New polymeric materials for reverse osmosis membranes

The results obtained from investigations on new aromatic polyamides containing carboxylic groups suitable for reverse osmosis are reported. The polymers are fabricated into asymmetric membranes by the Loeb-Sourirajan technique. The effects of fabrication conditions were also investigated to yield the optimum membranes for brackish water and sea water desalination, respectively. Performance characteristics of the membranes are 0.30 m³/m²·day above 98% rejection at 70 kg/cm²-35000 ppm NaCl, and 1.0–1.15 m³/m²· day, 97–98% rejection at 40 kg/cm²-5000 ppm NaCl. They exhibited good resistance in the tests carried out in alkaline medium (pH 10) and acid medium (pH 4).     

New membranes for reverse osmosis I. Characteristics of the base polymer: sulphonated polysulphones

Within the scope of our research programme on new materials for the preparation of reverse osmosis membranes, we have studied the specific characteristics and performance of a new polymer which is a sulphonated aromatic polysulphone.Film properties were determined in reverse osmosis, permeability constants were calculated from measurements carried out at 52 bars and 20°C on a 35 g/l NaCl solution:Water permeability D₁C₁ = 2.8 × 10^-7 g/cm/sSalt permeability D₂K = 1.9×10^-10 cm²/sThere the ratio (D₁C₁/D₂K) = 1.470 g/cm³Under the same conditions, a cellulose acetate film allows a ratio D₁C₁:D₂K of 1,200 g/cm³ which is in agreement with results in the literature.     

A centrifugal method for the evaluation of polymer membranes for reverse osmosis

A rapid and simple method employing the laboratory centrifuge shows promise for evaluation of membrane performance during reverse osmosis. Results are presented for cellulose acetate membranes for rejection of salt and urea dissolved solids. Implications of the study are to rapid screening of membrane performance, use in laboratories with limited facilities, and possible space waste water purification.     

Recent advances in polymer and polymer composite membranes for reverse and forward osmosis processes

Semipermeable membranes are the core elements for membrane water desalination technologies such as commercial reverse osmosis (RO) process and emerging forward osmosis (FO) process. Structural and chemical properties of the semipermeable membranes determine water flux, salt rejection, fouling resistance, and chemical stability, which greatly impact energy consumption and costs in osmosis separation processes. In recent years, significant progress has been made in the development of high-performance polymer and polymer composite membranes for desalination applications. This paper reviews recent advances in different polymer-based RO and FO desalination membranes in terms of materials and strategies developed for improving properties and performances.     

Improved cellulose acetate membranes for reverse osmosis

Performance of cellulose acetate membranes in reverse osmosis varies with the conditions under which they are cast. By varying casting solution composition and holding time in a systematic way, improvement in water flux at a given level of salt rejection has been obtained. Statistically designed experiments have been helpful in optimizing these two variables. A phase diagram of the cellulose acetateformamide-acetone casting system has been determined which gives the region of natural solubility of this three component system.     

反渗透膜的清洗

介绍了反渗透设备在应用中遇到的膜污染问题、清洗方案的制定及实施清洗后取得的良好效果。     

New composite reverse osmosis membranes made from poly-2-vinylimidazoline

Novel ultra-thin film (UTF) composite reverse osmosis (RO) membranes were fabricated in situ by the interfacial polymerization of poly-2-vinylimidazoline (PVI) precursors and aromatic acid chloride crosslinking agents. The syntheses of three different forms of PVI, which were used as precursors, are described. 3-(Chlorosulphonyl)benzoyl chloride, 3,5-di(chlorosulphonyl)benzoyl chloride and 1,3-benzenedicarbonyl dichloride were used as crosslinking agents. Both flat-sheet and tubular membranes were fabricated and tested for their salt retentions and permeate fluxes. The RO performances of the two sets of tubular membranes PVI I and PVI II crosslinked with 3-(chlorosulphonyl)benzoyl chloride, prepared according to optimized fabrication formulations, were recorded. These were 97.1 ± 0.1% retention; 730 ± 24 lmd flux and 98.2 ± 0.7% retention; 560 ± 150 L/m²/day (lmd) flux, respectively. The RO performances of initial tubular membranes made from PVI III and 3-(chlorosulphonyl)benzoyl chloride was 97.0 ± 1.1% retention and 643 ± 74 lmd flux.     

