Fouling in reverse osmosis membranes: monitoring, characterization, mitigation strategies and future directions

Abstract

Water scarcity has been a global challenge for many countries over the past decades, and as a result, reverse osmosis (RO) has emerged as a promising and cost-effective tool for water desalination and wastewater remediation. Currently, RO accounts for >65% of the worldwide desalination capacity; however, membrane fouling is a major issue in RO processes. Fouling reduces the membrane's lifespan and permeability, while also increases the operating pressure and chemical cleaning frequency. Overall, fouling reduces the quality and quantity of desalinated water, and thus hinders the sustainable application of RO membranes by disturbing its efficacy and economic aspects. Fouling arises from various physicochemical interactions between water pollutants and membrane materials leading to foulants' accumulation onto the membrane surfaces and/or inside the membrane pores. The current review illustrates the main types of particulates, organic, inorganic and biological foulants, along with the major factors affecting its formation and development. Moreover, the currently used monitoring methods, characterization techniques and the potential mitigation strategies of membrane fouling are reviewed. Further, the still-faced challenges and the future research on RO membrane fouling are addressed.

1. Introduction

Water occupies more than 70% of the earth's surface, wherein more than 97% is saline water. Far too little freshwater is found in rivers and lakes; i.e., less than 0.007% of the global water is available for possible domestic use, showing the scarcity of conventional sustainable freshwater sources [1]. During the past 40 years, the use of water has been quadrupled as a result of duplication of the world's population, which reached 8.009 billion people by January 2023; therefore, an extra annual supply of clean and low-salinity water is a must for normal agricultural, industrial, and domestic uses [1,2]. The agricultural and industrial sectors account for about 93% of the total global water consumption, whereas domestic uses account for the remaining 7% [[1], [2], [3]]. (Fig. 1), illustrates the expected water scarcity on the earth, by 2025 [4], where, approximately two-thirds of the world population will live in water-stressed areas, while over 1.8 billion people will suffer from clean water scarcity; however thereafter, these numbers are expected to rise dramatically [5]. Therefore, researchers are constantly looking for sustainable and cost-effective sources of clean and potable water [[6], [7], [8], [9]]. Saline waters of the oceans and seas account to >97% of the earth's water and represent a huge water reservoir that should be used effectively to overcome the current global water crisis [5,10]. Therefore, desalination represents the most efficient and cost-effective technique to overcome the global water stress [11]; however, wastewater reclamation is a feasible option for regions lacking brackish or seawater resources, to overcome the water shortage [12,13].

Fig. 1.

Projected water scarcity in 2025 [4].

Many cities in various countries now have large-scale desalination treatment plants [14,15]. Desalination production capacity has increased dramatically over the last decade, with an annual increase of about 7% from 2010 to 2019 [16]. Currently, There are about 18,000 desalination plants spread all over the world, with a total capacity about 99.8 million m³/day [17]. Membrane-based technologies, like nanofiltration (NF) and reverse osmosis (RO), and pressure retarded osmosis (PRO) are most widely used; however, thermal operations like multi-effect desalination (MED) [18], and multistage flash distillation (MSF), as well as new methods like membrane capacitive deionization (MCDI) and microbial fuel cell (MFC) have been recognized as promising processes to obtain sustainable clean water [19]. RO desalination processes have advanced considerably in the past few decades and have largely dominated the desalination field. Currently, RO desalination plants represent >65.5% of the overall worldwide desalination capacity. Fig. 2a illustrated share capacity of global desalination by process, revealing the large capacities produced through RO [20]; while (Fig. 2b) showed percentage of feed-water type in the desalination operation over the world. The major advantage of desalination plants based on RO is the energy-efficiency, where the energy consumption has been decreased, (Fig. 2c), from more than 15 kW h/m³ in the 1970s to less than 3 kW h/m³ after the 2000s [21]. Several approaches have been developed to reduce the energy consumption of RO plants, including the use of energy saving/recovery equipment, improving the separation efficiency of RO polyamide thin film composite (PA-TFC) membranes, and optimizing desalination plant design, e.g., by adopting staged desalination [21,22]. Further, guided by the environmental protection and public health regulations, RO has a dual feature of maintaining high water productivity and extremely large rejection rates for approximately all pathogenic species, inorganic, and organic pollutants [23]. Furthermore, thin-film membrane composites in common RO plants are ordered in spiral-wound systems to make the RO plant highly compact [24]. The RO membrane is typically composed of three layers: an active polyamide layer of 0.2 μm thickness, followed by layers of polysulfone and polyester fabrics of 40 and 120 μm in thickness, respectively [25]. The polyamide layer is responsible for the membrane selectivity, while the polysulfone interlayer allows the polyamide to withstand high pressures. The polyester layer, on the other hand, is too porous and irregular to serve as a direct backing for the active polyamide layer [26]. Despite the significant advances in membrane technology, the unavoidable fouling of membranes remains a major challenge for all membrane-based operations [5,27,28]. Membrane fouling has several adverse effects on the performance of RO plants, including deterioration of permeate quality, decreased permeability, and increased operation costs due to increased energy consumption, higher pressure requirements, increased cleaning of membranes, additional pretreatments, and a decrease in membrane lifespan [[29], [30], [31]]. For example, the annual costs of preventive measures to mitigate biofouling phenomena in the desalination industry are approximately US$15 billion worldwide [32]. Depending on the feed water type and quality, space design and membrane nature, chemical dosages, and plant design and operating parameters, one or more types of fouling can deposit on the surface of a membrane [33]. Colloidal matter accumulation, organic fouling, mineral deposition, and the formation of a biofilm comprised of a mixture of microorganisms and organic matter are the most common types of fouling in RO desalination plants [[34], [35], [36], [37]]. Determining the causes, development mechanisms, and nature of fouling layers is critical for taking appropriate corrective measures aimed at reducing fouling potential and, as a result, the overall economic costs of desalination operations. In general, the type of fouling can be identified using a non-destructive technique such as observing changes in plant performance, or a destructive technique such as autopsy of fouled membrane. An autopsy technique includes a variety of physical and chemical analyses of the membrane. Each analysis provides valuable information that can greatly aid in determining the causes of membrane deterioration [38,39]. Several studies discussed the autopsy of the membranes in RO plants [34,38,39]. An autopsy of a fouled membrane extracted from a full-scale plant fed with brackish water (BW), for example, revealed polysaccharides, organic-Al-P complexes, and aluminum silicates as foulants [40]. Another study reported that organic matter, with large amounts of iron, nitrogen, and silicates were the main cause for the membranes deterioration [41]. Despite its destructive nature, autopsies of fouled membranes lead to significant lowering in operating costs [42]. Thus, various methods are adopted to reduce fouling issues, such as novel RO membrane development, effective membrane cleaning and surface modifications, and pretreatment enhancement [[43], [44], [45], [46]]. The proper selection of pretreatment stages is critical when treating various types of waters that are used to remove dissolved organics, suspended solids, and pollutants from feed water. The pretreatment stages are determined by several factors, including feed solution quality, membrane material, recovery ratio, and frequency of membrane cleaning [47]. The pretreatment stage may be conventional like disinfection, scale inhibition, coagulation, and flocculation or non-conventional like nanofiltration (NF), microfiltration (MF), and ultrafiltration (UF) [48]. The cleaning strategy is commonly used to eliminate the foulants layer from pollutant membranes and consequently recover the unit performance [28]. Although significant efforts have been made to optimize the pre-treatment stages and design the membranes to prevent foulant problems, these methods do not completely eliminate them, necessitating the use of the cleaning process on a regular basis. Cleaning is typically used to restore unit efficiency when normalized permeate data is reduced by about 10%, differential pressure is increased by 15%, or product salinity is increased by 10% [49]. The selection of the appropriate cleaning depends on the composition of foulant layers, operating period, membrane type and structure and configuration of RO system [33]. Moreover, applying ineffective cleaning strategies will lower the membrane lifespan and increase its frequency of replacement, as well as increase the operational and maintenance costs. The fouling issue was related to the surface properties of the membrane; e.g., surface hydrophilicity and smoothness [50]. Modifying the surface of the membrane is one of the most promising methods that can help reduce the problem of fouling. Smooth and hydrophilic membrane surfaces show lower fouling potential than those with rough and hydrophobic membrane surfaces [50]. Several innovative modifications have been developed to customize and alter the surface structure and membrane properties including for example but not limited to the incorporation of nanomaterials or modification of membrane surface through chemical or physical methods. In practice, more than one mitigation strategy is usually used to lower fouling potential to the RO system [[51], [52], [53]]. According to SciFinder search when using fouling and reverse osmosis as keywords, over 4650 articles have been published in the past 23 years to discuss fouling and reverse osmosis, (Fig. 3). There were 77% journal articles, 15% patents, 10% conference and meeting documents and only 0.4% dedicated reviews among these new publications, indicating the high interest in this field. As a result, it was critical to conduct an up-to-date review of the RO fouling issue and its control strategies. Furthermore, according to the results of Scopus database searches, there is a high level of research interest in RO membrane fouling, foulant types and characterization methods, and membrane pretreatment steps, cleaning protocols, and modification methods, as shown in Table 1. The basic search string was “(fouling or foul*) and RO”; however, various additional strings were searched in the presence of the basic search string, and gave the indicated number of documents as shown in Table 1. These searches made the basis of the current review theme.

