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Home>Water University>Water Glossary>Water Glossary: V

Water Glossary: V


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Vacuum Breaker A mechanical device which automatically vents a water line to the atmosphere when subjected to a partial vacuum, thus preventing back-flow. (See back-flow, air gap, back-flow preventer.)
   
Valence A small positive or negative whole number, also called oxidation number, which indicates the net number of electrons gained or lost in the formation of an ion, or the number of electrons the substance can donate or accept in a chemical reaction, and thus the numbers of each kind of ion necessary for a balanced chemical reaction. For example, two hydrogen ions (each with a valence of +1) must be present for each ion of oxygen (-2) to form a molecule of water (H2O).
   
Variance State or EPA permission not to meet a certain drinking water standard. The water system must prove that: (1) it cannot meet a MCL, even while using the best available treatment method, because of the characteristics of the raw water, and (2) the variance will not create an unreasonable risk to public health. The State or EPA must review, and allow public comment on, a variance every three years. States can also grant variances to water systems that serve small populations and which prove that they are unable to afford the required treatment, an alternative water source, or otherwise comply with the standard.
   
Viable Alive and capable of continued life.
   
Violation A failure to meet any state or federal drinking water regulation.
   
Virus The smallest form of life known to be capable of producing disease or infection, usually considered to be of large molecular size. They multiply by assembly of component fragments in living cells, rather than by cell division, as do most bacteria.
   
Viscosity The resistance of fluids to flow, due to internal forces and friction between molecules, which increases as temperature decreases.
   
Void Volume  The volume of the spaces between particles of ion exchanger, filter media, or other granular material; often expressed as a percentage of the total volume occupied by the material.
   
Volatile Capable of vaporization at a relatively low temperature.
   
Volatile Organic Chemicals (V.O.C.'S) Chemicals that, as liquid, evaporate into the air. 
   
Volatile Solids Matter which remains as a residue after evaporation at 105 or 180oC, but which is lost after ignition at 600oC. Includes most forms of organic matter.
   
Volitile Organic Chemicals (Voc) VOCs are a category of cantaminants. These chemicals, which include tetrachloroethylene, benzene and xylenes, are used in solvents, cleaners and degreasers used in many industrial and household products. When spilled or dumped, a portion of VOCs enter the ground and may eventually reach the water table. According to the EPA, health effects of VOCs include liver, kidney or central nervous system problems. Some VOCs are also suspected carcinogens.
   
Volumetric Referring to measurement by volume rather than weight. (See gravimetric.)
   
Vulnerability Assessment  An evaluation of drinking water source quality and its vulnerability to contamination by pathogens and toxic chemicals.