Removing salt from seawater has always been possible and always been expensive. The energy required was the constraint, and as the cost of energy – particularly solar – has fallen, so has the cost of water.
The technology
Reverse osmosis dominates: forcing seawater through membranes that pass water and block salt. Membrane efficiency has improved steadily, energy recovery devices reclaim most of the pressure, and the energy per cubic metre has fallen substantially over two decades.
Where it is now viable
Coastal regions with abundant solar and scarce fresh water, which describes a great deal of the world. Plants pairing desalination with dedicated solar generation produce water at costs competitive with conventional sources in many locations.
What it changes
The assumption that water availability is fixed by geography. A city with a coastline and sunlight has an option it did not previously have, and that changes planning, agriculture, and the politics of water rights.
The problems
Brine. What comes out is concentrated salt, which has to go somewhere, and discharging it near the intake affects the local marine environment. Solutions exist and add cost.
Intake, which draws in marine life. Better screening mitigates this.
And dependence. A region that comes to rely on desalination has tied its water supply to its energy supply, which is a new kind of vulnerability.
What is coming
Smaller distributed plants rather than a few large ones. Integration with renewable generation as a matched pair. And in the longer run, extraction of useful minerals from the brine, turning a waste stream into a product.