Hold the Salt

Part of the Water: H2O = Life exhibition.

The oceans are vast, so it's no wonder that people have often looked seaward and wondered, "Isn't there a way we could drink that water?" Today, sophisticated filtration devices have made removing salt from seawater--a process called desalination--a reality in some places. Indeed, there are now more than 15,000 desalination plants in 125 countries.

But desalination has drawbacks. It uses a lot of energy, and energy production has costs--both financial and environmental. Marine life can be damaged, both from the powerful water intake systems and the discharge of salty brine. And there are those who think that increasing water supply attracts more people to fragile coastal areas, setting up a destructive cycle.

Ah Choo!

A number of lizards have special glands that allow them to desalinate themselves . . . naturally. In the marine iguana (Amblyrhynchus cristatus) these salt-removal glands drain into the nasal passages; occasionally the animal sneezes out a fluid that's much saltier than seawater. The dried salt sometimes leaves the iguana white-faced.

More Than a Pinch

The various compounds in a cubic foot of seawater weigh just over two pounds, or one kilogram--most of it the familiar substance we know as salt. But the salty wastewater--or brine--produced by a desalination plant after the fresh water is extracted may contain twice as much. That brine is either returned to the ocean or pumped into the ground. The environmental costs of desalination--marine organisms sucked into intake pipes, for example--are hard to quantify but may be serious.

Filtration Device

A pressure vessel is one part of the system used in a desalination plant to remove salt from seawater. Seawater at high pressure is forced through a special membrane that keeps salty water out but lets fresh water through. Half the water that begins this process emerges as fresh water; the other half is discharged as hyper-salty brine.

Desalinization filter
Adapted from Hydronautics, Oceanside, CA

Get the Salt Out

Seawater that has already been screened for impurities is forced through sheets of a special membrane, spiraled around a central pipe like the folds of a furled umbrella. This membrane keeps salt water out but lets fresh water flow into a central collection pipe.

Wind and Water

Perth, a rapidly growing city in hot and dry Western Australia, gets 20 percent of its water supply from the Indian Ocean. But Perth practices desalination with a difference: They use wind power--a clean, renewable source--for this energy-intensive process. The power comes from 48 huge wind turbines located 260 kilometers (160 miles) away.