Roger Leysen

Alumina and titania multilayer membranes for nanofiltration: preparation, characterization and chemical stability

Abstract The preparation and characterization of porous ceramic multilayer nanofiltration (NF) membranes is described. During preparation, special care was given to each sub-layer that forms a part of the multilayer configuration: the macroporous substrate, the membrane interlayers and the NF toplayers. High-quality macroporous supports are prepared from α-Al 2 O 3 . Three types of colloidal sol–gel derived mesoporous interlayers are considered: Al 2 O 3 , TiO 2 and mixed Al 2 O 3 –TiO 2 . The active NF toplayer is a very thin and fine textured polymeric TiO 2 toplayer. Optimized α-Al 2 O 3 /γ-Al 2 O 3 /anatase and α-Al 2 O 3 /anatase/anatase multilayer configurations show high retentions for relatively small organic molecules (molecular weight cut-off 2 O 3 layers is restricted to mild aqueous media (pH 3–11) or non-aqueous media (organic solvents). For NF applications in aqueous media with a lower or higher pH, the multilayer membrane composed of anatase on a α-Al 2 O 3 support is to be preferred.

Effect of the addition of ZrO2 to polysulfone based UF membranes

Abstract Flat sheet Zirfon® membranes composed of organic (polysulfone; PSf) and inorganic (zirconium oxide; ZrO2) constituents were prepared by the phase-inversion process. The casting solutions consist of 18 wt% polymer solutions of PSf in N-methyl-pyrrolidone to which different amounts of ZrO2 were added. The membranes were evaluated by measurements of their permeability, porosity and dextran retention. High resolution scanning electron microscopy was applied to obtain direct information on the skin morphology. The membrane permeability was found to increase for growting amounts of inorganic grains added to the casting solution. However, image analysis of the surface micrographs did not reveal corresponding changes in the surface porosity and pore dimensions of the skin. The observed flux behaviour is atributed to the disturbance of the normal phase-inversion process by the presence of the inorganic grains. From 40 wt% ZrO2, the hindrance by the inorganic grains is sufficiently high to cause the formation of a highly porous surface layer. A further increase in wt% ZrO2 influences the formation of the total skin layer, enhancing a higher throughput to the porous sub layer.

Polysulfone-Aerosil composite membranes - Part 1. The influence of the addition of Aerosil on the formation process and membrane morphology

Presented organo-mineral membranes are the result of the addition of Aerosil 200 spheres to a solution of polysulfone (PSf) in N-methylpyrrolidone (NMP). They are prepared using the wet phase inversion process by casting the suspension on a glass plate and immersing it in water. For the first time, transparent suspensions are used to investigate the effect of the filler on the membrane formation process. It allows the transport of nonsolvent into the membrane to be followed by light microscopy. The results prove that an increase of the filler concentration slows down the indiffusion rate. Cloud point measurements indicate that the addition of fillers changes the thermodynamic behavior of the polymer/solvent/nonsolvent system in such a way that demixing occurs at a lower concentration of nonsolvent. A special experimental ‘slide’-setup is designed and presented here to follow the instantaneous demixing process of the filled polymer solutions as a function of time. These experiments prove that the presence of a small amount of filler increases the demixing rate in the initial stage of membrane formation. A further increase of the amount of filler decreases the demixing rate of the instantaneous demixing process due to viscosity effects.

The use of bacteria immobilized in tubular membrane reactors for heavy metal recovery and degradation of chlorinated aromatics

Abstract Microbial treatments of waste water can be done in membrane reactors. A membrane installed outside the reactor is used to separate bacteria from the treated effluent. A new membrane reactor concept is presented. The separation membrane is introduced in the reactor and not outside as in a normal one. The membrane plays the role of a separator of two streams and is used at the same time as the immobilizing support for the bacteria. The reactor keeps the bacteria active via a specific nutrient stream that is provided on one side of the membrane. The bacteria grow in and on the membrane where they form an active biofilm. The bacteria can treat the effluent on one side and can be kept active via the nutrient stream at the other side without contamination of the effluent by the nutrient. In this work, the performance of the BICMER (Bacteria Immobilized Composite MEmbrane Reactor) is demonstrated via treatments of effluents containing heavy metals or organic xenobiotics. For heavy metal removal Alcaligenes eutrophus CH34 bacteria were used. These bacteria induce a metal bioprecipitation process that results in the formation of crystalline metal carbonates, which are recovered on a separate column in the reactor. In this way metals can be recovered without disturbing the biofilm on the membrane. Metals such as Cd, Zn, Cu, Pb and Y can be reduced to less than 50 ppb. The metals Co, Ni, Pd and Ge are reduced to below 100 ppb. For organic xenobiotics Alcaligenes eutrophus AE1308 bacteria or other strains (depending on the xenobiotic to be degraded) were used. This strain degrades the xenobiotic 3-chlorobenzoate (Cba) and 2,4-dichlorophenoxyacetic acid to CO2, H2O and chloride). Concentrations of 3 mM Cba could be reduced to less than 0.1 mM. For other toxic organic compounds, different biodegrading strains need to be used.

The influence of filler concentration on the compaction and filtration properties of Zirfon®-composite ultrafiltration membranes

Abstract During ultrafiltration, a transmembrane pressure difference is applied across a polymeric membrane. This pressure difference results in solute transport through the membrane and mechanical compression of the membrane. The ultrasonic time-domain reflectometry (UTDR) technique is used to measure mechanical compression of the membrane structure in real time while water permeability is simultaneously measured during ultrafiltration in a specially designed crossflow filtration cell. The membranes used in this study are Zirfon® organo-mineral membranes fabricated using 0.9-μm zirconia particles at three different loadings. Thickness changes and the permeability of composite ultrafiltration membranes are obtained simultaneously and in real-time as a function of filler concentration and transmembrane pressure. Results indicate that the presence of ZrO 2 particles in the polymeric matrix influences the membrane permeability as well as compression of the membrane structure. Increasing the filler concentration decreases the elastic strain component but increases the time-dependent strain component. Both the initial and the time-dependent strain increase with increasing pressure. The flux decline is more severe at higher pressures and appears to be due to localized deformation in the membrane skin layer. This study provides the first quantitative information regarding compaction of organo-mineral Zirfon® membranes.

