Flux Decline Research Articles

Surface modification of a PVDF membrane by co‐grafting hydroxyl and zwitterionic polymers to enhance wettability and antifouling property

AbstractPolyvinylidene fluoride (PVDF) membranes have been widely applied in the separation of various organic matters owing to their excellent properties. However, they possess low wettability and undergo fouling because of the hydrophobic nature of PVDF. In this study, poly (2‐hydroxyethyl methacrylate) (PHEMA) and poly (sulfobetaine methacrylate) (PSBMA) were co‐introduced to modify the highly hydrophobic PVDF membrane surface via UV photo‐irradiation. Facile UV photo‐grafting was performed by irradiating the pristine PVDF membrane immersed in a PHEMA/PSBMA mixture solution with UV light for 5 min. The performance of the as‐prepared PHEMA/PSBMA‐grafted membrane was compared with that of membranes modified with solely PHEMA or PSBMA. The hydroxyl and zwitterionic groups of the grafted membranes enhanced wettability and increased flux. A tightly bound water layer was formed because of the enhanced wettability, significantly suppressing protein adsorption. The initial flux of bovine serum albumin (BSA) solution through the PHEMA/PSBMA‐grafted membrane was 2861 LMH, which was 3.1 times higher than that obtained by the pristine PVDF membrane. Furthermore, the flux decline of the modified membrane caused by BSA fouling was 49% lower than that of the pristine PVDF membrane. Finally, the PHEMA/PSBMA‐grafted PVDF membrane showed antifouling properties after three BSA filtration cycles.

Gravity-driven membrane coupled with oxidation technology to modify the surface properties and biofilm formation: Biofouling mitigation

Biofilms caused by biological fouling play an essential role in gravity-driven membranes’ (GDMs) flux decline and rejection rate. The effects of ozone, permanganate, and ferrate (VI) in-situ pretreatment on membrane properties and biofilm formation were systematically studied. Due to the selective retention and adsorption of algal organic matter by biofilms and oxidative degradation, the rejection efficiency of dissolved organic carbon (DOC) in algae-laden water pretreated with permanganate by GDM was up to 23.63%. Pre-oxidation extraordinarily postponed flux decline and biofilm formation of GDM and reduced membrane fouling. The total membrane resistance decreased by 87.22%–90.30% within 72 h after pre-ozonation. Permanganate was more effective than ozone and ferrate (VI) in alleviating secondary membrane fouling caused by algal cells destroyed by pre-oxidation. Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory revealed that the distribution of electrostatic force (EL), acid-base (AB), and Lifshitz-van der Waals forces (LW) interactions between M. aeruginosa and the released intracellular algogenic organic matter (IOM) and ceramic membrane surface was similar. The membrane and foulants are always attracted to each other by LW interaction at different separation distances. The dominant fouling mechanism of GDM combined with pre-oxidation technology shifts from complete pore blocking to cake layer filtration during operation. After pre-oxidation of algae-laden water by ozone, permanganate, and ferrate (VI), GDM can treat at least 131.8%, 37.0%, and 61.5% more feed solution before forming a complete cake layer. This study provides new insights into the biological fouling control strategies and mechanisms for GDM coupled with oxidation technology, which is expected to alleviate membrane fouling and optimize the feed liquid pretreatment procedure.

Effectiveness of different CuO morphologies nanomaterials on the permeability, antifouling, and mechanical properties of PVDF/PVP/CuO ultrafiltration membrane for water treatment

