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Desalination Research





















The Irony of Water

By Jason Zheng

Introduction

Desalination is the process where minerals are removed from saline water. Generally speaking, desalination is also referred as the removal of salts and mineral in water and as well maintaining salinity levels in soils for agricultural productions. Salt water can be desalinated to produce fresh water that is suitable for human consumption and/or irrigation uses. One of the by-products of desalination is salt. Desalination is also used by marine water crafts, such as ships and submarines. Nowadays, the methods of desalination are applied to develop more cost-effective ways of providing fresh water from human use. However, not all waters sources are treated equally.

Sea water is generally more expensive to desalinate than the alternatives—fresh waters from rivers or groundwater, water recycling, and water conservation methods. However, these alternatives are not always available. Humans are overdrawing and depleting these water sources worldwide, thus leaving none for future use. The Global Water Intelligence coined that only 1% of the global population is dependent on desalinated water. The United Nations analyzed that by 2025, 14% of the world’s population would encounter problems for water scarcity.

Traditional Methods

The traditional process of desalination is vacuum distillation, which is literally boiling water at a less atmospheric pressure and much lower temperature than normal. The reason for this is because the boiling temperature of a liquid occurs when the vapor pressure equals to the ambient pressure and also vapor pressure increases with temperature. A more modernize method of desalinating water would be using the reverse osmosis technology. The process involves the usage of semipermeable membranes and pressures to separate the salt from the water. A reverse osmosis plant uses less energy than thermal distillation, which is why the cost of desalination has decreased over the decade. Any methods of desalinations require a source of energy. Thus the cost desalinizing water would fall under the cost of both energy and the technology available. 

Cogeneration Method

The cogeneration desalination method is the process of applying heat from electricity to create drinking water from seawater or brackish groundwater in power plants. Cogeneration has many forms, and majority of the energy sources available in the world can be used. These desalination plants can use either fossil fuels or nuclear power as their source of energy. Majority of these plans are located in the Middle East or North Africa, which their petroleum resources to offset limited water resources. Dual-purpose facilities have hybrid configurations, which the reverse osmosis desalination method is combined with the thermal desalination method. Basically it is two or more desalination processes that are combined with power production. These facilities have been established in Jeddah and Yanbu, Saudi Arabia.

Another desalination method is used by the United States military. A typical U.S. military aircraft carrier uses nuclear power to desalinate 400,000 gallons of seawater every day, enough to supply 2,000 homes. The desalination process is powered by onboard turbines, which generates electricity to power the ship’s electrical systems.

Energy Consumption

The minimum energy consumption for sea water desalination falls around 3kWh/m including pre-filtering and ancillaries. The minimum required energy consumption for sea water is determined by the Department of Chemical and Environmental Engineering of Yale University as 1kWh/m, which excludes pre-filtering and pumping. An energy consumption of 2kWh/m has been achieved with the current reverse osmosis membrane technologies. If the United States replaced domestic waters with methods of sea water desalination, this would increase energy consumption around 10%, which is about the same amount of energy use by domestic refrigerators. 

Cost of Desalination

The cost of desalinating sea water—infrastructure planning and constructing, energy and maintenance; is generally higher than the alternative methods—fresh water from rivers or ground water, water recycling and water conservation—however these alternative methods are not always readily available.

In 2013, the cost of desalination ranged from $0.45 to $1.00/cubic metre—1 cubic meter is about 264 gallons of water. However, more than half of the total cost of desalination comes directly from energy cost. Other factors contributing to cost includes capacity and type of facility used, location, feed water, labor, financing and concentrate disposal. For a large-scale area, nuclear powered desalination may be economical enough to provide the necessary fresh water. 

In November 2008, the Connecticut-based Poseidon Resource Corp. won the approval of the $300 million water desalination plant in Carlsbad, San Diego. This facility has the capabilities to produce 50,000,000 gallons of drinkable water every day, enough to supply 100,000 homes. However, in 2012 it is estimated that the price for desalinated water went up $2,329 per acre-foot. Each $1,000 per acre-foot works out to $3.06 for 1,000 gallons or $0.81 per cubic meter. Desalinating 1,000 gallons of water can cost as much as $3, the same amount of bottled water would cost $7,945 (purchased individually in half-liter bottles).

Desalination can provide flexibility to the water supply system. It gives people alternative method when the traditional method of filtering drinking water.

