Biofouling in sugarcane juice refining by nanofiltration membrane: Fouling mechanism and cleaning

Ultrahigh-throughput cross-flow filtration of solution-processed 2D materials enabled by porous ceramic membranes

Printed electronics is a disruptive technology in multiple applications including environmental and biological sensors, flexible displays, and wearable diagnostic devices. With superlative electronic, optical, mechanical, and chemical properties, two-dimensional (2D) materials are promising candidates for printable electronic inks. While liquid-phase exfoliation (LPE) methods can produce electronic-grade 2D materials, conventional batch separation processes typically rely on centrifugation, which requires significant time and effort to remove incompletely exfoliated bulk powders, hindering the scale-up of 2D ink manufacturing. While cross-flow filtration (CFF) has emerged as a promising continuous flow separation method for solution-processed 2D nanosheets, previously demonstrated polymer CFF membranes necessitate low 2D nanosheet concentrations to avoid fouling, which ultimately limits mass throughput. Here, we demonstrate a fully flow-based, exfoliation-to-ink system for electronic-grade 2D materials using an integrated cross-flow separation and concentration system. To overcome the relatively low-throughput processing concentrations of incumbent polymer CFF membranes, we employ porous ceramic CFF membranes that are tolerant to 10-fold higher nanosheet concentrations and flow rates without compromising separation efficiency. Furthermore, we demonstrate a concentration method via cross-flow ultrafiltration, where the retentate can be directly formulated into printable inks with electronic-grade performance that meets or exceeds centrifugally produced inks. Life cycle assessment and technoeconomic analysis quantitatively confirm the advantages of ceramic versus polymer CFF membranes including reductions of 97%, 96%, 94%, and 93% for greenhouse gas emissions, water consumption, fossil fuel consumption, and specific production costs, respectively. Overall, this work presents an environmentally sustainable and cost-effective solution for the fabrication, separation, and printing of electronic-grade 2D materials.

The removal of pathogenic bacteria and dissolved organic matter from freshwater using microporous membranes: insights into biofilm formation and fouling reversibility

Pathogenic bacteria in drinking-water pose a health risk to consumers, as they compromise the quality of portable water. Chemical disinfection of water containing dissolved organic matter (DOM) causes harmful disinfection by-products. In this work, 4-hydroxybenzoic acid (4-HBA) blended polyethersulfone membranes were fabricated and characterised using microscopic and spectroscopic techniques. The membranes were evaluated for the removal of bacteria and DOM from synthetic and environmental water. Permeate flux increased from 287.30 to 374.60 l m-2 h-1 at 3 bars when 4-HBA increased from 0 to 1.5 wt.%, suggesting that 4-HBA influenced the membrane's affinity for water. Furthermore, 4-HBA demonstrated antimicrobial properties by inhibiting bacterial growth. The membrane with 1 wt.% 4-HBA recorded 99.4 and 100% bacteria removal in synthetic and environmental water, respectively. Additionally, DOM removal of 55-73% was achieved. A flux recovery ratio (FRR) of 94.6% was obtained when a mixture of bacteria and humic acid was filtered, implying better fouling layer reversibility during cleaning. Furthermore, 100% FRR was achieved when a multimedia granular filtration step was installed prior to membrane filtration. The results illustrated that the membranes had a high permeate flux with low irreversible fouling. This indicated the potential of the membranes in treating complex feed streams using simple cleaning protocols.

Resource recovery from RO concentrate using nanofiltration: Impact of active layer thickness on performance

Modelling the removal of monovalent and divalent ions from seawater via nanofiltration is crucial for pre-treatment in seawater reverse osmosis systems. Effective separation of divalent ions through nanofiltration and allowing the permeate containing only monovalent ions to pass through the reverse osmosis system produces pure NaCl salt from the concentrate. However, the Donnan steric pore model and dielectric exclusion assume a uniformly distributed cylinder pore morphology, which is not representative of the actual membrane structure. This study analyzed the impact of membrane thickness on neutral solute removal and investigated the effect of two different methods for calculating the Peclet number on rejection rates of monovalent and divalent salts. Results show that membrane thickness has a significant effect on rejection rates, particularly for uncharged solutes in the range of 0.5-0.7 solute radius to membrane pore size ratio. Operating pressures above 10 bar favour the use of effective active layer thickness over the membrane pore size to calculate the Peclet number. At low pressures, using the effective active layer can lead to overestimation of monovalent salt rejection and underestimation of divalent salt rejection. This study highlights the importance of appropriate Peclet number calculation methods based on applied pressure when modelling membrane separation performance.

