Appl Water Sci

DOI 10.1007/s13201-016-0493-1

REVIEW ARTICLE

Biofouling in reverse osmosis: phenomena, monitoring,

controlling and remediation
Hisham Maddah1,2 • Aman Chogle2

Received: 16 April 2016 / Accepted: 11 October 2016

Ó The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract This paper is a comprehensive review of bio- Introduction

fouling in reverse osmosis modules where we have dis-
cussed the mechanism of biofouling. Water crisis is an Worldwide demand for drinking water is increasing
issue of pandemic concern because of the steady rise in rapidly. The world’s population tripled in the twentieth
demand of drinking water. Overcoming biofouling is vital century and is expected to increase by another 40–50% by
since we need to optimize expenses and quality of 2050. Hence, improving the performance of water purifi-
potable water production. Various kinds of microorganisms cation technology is necessary to compensate for our fresh
responsible for biofouling have been identified to develop water demands (Kang and Cao 2012). Reverse osmosis
better understanding of their attacking behavior enabling us (RO) has become a critical technology in purification of
to encounter the problem. Both primitive and advanced non-traditional water sources such as brackish, sea, and
detection techniques have been studied for the monitoring wastewater and it is the most efficient technique for sea-
of biofilm development on reverse osmosis membranes. water desalination purposes (Matin et al. 2011). Around
Biofouling has a negative impact on membrane life as well 20% of the world’s population lacks safe drinking water. It
as permeate flux and quality. Thus, a mathematical model is expected that by 2025, 1.8 billion people will find dif-
has been presented for the calculation of normalized per- ficulties in getting clean water or will live in areas where
meate flux for evaluating the extent of biofouling. It is water is scarce. Consequently, ensuring high performance
concluded that biofouling can be controlled by the appli- of RO plants is important and this is possible by adjusting
cation of several physical and chemical remediation parameters like feed pressure, permeability, system tem-
techniques. perature, flow rates, feed salinity, and controlling biofoul-
ing issues. Selecting the accurate operating conditions will
Keywords Biofouling  Reverse osmosis  Mechanism  allow us to determine the necessary membrane area and
Control  Consequences  Disinfection  Surface therefore reaching the optimum values for permeate water
modification flux and salt rejection. For instance, applying a high pres-
sure (DP) that is larger than the osmotic pressure (Dp)
across the membrane, results in an increase in water flux
and salt rejection (Qureshi et al. 2013). The most com-
mercially available RO membrane is the asymmetric cel-
lulose type (cellulose acetate, triacetate, cellulose diacetate
or their blend) and thin-film composite (TFC) type. TFC
& Aman Chogle aromatic polyamide membrane exhibits superior water flux
chogle@usc.edu
and salt rejection (Kang and Cao 2012).
1
Department of Chemical Engineering, King Abdulaziz Fouling occurs when dissolved and particulate matter in
University, Rabigh, Saudi Arabia feed water deposits on the membrane surface leading to an
2
Department of Chemical Engineering, University of Southern increase in the overall membrane resistance (El Aleem
California, Los Angeles, CA, USA et al. 1998). In other words, fouling happens when solutes

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 Appl Water Sci

in the flow are adsorbed reversibly or irreversibly onto the feed water and removing the developed biofilm on RO
membrane surface or within the pores of the membrane. membrane can be regarded as some other approaches that
The irreversible adsorption is the main issue and it pro- could be applied to solve the problem of biofouling in RO
duces a long-term flux decline (Matin et al. 2011). There modules.
are four categories for fouling sources (as seen in Table 1): Biofouling in a seawater reverse osmosis (SWRO) plant is
scale (inorganic), particulate, biological and organic com- controlled by the surrounding environment as well as pre-
pounds. Biofouling depends on the amount of biological, treatment of feed water. The population of bacteria in sea-
organic matter and colloidal particles in the feed water. water is dependent on various environmental factors such as
Eliminating these particles (through pretreatment) in feed light, temperature, tides, currents, turbidity and nutrients.
water is the main objective to avoid major biofouling SWRO module is more vulnerable to biofouling in hot cli-
problems in the final RO modules of the plant that are the matic conditions. For example, degradation of humic acid is
most affected elements. Another effective way to increase much easier and greater at a temperature of 35 than 18 °C.
the recovery rate is to have a partial membrane replace- Degraded small molecules are a source of nutrition for
ment (Qureshi et al. 2013). bacterial growth. Since RO feedwater and brine reject tem-
Saudi Arabia produces around one-third of the world’s peratures are always higher than that of seawater feed, a
capacity of desalinated water. Current desalination tech- higher biofouling potential is expected at the increased
nologies in the Kingdom of Saudi Arabia include multi- operation temperature. In addition, water samples near shore
stage flash method (MSF) and the RO process. RO process surface at Al-Birk plant in Saudi Arabia showed less nutrient
is preferable since it is simple, inexpensive and easy to content than water samples from the intake. It is important to
maintain. However, recent critical problems related to RO choose an intake site that is less in nutrients and silt to avoid
membrane processes are fouling, biofouling, and biocor- biofouling since the water source may have a negative
rosion (El Aleem et al. 1998). impact on the operation parameters. Studies showed that the
Gulf water is rich in microorganisms, organics and has a shortest bacterial growth generation time is *2.5 h meaning
high level of total dissolved solids (TDS) ([40,000 ppm). that biofouling is a biofilm problem. RO membranes have an
Thus, the main reason for flux decline in RO plants in the enormous surface area that increases the chances of a single
Middle East is biofouling. Biofouling reduces actual bacterium to reach a membrane surface and later colonize to
membrane performance through microbial generation in a form a biofilm (Saeed et al. 2000).
biofilm which is formed on the membrane surface. Biofouling causes severe losses in performance of RO
Wastewater recirculation in industrial treatment plants membranes and requires costly cleaning procedures to
results in having a higher concentration of TDS that pro- remove biofilms. Impact of biofilms on plant performance
motes bacterial growth and biofilm development. Further, is linked to the structure and composition of the biofilm.
the use of activated carbon system (GAC or PAC) before Microorganisms including bacteria are the main reason for
the RO modules increases biological fouling. Hence, biofouling and since bacteria is very adaptable, it is capable
proper pretreatment, disinfection, and micron cartridge of colonizing almost any surface at extreme conditions
filters are important to control bacterial growth during RO such as temperatures from -12 to 110 °C and pH values
treatment process (El Aleem et al. 1998). Reducing the between 0.5 and 13 (Qureshi et al. 2013). Table 2 shows
concentration of microorganisms and nutrients in the feed the most common microorganisms that can attack RO
to the RO membrane, adjusting the properties of the RO membranes.

