RESEARCH PAPER

 

Physicochemical autopsy and sequential cleaning optimization of seawater reverse-osmosis membranes: a study from Beni Saf desalination plant, Algeria

 

 

Abdessalam Radjai^I; Mourad Amara^{II, III}; Fouzia Houma^I; Hafida Hadjar^{IV, V}; Safia Chernai^I; Saifi Amirouche^{IV, VI}; Boualem Hamdi^I

^IENSSMAL, École Nationale Supérieure des Sciences de la Mer et de l'Aménagement du Littoral, BP 19 Campus Universitaire, Bois des Cars, Dely Ibrahim, 16320, Algiers, Algeria
^IILaboratory of Hydrometallurgy and Molecular Inorganic, Chemistry, Faculty of Chemistry, USTHB, El Alia, BP 32,16111, Algiers, Algeria
^IIINational School of Nanoscience and Nanotechnology (ENSNN), Sidi Abdellah, Technological Hub, Algiers, Algeria
^IVCRAPC, Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques, BP 384, Zone Industrielle, Bou-Ismail, RP 42004, Tipaza, Algeria
^VLPCMAE, Laboratoire d'Etude Physico-Chimique des Matériaux et Application à l'Environnement, Faculty of Chemistry, USTHB University, BP 32, El Alia Beb Ezzouar, 16111, Algiers, Algeria
^VIURMPE, Unite de Recherche, Matériaux, Procédés et Environnement, M'hamed Bougara University, Boumerdès, 35000, Algeria

Correspondence

 

 

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ABSTRACT

This study focused on the fouling of two seawater reverse osmosis (SWRO) membranes at the Beni Saf Water Company desalination plant in Algeria, which has a daily capacity of 200 000 m³ and a recovery rate of 45% using 17 920 membranes. Approximately 3 234 membranes are replaced annually due to fouling. A detailed study of the fouling agents of the two membranes was conducted using various analytical techniques, such as moisture analysis, loss on ignition (LOI), determination of calcium carbonate (CaCO₃) content, x-ray fluorescence (XRF), x-ray diffraction (XRD), and Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR). Surface characterization was also performed using scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy (SEM-EDS), FTIR-ATR, and XRD. The LOI analysis indicated that more than 30% of the fouling material was organic in nature. FTIR-ATR identified the presence of -OH groups, phenolic C-O groups, and amide bonds, suggesting the accumulation of organic substances such as proteins, humic substances, and polysaccharides. Additionally, SEM-EDS, XRF, and XRD revealed relatively high concentrations of silica, primarily in the form of quartz, confirming the formation of an organo-inorganic complex on the membrane surface. Based on these findings, a sequential chemical cleaning protocol was developed, incorporating alkaline (NaOH), metal chelator (EDTA), surfactant (SDS), oxidant (H₂O₂), and sulfuric acid (H₂SO4), each followed by rinsing with deionized water (DI). This cleaning regime effectively removed fouling from the membrane surface, resulting in an average weight loss of 15% for one membrane and 14% for the other.

Keywords: desalination, autopsy, membrane fouling, foulant characterization, chemical cleaning, Algeria

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INTRODUCTION

North African countries face a critical water crisis driven by natural and human factors, resulting in alarmingly low freshwater availability (less than 1 000 m³/inhabitant per year) (Leal Filho et al., 2022; Soula et al., 2021). Unsustainable water demand, fuelled by population growth, urbanization, and rising living standards (Gleick, 2014), is exacerbated by climate change, poor water network maintenance, and inadequate treatment infrastructure (Kusangaya et al., 2014). These challenges hinder socio-economic development and water security (Zlati et al., 2024). To address this crisis and achieve water and food security as part of sustainable development goals, governments must adopt innovative strategies, including non-conventional water resource treatment powered by renewable energy (Kusangaya et al., 2014; Al-Saidi et al., 2016). Among advanced desalination technologies (thermal, electrochemical, and reverse osmosis (RO)), RO membrane desalination stands out as the most promising, offering high-quality potable water while minimizing environmental impact (Lee et al., 2021).

