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。     

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.     

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.     

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.     

New polymer materials for reverse osmosis membranes

The results of investigations on new polymer materials suitable for reverse osmosis are reported. The studies particularly concerned polypiperazinamides derived from differently structured bicarboxylic acids. Such polyamides show a water permeability that may vary from 0.09 μg cm^?1 sec^?1 to 2.3 μg cm^?1 sec ^?1 depending on the structure.The results obtained from reverse osmosis tests on dense and homogeneous films are described. Anisotropic membranes were prepared from poly(trans-2,5-dimethyl-piperazin-thiofurazanamide). Their properties very closely approach those of the corresponding cellulose acetate membranes. The anisotropic structure of the membranes has been evidenced by stereoscan electron microscope analysis.     

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.     

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).     

Novel ion-containing reverse osmosis membranes. II. Reverse osmosis properties

The reverse osmosis properties of ion-containing membranes prepared by ⁶⁰Co mutual irradiation grafting of 2-vinylpyridine to poly(3,3-bis(chloromethyl)oxetane) (polyoxetane, Penton) followed by quaternization with methyl bromide are presented. In general, the volumetric fluxes varied linearly with pressures up to 55 atm. Katchalsky's treatment of membrane permeability was used to analyze the data. Membrane constants and diffusion coefficients of water and sodium chloride were determined with membranes containing different volume fractions of water. The diffusion coefficients of water in the membranes were of the same order of magnitude as the self-diffusion coefficient of water (～3 × 10^?5 cm²/s). The diffusion coefficients of sodium chloride in the membrane were of the order of 10^?7 cm²/s. The diffusion coefficients increased with hydration, and the salt rejections were markedly affected by the external salt concentrations. The apparent energies of activation for the volumetric flux were calculated in distilled water and in 0.5% sodium chloride solution.     

The electret effect in cellulose acetate reverse osmosis membranes

The electret potentials developed by reverse osmosis electret membranes help control the undesirable deposition of charged colloidal particles on the membrane surfaces during membrane desalination. These antifouling electret membranes should help prevent the costly flux declines normally associated with deposition of colloidal iron oxides on the reverse osmosis membrane surfaces. Homocharge and heterocharge behavior of cellulose acetate membrane electrets have been studied. Asymmetric reverse osmosis membranes and dense membrane films were studied. The homocharge and heterocharge of cellulose acetate reverse osmosis electret membranes have been explained.     

Reuse of reverse osmosis membranes in advanced wastewater treatment

In areas where tap water has a high salt content, wastewater is not appropriate for reuse in agriculture, particularly for sensitive crops. One alternative is reduction, via desalination, of the brackish character to the secondary effluent. A filtration stage is also required before desalination. On the other hand, used reverse osmosis membranes can be recycled and used as filters in the advanced treatment stage in order to reduce suspended matter contained in the secondary effluent—one advantage being the environmental recovery of solid waste. Used membranes can be treated with strong chemical oxidants to peel off the active separation layer in order to transform them into microfiltration or ultrafiltration elements. Preliminary tests have been carried out with 8″ elements, aimed at comparing membrane performance before and after the peeling process. An index denoted as peeling effectiveness (high flux, high salt passage) is used for comparison. It was soon observed that potassium permanganate was more effective than others, together with sodium hydroxide. Doses around 1000 mg/L KMnO₄ provided the best results. It was also concluded that membrane cleaning, done with sodium bisulphite prior to peeling, was better when recirculating the cleaning solution around the membrane rather than soaking it. Next steps in the research will test the actual filtration capability of the peeled membranes in actual wastewater.     

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。     

Investigations of silica scaling on reverse osmosis membranes

In reverse osmosis systems with high silica water severe and irreversible membrane scaling can be observed. But suitable test methods to find an appropriate antiscalant agent and the optimal dosage are missing. Usually only the silicate concentration in the feed water is regarded.In this paper the results of laboratory experiments about the influence of Ca²⁺- and Mg²⁺-ions on the behaviour of supersaturated solutions of silica in different test waters are discussed first. After that the new developed membrane-based test method to determine the effectiveness of antiscalants is presented.The applied different methods of analysis enabled the differentiation of three groups of silicates: ‘monomeric’, ‘polymeric’ and ‘filterable’.With the test method the strong impact of silicate scaling was proven, even if only small amounts of scale had been formed. It could be shown, that the ‘polymeric silica’ is mainly responsible for the membrane scaling. The kinetic of the formation of ‘polymeric silica’ is strongly influenced by the cations and the pH-value. It could be demonstrated that the use of a suitable antiscalant makes it possible to operate the plant at significantly higher recovery rates.     

Precipitation of calcium carbonate on reverse osmosis membranes

The precipitation of calcium carbonate from aqueous solutions was investigated in a laboratory reverse osmosis cell. Solutions of sodium chloride and calcium bicarbonate, resembling hard brackish water, were desalted in the batch test cell using cellulose acetate membranes at a pressure of 40 bar. Desalting of hard water resulted in a significant reduction of the water flux due to the precipitation of calcium carbonate on the membrane. Precipitation is caused by the high permeability of carbon dioxide through the membrane, resulting in a higher pH of the brine. The experimental results are related to the chemical equilibrium of calcium bicarbonate, the mass transfer of the various species through the membrane, and the crystallization kinetics of calcium carbonate.     

反渗透膜离线清洗影响因素

影响反渗透膜离线化学清洗效果的因素较多,通过对反渗透膜离线化学清洗实践的总结,对影响其清洗效果的因素进行分析,从而确定合适的离线清洗条件,为今后反渗透膜离线清洗提供一定的技术支持。     

反渗透膜的污染及其化学清洗

反渗透膜的污染是反渗透系统应用中最大的问题之一,及时的化学清洗可有效地恢复反渗透膜的性能、延长其使用寿命。对反渗透膜的污染及其化学清洗方法进行了综述,主要内容包括反渗透膜污染物的种类,污染物形成的原因,以及清洗时机的选取,清洗药品的选择,影响清洗效果的主要因素、清洗系统的构成,化学清洗的方法和清洗效果的评价指标。