Table 1 Types of fouling in RO membrane systems (Qureshi et al. 2013; Kang and Cao 2012)
Fouling type Mechanism Causing substances

Inorganic Deposition of inorganic materials Metal hydroxides, carbonates, sulfates, phosphates

Organic Deposition of organic substances Oil, proteins, humic acids, polysaccharides, lipids
Particulate and colloidal Deposition of debris and other substances Clay, silt, silica
Biofouling Adhesion and accumulation of microbes, forming biofilms Bacteria, fungi, yeast

Table 2 Common microorganisms identified in biofilms (Qureshi et al. 2013; Baker and Dudley 1998)
Bacteria Mycobacterium, Flavobacterium, Pseudomonas, Corynebacterium, Bacillus, Arthrobacte, Acinetobacter, Cytophaga, Moraxella,
Micrococcus, Serratia, Lactobacillus, Aeromonas
Fungi Penicillium, Trichoderma, Mucor, Fusarium, Aspergillus
Yeasts Occasionally identified in significant numbers

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Appl Water Sci

Fig. 1 Mechanism of biofilm

development

Biofilm development

Mechanism

Biofouling process or biofilm formation is a multistage

process that is complex, slow, reversible or irreversible
process where microbial growth can take couple of weeks
or months. However, the initial step (adsorption) is rela-
tively fast and can occur in about 2 h only. Mechanism of
biofilm development is illustrated in Fig. 1. Biofouling
process goes sequentially through the following steps
(Matin et al. 2011; El Aleem et al. 1998).
1. Adsorption of organics onto the wetted membrane
Fig. 2 Sequence of events leading to the formation of a Biofilm
surface (conditioning): Biofouling occurs through a
(Cunningham et al. 2011)
cascade of events including the transport, deposition
and adhesion of cells followed by exopolymer pro-
duction, cell growth and proliferation. Conditioning The primary induction phase is followed by the loga-
enhances attachment of cells to the surface. rithmic growth phase which contributes more to microbial
2. Transport and attachment of the microbial cells to the growth as compared to microbial adhesion; then plateau
conditioned surface: This step depends on different phase which is mainly controlled by the presence of
physical and chemical factors, but attachment generally nutrients. When plateau phase is attained, the membrane is
is more favorable with hydrophobic, non-polar surfaces. masked by the biofilm (Matin et al. 2011). More details
3. Growth (metabolism) of the attached microorganisms about each phase are summarized in Fig. 2 and below
and biofilm development: Biofilm formation stage (Flemming 1997).
takes place by auto-aggregation of the attached cells Induction phase refers to the primary colonization of the
and formation of microcolonies. Extracellular poly- membrane by microorganisms. The primary colonization is
meric substances (EPS) are continuously produced and followed by a primary plateau. The induction phase also
acts as a reactive transport barrier to chemical biocides refers to the time between two cleaning measures. Colo-
and promotes nutrient concentration/storage. nization takes place due to microbial adhesion which is
4. Detachment and limitation of biofilm growth by fluid proportional to the cell density in the water phase and
shear forces: Cell detachment is an active form of occurs owing to weak physicochemical interactions
dispersion of cells from the biofilm matrix and (Flemming 1997).
detached biofilm cells reinitiate biofilm formation on Logarithmic phase involves cell growth which con-
new sites. Understanding this step is important since it tributes more to biofilm accumulation than adhesion of
is related to the control of growth. planktonic cells (Flemming 1997; Schaule 1992).

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 Appl Water Sci

Plateau phase is governed by nutrient concentration and beneath the defined threshold of interference (Flemming
the resultant growth rate, mechanical stability of the bio- 1997).
film, and effective shear forces. It is independent of the Biofouling occurs due to the deposition and growth of
concentration of cells in the feed water. In this phase, we biofilms. However, biofilm generation starts when the
have another plateau which represents the balance between attached microorganisms excrete EPS. Biofilms are com-
biofilm growth and cell detachment. The concentration of posed primarily of microbial cells and EPS as shown in
assimilable organic carbon is the key parameter controlling Fig. 4. EPS constitutes 50–90% of the total organic carbon
the level of the plateau which is significant for process (TOC) of biofilms and is considered as the primary matrix
stability, energy consumption, and economics (Flemming material of the biofilm. EPS consists primarily of
1997). polysaccharides, proteins, glycoproteins, lipoproteins, and
Threshold of interference in Fig. 3 is the extent of bio- other macromolecules of microbial origin. The EPS matrix
film development above which the biofilm interferes with offers important advantages for bacteria like maintaining
the performance of a membrane system. Treatment tech- stable arrangements of the cell and enhancing the degra-
niques focus on getting the microbial concentration levels dation of complex substances (Matin et al. 2011).

Factors influencing microbial adhesion

Transport conditions play an important role in microbial

adhesion as they affect the accumulation of microorgan-
isms on the surface of the membrane. These transport
conditions also influence generation of shear forces. High
shear forces are desirable as they inhibit microorganism
adhesion and hence microbial growth at the membrane
surface (Al-Juboori and Yusaf 2012).
pH of solution affects the electrostatic double layer
interaction between the membrane and microorganisms
due to change in surface charge. Change in pH of the
solution has a slight effect on the surface charge of the
Fig. 3 Development of biofilm and accumulation of microbial matter membrane but has a substantially higher effect on colloids’
with respect to time (Flemming 1997) charge (Brant and Childress 2002).

