Reverse osmosis ultrafiltration

The solvated metals are large enough not to pass through the membrane. Consequently the membrane can separate solvent from solutes. The whole process is equivalent to evaporation but done at ambient temperature. In operation the effluent gradually becomes more concentrated in solutes, whereby it can be eventually collected or recycled. The removed solvent (usually water) is virtually contaminant free and can be recycled and used as rinse water. (Another application of membrane filtration is the purification of water for use in power plant steam circuits.) 

Ultrafiltration describes the process where the membrane is simply used as a filter. The filtration process usually occurs under the influence of gravity (or at slight pressure 1-8 bar) and, as a result, is slow. Reverse 

Ultrafiltration is currently used as a pretreatment technique for the removal of greases, oils and colloids before effluent is passed through an ion-exchange column for final clean-up. Otherwise, there are no other uses in industry for the direct use of ultrafiltration as a single clean-up step for metals. 

On the other hand, reverse osmosis finds extensive use for the removal of dissolved metals. In most cases the technique is used in crossflow conditions, in other words the effluent flow is parallel to the membrane plane. The technique is very popular in the electroplating industry where effluent is concentrated to a point where it can be reused this represents a considerable saving in terms of chemicals and water costs. Typically, in these situations effluent flow rates are of the order of 100-300 1 min  

There are restrictions for the use of reverse osmosis for effluent treatment. Nearly all the previous uses of reverse osmosis have been in metal recovery and recycling within the electroplating industry. The use of reverse osmosis for simple effluent treatment is rather restricted due to the chemical stability of the membrane. Often, industrial effluents are highly corrosive solutions containing hydroxide or acids. The membranes, which are essentially organic polymers, are quickly decomposed by hydroxide and acid. This can lead to rapid mechanical failure of the membrane especially under the high pressures used in reverse osmosis. Consequently the 

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In reverse osmosis membranes, the pores are so smaH, in the range 0.5— 2 nm in diameter, that they ate within the range of the thermal motion of the polymer chains. The most widely accepted theory of reverse osmosis transport considers the membrane to have no permanent pores at aH. Reverse osmosis membranes are used to separate dissolved microsolutes, such as salt, from water. The principal appHcation of reverse osmosis is the production of drinking water from brackish groundwater or seawater. Figure 25 shows the range of appHcabHity of reverse osmosis, ultrafiltration, microfiltration, and conventional filtration. 

Fig. 25. Reverse osmosis, ultrafiltration, microfiltration, and conventional filtration are related processes differing principally in the average pore diameter of the membrane filter. Reverse osmosis membranes are so dense that discrete pores do not exist transport occurs via statistically distributed free volume areas. The relative size of different solutes removed by each class of membrane is illustrated in this schematic.

Membrane Filtration. Membrane filtration describes a number of weU-known processes including reverse osmosis, ultrafiltration, nanofiltration, microfiltration, and electro dialysis. The basic principle behind this technology is the use of a driving force (electricity or pressure) to filter... 

Tangential crossflow filtration Process where the feed stream sweeps the membrane surface and the particulate debris is expelled, thus extending filter life. The filtrate flows through the membrane. Most commonly used in the separation of high-and-low-molecular weight matter such as in ultrapure reverse osmosis, ultrafiltration, and submicron microfiltration processes. 

Sourirajan, S., Matsuura, T, Reverse Osmosis/Ultrafiltration, Process Principles, National Research Council Canada, Ottawa, 1985... 

Fig. 16.2. Reverse osmosis, ultrafiltration, microfiltration and conventional filtration with distinct pore size.

Aeration, Stripping Lime Softening Anion Cation Reverse Osmosis Ultrafiltration Chemical Oxidation Disinfection GAC PAG Granular Ferric Hydroxide Activated Alumina... 

S. Souriiajan and T. Matsuura, Reverse Osmosis/ Ultrafiltration Principles, National Research Council of Canada, Ottawa, Canada, 1985. 

In either approach, the selection of isolation (e.g., solvent extraction, adsorption on carbon and synthetic resins) and concentration (e.g., lyophilization, vacuum distillation, reverse osmosis, ultrafiltration) methods is of paramount importance in properly assessing the potential toxicity of waterborne organics. A comprehensive literature review on the development and application of these and other methods to biological testing has recently been published by Jolley (3). 

Size ranges for membrane processing by reverse osmosis, ultrafiltration and microfiltration are shown in Figure 19.1. Reverse osmosis is effective in removing solvents away from dissolved molecules. Because of limitations in crushing strengths of membranes, pressures are limited to maxima of about 1000 psi... 

