The clothes washing industry generates large volumes of laundry wastewater that, in principle, can be well treated by ceramic membrane filtration. However, the fouling of ceramic membranes by fibers/fragments from laundry wastewater could result in a decrease of the water permeance across the membranes. In this study, synthetic wastewater containing cotton, linen, polyester, and nylon fibers and real wastewater were characterized and prepared for the filtration experiments, which were conducted at a flux of 70 Lm−2 h−1 using an alumina (Al2O3) membrane and a silicon carbide (SiC)-coated Al2O3 membrane. Results revealed that natural fabrics, particularly cotton and linen, released higher chemical oxygen demand (COD) loads than synthetic fibers when tested at equal mass, which was further supported by microscopic and SEM imaging. The SiC-coated membrane exhibited a relatively lower reversible and irreversible fouling, attributed to its highly negatively charged surface, which repels the fibers that are negatively charged by negatively charged surfactants. The observed fouling among different fibers corresponded well with the COD levels of the synthetic laundry wastewater containing those fibers. Laser direct infrared imaging (LDIR) analysis confirmed that natural fibers dominate in real laundry wastewater. Treating hot laundry wastewater was more effective in reducing both reversible and irreversible membrane fouling than treating it at room temperature. Moreover, the filtration of hot laundry wastewater could facilitate the recovery and reuse of water, surfactants, and heat, offering a sustainable solution to reduce both water consumption and energy costs. This study underscores the importance of paying closer attention to natural fibers, as they tend to cause more severe membrane fouling compared to synthetic fibers in ceramic membrane-based water treatment systems. ... Read more

Sulphate (SO42­) is a model ion due to its negative charge and multivalent nature. Its rejection behavior serves as an indicator of the separation performance for other analogous ions in modified membranes. In literature the rejection of the SO42­ by negatively charged polymeric nanofiltration (NF) membranes has been studied extensively with rejection percentages of >90 %. Silicon carbide (SiC) membranes have gained attention for wastewater treatment due to their high hydrophilicity and negative charge. However, no negatively charged ceramic ultrafiltration (UF) membranes have been tested yet for SO42­ retention. In this study, a commercial alumina (Al2O3) UF membrane was converted into a highly negatively charged tight-UF membrane by coating it with SiC. This was achieved by depositing a 5 μm SiC coating in a single-step via low-pressure chemical vapor deposition (LP-CVD). LP-CVD facilitates the preparation of a SiC at much lower temperatures (700–900 °C) compared to the sol-gel methods (ca. 2100 °C), and it does not require multiple coating cycles and sintering steps to achieve the desired selective layer thickness. Subsequently, properties and performance of the as-prepared tight-UF membrane coated with SiC were evaluated. The SiC coated membrane had a highly negative charge of −70 mV at pH of 6, and a pure water permeability (PWP) of 26 L.m−2.h−1.bar−1. The SiC coated membrane furthermore demonstrated a SO42­ rejection of 79 % despite having a large pore size of 7 nm, in comparison with the pore sizes of below 1 nm of NF membranes. These results highlight the potential of singe-step LP-CVD modification of commercial UF ceramic membranes to produce highly negatively charged SiC coated UF membranes with a high SO42­ rejection, and without a large loss of PWP normally associated with NF membranes. ... Read more

During the extraction of fossil fuels, a complex waste stream is produced simultaneously, also known as produced water (PW). Membrane filtration is a promising technology that can successfully enable the treatment and reuse of PW. Silicon carbide (SiC) membranes are preferred for PW treatment, due to their low (ir)reversible fouling compared to other ceramic membranes. However, full SiC membrane is expensive and thus economically less feasible. Therefore, we established a method for coating SiC on alumina (Al₂O₃) ultrafiltration membranes, based on low-pressure chemical vapor deposition at 860 °C. In the presented study the fouling resistance and behavior of these novel membranes, with various pore sizes and under different operating conditions, including flux and crossflow velocity, were evaluated. We also used Al₂O₃ membranes and SiC-coated Al₂O₃ membranes in constant flux mode to treat real oilfield PW with high salinity (142 mS/cm) and COD (22670 mg/L). Additionally, the fouling mechanisms in the SiC-coated and Al₂O₃ membranes were analyzed with the help of Focused Ion Beam-Scanning Electron Microscopy imaging. The major findings were that pore blockage served as the initial (irreversible) fouling mechanism and that the (reversible) cake layer, a mixture of organic and inorganic components, dominated the rest of the filtration cycle, where the SiC coated membrane performed better than the original alumina membrane. In addition, it was found that the application of the SiC coating, and the selection of the appropriate pore size (62 nm) and crossflow velocity (0.8 m/s) increased the fouling mitigation, potentially advancing the utilization of ultrafiltration in treating saline PW for reuse purposes. ... Read more