Fig. 4 EPS components of a

bacterium encountering a non-
biological surface in water
(Tamachkiarow and Flemming
2003)

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Ionic strength of solution also affects the electrostatic Table 3 Typical composition of biofilm (Baker and Dudley 1998)
double layer interaction between the membrane and the Parameter Composition
microorganisms. Most microorganisms are negatively
charged; so in order to avoid microbial adhesion and sub- Moisture content of dried deposit [90%
sequent growth on the membrane surface we desire that the Total organic matter (TOM) [50%
membrane should also be negative thereby inhibiting Humic substances as % of total organic matter B40%
adhesion due to repulsive forces (Al-Juboori and Yusaf Microbiological counts [106 cfu/cm2
2012; Lee and Elimelech 2006; Hong and Elimelech 1997).
The characteristics of interacting surfaces that play a
significant role in biofilm formation are hydrophobicity,
hydrophilicity, and surface roughness. Hydrophobicity and
hydrophilicity are analogous properties that determine the
membrane’s tendency to foul. As the name suggests,
hydrophobic membranes preferentially interact with
microbial matter which causes biofouling; while hydro-
philic membranes interact with water. Another crucial
factor is surface roughness of the membrane. Rough sur-
faces have larger number of sites convenient for microbial
adhesion in the form of peaks and troughs. Rough surfaces
also have larger surface areas than smoother surfaces
thereby increasing the number of sites for adhesion.
Moreover, the roughness of the membrane surface can Fig. 5 Spiral-wound RO module (Qureshi et al. 2013)
decrease the Lifshitz–van der Waals and electrostatic
double layer interactions of the membrane (Brant and various environmental factors such as temperature and
Childress 2002; Yu et al. 2010). humidity. In Table 3, we have a typical biofilm composi-
Nutrients in the bulk solution serve as food for tion from previous laboratory studies for brackish and
microorganisms; hence, concentration of nutrients should seawater treatment plants:
be low to avoid biofouling. While the presence of nutrients
is not directly detrimental to the membrane, it acts as a Reverse osmosis module
source of nutrition for microorganisms aiding their meta-
bolic activities and growth. It has been found that Biofouling in RO module elements include the formation
increasing the concentration of carbon in bulk solution, of biofilms in permeate surfaces of cross-flow membranes,
shortens the initial growth period of the biofilm resulting in woven polyester support fabrics, permeate collection
lesser microbial mass (Al-Juboori and Yusaf 2012). material, and feed channel spacer materials. The crucial
Higher concentration of microorganisms in the bulk biofouling type in RO module is the formation of biofilm in
solution leads to higher adhesion and microbial growth on the feed channel spacer material. This should be avoided to
the membrane surface as well as higher generation of EPS restrict the impact of biomass accumulation on the feed
which fouls the membrane and reduces membrane flux (Al- channel pressure gradient increase. Fig. 5 represents a
Juboori and Yusaf 2012). Factors affecting bacterial mul- spiral-wound RO module.
tiplication rate are feed water quality, temperature, pH, The spacer minimizes the problem of concentration
dissolved oxygen content, the presence of organic and polarization since it consists of a network of plastic fibers
inorganic nutrients, pollution, depth and location of the that separates the spiral wound membrane sheets from each
intake (Saeed et al. 2000; El Aleem et al. 1998). other to create turbulence and inhibit further biofouling.
Moreover, biofilm development is also influenced by the Channeling problems happen in hollow fiber bundles when
carbon: nitrogen: phosphorus ratio, and redox potential. we have individual fibers that are bounded together which
Physical structure of biofilm can be compact and gel like or causes rapid salt concentration leading to the precipitation
slimy and adhesive with large amounts of polysaccharide. of salts such as calcium carbonate and calcium sulphate
Generally, biofilm contains between 106 and 108 colony (Matin et al. 2011). Table 4 summarizes bacteria counts in
forming units (CFU) of bacteria per cm2 of membrane area. biofouled systems that produce potable water (Baker and
There is a strong relation between biofilm composition and Dudley 1998).

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Table 4 Typical microbial activity in biofouled spiral wound ele-

ments (Baker and Dudley 1998)
Range of viable bacteria Range of fungal
counts (cfu/cm2) counts (cfu/cm2)

Fouled membrane 1 9 102–1 9 108 0–1 9 103

a 2 6
Plastic spacer material 4 9 10 –5 9 10 0–1 9 103
2 6
Permeate carrier \10 –1 9 10 None
a
Viable bacteria computed per cm2 of the spacer mesh

Modeling and monitoring

Modeling of flux decline

In RO systems, the most important parameters in terms of

design and performance are the feed pressure and feed
concentration, respectively. A solution-diffusion model for
steady-state processes showed a good agreement between
Fig. 6 Curve fit of normalized permeate flux decline versus time
the experimental or measured results and simulated results (Qureshi et al. 2013)
(Qureshi et al. 2013).
Fouling analysis model with two constants is proposed for respectively. Since we have large error values, they can not
predicting the normalized decrease in permeate flux due to be neglected; further investigations and experimental works
fouling. Membrane fouls over time and fouling curve exhi- are needed to determine accurate constant values for specific
bits an asymptotic behavior. Fouling of RO membranes can RO applications. Practically, integration of the model into an
be modeled using a normalized permeate flux decline gJ that RO cleaning strategy helps in identifying the affected
follows the following relation and varies with time (Khan membrane points and whether a backwash with or without
and Zubair 2004; Qureshi and Zubair 2005). cleaning chemicals is needed or not (Qureshi et al. 2013).
gJ ¼ gJ ½1  expðt=sc Þ ð1Þ Fig. 6 demonstrates the normalized decrease in flux of
permeate with respect to time for different feed pH values.
where gJ is the asymptotic value of the normalized permeate
flux decline ðgJ Þ and sc is the time constant expressing the Monitoring and detection
time when the normalized permeate flux ðgJ Þ reaches 63.2%
of its asymptotic value. gJ and sc are two constants to be The first step towards addressing biofouling through
determined beforehand. This model is used to predict the treatment is to detect formation of biofilms and monitor
decrease in permeate flux as the membrane fouls over time. cell accumulation. Techniques by which this is done can
Literature shows that both constants depend on the feed range from primitive inspection through sight or smell,
concentration, cross-flow velocity, pH and transmembrane sampling and lab testing to more advanced techniques like
pressure drop (Qureshi et al. 2013; Khan and Zubair 2004; bioluminescence, epifluorescence microscopy, etc. Here
Qureshi and Zubair 2005). we will discuss the various techniques employed for
sc ¼ f ðCo ; u; DP; pH; T Þ ð2Þ detection and monitoring of biofouling (Al-Juboori and
Yusaf 2012).
gJ ¼ f ðCo ; u; DP; pH; T Þ ð3Þ
1. Physical inspection: RO systems such as the spiral
Koltuniewicz and Noworyta (Koltuniewicz and
wound membrane module may show signs of biofoul-
Noworyta 1994) suggested two equations for the
ing in smell and color which can be physically
calculation of both constants as follows:
inspected. Routine visual inspection of various plant
1 components such as pretreatment piping, cartridge and
¼ 0:298  104 Co0:567 DP ð4Þ
sc media filters should be done to detect accumulation of
3:875  106 biological matter. All of these inspections must be
gJ ¼ ð5Þ performed in wet conditions since microorganisms
Co1:21
thrive in it (Al-Ahmad et al. 2000).
However, authors reported a maximum relative error for 2. System performance analysis: EPS secreted by
Eqs. (4) and (5) which is about –13.1 and –20.1%, microorganisms cause a decline in membrane flux.

