Besides air, water is the resource most critical to sustaining life on Earth. Human beings depend on water not only for drinking, but for generating energy, growing crops, carrying waste, for washing and cleaning, and to run machinery. Clean water is also essential for many species of plants and animals living in streams, rivers, and lakes.

As the human population grows and the world becomes more and more industrialized, demands on fresh water resources are increasing sharply. At the same time, this very growth threatens the water supply through human and industrial pollution. Climate change is creating drought conditions in some parts of the world. Already, wars are being

fought over water. It seems clear that water will replace oil as the most precious and fought-over resource on the planet.

The importance of water cannot be overstated, yet people in industrialized nations take for granted the fact that they can turn on a faucet for fresh water on demand. But even in this water-on-demand society, contaminants and pathogens find their way into the