Reverse osmosis desalting

Figure 19 6 H versus 6 0 aKr values of reverse osmosis desalted seawater and the global meteoric waterline (Craig, 1961). The anthropogenic line lies on a possible mixing relationship between natural waters along the meteoric waterline (arrow) and the desalted seawater. Future formation of a large volume of desalted seawater expects to cause infiltration (e.g., leakage, reuse) of freshwater with high S H and 6 0 values relative to natural replenishment.

Fig. 13. A hoUow-fibet reverse osmosis membrane element. Courtesy of DuPont Permasep. In this twin design, the feedwater is fed under pressure into a central distributor tube where half the water is forced out tadiaUy through the first, ie, left-hand, fiber bundle and thus desalted. The remaining portion of the feedwater flows through the interconnector to an annular feed tube of the second, ie, right-hand, fiber bundle. As in the first bundle, the pressurized feedwater is forced out tadiaUy and desalted. The product water flows through the hoUow fibers, coUects at each end of the element, and exits there. The concentrated brine from both bundles flows through the concentric tube in the center of the second bundle and exits the element on the right.

Hyperfiltration (Reverse Osmosis) is a form of membrane distillation or desalination (desalting) operating with membrane pore sizes of perhaps 1 to 10 Angstrom units. The various individual RO component technologies have improved tremendously over the last 20 to 25 years, and resistance to fouling and permeate output rates have benefited. Nevertheless, all RO plants remain susceptible to the risk of fouling, and adequate pretreatment and operation is essential to minimize this problem. 

Belfort, G. In Synthetic Membrane Processes, Belfort, G. (ed.) (Academic Press, Orlando, 1984). Desalting experience by hyperfiltration (reverse osmosis) in the United States. 

All commercial types of processes, with the exception of freezing, namely, distillation, reverse osmosis and electrodialysis, are being applied in the above units with various kinds of distillation processes being used for seawater desalting. Two of them, horizontal tube multieffect distillation and vapor compression units were developed and manufactured locally by the Israel Desalination Engineering Ltd. Recently, two small RO units with a combined capacity of approx. 100 cu. m/day were also used to desalt seawater. The main aim of these units is to test and demonstrate the feasibility of this new technology. 

The comparison between the two major seawater desalting alternatives, reverse osmosis and distillation, is more complex then ever. The location, system size, time of implementation and economic parameters, especially the price of conventional energy and also the possibility of use of non-conventional energy in the future, such as solar or geothermal energy sources, may greatly affect the final decision. 

Durability Study of Cellulose Acetate Reverse-Osmosis Membrane Under Adverse Circumstances for Desalting... 

The rapid expansion of reverse osmosis technology during the past two decades has resulted in the development of a variety of new membranes. Unique polymer systems and fabrication methods have led to the production of membranes with significantly improved performance and reliability. In spite of these developments little is known about chemical sensitivity or life expectancy of reverse osmosis membranes used in desalting applications. Manufacturers are consequently reluctant to guarantee their products for long runs especially in unique chemical environments. 

Reverse osmosis is essentially the same process as ultrafiltration with low salt rejection. Kopfler and co-workers (12) used reverse osmosis in combination with solvent extraction and XAD adsorption. Solvent extraction with pentane and methylene chloride was used to remove organics from the reverse-osmosis concentrate (i.e., for desalting), and XAD was used for adsorption of the intractables from the extracted... 

Reverse osmosis is nsed as a method of desalting seawater, recovering wastewater from paper mill operations, pollution control, industrial water treatment, chemical separations, and food processing. This method involves application of pressure to the surface of a saline solution, thus forcing pure water to pass from the solution through a membrane that is too dense to permit passage of sodium and chlorine ions. Hollow fibers of cellulose acetate or nylon are used as membranes, since their large surface area offers more efficient separation. 

Depending on the enrichment term (E0) of the membrane, the modulus can be larger or smaller than 1.0. For reverse osmosis E0 is less than 1.0, and the concentration polarization modulus is normally between 1.1 and 1.5 that is, the concentration of salt at the membrane surface is 1.1 to 1.5 times larger than it would be in the absence of concentration polarization. The salt leakage through the membrane and the osmotic pressure that must be overcome to produce a flow of water are increased proportionately. Fortunately, modem reverse osmosis membranes are extremely selective and permeable, and can still produce useful desalted water under these conditions. In other membrane processes, such as pervaporation or ultrafiltration, the concentration polarization modulus may be as large as 5 to 10 or as small as 0.2 to 0.1, and may seriously affect the performance of the membrane. 

