrophobic fraction

of NOM. On the other hand, chemical cleaning agents were

not able to clean fouled membrane by hydrophilic fraction of

NOM, because of lack of electrostatic repulsion between NOM

acids and the negatively charged membrane surface[104].This

was due to high ionic strength of the feed solution masking the

membrane surface charge.

In general, alkaline cleaning recovers the flux, while the

introduction of alkaline chelating agent further increases the

flux. Liikanen et al.[109]reported that alkaline chelatant such

as EDTA increased the flux more than plain alkaline cleaning

(NaOH) due to membrane charge increase in EDTA alkaline

environment, which makes the membrane more open. Liika-

nen et al. concluded that alkaline and chelating cleaning agents

increased membrane flux, but they reduced the ion retention,

whereas acidic cleaning could be used in order to recover mem-brane ion retention. In a recent study, Li et al.[110]noticed that

combined simultaneous process of NaOH with sodium dodecyl

sulfate (SDS) demonstrated greater cleaning power and cleaning

efficiency by about more than 100% compared to that of single

cleaning with eachof NaOHor SDS alone.Thisis alsotruewhen

two step method in which SDS cleaning step was performed

after caustic treatment[111].Jacques et al.[112]reported that

hydrochloric acid cleaning showed better results than citric acid

in removal of the iron deposition on the membrane surface[32].

Song[51,113]reported that sequential use of both caustic and

acid cleaning was more effective, in terms of high flux recov-

ery, than caustic or acid alone in removing both acidic and basicfractions of NOM. Also he reported that the caustic cleaning

was found to more effective than acid cleaning in removal of

the NOM foulants. This is a result of the presence of hydroxyl

ions in caustic solutions, which could promote disruption of the

foulant layer by these mechanisms: (i) increasing ionic strength,

(ii) increasing solubility of NOM foulants, (iii) increasing pH.

Increasing the pH should result in an increased negative charge

of NOM, because of deprotonation of the carboxyl and pheno-

lic groups. Conversely, decreasing negative charge of NOM has

been observed as a result of adsorption of sodium ions to NOM

during cleaning with[81,113].EDTA and SDS were also used

as effective cleaning agents in order to remove virtually or all of

the NOM foulant material[51,114].The acid cleaning is effec-tive in removal of precipitated salts (scaling) from the surface

of the membrane and from the pore[10].

The polyamide thin film membrane (TFM) is very sensitive

to disruption by the oxidising agent. Powerful oxidation agents

have not been used in order to regenerate membrane perfor-

mance because of oxidation agent typically causes irreversible

damage to these membranes. However, there is a procedure

based on a patented chemical cleaning using NaOCl where a

known concentration is prepared and recirculated through the

membrane cells for 20 min at pH > 10, while chlorine oxidation

effects were almost negligible[115].This cleaning procedure at

high pH was effective to remove the organic foulant materials


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Table 5

chemical cleaning agents recommend by different manufacturers

Type of foulant Type of membranes

DuPont B-10 FilmTec FT-30 Fluid System Nitto Denko Toyobo

CaCO3 HCl at pH 4, citric acid(2%w) pH 4 (NH4OH),Nutek-NT 600 (5%w),citric acid(2%w)+ Na2EDTA

(2%w), pH 4 (NH4OH)

HCl (0.2%w), phosphoric acid,H3PO4(0.5%w), citric acid(2%w), pH 4, sulfamic acid,NH2SO3H (0.2%w)

Citric acid (1%w), pH2.5

Citric acid (2%w) pH 4(NaOH)

CaSO4/BaSO4/SrSO4/CaF2

Citric acid (2%w) pH 8(NH4OH), EDTA(1.5%w), pH 78(NaOH/HCl), sodiumhydrosulfite, Na2S2O4(1%w)

HCl (0.2%w), phosphoric acid,H3PO4(0.5%w), Citric acid(2%w), pH 4, sulfamic acid,NH2SO3H (0.2%w)

Sodiumtripolyphosphate,STP(2%w)+Na4EDTA(0.85%w), pH 10 (H2SO4

SiO2 NaOH, pH 11, Biz(0.5%w), pH 11 (NaOH)