The hydrophobic nature of Poly (vinylidene fluoride) (PVDF) is a significant barrier to use in ultrafiltration, resulting in fouling, flux decline, and reduced lifespan in water treatment. This study examines the effectiveness of different morphologies of CuO nanomaterials (NMs) (spherical, rod, plate, and flower), synthesized by the facile hydrothermal method, to modify PVDF membrane with PVP additive for improving the performance of water permeability and antifouling. Such membrane configurations with different morphologies of CuO NMs improved hydrophilicity with a maximum water flux of 222–263 L m−2h−1 compared to 195 L m−2h−1 for the bare membrane and exhibited excellent thermal and mechanical strengths. The characterization results exhibited that plate-like CuO NMs were dispersed uniformly in the membrane matrix, and their incorporation as a composite improved the membrane properties. From the antifouling test with the bovine serum albumin (BSA) solution, the membrane with plate-like CuO NMs had the highest flux recovery ratio (FRR) (∼91%) and the lowest irreversible fouling ratio (∼10%). The antifouling enhancement was due to less interaction between modified membranes and foulant. Further, the nanocomposite membrane showed excellent stability and negligible Cu2+ ion leaching. Overall, our findings provide a new strategy for developing inorganic nanocomposite PVDF membranes for water treatment.

Zero-Oil-Fouling Membrane With High Coverage of Grafted Zwitterionic Polymer for Separation of Oil-in-Water Emulsions.

Current hydrophilic modification strategies improve the antifouling ability of membranes but fail to completely eliminate the fouling of emulsified oil droplets with a wide size distribution. Constructing membranes with superior anti-oil-fouling ability to resist various oil droplets especially at high permeation fluxes is challenging. Here, the fabrication of a zero-oil-fouling membrane by grafting considerably high coverage of zwitterionic polymer and building defect-free hydration defense barrier on the surface is reported. A uniform layer of protocatechuic acid with COOH as abundant as existing in every molecule is stably deposited on the membrane so as to provide sufficient reactive sites and achieve dense grafting of the zwitterionic polymer. The coverage of zwitterionic polymer on the membrane plays a crucial role in promoting the antifouling ability to emulsified oil droplets. The poly(vinylidene fluoride) membrane with 93% coverage of the zwitterionic polymer exhibits zero oil fouling when separating multitudinous oil-in-water emulsions with ≈0% flux decline, ≈100% flux recovery, and a high water flux of ≈800 L m-2 h-1 bar-1. This membrane outperforms almost all of the reported membranes in terms of the comprehensive antifouling performance. This work provides a feasible route for manufacturing super-antifouling membranes toward oil/water separation application.

Thermodynamic and kinetic analyses of Janus membrane scaling in membrane distillation for zero liquid discharge engineering

Scaling phenomena are the principal obstacle to the factual application of membrane distillation (MD). In this study, we fabricated a Janus membrane composed of a commercial polytetrafluoroethylene substrate, a superhydrophobic surface layer, and a hydrophilic reverse side coated by MXene nanosheets via a simple spraying method. The virgin and modified membranes were carried out and compared in zero liquid discharge (ZLD) MD experiments. MD flux analysis showed that all the membranes experienced severe flux decline during the ZLD process. Saturation index, Gibbs free energy and nucleation energy barrier were calculated to explain the observed difference in scaling behavior. Screenage assessment of the homogeneous nucleation and heterogeneous nucleation showed that the severe scaling developed from some small clusters of the new phase, wherein the thermodynamic energy barrier played a critical role in determining the anti-scaling behavior for these unsmooth surfaces. Model analysis and data fitting were used to conclude the kinetic behavior of the virgin and modified membranes in the ZLD engineering. Finally, we evaluated the reversible and irreversible scaling after twice washing. Therefore, this study may provide instructive insights to design novel anti-fouling membranes and promote practical applications such as industrial brine treatment and ZLD engineering.

Effect of Electrochemical Pre-Oxidation for Mitigating Ultrafiltration Membrane Fouling Caused by Extracellular Organic Matter

Algal extracellular organic matter (EOM) will cause grievous membrane fouling during the filtration of algae-laden water; hence, boron-doped diamond (BDD) anodizing was selected as the pretreatment process before the ultrafiltration, and the EOM fouling mitigation mechanism and the purification efficiency were systematically investigated. The results showed that BDD oxidation could significantly alleviate the decline of membrane flux and reduce membrane fouling, and the effect was more notable with an increase in oxidation time. Less than 10% flux loss happened when oxidation duration was 100 min. The dominant fouling model was gradually replaced by standard blocking. BDD anodizing preferentially oxidizes hydrophobic organic matter and significantly reduces the DOC concentration in EOM. The effluent DOC was reduced to less than 1 mg/L when 100 min of BDD anodizing was applied. After the pre-oxidation of BDD, the zeta potential and interfacial free energy, including the cohesive and adhesive free energy, were all constantly increasing, which implied that the pollutants would agglomerate and deposit, and the repulsion between foulants and the ultrafiltration membrane was augmented with the extensive oxidation time. This further confirms the control of BDD on membrane fouling. In addition, the BDD anodizing coupled ultrafiltration process also showed excellent performance in removing disinfection by-product precursors.