Fig. 2.

Global desalination capacity (a) according to the used technology, (b) according to feed water source [239], and (c) Improvement in power consumption for reverse osmosis seawater desalination over the past decades [240].

Fig. 3.

Number of publications during the last 22 years using SciFinder data, (search words used: fouling and reverse osmosis).

Table 1.

Scopus database searches for RO membrane fouling, foulant types and characterization methods, and membrane pretreatment steps, cleaning protocols, and modification methods.

Search string	No. Of documents
Basic search string: “(Fouling OR foul*) and RO”	4123
Basic search string plus foulant type's string:
And organic	1426
And colloidal fouling	260
And biofouling or Bio-fouling	926
Scale or scaling	1273
And antiscal*	160
Basic search string plus characterization method's string:
And scanning electron or SEM	368
And transmission electron or TEM	477
And atomic force or AFM	145
And EDX	57
And confocal microscop*	55
And FTIR	183
And Zeta potential	173
And XRD	54
And XPS	84
And XRF	19
And Fluorescence	99
And contact angle	283
And organic carbon	375
Basic search string plus pretreatment/cleaning's string:
And membrane pretreatment	804
And membrane cleaning	826
And physical cleaning	118
And chemical cleaning	443
Basic search string plus sample modification's string:
And membrane modification	367
And thin film nanocomposite	148
And Graphene	97
And carbon nanotubes	61
And nanoparticles or NPs	175

The primary goal of this review is to use cutting-edge literature to provide a comprehensive coverage of fouling development and treatment in desalination technology. A thorough understanding of the factors that contribute to the causes of this problem will aid in the development of promising strategies to reduce or treat the fouling problem. The techniques used to identify and characterize foulant layers via autopsy of foulant membranes are discussed. We also discuss recent fouling remediation or mitigation strategies, such as cleaning, surface modification, and pretreatment enhancement. Finally, we hope that this review will assist researchers in understanding the various types of pollution and developing effective and cost-effective solutions to pollution problems, thereby assisting in the provision of sustainable sources of fresh water. Of course, the current review does not purport to cover all published papers on the subject; therefore, we sincerely apologize for any missed useful articles that were not included in the current review; however, some representative articles were included based on their featured ideas/data and ability to fit into the review's theme.

2. Membrane fouling

Fouling issue is a main limiting challenge that faces RO applications, which must be taken into consideration while designing and operating RO plants as it affects pre-treatment requirements, operating conditions, cleaning requirements, performance, and overall economic cost [54]. Membrane fouling depends on several factors including feed water quality, membrane features, and operational conditions (Fig. 4). Fouling problems in the RO unit reduce water permeability by partially blocking the membrane surface and pores, resulting in additional flow resistances as evidenced by a decrease in permeability at constant applied pressure or an increase in pressure to maintain a flux constant [55]. Since water is an essential component of the desalination operation and can contain many sources of pollution, it is vital to understand the water behavior as well as how ions are transported via membranes, which could illustrate how membranes fouling happen. Water permeation via the membrane could occur in the form of flush, jump-diffusion, and Brownian diffusion [56]. The rejected particles or solutes that accumulate near the membrane's surface may remain as singular entities in the formed concentrated layer due to repulsive forces among solute particles or because the particles or solute concentration is still low. Nevertheless, as particles or solute concentration increases, some particles or solute can react with each other or with the membrane's surface, resulting in the formation of a more structured “cake” layer [57]. As shown in (Fig. 5a, d), fouling can form on the membrane surface or cause pore clogging. Surface fouling is more likely than internal pore fouling due to the membrane's nonporous and compact nature. Surface fouling is easier to control than internal fouling (internal pore clogging) based on chemical cleaning and/or manipulation of feed water hydrodynamics [58]. In terms of foulants' nature, it can be divided into scaling, biological fouling, colloidal and organic fouling. The below subsections give a brief of the different fouling types and their effects on the membrane surface features and performance through seawater or brackish water desalination operation.

Fig. 4.

Factors affecting RO membrane fouling [241].

Fig. 5.

Possible mechanisms of fouling; (a) Pore-clogging, (b) partial pore-clogging, (c) internal pore-clogging, and (d) cake formation [172].

2.1. Organic fouling

Organic fouling can be defined as the accumulation of carbon-based species on a desalination membrane. There are different types of organic matter that can be present in the feed water, including natural organic matter (NOM) that is a mixture of fluvic acids (FAs) and aquatic HAs and other NOM coming from the decay of animal and plant material [59]. Organic material is often completely reactive, and the hazard that it forms as a foulant depends on many things, including its affinity to the membrane substance. The organic fouling issue can be minimized by applying an initial treatment of raw water, and/or choosing a membrane material that has the ability to reduce the adsorption of organic compounds on the membrane [60,61]. The accumulation of organic compounds on the surface of the membrane can be depicted by the model of cake filtration [62]. In the first step, the organic foulant materials are adsorption on the surface of the membrane, which leads to raised membrane hydrophobicity and decreased filtration area. Subsequent deposition of organics on membrane surfaces generates a fouling layer leading to an increase in membrane hydraulic resistance. Many studies were performed to examine the factors affecting the formation of organic fouling during RO desalination operation [63,64]. It has been observed that hydrodynamic operating parameters, the chemistry of feed water, and membrane features are the main factors affecting the organic fouling issue of RO membranes. Among these factors, concentrations of divalent cations and the pH value of feed water greatly affect the flux-drop behaviors during the formation of organic fouling [63,64]. Organic materials commonly possess functional groups like hydroxyl (−OH) and carboxyl (−COOH) groups which are greatly related to the concentration of divalent cation and pH value of feed water. The organic fouling is commonly accelerated by reducing pH value and raising the concentration of divalent cation [65]. Thus, it was noticed that the extent and rate of humic acid and protein adsorption are significantly affected by the pH value of the feed water [66]. Humic acid and protein adsorption onto the membrane are reduced as the pH raises due to raised electrostatic repulsion forces [66,67]. Other works reported more permeate flux drop in the presence of high concentrations of Ca²⁺ ions [68,69]. The impact of magnesium ion concentration on the organic fouling issue was also explored [65,70]. Membrane fouling raises slightly with magnesium ions compared to calcium [71]. Another study illustrated the higher intermolecular interactions of magnesium ions with humic compounds compared to alginates [18]. The bridging between organic matter and divalent cations usually results in the formation of a compacted layer and, as a result, a significant decrease in flux [72]. As a result, the membrane surface characteristics are important in controlling membrane fouling and the permeate flow tends to decrease as the membrane surface becomes more hydrophobic, rougher, and/or accumulating negatively charged or neutral foulants.

2.2. Colloidal fouling

Colloids are a major class of membrane foulants that are formed by the accumulation of particles with sizes ranging from 1 to 1000 nm; which are small enough to diffuse through different pre-treatments stages but still large enough to be retained and deposit on the surfaces or pores of membranes and adversely affect both the quality (solute concentration) and quantity (flux) of the permeate water [11,55]. Colloidal fouling can arise from organic macromolecules and/or inorganic foulants. The inorganic colloidal foulants include colloidal silt, silica, kaolin, clay, and iron hydroxides/oxides while colloidal organic macromolecules mainly arise from proteins, polysaccharides, and other natural organic matter [73]. These colloidal pollutants suspended in the feed water can be assessed by silt density index (SDI) or turbidity index. Colloidal fouling is usually influenced by several factors like the colloids shape, size, and charge, as well as the interactions with other colloids ions [74]. It is believed that the major mechanism by which colloidal contamination affects the performance of the membrane is the cake-enhanced concentration polarization [75]. In this regard, a porous cake layer increases concentration polarization near the membrane's surface, which raises the osmotic pressure and, as a result, decreases the driving force for product water passage, resulting in flux decline [76].

