Enhanced Filtration Performance Research Articles

Highly Efficient Oxygen-Activated Self-Cleaning Membranes Prepared by Grafting a Metal-Organic Framework-Derived Catalyst.

In this study, an efficient oxygen-activated self-cleaning membrane was successfully prepared by grafting a metal-organic framework-devised catalyst (CuNi-C) onto a membrane surface, resulting in enhanced filtration performance and self-cleaning capability based on oxygen activation under mild conditions. The pore features, surface roughness, and surface hydrophilicity of the prepared membrane were analyzed and used to determine the causes of the enhanced filtration performance; the results showed that an increase in the porosity and surface roughness enhanced the permeate flux, and enhanced adsorption capacity and surface hydrophobicity improved the membrane removal efficiency. The self-cleaning mechanism was elucidated by identifying the reactive oxygen species (ROS) and detecting catalytic element valences. The results revealed that zero-valent Cu embedded into the membrane surface effectively activated natural dissolved oxygen (DO) to generate ROS that degraded organic pollutants. In this study, catalytic oxidation with DO as the oxidant was successively integrated with membrane separation to prevent membrane fouling, providing a novel direction for the development of multifunctional membranes.

Rheology and Filtration Properties of Bentonite–Water Suspensions as Influenced by Lignocellulosic Nanomaterials and Water-Soluble Biopolymer Additives

Stability, structural morphology, and rheological and filtration properties of bentonite (BT)–water suspensions as influenced by the addition of lignocellulosic nanomaterials, including bleached nanofibrillated cellulose (NFC) and lignin-containing NFC (LNFC), and water-soluble polymers (WSPs) were investigated in this work. The performance of the formed fluids varied with the distinct fiber morphology and/or chemical composition of the NFC and LNFC. The NFC helped bridge BT particles for enhanced rheological performance but adversely affected filtration properties by forming thicker filter cakes. Reground NFC led to improved rheological properties owing to more individualized fibers with an enlarged surface area, but its effect on fluid loss was insignificant. LNFC enhanced viscosity, yield stress, and gelation properties of the fluid with effectiveness varying with the actual lignin content in the system. Enhanced filtration performance was seen with the presence of lignin in LNFC ascribed by its hydrophobic nature as well as the formation of more compact filter cakes. The applied biopolymers depicted better compatibility with smaller cellulosic fibril dimensions and lower lignin content. Thus, LNFC with moderate amounts of lignin in combination with biopolymer additives can help achieve much-improved rheological and filtration performance of BT-based fluids with lower cost and more environmental friendliness.

Modulating the structural characteristics of layered nonwoven fabrics devised by fibre fineness for enhanced filtration performance

This paper reports an investigatory study on the structure and properties of nonwoven fabrics made from different fibre fineness. The structure of the nonwoven fabrics was analysed using the Image analysis technique and Lindsley’s technique. Pore channel tortuosity was also calculated to quantify the porous path formed in the fibrous media. A relationship is developed between the structure and property of the nonwoven fabrics which helps in designing a suitable nonwoven filter media. In the later part, an attempt is made to regulate the structure by layering the batts made from different linear densities of fibres in various combinations to improve the filtration performance. It was interestingly established that an inverse gradient structure of batts having decreasing order of fibre orientation in the layered structure provided the lowest pressure drop along with near highest filtration efficiency. This study provides an insight into modulating the structure of the layered nonwoven fabric using different linear density fibres to prolong the life of the filter media.

Electrostatic Charge Retention in PVDF Nanofiber-Nylon Mesh Multilayer Structure for Effective Fine Particulate Matter Filtration for Face Masks.

