The reverse osmosis (RO) process has been a leading technology in the desalination field; however, an inevitable occurrence of membrane fouling during an operation reduces their efficiency in cost and productivity. Autopsy has been regarded as a practical and effective approach to examining the RO membrane conditions and foulant properties. Herein, we investigated the fouling behaviors of RO membranes in a pilot-scale brackish water reverse osmosis (BWRO) system after 142 h of operation through membrane autopsies and fouling layer characterization. X-ray diffraction (XRD) patterns disclosed Si and Al minerals responsible for inorganic foulants, while Fourier-transform infrared (FT-IR) spectra indicated the presence of organic foulants, including polysaccharides and proteins. Biofouling, such as coliform, was also detected using a liquid-medium culture technique. In addition, the fouled RO membrane was decontaminated by a chemical cleaning process, including three steps of acidic cleaning with Hydrex 4703, alkaline cleaning with Hydrex 4506, and neutral cleaning with RO water. The feed pressure and permeate flux were regenerated over 96%. This study may have fundamental implications for proposing an appropriate pretreatment setup

  • A BWRO system was operated without pretreatment to assess fouling characterization and cleaning efficiency.

  • After 142 h of operation, feed pressure increased to 9.9% and permeate flux decreased to 42.5%.

  • Al and Si minerals, polysaccharides, proteins, coliforms contributed to fouling membrane.

  • Chemical cleaning with Hydrex 4506 and 4703 restored >96% of feed pressure and permeate flux.

Ongoing research and development in brackish water and seawater desalination technologies aim to tackle water scarcity in various regions worldwide. Among various desalination technologies, reverse osmosis (RO) processes currently dominate the others, occupying over 60% of the global desalination installations because of their benefits concerning simple design and operation and low energy demand (Aryanti et al. 2023; Zolghadr-Asli et al. 2023). However, RO membranes are highly susceptible to impurities (i.e., inorganic, organic, and biological compounds) in water and can be easily fouled during operation (Ahmed et al. 2023). Inorganic fouling occurred when dissolved salt ions (e.g., Ca2+, Mg2+, , , and ) precipitated and accumulated on the membrane surface once their concentration reached their saturated solubility, resulting in the phenomenon of inorganic fouling (Matin et al. 2021). Organic fouling resulted from the interaction between macromolecular matters (e.g., humic and fulvic acids, proteins, and polysaccharides) and the RO polyamide membrane through Van de Waals and electrostatic forces (Goh et al. 2018). The attachment of various microorganisms in the influent on the RO membrane and their growth by means of the uptake of organic nutrients caused the formation of biofouling (Suresh et al. 2023). Membrane fouling gradually increased the hydraulic resistance for water passage, requiring more energy to maintain the constant permeate flux. In addition, the occurrence of foulants on the RO membranes decreased their longevity and performance, consequently accelerating the membrane replacement (Nthunya et al. 2022). Therefore, fouling processes impaired the productive efficiency of RO systems, resulting in an increase in the cost of water treatment.

Pretreatment processes were frequently installed before the RO system to upgrade feed water quality and alleviate membrane fouling. In addition, cleaning processes, especially chemical methods, were employed when the RO elements were polluted to remove foulants on the RO membrane and recover its performance (Abdel-Karim et al. 2021; Abushawish et al. 2023). Understanding the nature and characteristics of impurities plays a decisive role in designing the pretreatment configuration as well as selecting appropriate chemicals for cleaning. Autopsies of used membranes can disclose reliable information on the composition of fouling layers, which is beneficial to suggest strategies for preventing and removing the buildup on the membrane (Adel et al. 2022; Wang et al. 2023). For example, inorganic minerals, including halloysite, silica, and lithium chloride, were determined as the main foulants of brackish water reverse osmosis (BWRO) membrane operated for roughly a year (Balcik 2021). In addition, interactions between inorganic compounds of Fe and Al and fulvic acid were responsible for membrane fouling of a BWRO desalination plant producing drinking water for approximately five years (Kim et al. 2015). Furthermore, an autopsy of RO membranes after 11 years of operation in a BWRO desalination plant revealed that inorganic compounds (i.e., calcium carbonate and aluminosilicates) and biofilm of diatoms primarily contributed to membrane fouling (Ruiz-García et al. 2018). It was obvious that the reasons for membrane fouling varied depending on the sources of feed water. Besides, there have been very few studies on the situation of the RO module after a period of operation without pretreatment installation. Generally, the absence of pretreatment in RO systems led to a decline in permeate flow caused by fouling, which in turn reduced the operational lifespan of the RO membranes (Shaheen & Cséfalvay 2024). Nevertheless, these studies were primarily conducted under laboratory conditions with simulated feedwater (Yang et al. 2019; Sun et al. 2024), leading to discrepancies when compared to field studies due to the lack of real-world environmental variables. Additionally, the failure to perform a membrane autopsy has restricted the acquisition of detailed physicochemical data regarding the fouling constituents (Cornelissen et al. 2021; Costa et al. 2024).

