Large-scale desalination is an enticing dream in a warming world with a dwindling supply of drinking water. If we could simply remove the salt, Earth's 352 quintillion gallons of ocean water would become a potable pour.

The dream isn't new. The first patent for a desalination system was issued in England in the 1600s, and the first municipal plants opened in the Caribbean and the Middle East in the early 20th century. But almost 100 years later, we still don't have a robust global desalination network. In the United States, plants are clustered in Florida and California, despite the growing need elsewhere. In regions like Tampa Bay and San Diego—site of the largest desalination plant in the country—desalinated water still accounts for only 10 percent of drinking water.

How It Works

Scientifically, it is quite simple to create clean drinking water from salt water. The most common method is reverse osmosis, which puts water under immense pressure to filter out salt, minerals, and other unsafe bits. The large amount of energy required to create that pressure is a major reason we haven't seen a desalination-plant building spree. Energy accounts for as much as 40 percent of the hefty cost of the process.

How It Doesn't Work

If desalination plants are powered by fossil fuels, there is also a planetary cost, exacerbating future droughts and creating a circular climate issue. The annual emissions from a large desalination plant can equal those of more than 50,000 cars during the same period.

Even a desalination plant powered by renewable energy faces the second hurdle: brine. The leftovers from desalination are concentrated salts mixed with chlorine and copper. This saline sludge is dumped back into the ocean, which increases ocean acidification and can be deadly for marine life.

How It Could Work

The National Renewable Energy Laboratory proposes harnessing the power of the salty waves themselves as a net-zero way of pressurizing municipal desalination plants.

Another option might be off-grid, household desalination systems using low-cost materials. Researchers at MIT and Shanghai Jiao Tong University have designed one small enough to sit on a rooftop but efficient enough to make about a gallon of drinking water per hour. Instead of reverse osmosis, the system relies on distillation: Solar heat (captured by blackened glass panels, not solar cells) evaporates water, leaving the salt behind; the recycled heat then condenses the vapor back into a potable liquid. Instead of electrified pumps to bring the salt water in, the tool relies on the power of gravity to passively draw salt water through several filters. The byproduct is still salty brine but without toxic chemicals. Researchers are working on getting rid of the liquid entirely, leaving only salt crystals—the same kind one might use to season dinner.

The Upshot

Desalination—without excessive energy costs or toxic byproducts—is at present probably best achieved by scaling up passive, inexpensive systems designed for individual coastal households. Bonus: Such systems could be cheaper for a family to run and maintain than paying their usual municipal water bill.