2.3. Bio fouling

The most common and problematic issue that osmosis units face is biofouling, because even with extremely efficient pre-treatment operations that can remove 99.9% of all microorganisms, there are still enough cells that can colonize and produce a biofilm, reducing permeate flux and forming irreversible fouling [77]. A biofilm issue can be formed on the feed-spacers’ surfaces of RO membranes. Fig. 6 depicts the mechanism of biofouling development. Prior to bacterial attachment, a "conditioning film” is formed on the membrane surface as a result of the fast adsorption of NOM and other biogenic materials. Microorganisms exist in approximately all water systems where they stick to surfaces and feed primarily on the nutrients present in the water. Various kinds of microorganisms can contribute to the development of biofilms, such as fungi, bacteria, microalgae, and protozoa. Nevertheless, there is no typical biofouling organism since all have the ability of adhesion to membrane surfaces based on various factors such as the microbial type, the chemistry of the membrane, and the quality of the feed water. The stuck microorganisms that are stuck produce extracellular polymeric compounds (EPS) in which they are embedded, and produce biofilms [78]. As a result, the dissolved nutrients become immobilized and transit from a liquid to a semisolid state. EPS can contain a variety of substances depending on the bacteria communities and water environments, but it is most commonly composed of proteins, lipoproteins or lipids, polysaccharides, glycoproteins, and nucleic acids [37,79]. The biofilm thickness increases at high nutrient concentrations due to extremely high microorganism growth, which cannot be reduced by simple detachment [80]. In this case, a chemical cleaning strategy can reduce the thickness of the biofilm. However, after a certain stage of biofilm development, chemical cleaning strategies cannot completely remove it, and the biofouling problem becomes partially irreversible. The biofouling problem can have a number of negative effects on membrane systems, including a decrease in membrane flux, increased differential pressure drops, and membrane biodegradation caused by acidic compound byproducts that are concentrated at the membrane surfaces; for instance, cellulose acetate membranes have been found to be more liable to biodegradation [44].

Fig. 6.

Mechanism of biofouling development [147].

2.4. Mineral scales

RO membranes are capable of rejecting extremely high rates of inorganic salts. In seawater, the recovery rate can range from 35 to 45%, while in brackish water; it can reach more than 70–97%. Nevertheless, a high recovery ratio raises the concentration of the sparingly dissolved salts in the water, increasing the likelihood of their precipitation on the membrane surface and ultimately decreasing the RO operation efficiency [81]. Scales form in the desalination plant whenever the ionic product of sparingly dissolved salts in the concentrated flow equals or exceeds its solubility product. The extent and degree of scaling phenomena are determined not only by the supersaturation conditions that occurred, but also by the precipitation kinetics [82]. For example, even if supersaturation exceeds a critical value and nucleation for precipitation begins, the crystal formation rate will be slow if nucleation sites are scarce [83]. The likelihood of formation of mineral scales is enhanced by the phenomenon of concentration polarization (CP) that results in elevated particles concentration of dissolved salts in the vicinity of membrane surface when compared to the bulk of solution [84]. The riskiness of concentration polarization depends upon the rejection rate of salts that form scales; thus, almost all nonporous and dense membranes such as those of reverse osmosis and nanofiltration are most influenced by this phenomenon [85]. The concentration polarization increases the probability of several consequences such as deterioration of produced water quality, higher potential of scaling, a decline of product flux because of rise of osmotic pressure [86]. Among the common types of scaling in the desalination process are calcium carbonates, barium sulfate, calcium sulfate, calcium phosphate, and silica. Fig. 7 schematically illustrates the essential steps in the formation of mineral scales.

Fig. 7.

Schematic illustration of essential steps in the scales formation [242].

3. Methods of fouling characterization

Fouling lowers the separation efficacy of RO membranes as it affects both the surface characteristics and its pores. To better understand the mechanisms of fouling development in RO membranes and limit their effects, various measurement techniques have been used to characterize the foulant. Membrane autopsy is a destructive method that has been widely used to examine the nature and formation of foulants on the membrane top surface using surface characterization techniques. An autopsy can determine the real nature of the foulant, and with thorough characterization and analysis, future pretreatment, selection of membrane type, and clean-up strategies can be properly adopted. In order to carry out the autopsy, the fouled membrane is extracted from unit, and samples are taken from various places and stored in a cool environment until the analysis is carried out. Physical, chemical, and biological characterization approaches can provide insight into the underlying processes governing membrane fouling.

3.1. Electron microscopy techniques

To study the morphological structure of clean and fouled membranes, various electron microscopy techniques can be applied such as atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), etc. These approaches can provide large information that can be used to determine the type and source of foulants that have been formed on the membrane's surface.

3.1.1. Scanning and transmission electron microscopy

SEM is a popular technique for studying the cross-section and surface morphology of samples at the microscopic level [87]. In order to be examined by SEM, foulants do not need to be extracted from the membrane. Energy dispersive X-Ray Spectroscopy (EDX) is often combined with SEM and these techniques can give a detailed perception of the chemical composition, structure, size, and shape of foulant and membrane material. This can be applied to characterize fouling layers with a thin thickness, such as membrane scaling, and microbiological fouling, as well as identifying the physical damage and degradation of the membrane, e. g., abrasion marks from the spacer and membrane. Thus, SEM-EDX has been used to investigate the surface features of two membranes extracted from two plants with various feed waters [88]. The first membrane's surface was covered by inorganic particles that aggregated into various shapes, and EDX analysis revealed the presence of silicon and aluminum, whereas analysis of the second membrane revealed the presence of microbial clusters not only on the membrane surface but also on the feed spacer. In another study, significant difference in the thicknesses of the clean, moderately fouled and heavily fouled membranes were observed, as shown in (Fig. 8a–c) [41]. The membrane's filtering layer thicknesses were 49 μm, 38 μm and 25 μm for the clean, mildly fouled and fully fouled membranes, respectively, showing that the filtering layer thickness of the clean membrane was roughly double that of the fully fouled membrane. These results suggest that the membranes become compressed when present under pressure due to foulant layers. As the applied pressure rises, the polymers rearrange into a tighter structure and result in a decreased porosity with a subsequent reduction in the membrane system's efficiency. SEM micrographs, (Fig. 8 d, e), revealed the presence of abrasion marks caused by corrosion drag on the membrane surface [89]. Several studies used this method also to compare the effectiveness of various cleaning strategies [90,91]. TEM microscopy can be used as a supplement to SEM, especially when the fouling is extremely small and cannot be detected by SEM; moreover, TEM can also be used to investigate the presence of fouling inside membrane pores [92]. TEM was also used to investigate the foulants collected from RO membranes, which were made up of inorganic scaling and organic fouling [93]. In another study, TEM image analysis revealed that organic foulant-humic materials had not penetrated the polyamide layer of the membrane material [94].

Fig. 8.

(a–c)_SEM images of cross-sections of three membranes [41], (d, e) SEM images illustrating the presence of abrasion marks on membrane surfaces [89].

3.1.2. Atomic force microscopy (AFM)

Recently, atomic force microscopy (AFM) has become one of the fruitful tools for the development of membrane industry, as it helps to understand in depth the causes of membrane fouling [95]. AFM measurements provides information on pore size distribution, membrane surface roughness, fouling distribution, and membrane surface thickness before and after membrane cleaning processes [96]; additionally, AFM investigates the early stages of bacterial colonization that causes biofilm formation [97], as well as the effectiveness of membrane cleaning by inspecting the surface structure with high accuracy through three-dimensional analyses [98]. Several studies [99,100] have used AFM measurements to characterize the surfaces of fouled and virgin membranes, as well as comparing the changes in morphological characteristics of fouled membranes at the first and second stages [101]. During both stages, AFM images revealed the formation of fouling layers on membrane surfaces, but the fouling was more severe in the second stage [101]. Furthermore, AFM was used to investigate the changes in the morphology of fouled membranes after consecutive cleanings with alkali and acid [98]. The roughness values and AFM images illustrated that the surface of membranes recovered significantly after the second alkaline cleaning, but not to their initial state [98]. In addition, AFM was used to differentiate between physically and chemically damaged membranes through observation of the changes in roughness and topography compared to a virgin membrane (Fig. 9a, c) [102]. In case of physical damage, the difference between the valleys and maximums on the membrane was significantly higher than those of a virgin membrane, which appeared smoother, as shown in (Fig. 9b). On other hand, the chemically damaged membrane has a relief more similar to the virgin membrane (Fig. 9c), but with the much higher and wider distances between the maxima and valleys.

Fig. 9.

AFM data of (a) virgin, (b) physically, and (c) chemically damaged membranes [102].