North Africa and the Middle East dominate the global desalination market, representing 70% of its capacity (Philibert et al., 2024). Algeria, a regional leader, has constructed 21 desalination plants in less than 20 years, investing heavily in seawater desalination since 2000 (Mahmoudi et al., 2023). Today, Algeria ranks first in Africa and second in the Arab world (after Saudi Arabia) in desalinated water production capacity (Sayed et al., 2023). According to the National Statistics Office (ONS, 2024), Algeria's population reached 46.3 million in 2023, with projections of 47.4 million by 2024 and 52 million by 2030. This demographic growth, combined with ongoing drought conditions, has driven annual potable water demand to over 52 billion m³. To combat these challenges, the government has invested heavily in desalination in a staggered development programme (as shown in Fig. A1, Appendix): (i) initial phase - 23 monobloc plants (57 500 m³/day) and 14 large-scale plants (2.2 million m³/day), providing 18% of potable water (Amitouche and Remini, 2016); (ii) second phase - 5 mega-plants (300 000 m³/day each) entered service in 2025, increasing desalinated water's share to 42% of the country's drinking water production and total capacity to 3.7 million m³/day; (iii) third phase: 7 new plants by 2027, including 6 plants treating 300 000 m³/day and one 60 000 m³/day plant, raising total capacity to 5.61 million m³/day. To oversee these efforts, Algeria established the National Desalination Agency (ANDE) in 2023 (JORA, 2023). This agency is responsible for the completion, operation, and maintenance of desalination plants and related infrastructure, ensuring efficient and effective performance.

The rapid deployment of membrane-based seawater desalination plants has led to frequent technical shutdowns, primarily due to membrane fouling, causing significant financial losses and water distribution disruptions. These issues are exacerbated by poorly located plants near polluted areas, such as river mouths with high suspended solids and industrial spills. Poor seawater quality, inadequate pre-treatment, and marine hydrodynamics contribute to premature fouling, reducing potable water production by over 40% in some cases. Under optimal conditions, RO membranes are typically replaced every 2-3 years to maintain efficiency (Choi et al., 2009; Khoo et al., 2021). However, in some Algerian plants, membrane lifespans are less than 2 years, influenced by raw water quality, hydraulic conditions, fouling, and cleaning frequency (Ahmed et al., 2023). Premature replacements increase costs and disrupt operations.

The Beni Saf Water Company plant in Algeria, has a capacity of 200 000 m³ per day, consuming an average of approximately 3 234 membranes annually (18% of its total). Across Algeria's 21 desalination plants (producing 2.26 million m³/day), around 15 000-25 000 membranes are replaced yearly, resulting in an estimated 8-12 million USD in annual losses. Despite the significant impact of premature fouling, systematic studies on this issue are lacking. Membrane autopsies are essential to diagnose fouling causes and inform decision-making.

By foulant class, 4 types dominate: organic, colloidal, biofouling, and mineral scaling. Organic fouling (NOM: humic acids/ fulvic acids, proteins, polysaccharides) is strongly influenced by pH and divalent cations; lower pH and higher Ca²+/Mg²+ enhance adsorption/bridging and flux decline, and hydrophobic/ rough surfaces foul more readily (Yang et al., 2021; Hong and Elimelech, 1997). Scaling increases with recovery as sparingly soluble salts exceed solubility at the surface; severity depends on supersaturation and precipitation kinetics and is exacerbated by concentration polarization (common scales: CaCO₃, CaSO₄, BaSO₄, Ca-phosphate, silica) (Jawor and Hoek, 2009; Matin et al., 2019). Biofouling begins with a NOM followed by microbial attachment; biofilms thicken with nutrients and can become partially irreversible, causing flux loss, differential pressure (ΔΡ) rise, and occasional membrane biodegradation (Creber et al., 2010; Matin et al., 2011; Flemming, 1997). Colloidal fouling (11 000 nm; silt, silica, clays, iron oxides, macromolecules) is often tracked by SDl/turbidity and typically proceeds via cake-enhanced concentration polarization, which raises near-surface osmotic pressure and reduces driving force (Adel et al., 2022; Al-Amoudi and Lovitt, 2007).

This study aimed to conduct a detailed physicochemical autopsy of fouling on two SWRO membranes from the Beni Saf Water Company (BWC) desalination plant in Algeria, identifying the primary fouling agents. The results should help the plant operator improve management of operations. To achieve this goal, a comprehensive analysis of the extracted fouling material was performed using various techniques, including moisture analysis, loss on ignition (LOl), determination of CaCO₃ percentage, Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), x-ray diffraction (XRD), and x-ray fluorescence (XRF). Additionally, the membrane surface was also examined using scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy (SEM-EDS), FTIR-ATR, and XRD.

