Today's engineers still struggle with the problem, and some of them are coming up with smaller-scale solutions. A case in point is a relatively simple device invented by Ashok Gadgil, an Indian-born research scientist working at the Lawrence Berkeley National Laboratory in California. When a new strain of cholera killed more than 10,000 people in southeastern India and neighboring countries in 1992 and 1993, Gadgil and a graduate student assistant worked to find an effective method for purifying water that wouldn't require the cost-prohibitive infrastructure of treatment plants.

Their device was simplicity itself: a compact box containing an ultraviolet light suspended above a pan of water. Water enters the pan, is exposed to the light, and then passes to a holding tank. At the rate of 4 gallons a minute, the device kills all microorganisms in the water, with the only operating expense being the 40 watts of power needed for the ultraviolet lamp. Dozens of these devices, which can be run off a car battery if need be, are now in use around the world—from Mexico and the Philippines to India and South Africa, where it provides clean drinking water to a rural health clinic.  Regions using the simple treatment have reported dramatic reductions in waterborne diseases and their consequences.

Whatever their scale, from aqueducts and dams to desalination plants and portable ultraviolet devices, the notable successes in water management achieved in the 20th century continue to offer encouragement to a new generation of civil engineers worldwide as they face the challenge of our never-quenched need for clean water.