Most of Earth’s water—about 97 percent—is undrinkable saltwater. Much of the world’s population lives in close proximity to this coastal saltwater, yet only in the past 40 or 50 years has this vast resource become available for widespread human consumption.
Groundwater is often brackish or saline as well. Coastal aquifers may contain seawater from ancient times when their sediments were deposited. They are also susceptible to salt-water intrusion caused by over-pumping of freshwater, which pulls in saline water from underneath the oceans. Arid zone aquifers also may contain salty groundwater. In this case, the salts have often been concentrated by evaporation from closed basins or depressions.
Modern desalination technologies can remove salt from both seawater and brackish water, thus providing a new source of freshwater.
Many processes are used to remove salt from water. Distillation, of various types, is one of the more common methods. Boiling water becomes freshwater vapor and leaves its salt behind as residue. The steam can be cooled, condensed, and collected in a process that produces higher quality freshwater.
Techniques used to desalt water are also useful for removing other impurities common to contaminated source waters.
Desalination costs vary depending on water quality going in and coming out, methodology and energy costs—but they tend to be significant. The process is very energy intensive.
Desalination is especially common in water-poor but relatively wealthy Middle East nations such as Kuwait, Saudi Arabia, and Israel.
In Tunisia, four desalination plants produce nearly four percent of the country’s total water resource. The plants use reverse osmosis, a process that passes water through a membrane with such tiny pores that most of the dissolved salts as well as most organic compounds and microbiological contaminants cannot pass through.
Tunisia is also notable for its extensive research aimed at coupling alternative energy sources with desalination technology to produce more economical and environmentally friendly systems. Solar and wind energies have been explored, as each provides a potentially vast renewable energy source for Tunisia.
Desalination can exact an environmental toll. The process produces a highly concentrated brine or waste product, which must be disposed of properly. This waste product can damage ecosystems if it is not well managed.