 

MATERIAL AND METHODS

Description of the Beni Saf Water Company (BWC) plant

The BWC plant is located in the Aïn Témouchent Province on the coast of the Alboran Sea. lt serves the water needs of two provinces suffering from limited access to freshwater resources. The plant's flow diagram (Fig. 1) illustrates the various stages of the desalination process. The RO unit consists of 10 racks, each rack containing 265 tubes, with 7 membrane elements installed per tube. A total of 17 920 Hydranautics-SWC5-MAX RO membranes produce a total daily capacity of 200 000 m³.

Seawater is abstracted at a fixed intake tower located approximately 1.2 km offshore, with the intake ports positioned 5 m below the water surface. From the tower, water flows by gravity through a submarine polyethylene pipeline (diameter 2 400 mm) to the onshore catchment tank. The main physicochemical characteristics of seawater around the intake tower are summarized in Table 1. Most measurements are from a single sampling campaign (July 2024); SDI values are 1-year averages.

Membranes and fouling material sampling and preparation

Two Hydranautics SWC5-MAX elements (Memb01 and Memb02) from Rack No. 10 at the BWC plant were removed for autopsy; each occupied the No. 2 position in different pressure vessels. Elements were taken from the second position rather than the first to avoid lead-element bias from transient high turbidity events and operational anomalies, while still capturing a fouling profile representative of normal plant operation. Removal was prompted by a rack-specific 19% decrease in normalized permeate production, coincident with a progressive rise in rack ΔΡ, indicative of fouling; this decline does not reflect plant-wide performance. This sampling was designed to identify the plant's dominant fouling type. The membranes were visually inspected for physical deterioration, dissected to remove the outer casing, and unfolded for layer-by-layer examination. From each selected fouling layer, 9 square-shaped coupons (3cmx3 cm) were systematically cut from visibly fouled areas (Fig. 2). Two sample types were collected: (i) membrane coupons: the 3 cm x 3 cm coupons were cut from the fouled area; (ii) fouling material: remaining foulants on the membrane surface were carefully scraped and stored in Petri dishes (Díaz Salazar and Gorczyca, 2024). Both membrane coupons and fouling material samples were stored at 4°C for further analysis.

Membranes and fouling material characterization

To study the fouling material scraped up from the two membranes, several parameters were measured. Moisture content was determined by weighing the samples before and after drying at 105°C for 24 h. LOI was performed by incinerating the samples at 525°C for 4 h to estimate the percentage of organic matter (OM) (Fu et al., 2020; Heiri et al., 2001). CaCO₃ content was determined using Bernard's calcimeter through acid digestion, with CO₂ release measured according to the French AFNOR standard (AFNOR, 1996). Semi-quantitative XRF analyses were conducted on a SciAPS apparatus to determine the elemental composition of inorganic substances present. The crystalline structure of inorganic compounds was characterized by XRD using a BRUKER D8 Advance with CuKa radiation. The morphology and surface composition were studied using SEM-EDS, respectively, on a Zeiss Smart SEM EDX system. FTIR-ATR was performed using an Alpha-Bruker spectrometer, operating in the 4 000-300 cm^-1 wavenumber range at a 2 cm^-1 resolution, to identify the specific functional groups present in the fouling material. The coupons from both membrane modules were also characterized by SEM-EDS. The SEM provided detailed images of the membrane surface, while EDS identified the elemental distribution of the remaining fouling material. Additionally, FTIR-ATR and XRD were used to analyse the functional groups and crystalline phases of the close surface deposits.

