The Arid West—Where Water Is Scarce - Desalination—a Growing Watersupply Source
plant plants seawater desalting
According to an online fact sheet of the American Water Works Association (http://www.awwa.org/Advocacy/pressroom/Desalination.cfm, 2005), there are more than 12,500 desalination plants operating in 120 countries. These plants convert seawater, brackish water, and wastewater to freshwater suitable for a variety of purposes. Sixty percent of desalination plants are located in the Middle East. As of 2005 Saudi Arabia had twenty-seven plants desalinating 70% of the country's drinking water. Only 12% of the world's desalination capacity is located in the Americas, with most of the plants in the Caribbean and Florida.
In the past few years, desalination has become a rapidly growing alternative to water scarcity. With population growth and the threat of drought throughout the United States—particularly in the nineteen western states and Florida—desalination, once considered too expensive, is looking more attractive. Hundreds of desalination plants of all sizes are operating throughout the United States, and more are coming online everyday.
The growth in the use of desalination was fueled by the adoption of the Reclamation Wastewater and Groundwater Study and Facilities Act of 1992 (PL 102–575, Title XVI). The Act directed the Secretary of the Interior to undertake a program to investigate and identify opportunities for water reclamation and reuse and authorized participation in five water-recycling projects. In 1996 Congress reauthorized the Act, expanding it to include another eighteen projects, eight of which are in Southern California, an area in desperate need of water. At the same time, Congress enacted the Water Desalination Act of 1996. The Act is based on the fundamental need to find additional sources of potable (drinkable) water. Its primary goal is development of more cost-effective and technologically efficient means to desalinate water.
What Is Desalination?
Desalination is the removal of dissolved minerals (including, but not limited to, salts) from seawater, brackish water, or treated wastewater. A number of technologies have been developed for desalination. In the United
States, desalination research is directed by the Bureau of Reclamation, which is a branch of the Department of the Interior.
There are several desalination processes:
Reverse osmosis—filtered water is pumped at high pressure through permeable membranes, separating the salts from the water.
Distillation—water is heated and then evaporated to separate out the dissolved minerals. The most common methods of distillation are:
Multistage flash distillation, where the water is heated and the pressure lowered so that the water "flashes" into vapor that is drawn off and cooled to provide desalted water.
Multiple effect distillation, where the water passes through a number of evaporators in series with the vapor from one series being used to evaporate the water in the next series.
Vapor compression, where the water is evaporated and the vapor compressed; the heated compressed vapor is used to evaporate additional water.
Electrodialysis—electric current is applied to brackish water, causing positive and negative ions of dissolved salt to split apart.
The two most common desalination processes worldwide are multistage flash distillation and reverse osmosis. Although water of different quality, including seawater, brackish water, or impure industrial wastewater, can be desalinated, seawater and brackish water are the most common water sources.
In 1962 Buckeye, Arizona, became the first town in the United States to have all its water supplied by its own electrodialysis-desalting plant. The plant provides about 650,000 gallons of water daily at a cost of about $1 per 1,670 gallons. In 1967 Key West, Florida, opened a flash-evaporation plant and became the first city in the United States to draw its freshwater from the sea.
The Yuma desalting plant in Arizona is the second-largest reverse osmosis desalting plant in the world, producing about ninety-five mgd. The plant was built as a result of a dispute with Mexico over the salinity of drainage water from the Wellton-Mohawk Irrigation District in Arizona. The salinity of this irrigation return flow has caused marked deterioration in Colorado River water quality in Mexico. The problem was so severe that Congress enacted the Colorado River Basin Salinity Control Act to fund the plant's construction.
The saline drainage water from farmlands east of Yuma flows in a concrete drainage canal to the desalting plant. The drainage water enters the plant intake system where screens remove algae and large debris such as tree limbs. As the drainage water flows into the plant, it is treated with chlorine to kill organisms and stop the growth of algae, which would damage or plug the filters and membranes.
Before being desalted, the water passes through several pretreatment steps to remove all solids that would interfere with the membrane performance. Pretreatment extends the life of the reverse osmosis membranes three to five years. Without pretreatment, the membranes would last about one hour. It takes about 4.5 hours for a unit of water to travel through the plant, from where it enters as untreated drainage water, is pretreated and subjected to reverse osmosis, and then discharged to a small canal that empties into the Colorado River.
LIMITATIONS OF SEAWATER DESALINATION.
One major limitation of desalination projects is their cost. The cost to produce water through desalination depends on the technology used, the plant capacity, and other factors. Price estimates for water produced by desalination plants in California ranged from $1,000 to $4,000 per acre-foot. In comparison, traditional water source costs range from $27 to $269 per acre-foot. For new supplies that are developed, costs are about $600 to $700 per acre-foot. During the 1988 drought, however, Santa Barbara, California, paid $2,300 per acre-foot, while permanently tying into the California Water Project would cost about $1,300 per acre-foot. Given the cost of new supplies, the cost of desalting water becomes more competitive.
Desalination requires relatively large parcels of land, preferably near the coast. Pumping seawater and brine over long distances to avoid the need for a coastal location would add to desalination's already considerable expense. Nevertheless, the demand for water by a growing population, the effects of drought on the cost and availability of water, and the technical improvements in the desalination process have led city planners to consider the expensive alternative. As other sources of water become more expensive or less available, desalination becomes more attractive. Advocates of desalting plants claim that the price of water in the West will inevitably rise as demand outpaces supply, while the cost of desalting will fall as technology improves.
On February 12, 2003, the U.S. Bureau of Reclamation and the U.S. Department of Energy's Sandia National Laboratories announced the release of a "research road map" designed to guide future investments necessary to reduce the cost of desalination. The report, Desalination and Water Purification Technology Roadmap: A Report of the Executive Committee, also described related advanced water treatment technologies and enhanced uses of desalination.
While recognizing the high cost of desalination technologies—$1 to $3 per thousand gallons of desalinated water compared to prices as low as pennies per thousand gallons—researchers believe that by 2020 desalination and water purification technologies can contribute significantly to ensuring a safe, sustainable, affordable, and adequate water supply for the United States. In fact, the report stressed the increasing strain on water supplies in the United States and the possibility that desalination technologies will have to be used to keep up with demand.
Researchers suggested that while the desalination cost of $3 per thousand gallons of water might appear to be expensive, consumers have shown a willingness to pay the equivalent of $7,945 per thousand gallons for bottled water (based on a shelf price of $.99 per half-liter bottle).
Desalination is becoming increasingly important in this age of severe water shortages. On March 12, 2003, five major municipal water agencies in California joined together to form the U.S. Desalination Coalition. According to information on the group's Web site (www.usdesal.org), the coalition's mission is to pursue federal legislation to establish a new program to provide federal financial assistance to encourage the development of seawater desalination projects. Specifically, their goal is to develop and pass legislation to assist in funding the planning, design, construction, operation, and maintenance of projects to desalinate seawater and convey the treated water for municipal and industrial use.
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