3.1.3. Confocal laser scanning microscopy

CLSM is a non-destructive approach for in situ membrane visualization [103]. It is a very helpful technique in the field of fluorescence scanning and imaging for getting high-resolution images [104]. In CLSM a laser beam is focused on a small focal volume of the target surface, and the reflected light is collected and detected via a photodetection device, where the detected signal is converted to images and monitored on a computer. CLSM combined the laser scanning technology with 3D identification of biological objects identified with fluorescent markers to acquire images of membrane biofouling [105]. The membrane surface was stained with live/dead viability kit before measurements. The obtained results gave an estimated biovolume of 12 μm³/μm², where live and dead cells made up 37.3 and 62.7%, respectively, of this biovolume [105]. Dead cells were usually found in high-permeability areas of the membrane (valley regions), whereas live cells were found in lower-permeability areas (ridge regions) [105]. Further, CLSM was used to visualize the biofilms on various surfaces of the RO elements [106]; while, other studies used CSLM to investigate the membrane cleaning efficiency [107,108].

3.2. FTIR-ATR analysis

Fourier transform infrared spectroscopy (FTIR) can be coupled with attenuated total reflectance (ATR) and is widely applied to identify and characterize both inorganic and organic compounds in solids and liquid samples [12,109]. For example, FTIR was used to investigate fouled RO membrane extracted from a plant in Australia after nearly 1 year of operation in a water treatment facility [110]. Various observed bands at 1078, 1563, 1631, 2920, and 3428 cm⁻¹ were assigned to C–O stretching of polysaccharides, C–N stretching of amide II, C–O stretching of amide I, aliphatic C–H stretching, and OH- stretching, respectively. The band around 1400 cm⁻¹ can be attributed to C–O stretching, aliphatic C–H deformation, and O–H deformation of phenol. In addition, observed bands in the range 600–800 cm⁻¹ can be attributed to aromatic compounds. The obtained results indicated that the fouling layer was made up of polysaccharides, proteins, and aromatic and aliphatic compounds [110]. However, the foulant collected from another membrane surface was investigated and exhibited bands at 715 cm⁻¹, 871 cm⁻¹, 1066 cm⁻¹, and 1145 cm⁻¹ due to the presence of carbonate precipitation [34]. In addition, the IR spectra of two fouled membranes and the foulant collected from their surfaces as well as the IR spectrum of the washed membrane are shown in (Fig. 10), [88]. These spectra exhibited characteristic bands revealing that one membrane had a large amount of aluminosilicate (3695, 3620, and 469 cm⁻¹) and silica (1025 cm⁻¹) deposits, while the other membrane suffered from biogenic foulants; i.e., proteins (Amide I, II, 1652, 1548 cm⁻¹), aminosugars (CH₃ of acetyl group, 1380 cm⁻¹), DNA/RNA/free phosphate/phospholipid (>P=O, 1240 cm⁻¹), sugars (C–O, 1070 cm⁻¹) and lipids (CH₃, and CH₂, 2960, 2925, 2850 cm⁻¹) [88].

Fig. 10.

ATR-FTIR spectra of two fouled membranes and the foulant materials [88]. Membrane A is mainly fouled by aluminosilicates and silica; membrane B is fouled by biogenic materials of proteins, aminosugars, DNA/RNA/free phosphate/phospholipid, sugars and lipids.

3.3. Zeta potential measurement

The zeta potential is a crucial metric in defining membrane electrokinetic phenomena. The zeta potential value is estimated by measuring the difference in potential between the charged surface and the dispersion medium [111]. Several factors affect the value of the zeta potential including concentration, pH, and composition of the solution temperature, as well as the surface properties of the membrane (i.e., charge, chemical heterogeneity, roughness, etc.) [111,112]. Several studies examined the zeta potential value of virgin and fouled membranes and the removed foulants [11,113]. For example, the zeta potential values of a virgin and fouled membrane were found to be −24.0 and −3.1 mv, respectively [91]. Furthermore, other works used zeta potential measurements to evaluate the membrane cleaning efficiency [91,113,114].

3.4. XRD analysis

X-ray diffraction (XRD) analysis is used to identify the features of crystalline compounds in membrane foulants. It can provide important information about crystal size, structure, shape, among other features. The method can be applied to collected foulants from membrane surfaces or directly to the foulant adhering to the membrane. For example, XRD analysis of the foulant collected from sea water reverse osmosis (SWRO) membrane indicated the presence of quartz and dolomite in the foulant, which passed from the primary treatment and was deposited on the membrane surfaces [11]. However, in other study the XRD patterns of virgin and partially fouled membranes revealed the presence of hydrogen aluminosilicate and halite (NaCl), where the majority of foulants were amorphous and cannot be fully characterized by XRD measurements alone [115].

3.5. XPS analysis

XPS is a quantitative technique that can provide information of empirical formula, and the electronic and chemical states of the elements that present within a material. The main target of XPS here is to determine the type of foulants and their oxidation states, as well as examining the possible degradation of membrane by oxidizing agents or by-products resulting from bio-fouling. Thus, in a pilot study, XPS analysis explored the reason of degradation of three membranes made of polyamide and cellulose acetate [116]. The data revealed that the cellulose acetate membrane had been damaged by biological activity, as evidenced by the presence of nitrogen on XPS scans, whereas the polyamide membrane had been damaged by chlorine oxidant, as evidenced by C–Cl bonds [116]. Further, XPS analysis identified calcium phosphate as the mineral scale deposits in the last membrane element [117]. In addition, XPS scans revealed nitrogen, carbon, sulphur, and oxygen as the main constituent elements of the virgin RO membrane's polysulfone support and active PA layers; however, XPS scans of fouled membranes revealed the presence of high fractions of silica deposits, particularly on fouled membranes of the plant's first- and third-RO treatment stages [118]. Furthermore, the presence of proteinaceous foulants can explain the increase in the percentage of nitrogen in fouled membranes compared to virgin membranes [118].

3.6. XRF analysis

The elemental composition of substances can be determined using X-ray fluorescence, a semi-quantitative analytical approach. XRF identifies the chemistry of material by detecting the fluorescent (or secondary) X-ray released from analyte when it is excited by a primary X-ray beam. The target element in the membrane material released a series of distinctive fluorescent X-rays (“fingerprints”) that are characteristic of the specific element. Several studies applied XRF analysis to identify the constituent elements of both pristine and fouled membranes and hence determine the nature of foulants [61,115,119]. For example, XRF analysis demonstrated that Si and Al were the major constituents of the membrane foulant [115].

3.7. Fluorescence spectroscopy

Fluorescence Excitation−emission matrix (FEEM) can be applied to determine dissolved organic matter (DOM) in feed water and membrane foulant layer with great sensitivity, and high speed. This technique is very popular in tracking various compositions and sources of natural organic matter (NOM) and extracellular polymer substances (EPSs) in order to identify whether humic fouling or biofouling is the primary cause of membrane performance degradation. Fluorescence peaks in the EEM matrix have been divided into five distinguished areas, where peaks in each area have been linked to previously identified fluorophores, such as tyrosine, humic, phenol, and tryptophan-like moieties, Fig. 11a, [120]. Peaks at lower excitation wavelengths (<250 nm) and lower emission wavelengths (<380 nm) were attributed to aromatic proteins like tyrosine (Region I and Region II). Fluorescence peaks with Ex/Em in ranges of 250–300 nm and 300–380 nm are ascribed to soluble microbial byproducts. Peaks with longer wavelengths Ex/Em than 280 and 380 nm are ascribed to humic acid compounds, while the peaks with Ex < 300 nm and Em > 380 nm are attributed to fulvic acid-like materials. Further, FEEM has been applied to characterize the DOM in feed water and treated water, where the obtained results demonstrated the presence of humic-like fluorophore in feed water (intensity = 39 mV at Ex = 280 nm, Em = 450 nm) and its fluorescence intensity was decreased through dissolved ait floatation process (intensity = 28 mV at Ex = 280 nm, Em = 450 nm) and the UF unit (intensity = 24 mV at Ex = 270 nm, Em = 440 nm), and the peaks were not noticed for the RO effluent (Fig. 11b–e), [91]. Similarly, FEEM was used to characterize the organic foulants adsorbed on the membrane and led to the deterioration its performance [113]. Furthermore, the characteristic intense FEEM peaks of fluvic acid and humic acid were observed for the RO concentrate but not for the cleaning solutions of the last membrane of the third treatment stage, which revealed that humic substances were not responsible for the flux decline of the third stage [117].

Fig. 11.

(a) Location of EEM peaks (symbols) for five EEM regions [120], (b–d) 3D FEEM of DOM in: (b) feed water, (c) DAF effluent, (d) UF effluent, and (e) RO effluent [91].

3.8. Contact angle measurement

The contact angle method is used to examine the change in hydrophilicity and wetting properties of a membrane and its tendency to fouling. This technique does not require any sample preparation. A syringe tip is positioned near the surface sample and pushed to deposit a liquid drop (i.e., distilled water) of about 2 μL on it. A camera captures images of the drop, and special software determines the contact angles, where the surface of fouled membrane become more hydrophobic compared to pristine membrane [121]. Therefore, contact angle measurements’ were used to study the effects of various cleaning solutions on the surface characteristics of membranes [91].