In this regard, we ran the BWRO system at a pilot scale of 20 m3/day in the absence of pretreatment processes in a batch mode, with an influent from the Luong Quoi River. The feed pressure and permeate flux values were monitored to assess the fouling time of the RO membrane. The fouled RO membrane was disconnected from the system, and the fouling samples were obtained for further analysis. The morphologies and composition of foulants were characterized using various analytical techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), energy-dispersive X-ray (EDX), and Fourier-transform infrared (FT-IR). In addition, the efficiency of membrane cleaning using commercial chemicals of Hydrex 4506 and 4703 was investigated and discussed.

Operation of a BWRO system

The BWRO system at pilot scale with a capacity of 20 m3/day was located at Luong Quoi water plant, Giong Trom District, Ben Tre Province (i.e., latitude: 10.2020951, longitude: 106.4776562). The feed water was drawn from a container (i.e., containing water from Luong Quoi River) by a centrifugal pump (CM 3-5 A-R-A-E-AVBE F-A-A-N, Grundfos), and delivered to an RO module via a high-pressure pump (CR 3-29 A-FGJ-A-E-HQQE, Grundfos). The characteristics of the original RO membrane are supplied in Table 1. The RO system was operated in the absence of pretreatment processes to gather data on membrane fouling composition and fouling time, facilitating the selection of suitable pretreatment technologies. The pressure and permeate flow rate of the RO system were monitored using a pressure transmitter (A-10, Wika) and an electromagnetic water meter (CZ-EFM, Sence).

Table 1

Technical parameters of the RO membrane

ParametersProperties/Values
Supplier Dupont 
Model CR100 
Material Polyamide thin-film composite 
Active area 37 m2 
Feed spacer 34 mil 
Permeate flow rate 43.5 m3/day 
Salt rejection performance 99.4% 
ParametersProperties/Values
Supplier Dupont 
Model CR100 
Material Polyamide thin-film composite 
Active area 37 m2 
Feed spacer 34 mil 
Permeate flow rate 43.5 m3/day 
Salt rejection performance 99.4% 

Autopsies of the used RO membrane

A membrane autopsy is a highly effective way to visualize the membrane damage and identify the factors responsible for the decline in membrane performance. When the feed pressure exceeded its initial value (i.e., recorded at the time of first utilizing a new membrane) by 10%, based on the technical recommendation (DuPont 2023), the system was shut down, and the membrane autopsy was performed. The membrane element was separated from the pressure vessel, and the liquid sludge (Figure 1(a)) stagnated within the vessel was collected. In addition, fouling layers on the surface of membranes were scraped off using a steel knife, and the contaminated membranes at the head and tail sections as representative samples were cut into 2 × 2 cm in size. Both samples were dried in an oven (UF160, Memmert) at 80 °C for 24 h and stored for further analysis (Figure 1(b) and 1(c)).
Figure 1

(a) Liquid sludge, (b) dried foulants, (c) and fouled RO membrane.

Figure 1

(a) Liquid sludge, (b) dried foulants, (c) and fouled RO membrane.

Close modal

Characterization of fouling layers

The phase structure of impurities on the RO membrane surface was determined using an X-ray diffractometer (Empyrean, PANalytical). XRD spectra of samples were recorded in the range of 2θ = 5 − 80° with scanning speed = 1.6 °/min at an accelerating voltage of 45 kV and an applied current of 40 mA using a Cu anode material as an X-ray source with a Kα wavelength of 1.54 Å. The morphology and the elemental analysis of the fouled RO membrane were performed using a scanning electron microscope (SEM) (S4800, Hitachi) coupled with an EDX (H-7593, HORIBA). The analysis was carried out under an accelerating voltage of 10 kV, an emission current of 9,000 nA, and a working distance of 8 mm. The membrane samples were analyzed without any pretreatment or surface coating and were affixed to the specimen holder using conductive double-sided carbon adhesive tape. XRF (ZETIUM, PANalytical) analysis was employed to investigate the chemical composition of the dried foulant. Organic functional groups of fouling samples and RO membranes were detected using FT-IR spectroscopy (NICOLET 6700, Thermo). Contact angles of RO membranes were measured using optical contact angle goniometers (OCA50, Dataphysics). In addition, the presence of several microorganisms, including Escherichia coli and coliform, in the liquid sludge samples was examined by the liquid-medium culture technique based on the guidelines of International Organization for Standardization ISO 7251:2005 and ISO 4832:2007, respectively (ISO 2005, 2006).

