Water covers nearly three-quarters of Earth’s surface, yet only about 2.5% of it is fresh water. And the rising demand, coupled with climate change and drought, has put an increasing strain on the freshwater supply in some areas. So why not just remove the salt from–or desalinate–ocean water for human use? Well, it’s easier said than done. 

Seawater is on average 140 times saltier than drinking water. To efficiently remove those salts, desalination plants use specialized techniques that largely fall into two categories: thermal desalination and reverse osmosis. 

Thermal desalination involves heating seawater to its boiling point and condensing the salt-free steam for drinking water. Though there’s a range of efficiency in both methods, thermal desalination requires roughly 10 times as much energy as reverse osmosis, says Emily Tow, a mechanical engineer at Olin College of Engineering who studies water desalination and reuse. 

Most desalination plants today use reverse osmosis. In this process, seawater is pushed through a membrane to remove the salts. “The membranes are so fine that water can get through, but salts really can’t,” Tow says, and the clean water that comes through is taken for drinking water. Although this method requires less energy than thermal desalination, it still uses quite a bit of electricity, about “four times what we would typically spend on our municipal water use, including wastewater treatment,” Tow says. 

But for every gallon of seawater that plants process through reverse osmosis, only half is recovered as fresh water. That’s because the process has a built-in trade-off: As water is pushed through the membrane, the seawater on the starting side gets more concentrated, which makes it even harder to pass through the membrane. This, in turn, increases the energy needed to push the water through.

The super-concentrated seawater that’s left over, called brine, is a common byproduct of reverse osmosis plants. Once the filtration is done, plants typically pump the brine back into the ocean, far away from shore, while mixing it with seawater so it doesn’t create extra salty zones. Some researchers have pointed out that having multiple plants in one area operating for an extended period could lead to an increase in ocean salinity and negatively affect the surrounding ecosystem. But with proper design and monitoring the effect should be minimal.

Although seawater is plentiful, removing the salt is an energy-intensive process that may only make sense in specific scenarios. “Our need for water as humans is never-ending, but our supply of fossil fuels is limited,” Tow says.

One way to bring down desalination energy costs is by coupling plants with renewable energy sources. “Wouldn’t it be great if anytime you build a new seawater plant you have to build enough renewable energy capacity to power that plant,” Tow says, “so that we’re not just making things worse for the environment while we solve our short-term water problem.”