3.9. Liquid chromatography with organic carbon detection

Assessment of both DOM in feed water or backwash and the organic matter deposited into fouling layers on membrane surfaces can be achieved using liquid chromatography with organic carbon detection (LC-OCD). For example, LC-OCD was used to explore protein fouling of membranes as applied to uniform globular protein (bovine serum albumin (BSA)), non-uniform protein (algal protein), fibrillar protein (fish gelatin), and protein-polysaccharide (BSA-sodium alginate) mixtures [122]. Further, the characteristics of DOM that caused fouling in the PRO membrane were assessed using FEEM and LC-OCD [[123], [124], [125]].

4. Control of fouling issues

4.1. Pretreatment

Pre-treatment stages play an important role in reverse osmosis desalination plants as they enhance the quality and flux of water and extend the lifespan of RO membranes, as well as reduce the overall operating costs [126]. A thorough understanding of the characteristics, quality, and type of feed water resource as well as the intended use and the required production capacity are important for selecting the type and number of pretreatment stages required prior to installing a reverse osmosis system [127]. The pretreatment of feed-water is applied to make it compatible with the nature of membranes by reducing the concentrations of constituents such as bacterial species, inorganic colloids, organic compounds, total dissolved solids (TDS) to acceptable low levels [128]. Pretreatment methods can be classified as conventional including disinfection, dissolved air floatation (DAF), flocculation, coagulation, granular media, scale inhibition, and softening, or non-conventional processes such as membrane-based filtration including nanofiltration (NF), microfiltration (MF), and ultrafiltration (UF) [[129], [130], [131]]. Fig. 12 illustrates a schematic diagram of a RO unit with conventional and UF pretreatments. Ineffective pre-treatment of feed water generally leads to adverse effects such as an increased rate of membrane fouling with subsequent reduction of water flow and deterioration of water quality over time, requiring more frequent chemical cleanings as well as reduced membrane lifespan, which ultimately affects the economic aspects of the RO process [129].

Fig. 12.

A schematic diagram of RO unit with conventional and UF pretreatments [243].

Coagulation and flocculation are important stages in water treatment, which aim at reducing the turbidity load in the feed water [132]. Coagulation involves the use of special coagulants to neutralize the colloidal negative charges in order to destabilize the suspended colloids allowing its sedimentation [133]. Common colloids existing in feed water include colloidal clays, viruses, silica, proteins, and humic materials. Coagulants are generally injected before the sedimentation tanks, filters, or dissolved air flotation units. The most widely used coagulants are salts of ferric and aluminum and also organic coagulants [134]. Iron salts are more widely used than aluminum salts as it is difficult to keep aluminum concentrations low; additionally, lower aluminum concentrations can lead to the formation of irreversible fouling of the elements in down streams RO membrane elements [135]. Two major coagulation operations are available which are electrocoagulation and chemical coagulation. Flocculation is a post-coagulation treatment process in which charged colloidal particles are neutralized by the addition of flocculants and slow stirring, allowing them to collect together and eventually form more settleable compounds known as flocs. Thus, electrocoagulation was used for the elimination of turbidity, total suspended solids (TSS), and biological oxygen demand (BOD) compounds from municipal wastewater [136]. For example, injecting 3 ppm ferric chloride as a pretreatment coagulant caused significant improvement of feed water quality that marginally improved the BWRO plant performance; as evidenced by the improvement in the Silt Density Index (SDI) value where the SDI papers, after coagulation treatment, exhibited lower amounts of foulants [137], as shown in (Fig. 13a, b). In another study, the efficiency of polyaluminum chloride and aluminum sulfate in different doses of coagulants and pH values were examined to determine the best operating parameters for high and low turbidity water samples [138]. The results showed that the coagulation process could effectively remove turbidity from low to medium turbidity samples using relatively small concentrations of 10–20 mg/L of polyaluminum chloride or aluminum sulfate. The removal efficiency remained high with increasing the levels of turbidity to 500–1000 NTU [138].

Fig. 13.

SDI paper (a)before and (b) after using coagulant [137].

Dissolved air flotation (DAF) units are commonly used after coagulation-flocculation processes to eliminate particulate foulants like grease, oil, algal cells, or other floating pollutants before membrane filtration or conventional filtration [139]. DAF performs flotation by dissolving compressed air in the raw water and then releasing pressure which causes the solution to become supersaturated with millions of tiny bubbles of air [140]. These tiny bubbles are attached to pollutant particles in the water, making their density lower than that of water. The particles then quickly float to water surface to be removed, leaving the filtered water behind. The application of DAF before hollow-fiber microfiltration (MF) greatly improved the performance of MF unit [141]. In addition, the use of DAF marginally improved the elimination of algae contamination [142]. DAF also played an important role in improving the quality of oil-containing water, removing more than 90% of the oil [143].

Disinfection is an essential process to reduce biofilm development in RO plants. There are several types of disinfection methods such as ultrasonic [144], thermal [145], ultra-violet radiation [146], and chemical [147]. The commonly injected chemical disinfectants include oxidizing biocides, e.g., potassium permanganate, ozone, chlorine specie (e.g. hypochlorite, chlorine dioxide, and chloramines), hydrogen peroxide, or non-oxidizing biocides. Table 2, illustrates some examples of oxidizing biocides that are commonly used for controlling bio-fouling of RO membranes. Most oxidizing biocides are incompatible with membrane materials and can produce low amounts of residues that can destroy the polyamide layer of membranes and result in increased salt passage, so their use should be limited to pretreatment stages prior RO units [148,149]. On the other hand, some non-oxidizing biocides are safe to polyamide layers, while others have dangerous impacts on performance. Compatible biocides, e.g., 2,2-dibromo-3-nitrilopropionamide (DBNPA) and isothiazolones, can be applied intermittently (shock), continuously, or for off-line soaking. Off-line disinfection is typically applied after chemical cleaning operations or as a precaution in some industries based on their regulations; for example, beverage and food processes require the off-line application of non-oxidizing biocides [149,150]. In addition, the continuous dose application of DBNPA biocide was able to prevent increases in pressure drop and accumulation of biofilms during RO processes, while not damaging the membrane materials [151,152]. For example, applying 1000–100,000 mg/L DBNPA biocide to membrane surfaces for 24 h did not damage the polyamide layers of membranes [152].

Table 2.

Advantages and disadvantages of oxidizing biocides used for controlling bio-fouling of Ro membranes [150,247,248].

Oxidizing biocide	Advatanges	Disadavantages
HOCl, OCl−	• Deactivation of most bacteria and viruses in water	• Lower efficiency in turbid waters.
	• Can be used to decompose some organic species	• Corrsosive
	• Cost effective disinfection technique	• Produce hazardous disinfection byproducts, e.g., trihalomethanes, haloacetic acids, assimilable organic carbon
Chloramines	• Produced from the interaction of ammonia with free chlorine or hypochlorous acid	• lower biocidal power (0.4% of hypochlorous acid)
	• moderated formation of disinfection byproducts
	• Can still active for a long period
	• Of low hazard to membranes compared to free chlorine and hypochlorites
Ozone	• Significant oxidation ability for various microorganisms and organic matter	• Very lower half life
	• Required relatively lower contact times for inactivation	• React with humic and bromide and give hazardous compounds.
	• yield minimum DBPs in various applications	• Can damage polyamide layer
Hydrogen Peroxide	• It can be used directly to disinfect membranes but under certain conditions to avoid destruction of membrane	• Require high concentrations for complete disinfection.
	• Does not generate residues or gasses

Furthermore, scale inhibitors are typically injected during feed water pretreatment prior to the RO process to prevent the formation of mineral scales on membrane surfaces, thereby improving RO unit performance and lowering the overall operating costs. Antiscalants control the formation of scales through three mechanisms: crystal distortion, dispersion, and threshold inhibition [[153], [154], [155]]. The first one refers to the capacity of an antiscalant to delay or prevent nucleation of mineral crystal and/or crystal growth; e.g., scales of calcium carbonate, sulfate, phosphate … etc. The second type of antiscalant works by deforming the crystal forms leading to the formation of smooth, non-adherent scales. The third mechanism for antiscalant is the adsorption on colloidal particles or crystals and giving the resulting crystal a negative charge that maintains its repulsion and prevents its agglomeration. To prevent formation of mineral scale and related deposits on membrane surfaces, various antiscalants; e.g., organophosphonates, polyacrylic acid, polyphosphates, polymeric anhydride, and polyacrylamides have been widely used [[156], [157], [158]]. Although there are many effective antiscalants available, research is being conducted to improve existing ones, as well as to develop and apply environmentally friendly green antiscalants. As a result, carboxymethyl cellulose (CMC) was investigated as a green antiscalant to prevent the formation of gypsum scaling on the surface of RO membrane. XRD and optical microscopic data analysis revealed remarkable changes in crystal surface morphology and size after using CMC as an antiscalant, indicating that lattice distortion, chelation effects, and dispersion were all involved as anti-scaling mechanisms. Plant productivity remains relatively constant in the presence of CMC while declining sharply in the absence of CMC. The presence of coexisting anions such as PO₄³⁻ and CO₃²⁻ had no effect on the antiscalant influence of CMC during RO operation [159]. Furthermore, a number of new antiscalants based on copolymers of methacrylic acid, acrylic acid, maleic anhydride, polyaspartic acid, and various cross-linking agents were evaluated to control/prevent the formation of calcium carbonate scaling in RO units, with (poly)acrylic and polyaspartic acid-based antiscalants proved particularly effective [160].

