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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.) |
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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). |
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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. |
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Viable |
Alive and capable of
continued life. |
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Violation |
A failure to meet any state
or federal drinking water regulation. |
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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. |
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Viscosity |
The resistance of fluids to
flow, due to internal forces and friction between molecules, which
increases as temperature decreases. |
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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. |
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Volatile |
Capable of vaporization at
a relatively low temperature. |
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Volatile Organic
Chemicals (V.O.C.'S) |
Chemicals
that, as liquid, evaporate into the air. |
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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. |
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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. |
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Volumetric |
Referring to measurement by
volume rather than weight. (See gravimetric.) |
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Vulnerability Assessment |
An evaluation of drinking
water source quality and its vulnerability to contamination by pathogens
and toxic chemicals. |
