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Symbols and Acronyms

ultimate biochemical oxygen demand (BODu)   (1) Commonly, the total quantity of oxygen required to completely satisfy the first-stage BOD. (2) More strictly, the quantity of oxygen required to completely satisfy both the first-stage and second-stage BOD.

ultimate disposal  The final release of a biologically and chemically stable wastewater or sludge into the environment.

ultraviolet radiation (UV)  Light waves shorter than the visible blue-violet waves of the spectrum.

ultraviolet ray  Light rays beyond the violet of the spectrum; these are invisible to humans.

underdrain  A drain that carries away groundwater or the drainage from prepared beds to which water or wastewater has been applied.

unsteady nonuniform flow  Flow in which the velocity and the quantity of water flowing per unit time at every point along the conduit varies with respect to time and position.

upflow  Term used to describe treatment units in which flow enters at the bottom and exits at the top.

upflow clarifier  A treatment unit in which liquid containing suspended solids is passed upward through a blanket of settling sludge; mixing, flocculation, and solids removal are all accomplished in the same unit.

upflow coagulation  Coagulation achieved by passing liquid, to which coagulating chemicals may have been added, upward through a blanket of settling sludge.

upflow filter  A gravity or pressure filtration system in which the wastewater flows upward, generally first through a coarse medium and then through a fine medium, before discharging.

upflow tank  A sedimentation tank in which water or wastewater enters near the bottom and rises vertically, usually through a blanket of previously settled solids. The clarified liquid flows out at the top and settled sludge flows out the bottom; a vertical-flow tank.

user  The party who is billed, usually for sewer service from a single connection; has no reference to the number of persons served. Also called a customer.
user charge  Charge made to users of wastewater services supplied.
utilization equipment  Equipment that uses electrical energy for mechanical, chemical, heating, lighting, or similar useful purposes.