Unconventional membrane-based raw water pretreatment techniques have recently gained popularity due to their high separation efficiency and ability to overcome numerous limitations associated with conventional treatment methods, such as the passage of colloidal particles through conventional filters, which causes irreversible damage to RO membranes [11,128,161]. Thus due to their small pore size (100–5000 nm), microfiltration (MF) units were used to remove TSS, large colloids, suspended particles, flocculated materials, and bacteria, prior to RO units [143,162]. However, ultrafiltration membranes have pore sizes of 10–100 nm and are able to separate macromolecules, while passing smaller molecules and dissolved salts. Thus, UF units are used to reject proteins, large organic molecules, colloids, and microbiological contaminants. On the other hand, nanofiltration (NF) membranes has a pore size of 1–2 nm enabling the separation of TOC, color producing species, dissolved solids, divalent cations, and polymeric, organic and bio-materials from various feed waters and wastewaters [143,162]. Fig. 14 highlights the ability of various membrane filtration processes in removing various pollutants depending on their sizes. Several studies illustrated that using of UF, MF, and NF may lead to a100% removal of foulant under specific conditions [163]. For instance, the abundance of bacteria forming biofilm was reduced by about 30% and about 90% upon applying conventional and membrane pretreatments, respectively [164]. Furthermore, UF pre-treatment improved the SDI values of treated water to less than 2.5 and removed the turbidity by 98–99.5% [165]. A comparison of seven membrane systems for desalination, showed that the NF-RO hybrid plant showed increase in permeate flux with high recovery ratio (>50%) [166]. Herein, Table 3 gives an overview of published works on the performance of MF, UF, and NF units for the removal of various pollutants.

Fig. 14.

Pollutants that can be eliminated by membrane processes [244].

Table 3.

Overview of published works on the performance of MF,UF, and NF units for the removal of various pollutants.

Filtration configuration	Filter material	Observations	Ref
Microfiltration	Ceramic membrane made of a mixture of TiO2 and ZrO2	• MF removed bisphenol pollutant at an initial concentration of 0.3 ± 0.14 mg/L	[249]
		• MF removed suspended solids and 40–60% COD
hybrid system of multi-layer slow sand filter (MSSF), MF and UF	MSSF consists of (GAC, Silica, sand), MF consists of PVDF, and UF consists of PAN	• removal efficiencies for COD, LAS, TSS, and turbidity were 98.22, 99.97, 99.99, and 99.98%, respectively using the MSSF-MF-UF hybrid system.	[250]
Microfiltration	Fly ash based ceramic membranes	• 87–96% rejection for 200 mg/L oil emulsion (decreasing with increasing pressure)	[251]
Nanofiltrations	Polyamide	• the maximum removal efficiencies for Ba and Cr is 85.3% and 58.5%, respectively	[252]
Ultrafiltration	nanocomposites PVDF membranes	• exhibited high removal efficiency for dyes.	[253]
Ultrafiltration	cellulose acetate	• Exhibited high efficiency in removing high MWt compounds produced from the oxidation of 4-chlorophenol	[254]
Microfiltration	TiO2 composite membrane	• 99 0% rejection of oil emulsion	[255]
Ultrafiltration	PVDF/2-aminobenzothiazole modified UF membrane	• Exhibited high rejection rate of BSA 91.71%, and an adsorption capacity of 157.75 μg/cm2 towards chromium ion	[256]
Ultrafiltration	HPZNs-loaded PES membrane	• Exhibited effective removal of polyethylene glycol and colloidal gold pollutants	[257]
Nanofiltration	Polyamide composite	• Exhibited a good ability to eliminate pesticides from wastewaters	[258]

In summary, proper selection of pretreatment stage improves raw water quality prior to RO units and is critical to avoid membrane fouling problems. In practice, various pretreatment techniques have varying capabilities in removing various types of pollutants; as a result, a combination of different pretreatment methods is usually required to remove various pollutants and avoid their adverse effects on the performance of desalination plants [167].

4.2. Cleaning strategy

Fouling of RO units is unavoidable in the long run; even with proper pretreatment, cleaning of RO membrane units will be required occasionally due to its ease of application and efficacy in restoring plant performance [168]. To avoid increasing the percentage of irreversible membrane fouling that cannot be resolved, cleaning should be performed when permeate flux is reduced by about 10%, differential pressure is increased by 15%, or product salinity is increased by 10% [11]. Higher frequency of membrane cleaning, on the other hand, makes the membrane less robust, resulting in increased downtime and membrane damage risks, and thus increasing the economic cost of the desalination process [169]. The cleaning strategies depend on the nature of fouling and can be (i) chemical and (ii) physical, or both in some cases. The removal of hydraulic resistance and the restoration of plant productivity are the main indicators of the success of the applied cleaning process [170].

4.2.1. Physical cleaning

Physical cleaning methods rely on mechanical forces to remove loosely attached cake layers from the membrane surfaces [5]. Physical cleaning efficiency usually decreases as foulants age because some of the foulants become irreversibly attached to the membrane material. The most common physical cleaning methods include sponge ball cleaning, backwashing, forward and reverse flushing, and air sparging, as well as non-conventional methods such as ultrasonication, osmotic back washing, and application of electric, and magnetic fields.

The sponge ball method removes foulants by scrubbing the membrane surface with a sponge ball, which is typically made of polyurethane. According to a previous study, the optimal application of physical cleaning of the membrane with sponge balls (replacement every 3 weeks) greatly reduced the use of chemical cleaning [171].

The air sparging (flushing) method is used to reduce external membrane fouling and remove the accumulated cake layer from a membrane surface, which improves the separation performance [172]. This method can be employed to clean various type of membranes such as flat sheet, tubular, hollow fiber, and spiral wound types [173]. Air bubbles enhance the membrane performance by causing individual foulant particles to aggregate to form larger particles and reduce the potential of foulant accumulation on the membrane surface [174]; air bubbles penetrate and disrupt the foulant concentration-polarization layer [175,176]; additionally, pressure pulsations resulting from air bubbles movement near the membrane surface produce a similar pulsatile flow inside the membrane channels to remove internal fouling [175]. Therefore, due to the transient and high wall shear stress produced by the sparging process, air sparging increased both the RO membrane flux and the salt rejection properties by 166% and 91%, respectively [177].

In forward flushing [86], the permeate water is flushed by passing it through the feed channel of the membrane module at a high cross-flow speed, (Fig. 15a). Many adsorbed particulates on the surface of the membrane are usually removed due to shear forces induced by the high turbulence rate in the flow. In this method, the direction of water flow is from the feed side to the brine side and is often helpful in the elimination of colloidal matter. In reverse flushing, the permeate water is passed for several seconds in the forward direction and then, in the opposite direction, allowing the foulants liberated from the membrane's surface to be dumped on the feeding side; (Fig. 15b), [178]. The foulants particles that have diffused into the membrane pores are typically not removed by reverse and forward flushing; however, chemical cleaning or backwashing can remove them [179].

Fig. 15.

Flow direction in (a) forward flushing [178], (b) reverse flushing [178], and (c) backwashing [245].

The backwashing method involves pumping permeate water in the reverse direction across membranes to remove the majority of reversible fouling, as shown in (Fig. 15c). This can be accomplished by lowering the operating pressure below the osmotic feed pressure or increasing the permeate water pressure [180]. Backwashing hasn't been widely employed in RO desalination since it would necessitate a high back-pressure, which might cause damage to membranes [181]. Otherwise, a non-conventional osmotic backwashing method has recently been applied to RO membrane-based units. This can be accomplished by injecting solution with a high salinity into the feed channel [182]. Macleish and Spiegler were the first to propose osmotic backwashing [183]. Since then, many studies have dealt with this process from an experimental [184] and theoretical viewpoints [185]. For example, the efficiency of osmatic backwashing has been evaluated in a crossflow unit using SWRO membranes and alginic acid as the organic foulant model when using concentrated salt solution instead of RO feed water [186]. Experimental data revealed that osmotic backwashing gave approximately similar productivity recovery as chemical cleaning [186]. Furthermore, osmotic backflushing resulted in a significant decrease in foulant biovolume (70–79%) as well as significantly improved removal of proteins and total organic carbon (66 and 78%, respectively), while restoring permeates water flux by approximately 63% [187].