Currently, almost 70% of the world’s population occupies urban areas. Owing to the high population density in these regions, they are exposed to various types of air pollutants. Fine particle air pollutants (<2.5 μm) can easily invade the human respiratory system, causing health issues. For fine particulate matter filtration, the use of a face mask filter is efficient; however, its use is accompanied by a high-pressure drop, making breathing difficult. Electrostatic interactions in the filter of the face mask constitute the dominant filtration mechanism for capturing fine particulate matter; these masks are, however, significantly weakened by the high humidity in exhaled breath. In this study, we demonstrate that a filter with an electrostatically rechargeable structure operates with normal breathing air power. In our novel face mask, a filter membrane is assembled by layer-by-layer stacking of the electrospun PVDF nanofiber mat formed on a nylon mesh. Tribo/piezoelectric characteristics via multilayer structure enhance filtration performance, even under air-powered filter bending taken as a normal breathing condition. The air gap between nanofiber and mesh layers increases air diffusion time and preserves the electrostatic charges within the multi-layered nanofiber filter membrane under humid air penetration, which is advantageous for face mask applications.

Improvement of Filtration Performance of Foam Ceramics Using External Electric Field

To enhance filtration performance of foam ceramics, effects of electric field on filtration performance of foam ceramics were studied. Results show that external electric field can change the wettability between melt and foam ceramics and can improve filtration performance of foam ceramics. Meanwhile, chemical composition of melt is not affected by external electric field.

Synergistic effects of matrix-anchoring and surface-segregation behavior of poly(N-vinylpyrrolidone)-grafted-silica filler for PVDF membrane performance improvement

Three dimensional (3D) poly(N-vinylpyrrolidone)-grafted-mesoporous silica (MS-g-PVP) was first synthesized, and explored as a filler for enhancing filtration performance of poly(vinylidene fluoride) (PVDF) composite membrane in this study. Membrane structure, interfacial property and filtration performance of the composite membranes were studied in detail. The relationships of mesoporous silica properties with membrane performance was systemically revealed. The 3D morphology of MS-g-PVP, that stably anchored in PVDF, endowed the membrane with defects-free, uniform and straight finger-like pores structure, and thus greatly enhanced water flux (maximized to 80.7 L m-2h−1, over 5.7 times than that of pristine PVDF membrane), without BSA rejection efficiency changing (98.6%). Three reasons can be accounted for the membrane performance improvement: i. The hydrophilic MS-g-PVP well dispersed in PVDF membrane matrix, and facilitated the formation of the evenly porous structure for the composite membrane; ii. Surface segregation of MS-g-PVP with the amount and diversity of oxygen-/nitrogen-containing groups on membrane surface, thus improving membrane surface hydrophilicity with contact angle decreasing from 89.0° to 70.6°; iii. Numerous mesopores inside of MS-g-PVP provided additional water pathways. In addition, these synergistic effects also enhanced the fouling-resistance of PVDF/MS-g-PVP membrane, whose flux recovery ratio maintained at 85.8% even after three “fouling-cleaning” dynamic BSA solution (0.2 g L-1) filtration cycles.

Controllable morphology of electrospun nanofiber membranes with tunable groove structure and the enhanced filtration performance for ultrafine particulates

As researchers are striving to develop high-performance filtration membranes with hierarchical micro/nano structures, the challenges and costs of processing often limit creative innovation. Here, we propose a polyethersulfone/polyacrylonitrile (PES/PAN) nanofiber membrane with groove structure by electrospinning and facile post-processing. The resulted membrane can form a groove structure on the surface of the fiber after being soaked in chloroform, thereby increasing the collision probability and extending the residence time for ultrafine particulates and improving the filtration efficiency. The groove structure can be attributed to the solubility of PES constituent in chloroform, while PAN constituent will not be dissolved, thus forming a high-performance nanofiber membrane with high filtration efficiency (ca. 99.54%) and withstand pressure drop (ca. 133.9 Pa) for dioctyl phthalate aerosol particles with diameter of 0.3 μm. The results show that this convenient and low-cost fabrication technology can be used to prepare high-performance nanofiber membrane based air filters that have broad application prospects in respiratory protective equipment.

