Desalination, the process of removing salt from seawater, provides drinking water for millions of people, particularly in dry coastal regions. The most common method today is reverse osmosis, in which seawater is pushed at high pressure through membranes that allow water to pass but block salt. Although the technology has become more efficient over the past few decades, it still requires a considerable amount of energy. If that energy comes from fossil fuels, desalination contributes to greenhouse gas emissions. Another problem is the concentrated salty water, known as brine, that remains after fresh water has been extracted. When brine is released back into the sea, it can harm marine life near the outlet, especially if it is not mixed well with the surrounding water. Some new plants are powered by solar or wind energy, and researchers are looking for ways to extract useful minerals from brine. Still, experts often recommend that regions first reduce water leaks from pipes and encourage efficient water use, since saving water is usually cheaper than producing new supplies.

Reverse osmosis uses membranes that allow water to pass through but block salt.