NaOH (0.1%w) + Na2EDTA(0.1%w), pH 12, max 30 C

- Citric acid (2(NH4OH)

Metal oxides Citric acid (2%w) pH 4(NH4OH), sodiumhydrosulfite, Na2S2O4(1%w), citric acid

(2%w) + EDTA (2%w)pH 4 (NH4OH), v

Phosphoric acid, H3PO4(0.5%w), sodium hydrosulfite,Na2S2O4(1%w), sulfamic acid,NH2SO3H (0.2%w)

Citric acid (1%w), pH2.5

Citric acid (2%w) pH 4(NaOH)

Citric acid (2(NH4OH)

Inorganic colloids HCl at pH 4, citric acid(2%w) pH 4 (NH4OH),NaOH, pH 11, Biz(0.5%w), pH 11 (NaOH),Drewperse 738 (1%w),SHMP (1%w)

NaOH (0.1%w) + sodiumdodecylsulfate Na-DSS(0.05%w), pH 12, max 30 C

- Sodiumtripolyphosphate,STP(2%w)+Na4EDTA(0.85%w), pH 10 (H2SO4

Citric acid (2(NH4OH)

Biological matter Formalin (0.252%w)followed by Biz(0.25%w)

NaOH (0.1%w) + Na2EDTA(0.1%w), pH 12, max 30 C,NaOH (0.1%w) + sodiumdodecylsulfate Na-DSS(0.05%w), pH 12, max 30 C,sodium tripolyphosphate, STP

(1%w) + trisodium phosphate,TSP (1%w)+ EDTA (1%w)

Sodiumtripolyphosphate, STP(1%w)+ trisodiumphosphate, TSP(1%w)+ EDTA(1%w) pH 1011

(HCl)

Sodium Tripolyphosphate,STP (2%w)+ Na4EDTA(0.85%w), pH 10 (H2SO4),Sodium Tripolyphosphate,STP (2%w) + sodium dodecylbenzene sulfonate (0.25%w),

pH 10 (H2SO4

15 ppm chl6.57.5, Form(0.52%w)

Organics NaOH, pH 11, Biz(0.5%w), pH 11 (NaOH),SHMP (1%w)

NaOH (0.1%w) + Na2EDTA(0.1%w), pH 12, max 30 C,NaOH (0.1%w) + sodiumdodecylsulfate Na-DSS(0.05%w), pH 12, max 30 C,Sodium tripolyphosphate, STP(1%w) + trisodium phosphate,TSP (1%w)+ EDTA (1%w)

Sodiumtripolyphosphate, STP(1%w)+ trisodiumphosphate, TSP(1%w)+ EDTA(1%w) pH 1011(HCl)

Sodiumtripolyphosphate,STP(2%w)+Na4EDTA(0.85%w), pH 10 (H2SO4),sodium tripolyphosphate,STP(2%w)+ sodium dodecylbenzene sulfonate (0.25%w),pH 10 (H2SO4)

15 ppm chl6.57.5, form(0.52%w)

Flow rate velocity as high as possible, pressure as lowest as possible, temperature does not exceed manufacturer recommendation (


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Fig. 8. Schematic illustration of the change in the organic fouling layer structure by EDTA (a) compact fouling layer formed in the presence of Ca 2+. (b) Loose

structure of the fouling layer after EDTA addition[117].

from membrane surface[78].Hydrogen peroxide also used as

oxidizing cleaning agent at high pH in order to clean the mem-

brane from NOM. A combination of both Cl2and H2O2at high

pH were noticed that had remarkable increased in the product

flux[78].

4.3.2. Cleaning mechanismsIt is reported that the presence of Ca2+ with humic acid

increased the fouling rate of humic acid on the membrane

surfaces. Li and Elimelech [116], reported that the proposed

mechanisms of chemical cleaning with EDTA in order to clean

membrane from NOM(humic acid). Since EDTA forms a strong

complex with Ca2+, humic acid molecules associated with Ca2+

ions are replaced by EDTA via a ligand exchange reaction.

EDTA cleaning agent does reduce the intermolecular Ca2+-

humic acid complexes and humic acid molecules can be more

easily rinsed off the membrane surface as illustrated inFig. 8.