Large amounts of oily wastewater, which can be defined as produced water, are generated in oilfields. Ultrafiltration (UF) serves as an effective and economical method to purify produced water. Unfortunately, membrane fouling during produced water treatment is severe. In this paper, the effects of the ionic strength (1, 20, and 100 mM) as well as different surfactants on the membrane fouling are investigated. Four surfactants, including SDS (anionic), APG (non-ionic), CTAB (cationic) and DDAPS (zwitterionic), were selected for this study. The Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO (XDLVO) models were used to quantify interactions between the membrane-oil droplet and deposited oil layer-oil droplet surfaces and to compare these interactions with the fouling experiments. The (X)DLVO interaction energies of the membrane-oil droplet exhibited a strong agreement with the fouling tendencies at 1 mM salinity. The SiC-deposited (B20) membrane showed less reversible and irreversible membrane fouling than the Al₂O₃ (B0) membrane when filtering negatively charged O/W emulsions stabilized with SDS, APG, or DDAPS. The DLVO model predicted a higher fouling tendency at higher salinity levels during the filtration of SDS, APG, or DDAPS-stabilized O/W emulsions and a decreased fouling tendency for CTAB-stabilized emulsion with the B20 membrane. However, at higher salinity levels, the XDLVO energy barrier was affected by both the repulsive electrostatic double layer (EL) interaction and attractive Lewis acid-base (AB) interaction. By comparing both experiments and (X)DLVO modeling, this study improves the fundamental understanding of the effect of ionic strength and surfactant types on reversible and irreversible fouling of the Al₂O₃ and SiC-coated membranes fouling by O/W emulsions. ... Read more

Rapid industrialization and urbanization over the past two decades have made water scarcity and water pollution the most serious and persistent challenges for people around the world. Membrane technologies have emerged as crucial solutions to tackle the global water shortage crisis, especially for the re-use of industrial effluents. Inorganic ceramic membranes are gaining increasing attention in industry due to their high mechanical and chemical stability, hydrophilicity, water permeability, antifouling abilities. Silicon carbide (SiC) membranes have shown the lowest fouling compared with other ceramic membranes. Therefore, recently, new methods have been developed to fabricate SiC membrane at a low temperature of 860 ^oC, using low pressure chemical vapor deposition (LPCVD). This thesis focuses on the fabrication and application of SiC-coated membranes, detailing their preparation via LPCVD and their performance in treating nano-sized oil-in-water (O/W) emulsions, real produced water, and laundry wastewater.

First of all, a novel approach is presented for effectively separating microemulsions via SiC (3C-SiC)-coated alumina (Al₂O₃) membranes, fabricated based on LPCVD. With the increase in deposition time, up to 25 min, the pore size of the membranes decreased from 41 nm (without deposition) to 33 nm (deposition time of 25 min). The polycrystalline 3C-SiC-coated membranes also showed an improved hydrophilicity (water contact angle of 15º) and highly negatively charged surfaces (-65 mV). Oil-in-water (O/W) microemulsions filtration experiments were carried out at a constant permeate flux (80 Lm^-2h^-1) for six cycles with varying deposition time, pH, surfactant types, and pore sizes. The fouling of the SiC-coated membrane was, compared to the Al₂O₃ membrane, effectively mitigated due to the enhanced electrostatic repulsion and hydrophilicity. Surfactant adsorption mainly occurred when the surface charge of the microemulsion and the membranes were opposite. Therefore, the surface charge of the Al₂O₃ membrane changed from positive to negative when soaked in negatively charged microemulsions, whereas SiC-coated membranes remained negatively charged regardless of surfactant type. The membrane fouling was alleviated when the membrane and oil droplets had the same charge.