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Appl Water Sci

The measurement of this change in flux and pressure be in different phases and physical forms such as solid,
drop across the membrane is a very good way of colloidal or slimy films. Applicability of FT-IR spec-
monitoring biofouling. Performance of the module is troscopy irrespective of the physical nature of biofilm
gauged by measuring the flow rate and purity of makes it the best spectroscopic technique for monitoring of
permeate, salt rejection efficiency, and silt density biofouling (Brant and Childress 2002).
index (SDI) of feed water entering the module (Al- While FT-IR spectroscopy has drawbacks, the authors
Ahmad et al. 2000). believe that these do not have any consequences on the
3. Water sampling: Routine collection of feed, perme- legitimacy of this technique for monitoring of biofouling in
ate and retentate streams should be done right from RO systems. Since routine sampling is conducted to detect
the onset of operation of RO plant. The sampling early onset of biofilm formation, the microbial growth and
points should be chosen as to adequately cover the EPS secretion is highly unlikely to be significant enough to
entire system. This monitoring technique primarily form a biofilm which is thicker than the order of 1 lm
serves as a preventive measure. The main objective (Flemming 1997).
of this sampling and analysis technique is to locate Furthermore, even though FT-IR spectroscopy requires
or isolate the source of any bioactivity before it a library of spectra for each microorganism for its identi-
starts to spread and affect other parts of the RO fication after detection, owing to the culturing techniques
system. Presence and accumulation of different discussed earlier, we already know the different kinds of
species of microorganisms is measured along with microorganisms that are present in the feed. Hence, we
SDI, pH, COD, TOC, and dissolved oxygen content. need information on spectra of only those microorganisms
SDI is a measure of fouling potential; clean brackish which are present in the feed to the RO membrane and can
water will have SDI \5, whereas, seawater will have potentially cause biofouling.
SDI values ranging 6–20 (Al-Ahmad et al. 2000; This analysis of drawbacks presents the conclusion that
Abd 1998). FT-IR spectroscopy is the best spectroscopic technique for
4. Culturing techniques: These are employed to detect the monitoring of biofouling in RO systems as routine sam-
kind of microbial activity as well as the concentration pling of feed and culturing techniques can eliminate the
of those species affecting the RO system. Methods disadvantages associated with this technique.
usually used for this biological analysis are either for
measuring the total accumulation of biological matter
or for the detection of specific species of microorgan- Consequences of biofouling
isms through analysis of microbial activity on cultured
samples. Cultures are retained for 24–72 h at 25–30 °C Biofouling has diverse consequences on the entire RO
(Al-Ahmad et al. 2000). module, particularly the membrane system. It affects both
the process as well as physical components of RO module.
Table 5 summarizes most of the microscopic and
These effects are elucidated below (Baker and Dudley
spectroscopic techniques used for the inspection of biofilms
1998; El Aleem et al. 1998; Flemming 1997).
in reverse osmosis modules. While each technique has its
own advantages and disadvantages, Hoffman modulation 1. Membrane flux decline: This is because of the
contrast microscopy (HMCM) can be considered as the formation of a film of low permeability on the
single most beneficial microscopic technique for monitor- membrane surface.
ing of biofilm formation. HMCM (Fig. 7) has no significant 2. Membrane biodegradation: Microorganisms produce
drawbacks and has plentiful advantages. Being non-inva- acidic byproducts that damage RO membrane.
sive, HMCM technique does not interrupt normal RO plant 3. Increased salt passage: Accumulated ions of dissolved
operation and trumps most other techniques by offering salts on the membrane surface enhances concentration
high resolution imaging without the need of preparation of polarization and inhibits convectional transport.
any specific kinds of samples (Al-Juboori and Yusaf 2012). 4. Increase in the differential pressure and feed pressure:
Similarly, the authors believe that Fourier transform- This is due to biofilm resistance.
infrared (FT-IR) spectroscopy is arguably the best spec- 5. Increased energy requirements: High-pressure require-
troscopic technique to study the physiological behavior of ments are due to higher feed pressure, frictional energy
microorganisms. FT-IR spectroscopy (Fig. 8) is the most losses and drag resistance to tangential flow over the
commonly used spectroscopic technique as it not only membrane.
detects microbial presence but can also distinguish between 6. Frequent chemical cleaning: Imposes a large economic
live and dead cells, thereby, aiding the subsequent con- burden on RO membrane plant operation, up to 50% of
trolling and treatment techniques. Moreover, biofilms can the total costs, and shortens membrane life.