Energy efficient harvesting of Spirulina sp. from the growth medium using a tilted panel membrane filtration

Membrane fouling is one of the main drawbacks in membrane-based microalgae harvesting. This study assessed the tilted panel to enhance filtration performance of Spirulina sp. broth. The influences of the operating parameters including the tilting angle, aeration rate and membrane materials on filtration performance and energy consumption were evaluated. Results showed that the system was effective and energy-efficient for membrane fouling control. The permeability peaked at a tilting of 45° thanks to combination of aeration and panel tilting. The microfiltration performed better than the ultrafiltration membrane due to the effective impact of air bubbles for foulant scouring that maximized the membrane intrinsic property. Small aeration rate of 1.0 L/min offered a high plateau permeability of 540 L/(m2·hr·bar) in which reversible fouling almost fully absent. The high permeability could be achieved under a low energy input of 0.2 kWh/m3.

Preparation and Characterization of PTFE/PI Nanofiber Composite Assembled Sponges

Due to the ultralow density and high specific area, nanofibrous sponges show great potential in the field of filtration and separation. To prepare nanofibrous sponges for high temperature smoke filtration, polytetrafluoroethylene (PTFE) was chosen due to its corrosion resistance and high temperature resistance. PTFE nanofiber prepared via electrospinning shows irregular concave-convex pores in the fiber body, which can improve the specific surface area and further enhance filtration performance. However, sponges made of PTFE nanofiber suffered from severe shrinkage, which limited the development in sponge products. In this study, polyimide (PI) nanofiber converted from polyamic acids (PAAs) nanofiber was introduced into the PTFE sponges to reinforce the dimensional stability. The parameters that influenced the morphology, porosity, shrinkage ratio, and thermodynamic properties of the composite sponges were also investigated. Moreover, the filtration performance of the composite sponges was tested. Results indicated that PTFE/PI composite nanofiber sponges possessed high porosity (94.34–98.12 %), excellent thermal stability (above 550 °C), and decent filtration efficiency (the maximum filtration efficiency reached 99.97 % to PM 2.0) were prepared, which demonstrated the potential in the field of high-temperature filtration.

Sewage concentration via a graphene oxide modified thin-film nanocomposite forward osmosis membrane: Enhanced performance and mitigated fouling

• The GO modified TFN-FO membrane shows improved filtration performance. • The GO modified TFN-FO membrane shows an enhanced antifouling property. • The GO modified FO membrane has high and stable sewage concentration performance. • High water flux recovery rates can be achieved by simple membrane cleaning. Upstream intensification of wastewater pre-treatment could improve the efficiency of the biological wastewater treatment process by separating and concentrating the organic matter. Forward osmosis (FO) technology is a potential upstream intensification strategy for sewage concentration. Herein, a high-performance graphene oxide (GO) modified TFN-FO membrane was fabricated and evaluated. The GO modified TFN-FO membrane showed 41% enhancement in water flux without weakening the selectivity property, due to the improved membrane hydrophilicity, thinner polyamide layer and facilitated water transportation by GO. The enhanced filtration performance and antifouling property of the TFN-FO membrane were confirmed by using simulative foulant solution and raw sewage as feed solutions. Moreover, a 41 h semi-continuous filtration experiment was conducted for sewage concentration using real raw sewage as the feed solution. Results indicated that the TFN-FO membrane achieved 4.6 times sewage concentration with high rejection of ammonia, nitrogen and organics. In addition, with a simple in-line water cleaning-in-place (CIP) process, most of the pollutants could be washed off from the surface of the TFN-FO membrane in 15 min and ~ 90% water flux could be recovered after each cleaning cycle. This work demonstrates the potential application of TFN-FO membranes for sewage wastewater concentration.

Enhanced filtration performance using feed-and-bleed configuration for purification of antibody precipitates.

Precipitation can be used for the initial purification of monoclonal antibodies (mAbs), with the soluble host cell proteins removed in the permeate by tangential flow microfiltration. The objective of this study was to examine the use of a feed-and-bleed configuration to increase the effective conversion (ratio of permeate to feed flow rates) in the hollow fiber module to enable more effective washing of the precipitate. Experiments were performed using human serum Immunoglobulin G (IgG) precipitates formed with 10 mM zinc chloride and 7 wt% polyethylene glycol. The critical flux was evaluated as a function of the shear rate and IgG concentration, with the resulting correlation used to predict conditions that can achieve 90% conversion in a single pass with minimal fouling. Experimental data for both the start-up and steady-state performance are in good agreement with model calculations. These results were used to analyze the performance of an enhanced continuous precipitation-microfiltration process using the feed-and-bleed configuration for the initial capture / purification of a mAb product.