New thin-film composite reverse osmosis membranes and spiral wound modules

Two new series of thin-film composite reverse osmosis membranes have been developed and fabricated into spiral wound modules. The NTR-7100 series membrane is able to desalt sea and brackish water. The NTR-7250 membrane is designed for use at pressures below 20 kg/cm². The membrane has a very high water permeability and is resistant to chemical and microbiological attack. In particular, the membrane is stable to chlorine, as shown by long-term reverse osmosis tests with tap water containing about 1 ppm of residual chlorine. The membrane has an unusual pattern of solute rejection. Salts containing divalent anions, such as sodium sulphate or magnesium sulfate, are rejected more than 98%; while salts with monovalent anions and bivalent cations, such as magnesium chloride, are rejected about 90% and salts with monovalent anions and cations, such as sodium chloride, are rejected 30–50%. Neutral solutes have relatively high rejection; for example, glucose, 90% and sucrose, <99%.     

Surface-tailoring chlorine resistant materials and strategies for polyamide thin film composite reverse osmosis membranes

Polyamide thin film composite membranes have dominated current reverse osmosis market on account of their excellent separation performances compared to the integrally skinned counterparts. Despite their very promising separation performance, chlorine-induced degradation resulted from the susceptibility of polyamide toward chlorine attack has been regarded as the Achilles’s heel of polyamide thin film composite. The free chlorine species present during chlorine treatment can impair membrane performance through chlorination and depolymerization of the polyamide selective layer. From material point of view, a chemically stable membrane is crucial for the sustainable application of membrane separation process as it warrants a longer membrane lifespan and reduces the cost involved in membrane replacement. Various strategies, particularly those involved membrane material optimization and surface modifications, have been established to address this issue. This review discusses membrane degradation by free chlorine attack and its correlation with the surface chemistry of polyamide. The advancement in the development of chlorine resistant polyamide thin film composite membranes is reviewed based on the state-of-the-art surface modifications and tailoring approaches which include the in situ and post-fabrication membrane modifications using a broad range of functional materials. The challenges and future directions in this field are also highlighted.     

Recent advances in reverse osmosis membranes

A review is presented of the developments of the past several years in the area of reverse osmosis membranes. These developments have occurred along two lines: new membrane materials and new membrane fabrication methods. Among the new materials can be cited aromatic polyamides, polybenzimidazole, polyphenylene oxide, hydrated metal oxides, crosslinked and blended cellulose acetates, crosslinked polyvinylpyrrolidone, crosslinked polyethylenimine, and derivatives of several of these materials. The water and salt permeabilities of many of these materials and of the cellulose acetates are compared.Important developments in the area of fabrication methods include the extension of asymmetric membrane techniques to new materials, supported ultra-thin films, hollow fibers and asymmetric hollow fibers, and dynamically formed membranes. A comparison of these materials and fabrication methods requires some kind of figure of merit and one such parameter is discussed.     

Colloidal fouling of reverse osmosis membranes

Small colloidal particles intermediate in size between suspended solids, and true dissolved solids have been causing serious problems to many reverse osmosis systems throughout the world.These tiny negatively charged particles become concentrated at the surface of the membrane during the R.O. process and begin to coagulate, plugging the membrane. If this fouling is allowed to go on unchecked, the quantity and quality of the product water will begin to deteriorate until the membranes will eventually need to be replaced.In order to prevent this from happening, it is suggested that the R.O. pretreatment system be designed properly. If the pretreatment is not adequate, it can lead to high maintenance costs and an increase in membrane replacements.     

Hybrid reverse osmosis — ultrafiltration membranes

A new group of asymmetric non-cellulosic membranes having a performance intermediate between that of a conventional RO membrane and an ultrafilter has been developed. Its main characteristics are high fluxes (4–10 m³/m² day) at low pressures (6–10 at) and moderate rejections for various salts. From 200 mg/l solutions of NaNO₃, Na₂SO₄ and KH₂PO₄ rejections up to 40%, 70% and 90%, respectively, were obtained. The new membranes can withstand large variations in pH (1 to 13) and have excellent chemical and biological stability. The membranes have been tested in a mobile SUF pilot plant operating on oxidation pond effluent. High rejections for BOD, COD, bacteria and suspended solids as well as a 20% reduction in salinity have been obtained.     

