Desalination of seawater

Desalination of seawater to water suitable for human consumption and/or agricultural uses is a vital operation in many parts of the world. Table 3.1 gives the typical salt content of seawater and the maximum salt content of drinking water. The chief component of sea salt is NaCl seawater contains 1.1 wt% Na and 1.9 wt% Cl. 

What are the flow rates of the water and salt streams that leave the ideal desalinator The composition given in Table 3.1 tells us that the 100 kg of seawater that enters the process each minute contains 96.5 kg of water and 3.5 kg of salt. By inspection, 

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Reverse osmosis is used for desalination of seawater, treatment of recycle water in chemical plants and separation of industrial wastes. More recently the technique has been applied to concentration and dehydrogenation of food products such as milk and fruit juices. See ultrafiltralion. 

J. Scott, ed.. Desalination of Seawater by Reverse Osmosis, Pollution Technology Review No. 75, Noyes Data Corp., Park Ridge, N.J., 1981. 

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. 

In some places and under certain conditions, freshwater can be obtained more cheaply by desalination of seawater than by transporting water. This is tme when all the costs of extremely large monetary investments in dams, reservoirs, conduits, and pumps to move the water are considered. Before the rapid escalation of fuel costs between 1973 and 1980, the cost of desalination of seawater to adequately supply southern California would have been less than that of transport to the Peripheral Canal. This would have been the case even if there were an unlimited supply of water in the mountains of northern California, a condition that does not appear to exist. It has been shown that before 1973 a seacoast town could have been suppHed with 7-12 x lO" /d of freshwater more cheaply by desalination than by damming and piping water a distance of >160 km km (7). Indeed, the 1987—1992 drought in California has compelled the city of Santa Barbara to constmct a water desalination plant, and a 76,000-m /d plant is plaimed for the western coast of Florida (8). 

Although desalination technologies ate diverse, MSF has been for some time, and will remain well into the next century, the main process for desalination of seawater. Inroads ate being made by the multi-effect processes and, in particular, by the low temperature ME processes. 

To illustrate the options that are made available when there is an ample energy supply, we will consider a rather extreme case - the desalination of seawater. It would make little sense to undertake this in the United States in the predictable future, except in limited local situations, because we are not faced with imminent national water shortages, but other countries have little alternative to desalination and even in the United States there are already some desalination projects. 

If you were to place a solution and a pure solvent in the same container but separate them by a semipermeable membrane (which allows the passage of some molecules, but not all particles) you would observe that the level of the solvent side would decrease while the solution side would increase. This indicates that the solvent molecules are passing through the semipermeable membrane, a process called osmosis. Eventually the system would reach equilibrium, and the difference in levels would remain constant. The difference in the two levels is related to the osmotic pressure. In fact, one could exert a pressure on the solution side exceeding the osmotic pressure, and solvent molecules could be forced back through the semipermeable membrane into the solvent side. This process is called reverse osmosis and is the basis of the desalination of seawater for drinking purposes. These processes are shown in Figure 13.1. 

RO process for the desalination of seawater was proposed for the first time by Reid in 1953, but no significant advancement was observed until the invention of asymmetric membrane with high water flux by Loeb and Sourirajan in I960. Since that time, RO process showed remarkable progresses in practical applications in the field of desalination of brackish water for potable and pure water. 

The fact that solutes are excluded from the hydrate lattice motivated work on using hydrates for the desalination of seawater and the concentration of aqueous solutions in general. Most of that work occurred in the 1960s and 70s and was reviewed by Englezos... 

Rautenbach, R. Pennings, P. (1973). Development and optimization of a hydrate process for desalination of seawater. Chemie Ingenieur Technik, 45 (5), 259-264. 

Potential applications for CA-CDI technology include the purification of boiler water for fossil and nuclear power plants, volume reduction of liquid radioactive waste, treatment of agricultural wastewater containing pesticides and other toxic compounds, creation of ultrapure water for semiconductor processing, treatment of wastewater from electroplating operations, desalination of seawater, and removal of salt from water for agricultural irrigation. 

