unaccounted-for water - A term used in municipal water systems to describe the difference between the water produced and that metered and sold. It usually includes losses due to leakage, water used for fire fighting, public sprinkling and other municipal purposes, and usually ranges from 10 to 35 percent of the water produced.

uniformity coefficient - A measure of the variation in particle sizes of ion exchange resins and filter media. It is defined as the ratio of the size of particle which has 60 percent of the material finer than itself, to the size of the particle which has 10 percent finer than itself.

upflow - A term used to indicate the direction (up) in which water or regenerant flows through an ion exchanger or filter media bed during any phase of the operating cycle.

USEPA - The abbreviation for "United States Environmental Protection Agency".

USPHS - The abbreviation for "United States Public Health Service".