The proposed mechanisms of SDS solubilization of Ca2+-

humic acid fouled surface at low, moderate and high

concentration are illustrated in Fig. 9. Low concentration ofSDSis notsufficient to break theintermolecular bridging formed

between humic acid and Ca2+. When moderate concentration of

SDS is used, more SDS moleculespartition into the foulant layer

results in breakup of some Ca2+ binding. Once the SDS concen-

tration exceeds thecritical mycella concentration CMC, it is then

strong enough to break up all the Ca2+-induced bridges, result-

ing in the dissociation of humic acid to the aqueous phase (as

indicated by the zero adhesion with SDS shown in Fig. 10). Chil-

dressand Elimelech[117], exploredthe mechanisms of chemical

cleaning with SDS and dodecyl trimethylammonium bromide

(DTAB, cationic surfactant) at high pH and low pH. Fig. 11

shows the differences in SDS adsorption at low pH and high pH.

At low pH of 3, the membrane initially has a slight positive and

adsorption occurs as a result of electrostatic attraction between

the positively charge membrane surface and the negative charged

polar head of surfactant ions. The surfactant ions start asso-

ciating with each other and form surfactant aggregates when

the concentration of SDS increases causing dramatic change in

the surface charge potential. At pH 8 the membrane has nega-tively charge and the adsorption will be a result of hydrophobic

interaction between membrane surface charge and surfactant

tail. When the SDS concentration increases, the membrane sur-

face becomes slightly more negative due to a larger number of

adsorbed surfactant molecules. Schematics of adsorption mech-

anisms of DTAB molecules onto membrane surface are shown

inFig. 12.As the concentration increases the membrane surface

charge become more positive due to hydrophobic interactions

at low pH. At high pH of 8, the adsorption occurs between the

membrane surface and charge polar head of the surfactant will

be due to electrostatic attraction. Hemi-micelle formation may

take place at the very high concentrations[117].

In allcasesthe cleaning process depends on thetype of foulantdeposited on the membrane surface, and for a successful clean-

ing of fouled membranes, identification of the type of foulant is

essential which is done by extensive analysis of the foulants. A

destructive autopsy, which can provide a scientific foundation

on which to optimize the cleaning procedure, is done as a last

resort, when cleaning fails to restore membrane performance.

4.3.3. The impact of cleaning on NF permeate quality

An impact of cleaning on NF permeate quality has also been

observed. According to Liikanen et al. who performed anal-

ysis for alkalinity, hardness and conductivity found that the

permeate conductivity generally increased after cleaning[118].


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Fig. 9. Mechanism of humic acid solubilization by SDS (a) low SDS concen-

tration allows association of humic acid (b) moderate SDS concentration allow

partial breaking,and (c) SDS concentrationexceedingthe CMCallowing solubi-

lization of humic acid.The binding sitsshows are solely for illustration purposes

[117].

Al-Amoudi et al. [24] also recognized that theincreasein perme-

ate conductivity after each chemical cleaning specifically after

high pH cleaning when it was carried out in commercial NFplant at UmmLujj. However, the acid cleaning following high

pH cleaning assisted in partial restoration of the ions reten-

tion property of membrane[24,118].This suggests that acidic

cleaning had a role in preserving the membrane ion retention

capability, probably by making the membranes tighter by charge

neutralisation.

4.4. Methods of assessing the cleaning effectiveness

There is several assessment methods of cleaning have been

usedand well established in orderto evaluate cleaning efficiency.

The most common methods are flux measurements, or forms

Fig.10. Interaction forces between theCML colloid probe andthe SRHAfouled

membrane surface in the presence of various chemical cleaning agents. The

test solution contained 20 mg/l SRHA, cleaning chemical as indicated 1 mM

NaHCO3, 1 mM CaCl2and NaCl to adjust the total ionic strength to 10 mM. the

solution pH during the measurements was fixed at 8.1[117].

Fig. 11. Schematic of adsorption of sodium dodecyl sulfate (SDS) molecules

into the membrane surface[118].

of surface analysis such as atomic force microscopy (surface

characterization by visualization and measurement of the sur-

face characteristics), FTIR and zeta potential. The three types

of measurement are complementary.