Chemical cleaning procedures

Chemical cleaning processes were carried out to remove various foulants from the contaminated membrane surface using Hydrex 4703 (Veolia) and Hydrex 4506 (Veolia) as acidic and alkaline cleaning agents, respectively. First, 50 L of cleaning solutions (i.e., Hydrex 4703 3% or Hydrex 4506 3% or clean water produced from the RO system) was prepared in a 100-L polyethylene tank. The solution pH was adjusted to the desired values using a 20% solution of citric acid (Xilong). The solution was circulated in the system between the tank and the RO unit for a predetermined time by a centrifugal pump (CM 10-1 A-R-A-E-AVBE F-A-A-N, Grundfos). Membrane cleaning procedures included three steps: first, rinse the RO membrane at acidic conditions for 60 min using Hydrex 4703 3%; second, rinse the RO membrane at basic conditions for 60 min using Hydrex 4506 3%; final, rinse the RO membrane using clean water until the pH of the solution reaches around 7. The efficiency of the cleaning process was evaluated based on a recovery in permeate flux and feed pressure.

Visual and scanning electron microscopic observation of RO membrane

A visual examination of the fouled RO membrane revealed no physical or pressure-induced deformation in key parts such as permeate tubes, brine seals, and fiberglass. However, a noticeable accumulation of brown mud was distributed on the entire outer wrapping (Figure 2(a)). The membrane element was cut with a saw at the head and tail positions and separated into individual membrane sheets (Figure 2(b)). Observational findings indicated pronounced fouling layers present across all RO membrane surfaces, at both the upstream and downstream positions, resulting from the direct introduction of untreated river water into the RO system. A cross-sectional image of the fouled membrane further confirmed uniform fouling across all sections of the membrane (Figure 2(c)). Dense layers with black color and muddy odor occurred on the surface of the membrane sheets (Figure 2(d)), indicating that they experienced a heavy incrustation and fouling of impurities in the feed water.
Figure 2

(a) Outer wrapping, (b) autopsied RO membrane at the head and tail, (c) cross-sections of fouled RO membrane, and (d) fouled RO membrane sheets.

Figure 2

(a) Outer wrapping, (b) autopsied RO membrane at the head and tail, (c) cross-sections of fouled RO membrane, and (d) fouled RO membrane sheets.

Close modal
SEM measurements under different magnifications were conducted to scrutinize the fouled membrane sheet. Figure 3(a) displayed the presence of foulants with various morphologies across different areas of the membrane surface. Several cracks randomly formed on the membrane surface in various directions (Figure 3(b)), possibly due to high-pressure stresses from dense fouling layers. The structures of foulants appeared layered and amorphous at a high magnification of 8.0 mm × 60,000 (Figure 3(c)). However, it is challenging to elucidate the nature of the foulants using only the SEM technique, necessitating further analysis. These results suggest that operating the RO unit without pretreatment processes caused severe membrane fouling, significantly reducing its performance.
Figure 3

SEM images of fouling layers on the RO membrane at a magnification of (a) 8.0 mm × 250, (b) 8.0 mm × 100, and (c) 8.0 mm × 60,000.

Figure 3

SEM images of fouling layers on the RO membrane at a magnification of (a) 8.0 mm × 250, (b) 8.0 mm × 100, and (c) 8.0 mm × 60,000.

