Diagram of reverse osmosis desalination

Put simply, desalination purifies water by removing dissolved mineral salts and other solids. In the Middle East, most desalted water is produced by means of distillation, which imitates the natural water cycle: Salt water is heated to produce water vapor, which then condenses to form freshwater. American desal plants favor a different technology -- reverse osmosis -- which forces the water through a series of membranes, leaving the salts behind.

Illustration: Colin Hayes

OnEarth. Summer 2004
Copyright 2004 by the Natural Resources Defense Council