A comparison between polysulfone, zirconia and organo-mineral membranes for use in ultrafiltration

Abstract In this paper, representative polymeric (a PSf/PVP membrane), ceramic (a ZrO 2 membrane) and organo-mineral (a ZrO 2 /PSf membrane) ultrafiltration membranes, all in the tubular configuration, are being compared for their basic membrane properties, and for the typical ultrafiltration application of protein recovery of cheese whey. These three different membranes with a quite similar pore size (the cut-off values for each of the three membranes were comprised between 25 000 and 50 000 Dalton) showed pure water permeability coefficients between 135 and 1250 l/h m 2 bar. The highest pure water flux was found for the organo-mineral membrane, the lowest for the polymeric membrane. By FESEM analysis of the top-surfaces (skin) of both the PSf/PVP and the ZrO 2 /PSf membrane a strong difference in surface-porosity was found. These results were claimed to partially explain the difference in pure water flux. From SEM pictures of the cross-section of the ZrO 2 /PSf membrane it could also be seen that the skin layer thickness is smaller, at these places where particles are present near the skin-surface, compared to the rest of the membrane as well as to the skin of the PSf/PVP membrane. These latter observations were also used to further explain the flux difference between the PSf/PVP and the ZrO 2 /PSf membrane. In application tests (ultrafiltration of a sweet Gouda cheese whey) these three rather different membranes surprisingly showed practically the same gel-layer controlled or plateau fluxes, the same flux stability, and flux/concentration factor behaviour. The protein retention in all the experiments was 99% or more. The permeability coefficient however for this sweet Gouda whey was identical for the PSf/PVP and the ZrO 2 membrane and equal to 50 l/h m 2 bar. On the contrary for the ZrO 2 /PSf organo-mineral membrane a nearly three fold higher permeability coefficient of 135 l/h m 2 bar was found. This property is partly attributed to the much higher surface porosity of the organo-mineral membrane as compared to the polymeric membrane. From this comparison one may conclude that for high fouling applications, the only positive effect upon using membranes with high permeability coefficients is a reduced transmembrane pressure for a given flux. However, for low fouling applications distinct gains in terms of flux can be expected upon using such membranes.

Porous Ceramic Membranes: Preparation, Transport Properties and Applications

The preparation and characterization of porous ceramic membranes is presented. These membranes consist of a macroporous support system, with or without a mesoporous intermediate layer, and a microporous top layer. For the macroporous support membranes two manufacturing routes are described: a conventional and a RBAO (Reaction Bonded Aluminium Oxide) route. The mesoporous γ-Al2O3 layer is obtained by means of a sol-gel dipcoating technique. Three microporous top layers are considered: SiO2, Al2O3-pillared montmorillonite and Laponite. These top layers have different pore structures which results in different gas transport properties. A SiO2 membrane can be used for H2 removal from a gas mixture. Al2O3-pillared montmorillonite and Laponite membranes do not show specific gas separation properties. Dehydration of water/2-propanol mixtures by means of pervaporation also proved a different behavior for these microporous membranes.

A new method of measuring and presenting the membrane fouling potential

The silt density index (SDI) and modified fouling index (MFI) characterisation methods are well known for the evaluation of membrane fouling potential of dispersed particulate matter (suspended solids, colloids) in a feed. The SDI and MFI methods, however, reduce the overall and very complex fouling phenomena into a one number value, on which the interpretation of the fouling potential of the feed is based. Considering such a one number characteristic, a significant amount of information from the fouling measurement (data) is lost. In this paper a concept is introduced in order to preserve such information and supplement the existing indexes. The proposed method measures, processes and presents data in a specific format. To illustrate the concept, some results are shown from measurements on three types of feed. Future systematic research will also include the measurement on some model feeds with, for example, well characterised dispersions for comparison purposes. However, based on the contents of this paper, a discussion on the method could be initiated.

Shaping of multilayer ceramic membranes by dip coating

Abstract For the development of new ceramic membranes for gas separative and catalytic membrane reactors, multilayer membranes were synthesized. These membrane configurations consist of three layers: a porous support (0.1 μm In this contribution, the characterization of the membranes and the optimization of the manufacturing route for the different material layers are discussed. Some special attention was given to the development of an alternative porous support, adapted from a Reaction Bonded Al2O3 (RBAO)-process and to the tailoring of the pore texture of the γ-Al2O3 interlayer. Preliminary dip coating experiments proved that the formation of a thin clay or pillared clay layer on an (α + γ)-Al2O3 support is possible.

Development of a membrane biofilm reactor for the degradation of chlorinated aromatics

Many wastewaters generated by industry contain a limited amount of very toxic and recalcitrant pollutants which can upset biological treatment systems. A new membrane biofilm reactor concept is studied to treat these wastewaters. A biofilm is grown on an ultrafiltration membrane supporting layer that separates wastewater from an additional nutrient stream. The biofilm bacteria are in close contact with the wastewater which allows an efficient degradation of organic pollutants. Meanwhile, the diffusion of specific nutrient compounds keeps the bacteria active. The study is focused on modelling and optimization of the reactor operation considering both mass transfer and degradation kinetics. The biofilm structure and characteristics are very important in this optimization study.