Chemical cleaning

Several studies (Filloux et al., 2015; Gul et al., 2021; Unal, 2022; Wang et al., 2015) have explored the efi^ectiveness of various cleaning agents by immersing membrane coupons in difi^erent solutions to evaluate their capacity to remove fouling under controlled conditions. A comprehensive examination was conducted with a range of chemical agents, including alkaline agents (sodium hydroxide, NaOH; potassium hydroxide, KOH), acids (hydrochloric acid, HCl; acetic acid, CH₃COOH; sulfuric acid, H₂SO₄), surfactant (sodium dodecyl sulfate, SDS), chelating (ethylenediaminetetraacetic acid, EDTA), and oxidizing (hydrogen peroxide, H₂O₂) agents. Each agent was evaluated for its cleaning potential. After accurate preparation of chemical cleaning solutions was completed, the effectiveness of the cleaning process was assessed by exposing small coupons (4 cm²) cut separately from various areas of the fouled membranes to the following steps:

• Preparation of the chemical solutions

• Precise weighing and photography

• Immersion in 70 mL of each cleaning solution

• Agitation for 6 h at a controlled temperature

• Drying at 40°C for 48 h

• Weighing after cleaning to determine the residual mass

• Post-cleaning photographing to document any changes in colour and/or aspect

Following the initial cleaning process, the selection of suitable agents for sequential cleaning was based on comparing observed weight loss and changes in the membrane colour. A series of sequential cleaning tests were conducted to establish the most effective chemical cleaning regimen.

 

RESULTS AND DISCUSSION

Visual inspection

Visual inspection of the fouled membranes revealed no noticeable physical damage. A systematic examination of each layer confirmed this observation, alleviating concerns about performance degradation due to structural alterations. Optical images of the fouling on Memb01 and Memb02 (Fig. 3) show a gradual accumulation of dark brown gelatinous deposits, with a concentration gradient along the flow path. This likely results from complexes formed between humic acids and iron, creating a gelatinous brown layer (Nyström et al., 1996). Fouling was most severe at the flow inlet, diminishing toward the outlet. The significant deposits indicate substantial fouling on both membranes (Fig. 3).

Characterization of fouling material

Moisture content, LOI, and CaCO₃

The moisture content was approximately 2% for Memb01 and 1.62-3.18% for Memb02, likely due to the presence of fine particles, such as clays, known for their hydrophilic properties (Paul and Naidu, 2022). Generally, residual moisture typically increases with higher clay and OM content (Baize, 2018). For external context, clayey textures commonly show 4-8% residual moisture, whereas sandy horizons can be <1%; our measurements for both membranes are within the low single-digit range, consistent with a mixed salt-clay-organic matrix (Baize, 2018). LOI analysis showed an average presence of 34.36% OM in the fouling of the first membrane and 32.44% in the second (Fig. 4). This OM content is likely due to the formation of organo-inorganic complexes during the desalination process (Baize, 2018).

 

 

Scaling from CaCO₃ can occur in nearly all types of feedwaters, including seawater, and can form resistant deposits on membrane surfaces (Tzotzi et al., 2007). However, CaCO₃ represented only 4% of the total fouling material in both membranes (Fig. 4), suggesting effective anti-scalant dosage upstream of the cartridge filters or removal during acid-based cleaning (pH 2.5-3.5).

FTIR-ATR analysis of fouling material

Analysis of the fouling materials from both membranes (Fig. 5) revealed distinct peaks at specific wavenumbers.

FTIR-ATR analysis (Table 2) indicates that fouling is predominantly organic in both membranes, derived from POM in seawater (Table 1). Peaks associated with hydroxyl groups, phenolic compounds, amide bonds, and C-C bonds suggest the accumulation of rubber proteins, humic substances, and polysaccharides. Inorganic fouling, indicated by SiO₂ peaks, results from Si-based mineral deposits. Memb01 and Memb02 share the same set of diagnostic bands, indicating the same foulant classes. However, band depths differ between the spectra - several features are more pronounced on Memb01, whereas others are stronger on Memb02. Because band positions coincide and the spectra were compared, these intensity differences are attributed to the relative concentration of the same organic functionalities, rather than different chemistries. Overall, the deposits comprise a mixed organic-inorganic layer.

XRF investigation of deposited fouling material

To evaluate longitudinal variability, fouling material was scraped at the inlet, middle, and outlet of each membrane and analysed by XRF. The results (Table 3) showed that Si, Al, and Fe were the predominant metallic elements in the fouling materials of both membranes. Minor elements (S, P, K, Ca, Ti, Cu) had a total concentration below 2%, while trace elements (Mn, Ni, Zn, As, Rb, Sn, Cr, Mo, Co, Sr, Nb, Zr) had a combined concentration of less than 0.2%. Collectively, these findings indicate minimal contributions from inorganic colloids and metal oxides to fouling, thereby confirming a common fouling type. The high concentrations of Si and Al indicate that clay minerals may play a role in fouling (Leger and Hawker, 1987). Prolonged operation could lead to the accumulation of insoluble aluminosilicate deposits (Melián-Martel et al., 2012). Most heavy metals are likely by-products of nearby industrial activities (Kabata-Pendias and Mukherjee, 2007), particularly discharges from the Beni Saf cement factory near the desalination plant. Across positions, Memb02 exhibited higher inorganic fractions than Memb01. These results indicate that inorganic foulants are primarily captured at the inlet part of the elements, while the observed differences reflect differences in concentration rather than a change in composition.

