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Desalination

 

History of Desalination

The idea that pure water could be made from seawater has been tantalizing thirsty humans for hundreds, if not thousands, of years. The first record of anyone trying a desalting process is actually found in the bible? It is questionable, of course, but it reads of how Moses and the people of Israel came upon the waters of Marah which were bitter. The quote reads "And he cried unto the Lord; and the Lord showed him a tree, which when he had cast unto the waters, the waters were made sweet." One might consider this to be desalination. (Exodus)  

The original premise was based on the idea that boiling or evaporating water separates the water from the salt. That theory—vaporization or distillation—was behind the technology for the first large-scale desalination plants that sprouted in desert areas in the 1950s and 1960s, primarily in the Middle East. Researchers throughout the early 1900s had been studying the idea of using a membrane to separate out salt from seawater. This is based on the osmotic nature of cell walls: certain semi permeable membranes, such as animal and plant cell walls, allow water to pass through, creating equilibrium between a highly concentrated solution on one side of the membrane and a diluted concentration on the other. In the 1960s, researchers in the U.S. and Japan who developed membranes for industrial purposes soon realized that those same semi permeable man-made membranes could be used in desalination. By the 1970s, desalination-plant developers adopted reverse osmosis (RO) for use in new desalination plants. Today, there are more than 15,000 desalination plants in the world

Technologies

Desalination plants are being build in countries that have sever water shortages like in Israel and Saudi Arabia. There are four main technologies available for desalination, distillation, freezing-thawing, reverse osmosis, and electro-dialysis.

Distillation

Distillation-based desalination is a two-step process. Seawater is heated in large, closed tanks to produce steam, which is a purified form of water. Various improvisations of the basic distillation scheme have been devised to reduce the net energy consumption. In multiple-effect distillation, the steam generated in the first distillation tank is used to vaporize the water in a second distillation tank and the chain is repeated. The net energy consumption is reduced by a factor equal to the number of distillation effects. The distillation equipment is inherently expensive because of the large size of the vessels and because of the necessity to use exotic, corrosion resistant materials.

Freeze thawing

Desalination by freeze thawing is a three-step process.

1.      The seawater is cooled until the water forms ice.

2.      The ice is separated from the slurry of ice and concentrated brine.

3.      The ice is molten to obtain liquid water.

Freeze thawing is not utilized commercially because of the complexity of the refrigeration process and the difficulties in handling ice-brine slurries.

Reverse osmosis

Reverse osmosis desalination is energy-efficient because no heating or cooling is involved. Reverse osmosis is a preferred choice for desalinating seawater due to its low operating costs and low capital requirements. It is also ideal for on-board and mobile applications, due to its low space needs.

Electro-dialysis

Electro-dialysis technology is based on the principle of migration of ions towards positively and negatively charged electrodes in an electrolytic cell. Electro-dialysis is not a true water purification process, since it merely extracts ionized salts from the seawater, leaving everything else behind in the desalinated water. This method, therefore, is not recommended for producing fresh water for municipal uses, but it could be used to produce water for irrigation.

Case Studies of Two Desalination Plants

There are two countries that use the desalinization process the most with they have build many plants on their respective countries and like everything else Israel and Saudi Arabia do not want to have anything in common.

·        Israel used the technology know as reverse osmosis

·        Saudi Arabia used the technology know as thermal-based or desalination

Israel's Hadera facility which is jointly owned by IDE Technologies and Shikun & Binui, said its plant will supply 127 million cubic meters of desalinated water a year, or about 20 percent of the yearly household consumption in Israel.[1]

Initially designed to produce about 100 million cubic meters of water per year, the plant was recently expanded to produce an addition 27 million cubic meters per year, making it the largest reverse osmosis desalination plant in the world.  In the reverse osmosis process, water is forced through a fine-pored membrane, dense barrier layers of polymer matrix through which most of the separation occurs.  The process purifies water up to 98%.

