Water desalination processes

The major water desalination processes that ate currendy in use or in advanced research stages are described herein. Information on detailed modeling can be found in the Hterature cited. The major texts on water desalination written since the 1980s are those by Spiegler and Laird (47), Khan (48), which contains many practical design aspects, Lior (49) on the measurements and control aspects, Heitman (40) on pretreatment and chemistry aspects, and Spiegler and El-Sayed (50), an overview primer. Extensive data sources are provided in References 39 and 51. 

Water desalination. See also Distillation processes, Water problem Water desalination processes, 26 59, 61 Water-dominated hydrothermal resources, 12 530-533... 

Cotravo, J.A. Water Desalination Processes and Associated Health and Environmental Issues. Water Cond. Purif. (January), 13-17 (2005). 

Currently, the most important application area for membrane distillation is water desalination technology. Figure 10 shows one of the water desalination processes developed by a Japanese organization, the Water Re-Use Promotion Center, in cooperation with Takenaka Corporation and Organo Corporation (204). The process uses solar energy and can therefore be installed at locations without an electricity supply. Other application areas for membrane distillation reported in the literature are summarized in Table 8. 

Strathmann, H. (2004) Assessment of electrodialysis water desalination process costs. Proceedings of the International... 

Reverse osmosis membrane is widely used in seawater and brackish water desalination processes. Compared to traditional distillation, there is no energy-intensive phase change involved in membrane processes. Therefore, desalination with RO membrane is more energy efficient. In addition to the traditional desalination processes, RO membranes have also found wide application in industrial and municipal wastewater treatment, in pure water production for the electronic and pharmaceutical industries, and in the food industries. 

R. Singh, Analysis of high recovery brackish water desalination processes using fuel cells, Sep. Sci. Technol. 44 (2009) 585-598. 

Nakao, S. (1996). Sea water desalination process for high recovery of fresh water by reverse osmosis. Bull. Soc. Sea Water Sci. Japn. 50(6), 406-412. 

R Macedonio, L. Katzirc, N. Geismac, S. Simone, E. Drioli, and J. Gibon, Wind-Aided Intensified evaporation (WATV) and Membrane Crystallizer (MCr) integrated brackish water desalination process Advantages and drawbacks. Desalination 273 (2011) 127-135. 

Desalination. Desalination of seawater and brackish water has been and, as of the mid-1990s, is the primary use of RO. Driven by a need for potable water in areas of the world where there is a shortage, this industry has developed. Desalination involves the reduction of the total dissolved soHds (IDS) concentration to less than 200 mg/L. RO offers several advantages over other possible desalination processes such as distillation (qv), evaporation (qv), and electro dialysis. The primary advantage of RO over the traditionally used method of distillation is the energy savings that is afforded by the lack of a phase change in RO. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

Potable Water RO and NF both play a major role in providing potable water, defined either by the WHO criterion of <1000 ppm total dissolved solids (TDS) or the U.S. EPA limit of 500 ppm TDS. RO is most prominent in the Middle East and on islands where potable-water demand has outstripped natural supply. A plant awaiting startup at Al Jubail, Saudi Arabia produces over 1 mVs of fresh water (see Table 22-17). Small units are found on ships and boats. Seawater RO competes with multistage flash distillation (MSF) and multieffect distillation (MED) (see Sec. 13 Distillation ). It is too expensive to compete with conventional civil supply (canals, pipelines, w ls) in most locations. Low-pressure RO and NF compete with electrodialysis for the desalination of brackish water. The processes overlap economically, but they are sufficiently different so that the requirements of the application often favor one over the others. 

Processes in which two phases react and result in the formation of a third form an important group of gas-liquid-particle processes. In the production of acetylene, a gaseous phase is formed by reaction between a liquid and a particle phase water and carbide. In the production of gas hydrates in desalination processes, a particle phase is formed by reaction between a liquid and a gaseous phase sea water and, for example, propane. In the melting of gas-hydrate or ice crystals a liquid phase is formed when gaseous and particle phases are brought in contact. 

Water desalination, 26 51-102 economic aspects of, 26 96 energy requirements for, 26 59-61 freshwater manufacture via, 26 58-61 future prospects for, 26 96-98 membrane desalination processes, 26 73-87... 

Crystallisation by freezing, or freeze crystallisation, is a process in which heat is removed from a solution to form crystals of the solvent rather than of the solute. This is followed by separation of crystals from the concentrated solution, washing the crystals with near-pure solvent, and finally melting the crystals to produce virtually pure solvent. The product of freeze crystallisation can be either the melted crystals, as in water desalination, or the concentrated solution, as in the concentration of fruit juice or coffee extracts. Freeze crystallisation is applicable in principle to a variety of solvents and solutions although, because it is most commonly applied to aqueous systems, the following comments refer exclusively to the freezing of water. 

The majority of the discharge from a desalination processes is concentrated brine from the membrane process, and this may contain quantities of treatment chemicals used. Treatment of water is necessary in all desalination plans for variety of reasons feed water treatment, membrane protection, membrane cleaning, permeate treatment and concentrate treatment prior to discharge. Although non-chemical treatment is possible, chemical treatment is widely practiced. 

NF has been also been studied as a potential form of pretreatment for reverse osmosis desalination processes (Hassan et al. 1998, 2000). Based on the feed water, it may be a suitable pretreatment method that allows for operation with little or even no use of antisealants. 

Schliephake, K., Brown, P., Mason-Jefferies, A., Lockey, K., Farmer, C. Overview of Treatment Processes for the Production of Fit for Purpose Water Desalination and Membrane Technologies, ASIRC Report No. R05-2207. Australian Sustainable Industry Research Centre Ltd., Churchill (2005)... 

For this reason, two pass seawater desalination process have been necessarily employed till quite recently, and the results obtained have been satisfactory to some extent with regard to water quality and practical operation. However, one pass process has advantages over two pass process for simple and compact plant, simple operation, easy maintenance and lower energy consumption. 

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