XRD analysis of foulant material

XRD analysis of fouling material from both membranes revealed several peaks (Fig. 6), with diffraction angles at 20.87°, 26.61°, 31.70°, 39.42°, 45.58°, 50.16°, 60.00°, and 68.28°.

According to Khamis and Arafah (2017), the sharp peaks at 20.87°, 26.61°, 50.16°, 60.00°, and 68.28° indicate the presence of crystalline quartz (SiO₂), which typically originates from marine sediments. These results suggest that the inorganic component of the fouling material contains a significant amount of marine-derived quartz. Additionally, the peaks at 31.70° and 45.58° are attributed to NaCl, a residual salt from seawater, as identified by Al-Abri et al. (2022). The smaller peak at 39.42° corresponds to CaCO₃ (Sahadat Hossain et al., 2023), and the relatively low concentration of CaCO₃ in the fouling material aligns with the results from the Bernard calcimetry analysis, confirming the effectiveness of the anti-scalant used prior to cartridge filtration (Laqbaqbi et al., 2017).

Membrane surface characterization

SEM-EDS analysis of fouled membrane surface

The fouled membrane surfaces were analysed using SEM-EDS (Fig. 7). A thick and heterogeneous fouling layer was observed embedded with white particles, which could significantly impact permeate flow and potentially cause membrane deformation over time (Unal, 2022). Irregular particles were concentrated in the upper regions of the fouling layer; qualitative visual inspection indicated that their apparent density increased from the membrane inlet toward the outlet. Notably, white deposits larger than 100 μm and particles exceeding 5 μm were observed, despite pretreatment with cartridge filters before RO.

EDS analysis detected various elements in the fouled membranes, as shown in Fig. 8. The presence of C and O indicates an organic or biologically introduced contamination, while elements such as Na, Cl, Mg, Al, Si, K, Mo, Fe, and Ca suggest inorganic colloids and crystals (Lee et al., 2021; Adel et al., 2022). Ti, Nb, and S were also observed in some samples. Both membranes exhibited high levels of carbon (30-39%) and oxygen (38%) (Fig. 8), strongly suggesting organic fouling, such as from humic-like substances (Lee et al., 2021). The consistent carbon content (32%) aligns with LOI results.

Na and Cl, even at low percentages, likely originate from seawater, indicating NaCl deposition. Ca and S suggest potential scale formation (e.g., CaCO₃ and CaSO₄) (Butt et al., 1997). K and Ti further highlight the diversity of inorganic fouling.

Low percentages of Si, Al, Mg, and Fe suggest complex forms such as quartz or metallic silicates (e.g., Fe₂SiO₄, Al₂SiO₅, Mg₂SiO₄) (Adel et al., 2022). Their formation may be influenced by anti-scalants and sodium bisulfite (SBS) (Fortunato et al., 2020). Mo is likely from wastewater discharges of the nearby Beni Saf cement plant, as industrial facilities often release Mo residues (Gao et al., 2020). This underscores the impact of external sources, particularly industrial activities, on fouling characteristics.

Apparent differences in wt% between the two elements are consistent with non-uniform deposit thickness/packing and axial transport, rather than different chemistries. These variations also reflect characteristics of SEM-EDS: it is surface-sensitive and semiquantitative, and signals are influenced by local heterogeneity of the fouled layer (Ahmed et al., 2023). Accordingly, the EDS data are interpreted qualitatively and indicate that both membranes experienced the same fouling type; differences in concentration reflect the extent and spatial distribution of the same foulant classes (Martin et al., 2014).

FTIR-ATR spectral analysis of fouled membrane surface

The FTIR-ATR spectra from both membrane surfaces revealed several peaks that provide insight into the composition of the fouling material (Fig. 9). The most significant peaks observed are mentioned in Table 4.