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Table 5 Microscopic and spectroscopic techniques for the detection of biofouling in RO membranes (Al-Juboori and Yusaf 2012; Khan et al.
2010; Wolf et al. 2002; Griffiths and De Haseth 2007; Chambers et al. 2006)
Technique Advantages Disadvantages

Microscopic techniques
Epifluorescence Rapid analysis, provides information on the structure– Unable to measure the depth of the biofilm, low resolution
microscopy function relationships in biofilm and the requirement of removing the biofilm (invasive
technique)
Electron microscopy Produces images with high resolution, and provide cross- Unable to study biofilm structure, slow analyses, may
sectional details of the biofilm, which allows visualizing damage the biofilm
the spatial distribution of microorganisms in the biofilm
matrix
Confocal laser Able to produce 3D images of biofilm efficiently Overlapping of the fluorescence signals of the auto-
scanning monitoring bacterial growth, metabolic activity and gene fluorescence biomolecules and fluorophores, limitations
microscopy (CLSM) expression in biofilm, and allows studying the physio- over the number of the fluorescence filters combinations
chemical and biochemical aspects of biofilm and unsuitable for use with opaque and very thick
microenvironments biofilm
Atomic force Has a high resolution and it can be used in vivo studies Sample dehydration during the examination which may
microscopy affect the accuracy of the extracted biofilm information
X-ray microscopy High resolution, simplicity in preparing the samples and Unsuitable for thick biofilms (\10 lm), and a destructive
maintenance of hydration of biofilm sample mode of analysis
Raman microscopy Can examine the spatial distribution of microorganisms in Restricted to infrared wavelength. There is also a lack of
the biofilm matrix in a non-invasive way. Capable of spectral database of microbes without which we cannot
yielding spatially resolved chemical information of the differentiate between species of microbes
biofilm
Hoffman modulation Non-invasive microscopic technique, ability of HMCM to No notable drawbacks
contrast microscopy produce 3D image, HMCM has other advantages such as
(HMCM) high contrast resolution, suitability to use with dense
biofilm and no requirements for sample preparation
Differential Rapid way for monitoring biofilm and it has the capacity to It is fragile and sensitive to heat. Uses expensive quartz
interference contrast produce 3D images of in situ biofilm Wollaston prisms. The signal is reduced by the presence
microscopy (DICM) of the polarizer. Image contrast is reduced by the
presence of birefringent materials. Varying ellipticity of
polarization of laser light causes fluctuations in
brightness of produced DIC images
Environmental Can analyze hydrated biofilms Cannot be used for in vivo and on-line monitoring systems.
scanning electron Poor distinguishing between small cells and the texture
microscopy (ESEM) of the substrate in a biofilm with random topography
Digital time-lapse Can study the effect of membrane surface properties on Observed area in the flow cell is very limited which may
microscopy initial adhesion of bacteria, effect of nutrients and flow not give an accurate representation for the case.
conditions on deposition of microorganisms on RO Limitation of depth in the flow cells restricts the flow in
membrane the cell to laminar conditions
Spectroscopic techniques
Fourier transform- Required volume of sample is very small (range of ng-lg), Can only detect thin biofilms of the order of 1 lm and for
infrared (FT-IR) can analyze samples of different phases and identify if accurate analysis, a complete library of the spectra for
spectroscopy microorganisms are dead or alive each microorganism is required
Bioluminescence Can identify characteristics of biofilm such as bacterial Confined to environments possessing microorganisms that
biomass, cellular activity and gene expression in are naturally or genetically modified to emit light under
genetically modified bacteria the effect of biochemical reactions
Nuclear magnetic Non-destructive and non-invasive. Can monitor growth Low signal/noise ratio, long time required for data
resonance (NMR) state of microorganisms in biofilm, the architecture of acquisition and the quality of the produced images by
spectroscopy the biofilm and the detachment rate of the biofilm under NMR is affected by the surface curvature of the biofilm.
starvation conditions as well as effect of biofilm on the Expensive technique because isotopes required in NMR
hydrodynamics of the surrounding liquid spectroscopy are naturally scarce
Pressure drop Cost effective technique for monitoring early stage Cannot specifically detect biofilm formation on the
measurements biofouling in membrane systems membrane as pressure drop can be due to factors other
than biofouling too

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Fig. 7 Permeate flux and TOC removal upon growth of biofilm on an

RO membrane (Herzberg and Elimelech 2007)

7. Serious decline in the quality of permeate: This is

because of all the factors previously listed. Fig. 8 Death of a cell caused by PEF (Guyot et al. 2007)
8. Higher treatment costs: This results from high energy
requirements, cleaning demand, and membrane permeate flux, the membrane development and the solute
replacement. diffusion coefficient in the boundary-layer fluid. Concen-
Permeate flux decline exhibits two phases; initial rapid tration polarization results in the following effects: reduces
decline followed by a more gradual decay. The rapid the net driving pressure differential across the membrane,
decline takes place in the early attachment stage while the thus, lowering the permeate flow rate, increases salt flow
slow decline occurs during the plateau phase. In the pres- across the membrane, and increases precipitation that
ence of bacteria, the higher the permeate volume required, causes membrane scaling (Qureshi et al. 2013).
the greater the flux decline is observed, Fig. 7. System Concentration polarization strongly affects the perfor-
pressure will increase to compensate for the flux decline mance of the separation process. First, concentration
and this will add more treatment costs. The main reason for changes in the solution reduce the driving force within the
the decline in flux or salt rejection is that bacterial cells membrane, hence, affecting the useful flux/rate of separa-
hinder the back diffusion of salts by secreting EPS which tion. In the case of pressure driven processes, this phe-
then increases hydrolytic resistance of the membrane. In nomenon causes an increase in the osmotic pressure
particular, EPS fouling only showed salt rejection decrease gradient of the membrane reducing the net driving pressure
by 2%, but with dead cells, reduction could reach up to gradient. In the case of electromembrane processes, the
5–6%. Membrane biodegradation is another reason for the potential drop in the diffusion boundary layers reduces the
decrease in salt rejection in RO cellulose acetate modules gradient of electric potential in the membrane. Lower rate
(Matin et al. 2011; Herzberg et al. 2009). of separation under the same external driving force means
Gradual accumulation of dissolved substances retained increased power consumption (Baker 2012).
by the membrane at the raw waterside initiates concen- A case study showed that, because of an additional
tration polarization phenomenon. The increase in hydraulic hydraulic resistance of the biofouling layer, Water Factory
resistance also results in reducing permeate flux and 21, Orange County, CA, operates at about 150% of their
enhancing concentration polarization which causes a initial operating pressure (roughly 200 psi). It was observed
decrease in salt rejection (Matin et al. 2011). Concentration that the $1 million membrane inventory lasted only for
polarization occurs when the salt concentration near the 4 years instead of its theoretical life-span of 8 years. This
membrane surface exceeds the salt concentration in the amounts to an added cost of $125,000 per year due to
bulk solution because of flow of water through the mem- biofouling (Flemming 1997; Flemming et al. 1994). Mad-
brane and rejection of salts (Flemming 1997). We have dah et al. showed in their membrane cost study analyses
four key factors to determine the magnitude of concentra- that integrated UF-RO membranes have the lowest treat-
tion polarization: the boundary-layer thickness, the ment cost of $0.3/m3 compared to MF-RO and MBR types