An Alternating, Current-Induced Electromagnetic Field for Membrane Fouling and Scaling Control during Desalination of Secondary Effluent from Municipal Wastewater

Membrane treatment of secondary effluent for reuse applications is a promising approach to expand water supplies and provide flexibility to water resources management. However, effective control of membrane fouling and scaling is crucial for cost-effective treatment and system resilience. This study compared the performance of antiscalants to an alternating, current-induced electromagnetic field (EMF) as an alternative pretreatment method to reverse osmosis. Compared to the no-EMF control experiments, the EMF device resulted in 13% higher water recovery and 366% lower flux decline at 60% of water recovery, along with 2–8 times lower precipitation of fouling and scaling, as evidenced by scanning electron microscope, energy dispersive X-ray spectroscopy, and chemical extraction analysis. The combination of the EMF with antiscalant was more effective for reducing membrane fouling and scaling, increasing water recoveries up to 89.3%, as compared to the EMF (67.5%) and antiscalant-only (73.6%) configurations. This is the first study to demonstrate synergistic effects of using an EMF in combination with antiscalants and could lead to lower pretreatment costs. Additional research is required to quantify the economics of this approach and to fully understand the fundamental mechanisms governing fouling and scaling control by an EMF.

Robust Zwitterionic Poly(imidazolium)-Coated PVDF Membrane for Oily Water Treatment with Antibacterial and Anti-Biofouling Performances

Membrane fouling remains a major challenge during oily wastewater purification, resulting in reduced water flux and separation efficiency. Zwitterionic polymers have been introduced to the PVDF membrane and shown their fouling resistance ability. However, most current zwitterions lack bactericidal effects and tend to be fouled by the attachment of bacteria and biofilm formation in long-term service. Herein, a synthetic zwitterionic poly(imidazolium), poly(ZIM-co-VTS), was introduced onto the PVDF membrane by co-assembly with polydopamine (PDA) and tetraethyl orthosilicate (TEOS) as a silica source. The in situ covalent bond can be formed between the catechol from PDA and silanol groups from both TEOS and imidazole-based polymers. Further, Fe3+ cross-linking contributes to the robust chemical stability of modified PVDF. The as-prepared coating exhibits superhydrophilicity and underwater superoleophobicity under harsh conditions (pH = 1 and 13), remaining high separation efficiency (>99%) toward various surfactant-stabilized oil-in-water emulsions without an apparent water flux (>1013 L m–2 h–1 bar–1) decline. More importantly, the cationic imidazolium in poly(ZIM-co-VTS) endues the excellent bacterial killing efficiency toward Staphylococcus aureus (4.5 log10 reduction) and Escherichia coli (4.7 log10 reduction). The zwitterionic poly(imidazolium)-modified PVDF exhibited long-term durability with anti-biofouling properties and was a promising candidate for practical oily wastewater remediation.

Preparation and dye separation performance of ZIF-67/mesoporous silica ceramic nanofiltration membrane by liquid phase epitaxy (LPE) growth method

Ceramic membrane plays an important role in separation process with advantages of high thermal and chemical resistances, but its applications on the nanofiltration are restricted because of the limitation of pore size. In this study, a commercial tubular ceramic membrane with pore diameter of 0.1 μm was firstly coated with mesoporous silica substrates, in which ZIF-67 nanocrystals were generated layer-by-layer inside the mesochannels via liquid phase epitaxy growth method. The open pore size of ceramic membrane can be precisely controlled by increasing the growth cycles of ZIF-67 nanocrystals. The influence of preparation conditions on the dye separation was investigated, including growth cycles, precursor concentration, and molar ratio of Co2+/2-methylimidazole (Co2+/Hmim). Under the optimized conditions, the rejection of methylene blue, methyl orange, chromium black T, rhodamine B, acid fuchsin, and Congo red were in the range of 92–99 % with membrane fluxes of 0.45–0.73 L m−2 h−1. Bovine serum albumin solution was used to evaluate the anti-fouling abilities of the ceramic membrane. The flux recovery ratio, which was calculated according to the ratio between the water permeance after washing and before filtration, was 52.3 % with the flux decline rate of 66.3 %.

