Desalination (also called “desalinization” and “desalting”) is the process of removing dissolved salts from water, thus producing fresh water from seawater or brackish water. Desalting technologies can be used for many applications. The most prevalent use is to produce potable water from saline water for domestic or municipal purposes.
Of the world’s water, 97.5 percent is salt water from its oceans. Only 2.5 percent is fresh water. Of that 2.5 percent, approximately 69 percent is frozen in glaciers and ice caps, leaving less than .75 percent in fresh groundwater.
A Natural Process
Desalting is actually a natural, continual process and an essential part of the water cycle. Rainwater falls to the ground and eventually flows to the sea, moving over and through the earth, dissolving minerals and other materials along the way, and becoming increasingly salty. As water evaporates through the sun’s energy, it leaves the salts behind, and the resulting water vapor forms clouds that produce rain, thus continuing the cycle.
The Quest for Water
People have been desalinating water for centuries. One of the first mentions was by Aristotle, who wrote of seawater distillation in 320 BC. Different techniques were used during the ages: Rome’s Pliny the Elder described seawater distillation with condensation on fleece in 70 AD, Greece’s Alexander of Aphrodisias described seawater distillation with condensation on sponges 130 years later, French explorer Jean De Lery reported the successful distillation of seawater during a voyage to Brazil in 1565, and James Cook desalinated seawater during his circumnavigation of the world.
Developments in Desalination Technology
Through the mid-1900s, the most commonly used techniques involved evaporation and distillation. The development of desalination processes took a major step forward in the 1940s during World War II, when military establishments operating in arid areas needed a way to supply their troops with potable water. By the late 1960s, commercial desalting units producing up to 8,000 cubic meters per day (m3/d) – approximately 2 million U.S. gallons per day – were beginning to be installed in various parts of the world. Most of these installations used thermal (distillation) processes.
In the post-war years, however, scientists began studying osmotic processes to desalinate water. The first reported use of the term “reverse osmosis” – now a popular desalination technology – appeared in the US Department of Interior’s Office of Saline Water 1955 annual report. Development continued, and in the 1970s, commercial membrane processes, such as reverse osmosis (RO) and electrodialysis (ED), began to be used more extensively. Since ED could desalt brackish water more economically than distillation, more interest was focused on using desalination as a way to provide water for municipalities with limited fresh water supplies and the availability of brackish water sources.
By the 1980s, desalination technology became a fully commercial enterprise and by the 1990s, the use of desalination technologies for municipal water supplies was commonplace. The major desalination processes employ membrane and thermal technologies.
Currently, reverse osmosis (RO), a membrane process, accounts for nearly 60 percent of installed capacity, followed by the thermal processes multi-stage flash (MSF) at 26 percent and multi-effect distillation (MED) at 8.2 percent. Other processes include electrodialysis (ED) at 3.4 percent, hybrid technologies combining membrane and thermal processes at 0.7 percent, and electrodeionization (EDI) at 0.4 percent.
Desalination: Solving the World’s Water Scarcity Issues
Growth in desalination has increased dramatically as countries seek solutions to water scarcity caused by population growth, climate change, pollution and industrial development. In addition, the industry has done much to lower the cost of desalination. Advances in technology have led to increased energy efficiency, and greater economies of scale have also helped lower costs. The majority of new commissioned capacity is seawater desalination.
As of June 30, 2011, there were 15,988 desalination plants worldwide, and the total global capacity of all plants online (e.g., in operation) was 66.5 million cubic meters per day (m3/d), or approximately 17.6 billion US gallons per day.
Reducing Costs and Addressing Environmental Concerns
Today, developments in desalination technologies are specifically aimed at reducing energy consumption and cost, as well as minimizing environmental impacts. New plants are incorporating renewable energy; for example, the plant in Kwinana, Australia, is powered entirely by wind power. New technologies are being used to lessen disruptions to marine life during the intake and outfall processes, and the results are positive. Studies of the Kwinana plant, for example, show that even highly sensitive marine flora and fauna are flourishing around its intake and outfall.