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(*$0.5/m3) since UF membranes can control foulants can be resolved by the application of several physical and
before they reach at the RO module and damage it. chemical disinfection techniques which are categorized and
Therefore, fouling costs were eliminated in UF-RO summarized in Table 6 (Matin et al. 2011; Al-Juboori and
reducing the overall treatment cost for the UF-RO modules Yusaf 2012; Young 1999).
(Maddah and Chogle 2015). Biocides are materials and substances that are used for
the purpose of feed pretreatment and are categorized as
oxidizing and non-oxidizing biocides. Oxidizing agents
Control and remediation include chlorine, bromine, chloramine (NH2Cl), chlorine
dioxide (ClO2), hydrogen peroxide, peroxyacetic acid,
After detection and monitoring of biological matter that is hypochlorous acid (HOCl), and ozone while non-oxidizing
responsible for forming biofilms, the next stage is suc- agents include formaldehyde, glutaraldehyde, quaternary
cessful enactment of remediation techniques for controlling ammonium compounds, etc. Oxidizing agents are applied
biofouling in RO systems. Techniques employed for con- to industrial water treatment plants, but are incompatible
trolling biofouling include the following: with polyamide RO membranes since they may break
down humic acids into smaller components that serve as
Membrane cleaning nutrients to bacteria. On the other hand, non-oxidizing
agents are more relevant to industrial wastewater treatment
Membrane cleaning involves physical cleaning, back- plants since they are more compatible with RO membranes.
washing, chemical cleaning, removal of organic films, It is recommended to avoid using low levels of biocides on
slimes, and biological fouling. It contributes to 5–20% of microbes because continuous low dose rates often cause
the operating cost. Chemical cleaning agents are com- microbial resistance (Matin et al. 2011).
mercially available and they are included in six categories: Chlorine is another biocide which is used for chlorina-
alkalis, acids, metal chelating agents, surfactants, oxidation tion; another technique that is not viable anymore because
agents, and enzymes. The most effective combination is it is found that chlorine is responsible for the degradation
enzyme–anti-precipitant–dispersant and bactericidal agent of humic acids to smaller molecules that are used as
with an anionic detergent for cellulose acetate RO mem- nutrients to bacteria. Another reason is related to the
branes. Another noteworthy combination is chelating agent aftergrowth mechanism in which there is a sharp increase
surfactant with alkali for polyamide RO membranes (Matin in bacteria after dechlorination with sodium metabisulfite
et al. 2011). (SBS) since surviving bacteria utilize the degraded mole-
Cleaning chemicals should be used wisely in RO cules and use them as nutrients (Abd 1998). However,
membranes as they could be harmful to the membrane disinfectants like chloramine and copper sulfate would be
material since frequent cleaning may cause conditioning or excellent substitutes for chlorine. Stopping chlorination/
hardening of foulant layers (Baker and Dudley 1998). dechlorination altogether is the most recommended
Moreover, cleaning techniques are employed after bio- approach to achieve more successful operations and
fouling has already occurred. Therefore, since prevention is improved performances. Intermittent or shock dosing
better than cure, focusing on feed pretreatment is the chlorination is an excellent alternative to plants which
optimal approach to prevent biofouling repercussions. operate without chlorine; it is suggested to chlorinate for
Feed pretreatment includes acid dosing for pH control, 6–8 h per week with a residual chlorine level of 1 mg/l
coagulation and flocculation, media filtration, chlorination, (Saeed et al. 2000). Similarly, shock dosing is also per-
ozonation, UV radiation, addition of antiscaling com- formed by using sodium bisulphite (NaHSO3) for an
pounds or inhibitors, cartridge filters, activated carbon exposure time of 30 min at a concentration of 500 ppm
adsorption, etc. Practically, in RO systems disinfection is with kill rates up to 99% for seawater microflora (Baker
done by chlorine and copper sulphate while coagulation is and Dudley 1998).
carried out by alum (El Aleem et al. 1998). On the contrary, under physical methods we have
electrical techniques used for water disinfection that
Disinfection include electro-chemical techniques and pulsed electric
field (PEF). Electro-chemical techniques can be catego-
Biofouling cannot be eradicated by pretreatment alone. rized into two groups, namely, methods that use direct
Even if 99.99% of all bacteria are eliminated by pre- electrolysers which interact directly with microbes, and
treatment, a few surviving cells will enter the system and other methods that use mixed oxidant generators producing
multiply. Biofouling occurs even after significant disin- oxidizing species for damaging microbes. PEF as seen in
fection with chlorine. In the Middle East, about 70% of the Fig. 8 is a disinfection technique that involves maintaining
seawater RO plants suffer from biofouling problems which the suspension of microorganisms between electrodes and

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Table 6 Summary of disinfection techniques (Matin et al. 2011; Al-Juboori and Yusaf 2012; Young 1999; Kim et al. 2009)
Disinfection Advantages Disadvantages