Many synthetic membranes are known to be useful for separation of water and various sizes of solutes from aqueous solutions by selective separation, for examples reverse osmosis, ultrafiltration, dialysis and so on 1 7). The permeability is much dependent on both of chemical and physical structures of the membranes. The choice of the barrier materials for membranes and the control of their morphology are important to get effective permselective membranes. 

The range of application of the three pressure-driven membrane water separation processes—reverse osmosis, ultrafiltration and microfiltration—is illustrated in Figure 1.2. Ultrafiltration (Chapter 6) and microfiltration (Chapter 7) are basically similar in that the mode of separation is molecular sieving through increasingly fine pores. Microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 pm in diameter. Ultrafiltration membranes can be used to filter dissolved macromolecules, such as proteins, from solutions. The mechanism of separation by reverse osmosis membranes is quite different. In reverse osmosis membranes (Chapter 5), the membrane pores are so small, from 3 to 5 A in diameter, that they are within the range of thermal motion of the polymer... 

Although reverse osmosis, ultrafiltration and microfiltration are conceptually similar processes, the difference in pore diameter (or apparent pore diameter) produces dramatic differences in the way the membranes are used. A simple model of liquid flow through these membranes is to describe the membranes as a series of cylindrical capillary pores of diameter d. The liquid flow through a pore (q) is given by Poiseuille s law as ... 

Figure 4.1 shows the concentration gradients that form on either side of a dialysis membrane. However, dialysis differs from most membrane processes in that the volume flow across the membrane is usually small. In processes such as reverse osmosis, ultrafiltration, and gas separation, the volume flow through the membrane from the feed to the permeate side is significant. As a result the permeate concentration is typically determined by the ratio of the fluxes of the components that permeate the membrane. In these processes concentration polarization gradients form only on the feed side of the membrane, as shown in Figure 4.3. This simplifies the description of the phenomenon. The few membrane processes in which a fluid is used to sweep the permeate side of the membrane,...

The final parameter in Equation (4.9) that determines the value of the concentration polarization modulus is the diffusion coefficient A of the solute away from the membrane surface. The size of the solute diffusion coefficient explains why concentration polarization is a greater factor in ultrafiltration than in reverse osmosis. Ultrafiltration membrane fluxes are usually higher than reverse osmosis fluxes, but the difference between the values of the diffusion coefficients of the retained solutes is more important. In reverse osmosis the solutes are dissolved salts, whereas in ultrafiltration the solutes are colloids and macromolecules. The diffusion coefficients of these high-molecular-weight components are about 100 times smaller than those of salts. 

This membrane industry is very fragmented. Industrial applications are divided into six main sub-groups reverse osmosis ultrafiltration microfiltration gas separation pervaporation and electrodialysis. Medical applications are divided into three more artificial kidneys blood oxygenators and controlled release pharmaceuticals. Few companies are involved in more than one sub-group of the industry. Because of these divisions it is difficult to obtain an overview of membrane science and technology this book is an attempt to give such an overview. 

Membrane separation techniques, which are used mainly in industrial processes, include dialysis, electrodialysis, reverse osmosis, ultrafiltration,... 

Ion exchange, reverse osmosis, ultrafiltration, and air stripping can also be used for separating waste components, especially for waste-water treatment. 

For water for injection preserved in containers and sterilized, the JP 2001 provides separate tests for acid or alkali, chloride, ammonium, and residue on evaporation within the monograph. For water for injection prepared by reverse osmosis-ultrafiltration. 

T. S. Chung, J. J. Shieh, J. Qin, W. H. Lin, and R. Wang, Polymeric membranes for reverse osmosis, ultrafiltration, microfdtration, gas separation, pervaporation, and reactor applications. In Advanced Functional Molecules and Polymers, H. S. Nalwa (ed.). Chapter 7, Gordon Breach, pp. 219-264 (2001). 

M. Perry and C. Linder, Intermediate reverse osmosis ultrafiltration membranes for concentration and desalting of low molecular weight organic solutes. Desalination, 71 (1989) 233. 

In pervaporatlon, reverse osmosis, ultrafiltration and to a lesser extent microfiltration, both steric and chemical factors Influence permeation and separation. Thus, proper membrane material selection la Important. While the physical structure of the membrane is in large part a function of membrane preparation procedures, the chemical nature and, to some degree, the physical properties of the membrane are dependent upon the chemical nature of the membrane material. Thus, membrane material selection based on the chemical nature of the polymer and the solution components to be separated is feasible. 

Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

Wong, E.W. Urethane-Polyether Block Copolymer Membranes for Reverse Osmosis, Ultrafiltration and Other Membrane Processes, ORF Record of Invention No. 335, 1969. 

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