Equation (4.9) shows that concentration polarization increases exponentially as the total volume flow Jv through the membrane increases. This is one of the reasons why modem spiral-wound reverse osmosis membrane modules are operated at low pressures. Modem membranes have two to five times the water permeability, at equivalent salt selectivities, of the first-generation cellulose acetate reverse osmosis membranes. If membrane modules containing these new membranes were operated at the same pressures as early cellulose acetate modules, two to five times the desalted water throughput could be achieved with the same... 

Currently, approximately one billion gal/day of water are desalted by reverse osmosis. Half of this capacity is installed in the United States, Europe, and Japan, principally to produce ultrapure industrial water. The remainder is installed in the Middle East and other desert regions to produce municipal drinking water from brackish groundwater or seawater. In recent years, the interfacial composite membrane has displaced the anisotropic cellulose acetate membrane in most applications. Interfacial composite membranes are supplied in spiral-wound module form the market share of hollow fiber membranes is now less than... 

The comparative performance of high-pressure, high-rejection reverse osmosis membranes, medium-pressure brackish water desalting membranes, and low-pressure nanofiltration membranes is shown in Table 5.2. Generally, the performance of a membrane with a particular salt can be estimated reliably once the... 

Figure 5.19 Skid-mounted reverse osmosis plant able to produce 700 000 gal/day of desalted water. Courtesy of Christ Water Technology Group...

Electrodialysis is now a mature technology, with Ionics remaining the worldwide industry leader except in Japan. Desalting of brackish water and the production of boiler feed water and industrial process water were the main applications until the 1990s, but electrodialysis has since lost market share due to stiff competition from improved reverse osmosis membranes. Beginning in the 1990s,... 

Reverse Osmosis/Electrodialysis. Very recently, Perdue and co-workers have developed a novel process in which reverse osmosis (RO) is used to remove water from seawater and electrodialysis (ED) is used to desalt seawater. The coupled RO/ED process was developed initially by Koprivnjak et al. (2006) to purify RO-concentrated samples of DOM from freshwaters, which are nearly always contaminated with sulfuric acid (H2S04) and silicic acid (H4Si04). It was possible to remove nearly all H2S04 and H4Si04 from such samples with an average 85% overall recovery of DOC. 

Lorain, O., B. Hersant, F. Persin, A. Grasmick, N. Brunard, and J. M. Espenan. 2007. Ultrafiltration membrane pre-treatment benefits for reverse osmosis process in seawater desalting. Quantification in terms of capital investment cost and operating cost reduction. Desalination 203 277-285. 

In electrodialysis, an applied electric field rather than a concentration gradient is used to draw ions across the membrane. Because it is faster than ordinary dialysis, electrodialysis is often used in biochemical analyses for purposes of fractionation, concentration, and desalting. Reverse osmosis (RO) is a process that uses semipermeable membranes to allow water permeation however, the membranes act as a barrier to the passage of dissolved and suspended particles. Typically, RO membranes are used to extract pure water from aqueous solutions of dissolved salts, such as seawater. The particle size cutoff is typically 0.0001 yum with driving pressures of 200 to 800 psi (1.4 to 5.5 MPa).61... 

Osmosis involves the transfer, by a concentration gradient, of a solvent through a membrane into a mixture of solute and solvent. The membrane is almost non-permeable to the solute. In reverse osmosis, transport of solvent in the opposite direction is effected by imposing a pressure, higher than the osmotic pressure, on the feed side. Using a non-porous membrane, reverse osmosis successfully desalts water. 

Ko, Albino, and Donald B. Guy, "Brackish and Seawater Desalting," in Reverse Osmosis Technology, Bipin S. Parekh ed., Marcel Dekker, Inc. New York, New York, 1988. 

Polyelectrolyte complexes are very promising materials for preparing semi-permeable membranes of definite permeability and selectivity The methods of preparation and the properties of membranes made of polyelectrolyte complexes based on strong polyelectrolytes, e.g. poly(sodium sterene sulfonate) and poly(vinylbenzyl-trimethyl ammonium chloride) were described These membranes may be applied for reverse osmosis in the desalting of sea-water, for dialysis and ultrafiltration in purifications and concentration of water solutions containing coUoids or micro-and macroparticles ... 

Like many other specialities, electrodialysis plants are purchased as complete packages from a few available suppliers. Membrane replacement is about 10% per year. Even with prefiltering the feed, cleaning of membranes may be required at intervals of a few months. The comparative economics of electrodialysis for desalting brackish waters is discussed by Belfort (1984) for lower salinities, elecfrodialysis and reverse osmosis are competitive, but for higher ones elecfrodialysis is inferior. Elecfrodialysis has a number of important unique applications, for removal of high contents of minerals from foods and pharmaceuticals, for recovery of radioactive and other substances from dilute solutions, in electro-oxidation reduction processes and others. 

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