vacuum breaker – A mechanical device which automatically vents a water line to the atmosphere when subject to a partial vacuum, thus preventing back siphonage.
vacuum distillation – Distillation that occurs at a pressure somewhat below atmospheric pressure. Lowering the pressure also lowers the boiling point of water, thus conserving energy by requiring less heat to bring about distillation.
vacuum filtration – The filtration process in which a partial vacuum is applied to increase the rate of filtration by causing the water to be sucked through the filter medium. This is one of the oldest mechanical dewatering techniques in continuous use. In municipal softening, this process is used to separate water from the lime sludge for sludge disposal.
vacuum freezing – A form of desalination using a vacuum to help cool and fast freeze high TDS source water which separates the solids by concentrating them in the portion of the water that doesn't freeze or that freezes last in a similar manner to what occurs in the cloudy centers of ice cubes.
vacuum pan – An airtight container used to produce granulated water softener salt using a process involving the evaporation of brine-turned-to-steam in a partial vacuum.
vacuum pump – A pumping apparatus which exhausts gas or air from an enclosed space to achieve a desired degree of vacuum.
valence – A whole number (positive or negative) representing the power of one element to combine with another. In general terms, the valence number represents the number of electrons in an atom or combined group of atoms which can be easily given up or accepted to react with or bond to another atom or group of atomes to form a molecule.
validation – 1. (water treatment industry) Determination upon testing that a representative sample of a water treatment equipment model has met the requirements of a specified standard. 2. (pharmaceutical industry) The requirement of certain quality control testing and record keeping procedures to ensure compliance not only with a specific quality but also with a specific means to achieve and encore that quality.
Van Der Waal's forces – Weak attractive forces acting between molecules. These forces are somewhat weaker than hydrogen bonds and far weaker than interatomic valences.
vapor – 1. The gaseous form of any substance whose usual forte is a liquid or a solid. 2. Visible particles of moisture suspended in air, such as mist or fog.
vapor pressure – The pressure, often expressed in millimeters of mercury (mm Hg, at which a vapor is in a state of balance with its liquid or solid form.
variable costs – Input costs that change as the nature of the production activity of its circumstances change; for example, as production levels vary.
variance – A State with primary enforcement responsibility under the Safe Drinking Water Act may relieve a public water system from a requirement respecting an MCL by granting a variance if certain conditions exist. These are: 1) the system cannot meet the MCL in spite of the application of best available treatment technology, treatment techniques, or other means (taking costs into consideration), due to the characteristics of the raw water sources which are reasonably available to the system, and 2) the variance will not result in an unreasonable public health risk. A system may also be granted a variance from a specified treatment technique if it can show that, due to the nature of the system's raw water source, such treatment is not necessary to public health.
vegetative controls – Nonpoint source pollution control practices that involve planet (vegetative cover) to reduce erosion and minimize the loss of pollutants.
velocity (general water treatment) – The time measurement of linear motion (flow) in a given direction. For example, water flowing 60 feet in a conduit each minute has a velocity of 60 feet per minute (fpm) or one foot per second (1 fps).
velocity profile – The relationship between the velocity of fluid flowing adjacent to the conduit wall or membrane surface and that flowing at a distance from the wall or surface.
venturi – A tube with a narrow throat (a constriction) that increases the velocity and decreases the pressure of the liquid passing through it, creating a partial vacuum immediately after the constriction in the tube. The vacuum created has a sucking effect (eduction), and a venturi is commonly used to introduce a liquid (such as a regenerant) or gas (such as air) into a flowing water scream.
versenate – A chemical substance used in water analysis for water hardness or with an indicator to colorimetrically measure hardness quality.
viable – 1. Capable of living independently (as in a fetus capable of living outside the womb) and being reproductive (as microorganisms capable of colonizing and thriving) 2. workable (as a viable idea).
viable treatment process – A water or waste water treatment process capable of accomplishing the desired water quality.
virulence – Degree of ability to cause disease.
virus – A parasitic infectious microbe, composed almost entirely of protein and nucleic acids, which can cause disease(s) in humans. Viruses can reproduce only within living cells. They are 0.004 to 0.1 microns in size, and about 100 times smaller than bacteria.
viscosity – The tendency of a fluid to resist flowing due to internal forces such as the attraction of the molecules for each other (cohesion) or the friction of the molecules during flow. Viscosity varies with water temperature.
VOCs – volatile organic chemicals.
voids – SEE void volume.
void volume – The volume occupied by the interstitial spaces between the particles of ion exchangers, filter media, or other granular materials in a bed or column. Often expressed as percent of the total volume occupied by the medium bed.
volatile – Capable of becoming vapor at relatively low temperatures.
volatile acids – Acids produced during digestion. Fatty acids which are soluble in water and can be steam-distilled at atmospheric pressure. Also called organic acids. Volatile acids are commonly reported as equivalent to acetic acid.
volatile liquids – Liquids which easily vaporize or evaporate at room temperatures.
volatile organic chemicals (VOCs) – Organic chemicals that turn into vapor at relatively low temperatures.
volatile solids – The term used in the laboratory analysis of the solids content of a substance (such as water) to define the portion of the total suspended and/or dissolved solids that become expulsed or driven off after heating or burning a given sample of the substance at a specified temperature and for a specified time.
volatilization – Loss of a substance through evaporation.
voltage – The electrical pressure available to cause a flow of current (amperage) when an electrical circuit is closed.
volumetric – A measurement based on the volume of some factor. Volumetric titration is a means of measuring unknown concentrations of water quality indicators in a sample by determining the volume of titrant or liquid reagent needed to complete particular reactions.
vortex – A revolving mass of water which forms a whirlpool. This whirlpool is caused by water flowing out of a small opening in the bottom of a basin or reservoir. A funnel-shaped opening is created downward from the water surface.