Fig. 12. Schematic of adsorption of sodium dodecyl trimethyl ammonium bro-

mide (DTAB) molecules into the membrane surface[118].


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4.4.1. Flux measurement (non-destructive)

Flux measurement is a directassessmentof fouling andclean-

ing process and can be made in the applied situation. There is

typically a linear relationship between the flux decline and the

deposited mass indicating that the flux decline is due to NF foul-

ing resulting from deposition heterogeneous crystallization[30].

Typically the product water flux declines drastically at higher

permeation rates in the presence of NOM refers and relates to

the transport processes driven by the hydrodynamics force that

acts perpendicular to the membrane surface[18,64].

It is important to establish effectiveness of a particular

cleaning protocol. The clean water flux can be measured and

compared to the flux of the original steady state process. The

water flux recovery (WFR) can be calculated[118]as:

WFR =Jc

J0(1)

whereJcis the flux after cleaning and J0is the flux of the virgin

membrane. The measurement of initial flux and the flux after

cleaning has to be carried out at the pressure and temperature.Several authors[119,4,17]have proposed the comparison of

the hydraulic resistance of the cleaned membrane, Rcw, and the

intrinsic hydraulic resistance of the membrane to evaluate the

cleaning efficiency. Permeate flux data was used to evaluate the

hydraulic resistance of the membrane (R), according to Darcys

law:

R =P

J(2)

and

Ruf=Rm +Rf=Rm + Rif+Rrf (3)

wherePis the transmembrane pressure; Jthe permeate flux;

and Ruf, Rm and Rf, respectively, Ruf is the total resistance

of the intrinsic hydraulic resistance of the membrane plus the

total resistance of the total fouling membrane (Rf), the intrinsic

hydraulic resistance of the membrane (Rm), thefRresis residual

resistance after cleaning, and the resistance due to membrane

fouling, which combines reversible (Rrf) and irreversible (Rif)

phenomena. The variation of membrane resistance is depicted in

Fig.13. Cleaning canbe assumed to be complete whenRcwRmallowing for experimental error (Fig. 13)[120].

Cleaning efficiency (ERW) can be determined as

ERW=R

if R

resRif

100 (4)

Both WFR andERWhave been used as a measure of clean-

ing efficiency. There is no difference between the above two

methods. However, the hydraulic resistances give a more details

to understandings fundamental to the flux. By knowing the

membrane hydraulic resistance and other fouling resistance, an

understanding of some fouling properties could be obtained.

The efficiency of membrane cleaning is mostly evaluated by

flux measurements[121].Song et al. reported that the chemical

cleaning agents tested could not achieve complete flux recov-

ery as a result of residual foulants were strongly embedded in

the concavities of membrane surface[113].However, Zhu et al.

Fig. 13. Graphical depiction of resistance in filtration, rinsing and cleaning

[121].

concluded that most of cleaning agents used improved the mem-

brane flux after fouling and some of them even restored the flux

up to about 95%. Recently, Al-Amoudi and Lovitt et al. [95]

from the results of the permeability of the fouled NF-DK mem-

brane before and after cleaning showed that the cleaning process

restored the declined flux close to its original value. Moreover,

it was also found that the SDS cleaning agents triple the perme-

ability of the virgin membrane. These results suggest that the

chemical cleaning does have a major effect on the flux of NF

membrane as well on its surface properties. It has been noticed

that cleaning often increases the flux and even the permeability

of the virgin membrane.

4.4.2. Atomic force microscopy (AFM) (destructive method)

There are now many new surface analysis techniques avail-

able for assessing membrane fouling and cleaning processesthat

based on the visualization of the surface of membrane down to

the nanometer scale[122]. These imagesallow the assessment of

surfaces and pore by direct measurement of surface morphology

in air and in liquid (process relevant) environment. Analysis of

these images can be carried out in a number of ways, the most

useful being various measurement and/or dimension of pores

[122124].Atomic force microscopy can be used as good tool

to evaluate chemical cleaning procedures performance. Using

AFM it was showed that there was an accumulation of the par-ticles in the valleys of rough membranes causing more severe

flux decline than smooth membranes [125]. Song et al. [113]

reported that significant difference between the surface mor-

phologies of the virgin and fouled membrane is recognized by

AFM. Here the root mean square (RMS) surface roughness of a

virgin membrane was about 48 nm whereas; RMS of the fouled

membrane was about 124 nm. This increase in surface rough-

ness was observed as a result of the presence of humic acid

with calcium on the negatively charged membrane surface. Also,

workers reported that, RMS of cleaned fouled membrane was

about 40 nm compared the virgin of about 48 nm. This decrease

in surface roughness was possibly due to the presence of resid-


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Fig. 14. Scanning electron microscope image of a silicon collide probe [128].