Ultrasonic waves can be used to promote penetration in the membrane as a pretreatment stage to reduce fouling potential resulting from organic matter and other solid particles. Sound waves with high frequency are applied in order to vibrate the aqueous medium, and disintegrate adsorbed foulants on the surface of the membrane [188]. This method has several advantages, including the ability to clean membrane foulants online, the absence of by-products, and the absence of risk, such as the handling and transportation of chemicals used in chemical cleaning [172]. For example, when carboxyl cellulose, CaSO₄, and Fe³⁺ solutions were cross-flow filtered, on-line cleaning by ultrasonication successfully treated fouling of a conventional PA membrane, resulting in a significant increase in permeate flux while not decreasing the membrane rejection ability [189].

4.2.2. Chemical cleaning

When fouling causes a significant decrease in flux, chemical cleaning becomes the most important membrane maintenance strategy [190]. Alkalis, acids, chelating agents, inorganic salts, enzymes and surfactants, are among the most widely used chemical cleaning agents [191]. The concentration of cleaning agent in a cleaning solution is usually in the range of 0.03–2.0% [192]. Following the selection of the appropriate chemical agent, several parameters should be adjusted and monitored during the cleaning operation, including temperature, crossflow velocity, cleaning time, pH, and reagent's concentration. The cleaning agent's effectiveness in removing foulants will be determined by a variety of factors, including but not limited to interactions between pollutants deposited on the membrane surface and the type of application in which the membrane has been used [168]. Alkalis, such as NaOH, act by increasing the pH of the solution, which increases the negative charge density of organic foulant compounds accumulated on the membrane surface due to deprotonation of carboxylic groups, among other things, and thus their solubility. Acid cleaning, on the other hand, dissolves inorganic scales such as calcium sulfate and carbonate. Sequestration/chelating agents are one of the important additives that greatly improve the cleaning process's efficiency. Enzymes are non-toxic materials that have recently been used in chemical cleaning processes to remove bio-contaminants from RO membranes. Enzymes have been reported to be highly effective materials for removing EPS proteins from membrane surfaces through protein fractionation into tiny fragments [193]. The hydrolysis of glycoprotein and proteinaceous exopolymers that encircle bacteria embedded in a biofilm matrix is thought to be the mechanism of enzyme catalyzed cleaning [194]. Thus, different cleaning solutions were used to remove organic and inorganic compounds from a hydrophilic FT-30 membrane installed in an industrial wastewater treatment plant [195]. A variety of chemicals were used in the cleaning solutions, including a base (NaOH), acids (citric, HCl, HNO₃, and H₂SO₄), surfactant (SDS), complexing agent (citric acid, EDTA), and their combinations. The cleaning efficiency was confirmed by calculating flux recovery (FR) and resistance removal (RR). According to the reported data, acid cleaning solutions were ineffective in restoring the flux, whereas two stages of detergent and caustic cleaning (NaOH-SDS) followed by acid cleaning provided effective recovery [195]. Further, it was found that using concentrated salt solutions was very effective in cleaning organic foulants from RO membranes [196]. The mechanism of foulant treatment by salt solution cleaning includes swelling of the foulant layer, which reduces the structural stability of the gel network; additionally, an ion-exchange process occurs between the calcium-polysaccharide complex present in the fouling layer and sodium ions, as shown in Fig. 16. Furthermore, the impact of sequential cleaning protocols (SCP) on cleaning of fouled membranes was investigated and it was found that SCP I (0.1 N NaOH → 0.1 N HCl → deionized (DI) water) was more effective than SCP II (0.1 N HCl → 0.1 N NaOH → DI water) in desorbing both inorganic and organic foulants from RO membrane surfaces [91]. On the other hand, the effectiveness of various enzymes in cleaning RO membranes that were continuously run in a rotating disc reactor for sixty days was evaluated [197]. Following a preliminary screening, three efficient enzymes were chosen and used to remove biofoulants deposited on the membrane's surface at low enzyme doses of 50–150 ppm, and neutral pH's. According to the results of bacterial quantification and biofilm analysis, lipase and protease-based enzymes had the best cleaning performance and were able to restore favorable surface features (roughness, wettability, etc.) to their initial values. Even at a lower concentration of 50 ppm, these enzymes reduced the number of culturable cells by five logs [197]. Despite the wide range of chemical cleaning agents and their ability to treat various types of irreversible foulants, they have some drawbacks. Particularly when using harsh cleaning parameters such as high temperature, concentration, and pH, which can cause chemical attack on membranes and reduce the overall membrane efficiency [181]. The total estimated costs that include transportation and handling, may reach about 5–20% of the RO unit's maintenance and total operating costs [11]. Moreover, the disposal of the cleaning solutions used in the process poses a significant environmental risk [198]. It is worth noting that even when aggressive chemicals are used, a full 100% recovery of performance is rare.

Fig. 16.

Mechanism of salt cleaning [196].

4.3. Membrane modification

The most fundamental fouling control strategy is the development of robust antifouling membranes. Membrane properties such as its charge, morphology, hydrophilicity, and chemical groups that attach to the membrane, greatly affect the membrane tendency to fouling. Hence, constructing antifouling membrane surfaces can be achieved by properly tailoring the inherent surface physical and chemical properties. Since the fouling mechanism varies from foulant to foulant, antifouling membrane fabrication should be based on a variety of strategies and mechanisms. Among the approaches of membrane modification covered in this review are surface modification, and the incorporation of nanoparticles in thin film composite membranes (TFC).

4.3.1. Surface modification

Modification of the active layer of reverse osmosis membrane is one of the most promising approaches to control properties of the membrane surface, which strongly affect the performance of the membrane including, salt rejection, permeate flux, and membrane antifouling characteristics [50,199]. Several works modified PA active layer through physical and chemical treatments to enhance the performance of RO membranes (surface deposition, coating, and grafting) [[200], [201], [202], [203]]. In a physical treatment, the RO polyamide layer interacts with modifiers by electrostatic interaction, hydrogen bonding, or van der Waals forces; however, this may deteriorate in long-term operations [204]. On the other hand, during chemical treatment, the RO polyamide layer is chemically bonded to modifiers by strong bonds, resulting in improved structural and chemical stabilities [204]. Physical treatment of RO membrane surfaces is achieved by surface coating or surface adsorption. The surface coating process consists of two steps: (i) spreading of coating liquid on the surface of the membrane, and (ii) evaporating the residual solvent from the membrane surface to solidify the coating agents onto the surface of the membrane to obtain a thin film layer of coating material [43]. The compounds used for coating membrane surfaces usually are hydrophilic polymers having carboxyl, amino, hydroxyl, or enzyme-mimic moieties. For instance, the membrane surface was modified by coating with polyvinyl alcohol (PVA) and 3-mercaptopropyl triethoxysilane by a sol-gel method followed by a thermal crosslinking process [205]. However, to enhance the modified TFC membrane hydrophilicity, it was immersed in H₂O₂ solution to convert the –SH groups into –SO₃H [205]. The modified membrane showed higher rejection ability to NaCl (99.29%) compared to the pristine membrane (97.20%), at the same conditions. Furthermore, in the presence of BSA organic foulants, the modified and pristine PA membranes lost approximately 13% and 42% of their initial productivity, respectively, after 12 h of operation, indicating that the modified membrane exhibited enhanced resistance to organic foulants [205]. Moreover, the modified membrane exhibited enhanced productivity recovery of 94% after the normally applied cleaning process [205]. Furthermore, coating the PA membrane surfaces with polyamidoamine (PAMAM) and PAMAM–PEG with bifunctional PEG cross linkers reduced the contact angle of virgin membrane without affecting its % salt rejection ability, while slightly decreasing its permeate flow [206]. The decrease of contact angle demonstrated increased resistance toward hydrophobic foulants such as organic and biofouling pollutants. In another case, to improve the antifouling properties of RO membranes, a hydrophilic PVA coating was applied, which reduced the zeta potential value of the membrane surface at pH 6 from 25 mV to 0 mV, allowing it to be used in the treatment of industrial wastewater containing a cationic surfactant [207]. Also, RO membrane surface modifications by adsorption of polyelectrolytes and surfactants have been described [208,209]. Thus, PA membrane was modified by electrostatic self-deposition of polyethyleneimine (PEI) to achieve charge reversal and enhance the membrane hydrophilicity and fouling resistance [209]. On the other hand, in the chemical modification of membranes, the membrane surface is activated by chemical reactions and then grafted to various modifiers and offer long-term chemical stability compared with physical modification methods [210]. Chemical methods of modifying membrane surfaces include plasma treatment, chemical coupling, sol-gel grafting, graft polymerization, hydrophilization, and atom transfer radical polymerization [202,[211], [212], [213], [214]]. For instance, tobramycin (TOB) and 3-amino-1,2-propanediol (APD) were grafted on the surface of TFC membrane to enhance the removal rate of boron and the fouling resistance [215]. When compared to the commercial TFC membrane, the modified TFC-TOB₂/APD₁₀ membrane increased water productivity by 33.4%, and boron rejection rate by 8%, while maintaining a high NaCl rejection of 98.0%. Moreover, the modified membrane showed excellent antifouling features such as scaling control, organic fouling resistance, and enhanced bactericidal capacity [215]. Further, the PA layer of commercial RO membranes has been modified by redox-initiated graft polymerization with N-isopropyl acrylamide (NIPAm) followed by graft polymerization with acrylic acid [216]. The modified membrane's surface carried negative charges and became more hydrophilic, increasing salt rejection rate, permeate flux, chlorine resistance, and membrane resistance to protein fouling [216]. Some works used plasma treatment to enhance the membrane's performance [217,218]. Table 4 shows representative examples of RO membrane surface modification using chemical and physical methods, emphasizing the impact of modification on membrane performance.