Close modal

Characterization of foulants

The crystalline phase structure of inorganic minerals present in the dried fouling sample on the RO membrane surface was investigated using an XRD analysis (Figure 4). Four distinct crystalline phases were identified in the XRD pattern of the fouling sample. The peaks at 2θ = 20.9, 26.7, 36.6, 39.5, 40.3, 42.5, 45.8, 50.2, 55.6, 60.0, 64.2, 67.8, 68.1, and 68.4° correspond to the crystal faces (100), (101), (110), (012), (111), (200), (021), (112), (103), (211), (113), (212), (023), and (301) of α-SiO2 (Quartz) (PDF#01-075-0443) (Hamed et al. 2024). The peaks located at positions of 8.8, 17.7, 22.6, 28.0, 30.0, 31.3, and 45.4° are attributed to KAl2(Si, Al)4O10(OH)2 (Muscovite-2M1) (PDF#00-058-2037) (Sanguanpak et al. 2022). The peaks of 12.4, 20.0, 24.8, 35.0 and 62.3° belong to the crystal structure of Al2Si2O5(OH)4 (Kaolinite) (PDF#00-001-0527) (Tome et al. 2024), while those of 12.4, 24.8, and 37.7 match well to crystal faces (002), (004), (006) of Al2Si2O5(OH)4 (Dickite-2M1) (PDF#00-058-2002) (Vasić et al. 2023). This result indicates that the detected inorganic foulants primarily consisted of minerals containing Si and Al, which is consistent with the fouling composition on RO membranes observed in other studies (Karmal et al. 2020; García-Triñanes et al. 2022; Tapiero et al. 2023). However, several colloidal or amorphous solids were not detected using the XRD technique.
Figure 4

XRD pattern of the dried fouling sample.

Figure 4

XRD pattern of the dried fouling sample.

Close modal
EDX spectroscopy was used to analyze the elemental composition of the fouling layer formed on the surface of the RO membrane obtained at the head and tail positions. Figure 5 shows the characteristic peaks of O, C, Si, Al, Mg, Na, and Fe elements with a corresponding decrease in atom percent. The one-way ANOVA analysis revealed no statistically significant variations (p < 0.05) in the elemental composition of the fouling samples obtained from the head and tail sections of the fouled RO membrane. The presence of C and O could be due to organic compounds such as proteins, polysaccharides, natural organic compounds (i.e., humic and fulvic acids) and/or biological agents such as microorganisms, cells, and algaes. Si and Al elements were detected with higher percentages than Na, Mg, and Fe, confirming that the inorganic foulants mainly exist in the form of Si and Al compounds. Mg and Fe could originate from respective inorganic minerals of dolomite and iron oxides, which were not detected by XRD measurement because of their weak crystallinity or low concentrations in the sample.
Figure 5

EDX spectra of fouled RO membrane at the (a) head and (b) tail positions.

Figure 5

EDX spectra of fouled RO membrane at the (a) head and (b) tail positions.

Close modal

In addition to the EDX results, an XRF analysis was also carried out to provide a more comprehensive assessment of the elemental composition in the dried fouling sample (Table 2). It should be noted that light elements such as C, N, and O cannot be found using the XRF technique. The other elements present in the EDX spectra were also detected by the XRF and substantially contributed to the composition of the fouling sample. The detection of P, S, Cl, Ca, and Na elements indicates that inorganic ions frequently found in water, such as , , Cl, Ca2+, and Na+, were precipitated on the membrane surface. In addition, a small amount of transition metals, such as Ti, Pb, Co, Ni, Zr, Cr, and Zn, were also present in the dried fouling samples.

Table 2

XRF results of dried fouling sample

Element%Atom
Fe 9.125 
Al 25.062 
Si 57.323 
0.283 
0.492 
Cl 0.900 
3.071 
Ca 0.593 
Ti 0.936 
Mg 1.792 
Pb 0.220 
Co 0.008 
Ni 0.003 
Cu 0.012 
Zn 0.009 
Ga 0.027 
Br 0.005 
Sr 0.004 
Zr 0.020 
Ba 0.026 
Cr 0.038 
Element%Atom
Fe 9.125 
Al 25.062 
Si 57.323 
0.283 
0.492 
Cl 0.900 
3.071 
Ca 0.593 
Ti 0.936 
Mg 1.792 
Pb 0.220 
Co 0.008 
Ni 0.003 
Cu 0.012 
Zn 0.009 
Ga 0.027 
Br 0.005 
Sr 0.004 
Zr 0.020 
Ba 0.026 
Cr 0.038 

