Desalination Is No Panacea, But Holds Potential as Water Shortage Solution

"Water, water everywhere, and not a drop to drink," said the Ancient Mariner in Samuel Taylor Coleridge's famous poem. Indeed, we are surrounded by water – about 80 percent of Earth's surface is covered by water – yet only 2.5 percent is freshwater. Of this freshwater, about 70 percent is frozen in glaciers and permafrost. Therefore, less than one percent of the planet's total water is non-frozen freshwater, most of which consists of groundwater (water stored beneath the Earth's surface).

In 2000, the UN officially established the UN Millennium Development Goals, a set of goals that UN countries strive to achieve by 2015. The sixth UN Millennium Development Goal, ensuring environmental sustainability, includes a specific target: "to halve, by 2015, the proportion of people without sustainable access to safe drinking water and sanitation [from 1990 levels]." By 2002, 1.1 billion people had gained access to improved drinking water, meaning the world is currently "globally on-track to achieve the MDG water target," according to the UN. However, challenges still lie ahead for meeting the UN's 2015 goal as human demand for water increases.

As evermore quantities of water are being pumped out of the ground at an unsustainable rate, the world is facing a growing problem of water shortages. Groundwater is being drained for human use faster than it can be naturally replenished. In 2003, the UN estimated that 40 percent of the population was living in areas with moderate-to-high water stress. The UN predicts that about two thirds of the population, or 5.5 billion people, will live in areas with moderate-to-high water stress by 2025. Pollution is also contributing to water scarcity. According to a 2003 Executive Report published by the World Water Assessment Programme, 2 million tons of waste are released into surface bodies of freshwater daily, which contaminates an estimated 12,000 km3 of our freshwater worldwide. This means that everyday a volume of water the equivalent of Lake Superior, the largest lake in the U.S., is polluted.

With freshwater becoming scarce, could we tap into the 97.5 percent of the world's water that is contained in the oceans? As countries begin to deplete their freshwater sources, many are turning to desalination to meet growing water demands despite its high cost.

Choosing a suitable desalination system depends on the consumer's needs and the quality of the available water. The most common desalination methods involve distillation or membrane filtration to purify water.

One form of desalination is distillation, which involves bringing water to its boiling point through the application of heat and sometimes with an alteration in pressure. This causes the water to boil and form water vapor, which is then cooled and condensed into water without its original impurities. Today, about 40 percent of desalted water is produced through thermal distillation, according to the Pacific Institute, a California-based nonpartisan research institute. Distillation can produce water with a lower salt content than with other desalination technologies, making it a useful technique for generating the high-purity water required for industrial use. However, the energy costs of distillation are relatively high. Craig Bartels, Vice President of Technology of Hydranautics, a California-based manufacturer of water purification products, notes that countries like those in the Middle East with relatively inexpensive energy sources have used distillation for water desalination in the past. However, the energy costs of such a technique make distillation an unaffordable option for many countries, particularly with rising oil prices.

Membrane filtration is another desalination technique. This method, which incurs lower energy costs than distillation, incorporates the separation process of reverse osmosis to remove salt and other impurities from water. In reverse osmosis, pressure is applied to a solution to force it through a semipermeable membrane. A semipermeable membrane only allows certain ions or molecules to pass through. Water molecules, for example, can pass through the membrane, but impurities such as salt and some organic contaminants are unable to cross the membrane. Increasing research in reverse osmosis membranes has increased the reliability and cost-effectiveness of the technology, said Manuel Schiffler, Senior Economist at the World Bank, in a 2004 paper published in Desalination. As reverse osmosis filtration requires less energy than distillation, its use will be favored in future desalination development, especially as energy costs continue to rise.

Countries that use desalination for obtaining water include those with "very little [freshwater] and cheap energy," said Richard S.J. Tol, professor at the Economic and Social Research Institute in Dublin, Ireland. Countries in North Africa and the Middle East are currently using this method, as well as some small island communities. In addition, countries such as Australia, the U.S., South Africa, and China are increasingly using desalination for reasons that include severe water shortages or the wasteful management of existing water resources, said Tol. In inland areas or places far from the coast, desalination can be used on available brackish water (water of a lower salinity than seawater but higher than freshwater).

Other than determining the accessibility of freshwater and energy, the environmental impact of desalination technology is another factor in determining its usefulness in water purification. Desalination produces brine, a concentrated solution of salt and chemicals that is left after filtration. Desalination plants sometimes dispose of brine in the ocean or other surface bodies of water, or into the ground. Discharging brine into the environment may be harmful to local ecosystems because some marine organisms are sensitive to salinity changes. The full extent of brine's effect on the environment is under investigation. In addition, the greenhouse gases emitted in producing electricity for desalination systems contribute to air pollution.

Desalination is not always the most cost effective solution to water shortages. The 2006 United Nations World Water Development Report estimated that newer large-scale desalination plants produce freshwater at costs ranging from $0.45 to $1.00/m3 of freshwater, though exact costs depend on the quality of available water, plant size, energy costs, and other local factors. Energy costs are particularly significant as roughly half the cost of desalinating water goes into producing energy for the desalination process, according to Tol.

Despite advances in technology, desalination is still an "option of last resort" for communities without any other cost-effective source of potable water, according to Tol. Other methods of obtaining fresh water, such as wastewater reclamation, may be a more effective method of increasing a community's water supply. Bartels said that importing water from San Francisco to Los Angeles costs about $0.53/m3, including the costs of pumping and conventional water treatment, but not including the cost of building existing transport infrastructure. Wastewater treatment, by contrast, costs about $0.45/m3. "The energy consumption of reclaiming wastewater is about half of the energy required to treat seawater," Bartels explained. Wastewater reclamation would therefore be a useful source of freshwater for a community like Los Angeles. "This is quite an economic incentive to reclaim wastewater," he said.

The lament of the Ancient Mariner may be heard around the world, as water shortages plague millions with limited access to freshwater. However, desalination technology advances offer hope, not as a cure-all, but as one additional tool in the mission to alleviate the global water crisis.

References:

1st United Nations World Water Development Report: Water for People, Water for Life. United Nations. 2003. Available http://www.unesco.org/water/wwap/wwdr/wwdr1/ex_summary/index.shtml.

2nd United Nations World Water Development Report: Water, a shared responsibility. United Nations. 2006. Available http://www.unesco.org/water/wwap/wwdr/wwdr2/.

Desalination, With a Grain of Salt – A California Perspective. Pacific Institute. June 2006. Available http://www.pacinst.org/reports/desalination/index.htm.

Schiffler, Manuel. "Perspectives and challenges for desalination in the 21st century." Desalination 165 (2004): 1-9.

"Water for Life Decade, 2005-2015." United Nations. 2006. Available http://www.un.org/waterforlifedecade/factsheet.html.

"What on Earth Do You Know About Water?" Environmental Protection Agency. 10 Mar. 2008. http://www.epa.gov/gmpo/edresources/water_5.html.

Written by: Amy Liu

Edited by: Jeff Kost

Published by: Hoi See Tsao

JYI publishes undergraduate research from the natural sciences, mathematics, engineering, and from some of the social sciences, such as psychology and the history of science.
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