Effects of Critical Operation and Cleaning Parameters on Performances and Economic Benefits of Biogas Slurry Concentration by Forward Osmosis Membrane

Forward osmosis membrane technology (FO) shows potential application prospects in biogas slurry concentration, which is conducive to promoting the sustainable development of biogas projects. However, at present, the key influencing factors of membrane concentration using FO are not well understood. Therefore, this study analyzed the influence of draw solution concentration, pH, temperature and cross-flow velocity on the concentration efficiency of FO membrane, and optimized the operation parameters of FO membrane. The results showed that the concentration effect of the NaCl draw solution at pH 5 or 9 was better than that at pH 7. The order of factor influencing the water flux was as follows: draw liquid concentration > cross-flow velocity > operating temperature. The optimal combination obtained by orthogonal analysis was under 45 °C, with a cross-flow velocity of 1 L/min and the use of 1.5 mol/L NaCl as draw solution. The results of the membrane cleaning implied that the recovery rate of the fouled membrane after acid-base cleaning is significantly higher (88%) than other cleaning solutions. This research offers a scientific reference for applying positive osmosis technology to re-utilize biogas slurry resources.

The Application of the Nanofiltration Membrane NF270 for Separation of Fermentation Broths

The potential for nanofiltration (NF) in removing both relatively low molecular weight (MW) organic species and charged solutes from complex media is noteworthy. The main aim of the current work was to improve understanding of the separation mechanisms of fermentation broths components in the NF process. For this purpose, the experimental investigations were performed using the commercial polyamide NF270 membrane. The feed solution was ultrafiltered 1,3-propanediol (1,3-PD) broths. The separation results were analyzed and discussed in light of the detailed characteristics of both the membrane and the broth components. It has been noted that the membrane ensured the complete 1,3-PD permeability and significant rejection of some feed components. A thorough analysis showed that the retention of carboxylic acids was based on both the Donnan effect and sieve mechanism, according to the following order: succinic acid > lactic acid > acetic acid > formic acid. Indeed, acids retention increased with increasing charged acids ions valency, Stokes radius (rS) as well as MW, and decreasing diffusion coefficient (D). In turn, for ions, the following orders retention was determined: SO42− = PO43− > Cl− and Ca2+ > Na+ > NH4+ ~ K+. It indicated that the ions retention increased with increasing ions charge density, hydrated radius (rH), and hydration energy (Eh). It showed that the separation of the ions was based on the Donnan exclusion, sieving effect, and dielectric exclusion.

Fouling characteristics and cleaning approach of ultrafiltration membrane during xylose reductase separation

Many operating parameters of ultrafiltration (UF) are playing a crucial role when using a polyethersulfone membrane to separate xylose reductase (XR) enzyme from reaction mixtures during xylitol synthesis. The present study focuses on the separation of XR enzyme using a cross-flow ultrafiltration (UF) membrane. The filtration process was analyzed using the three effective variables such as filtration time, cross-flow velocity (CFV), and the transmembrane pressure (TMP), which were ranging from 0 to 100 min, 0.52 to 1.2 cm/s and 1-1.6 bar, respectively. Then, using the resistance in series model, the hydraulic resistance for alkali chemical cleaning during XR separation was estimated. During separation, increased TMP showed a positive-flux effect as a driving force, however, fouling and polarized layer were more prominent under higher TMP. Increased CFV, on the other hand, was found more efficient in fouling control. In terms of the membrane cleaning techniques, an alkaline solution containing 0.1 M sodium hydroxide was shown to be the most effective substance in removing foulants from the membrane surface in this investigation. Cleaning with an alkaline solution resulted in a maximum flux recovery of 93% for xylose reductase separation. This work may serve as a useful guide to better understand the optimization parameters during XR separation and alleviating UF membrane fouling induced during XR separation.

A Review on the Use of Membrane Technology Systems in Developing Countries

Fulfilling the demand of clean potable water to the general public has long been a challenging task in most developing countries due to various reasons. Large-scale membrane water treatment systems have proven to be successful in many advanced countries in the past two decades. This paves the way for developing countries to study the feasibility and adopt the utilization of membrane technology in water treatment. There are still many challenges to overcome, particularly on the much higher capital and operational cost of membrane technology compared to the conventional water treatment system. This review aims to delve into the progress of membrane technology for water treatment systems, particularly in developing countries. It first concentrates on membrane classification and its application in water treatment, including membrane technology progress for large-scale water treatment systems. Then, the fouling issue and ways to mitigate the fouling will be discussed. The feasibility of membrane technologies in developing countries was then evaluated, followed by a discussion on the challenges and opportunities of the membrane technology implementation. Finally, the current trend of membrane research was highlighted to address future perspectives of the membrane technologies for clean water production.