Reverse osmosis is now extensively used to reduce salt concentrations in brackish waters and to treat industrial waste water, for example, from pulp mills. Reverse osmosis has also proved economical (the cost can be as low as about 1 per 1000 liters) for large-scale desalination of seawater, a proposition of major interest in the Middle East, where almost all potable water is now obtained by various means from seawater or from brackish wells. Thus, at Ras Abu Janjur, Bahrain, a reverse osmosis plant converts brackish feedwater containing 19,000 ppm dissolved solids to potable water with 260 ppm dissolved solids at a rate of over 55,000 m3 per day, with an electricity consumption of 4.8 kilowatt hours per cubic meter of product. On a 1000-fold smaller scale, the resort community on Heron Island, Great Barrier Reef, Australia, obtains most of its fresh water from seawater (36,000 ppm dissolved salts) directly by reverse osmosis, at a cost of about 10 per 1000 liters. 

The greatest use of membranes is for reverse osmosis desalination of seawater and purification of brackish waters. Spiral wound and hollow fiber equipment primarily are applied to this service. Table 19.6 has some operating data, but the literature is very extensive and reference should be made there for details of performance and economics. 

FIGURE 11.16 A schematic for the desalination of seawater by reverse osmosis. By applying a pressure on the seawater that is greater than osmotic pressure, water is forced through the osmotic membrane from the seawater side to the pure water side. 

Colligative properties have many practical uses, including the melting of snow by salt, the desalination of seawater by reverse osmosis, the separation and purification of volatile liquids by fractional distillation, and the determination of molecular mass by osmotic pressure measurement. 

Solar energy has been used in many traditional technologies for centuries and has come widespread where other power supplies are absent, such as in remote locations and in space. Solar energy is currently used in a number of applications such as heat production, electricity generation, desalination of seawater, and lighting. 

Lamendolar, M. F., and Tua, A. (1995). The Malta experience Desalination of seawater by reverse osmosis T Desalination and Water Reuse 50, 18. 

What do we think of when we hear fiber Clothing, certainly, and other textiles such as sheets and blankets, curtains and upholstery. Some fiber goes into carpeting—for offices, homes, automobiles, and recreational vehicles. Automobile, truck, and bus tires are strengthened with the use of fibers called cords. Some fiber is used for industrial purposes such as insulation and filtration. Hollow fibers that act as membranes are used in the desalination of seawater by reverse osmosis and in kidney dialysis. As we have already seen, extremely high-strength fibers are used to make bulletproof safety equipment and to reinforce polymers for high performance ap-... 

Desalination of seawater and brackish water for potable use. This is very common in coastal areas and.the Middle East where supply of fresh water is scarce. 

The most common uses of RO are for desalination of seawater and brackish water for potable and industrial applications. However, as demand for fresh water grows, RO is being pressed into service for wastewater and reuse applications. These will require extensive pretreatment, sometimes involving other membrane technologies such as micro- or ultrafiltration, to minimize fouling of the RO membranes (see Chapter 16). 

Some joined companies that manufacture polymers (plastics). One is working on the development of membranes for desalination of seawater (fresh water pa.sses through, salt is kept out) and for gas separations (hydrogen passes through and hydrocarbons are kept out, or vice versa) another is developing membranes to be used in hollow-tube artificial kidneys (blood flows from the patient s body through thin-walled tubes metabolic wastes in the blood pass through the tube walls but proteins and other important body chemicals remain in the blood, and the purified blood is returned to the body). 

Membrane materials for reverse osmosis and ultrafiltration applications range from polysulfone and polyethersulfone, to cellulose acetate and cellulose diacetate [12,18-23]. Commercially available polyamide composite membranes for desalination of seawater, for example, are available from a variety of companies in the United States, Europe, and Japan [24]. The specific choice of membrane material to use depends on the process (e.g., type of liquid to be treated and operating conditions) and economic factors (e.g., cost of replacement membranes and cost of cleaning chemicals). The exact chemical composition and physical morphology of the membranes may vary from manufacturer to manufaemrer. Since the liquids to be treated and... 

Cost Comparison of Different Processes for Desalination of Seawater... 

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