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The growing fear of water shortage in many parts of the world has made water reuse a popular topic of discussion. However, reuse actually is an ancient practice. Since there is a fixed amount of water on the earth, it is clear from the law of conservation of matter that the same water must be used over and over again to fulfill the many needs. The perpetual cycling of water from atmosphere to earth to ocean and back again to the atmosphere of course is not uniform throughout the world. In some places, water evaporated from the oceans is returned rather quickly by mountain streams with little or no use by industry. In other cases, precipitation may percolate into ground-water systems, from which much of it may not emerge again for thousands of years. But sooner or later the natural laws which control the hydrologic cycle guarantee that all water will return to a point of use. Reuse is the inevitable result of this process.