An electro-oxidation reactor for treatment of nanofiltration concentrate towards zero liquid discharge

Nanofiltration (NF) concentrate generated from the secondary wastewater treatment contains high concentration of ammonium nitrogen and refractory organics, thus having great environmental risks. In this study, an electro-oxidation (EO) reactor built up with a boron-doped diamond (BDD) anode is utilized to treat the NF concentrate. To reach "zero liquid discharge", a mixture of the electrolytic effluent and the raw secondary wastewater was collected and transported back to the NF module. Results show that under the current density of 30 mA·cm^-2, most of ammonia nitrogen was decomposed into N-gases within 30 min due to the active chlorine radicals generated in the electrochemical process. Moreover, the EO reactor completely eliminated antibiotics, humic acids and bacteria in the NF concentrate under long electrolysis time of 60 min. In particular, the organic pollutants removal rate was kept at a stable value in the EO reactor for a long-term operation of up to 120 h. In addition, the NF membrane remained a constant permeate flux without being affected by the membrane biofouling caused by organic components in wastewater. Our study highlights the potential of the NF-EO process as a "zero liquid discharge" approach for treatment of the secondary wastewater.

New insights into effect of alkaline cleaning on fouling behavior of polyamide nanofiltration membrane for wastewater treatment

Membrane fouling is an intractable issue in wastewater treatment by nanofiltration (NF) membrane, and alkaline cleaning is the most effective approach to remove organic fouling on NF membrane. However, it was found that pore swelling of NF membrane induced by alkaline cleaning might reduce cleaning efficiency, and it is never quantified and its effect on membrane fouling behavior is still mysterious. In this work, membrane pore swelling effect (~9.7%, increment of effective pore size) induced by alkaline cleaning (pH 11) is confirmed and its effect on fouling behavior of the polyamide NF membrane is investigated based on experimental and modelling results. It is found that the alkali-induced pore swelling phenomenon would disappear after water filtration at neutral pH for 30 min, and if such cleaned membrane is faced by the small foulants during this pore shrinkage period, the concentration polarization and membrane fouling would be severer, and the subsequent alkaline cleaning is less effective because more foulants enter the enlarged pores and are tightly embedded in the membrane. Thus, the irreversible fouling of the NF membrane increases from 20% to 40% while its permeability recovery declines from 100% to 67% after six fouling/cleaning cycles. When an anionic surfactant sodium dodecyl sulfate (SDS, 10 mM) is added in the alkaline cleaning solution, the adsorption of SDS in/on the membrane can not only improve its hydrophilicity and negative charge, but also quickly eliminate the alkali-induced pore swelling effect and avoid the accumulation of foulants in the pores, thereby enhancing the antifouling performance of the NF membrane. Using the alkaline SDS cleaning, the irreversible fouling of the NF membrane maintains below 10% while its permeability recovery keeps above 100% in six continuous fouling/cleaning cycles.

Stimuli-Responsive Lysozyme Nanocapsule Engineered Microfiltration Membranes with a Dual-Function of Anti-Adhesion and Antibacteria for Biofouling Mitigation

Biofouling remains as a persistent problem impeding the applications of membranes for water and wastewater treatment. Green anti-biofouling of membranes made of natural and environmentally friendly materials and methods is a promising strategy to tackle this problem. Herein, we have developed a functionalized PVDF membrane with stimuli-responsive lysozyme nanocapsules (NCP). These nanocapsules can responsively release lysozyme according to environmental stimuli (pH and redox) induced by bacteria. Results showed that (i) the surface of the functionalized membrane with NCP had enhanced hydrophilicity, reduced roughness, and negative charge, (ii) a remarkable reduction of adsorption of proteins, polysaccharides, and bacteria was achieved by the functionalized membrane, and (iii) the colony forming unit (CFU) of bacteria on a membrane surface was reduced more than 80% within 24 h of contact. In addition, the NCP membrane showed excellent anti-biofouling activity regarding the bacterial viability being 12.5 and 8.3% on the membrane after filtration with 10⁸ CFU mL^-1 Escherichia coli and Staphylococcus aureus solution as feed, respectively. The coating layer and assembled nanocapsules endowed the membrane with improved lysozyme stability, anti-adhesion performance, and antibacterial activity. Stimuli-responsive lysozyme nanocapsule engineered microfiltration membranes show great potential for anti-biofouling in future practical application.