The two membranes show similar band sets; several peaks appear stronger on Memb01, consistent with differences in areal coverage and concentration of the fouling material, and with the qualitative nature of ATR-FTIR. Thus, both Memb01 and Memb02 exhibit the same foulant classes, while intensity differences reflect the extent and spatial distribution of those classes.

The analysis reveals that fouled membranes consist of both organic and inorganic components. Organic fouling, derived from POM in seawater, dominates, while inorganic fouling results from mineral deposits.

The band assignments on the membrane surfaces (phenolic, amide, aromatic features, and carbonate) align with those observed for the scraped fouling material in Table 2, including Si-O-Si and broad O-H signatures. This concordance indicates that both approaches identify the same organic-dominated fouling with an inorganic contribution (silica/silicate and minor carbonate).

XRD investigation of membrane surface

The XRD patterns (Fig. 10) of the membrane coupons (MembOl and Memb02) show peaks in the 2θ range of 15° to 30°, along with peaks similar to those observed in the fouling material.

 

 

For MembOl, peaks were identified at 26.96°, 31.64°, 45.56°, 50.13°, and 60.20°. Memb02 presented peaks at 26.47°, 31.69°, 45.50°, 50.36°, 60.08°, and 68.32°. The peaks in the range 2θ = 15-30° are characteristic of the amorphous nature of polyamide membranes, as previously reported by Melián-Martel et al. (2012). Additional peaks observed in both membrane coupons, which closely align with those identified in the fouling material, reaffirm the presence of SiO₂, NaCl, and traces of CaCO₃, as noted earlier.

Chemical cleaning

Single chemical agent cleaning tests

The experiment aimed to evaluate the effectiveness of 8 different chemical agents for cleaning fouled membranes. The concentrations of the agents were as follows: NaOH (0.1 N), KOH (0.1 N), HCl (0.1 N), acetic acid (0.1 N), sulfuric acid (0.1 N), SDS (10 mM), EDTA (1 mM), and H₂O₂ (1 N). The membrane coupons were agitated in these solutions for 6 h, dried, and their weight loss (Fig. 11) and colour changes recorded.

 

 

The use of NaOH and KOH at 0.1 normality led to significant colour changes in the fouling of both membranes, especially with NaOH, which resulted in a 12% weight loss (Fig. 11). The alkaline agents raise the solution's pH, increasing the negative charge density of organic foulants through carboxyl group deprotonation, enhancing foulant solubility (Ahmed et al., 2023).

No significant colour changes were observed in membrane fouling for the acidic agents, including HCl, sulfuric acid, and acetic acid, and the weight loss was negligible. Acidic cleaning is typically employed to remove precipitated salts or scales, such as CaCO₃ (Lin et al., 2010). The relatively low percentage of CaCO₃ identified in the fouling material confirms that acid cleaning was less effective due to the minor role of scaling in the fouling of these membranes.

Both EDTA and SDS showed effectiveness in removing fouling material from the membrane surfaces (Ang et al., 2006). SDS's ability to clean RO membranes fouled by OM has already been reported (Hong and Elimelech, 1997).

At pH 11, H₂O₂ effectively removed fouling by oxidizing NOM into more oxygenated functionalities (carboxyl, ketonic, aldehydic) that, under alkaline conditions, are deprotonated and more susceptible to base-catalysed hydrolysis, increasing hydrophilicity and solubility (Kramer et al., 1984; Porcelli and Judd, 2010). This supports the superior cleaning efficiency of combined oxidants and alkaline agents over oxidants alone, especially for organic fouling.

Sequential cleaning

Based on the comprehensive analysis of the fouling material's composition and the effectiveness of chemical agents, the key to effective membrane cleaning lies in the successful removal of OM. Vendor guidance recommends avoiding low-pH cleaning initially, as it could worsen organic fouling by solidifying OM (Hydranautics, 2014). The literature indicates that alkaline cleaning is more effective than acidic cleaning for organic fouling because alkaline agents promote OM hydrolysis and solubilization (Madaeni and Samieirad, 2010). A sequential cleaning regimen, starting with SDS, followed by EDTA, proved highly effective for membranes fouled with OM under alkaline conditions (Al Ashhab et al., 2017). In addition, H₂O2 at high pH degrades NOM functional groups, making them more susceptible to hydrolysis (Kramer et al., 1984; Porcelli and Judd, 2010). By contrast, acidic cleaning is most effective for removing precipitated salts (Gan et al., 1999). Prior work also reports that combining NaOH and SDS, followed by acidic treatment, effectively regenerates membranes (Madaeni and Samieirad, 2010).