FT-IR spectra reveal information on the vibrations in atomic bonds induced by infrared radiation, allowing for the identification of specific functional groups in both organic and inorganic compounds in the fouling sample (Figure 6). The characteristic bands at positions 1,100 and 473 cm−1 correspond to the Si − O − Si asymmetric stretching and Si − O bending vibrations of the SiO2 molecule, respectively (Ponnilavan et al. 2015; Fallah et al. 2023). The regions 1,044 and 916 cm−1 correspondingly characterize the relaxation vibrations of the C − O functional group and ring vibrations in the polysaccharides, which are significant components of extracellular polymer substances associated with microbial growth (Fallah et al. 2023). The signals around 1,645 and 1,440 cm−1 are attributed to the respective vibrations of the amide and C = C bonds of the aromatic ring in the protein molecules (i.e., a form of biofouling) (Sannigrahi et al. 2018). The range from 800 to 600 cm−1 indicates aromatic skeletons of organic fouling (Melián-Martel et al. 2012). The two peaks located at 3,700 and 3,625 cm−1 belong to Al − OH − Si and Al − OH − Al bonds in Al and Si minerals (Khan et al. 2013), also found in the XRD results. In addition, the broad peak of 3,400 cm−1 may be related to relaxation vibrations of –OH originating from water, alcohol, or carboxylic acid molecules or from N − H of amine (Gonzalez-Gil et al. 2021). Results from FT-IR spectra confirm the presence of Si and Al minerals in inorganic fouling. In addition, organic matters such as polysaccharides, proteins, and aromatic compounds could be involved in fouling formation on RO membranes. The presence of two microorganisms of E. coli and coliform frequently existing in water was examined for the liquid sludge sample. The results showed that E. coli was absent; however, coliform was found with a concentration of 4.6 × 104 CFU/mL, confirming the contribution of biofouling to in the makeup of the deposited fouling.
Figure 6

FT-IR spectra of the dried fouling sample.

Figure 6

FT-IR spectra of the dried fouling sample.

Close modal

A comparative analysis was conducted to examine the primary fouling constituents observed on RO membranes used in the brackish water treatment system at the Luong Quoi Water Plant, located in Giong Trom, against those identified in other RO systems treating both brackish and seawater sources (Table 3). The findings indicate that the predominant foulants are generally consistent across systems and include primary inorganic deposits based on Si, Al, and Fe elements, as well as organic and biological substances such as humic acids, proteins, and polysaccharides. Despite this apparent similarity, it is essential to note that such comparisons serve only as general references due to substantial differences among the systems. Variations in feed water characteristics, operational scale, membrane types, and pretreatment processes can significantly influence the composition and severity of membrane fouling. Therefore, site-specific evaluations remain critical to accurately identifying fouling mechanisms and implementing effective mitigation strategies.

Table 3

Characterization of principal fouling agents across various RO systems

ApplicationsPositionCapacity (m3/day)Primary foulants and elementsReference
SWRO Mediterranean coast 2,500 SiO2, polysaccharides, humic acids, proteins Adel et al. (2022)  
SWRO Taizhou, Zhejiang, China – Si, Fe, Al, polysaccharides, proteins Wang et al. (2023)  
SWRO Red Sea coast 40,000 Al, Fe, Mn, Si, low molecular weight acids Fortunato et al. (2020)  
SWRO Gijang, Busan, Republic of Korea 9,000 Al, Ca, Cu, Fe, Mg, Mn, Zn, humic acids, proteins Lee et al. (2021)  
SWRO Saudi Arabia 1,800 Fe, polysaccharides, proteins Gonzalez-Gil et al. (2021)  
BWRO Turkey 40 Al, Si Balcik (2021)  
BWRO London, UK 1.2 Al, Ca, Si, diatoms, pseudomonas, polysaccharides García-Triñanes et al. (2022)  
BWRO Algeria 8,500 Si Arras et al. (2009)  
BWRO Tunisia 15,000 SiO2, CaSiO3, Fe3O4, AlPO4, CaSO4, polysaccharides, proteins Karime et al. 2008
BWRO Djerba Island, Tunisia 3,000 Si, Fe, Cr, Al, organic material Boubakri & Bouguecha 2008)  
BWRO Luong Quoi, Giong Trom, Ben Tre, Viet Nam 480 Si, Fe, Al, polysaccharides, proteins, aromatic compounds, coliform This work 
ApplicationsPositionCapacity (m3/day)Primary foulants and elementsReference
SWRO Mediterranean coast 2,500 SiO2, polysaccharides, humic acids, proteins Adel et al. (2022)  
SWRO Taizhou, Zhejiang, China – Si, Fe, Al, polysaccharides, proteins Wang et al. (2023)  
SWRO Red Sea coast 40,000 Al, Fe, Mn, Si, low molecular weight acids Fortunato et al. (2020)  
SWRO Gijang, Busan, Republic of Korea 9,000 Al, Ca, Cu, Fe, Mg, Mn, Zn, humic acids, proteins Lee et al. (2021)  
SWRO Saudi Arabia 1,800 Fe, polysaccharides, proteins Gonzalez-Gil et al. (2021)  
BWRO Turkey 40 Al, Si Balcik (2021)  
BWRO London, UK 1.2 Al, Ca, Si, diatoms, pseudomonas, polysaccharides García-Triñanes et al. (2022)  
BWRO Algeria 8,500 Si Arras et al. (2009)  
BWRO Tunisia 15,000 SiO2, CaSiO3, Fe3O4, AlPO4, CaSO4, polysaccharides, proteins Karime et al. 2008
BWRO Djerba Island, Tunisia 3,000 Si, Fe, Cr, Al, organic material Boubakri & Bouguecha 2008)  
BWRO Luong Quoi, Giong Trom, Ben Tre, Viet Nam 480 Si, Fe, Al, polysaccharides, proteins, aromatic compounds, coliform This work 