Electrically-conductive, Joule-heated pervaporation membranes for desalination: Investigating energy-saving, self-cleaning and anti-swelling trifecta

Desalination pervaporation (PV) process can be a promising technique to address global water-energy challenges. However, the energy-intensive nature of the process, which typically requires heating of the entire bulk feed, has been a significant challenge. Here, we reveal a novel self-heated PV membrane utilizing carbon nanostructures (CNS)-alginate (CaAlg) membrane Joule heater that merges interfacial heating of a feed solution and conventional PV separation. Low voltage is supplied to the electrically conductive alginate membrane to initiate resistive heating, which directly heats the solution at the feed/membrane interface. The developed self-heated membranes produce a high flux of 36 L m−2 h−1 at 50 % CNS loading while supplying a voltage of 5 V to the membrane to treat 1000 ppm NaCl solution. A flux of 24 L m−2 h−1 is obtained for the same membrane and the same conditions while treating hypersaline feed of 100,000 ppm. The self-heated PV membrane eliminates the preheating of the entire feed solution while enhancing permeate flux, resulting in a significantly reduced energy requirement. To produce a flux of 10 L m−2 h−1, electrothermally-heated PV membrane achieved exceptionally higher Gain output ratio (GOR) of 2.19 compared to the conventionally-heated (0.37). Besides their unique self-heating capability, the developed membranes are resistant to swelling and showed impressive antifouling/cleaning characteristics. Joule-heated CaAlg/CNS membranes showed a reduction in swelling degree from 150 % (Pristine CaAlg) to 50 % after 2-h exposure to water. Whether electrolysis (in-situ or cleaning cycles) or joule-heating cleaning is used, CaAlg/CNS membranes exhibited immaculate capabilities to self-clean surface foulants and maintain steady flux with only 3–8 % flux decline compared to pristine membranes that suffers 45–47 % flux decline.

Application of the electrical localized heating method in direct contact Membrane Distillation Crystallization process

Membrane Distillation Crystallization (MDCr) was the latest technology which shows great ability in the salt recovery area, but its practical application is currently confined to the laboratory due to the energy loss during long hydraulic circulation, and the high energy consumption caused by the long-term thermal requirements of the feed solution. In this study, the electric heating method was applied in the MDCr system as a potential candidate to increase the solution temperature and enhance the thermal efficiency. Compared to the normal water bath heating, the electrothermal MDCr system exhibited better permeate flux performance, higher thermal efficiency and high controllability. Additionally, the permeate flux decline under the control of AC 20 V was 11.1 %, which was far below the flux drop of 31.6 % obtained by the water bath method. Noticeably, the accumulation of NaCl crystals on the membrane was greatly alleviated according to the in-situ monitor of OCT due to the constant ion migration responding to the electric field within the conducting membrane. We hold the belief that this electrothermal direct contact MDCr system has a promising potential in the treatment of saline wastewater, building an effective foundation for efficient and energy-saving electrical heating MDCr systems.