Organic fouling assessment of novel PES/SPSf/Double layered hydroxide mixed matrix membrane for water treatment application

Fouling mitigation is of critical importance to warrant the widespread application of membrane filtration. Mixed matrix membranes (MMMs) have emerged as the attractive modified edition of traditional membrane due to their enhanced filtration performance, higher rejection and less fouling potential. This study examined the performance of MMMs by incorporating double layered hydroxide nanomaterials (DLH NMs) within the polyethersulfone/sulfonate polysulfone (PES/SPSf) blend matrix. The MMM with a loading rate of 0.5 wt.% DLH NMs showed an improved performance. The MMM exhibited enhanced antifouling ability than the pristine PES/SPSf membrane for terrestrial-derived organic matter. For sodium alginate (SA), the two membranes showed similar fouling potential, and the MMM even displayed worse irreversible fouling ascribed to higher roughness, reduced negative charge alongside the specific gel-box structure of SA-Ca2+. The fouling potential was further investigated by the thermodynamic analysis. The obtained results suggested that the specific free energies (ΔGho) should be used with the free energies as a function of distance (U) to prevent any fouling misinterpretation. The work clearly implied that the organic fouling behavior during water treatment is somewhat complex and DLH NMs could be promising materials for MMMs to address terrestrial organic matter dominated water resources.

Orifice-based membrane fouling inhibition employing in-situ turbulence for efficient microalgae harvesting

In this study, a simple way of generating turbulence via the use of an orifice was developed for the purpose to mitigating membrane fouling in microalgae harvesting. When an orifice with given number of holes was installed at the inlet of a membrane module, vigorous turbulence was observed on the surface of the membrane, which caused to detach the cake layer and limit further formation of it. Experiments on Chlorella sp. HS-2 showed that the filtration efficiency increased up to 299% compared to the conventional cross-flow operation. Through computational fluid dynamics, it was proven that the orifice increased the shear stress over the membrane surface area from 0.7 Pa to maximum average shear stress of 52.3 Pa. With regards to the operation time, microalgae harvesting could be finished nearly 3 times faster with the orifice. Faster harvesting time implies more quantity at a given time, not to mention prevention of quality degradation associated with cell decay. Low power consumption with enhanced filtration performance supported that membrane filtration equipped with an orifice can offer a promising means for microalgae harvesting, in terms of surpassing effectiveness and simplicity.

The Recent Progress in Modification of Polymeric Membranes Using Organic Macromolecules for Water Treatment

For decades, the water deficit has been a severe global issue. A reliable supply of water is needed to ensure sustainable economic development in population growth, industrialization and urbanization. To solve this major challenge, membrane-based water treatment technology has attracted a great deal of attention to produce clean drinking water from groundwater, seawater and brackish water. The emergence of nanotechnology in membrane science has opened new frontiers in the development of advanced polymeric membranes to enhance filtration performance. Nevertheless, some obstacles such as fouling and trade-off of membrane selectivity and permeability of water have hindered the development of traditional polymeric membranes for real applications. To overcome these issues, the modification of membranes has been pursued. The use of macromolecules for membrane modification has attracted wide interests in recent years owing to their interesting chemical and structural properties. Membranes modified with macromolecules have exhibited improved anti-fouling properties due to the alteration of their physiochemical properties in terms of the membrane morphology, porosity, surface charge, wettability, and durability. This review provides a comprehensive review of the progress made in the development of macromolecule modified polymeric membranes. The role of macromolecules in polymeric membranes and the advancement of these membrane materials for water solution are presented. The challenges and future directions for this subject are highlighted.

Polyimide aerogels with novel bimodal micro and nano porous structure assembly for airborne nano filtering applications.

