Reverse osmosis, alternatives

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

Electrodialysis. In reverse osmosis pressure achieves the mass transfer. In electro dialysis (qv), dc is appHed to a series of alternating cationic and anionic membranes. Anions pass through the anion-permeable membranes but are prevented from migrating by the cationic permeable membranes. Only ionic species are separated by this method, whereas reverse osmosis can deal with nonionic species. The advantages and disadvantages of reverse osmosis are shared by electro dialysis. 

Much as reverse osmosis (qv) can compete with evaporation in desalination appHcations, osmosis should also be considered as an alternative for process evaporation. Reverse osmosis is particularly attractive where the inlet stream is greater than 99% water. 

Grade 2 water to reverse osmosis or de-ionisation, followed by filtration through a membrane filter of pore size 0.2 jum to remove particulate matter. Alternatively, Grade 2 water may be redistilled in an apparatus constructed from fused silica. 

An alternative method of purifying water is by reverse osmosis. Under normal conditions, if an aqueous solution is separated by a semi-permeable membrane from pure water, osmosis will lead to water entering the solution to dilute it. If, however, sufficient pressure is applied to the solution, i.e. a pressure in excess of its osmotic pressure, then water will flow through the membrane from the solution the process of reverse osmosis is taking place. This principle has been... 

In addition to these three treatments, there are several alternative treatment technologies applicable to the treatment of common metals wastes. These technologies include electrolytic recovery, electrodialysis, reverse osmosis, peat adsorption, insoluble starch xanthate treatment, sulfide precipitation, flotation, and membrane filtration.1516... 

Electrodialysis can be an alternative to reverse osmosis, as discussed in Chapter 10. 

Wastewater reverse osmosis reclamation systems, 26 79-80 Wastewater treatment, 25 882-920 advances in, 25 910-912 alternative biological technologies in, 25 902-905... 

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. 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

Reverse osmosis as an alternative or substitute method for the purification 1-2 US5221485 Johnson/ 19... 

In hot-climate viticulture it is a common practice to lower the high ethanol content of wines made from overripe fruit by partial dealcoholisation. This objective can be achieved by vacuum distillation, where the spinning cone column technique allows even more viscous liquids to be processed. Alternatively, a water-ethanol fraction can be separated from wine by reverse osmosis, followed by distillation of the water-ethanol permeate to yield high-grade ethanol and pure water. The latter will be added back to the treated wine. 

Metal removal from surface water, groundwater or wastewater streams is more straightforward than that from soils. Typically, removal is achieved by concentration of the metal within the wastestream using flocculation, complexation, and/or precipitation. For example, the use of lime or caustic soda will cause the precipitation and flocculation of metals as metal hydroxides. Alternatively, ion exchange, reverse osmosis, and electrochemical recovery of metals can be used for metal removal (Chalkley et al., 1989 Moore, 1994). Unfortunately, these techniques can be expensive, time-consuming and sometimes ineffective, depending on the metal contaminant present. 

Laboratory-grade distilled or reverse osmosis water should be further treated using a deionization cartridge. The water plant must be well maintained to avoid bacterial growth in deionization columns, plumbing, and storage tanks. Alternatively, commercial HPLC-grade water has been found to be satisfactory. 

Plate-and-frame units have been developed for some small-scale applications, but these units are expensive compared to the alternatives, and leaks through the gaskets required for each plate are a serious problem. Plate-and-frame modules are now only used in electrodialysis and pervaporation systems and in a limited number of reverse osmosis and ultrafiltration applications with highly fouling feeds. An example of one of these reverse osmosis units is shown in Figure 3.39 [111],... 

A second potential application is pressure-driven desalination. When a pressure difference is applied across the membrane, the concentrated ionic groups in the ion exchange domains are swept through the membrane, producing a salt-enriched permeate on the low-pressure side. This process, usually called piezodialysis, has a number of conceptual advantages over the alternative, conventional reverse osmosis, because the minor component (salt), not the major component (water), permeates the membrane. 

Membrane distillation offers a number of advantages over alternative pressure-driven processes such as reverse osmosis. Because the process is driven by temperature gradients, low-grade waste heat can be used and expensive high-pressure pumps are not required. Membrane fluxes are comparable to reverse osmosis fluxes, so membrane areas are not excessive. Finally, the process is still effective with slightly reduced fluxes even for very concentrated solutions. This is an advantage over reverse osmosis, in which the feed solution osmotic pressure places a practical limit on the concentration of a salt in the feed solution to be processed. 

Although reverse osmosis can be used to separate organic and aqueous-organic liquid mixtures, very high pressures are required. Alternatively, pervaporation can be used in which the species being absorbed by, and transported through, the non-porous membrane are evaporated. This method, which uses much lower pressures than reverse osmosis, but where the heat of vapourisation must be supplied, is used to separate azeotropic mixtures. 

An interesting alternative development is that of forward osmosis. Whereas in reverse osmosis a high pressure is required to oppose the natural tendency of freshwater to move across such a membrane via osmosis to dilute the seawater, in forward osmosis the system takes advantage of this natural tendency. Here, salt water sits on one side of the membrane, but the freshwater on the opposite side is transformed into a high-concentration solution by adding NH3 and CO2. Water naturally flows from the salt water to what is now the draw solution, which can have a solute concentration as high as 10 times that of the salt water. There is no need for an external pressure. The diluted draw solution is then heated to evaporate off the CO2 and NH3 for reuse, leaving behind freshwater. (See Patel-Predd, 2006). 

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