A method of cross flow filtration (similar to reverse osmosis but using lower pressures) which uses a membrane to separate small colloids and large molecules from water and other liquids.     The ultrafiltration process falls between reverse osmosis and microfiltration in terms of the size of particles removed, with ultrafiltration removing particles in the 0.002 to 0.1 micron range, and typically rejecting organics over 1,000 molecular weight while passing ions and smaller organics.

Ultrapure Water

Highly-treated water that is deionized and mineral-free with high resistivity and no organics; it is usually used in the semiconductor and pharmaceutical industries.     Ultrapure water is NOT considered biologically pure (potable) or sterile.     There is no set numerical standard to determine exactly what "ultrapure" water is or should be.

Ultraviolet (UV)

Pertaining to ultraviolet light.

Ultraviolet (UV) Light

Radiation (light) having a wavelength shorter than 3900 angstroms, the wavelengths of visible light, and longer than 100 angstroms, the wavelengths of x-rays.     This wavelength puts ultraviolet light at the invisible violet end of the light spectrum.     Ultraviolet light is used as a disinfectant.

Ultraviolet (UV) Light Spectroscopy

See Spectroscopy

Ultraviolet Absorber

Substances which absorb ultraviolet radiation (light). Ultraviolet absorbers are added to plastic (such as used in plastic tanks and fittings) and rubber products to make them less likely to decay as the result of absorbing ultraviolet rays.

Ultraviolet Chamber

The area where the water is irradiated with ultraviolet rays.

Ultraviolet Demand

The amount of ultraviolet rays required to inactivate certain microorganisms.

Ultraviolet Dosage

The amount of disinfectant ultraviolet rays delivered to the organisms in the water being disinfected. Dosage is a combination of UV intensity times the contact time and is measured in watt-seconds per square centimeter.

Unaccounted-for Water

A term used by public and municipal water systems to describe the difference between the amount of finished water produced and the amount registered on meters as sold.     Unaccounted-for water may range from 10 percent-35 percent of finished water produced by the utility and usually includes water lost from leaky water mains, water lost in firefighting or from fire hydrants, or other public or municipal uses.

Uncertainty Factor

A number (equal to or greater than one) used to divide the NOAEL or LOAEL value derived from measurements in animals or small groups of humans, in order to estimate a NOAEL value for the whole human population.

Unconfined Aquifer

An aquifer containing water that is not under pressure; the water level in a well is the same as the water table outside the well.


A layer of gravel or grout used to fill the bottom curved base of a larger filter or softener tank, usually in a system with a header-lateral design.     Underbed is not necessarily the same as the media support bed.


The drainage piping arrangement to collect treated water at the bottom of the ion exchange or filter media beds.

Uniform Flow

A flow in which the feet per second velocity rates and directions are the same from point to point along the conduit.

Uniform Particle Size

The particle size distribution screen sizing for exchanger and filtration media as established by U.S. Mesh Standards.

Uniformity Coefficient

The measure of the variation in particle sizes of filter and ion exchange media.  The uniformity coefficient is defined as the ratio of the sieve size that will permit passage of 60 percent of the media material by weight to the sieve size that will permit passage of 10 percent of the media material by weight.  A uniformity coefficient of 1.00 denotes a material having particle grains all the same size; numbers increasingly greater than one denote increasingly less uniformity.

Unit Cancer Risk

Estimate of the lifetime risk caused by each unit of exposure in the low exposure region.

Unit Hydrograph

The hydrograph of one inch of storm runoff generated by a rainstorm of fairly uniform intensity within a specific period of time.

United States Pharmacopeia (USP)

The official publication for drug product standards including six water quality standards for pharmaceutical uses.  The USP was established by the U.S. Congress in 1884 to control makeup of drugs.


Having a valence of one. Also called monovalent.

Unsaturated Zone

The area between the land surface and water table in which the pore spaces are only partially filled with water. Also called zone of aeration.

Unslaked Lime

See Lime


A pattern of water flow in which a solution (water or regenerant usually) enters at the bottom of the vessel or column and flows out at the top of the vessel or column during any phase of the treatment unit's operating cycle.     The term is used to describe ion exchange system flow patterns or water flow through filter media. A system can have upflow during the treatment cycle and downflow during regeneration. Upflow is also called counter current flow.     Countercurrent flow means regeneration flows and service flows are in the opposite directions.

Upflow Brining

Is a means of forcing the brine solution upward through the cation exchanger for regenerating the resin. Where the softening flow is downward and the regenerating brine flow is upward, the mode is also called countercurrent flow.     Countercurrent flow means regeneration flows and service flows are in the opposite directions.

Upflow Softening

A pattern of water flow used in softeners in which the service water flows upward through the ion exchange bed; the media is restricted in movement, usually because of a packed bed. The regeneration brine usually flows downward in such systems.     Upflow softening is normally used to achieve higher operating efficiency.

Upper Bound Estimate

Estimate not likely to be lower than the true risk.

Upper Distributor

The piping arrangement inside and at the top of softeners and filters to more uniformly distribute the incoming water over the resin or filter media bed.     In small domestic units, this distributor also distributes the brine for regeneration.

Uranium (U)

A radioactive metallic element found naturally only in combination with other substances. Uranium 238 (U-238) is the most common form, but about 0.7 percent of natural uranium is present as U-235, which is the important fissionable component in work with atomic energy.     Uranium in natural water exists as anionic complexes UO<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub><sup>2-</sup> and UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup>.

Urban Runoff

Stormwater from city streets and adjacent domestic or commercial properties that may carry pollutants of various kinds into the sewer systems and/or receiving waters.


The product water consumer.

User Fee

A fee which is collected only from those persons who use a particular service, as opposed to one collected from the public in general.     User fees generally vary in proportion to the degree of use of the service.


United States Pharmacopeia.

USP Grade Water

See Pharmaceutical Grade Water.


Underground Storage Tanks.

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Updated on Mon, 22 Jun 2020 by Jonathan

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