Aerogels have presented a very high potential to be utilized as airborne nanoparticles' filtration media due to their nanoscale pore size and extremely high porosity. The filtering performance of aerogels, such as air permeability and filtration efficiency, is highly related to the configuration of aerogels' nanostructure assembly. However, as aerogel morphology is formed with respect to the intermolecular forces during the gelation stage, tailoring the aerogel nanostructure assembly is still a challenge. In this work, a novel strategy for tailoring polyimide aerogel nanostructure assembly is proposed by controlled disturbing of the intermolecular forces. From the results, the nanostructure assembly of the 4,4′-oxydianiline (ODA)–biphenyl-tetracarboxylic acid dianhydride (BPDA) polyimide aerogel is tailored to a uniform bimodal micro and nano porous structure. This was achieved by introducing the proper fraction of thermoplastic polyurethane (TPU) chains to the polyimide chains in the solution state and through a controlled process. The fabricated polyimide/TPU aerogels with bimodal morphology presented enhanced filtration performance, with 30% improved air permeability and reduced cell size of 3.51 nm over the conventional ODA–BPDA polyimide aerogels. Moreover, the fabricated bimodal aerogels present the reduced shrinkage, density, and effective thermal conductivity of 6.3% and 0.063 g cm−3, 28.7 mW m−1 K−1, respectively. Furthermore, the bimodal polyimide/TPU aerogels show the higher porosity of 96.5 vol% along with increased mechanical flexibility over the conventional polyimide aerogel with comparable backbone chemistry.

Real-time monitoring the spatial distribution of organic fouling using fluorescence imaging technique

Many studies have demonstrated a close relationship between fouling and fluid dynamics to control fouling formation. However, few have been carried out to validate the relationship in practice, owing to the lack of techniques to evaluate fouling with hydrodynamics. We investigated the organic fouling of humic acid (HA) and tannic acid (TA) at three crossflow velocities using a fluorescence-labelled organism bioimaging instrument (FOBI). Higher crossflow velocity generally enhanced filtration performance; however, the type of foulants induced different fouling formation patterns on the membrane. The monitoring system and computational model revealed that HA fouling had strong (R2 = 0.82) correlation with the shear rate (obtained from a velocity of 10.42 cm/s), while TA had weak correlation (R2 = 0.13). HA fouling showed a high correlation with velocity profile at 10 cm/s, but TA fouling occurred in the over 20 cm/s velocity zone. This is because TA had relatively high adhesive force acting to the membrane, resulting in irreversible fouling that was hardly mitigated by crossflow control. Therefore, this study demonstrated that fouling development was predominantly controlled by hydrodynamic conditions, but the actual fouling distribution could be influenced by the physicochemical properties of dissolved organic matters.

Investigation on the spatial filtration performance in spiral-wound membranes – Influence and length-dependent adjustment of the transmembrane pressure

During microfiltration, the length-dependent transmembrane pressure (ΔpTM) causes an inhomogeneous filtration performance along the module. Elimination of this length effect was found to increase the overall filtration efficiency of ceramic membranes significantly. Up to the authors’ knowledge, a systematic investigation on the spatial behavior of polymeric spiral-wound membranes (SWM) during microfiltration and the potential of a homogeneous ΔpTM has not been carried out yet. A modified SWM with permeate pockets divided in four hermetically separated sections was used to assess the length-resolved filtration performance during skim milk fractionation. Along the membrane, flux and fouling resistance decreased, whereas protein permeation increased. By adjusting the permeate pressure comparable to the uniform transmembrane pressure mode in ceramic membranes, the length dependency of ΔpTM along the module was reduced. In turn, the filtration performance was found to be more balanced. The overall whey protein mass flow increased as all sections were operating at the same ΔpTM under optimal condition. For the first time, we have proved with experimental data that SWM behave similar to ceramic membranes with regard to the length dependency of the filtration performance. Thus, the concept of a length-independent ΔpTM seems promising to enhance filtration performance of SWM at industrial scale.