 

Israel has three other desalination facilities, in addition to Hadera, producing more than 200 million cubic feet of water per year.  In spite of this massive production, Israel still needs more potable water.[2]

 

Figure 1   Aerial view of Israel’s Hadera facility

 

In Saudi Arabia has always had an acute fresh water shortage problem. The problem has been so severe that a proposal was once considered literally tow an iceberg from Antarctica all to way to the Kingdom for use as fresh water. The practicality of constructing desalination plants to extract salt and other minerals from sea water became a much more practical plan, and 27 have now been constructed in the Kingdom, supplying 70% of the country’s drinking water as well as more than 28 million megawatts of electricity. A new desalination plant, hailed as being the world’s largest, has now been completed in the new Jubail II Industrial Zone in the Kingdom’s Eastern Province. [3]

Figure 2 Schematic of Al Jubail Desalination facility

 

Environmental Impact of Desalination Plants

Recent advances in technology have made removing salt from seawater and groundwater a realistic option for increasing water supplies in some parts of the of the world, and desalination will likely have a niche in meeting our nation’s future water needs, says a new report from the National Research Council. However, a coordinated research effort with steady funding is required to better understand and minimize desalination’s environmental impacts. But one of the main environmental considerations of ocean water desalination plants is the impact of the open ocean water intakes, especially when co-located with power plants. Many proposed ocean desalination plants' initial plans relied on these intakes despite perpetuating ongoing impacts on marine life.

In the United States, due to a recent court ruling under the Clean Water Act, these intakes are no longer viable without reducing mortality, by ninety percent, of the life in the ocean; the plankton, fish eggs and fish larvae. There are alternatives, including beach wells that eliminate this concern, but require more energy and higher costs while limiting output. Other environmental concerns include air pollution and greenhouse gas emissions from the power plants. Fish habitats are eroded when water from lakes, rivers and groundwater is diverted for human use. Increased volumes of water obtained through the desalination of seawater would allow for the restoration of these habitats. However, these benefits must be weighed against the destructive impact desalination plants have on the marine ecosystem.[4]

 

Unintended Consequences From Desalination

The primary consequence from desalination is that widespread desalination could take a heavy toll on ocean biodiversity. Since ocean waters are filled with living creatures, and most of them could be lost in the process of desalination. Many microbial lives could be lost too also intake pipes to desalination plants take up the larvae of a cross section of life in the sea, as well as some fairly large organisms. In addition, there are other consequences from desalination like the following:

·        Air quality

·        Commercial and recreational fishing

·        Energy use

·        Growth-inducing effects

·        Marine resources impacts from feed water intake and ocean discharge

·        Navigation

·        Noise

·        Potential hazardous releases from accidents

·        Public access

·        Recreation

·        Visual quality

·        Water quality

Economics

The economic impact is huge since we could have endless water supply it could be beneficial for the following:

·        Drought Relief

During times of severe drought, the water made available through desalination would protect against water shortages.

 

·         Agriculture

Increased water supply from desalination plants would decrease the need for municipalities to re-route water that is needed for agriculture in times of water shortages.

• Self-Sustainability

For many coastal communities that have an inadequate local water supply, a desalination plant could free them from dependence on outside sources for their water. Local control of water resources is critical to a community’s ability to be self-sustaining. ·       

• Diversification

When cities have diversified sources for water, they are less vulnerable to the fluctuations from any one source. This allows for greater economic stability for the municipalities, greater reliable availability and more consistent rates for the consumer.

• Cost   

The current average of a US household of 4’s has a water consumption for one billing cycle an average daily water consumption of 600 gallons of water and average billing cycle water bill of $72 dollars. There is an unofficial estimate that there are over 114,825,428 homes in the United States at the end of 2010 which will make the final prices per water billing cycle at $ 8,267,430,816.[5]

Desalination Compared to Other Water Supply Options

Increased water conservation and water use efficiency remain the most cost-effective priorities in areas of the world where there is a large potential to improve the efficiency of water use practices While comparing ocean water desalination to waste water reclamation for drinking water shows desalination as the first option, using reclamation for irrigation and industrial use provides multiple benefits Urban runoff and storm water capture also provide benefits in treating, restoring and recharging groundwater. A proposed alternative to desalinization in the state of California and other areas in the American Southwest is the commercial importation of bulk water either by very large crude carriers converted to water carriers, or via pipelines[6]

Figure 3 Comparison of costs of fresh water[7]

Future

Energy recovery has taken over a big part of desalinization world. New devices will likely be develop with more sophisticated technologies that will utilize less energy and actually be able to recover energy back.  There will be more companies developing new technologies that will enable countries or regions of the world with fresh water shortages to be able develop their economies into a more competitive state.

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