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Hierarchical ceramic UF membranes
RESEARCH TRENDS which retention is negligible. However, on the basis of the RO fouling data, it appears that the SDI is not sufficiently conservative. Furthermore, since the SDI cannot predict fouling rates, it cannot discriminate between different types of membranes. S.G. Yiantsios and A.J. Karabelas: Desalination 151(3) 229–238 (20 January 2003).
Optimization studies for secondary effluent reclamation Physical-chemical pre-treatment was applied to a reverse osmosis unit that is used for reclamation of secondary effluents. The pilot plant was equipped with a variety of tertiary treatment units to prevent fouling and bio-fouling of the cellulose-acetate RO that were being used. The optimization of pre-treatment involved the application of various concentrations of lime to raise the pH to 10.3–12.1 and stabilize the sludge generated, as well as different dosages of ferric chloride (15 mg/l, 20 mg/l and 25 mg/l) for coagulation and solid–liquid separation. Sodium hypochlorite (8 mg/l) and ultraviolet light disinfection were used for microbiological control. The water quality obtained, under the optimum conditions (pH = 10.5; FeCl3 at 25 mg/l; anionic flocculant at 0.5 mg/l; sodium hypochlorite at 8 mg/l) was high, showing an average conductivity of 66 µS/cm and low COD values of 4 mg O2/l. The product water is suitable for injection into a groundwater aquifer to counteract sea water intrusion. J.A. López-Ramírez, S. Sahuquillo, D. Sales and J.M. Quiroga: Water Research 37(5) 1177–1184 (March 2003).
Characteristics of MF membranes Factors affecting filtration performance were investigated in a sequencing batch reactor (SBR) coupled with a submerged microfiltration (MF) module. Special bioreactors for aerobic and anoxic phases were specifically designed in order to differentiate the effect of dissolved oxygen (DO) from that of mixing intensity, on membrane filterability. When the filterability of a submerged microfilter
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was examined at each SBR phase, the DO concentration and mixing intensity proved to have a major influence on the membrane performance regardless of the SBR phase. A higher DO concentration resulted in a slower rise in TMP, corresponding to less membrane fouling, which was investigated in depth through a series of analyses including resistance measurements and compressibility of the cake layer, and looking at particle sizes as a function of DO for both aerobic and anoxic phases in SBR. I.-J. Kang, C.-H. Lee and K.-J. Kim: Water Research 37(5) 1192– 1197 (March 2003).
Hierarchical ceramic UF membranes Asymmetric alumina ultrafiltration membranes, with a hierarchical structure, were fabricated using carboxylic acid surface-stabilized alumina nanoparticles (alumoxanes). Pre-formed hollow α-alumina spheres (3 µm nominal diameter) were prepared, using 2 wt%, 5 wt% or 8 wt% alumoxane solutions, and suspended in an aqueous solution of acetic-alumoxane (A-alumoxane) nanoparticles. This suspension was contacted with an α-alumina support. Firing to a temperature of 600°C gave a defect-free alumina hierarchical membrane with a total thickness of about 2 µm. The flux and permeability for the membrane containing alumina spheres, and derived from 2 wt% solution of Aalumoxane, is comparable with the porous support, while those derived from 5 wt% and 8 wt% solution of alumoxane are similar to a ‘flat’ alumoxane-derived membrane. An alternative route to increasing the flux and permeability of the asymmetric membranes was developed. Colloidal polystyrene beads of either 0.75 µm, 3 µm or 15 µm diameter were suspended in an aqueous solution of either A-alumoxane (1 wt%) or methoxy(ethoxyethoxy)acetic acid alumoxane (MEEA-alumoxane, 10 wt%). The surface of an α-alumina support was dip-coated in the polystyrene/alumoxane colloidal solution, dried and then fired to 600°C, resulting in an asymmetric alumina membrane with a hierarchical tertiary structure. As the polystyrene out-gasses during pyrolysis the top
of the coated spheres burst, resulting in a macroporous membrane in which the ceramic walls have a pore size, and hence MWCO, that is defined by the alumina formed from the alumoxane nano-particles (A-alumoxane versus MEEA-alumoxane) rather than the macroporous structure of the membrane itself. The permeability of these membranes is equivalent to or better than that of the support. K.A. DeFriend and A.R. Barron: J. of Membrane Science 212(1–2) 29–38 (15 February 2003).