Evaluation of efficiency of chemical cleaning process

The BWRO system was operated in a batch mode (i.e., 10 m3 feed water/batch) without a pretreatment installation, and the feed pressure and permeate flux were monitored as a function of operating time (Figure 7). During the operation, the feed pressure gradually increased while the permeate flux rapidly decreased. After 142 h of operation, the feed pressure reached an increase of 9.9% from 14.1 to 15.5 bar, while the permeate flux experienced a decrease of 42.5% from 10.6 to 6.1 L/min, signifying membrane fouling and cleaning requirements. Clearly, the absence of pretreatment processes accelerated the fouling time and impaired the RO membrane efficiency.
Figure 7

Feed pressure and permeate flux of the BWRO system as a function of operating time.

Figure 7

Feed pressure and permeate flux of the BWRO system as a function of operating time.

Close modal
After the RO membrane was fouled, a chemical cleaning process using Hydrex 4506 and 4703 agents was initiated. These chemicals were diluted to a 3% solution to achieve the recommended active ingredient concentrations (i.e., [citric acid in Hydrex 4703] ∼ 2.0 wt%) and [sodium lauryl sulfate (SLS) in Hydrex 4506] ∼ 0.025 wt%) as specified by the membrane manufacturer (DuPont 2023). Citric acid in Hydrex 4703 can dissolve minerals and oxides and form complexes with metal ions, removing inorganic foulants from the membrane surface. SLS combined with sodium hydroxide in Hydrex 4506 facilitates the removal of organic compounds and prevents the re-deposition of impurities onto the cleaned membrane. In addition, ethylenediaminetetraacetic acid (EDTA) in Hydrex 4506 can eliminate the water hardness ions such as Ca2+ and Mg2+ through the stable complexation. Sequential acid and base treatments are commonly employed in chemical cleaning processes due to their effectiveness in disrupting interactions between inorganic and organic contaminants while minimizing the risk of reactions between the acidic and basic cleaning agents (Woo et al. 2015; Leong et al. 2016; Hacıfazlıoğlu et al. 2019). In this study, cleaning processes were conducted with Hydrex 4703 at pH ∼ 3, Hydrex 4506 at pH ∼ 11, and RO water at pH ∼7 in sequence. The suggested total circulation duration for both cleaning agents is approximately 2 h, ensuring effective removal of most fouling materials from the membrane surface (DuPont 2023). After cleaning, the feed pressure and permeate flux recovered 100.0 and 96.8%, respectively, compared with the initial values. However, the cleaned RO membrane was contaminated more rapidly than the virgin one (i.e., 93 vs. 142 h). This could be attributed to the change in the RO membrane surface characteristics such as hydrophilicity, zeta potential, permeability, and porosity, which are frequently encountered in chemical cleaning processes (Wang et al. 2020; Huang et al. 2021; Khan & Kim 2023). To evaluate the impact of chemical cleaning on membrane properties, fouled membrane samples (i.e., 2 × 2 cm in size) were cleaned in a 100-mL beaker under conditions mimicking the cleaning-in-place procedure. The FT-IR spectra and contact angles of membranes subjected to one and two cleaning cycles were analyzed, with the results presented in Figure 8(a). The peaks observed at 1,663, 1,609, and 1,541 cm−1 correspond to the vibrations of amide I, aromatic amide, and amide II groups of the polyamide layer, respectively, while peaks at 1,245 and 1,180 cm−1 are associated with aryl − O − aryl and SO2 groups of the polysulfone support layer (Kwon & Leckie 2006; Tang et al. 2009). A reduction in the intensity of these hydrophilic functional groups was observed following the second cleaning cycle, suggesting that chemical cleaning can degrade the functional groups on the RO membrane. Additionally, the contact angle of the membrane increased after the second cleaning compared to the first (40.0° vs. 35.3°), indicating a decrease in membrane hydrophilicity (Figure 8(b)). These findings imply that repeated chemical cleaning reduces the hydrophilic nature of RO membranes, thereby increasing their susceptibility to foulants and accelerating membrane fouling. While the cleaning process utilizing both acidic and caustic agents demonstrated high efficiency in restoring feed pressure and permeate flux, making it suitable for practical applications, its adverse effects on the membrane surface warrant further optimization and improvement.
Figure 8

