High Flux Microfiltration Research Articles

Microfiltration of polymeric microgels as soft model colloids through sterile filter membranes

In this work, filtration studies were performed with particles of soft or super-soft nature, made from polyacrylamide copolymers, and hard polystyrene reference particles, all having average diameters between 180 and 200 nm, at relatively high particle concentrations and different transmembrane pressures under stirred and non-stirred dead-end filtration conditions in order to evaluate the separation and fouling behavior of a commercially available high-flux microfiltration membrane (experimentally determined barrier pore diameter 210 nm). In order to identify the underlying fouling processes the dependencies of the flux on cumulative volume or time were analyzed in the frame of established models. It was found that low transmembrane pressures of 0.1 bar lead to immediate filter cake formation, that facilitates a high retention of the particles and leads to less fouling that cannot be removed by external washing. Medium to high pressures (0.5–2.0 bar) resulted in a pronounced penetration of microgel particles into the membrane structure and pore blocking in the first phase (I) of the filtration, before entering the cake formation phase (II); the particle rejection in phase I was lower and the extent of fouling remaining after washing was larger. Super-soft microgels showed a significantly more pronounced pore blocking phase (I), compared to their soft counterparts or the hard sphere reference particles. The results of the study yield deeper insights into retention and blocking mechanisms during filtrations of deformable microgels as model colloids with a benchmark sterile filtration membrane. This is relevant for purification of such polymeric materials after their synthesis or for the development of test systems mimicking the removal of microorganisms by sterile filtration.

Antifouling Fibrous Membrane Enables High Efficiency and High-Flux Microfiltration for Water Treatment.

Membrane biofouling has long been a major obstacle to highly efficient water treatment. The modification of the membrane surface with hydrophilic materials can effectively enhance biofouling resistance. However, the water flux of the membranes is often compromised for the improvement of antifouling properties. In this work, a composite membrane composed of a zwitterionic hydrogel and electrospinning fibers was prepared by a spin-coating and UV cross-linking process. At the optimum conditions, the composite membrane could effectively resist the biofouling contaminations, as well as purify polluted water containing bacteria or diatoms with a high flux (1349.2 ± 85.5 L m-2 h-1 for 106 CFU mL-1 of an Escherichia coli solution). Moreover, compared with the commercial poly(ether sulfone) (PES) membrane, the membrane displayed an outstanding long-term filtration performance with a lower water flux decline. Therefore, findings in this work provide an effective antifouling modification strategy for microfiltration membranes and hold great potential for developing antifouling membranes for water treatment.

Gravity assisted super high flux microfiltration polyamide-imide membranes for oil/water emulsion separation

Oil/water separation is a critical global challenge due to the ever-increasing generation of oily wastewaters from industrial activities and daily life. Membrane separation processes can effectively remove oil from water; however, synthesizing energy-efficient and anti-oil fouling membranes remains a huge enigma among researchers. In this work, we prepared novel polyamide-imide (PAI) microfiltration membranes using our recently developed hydrogel-facilitated phase separation (HFPS) technique and conventional nonsolvent induced phase separation (NIPS) method. The external surface of the hydrogel mold could be patterned as desired. The prepared membranes, including HFPS-patterned, HFPS-unpatterned, and NIPS, showed high porosity, superhydrophilicity, and underwater superoleophobicity. The underwater oil contact angles of n-hexadecane and mineral oils were higher than 150°, while a complete repellency for diesel oil was observed for all membranes. The ultra-high-water flux of patterned HFPS membranes, 440 L m-2 hour-1 (LMH), made them outstanding candidates for separating oil/water emulsions. For all fabricated membranes, gravity-driven filtration experiments of 9 consecutive oil cyclic filtration tests yielded >99.9% oil removal efficiency. Moreover, after 18 filtration experiments, the flux recovery ratio and flux decline were in the range of 90–100% and 3–20%, respectively.

An ultrathin bacterial cellulose membrane with a Voronoi-net structure for low pressure and high flux microfiltration.