Composite membranes for seawater desalination by reverse osmosis

A composite reverse-osmosis membrane has been developed for seawater desalination having a 400-Å semipermeable barrier. The membrane is prepared by directly forming a very thin film of a polymer, generally cellulose triacetate, upon the finely porous surface of a supporting membrane. The composite membrane, capable of desalinating seawater in a single pass, has demonstrated improved flux stability at high pressures over modified membranes currently used to desalinate brackish water.     

New reverse osmosis membrane materials with higher resistance to chlorine

Copolyamides were prepared from mixed diamine components of 3,3′‐ or 4,4′‐diaminodiphenylsulfone, piperazine, and dichlorides such as isophthaloyl or terephthaloyl. The obtained copolyamides are random copolymers, which have good solubilities in organic solvents and good mechanical properties, even in water. The asymmetric membranes prepared from the copolyamides [ex: 4I‐PIP(20)] not only have more excellent reverse osmosis performance, but also higher chlorine resistance than NOMEX‐type aromatic polyamide. New membrane materials with excellent reverse osmosis performance and higher chlorine resistance than the NOMEX‐type aromatic polyamide could be demonstrated successfully. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1357–1364, 2000     

反渗透膜有机污染的控制方法

综述了近年来反渗透膜有机污染控制方法的研究进展,其中进水预处理、优化操作条件、添加阻垢剂、膜表面改性和清洗对反渗透膜的有机污染均能进行有效的控制,提出了反渗透膜有机污染控制方法的研究方向。     

Ionic reverse osmosis membranes of grafted polyethylene

Membranes of graft copolymers of polyethylene with poly(sodium styrene sulfonate), poly(4-vinylpyridinium methyl bromide), and poly(sodium acrylate) were prepared by using the technique of peroxide grafting. The reverse osmosis characteristics of the membranes were examined as a function of grafting yield. In these membranes, the grafting can be considered as a process of introducing ionic sites, and it depends on the conditions of the grafting reaction, such as monomer concentration and temperature. However, the overall reverse osmosis characteristic is not only dependent on the number of ionic sites introduced but also on the swelling capability of the membrane. Consequently, the salt rejection of grafted membrane of a fixed graft yield depends on the conditions of the grafting reaction. All grafted membranes which have grafting yields above a certain value behave as normal ionic polymer membranes, and their interrelationship of salt rejection and water permeability follow the general dependence found for ionic polymer membranes.     

Dynamic polyacrylamide membranes in the reverse osmosis

Dynamic formation of Polyacrylamide membranes on a Millipore filter in the reverse osmosis was optimised concerning rejection of a standard NaCl water solution.Optimised membranes were tested under 70 bars with selected salts in single-solute water solutions.Exclusion of salts with alcali cations depend on the kind of anion and is in the order SO^2-₄ > F^- > HCO^-₃ > Cl^- > NO^-₃ > J^-.Rejection of salts with alkaline earth cations is generally lower than corresponding salts with alcali cations.Especially concentration dependence of rejection of SO^2-₄ and NO^-₃ through these membranes was tested using different cations.Remarkable is the relatively high rejection of Na₂SO₄, which is 93,6% for a 5 g/l water solution under a flux of 820 l/m²· d.Dynamic PAA membranes are not useful for sea water desalination but they could be used for treatment of water with higher SO^2-₄ content.     

Predicting flux decline of reverse osmosis membranes

A mathematical model predicting flux decline of reverse osmosis membranes due to colloidal fouling has been verified. This mathema- tical model is based on the theory of cake or gel filtration and the Modified Fouling Index (MFI). Research was conducted using artificial colloidal solutions and a pilot plant equipped with ultrafiltration membranes. Polystyrene latex spheres, having a size of about 0.05-0.08 μm were used as a foulant.The result of this research was, that the measured and calculated values of flux decline of the ultrafiltration membranes as a function of time agree reasonably well. The difference between the measured and calculated values may be explained by the assumption, that initially blocking filtration occurs instead of cake or gel filtration.