Humans have been separating salt from water for thousands of years, producing both salt and fresh drinking water from salty seawater. But there are limits to what can be achieved - sometimes with drastic consequences. When the people of ancient Mesopotamia couldn't figure out how to desalinate their irrigation water and prevent salts from building up in their soil, their society collapsed. “It's kind of the oldest, most boring, but serious problem in the world,” says Sujay Kaushal, a hydrologist at the University of Maryland in College Park.

This problem is now becoming more pressing as freshwater salinities increase for a variety of reasons. Rising sea levels are pushing salt into coastal groundwater, while elsewhere excessive groundwater extraction is drawing deeper, saltier waters into aquifers. And human activities — from de-icing roads to washing clothes to fertilizing fields — pollute surface waters with many types of salt. Last October, Kaushal and his colleagues reported that salinity levels in major streams and rivers around the world were skyrocketing; some waters are now several times saltier than they were a few decades ago 1. Freshwater salinization is a massive global problem, not a regional one, he says.

A second, related problem is the growing burden of problematic waste alkalis. A variety of industries – from oil and gas extraction to the desalination plants that produce drinking water – produce saline wastewater that is costly to dispose of. “We have to do something with the lyes,” says Menachem Elimelech, an environmental engineer at Yale University in New Haven, Connecticut.

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Price threshold

Whether any of these ideas will be successful depends on economic factors. If the SWCC extracted all the available sodium chloride from Saudi Arabia's desalination wastewater, Fellows notes, it would be enough to supply a third of the world market. On the other hand, waste liquors from the desalination of brackish water could provide the abundant mineral gypsum, but it is unlikely that brine extraction from conventionally crushed 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 waste caustic disposal more expensive or encouraging the use of brine-based salts in various applications, such as brine-based gypsum in road salts.

One thing is clear: the need for fresh water is increasing. New technologies to address the current limits of desalination are important, researchers say. But it is not an alternative to the still essential step of conserving fresh water. It will always take energy, time or land area to separate salt from water, so there will always be a price for desalination. “There is no magic,” says Elimelech.