Electrochemically activated peroxymonosulfate (PMS) pretreatment for fouling remediation during membrane distillation of surface water: Role of PMS concentration and dosing mode

Membrane distillation (MD) provides a potential method to supply large volumes of good quality potable water from surface water resources, avoiding the risk of long-term water shortages. However, the high abundance of dissolved natural organic matter (DOM) in surface water induces severe membrane fouling, which reduces productivity and increases energy consumption. Therefore, for the first time, electrochemically activated peroxymonosulfate (PMS) treatment was applied prior to the direct contact (DC)-MD process with the aim of reducing the deposition of organics and inorganics on the membrane and reducing membrane fouling. Results showed that supplying PMS continuously at a rate of 0.10 mM/min, eliminated 30.4% of dissolved organic carbon (DOC). Furthermore, fluorescence parallel factor (PARAFAC) analysis showed that >60% of two fluorescent substances were removed after 60 min of the electrochemically activated PMS pretreatment at a current density of 5 mA/cm2. Electrochemical activation of PMS generated •OH directly, facilitating the removal and degradation of DOM. In terms of the DCMD performance, almost no normalized flux decline was observed for the DCMS process after 0.01 mM/min pre-oxidation, while 44.8% flux decay occurred in the standalone DCMD treating raw surface water. Compared with continuous dosing, the use of a one-time dosing mode achieved a better outcome, as more small organic molecules generated in continuous dosing mode are not conducive to membrane fouling control. Therefore, electrochemically activated PMS could be an effective tool to mitigate membrane fouling in MD processes, although the characteristics of membrane and feed should be considered.

Improved separation performance, anti-fouling property and durability of polyamide-based RO membrane by constructing a polyvinyl alcohol/polyquaternium-10 surface coating layer

In this work, a novel coating layer of polyvinyl alcohol (PVA)/polyquaternium-10 (PQ10) was constructed onto the surface of polyamide-based RO membrane via the method of surface coating followed by cross-linking. Permeation tests demonstrated that the incorporation of PQ10 into PVA could efficiently improve the water permeation ability of surface coating layer, showing a significant decrease of hydraulic resistance from 5.9 × 1012 m−1 of the PVA-1.0 coating layer to 2.5 × 1012 m−1 of the PVA-1.0/PQ10-0.2 coating layer. Anti-fouling performance evaluation revealed that the PVA/PQ10-coated membrane not only possessed the good anti-fouling property of the PVA-coated membrane to negatively charged foulants and protein but also exhibited more excellent anti-fouling performance to positively charged foulant due to its relatively lower surface charge density and pseudo-zwitterionic surface character. Long-term filtration tests with dye wastewater illustrated that although the initial water permeability of membrane coated with PVA-1.0/PQ10-0.2 surface layer was lower than that of the virgin RO membrane under the same operation condition, the PVA-1.0/PQ10-0.2-coated membrane exhibited a higher salt rejection, lower flux decline rate and larger cumulative volume of permeated water during a filtration period of 48.0 h. Soaking tests further demonstrated the improvement of membrane chemical durability against acid, alkaline and chlorine by the PVA/PQ10 coating layer.

Composite membranes with multifunctionalities for processing textile wastewater: Simultaneous oil/water separation and dye adsorption/degradation

In this work, we report on a novel membrane design with both photocatalysis and electrocatalysis for simultaneous oil/water separation and dye adsorption/degradation to treat oily wastewater from textile dyeing and finishing. The membrane (designated as PAN-CF/MWCNT/FeOOH) comprised a cotton fabric (CF) dopped with carbon nanotubes (MWCNT) and FeOOH and a microporous polyacrylonitrile (PAN) top layer. Under a flow-through dynamic filtration mode, the membrane allowed for separation of oil-in-water emulsions by the PAN layer via wettability sieving and coalescence demulsification, while hydrosoluble dyes were captured and degraded by the CF/MWCNT/FeOOH via adsorption and in situ electro-degradation. The kinetics of adsorptive capture and electro-induced degradation of hydrosoluble dyes on the PAN-CF/MWCNT/FeOOH membrane was determined, which confirmed that the flux decline during flow-through filtration caused by the accumulated foulant in the membrane was alleviated effectively. A flow-through filtration test with methylene blue (MB) dyed petroleum ether-in-water emulsion showed that a high removal rate of oil (99.9 %) and dyes (97.6 %) was achieved simultaneously in a single pass at a high water permeability (17.2 m3/m2.h.bar) under an operating pressure of 6.5 kPa and a DC current of 10 mA. In addition, the fouled membrane could be cleaned easily via photo-Fenton degradation under light irradiation. This novel approach to the design and fabrication of membranes with multifunctionalities is expected to offer numerous advantages for treating complex oily wastewater from textile processing.