Chemical Chlorine (HOCl, Initial removal of biofouling prior to Dechlorination may enhance biofouling, chlorination gives
NH2Cl, ClO2) dechlorination, relatively low cost, less or no carcinogens (THMs, HAAs), chemically corrosive,
(Matin et al. 2011) damage to membrane chlorite toxicity, low efficiency
Ozone (Matin et al. High oxidation, ideal when combined with GAC Costly and generates carcinogens (bromate), very small
2011) half-life
Physical UV (Matin et al. No by-products, enhanced performance when Scale formation and may produce mutagenic components
2011; Al-Juboori combined with sodium hypochlorite, easy
and Yusaf 2012) installation and maintenance
Sand filtration Low installation and operation cost Low bacteria removal efficiency
Electrical (Al- Lower energy requirement, do not produce a new It may produce mutagenic components in the treated water,
Juboori and Yusaf generation of microbes that are tolerant to the cathode fouling
2012) treatment
Ultrasound (Young Can be combined with other techniques to High cost, requirement of cooling processes
1999) enhance performance, used for solutions
having suspended solids

subjecting them to a high intensity of electric field for a

short period to degrade the microorganisms directly by
decomposing DNA or RNA of their cells (Al-Juboori and
Yusaf 2012).
The advantages of electrical disinfection methods
include lower energy requirement, which unlike thermal
techniques, do not require energy expenditure in the form
of cooling. In addition, these methods do not produce a
new generation of microorganisms that are tolerant to
electrical treatment. However, it may produce mutagenic Fig. 9 Acoustic cavitation process (Young 1999)
components in the treated water as well as cathode fouling
(Al-Juboori and Yusaf 2012).
Ultrasound Techniques are employed primarily as for 1 cycle of the sound pressure in which bubbles expand
replacements for UV light and chlorination treatments for to at least double their size and collapse severely often
water disinfection, but can also be used for performance disintegrating into small bubbles. Whereas, stable bubbles
enhancement. These techniques include acoustic cavitation can oscillate for more than one cycle of sound pressure and
phenomenon (Fig. 9) that occurs due to the fall of the grow due to mass diffusion through the bubble (Young
ambient pressure under the saturated vapor pressure of the 1999).
liquid because of ultrasound waves passing through the Mechanism of ultrasound involves three stages. First,
liquid. There is an oscillation of pressure due to ultrasound the mechanical effects stage results from the cavitation
waves; the positive swing of pressure is called compression phenomena. Second, the chemical effects of cavitation
period and the negative swing is called rarefaction period. phenomena occur, which involve the generation of free
Formation of voids takes place during the rarefaction per- radicals in the medium. Third, heat effects represented by
iod, while the collapse of bubbles takes place during the the generation of localized hot spots developed by rapid
compression period (Young 1999). explosion of the bubbles (Young 1999).
Cavitation can be homogeneous, where the generation It was observed that mechanical effects play the main
of bubbles is due to interaction between liquid and vapor, role in destroying microorganisms while chemical effects
or heterogeneous, where the interaction is between solid, and heat effects play only a supporting role. Implosion of
liquid, and vapor phases simultaneously. The surface ten- bubbles generates mechanical effects such as high pressure,
sion of the liquid at the nucleation sites (where cavitation turbulence due to liquid circulation, and shear stresses.
occurs) is weak which allows the negative pressure of Micro-streaming resulting from bubble oscillation can
sound waves to rupture the liquid and generate bubbles. generate stresses that have the potential to rupture
The bubbles forming in the liquid as a result of irradiation microorganisms. It was proposed that in ultrasound treat-
may collapse either gently (stable cavitation) or violently ment, cell rupture occurs due to exposure of cells to vis-
(transient cavitation). Transient cavitation exists normally cous dissipative eddies that generate from the shock waves

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 Appl Water Sci

of bubble collapse. The main cause of cell disruption in hydrophilicity, but also can be reactive in nature (Malai-
ultrasound treatment was later confirmed to be the collapse samy et al. 2010). The surface roughness of RO mem-
pressure that results from bubble implosion (Young 1999). branes is also positively correlated with colloidal fouling
(Kang and Cao 2012).
pH adjustment Most species of bacteria are negatively charged and
hence, to reduce microbial adhesion, the theory of making
pH adjustment is recommended to control adhesion of membranes negatively charged was proposed. The elec-
microbes on the RO membrane. pH can either be increased trostatic repulsion existing between microorganisms and
by addition of a strong base like NaOH or decreased by the negatively charged membrane will inhibit adhesion and
addition of a strong acid like HCl. The addition of an acid hence, biofouling (Kang and Cao 2012).
is not recommended as it can lead to corrosion of the Increasing the hydrophilicity of a membrane leads to
membrane. It is also known that organic fouling is usually decrease in the attachment of microorganisms to the
accelerated with decrease in pH and increase in divalent membrane surface as the hydrophilic membrane favors
cation concentration. In low pH and high divalent cation interaction with water molecules in lieu of microorganisms.
concentration, charge property of organic matters dimin- In other words, hydrophobic membranes prefer interacting
ishes through the neutralization of functional groups as with microorganisms resulting in greater microbial adhe-
well as organic-calcium complexation. Moreover, it has sion. The hydrophilicity of a membrane can be increased
been found that increasing pH of feed water is not as by physically coating the membrane surface with a thin
helpful as initially presumed. Feed water pH affects both polymeric film.
the charge properties of bulk organic foulants as well as the Improvement of membrane surface is possible by adding
interfacial interaction between organic foulants and mem- active organic modifiers into trimesoyl chloride (TMC) or
brane surfaces. The former leads to the formation of thick m-phenylenediamine (MPD) solution. Currently, TMC and
and dense fouling layers on the membrane surface due to MPD are the most commonly used active monomers to
the favorable multi-layer accumulation of organic foulants. form functional polyamide layer in RO membrane. An
The latter results in the reduction of electrostatic repulsion earlier study showed that a novel prepared composite RO
between organic foulants and membrane surfaces leading membrane from 5-isocyanato-isophthaloyl chloride (ICIC)
to accelerated accumulation of the foulants on the mem- and MPD had favorable hydrophilicity and smoother sur-
brane surface (Al-Juboori and Yusaf 2012). face, and therefore ICIC-MPD membrane showed better
The effect of pH is noticeable only when the feed water resistance to fouling (Kang and Cao 2012).
has low ionic concentration. Increasing pH in such a feed Interestingly, Yang et al. (2011) synthesized a modified
can lower the flux decline rate. However, when the ionic RO membrane which was chemically grafted with poly-
concentration of feed water is high, there is a negligible (sulfobetaine) zwitterionic groups for surface development.
change in flux decline rate. As reverse osmosis is used for The modified RO membranes exhibited superior antifoul-
desalination of seawater, variations in pH are not beneficial ing performance against E. coli and showed long-term
since seawater has high ionic concentration. Thus, feed operation compatibility because the modifiers were cova-
water pH is not a significant factor affecting organic or lently connected with the membrane surface. Practically,
biological fouling during seawater desalination (Herzberg the coating layer must be synthesized sufficiently thin to
and Elimelech 2007; Baek et al. 2011). maintain the water flux and water permeability as high as
possible (Kang and Cao 2012).
Membrane surface modification Malaisamy et al. (2010) used polymeric films for
membrane modification to produce acrylic acid (AA)
Surface modification techniques are employed to improve modified and [2-(acryloyloxy)ethyl] trimethyl ammonium
certain membrane characteristics such as surface rough- chloride (AETMA) modified membranes. AETMA-modi-
ness, surface charge and membrane hydrophilicity. fied membranes, in addition to having higher flux than AA-
Surface roughness as discussed earlier, increases modified membranes, possess antibacterial properties that
microbial adhesion due to higher surface area as compared minimizes the biofoulant growth (Hyun et al. 2006; Lee
to a smooth surface. Moreover, the peaks and troughs of et al. 2010; Yang et al. 2010) Moreover, AA-modified
rough surfaces provide higher frequency of susceptible membranes, when fouled even with trace levels of bacteria,
sites for microbial adhesion. This problem can be consid- cannot prevent their growth. Hence, AETMA-modified
erably reduced by smoothening the membrane surface with membranes are most desirable for increasing hydrophilicity
the application of a thin layer of polymer. Thin polymeric along with anti-bacterial behavior (Malaisamy et al. 2010).
film is physically coated on the membrane surface. This Thin-film polyamide composite RO membranes can be
polymer can not only possess characteristics such as high modified by the addition of aliphatic and aromatic groups.