ual foulants within the concavitiesof the membrane surface after

cleaning. Warczok et al.[111]recently reported that it is possi-

ble to determine from AFM images the mean pore distribution

and roughness so indicating whether the cleaning procedure wascorrectly designed or not.

The AFM colloid probe which is powerful technique, has

been used to measure the force of interaction between colloid

particlesand thesurface of themembrane. UsingAFM it is possi-

bleto directly measure theforceof interaction in process relevant

environments where the cantilever tip of AFM is brought in a

contact with membrane surface.

The most commonly used AFM tips for force measurement

are the sharp silicon tips which provide high resolution when

measuring surface topography and force in a process relevant

environment, However, attaching a sphere to a tipless AFM

cantilever has been used to quantify the surface interactionsbetween a sphere and a flat surface as well as different material

(Fig. 14). These so called colloid probes give a known geometry

when approaching or leaving the surface. These probes typi-

cally are about 15m diameter whose surfaces can be treated

with many materials including foulants. With different foulants

as probe coatings in liquid medium of different salt solutions

or cleaning solution, this technique allows an assessment of

the foulant membrane interactions and the chemical cleaning

processes[126,127].

The study of the electrical double layer interaction between

a particle and membrane by AFM also allows assessment of

the propensity of the surface to fouling when in use. Force

measurement in conjunction with colloid probe technique alsoallow a direct quantification of membrane fouling through the

measurement adhesive force when the probe is retracted from

the surface after contact has been made[128].Adhesive force

measurements were performed utilizing carboxylate modified

latex (CML) Colloid probe in the presence of various chemical

cleaning agents in order to look into the effect of chemical clean-

ing on foulantfoulant interactions in the fouling layer. Fig. 10

shows that the adhesive forces were measured with and without

chemical cleaning agent addition. The eliminated adhesive force

was in the presence of EDTA and SDS gave rise to a complete

flux recovery. While the remaining adhesive force with NaOH

addition indicates a poor cleaning efficiency, although, the adhe-

sive force was reduced significantly by NaOH compared to that

without chemical cleaning addition.

4.4.3. Fourier transform infrared technique (FTIR)

(destructive method)

FTIR technique is used to investigate the membrane surface

properties and the cleaning efficiency. Her et al. used FTIR

techniques in combination with other techniques to study pre-

cipitation scaling attributed to inorganic scales such as CaCO3and CaSO4 [32]. Zhu and Nystrom [21]have used the FTIR

technique to characterize the chemical cleaning efficiency.

They concluded from the FTIR results that the fouling notably

changed the FTIR spectrum. New peaks appeared and foul-

ing obscured some of the peaks of the polysulfone membrane.

The results clearly showed that not all of the fouling had been

removed by cleaning. Song et al.[113]reported using FilmTec

NF-70 membrane that from the FTIR results of virgin, fouled

and cleaned membrane that the peak intensity of the virgin

were eliminated or severely attenuated due to coating by NOM

foulant, whereas the peaks intensity of caustic cleaned mem-brane tend to be slightly close to the peak intensity of the virgin

membrane.

The cleaning agent may not remove the NOM completely

from the membrane surface by using EDTA. This is supported

by FTIR spectra measurements for virgin membranes, fouled

membrane and cleaned membrane with and without preoxidiz-

ingwaterwherefouledmembrane, exposed to preoxidized water

and cleaned with caustic solution, had better peak recovery com-

pared to the fouled membrane exposed to raw water and cleaned

with caustic solution[113].

4.4.4. Ze

14 Fouling Strategies and the Cleaning

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