A review of reverse osmosis membrane fouling and control strategies

TL;DR: This paper reviews membrane fouling types and fouling control strategies, with a focus on the latest developments, including biofouling, organic fouling, inorganic scaling and colloidal fouling.

About: This article is published in Science of The Total Environment.The article was published on 2017-10-01 and is currently open access. It has received 567 citations till now. The article focuses on the topics: Membrane fouling & Fouling mitigation.

Summary

1. Introduction

• RO membrane technology is widely used in seawater desalination, drinking water production, water treatment and wastewater treatment.
• A lot of RO projects reusing wastewater with high levels of phosphate are in operation worldwide (Chesters, 2009).

2. Membrane fouling

• Generally fouling is the accumulation of undesired deposits on the membrane surface or inside the membrane pores, causing decrease of permeation flux and salt rejection (Malaeb and Ayoub, 6 / 41 2011).
• Water permeation through membrane could take place in the form of Brownian diffusion, flush and jump diffusion (Gao et al., 2015).
• The fouling mechanisms of low pressure membranes (i.e., MF and UF) are some kind of different from those of high pressure membranes (i.e., NF and RO).
• Nevertheless, it should be clarified that depending on feed water compositions and their interactions with membrane, both surface fouling and internal fouling can be irreversible.

2.2 Organic fouling

• Just as its name implies, organic fouling is caused by organic matters.
• These organic matters usually consist of humic substances, polysaccharides, proteins, lipids, nucleic acids and amino acids, organic acids, and cell components (Cho et al., 1999; Jeong et al., 2016).
• The fouling mechanism of BSA for RO is different with other membranes such as MF.
• The fouling behaviors of alginate and humic acid are similar to that of BSA.
• As revealed by Table 1, the contributions of different organic matters on RO fouling could be different in different situations, with one kind of organic matter being the dominant foulant in one situation but replaced by another organic pollutant in another situation.

2.3 Inorganic scaling

• Inorganic scaling is the deposition of inorganic substances on membrane surface or inside the membrane pores (Henthorne and Boysen, 2015; Khayet, 2016).
• To be specific, the inorganic ions in water which exceed the equilibrium solubility product firstly reach the nucleation stage, and then go through homogenous or heterogeneous crystal growth processes (Al-Amoudi and Lovitt, 2007).
• Statistical analysis revealed that calcium sulfate and calcium carbonate were most studied as inorganic scalants by researchers in the past 10 years, indicating their important roles in causing RO inorganic fouling (shown in Fig. 5).
• There are many factors which could affect inorganic scaling on membrane, such as membrane traits, compositions and features of feed water as well as operating conditions (Rabie et al., 2001; Shaalan, 2003).
• Basically, the compositions of salt deposits on RO membranes are determined by inorganic compositions in feed water, chemicals added during pretreatment, as well as the chemical properties of the sparingly soluble inorganic salts (Schneider et al., 2005).

2.4 Colloidal fouling

• Colloids are fine suspended particles, the size of which ranges from a few nanometers to a few micrometers, although some references define the size of colloids ranges from one nanometer to one micrometer (Al-Amoudi and Lovitt, 2007; Zhu and Elimelech, 1997).
• The common colloidal foulants can be divided into two types, i.e., inorganic foulants and organic macromolecules.
• Colloidal fouling could be influenced by many factors such as the colloids size, shape, charge as well as interactions with ions of the colloids (Buffle et al., 1998).
• The charge properties of polyamide based membranes can be effected by cations such as calcium and magnesium (Wang et al., 2014).
• The frequency of particle collision and the attachment coefficient could decide the rate of colloidal aggregation, while the coefficient is the reflect of the energy barrier that results from the summation of the van der Waals force and the electrostatic interaction force (Tang et al., 2011).

3. Membrane fouling control strategies

• Through improving hydrodynamics of the filtration process, membrane fouling could be reduced.
• A 13 / 41 detailed discussion on hydrodynamics can be found in another review paper (She et al., 2016) and is not the focus of this paper.
• Generally, the difficulty to mitigate fouling is different depending on fouling types.
• Inorganic scaling could be easily reduced through chemical and physical methods.

3.1 Pretreatment technologies

• Pretreatment has been widely used in RO systems and it has the advantage of improving the feed water quality greatly to ensure reliable RO operation as well as to prolong membrane life.
• As shown in Fig. 6, UF, coagulation/flocculation and MF are the three technologies that have been most studied by researchers as RO pretreatment methods.
• Following coagulation is flocculation, which is a slower mixing stage of microflocs to form larger visible particles and then these macroflocs can be removed by sedimentation, flotation or filtration.
• Activated carbon can effectively remove free chlorine, a common chemical existing in tap water after water chlorination (Jamaly et al., 2014).
• If pretreatment combinations and steps before RO are not selected properly, then more contaminants could reach to RO membrane surface.