Revisiting a flotation cell benchmark

Developing a flotation benchmark to compare different flotation systems has been challenging due to the variation that exists in flotation cell types, geometry, and internal hydrodynamics. Parkes et al (2022) focused on the product flux passing through the upper surface as a Performance Measure, interpreting a batch mechanical cell as a steady-state plug flow device, equivalent to infinite steady-state tanks in series. This work reveals a conundrum that arises out of the tanks in series analysis when applied to a system of fixed total volume. The free surface of the system increases with the number of stages, therefore leading to a decline in the product flux and hence the Performance Measure. This Technical Note resolves the issue by always using tanks of one size, comparing the result for N tanks in series with an equivalent number of batch cells operating in parallel. The analysis provides a much stronger and clearer framework for the flotation benchmark.

Harnessing the power of metal-organic frameworks to develop microplastic fouling resistant forward osmosis membranes

With the gradual increase of microplastics (MPs) in water and wastewater streams, it is imperative to investigate their removal using tertiary treatment systems to minimize and preferably prevent their entrance into aquatic environments. Forward osmosis (FO) is a non-pressurized membrane process with potential applications in MPs removal from wastewater. However, efficient application of FO systems relies on developing high-performance FO membranes with low fouling tendency. MPs are proven as emerging foulants in membrane systems, diminishing their performance and lifetime and this highlights the need to consider MP fouling in developing sustainable membranes. The current study focuses on a novel modification of thin film composite (TFC) FO membranes by MIL-53(Fe) as a water-stable and hydrophilic metal-organic framework. Experimental results demonstrated that the optimized FO membrane (0.2 wt% MIL-53(Fe)) achieved a significantly higher water flux (90% increase) with a 23% less reverse salt flux. The modified membrane also had significantly less flux decline in fouling experiments and higher flux recovery after physical cleaning compared to the control membrane affirming its higher antifouling efficiency. MIL-53(Fe) integration in the FO substrate proved to be a practical method for developing high-performance TFC FO membranes with improved antifouling properties against MPs and organic foulants.

Low-cost ceramic filter bioreactor for treatment and reuse of residential septic tank effluent: A decentralized approach for small communities

Membrane biological reactors (MBRs) have recently become widely accepted as an advanced technology for treating domestic wastewater for improved water recovery, reuse, and recycling. The present study investigated the application of a low-cost ceramic filter bioreactor (CFBR) to treat septic tank effluent for smaller communities in Saudi Arabia (KSA). Filter fouling behavior, pollutant removal, and effluent quality were assessed under varying flux (0.2, 0.4, and 0.65 m/d) and aeration on/off cycle (60 min/60 min, 40 min/80 min, 30 min/90 min, 24 h/24 h) conditions. Operating CFBR at a flux value of 0.4 m/d ensured moderate fouling propensity, reasonable flux decline, and a higher volume of treated effluent. In the continuous aeration mode experiments, CFBR demonstrated the removal of various pollutants. Except for NO3N, CFBR effectively reduced all other pollutants of concern (BOD, COD, TSS, NH3N) in the effluent to acceptable levels meeting the water quality standards for unrestricted irrigation set by the KSA. An enhanced NO3N removal at the 24 h/24 h on/off cycle, indicating the effectiveness of cyclic aeration for promoting both nitrification/denitrification in the CFBR, with sufficient removal of other pollutants, is an original finding of the present study. Simple and cost-effective operations, high-quality effluent, and reduced energy consumption with shorter aeration duration promote the sustainability of the CFBR system for wastewater reclamation and reuse in small communities and rural settlements.