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Appl Water Sci

Khan et al. (2010) have found that the addition of aliphatic

hydrocarbon groups on the polyamide layer of RO mem-
branes increased biofouling compared to the addition of
aromatics.
Moreover, RO membrane from 3-monomethylol-5,5-
dimethylhydantoin (MDMH) is characterized with improved
surface hydrophilicity as well as substantial biofouling pre-
vention which is confirmed by testing the membrane with
Escherichia coli (E. coli) as a model for microorganism fou-
lants. Not only this, MDMH-modified RO membrane offered
substantial chlorine resistance making this membrane ideal in
chlorine resistant and anti-biofouling applications (Kang and
Cao 2012; Wei et al. 2010a, b).
Hybrid organic/inorganic RO membrane process is
carried out by coating RO membranes with inorganic
particles by direct deposition or via interfacial polymer-
ization process. Inorganic particles include photocatalytic Fig. 10 Ratio of flux to their initial values during fouling experiment
titanium dioxide (TiO2), SiO2, Zeolite A, and silver (Matin et al. 2011)
nanoparticles (Kang and Cao 2012). Nanomaterials also
include chitosan, aqueous fullerene nanoparticles and car- chemically covalent bonds with membrane can withstand
bon nanotubes (Matin et al. 2011). longer than modifiers with physical bonds such as van der
Hybrid membrane with TiO2 nanoparticles can be Waals attractions, hydrogen bonding or electrostatic
introduced as a commercial RO membrane and they are interaction (Kang and Cao 2012; Rana et al. 2011).
capable of increasing the water permeability by 20%.
Fig. 10 confirms that the combination of TiO2 and UV light Biochemical action
is the optimal choice for decimation of E. coli population.
Silver compounds are strong bacterial growth inhibitors Biochemical materials like enzymes and bacteriophages
since silver ions can react with thiol (-SH) groups in can be used to alleviate detrimental effects of biofouling.
microbial cells for the inactivation of bacterial growth. While EPS may consist of exopolysaccharides, proteins,
Bacterial colonies were found to be at least 98% less in glycol-proteins, released nucleic acid, phospholipids and
coated silver nanoparticles substrates compared to the other surfactants, polysaccharides and proteins are the two
surrounding uncoated regions (Matin et al. 2011). main components of EPS. Hence, enzymatic action is
Furthermore, hybrid membranes are very promising in directed towards them. These enzymes are of two main
commercial use since they are capable of enhancing per- types, namely, polysaccharide lyases and hydrolases. For
meability characteristics and antifouling as discussed ear- proteins, there are degrading enzymes called proteases,
lier. Besides, they are characterized with self-cleaning which are categorized as exopeptidases and endopepti-
properties. For example, depositing TiO2 nanoparticles dases. Along with polysaccharide lyases, hydrolases, and
onto aromatic polyamide RO membrane surfaces showed proteases, bacteriophages are also employed for biochem-
an excellent antibacterial fouling potential and this is ical control of biofouling (Al-Juboori and Yusaf 2012;
confirmed by Madaeni and Ghaemi (Madaeni and Ghaemi Richards and Cloete 2010).
2007) who created a self-cleaning RO membrane using
TiO2 as a coating. Moreover, hybrid zeolite-polyamide
membranes (Jeong et al. 2007) showed enhanced surface Conclusion
hydrophilicity with greater negative charge and lower
roughness which implies that zeolite-polyamide mem- Microorganisms such as bacteria, fungi and yeast are major
branes have a strong potential to be used as antifouling enemies of desalination plants that involve reverse osmosis
membranes (Kang and Cao 2012). modules. Biofilm formation occurs in a series of events that
Rana et al. (2011) added 0.25 wt% of silver salt into are conditioning (adsorption), transport and attachment of
aqueous MPD phase to improve membrane surface microbes, growth and detachment. In the growth stage,
hydrophilicity and achieve better anti-biofouling property. extracellular polymeric substances are produced continu-
However, deposited inorganic particles onto RO membrane ously to provide nutrients to bacteria and offer defense
surface may face a problem of loss or leaching in long-term against biocides. It has been observed that increasing pH of
operations. It is worth mentioning that modifiers with feed water would reduce the permeate flux decline rate.

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 Appl Water Sci

Hoffman modulation contrast microscopy and Fourier appropriate credit to the original author(s) and the source, provide a
transform-infrared spectroscopy are determined to be the link to the Creative Commons license, and indicate if changes were
made.
best microscopic and spectroscopic techniques, respec-
tively, for the detection of biofouling in reverse osmosis
membranes as their disadvantages are either negligible or
can be minimized. Biofouling causes permeate flux and References
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