Ocean water desalination

There are three potential types of OTEC power plants opcii-cyclc, closed-cycle, and hybrid systems. Open-cycle OTEC systems exploit the fact that water boils at temperatures below its normal boiling point when it is under lower than normal pressures. Open-cycle systems convert warm surface water into steam in a partial vacuum, and then use this steam to drive a large turbine connected to an electrical generator. Cold water piped up from deep below the oceans surface condenses the steam. Unlike the initial ocean water, the condensed steam is desalinated (free of salt) and may be collected and used for drinking or irrigation. 

Desalination In some areas of the world, such as the Middle East, freshwater is scarce. Can the people in these areas drink the much more abundant ocean water Because seawater has a high salinity, it can t be consumed by living organisms. If humans are to use ocean water for drinking and for irrigation of crops, the salts must first be removed. The removal of salts from seawater to make it usable by living things is called desalination. 

Reverse osmosis can be used to desalinate ocean water and make it fit for human use. 

Freshwater sources generally consist of ground and surface water sources. In rare cases, water may be obtained from sea or ocean water by desalination processes that are relatively costly. Most municipal systems utilize surface water whereas most of the industrial consumers of water prefer to pump water from ground sources than to obtain it from municipal systems. Tap water provided by the municipal system to the community is treated to an extent which generally finds a balance between the economy and the practicality of the treatment and the safety of the water delivered. Nevertheless, the basic principle is that the quality of the water should be suitable for consumers to drink and use for domestic purposes without subsequent risk of adverse effects on their health throughout their lifetime. Also, special attention is necessary to protect vulnerable groups, such as pregnant women and children. 

The "Chemistry in Focus" segment Water, Water Everywhere, But... discusses the desalinization of ocean water. Explain why many salts are soluble in water. Include molecular-level diagrams in your answer. 

During the process of freezii dissolved salts are naturally excluded during the formation of ice crystak. Seawater, therefore, can be desalinated by cooling the water to form crystals, which can then be melted to produce fresh water. Before a given sample of ocean water has been frozen, however, the mixture needs to be rinsed with fresh water to remove the salts. Otherwise, the salts collect within tiny pockets within the ice. Unfortunately, the amount of fresh water required to rinse off the freezing water is comparable to the amount of fre.sh water obtained by thi.s process, making die process relatively inefficient. 

Desalination, the process of removing salt from the water, is often employed to make ocean water drinkable in places where freshwater is scarce. The salt is removed by microfiltration and reverse osmosis. 

Research projects in sanitary engineering include seeking processes and equipment for improved purification efficiency. One example is the development of large, portable water-treatment systems that are suitable for providing clean water to survivors of natural disasters and the bivouac medical units that treat them. Another example is a nanofiltration system that desalinates ocean water for use on naval ships, especially during times of conflict, and extended private offshore operations such as oil drilling. A related nanofiltration system is necessary for oil-spill cleanup. A third example is the specialized absorbent removal of microcontaminants that may be present in small yet detrimental amounts. These may include elements such as arsenic and lead, industrial solvents, and radioactive particles. 

The process of reverse osmosis has been applied to the problem of purifying water. In particular, the method has been used to desalinate ocean water (that is, remove salts from seawater to make drinkable or industrially usable water). In normal osmosis, the solvent flows from a dilute solution through a membrane to a more concentrated solution. By applying a pressure equal to the osmotic pressure to the more concentrated solution, the process of osmosis can be stopped. By applying an even greater pressure, the osmotic process can be reversed. Then, solvent flows from... 

The water that humans use is primarily fresh surface water and groundwater, the sources of which may differ from each other significantly. In arid regions, a small fraction of the water supply comes from the ocean, making use of huge water desalination plants, which is a source that is likely to become more important as the world s supply of freshwater dwindles relative to demand. Saline or brackish groundwater may also be utilized in some areas. 

Note the similarity of the above formula to the ideal gas law and also that osmotic pressure is not dependent on particle charge. This equation was derived by van t Hoff Osmotic pressure is the basis of reverse osmosis, a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it. Part of the chamber opens to a differentially permeable membrane that lets water molecules through, but not the solute particles. The osmotic pressure of ocean water is about 27 atm. Reverse osmosis desalinators use pressures around 50 atm to produce fresh water from ocean salt water. 

Seawater desalination is the production of fresh, low-salinity potable or industrial-quality water from a saline water source (sea, bay, or ocean water) via membrane separation or evaporation. Over the past 30 years, desalination technology has made great strides in many arid regions of the world such as the Middle East and the Mediterranean. Today, desalination plants operate in more than 120 countries worldwide, and some desert states, such as Saudi Arabia and the United Arab Emirates, rely on desalinated water for over 70% of their water supply. According to the 2004 desalination plant inventory report prepared by the International Desalination Association (Wagnick Consulting, 2004), by the end of 2003 worldwide there were over 17,000 desalination units with total installed treatment capacity of 37.8 million m /day. Seawater desalination plants contribute approximately 35% (13.2 million m /day) of this capacity. 

The mineral/salt content of the water is usually measured by the water quality parameter total dissolved solids (TDS), in milligrams per liter (mg/L) or parts per thousand (ppt). Natural water sources such as sea, bay, and ocean waters usually have TDS concentration higher than 15,000 mg/L. Seawater TDS and temperature are the two key source water quality parameters that have the most significant influence on the cost of seawater desalination. Table 3.2 presents typical TDS concentration and temperature for various seawater sources. 

Discharge to surface water is the most economical form of concentrate management for seawater desalination plants, regardless of the discharge volume. Due to the availability of ocean discharge for seawater desalination plants, the cost of disposal tends to be less costly than for inland desalination. Costs include pumps and pipes. 

According to the latest estimates of Skinner [18], elements potentially recoverable from seawater are sodium, potassium, magnesium, calcium, strontium, chlorine, bromine, boron, and phosphorus because of their practically unlimited presence in the ocean. After improving respective technologies, recovery of the following elements is expected to become profitable as well lithium, rubidium, uranium, vanadium, and molybdenum. Additional profit can be gained since desalinated water will probably be obtained as a by-product. This could be important for countries with a very limited number of freshwater sources (e.g., Israel, Saudi Arabia). 

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