Assessment of the DCMD performances of poly(vinylidene difluoride) vapor-induced phase separation membranes with adjusted wettability via formation process parameter manipulation

Poly(vinylidene difluoride) (PVDF) membranes for direct contact membrane distillation (DCMD) are commonly modified to decrease their wettability by water. Yet, the vapor-induced phase separation (VIPS) process may be a suitable alternative to avoid complex and expensive modifications. Superhydrophobic membranes with a water contact angle >152° were prepared in one single step by adjusting the polymer concentration, dissolution temperature, relative humidity, and exposure time to vapors to 10 wt%, 40 °C, 80 % and 10 min, respectively with N-Methyl-2-pyrrolidone (NMP) as the solvent. Compared to commercial polytetrafluorethylene (PTFE) membrane, the VIPS PVDF membrane showed higher water vapor flux (38 LMH vs 36 LMH), lower flux decline ratio (22 % vs 39 %) and superior salt rejection (99.99 % vs. 99.90 %) in a 3 h-DCMD process. In wetting tests, the VIPS membrane failed for a sodium dodecyl sulfate concentration of 0.2 mM while 0.1 mM was enough to trigger wetting of the commercial membrane. The energy barrier to heterogeneous crystallization was slightly larger for the VIPS membrane, and a higher rejection was measured during scaling tests (99.8 % vs. 94.2 % for PTFE membrane). Finally, the VIPS membrane outperformed the commercial membrane in a long-term test (100 h). These results evidence the suitability of VIPS PVDF membranes for DCMD.

Linking water quality, fouling layer composition, and performance of reverse osmosis membranes

Fouling of polyamide membranes during reverse osmosis (RO) is a major challenge for adopting membrane technologies to treat highly contaminated waters, especially those containing organic foulants (e.g., natural organic matter (NOM), polysaccharides) and dominant cations (e.g., sodium, magnesium, calcium). This work combines bench-scale membrane fouling experiments with detailed characterization of feedwater chemistry and fouling layer composition/morphology to reveal fundamental mechanisms of (in)organic fouling during RO. Divalent cations are shown to promote fouling by hydrophobic NOM containing aromatic and carboxyl groups, while NOM fouling in the presence of a monovalent cation, sodium, occurs by smaller fulvic acids containing larger fractions of carboxyl groups and other oxygen-rich moieties. Calcium-carboxyl bridging occurs in solution and near the membrane surface to induce NOM aggregation on nanometer length scales. In complex waters containing foulant mixtures, co-fouling by calcium-carboxyl bridging and CaCO3 precipitation influence membrane performance at longer timeframes. However, the flux decline observed for the co-fouling mechanism was less significant than the sum of its parts, suggesting both synergistic and antagonistic fouling mechanisms should be considered in membrane design/operation. These results encourage the design of pretreatment processes to reduce concentrations of multivalent ions and hydrophobic NOM in RO feedwaters, and of membrane materials to limit attachment/deposition of aggregates to/on polyamide surfaces.

A novel fouling control strategy for forward osmosis membrane during sludge thickening via self-forming protective layer

Severe membrane fouling significantly limited forward osmosis (FO) process for sludge thickening (FST). Here, a novel antifouling strategy by loading a self-forming protective layer on FO membrane surface was proposed in this study. The protective layer was coated on FO membrane surface via a short-term FO process using activated sludge as the feed solution. Results indicated that the self-forming protective layer made FO membrane surface more negatively charged and more hydrophilic. As for the control FO (C-FO) membrane, the water flux decreased approximately 66% of the initial flux in Cycle 2 of FST process and thus resulting in the sludge concentration only increased to 40 g/L. In contrast, the flux decline rate of modified FO (M−FO) membrane was about 22%, and the corresponding sludge concentration still rose to 50 g/L. It suggested that the M−FO membrane with a protective layer had a better sludge thickening efficiency and a lower flux decline during sludge thickening compared to the C-FO membrane. Furthermore, less deposited foulants and better fouling reversibility were observed for M−FO membrane after two cycles of sludge thickening, implying that the protective layer effectively mitigated FO membrane fouling during sludge thickening. This phenomenon could be attributed to the excellent barrier effect of the protective layer and the enhanced surface hydrophilicity and negative charge, which improved the antifouling performance of the FO membrane. The findings of this study were conducive to better understanding of FO membrane fouling mechanisms and further development of fouling mitigation strategy via a protective layer.