Developing a porous membrane to effectively remove sub-micron sized contaminants from water while maintaining a high permeate flux with energy-saving properties is of great significance but extremely challenging. Herein, we describe a feasible strategy to create a bacterial cellulose (BC) membrane with a continuous Voronoi-net structure via combining evaporation-induced self-assembly with chemical cross-linking. This presented approach allows micro-length BC nanofibers to self-assemble in the electrospun polyacrylonitrile (PAN) nanofibrous frameworks to form stable and continuous Voronoi-like nanonets, endowing the obtained membrane with small pore size, stable pore structure, high porosity, favourable interconnectivity, and ultrathin membrane thickness. By virtue of these unique structural advantages and superhydrophilicity from BC Voronoi-like nanonets, the resulting membrane exhibits integrated performances of high rejection efficiency (>99.63%) for 0.3 μm TiO2 microparticles, robust permeate flux (5541 L m-2 h-1) at a low driving pressure of 20 kPa, intriguing reusability, excellent bacterial rejection efficiency (log reduction value of 8.2), and promising antifouling function to bacteria. It is expected that the proposed strategy can provide a facile approach for the development of next-generation high performance microfiltration membranes.

High flux electrospun nanofiberous membrane: Preparation by statistical approach, characterization, and microfiltration assessment

Preparation, characterization and evaluation of new generation of micro-filters based on polyacrylonitrile electrospun nanofiberous membrane (ENM) were thoroughly investigated. First, quantitative relationships between average diameter, bead area density of nano-fibers and certain electrospinning parameters, i.e., concentration, voltage, spinning distance, and feed rate, were established by empirical modeling based on a central composite design. The analysis revealed that concentration, voltage and distance are the significant parameters. Also, adequacy checking indicated the appropriateness of fit for the models. Afterwards, bead-free ENMs with diameter of 100–500 nm were prepared and characterized in terms of porosity, pore size and mechanical properties. The results indicate that as the nano-fiber diameter increases from 100 nm to 500 nm, porosity decreases from 74% to 61%, pore radius increases from 0.48 µm to 1.40 µm and tensile properties slightly decrease. Moreover, pure water flux increased with increasing nano-fiber diameter and membrane compaction was observed with increasing applied pressure for each membrane. Finally, ENM with fiber diameter of 100 nm showed the highest rejection rate of 99% and steady permeate flux of 118 l/m2h using TiO2 micro-particles suspension. Such finding demonstrates that ENMs with proper fiber diameter and morphology are excellent choices for high flux microfiltration applications.

Super high flux microfiltration based on electrospun nanofibrous m-aramid membranes for water treatment

The effects of ultrathin fibers using electrospun m-aramid nanofibrous membranes were evaluated to determine their performance as filters for water purification. The M150 membrane (average fiber diameter: 150 nm) was higher air-permeability than the M120 membrane (average fiber diameter: 120 nm). The commercial GSWP filter had much lower air-permeability than the electrospun membranes. The mean flow pore diameters of the electrospun membranes were much larger than the GSWP due to their high air permeabilities. Increased membrane thickness resulted in decreased airpermeability, mean pore size, and largest detected pore size. The M150 and M120 membranes with thickness of 10 μm showed approximately 5 times higher water flux than the GSWP filter. However, these membranes surprisingly showed higher particle rejections (99.32% and 99.62%, respectively) in the filtration of 0.2-μm PS latex particles compared with the GSWP filter (95.91%). This result indicates that highly branched, ultrafine fibrous structures of the m-aramid nanofibrous membranes contribute to the great increase in the particle rejection efficiency without loss of water flux (or airpermeability). From these results, the electrospun m-aramid nanofibrous membranes are expected to be used as a novel microfilter with a super high flux and high rejection efficiency.