(a) FT-IR spectra and (b) contact angles of RO membrane after first and second cleaning.

Figure 8

(a) FT-IR spectra and (b) contact angles of RO membrane after first and second cleaning.

Close modal

Table 4 presents a comparative summary of the effectiveness of various chemical cleaning protocols for restoring membrane permeate flux, as reported in previous studies, in relation to the findings of the present investigation. Typically, membrane cleaning procedures involve the sequential or combined application of acidic and alkaline agents. Moreover, the inclusion of chelating agents, such as EDTA and citric acid or surfactants, such as sodium dodecyl sulfate (SDS) and SLS, can enhance cleaning efficacy. In scenarios involving severe biofouling or organic contamination, oxidizing agents like chlorine or hydrogen peroxide may be employed to degrade fouling compounds. However, due to their potentially deleterious effects on membrane integrity, the use of such oxidants should be carefully controlled. The combination of the SLS surfactant, EDTA as a chelating agent, and an alkaline cleaning solution (Hydrex 4506), along with citric acid (Hydrex 4703), had demonstrated a high degree of cleaning efficiency across a broad range of foulants. Consequently, this integrated cleaning approach achieved near-complete restoration of membrane permeate flux and is particularly suitable for treating membranes fouled by complex mixtures of inorganic, organic, and biological substances.

Table 4

Effectiveness of chemical cleaning agents in restoring permeate flux of RO membranes

Type of RO membranesChemical cleaning agentsCleaning time (min)Permeate flux recovery (%)Reference
BW30 (Dow) NaOH (3.0 ppm) 105 87.5 Hacıfazlıoğlu et al. (2019)  
NaOCl (5.3 ppm) 
Citric acid (1,000 ppm) 
SWC3 (Hydranautics) NaOH (2%) 120 74.4 Garcia-Fayos et al. (2015)  
sodium dodecyl sulfate (1%) 
TM820-400 (Toray) Hydrex® 4730 (2%) 60 99.0 Adel et al. (2022)  
RE 16040-SHF (Toray) NaOH (0.1 N) 360 – Lee et al. (2021)  
HCl (0.1 N) 
ESPA (Hydranautics) Basic solution (1.5%,) Prime-Tech, Korea) 1,440 87 Kim et al. (2017)  
Acidic solution (1.5%,) Prime-Tech, Korea) 
RE8040-BLR (CSM) EDTA (0.2%) 20 86.6 Park et al. (2018)  
CR-100 (DuPont) Hydrex 4506 (3%) 120 99.7 This work 
Hydrex 4703 (3%) 
Type of RO membranesChemical cleaning agentsCleaning time (min)Permeate flux recovery (%)Reference
BW30 (Dow) NaOH (3.0 ppm) 105 87.5 Hacıfazlıoğlu et al. (2019)  
NaOCl (5.3 ppm) 
Citric acid (1,000 ppm) 
SWC3 (Hydranautics) NaOH (2%) 120 74.4 Garcia-Fayos et al. (2015)  
sodium dodecyl sulfate (1%) 
TM820-400 (Toray) Hydrex® 4730 (2%) 60 99.0 Adel et al. (2022)  
RE 16040-SHF (Toray) NaOH (0.1 N) 360 – Lee et al. (2021)  
HCl (0.1 N) 
ESPA (Hydranautics) Basic solution (1.5%,) Prime-Tech, Korea) 1,440 87 Kim et al. (2017)  
Acidic solution (1.5%,) Prime-Tech, Korea) 
RE8040-BLR (CSM) EDTA (0.2%) 20 86.6 Park et al. (2018)  
CR-100 (DuPont) Hydrex 4506 (3%) 120 99.7 This work 
Hydrex 4703 (3%) 

