People have been separating salt and fresh drinking water from salty sea water for thousands of years. But there are limits that can be reached - sometimes with drastic consequences. When the people in ancient Mesopotamia could not find out how to desalcinate their irrigation water and prevent the accumulation of salts in their soils, their society collapsed. "It's kind of the oldest, boring but serious problem in the world," says Sujay Kaushal, hydrologist at the University of Maryland in College Park.

This problem is now becoming more and more urgent, since the salt content in sweet waters increases from a variety of reasons. The rising sea level presses salt into coastal ground water, while in other places the excessive groundwater extraction pulls deeper, saletier waters in aquifers. And human activities - from the de -icing of streets to washing clothes to fertilization of fields - dirty surface waters with many types of salt. Last October, Kaushal and his colleagues reported that the salt content in large streams and rivers all over the world increased; Some waters are now saltier several times than a few decades ago 1 . Salvation of fresh water is a massive global problem, not a regional, he says.

A second, related problem, is the growing burden of problematic waste levers. A large number of industries- from oil and gas extraction to the desalination systems that produce drinking water- create salt-containing waste water whose disposal is expensive. "We have to do something with the alkalis," says Menachem Elimelech, environmental engineer at Yale University in New Haven, Connecticut.

…

price threshold

whether one of these ideas will be successful depends on economic factors. If the SWCC gained all the available sodium chloride from the salt water salaries of Saudi Arabia, Fellows notes, this would be enough to provide a third of the world market. On the other hand, waste leverage from the desalination of brackish water could offer the abundant mineral plaster, but it is unlikely that the brine extraction from conventional broken rock could compete economically.

New markets, such as the introduction of salt-powered technologies, including zinc bromine batteries, could create fresh demand for certain salts, says Fellows. Regulations could also play a role, either by making the disposal of waste eyes more expensive or promoting the use of the brin -based salts in various applications, for example the brine -based plaster in street salts.

One thing is clear: the need for fresh water increases. New technologies to deal with the current limits of desalination are important, say the researchers. But it is not an alternative route to the still essential step of maintaining fresh water. Energy, time or land area will always cost to separate salt from water, so there will always be a price for desalination. "There is no magic," says Elimelech.