High flux microfiltration membranes with silver nanoparticles for water disinfection

Biofouling has a negative impact on the membrane water treatment since it reduces permeability, increases energy costs, and decreases the lifetime of the membranes. To minimize biofouling microfiltration (MF), polyethersulfone (PES) membranes were prepared by phase inversion method, and silver nanoparticles (AgNps) were synthesized by chemical reduction and incorporated into these MF membranes. Membranes with different pore size and porosity were selected and the presence of silver in the transversal section was confirmed. The antibacterial properties were analyzed against Pseudomonas fluorescens by the inhibition zone method after 24 and 96h of incubation and also by the bacterial count method. It was observed an inhibition ring of 20–30% of the sample diameter only around the membranes containing AgNps, indicating the suppression of micro-organism activity nearby the membrane. The permeate was analyzed using Colilert methodology to count the total number of coliforms. The coliform retention was 99.9 and 99.999% for membranes without and with AgNps, respectively. These results confirm the bactericide properties of the PES–AgNps membranes and their great potential in application for water treatment.

A novel high flux poly(trimethylene terephthalate) nanofiber membrane for microfiltration media

A novel high flux poly(trimethylene terephthalate) (PTT) nanofiber membrane for microfiltration media was prepared and characterized in this study. The PTT nanofibers for the membrane preparation were fabricated based on a high throughput, cost effective and environmental friendly method developed by us. The PTT nanofiber membranes were then prepared by a wet-laid process. To enhance the mechanical properties of fabricated PTT nanofiber membrane, two strategies were employed. The structures of PTT nanofiber membrane were compacted by making PTT nanofibers shrink together under heat. The other one is to facilitate the formation of locally physical cross-linking points by blending PP nanofibers having lower softening points into PTT nanofiber membranes and the following heat-treatment. The properties of the original nanofiber membrane and two modified nanofiber membranes, including morphology, apparent density, porosity, contact-angle, pore size distribution, water flux and filtration efficiency, were thoroughly investigated. The rejection rates of the prepared nanofiber membranes to TiO2 suspensions with the average diameter of 100nm were above 99.6%. The results indicated that the PTT nanofiber membranes prepared by this novel method have great potential for the application of high flux microfiltration media.

Overcoming the drawbacks of microsieves with micromeshes for in situ product recovery

During bioconversion processes in situ product recovery using membranes has been proven to be very effective for continuous product recovery from bioconversion processes. In particular, submerged membrane filtration offers an advantage over the conventional processes. Here it is crucial to provide effective fouling control to avoid loss in filtration performance. We recently developed an operation concept called reverse-flow diafiltration. It is characterized by a periodic flow reversal by substrate feeding, so cake-layer formation is prevented and substrate is supplied over the same membrane. In the presented research work, we focus on the use of high-flux microfiltration membranes, namely microsieves and micromeshes, using model yeast suspensions. These membranes provide of factor 5 higher permeabilities than conventional microfiltration membranes. Both membranes were tested in conventional submerged operation with backflushing and with reverse-flow diafiltration. For process evaluation transmembrane pressure profile and cell retention were analysed. The results show that (1) reverse-flow diafiltration allows easier process control at factor 2 lower transmembrane pressures than in submerged filtration and (2) micromeshes are advantageous in terms of fouling control and operability. TMP increase rates for micromeshes were about half as high as for microsieves in submerged filtration. Cell retention of the micromeshes could be improved by pore size reduction with layer-by-layer polyelectrolyte coating.

High-flux microfiltration filters based on electrospun polyvinylalcohol nanofibrous membranes

A novel class of high-flux microfiltration filters consisting of an electrospun nanofibrous membrane and a conventional non-woven microfibrous support is being presented. The nanofibrous non-woven layer was fabricated by electrospinning of polyvinylalcohol (PVA) directly onto the microfibrous support and then followed by chemical cross-linking with glutaraldehyde (GA) in acetone. By altering the processing parameters, such as the applied voltage and the distance between the spinneret and the collector, as well as the concentration of PVA solution, electrospun PVA membranes with an average fiber diameter of 100 ± 19 nm were obtained. Characterizations revealed that the mean pore size of the electrospun PVA membranes ranged from 0.30 μm to 0.21 μm with the electrospun PVA membrane thickness varying from 10 m to 100 μm. Due to the high porosity, microfiltration filters based on these electrospun membranes showed 3–7 times higher pure water flux than the Millipore GSWP 0.22 μm membrane. The nanofibrous PVA membranes with an average thickness of 20 μm could successfully reject more than 98% of the polycarboxylate microsphere particles with a diameter of 0.209 ± 0.011 μm, and still maintain 1.5–6 times higher permeate flux than that of the Millipore GSWP 0.22 μm membrane.

