Drink Deep

New tech pulls fresh water from the bottom of the sea

- —Vanessa Bates Ramirez

FROM CAPE TOWN TO TEHRAN to Lima to Phoenix, dozens of cities across the globe have recently experienced water shortages. In the next five years the world’s demand for fresh water could significantly outpace supply, according to a United Nations forecast. Now several companies are turning to an unexpected source for a solution: the bottom of the ocean.

Called subsea desalination, the idea is to remove the salt from water in the deep sea. If it worked at scale, the technology could greatly alleviate the world’s water-access problems.

Costs and energy requirements have kept desalination from going mainstream in most of the world. Early desalination involved boiling seawater and condensing the steam, a purely thermal method that used loads of energy. This approach was later replaced by multistage flash distillation, in which temperature and pressure “flash” salt water into steam. In the past 25 years reverse osmosis has become more common. This process uses high pressure to push seawater through a membrane with holes so small that only water molecules squeeze through, leaving salt behind.

Reverse osmosis is more efficient than distillation, but it takes a lot of energy to pressurize millions of gallons of seawater to force it through filters. What if we could let that movement happen naturally by harnessing the pressure hundreds of meters underwater?

That’s the concept behind subsea desalination. Reverse osmosis pods are submerged to depths of around 500 meters (1,600 feet), where immense hydrostatic pressure does the hard work of separating water from salt. Purified water is then pumped back to shore. Far-fetched as the setup may sound, there are multiple prototypes already at work; the companies behind them aim to take cheap, large-scale desalination from pipe dream to reality.