Formation of filter dust during CA membrane preparation Membranes were prepared from six samples of cellulose acetate (CA), differing in their average molecular weight (75–260 kg/mol) and molecular weight distribution using methyl acetate as solvent and 2propanol as precipitant. The routes through the phase diagram and the evaporation times were varied in these experiments. Electron microscopy demonstrates that the amount of filter dust (CA particles deposited on the membrane surface) decreases as the fraction of low molecular weight material drops in the starting polymer. For low average molar mass of CA and moderate polymer concentrations in the casting solution, the dust consists of individual spheres of relatively uniform size (1–2 µm) which cover the surface in a number of layers. With rising molar mass of the polymer the dust particles increasingly consist of strings of cohering and deformed beads of comparable size. Under these condition, some considerably smaller particles (≤0.1 µm) are also formed. Possible mechanisms of filter dust formation are discussed and measures for its prevention are proposed. J. Eckelt, S. Loske, M.C. Gonçalves and B.A. Wolf: J. of Membrane Science 212(1–2) 69–74 (15 February 2003).
Fouling mitigation in desalting electrodialysis Membrane fouling in electrodialysis (ED) can be minimized by using optimized pulsed power instead of direct current (DC) power. For the determination of an
optimum frequency, the effects of the square wave power (with various frequencies) on fouling mitigation were studied in the presence of humate as a foulant, in a NaCl solution. In desalting of a NaCl solution with humate an optimum frequency of the square wave power was found to be 30 Hz, determined by the membrane fouling index for Neosepta AMX and CMX membranes. The AMX membrane has a lower optimum frequency than that of the relatively loose AM-1, which was near 100 Hz in a previous study. The molecular weight distribution of humate transported to the concentrate compartment, through AMX and AM-1 membranes, indicated the structural difference between the two membranes. Through cell tests and desalting ED, a higher performance was achieved with the optimized square wave power than the DC power in terms of conductivity and cell resistance. J.-S. Park, H.-J. Lee and S.-H. Moon: Separation & Purification Technology 30(2) 101–112 (1 February 2003).
Next month News Business activities Industry trends Commercial news New products
Features Developments and applications around the world
Research Trends The latest membrane research published in the key journals
Patents Recently published US and WO patents
Events Calendar Conferences and shows relevant to the membranes industry
Membrane Technology May 2003
Optimization studies for secondary effluent reclamation Physical-chemical pre-treatment was applied to a reverse osmosis unit that is used for reclamation of secondary effluents. The pilot plant was equipped with a variety of tertiary treatment units to prevent fouling and bio-fouling of the cellulose-acetate RO that were being used. The optimization of pre-treatment involved the application of various concentrations of lime to raise the pH to 10.3–12.1 and stabilize the sludge generated, as well as different dosages of ferric chloride (15 mg/l, 20 mg/l and 25 mg/l) for coagulation and solid–liquid separation. Sodium hypochlorite (8 mg/l) and ultraviolet light disinfection were used for microbiological control. The water quality obtained, under the optimum conditions (pH = 10.5; FeCl3 at 25 mg/l; anionic flocculant at 0.5 mg/l; sodium hypochlorite at 8 mg/l) was high, showing an average conductivity of 66 µS/cm and low COD values of 4 mg O2/l. The product water is suitable for injection into a groundwater aquifer to counteract sea water intrusion. J.A. López-Ramírez, S. Sahuquillo, D. Sales and J.M. Quiroga: Water Research 37(5) 1177–1184 (March 2003).
Characteristics of MF membranes Factors affecting filtration performance were investigated in a sequencing batch reactor (SBR) coupled with a submerged microfiltration (MF) module. Special bioreactors for aerobic and anoxic phases were specifically designed in order to differentiate the effect of dissolved oxygen (DO) from that of mixing intensity, on membrane filterability. When the filterability of a submerged microfilter
16
was examined at each SBR phase, the DO concentration and mixing intensity proved to have a major influence on the membrane performance regardless of the SBR phase. A higher DO concentration resulted in a slower rise in TMP, corresponding to less membrane fouling, which was investigated in depth through a series of analyses including resistance measurements and compressibility of the cake layer, and looking at particle sizes as a function of DO for both aerobic and anoxic phases in SBR. I.-J. Kang, C.-H. Lee and K.-J. Kim: Water Research 37(5) 1192– 1197 (March 2003).