Table 4.

RO membrane surface modifications via different chemicals/physical methods.

Chemical modifier	Method	Performance after modification	Ref
Polydopamine (PDA)	Surface coating	Charge density and surface hydrophilicity of modified membrane were not affected, while biofouling resistance increased, bacterial adhesion resistance improved.	[259]
Tobramycin and polyacrylic acid (PAA)	Layer-by-layer assembly	Hydrophilicity improved, the productivity raised by 18%, salt rejection enhanced and anti-microbial characteristics improved showing a bacterial killing ratio of 99.6% for both Gram-positive Bacillus subtilis and Gram-negative Escherichia coli.	[260]
Silane coupling agents	Sol–gel process	Exhibited less flux decline upon membrane fouling.	[261]
Natural polymer sericin	Surface coating	Enhancement of salt rejection rate and surface negative charges, smoothness, hydrophilicity, and fouling resistance to BSA	[262]
p(4-VP-co-EGDA), pCBAA	Surface coating	Improved biofouling resistance	[263]
Hydrophobic PFDA and HEMA	Surface coating	Decreased the contact angle and flux rate, increased the hydrophilicity, while salt rejection rate remains essentially unchanged.	[264]
Silane	Dip-coating and quaternization	Increased hydrophilicity, flux rate, bacterial killing rate and biofouling resistance	[265]
PDA	Surface deposition	Charge density and hydrophilicity were not affected, while ant-adhesion against bacteria was enhanced.	[259]
3-glycidoxypropyltrimethoxysilane (GPPTMS)	surface grafting	Increased resistance to casein fouling	[202]
Polyethyleneimine (PEI)	Chemical coupling	High antifouling characteristics to positively-charged foulants	[266]
Phosphoryl choline	surface grafting	High anti-biofouling properties against Gram-negative bacteria, at the same water productivity.	[267]
Amphiphilic hydroxyethyl methacrylate (HEMA)-co-hydrophobic perfluoro decylacrylate (PFDA) copolymers	Initiated chemical vapor deposition	High resistance to biopolymer fouling and microbial attachment.	[268]
Polyvinyl alcohol (PVA)	Thermally initiated free radial grafting	High surface hydrophilicity, smoothness, and organic and biofouling resistance, as well as increased tolerance to chlorine exposure.	[268]

In conclusion, despite the abundance of research work to modify RO membrane surfaces using chemical and physical methods, the fouling problem could not be completely solved because, in many cases, the coating layer generated additional resistance to the membrane, clogging the membrane pores and reducing the permeate flux. The development of anti-fouling RO membranes with high water permeability and excellent salt rejection via surface modification methods remains a challenge, but it is critical.

4.3.2. Incorporation of nanomaterials in thin film nanocomposite membranes

Nanomaterials (NMs) are used in variety of fields due to their unique properties, such as their high and effective surface area, which positively affects their chemical and physical properties. Nanomaterials play an important role in increasing membrane fouling resistance in water desalination [219,220]. Inorganic nanoparticles, such as silica, titanium dioxide, zinc oxide, nanosilver (nAg), magnetite, boehmite, and zeolite, and metal–organic frameworks, as well as carbon-based nanomaterials like graphene oxide, and carbon nanotubes, have received wide attention due to their excellent functionalities in improving the fouling resistance of the surfaces of RO membranes [[221], [222], [223], [224], [225]]. In recent works, nanomaterials have been incorporated into polysulfone (PS) porous support layers or as surface modifiers, (Fig. 17). Several works have analyzed enhancement in membrane performance after the incorporation of various nanoparticles. For instance, GO has an excellent hydrophilicity because of the abundances of polar hydroxyl and carboxyl groups in the GO sheets [226]. Further, a thin-film nanocomposite (TFN) membrane was synthesized by incorporating p-aminophenol/graphene oxide (AP/GO) into the PA skin layer through interfacial polymerization [227]. The introduction of AP/GO decreased the contact angle from 69.6° to 48.2° and reduced the functional skin layer thickness from 240 nm to 50 nm, relative to virgin RO membrane. The as-synthesized TFN RO membrane increased water productivity by 24.5% and had excellent bacterial killing ratios of 96.78% and 95.26% against S-aureus and E-coli, respectively, when compared to the virgin membrane, which had killing ratios of only 4.95% and 2.48%, respectively. Other studies have shown that incorporating GO and its derivatives into RO membranes improves chlorine resistance [[228], [229], [230]]. Carbon nanotubes (CNTs) are another promising material because of their ease of functionalization, large surface area, superior mechanical strength, enhanced antifouling behaviors, remarkable water transport characteristics, and significant sieving capabilities [231,232]. CNTs can be used as direct filters or as nanofillers to raise the performance of the desalination operation [233]. The incorporation of CNTs with various ratios increased the hydrophilicity and fouling resistance of membrane surfaces [234]. Recent studies revealed that the addition of inorganic NPs also enhanced the overall RO membrane performances [224,235,236]. Furthermore, the use of SiO₂ or TiO₂ NPs as nanofillers in PA membranes favorably improved organic matter rejection, salts rejection, permeate flux rate, and consequently the membrane porosity, hydrophilicity, and permeability [237]. Permeability was raised by 24 and 58% with the incorporation of TiO₂ and SiO₂, respectively. Also, incorporating zeolite in the polyamide layer of RO membranes decreased the contact angle and increased water flux [238]. Additionally, the zeta potential value was increased and led to an increase of rejection of Cl⁻ and SO₄²⁻ [238].

Fig. 17.

Schematic illustration of the synthesis nanosilver - thin film nanocomposite membrane [246].

Aside from the materials mentioned above, researchers are attempting to develop new RO membrane materials as alternatives to existing polyamide in order to reduce fouling problems and lower the cost of the RO process. Nonetheless, the development is still in its early stages, and many obstacles must be overcome to be economically more feasible.

5. Conclusion and future prospectives

RO membranes are among the most advanced technologies for desalination and waste treatment remediation. Membrane fouling is a significant issue in membrane technology. Despite the fact that different types of foulants have distinct formation processes, there are no clear boundaries between them and they are synergistic or interrelated and generally give a combined foulant layer. As a result, more research on the behavior of foulants is required in order to understand the mechanisms of their formation, which will aid researchers in developing advanced antifouling strategies. This review discusses a variety of foulant control methods that have been used in practice, such as pre-treatment, cleaning processes, and membrane surface modification, and these strategies have played an important role in mitigating fouling effects. The review covered various methods used in the pretreatment step to reduce the possibility of foulant contamination of the membrane, including DAF units, antiscalants, disinfection, coagulation, flocculation, and membrane filtration. When the membrane is contaminated, a proper cleaning process must be performed to restore the plant's performance. The review presents a number of physical and chemical cleaning methods. Despite the high efficiency of chemical solutions in removing residues, it is preferable to reduce the frequency of chemical washing by using other means to avoid deterioration of membranes. Modification of membrane is another established strategy to increase the membrane fouling resistance, where the membrane surface is modified by physical or chemical methods. The incorporation of various nanoparticle additives was also discussed. Nevertheless, developing cost-effective and state of the art membranes with high performance and antifouling characteristics is highly desirable for future applications. In addition, modeling the properties, performance characteristics and RO cost aspects before and after membrane cleaning and modification should allow for optimal, sustainable and low-cost RO operations.

Author contribution statement

All authors listed have equally contributed to the development and the writing of this article.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data will be made available on request.