Environmental Impacts

Under the Environmental Protection Agency, cooling water intake is regulated by the Clean Water Act under Section 316(b). These intake infrastructures have similar impacts to the environment as desalination facility intakes. According to the EPA, water intake structures cause adverse environmental impact by pulling large numbers of fish and shellfish or their eggs into an industrial system. Therefore, organisms may be killed or injured by heat, physical stress or chemicals. Alternative intake types to avoid these environmental impact include beach wells, however they require more energy, thus higher cost to regulate. All desalination techniques produce large quantities of a concentrate. Chemical pretreatment and cleaning is necessary for most desalination plants, which includes treatment against biofouling, scaling, foaming and scale deposit in membrane plants. 

To limit the environmental impact of returning brine into the ocean, the by product can be diluted with another stream of water entering the ocean, such as by wastewater treatment or power plant. Brine is denser than seawater due to higher solute concentration. The ocean bottom would be at risk because brine would sink to the bottom and damage the ecosystem. 

Evaporation ponds, solar still and condensation trap (solar desalination) are all alternative methods that do not discharge brine. They do not require chemicals or the burning of fossil fuels in their process. In addition to this, these methods do not require membranes or other critical parts, such as components that include heavy metals, which does not cause toxic waste.

So Why Desalinating?

Why should we remove salt from sea water in the first place? The answer is simple, drinking salt water can kill you. Ingesting salt signals your cells to flush water molecules to dilute the mineral. Consuming too much salt places great stress on your body. Your cell will dehydrate because of the salt, your kidneys will shut down and your brain becomes damaged. The only way to counter this health problem is to urinate more often to expel the salt. However, this remedy will only work if the persons has access to lots of fresh drinking water. 

At least 70 percent of the world is covered with water, but only 3 percent out of 97 percent is suitable for drinking. The inequalities of water distribution and availability means that water scarcity is reality for many people. The World Health Organization stated that four out of 10 people in the world do not have access to sanitized drinking water. Another words, these people are currently and can be victims of waterborne diseases such as cholera, legionella, typhoid fever or salmonella.

The Future of Desalination

People that live in water-starved parts of the world have been searching for fresh water solution for centuries. This means the same people that built giant sphinxes and drove horse-drawn chariots also sought out for clean, pure water. The modern day population struggles with an irony—which we are surrounded by salt water, but lack drinking water.

The United States currently has numerous desalination plants erected in Texas, California, Florida, Arizona, and Trinidad and Tobago. The numbers of desalination plants will continue to grow to accommodate the need of fresh water. Large-scale desalination methods require a large source of energy and often accompanied with high maintenance. The primary culprits are membranes because they tend to foul frequently.

The cost of desalination is has and will be the major concern. Going back fifty-years, public and private companies invested to develop desalination technologies that research more than billion dollars worldwide. Even with the progress we have made, the idea of using desalination to solve water scarcity is fairly low. The Water Science and Technology Board stated that desalination plants are expensive to plan, building and operate. In addition to this, the American Chemical Society determined the average cost of a one acre-foot (which is estimated as 325,000 gallons) of salt water into fresh water would be around $800 to $1,400 and also requires a significant amount of energy.

Using reverse osmosis to produce fresh water cost about one-third less than the multistage flash, because of the cost of thermal energy used by the boiling method that is discussed in the previous paragraph. However, both process of desalination creates brine (or salt). This by-product from desalination contains high levels of salt and if it released back into the natural body of water, it can cause a significant damage to marine life. This is because brine is denser than water, and when it is released, it settles atop low-lying sediment which it depletes oxygen source.

Researchers in Korea and the Massachusetts Institute of Technology are creating light-weight portable desalination devices that can be used to decrease the cost of energy consumption. These small devices are made to fit into a person’s palm. However, the capability these devices have is surprising. The process involves gravity, which salt water is poured on top of the device and it removes salt and sediments. 1,600 of these units on an 8-inch-diameter wafer can produce about 15 liters of water per hour. 

Large-scale reverse osmosis plants are still under development; however this method can greatly decrease desalination plants’ uses of fossil fuels for energy.  The National Science Foundation granted the University of Michigan team $2.5 million to study how solar energy impact desalination technologies. In addition to government funding, private source of funds are beginning to allocate to research more energy efficient desalination methods.

However, we are certain by one this, if the desalination process improves by trial and error, then we could have the potential to change the Third World’s accessibility to water. 


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