Guided by the above, we devised a sequential approach using agents tailored to OM removal and subsequent scale removal. The chosen agents included deionized water (DI), NaOH (pH = 12), SDS (10 mM, pH = 11), EDTA (1 mM, pH = 11), H₂O₂ (1 M, pH = 11), and sulfuric acid (pH = 4). The sequence was applied as shown in Fig. 12.

Figure 13 illustrates the significant improvement in membrane surface cleanliness after sequential cleaning. Nearly all fouling material was removed, with an average weight loss of 15% for Memb01 and 14% for Memb02. After optimizing chemical cleaning at the laboratory scale, future research will focus on scaling up the process using a pilot system.

 

 

 

CONCLUSIONS

Our study aimed to evaluate the fouling state of two used membranes taken from a desalination plant located in the province of Aïn Témouchent, Algeria. The investigation into the nature of the fouling products and the surfaces of the two fouled SWRO membranes was conducted to determine the type of fouling on the membrane surfaces and to establish an effective cleaning protocol. This is the first time such a study has been undertaken for Algerian reverse osmosis desalination plants. The physicochemical, morphological, and structural analysis of the polyamide-based surface material required several techniques ((SEM-EDS, FTIR, XRD, and XRF). The membrane autopsies revealed the formation of an organo-inorganic complex on the surface layer of the membrane. Loss-on-ignition and calcimetry analysis indicated that the foulants contained over 30% OM and approximately 4% CaCO₃. FTIR-ATR analysis of the fouling material and membrane surfaces identified peaks corresponding to hydroxyl groups, phenolic compounds, amide bonds, and C-C bonds, suggesting the accumulation of organic substances such as proteins, humic substances, and possibly polysaccharides. Additionally, XRF, XRD, and EDS analysis revealed the presence of inorganic scale-forming compounds, primarily Na, Cl, Mg, Al, Si, K, Mo, Fe, and Ca. Diffraction techniques confirmed that the inorganic portion of the fouling material contained a significant amount of SiO₂, mainly of marine origin. Surface analysis of the fouled membrane samples using SEM-EDS confirmed the presence of a variety of inorganic chemical elements. Based on these results, it is recommended to effectively reduce fouling on the membrane surface by using a chemical cleaning protocol that includes alkaline NaOH, a metal chelating agent (EDTA), a surfactant (SDS), an oxidant (H₂O₂), followed by H₂SO₄ acid, with DI rinsing after each agent. This approach successfully removed almost all fouling materials from the membrane surface.

 

ACKNOWLEDGEMENTS

We thank the Thematic Research Agency in Science and Technology (ATRST) for its support. We are grateful to the Beni Saf Water Company (BWC) for facilitating sampling at the station. We also acknowledge the technical staff of the Center for Scientific and Technical Research in Physico-Chemical Analyses (CRAPC): M Boutaiba and N Yezli (cRAPC), A Bouchama (CRAPC-PTAPC Mostaganem), and H Boulahbal (CRAPC-PTAPC Ouargla), for conducting most of the analyses which significantly contributed to the outcomes of this study.

 

AUTHOR CONTRIBUTIONS

Abdessalam Radjai: conceptualization, formal analysis, investigation, methodology, visualization, writing original draft. Mourad Amara: investigation, methodology, conceptualization, validation, supervision. Fouzia Houma: project administration, resources. Hafida Hadjar: data curation, formal analysis, investigation, project administration, visualization, writing review and editing. Safia Chernai: supervision, writing original draft. Saifi Amirouche: formal analysis. Boualem Hamdi: conceptualization, investigation, methodology, project administration, validation, supervision, visualization, writing original draft.

 

ORCID

Abdessalam Radjai: https://orcid.org/0000-0001-7999-0331

 

 

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Correspondence:
Abdessalam Radjai
Email: abdesselam.radjai@enssmal.edu.dz

Received: 7 February 2025
Accepted: 10 December 2025

 

 

APPENDIX

 

Figure A1 - Click to enlarge