Hierarchical ceramic UF membranes Asymmetric alumina ultrafiltration membranes, with a hierarchical structure, were fabricated using carboxylic acid surface-stabilized alumina nanoparticles (alumoxanes). Pre-formed hollow α-alumina spheres (3 µm nominal diameter) were prepared, using 2 wt%, 5 wt% or 8 wt% alumoxane solutions, and suspended in an aqueous solution of acetic-alumoxane (A-alumoxane) nanoparticles. This suspension was contacted with an α-alumina support. Firing to a temperature of 600°C gave a defect-free alumina hierarchical membrane with a total thickness of about 2 µm. The flux and permeability for the membrane containing alumina spheres, and derived from 2 wt% solution of Aalumoxane, is comparable with the porous support, while those derived from 5 wt% and 8 wt% solution of alumoxane are similar to a ‘flat’ alumoxane-derived membrane. An alternative route to increasing the flux and permeability of the asymmetric membranes was developed. Colloidal polystyrene beads of either 0.75 µm, 3 µm or 15 µm diameter were suspended in an aqueous solution of either A-alumoxane (1 wt%) or methoxy(ethoxyethoxy)acetic acid alumoxane (MEEA-alumoxane, 10 wt%). The surface of an α-alumina support was dip-coated in the polystyrene/alumoxane colloidal solution, dried and then fired to 600°C, resulting in an asymmetric alumina membrane with a hierarchical tertiary structure. As the polystyrene out-gasses during pyrolysis the top
of the coated spheres burst, resulting in a macroporous membrane in which the ceramic walls have a pore size, and hence MWCO, that is defined by the alumina formed from the alumoxane nano-particles (A-alumoxane versus MEEA-alumoxane) rather than the macroporous structure of the membrane itself. The permeability of these membranes is equivalent to or better than that of the support. K.A. DeFriend and A.R. Barron: J. of Membrane Science 212(1–2) 29–38 (15 February 2003).
Formation of filter dust during CA membrane preparation Membranes were prepared from six samples of cellulose acetate (CA), differing in their average molecular weight (75–260 kg/mol) and molecular weight distribution using methyl acetate as solvent and 2propanol as precipitant. The routes through the phase diagram and the evaporation times were varied in these experiments. Electron microscopy demonstrates that the amount of filter dust (CA particles deposited on the membrane surface) decreases as the fraction of low molecular weight material drops in the starting polymer. For low average molar mass of CA and moderate polymer concentrations in the casting solution, the dust consists of individual spheres of relatively uniform size (1–2 µm) which cover the surface in a number of layers. With rising molar mass of the polymer the dust particles increasingly consist of strings of cohering and deformed beads of comparable size. Under these condition, some considerably smaller particles (≤0.1 µm) are also formed. Possible mechanisms of filter dust formation are discussed and measures for its prevention are proposed. J. Eckelt, S. Loske, M.C. Gonçalves and B.A. Wolf: J. of Membrane Science 212(1–2) 69–74 (15 February 2003).
Fouling mitigation in desalting electrodialysis Membrane fouling in electrodialysis (ED) can be minimized by using optimized pulsed power instead of direct current (DC) power. For the determination of an
optimum frequency, the effects of the square wave power (with various frequencies) on fouling mitigation were studied in the presence of humate as a foulant, in a NaCl solution. In desalting of a NaCl solution with humate an optimum frequency of the square wave power was found to be 30 Hz, determined by the membrane fouling index for Neosepta AMX and CMX membranes. The AMX membrane has a lower optimum frequency than that of the relatively loose AM-1, which was near 100 Hz in a previous study. The molecular weight distribution of humate transported to the concentrate compartment, through AMX and AM-1 membranes, indicated the structural difference between the two membranes. Through cell tests and desalting ED, a higher performance was achieved with the optimized square wave power than the DC power in terms of conductivity and cell resistance. J.-S. Park, H.-J. Lee and S.-H. Moon: Separation & Purification Technology 30(2) 101–112 (1 February 2003).
Next month News Business activities Industry trends Commercial news New products
Features Developments and applications around the world
Research Trends The latest membrane research published in the key journals
Patents Recently published US and WO patents
Events Calendar Conferences and shows relevant to the membranes industry
Membrane Technology May 2003