A protein-functionalized microfiber/protein nanofiber Bi-layered air filter with synergistically enhanced filtration performance by a viable method

Realization of high filtration efficiency and low pressure drop is in critical need for advanced air filters. Electrospun nanofiber-based air filters create high surface area which can efficiently capture pollutants, but the production rate is difficult to improve and it’s challenging to reduce the pressure drop. In this study, we demonstrate that a protein-functionalized cotton-fiber composite air filter with high filtration efficiency and low normalized pressure drop can be fabricated via an evaporation-controlled strategy. It is the first report that a “green” solvent system (ethanol aqueous), an optimized protein concentration, and evaporation temperature can lead to protein-functionalized cotton filters with improved air filtration efficiency. The resulting loose micro-fibrous structure and large pores of cotton fiber mats give rise to a low normalized pressure drop. The high filtration performance results from the unique protein morphology on the cotton fiber surface, e.g. homogeneously distributed protein particles via denaturation process, creating an optimized surface area for interactions between protein and pollutants. In addition, to enhance the filtration efficiency for capturing fine particulate pollutants (i.e. PM1.0, PM0.3), a synergistic bi-layered air filter which combines the zein-functionalized cotton fibers with a thin layer of protein nanofibers is fabricated. It is further demonstrated that the unique configuration of the composite air filter at the upstream side for airflow and protein nanofiber at the downstream side leads to remarkable filtration efficiencies over 99.0% for PM of a broad range of sizes due to a synergistic effect. This study provides a new and cost-effective method with a “green” solvent involved for efficiently fabricating “green” and efficient air filters by combining microfibers and nanofibers.

The preparation of bifunctional electrospun air filtration membranes by introducing attapulgite for the efficient capturing of ultrafine PMs and hazardous heavy metal ions

The comprehensive sources of particulate matter (PM) require air purification materials to possess both high filtration efficiencies and low air resistances in an effort to provide healthcare. However, the assembly of multiple-layered filters with different functions leads to high pressure drop and high operating cost. Therefore, a multifunctional air filter that can provide excellent air filtration capacity and healthcare is highly desired. Here, a novel bifunctional polyacrylonitrile/attapulgite hierarchical-structured filter with low air resistance and high adsorption capacity was designed and fabricated by embedding attapulgite nanorods during a facile electrospinning process. The hierarchical polyacrylonitrile/attapulgite membranes showed only a ∼64 Pa resistance for 0.1 μm PM. Another benefit of using the attapulgite nanorods is an adsorption effect for hazardous heavy metal ions that accompany airborne ultrafine PMs. Thereby this hierarchical membrane simultaneously exhibits an enhanced filtration performance and hazardous protection ability. Furthermore, due to the electret effect of the attapulgite nanorods, the surface potential of the membrane remains at above 2.2 kV after 600 min of continuous use, which could improve the air filtration efficiency and ensure the long-term service life of the filters. This work may provide a new approach for the design and development of multifunctional air filters for simultaneously capturing ultrafine PMs and any other accompanying hazardous chemicals.

Superior fouling resistant PVDF membrane with enhanced filtration performance fabricated by combined blending and the self-polymerization approach of dopamine

This work presents a new and simple modification approach to fabricate a Polyvinylidene Fluoride membrane with superior permeation, separation, and antifouling performance. Dopamine was introduced by blending into the membrane casting solution and the membrane was prepared using a classic non-solvent induced phase separation technique. The prepared flat-sheet membrane was then dipped into a Tris-HCl buffer solution with a pH of 8.5 to trigger the polymerization of dopamine into polydopamine. The effects of the modification on the membrane characteristics were analyzed in terms of a change in the chemical components, hydrophilicity, surface porosity, and mechanical strength. In addition, the permeation, separation, and fouling resistant performance were studied by means of a filtration experiment using deionized water and humic acid solution as feeds. The results revealed that the addition of dopamine successfully increased the porosity by 300%, hydrophilicity by 180%, and water flux performance by 600% compared to the original membrane. Polymerization of dopamine into the polydopamine layer recovered the loss of rejection due to pore enlargement by dopamine blending from 91 to 93%. In addition, the blended and polymerized membrane successfully recovered up to 92% of pure water flux after backwash, which indicates the superior antifouling performance. It can be concluded that this new technique can be a potential approach to produce membranes for the water treatment process with overall excellent performance.