3.2.1 Membrane monitoring

• Normalization of bulk observations of pressure, flow and conductivity is reported to be the most effective way for insitu and real-time monitoring the RO performance (Hu et al., 2013).
• Besides, as RO membranes developed in recent years possess high permeability and low resistance, it is becoming a less appropriate method to detect fouling by flux decline.
• Besides UTDR, ex-situ scale observation detector , which uses high resolution digital photography to detect scale crystals before flux decline happens, is another real-time monitoring technique that has a great potential for industrial applications (Hu 18 / 41 et al., 2013; Malaeb and Ayoub, 2011; Uchymiak et al., 2007).
• Another monitoring technology gaining popular recently is the electrical impedance spectroscopy (EIS), a novel non-invasive method to monitor the membrane fouling process (Antony et al., 2013; Chilcott et al., 2015; Jing et al., 2016).

3.2.2 Membrane cleaning

• Periodic membrane cleaning is of great importance during water and wastewater treatment processes (Sadhwani and Veza, 2001; Yang et al., 2013).
• The commonly used chemical agents include acids, bases, surfactants and chelating agents (Varin et al., 2013).
• The cleaning efficiencies by different chemical agents treating different foulants have been investigated widely (Piasecka et al., 2015; Ramon et al., 2013; You et al., 2016).
• Lee and Elimelech (Lee and Elimelech, 2007) found that in the presence of calcium, sodium chloride was a very effective salt in removing the alginate gel layer formed 20 / 41 on the RO membrane surface where alkaline cleaning was not effective.

3.3.1 Surface modification

• Membrane fouling in RO systems is closely related to surface characteristics (Saqib and Aljundi, 2016).
• But it should be clarified that in certain situations hydrophilic membranes are more inclined to attract hydrophilic substances and thus induce fouling (Kwon et al., 2005).
• Surface modification is a very common strategy used to reduce membrane fouling (Cheng et al., 2013; Jee et al., 2016).
• Indicated by green and red color, respectively (Saeki et al., 2014).
• Moreover, a lot of chemicals were used and their side effects to human beings and the environment were less understood.

3.3.2 Novel membrane materials

• Polyamide thin-film composite (TFC) RO membranes have long been the dominant RO membranes used in practice (Cohen-tanugi and Grossman, 2015).
• Fig. 11 shows novel RO materials that are most studied over the past 10 years.
• The left bar represents publications of related materials without testing their fouling performance while the right bar means publications of related materials whose fouling performance were studied.
• 24 / 41 CNTs have a great potential in increasing membrane surface hydrophilicity and reduce membrane fouling (Vatanpour and Zoqi, 2017).

4. Concluding remarks

• RO membrane technology is one of the best technologies for wastewater treatment and desalination.
• Depending on feed water qualities, operation conditions and membrane characteristics, one or several types of fouling could occur, such as biofouling, organic fouling and inorganic fouling.
• Sometimes there are no distinct boundaries between these foulants and they are interconnected or synergistic.
• Ongoing research on fouling behaviors are needed to gain a better understanding of fouling mechanisms, which could provide better foundation for improvement or even revolutionary development of fouling control strategies.
• Currently there are a variety of fouling control techniques that have been applied in practice (e.g., membrane pretreatment, membrane monitoring and cleaning, membrane surface modification) and these techniques are playing a very important role in RO fouling mitigation.

Figures

Table 1

Table 2 RO membrane surface modifications via different chemicals/polymers and methods

Fig. 7. Schematic diagram of RO pretreatment processes and their roles in fouling control

Fig. 6. Common studied RO pretreatment technologies in the past 10 years

Fig. 3. Factors affecting bacteria attachment to membrane surface

Fig. 11. Common studied novel RO materials over the past 10 years

Fig. 4. Cumulative number of publications related to three common RO organic foulants

Fig. 9. Schematic diagram of membrane surface smoothness and hydrophilicity

Fig. 2. SEM of four fouling types on membrane surfaces. (A) Biofouling. (B) Organic fouling. (C) Inorganic scaling. (D) Colloidal fouling. Adapted from (Ho et al., 2016; Hu et al., 2013; Shafi et al., 2017; J. Xu et al., 2013).

Fig. 5. Common studied inorganic foulants for RO in the past 10 years

Fig. 10. Microscopic images of bacteria on non-coated and coated membranes after the bacterial adhesion test. In the CLSM images (C and F), the living and dead bacteria were

Fig. 1. Number of publications per year and cumulative number of publications on RO fouling over the past 25 years

Frequently asked questions

Q1. What are the contributions mentioned in the paper "A review of reverse osmosis membrane fouling and control strategies" ?

This paper reviews membrane fouling types and fouling control strategies, with a focus on the latest developments. The fundamentals of fouling are discussed in detail, including biofouling, organic fouling, inorganic fouling and colloidal fouling. Pretreatment is widely used in practice to reduce the burden for the following RO operation while real time monitoring of RO has the advantage and potential of providing support for effective and efficient cleaning. Especially in this review paper, statistical analysis is conducted where appropriate to reveal the research interests in RO fouling and control. Furthermore, fouling mitigation technologies are also discussed comprehensively. Surface modification could slow down membrane fouling by changing surface properties such as surface smoothness and hydrophilicity, while novel membrane materials and synthesis processes build a promising future for the next generation of RO membranes with big advancements in fouling resistance. 

Q2. What are the future works in "A review of reverse osmosis membrane fouling and control strategies" ?

Although there are still many challenges, novel membrane materials and synthesis processes provide a promising solution for solving fouling problem and future research in this topic is expected to produce fruitful findings.