Design proposal for a pretreatment system

Based on the comprehensive analysis of fouled RO membranes and associated solid and liquid sludge samples using a variety of analytical techniques including SEM-EDX, XRD, XRF, FT-IR, as well as assessments of organic and microbiological indicators, the principal foulants were identified. These included inorganic mineral scales such as Quartz, Muscovite-2M1, Kaolinite, Dickite-2M1, among others; organic constituents such as polysaccharides and proteins; and biological contaminants including microorganisms like coliforms. It is evident that in the absence of a pretreatment system, the fouling of RO membranes occurs more rapidly and severely. Although chemical cleaning methods were effective in restoring membrane performance, they might compromise the physical and chemical integrity of the membrane surface, accelerate subsequent fouling, and increase operational costs due to chemical and energy consumption.

Accordingly, the implementation of an appropriate pretreatment system is essential for enhancing feed water quality prior to RO membrane filtration. In alignment with recommended design principles for RO desalination systems (Henthorne & Boysen 2015; Kavitha et al. 2019; Abushawish et al. 2023; Pesarakloo et al. 2024), a pretreatment process tailored to the pilot-scale RO unit utilizing Luong Quoi River water is proposed. The feed water will first undergo coagulation and flocculation, followed by storage in a 10 m3 tank. Subsequently, the water will pass through a dual-media filter (comprising quartz sand and gravel), a triple-media filter (consisting of greensand, quartz sand, and gravel), and finally a cartridge filtration unit equipped with a filter element having a pore size of 5 μm.

The coagulation–flocculation stage is particularly effective in removing a wide range of suspended solids, organic matter, and microbial contaminants. Greensand media exhibit strong adsorption capacity for heavy metals, while the presence of oxidizing agents (e.g., dissolved oxygen) facilitates the degradation of organic and microbial foulants. The integrated use of flocculation and multi-layer filtration has demonstrated high efficiency in mitigating common fouling agents. Furthermore, the final-stage cartridge filter acts as a physical barrier to particles larger than 5 μm, reducing the particulate load on the RO membrane.

To further improve the removal of microbial and inorganic foulants, a chlorine dosing system can be integrated for disinfection purposes. However, dechlorination using sodium bisulfite is required prior to membrane contact to prevent oxidative damage. Reducing the pore size of the cartridge filter to values such as 0.2 μm could enhance fine particle removal, albeit at the expense of increased system pressure. This would necessitate higher-capacity feed pumps and reinforced piping to accommodate the pressure demands. Therefore, a careful evaluation of the trade-offs between treatment efficiency and economic feasibility is crucial in optimizing the pretreatment strategy.

The BWRO system at Luong Quoi water plan was performed in a batch mode without pretreatment processes. The feed pressure increased to 10% of the initial value, and the permeate flux decreased to 45% after 142 h of operation. The visual and scanning electron microscopic observation on the RO module revealed serious foulants on the RO membrane, but no physical damage on the other parts. The analytical results from XRD, XRF, EDX, and FT-IR indicated the contribution of inorganic minerals, organic substances, and microorganisms to the fouling formation on the RO membrane, which can provide valuable data for pretreatment configuration design. A combination of Hydrex 4703 and 4506 chemicals in the chemical cleaning method efficiently removed the foulants on the RO membrane, which can be applied in practice. However, the repeated use of membrane cleaning chemicals can gradually degrade the hydrophilic functional groups on the membrane surface, leading to an increased rate of re-fouling. Consequently, it is essential to carefully assess the frequency of membrane cleaning and determine the optimal membrane replacement schedule to ensure economic efficiency. Furthermore, exploring the integration of physical cleaning techniques into membrane cleaning protocols may help mitigate the adverse effects of chemical agents on the properties of RO membranes. The scalability of the cleaning approach for industrial RO systems requires adjustments in chemical dosing strategies and hydraulic configurations. Additionally, incorporating the cleaning protocol into automated frameworks may enhance operational reliability and reduce labor-intensive maintenance.

We would like to express our sincere gratitude to the Ministry of Science and Technology, the Department of Science and Technology of Ben Tre, and Nguyen Tat Thanh University for their support in terms of time, material resources, and experimental laboratory facilities for this research.

This research is funded by the project ‘Study on the construction of a saline water treatment system with IoT application to supply domestic water for people in saltwater intrusion areas of Ben Tre province and some neighboring provinces,’ with project code ĐTĐL.CN–123/21.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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