Subsequently, the effects of the ionic strength (1, 20, and 100 mM) as well as different surfactants in O/W emulsions on the membrane fouling were studied. Four surfactants, including sodium dodecyl sulfate (SDS, anionic), alkyl polyglycoside (APG, non-ionic), cetyltrimethylammonium bromide (CTAB, cationic) and N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS, zwitterionic), were selected for this study. The Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO (XDLVO) models were used to quantify interactions between the membrane-oil droplet and deposited oil layer-oil droplet surfaces and to compare these interactions with the fouling experiments. The (X)DLVO interaction energies of the membrane-oil droplet exhibited a strong agreement with the fouling tendencies at 1 mM salinity. The SiC-deposited membrane showed less (ir)reversible membrane fouling than the Al₂O₃ membrane when filtering O/W emulsions stabilized with SDS, APG, or DDAPS. The DLVO model predicted a higher tendency at higher salinity levels during the filtration of SDS, APG, or DDAPS-stabilized O/W emulsions and a decreased fouling tendency for CTAB-stabilized emulsion with the SiC-deposited membrane. However, at higher salinity levels, the XDLVO energy barrier was affected by both the repulsive electrostatic double layer (EL) interaction and attractive Lewis acid-base (AB) interaction. For the Al₂O₃ membrane, the XDLVO model obscured EL and Lifshitz-van der Waals (LW) interactions since the AB component was dominant, confirmed by the diminished XDLVO energy barrier, whereas for the SiC-deposited membrane, the EL interaction prevailed since the energy barrier value was positive.

Then real oilfield produced water with high salinity (142 mS/cm) and COD (22670 mg/L) was successfully treated by SiC-coated Al₂O₃ membranes in constant flux mode. The major findings were that pore blockage served as the initial (irreversible) fouling mechanism and that the (reversible) cake layer, a mixture of organic and inorganic components, dominated the rest of the filtration cycle, where the SiC-coated membrane performed better than the original Al₂O₃ membrane. In addition, it was found that the application of the SiC coating, and the selection of the appropriate pore size (62 nm) and crossflow velocity (0.8 m/s) increased the fouling mitigation, potentially advancing the utilization of ultrafiltration in treating saline produced water for reuse purposes.

Finally, synthetic wastewater containing cotton, linen, polyester, and nylon fibres and real laundry wastewater were characterized and prepared for filtration experiments, which were conducted at a flux of 70 Lm^-2h^-1 using an Al₂O₃ membrane and a SiC-coated membrane. Results revealed that natural textiles, particularly cotton and linen, released higher COD loads than synthetic fibers when tested at equal mass, in the trend of, cotton>linen>polyester>nylon, which was further supported by microscopic and SEM images. Both the Al₂O₃ membrane and the SiC-coated membrane showed a high fiber rejection (100 %), whereas the SiC-coated membrane showed lower reversible and irreversible fouling than the Al₂O₃ membrane, due to highly negatively charged surface. The fouling order of the fibers were in line with the COD concentration of the synthetic laundry wastewater containing these fibers. Finally, treatment of hot real laundry wastewater by the ceramic membranes not only mitigated membrane reversible and irreversible fouling, but also enabled the simultaneous recovery and reuse of water, surfactants, and thermal energy, offering a sustainable strategy to reduce both water consumption and energy costs.

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Worldwide, a considerable amount of oily wastewater is generated, with oil droplets from 2 to 200 nm that are difficult to separate because of their size and colloidal stability. This study presents a novel approach for effectively separating microemulsions via cubic silicon carbide (3C-SiC)-coated alumina (Al ₂O ₃) membranes fabricated based on low pressure chemical vapor deposition (LPCVD). SiC was deposited at a relatively low temperature at 860 °C on 100 nm Al ₂O ₃ membranes using two precursors: SiH ₂Cl ₂ and C ₂H ₂. With the increase in deposition time, up to 25 min, the pore size decreased from 41 nm to 33 nm, which is a smaller pore size of a SiC membrane than previously used for oil/water separation. The polycrystalline 3C-SiC-coated membranes showed improved hydrophilicity (water contact angle of 15°) and highly negatively charged surfaces (−65 mV). Microemulsion filtration experiments were carried out at a constant permeate flux (80 Lm ⁻² h ⁻¹) for six cycles with varying deposition time, pH, surfactant types, and pore sizes. The fouling of the SiC-coated membrane was, compared to the Al ₂O ₃ membrane, effectively mitigated due to the enhanced electrostatic repulsion and hydrophilicity. Surfactant adsorption mainly occurred when the surface charge of the microemulsion and the membranes were opposite. Therefore, the surface charge of the alumina membrane changed from positive to negative when soaked in negatively charged microemulsions, whereas SiC-coated membranes remained negatively charged regardless of surfactant type. The membrane fouling was alleviated when the membrane and oil droplets had the same charge. Lastly, the 62 nm SiC-coated membrane with 20 min coating time was the best choice for the filtration of the microemulsion, because of the high rejection of the oil droplets and low fouling tendency. ... Read more