Solution blown nanofibrous membrane for microfiltration

Nanofibrous membranes have been paid attention in microfiltration. Solution blowing process is a new nanofiber fabricating method with high productivity. In this study, polyvinylidene fluoride (PVDF) nanofibrous mat was successfully solution blown using a multiorifices die. The fibers were mostly 60–280nm in diameter and three-dimensional curly which resulted in a loose construction with high porosity of 95.8%. The nanofibrous mat was further hot-pressed to increase its integrality. The structure and microfiltration performance was evaluated. The results showed the crystallinity of the membranes increased, the porosity and pore size decreased after hot-pressing treatment. The hot-pressed membranes showed high retention ratio against microparticles and high pure water flux which will help the solution blown membrane find application in high flux microfiltration.

Electrospun nanofibrous membranes for high flux microfiltration

Highly porous electrospun nanofibrous membranes have gained considerable interest in water filtration applications. To understand the effects of electrospun nanofibrous structures on the filtration performance, a series of nanofibrous membranes with different fiber diameters, diameter distributions and membrane thicknesses were prepared and studied. The results indicate a strong correlation between the physical parameters of the membrane and the filtration performance. For example, a thicker membrane with a smaller average fiber diameter greatly favors the formation of a smaller pore size and narrower pore size distribution, although the influence of the membrane thickness is relatively limited. Based on successful control of the total composite structure (electrospun polyacrylonitrile (PAN)/non-woven polyethylene terephthalate (PET)) containing the electrospun layer thickness of 200 ± 10 μm and a mean fiber diameter of 100 ± 20 nm, a high flux microfiltration (MF) membrane with a maximum pore size of 0.62 ± 0.03 μm and a mean pore size of 0.22 ± 0.01 μm was obtained. The PAN/PET nanofibrous MF membranes performed significantly better than the commercial MF membrane of the same mean pore size (0.22 μm), with two to three times higher flux (∼1.5 L/m 2 h). The nanofibrous MF filter could maintain a very high rejection ratio of micro-particle and bacteria (LRV = 6). The results suggest that electrospun nanofibrous membranes are excellent materials for high-flux MF applications.

Semi-continuous protein fractionating using Affinity Cross-Flow Filtration

Protein purification by means of downstream processing is increasingly important. At the University of Twente a semi-continuous process is developed for the isolation of BSA out of crude protein mixtures. For this purpose an automated Affinity Cross-Flow Filtration, ACFF, process is developed. This process combines the advantages of both affinity adsorption, having high resolution, and microfiltration, for rapid and large continuous processing capacity. The overall process contains 4 sequential steps: affinity binding; purification; dissociation; regeneration. In the first step binding of the desired protein to a tailor-made ligand-immobilized particle is established. This temporary fixation of target proteins on a large entity allows us to wash out the unbound components in the purification step using high flux microfiltration membranes. After washing out the unbound proteins the target protein-ligand immobilized particle complex is dissociated by adding an appropriate salt. In a second filtration cycle the purified protein can be collected as the permeate. In the last step the ligand-immobilized particle is regenerated for the next cycle with buffer. Using this process it was possible to obtain the target protein BSA with a purity of more than 95%.

Development and applications of very high flux microfiltration membranes

Inorganic microfiltration membranes with a pore size down to 0.1 μm have been made using laser interference lithography and silicon micro machining technology. The membranes have an extremely small flow resistance due to a thickness smaller than the pore size, a high porosity and a very narrow pore size distribution. They are relatively insensible to fouling, because they have a smooth surface, short pore channels and because they can be operated in cross flow configuration at very low transmembrane pressures. Experiments with yeast cell filtration of beer show a minimal fouling tendency and a flux that is about 40 times higher than in conventional diatomaceous earth filtration. The uniform pore distribution makes the membranes suitable for many other applications like critical cell to cell separation, particle analysis systems, absolute filtrations and model experiments.