V...

Vapor : The gas given off by substances that are solids or liquids at ordinary atmospheric pressure and temperatures.
Vapor Dispersion : The movement of vapor clouds or plumes in the air due to wind, gravity, spreading, and mixing.
Vapor Recovery System : A system by which the volatile gases from gasoline are captured instead of being released into the atmosphere. Recovery systems may be required for gasoline stations in some cities and other non-attainment areas.
Vector : An insect or other organism capable of transmitting germs or other agents of disease.
Vegetative : Actually growing state.
Vent : The connection and piping through which gases enter and exit a piece of equipment.
Virion : A virus particle. Viral DNA or RNA enclosed in an organic capsule.
Virus : A submicroscopic genetic constituent which can alternate between two distinct phases. As a virus particle, or virion, it is DNA or RNA enveloped in an organic capsule. As an intracellular virus, it is viral DNA or RNA inserted into the host organisms DNA or RNA.
Volatile : Any substance which evaporates quickly. A volatile substance is one that is capable of being evaporated or changed to a vapor at a relatively low temperature. Volatile substances also can be partially removed by air stripping.
Volatile Organic Compounds (VOC) : Any organic compound which evaporates readily to the atmosphere. VOCs contribute significantly to photochemical smog production and certain health problems.
Volatile Solids (VS) : Is the amount of matter which volatilizes (or burns) when a water sample is heated to 550 C.
Volatile Suspended Solids (VSS) : Is the non-filterable residue remaining after firing the total suspended solids at 550 C.

W...

Waste Activated Sludge (WAS) : The excess growth of microorganisms which must be removed from the process to keep the biological system in balance.
Waste Minimization : The elimination or reduction of a waste prior to its generation. This is accomplished by process changes rather than waste treatment methods.
Wasteload Allocation (WLA) : The portion of a strea's total assimilative capacity assigned to an individual discharger.
Wastewater : Consumed or used water from a municipality or industry that contains dissolved and/or suspended matter. The used water and solids from a community that flow to a treatment plant. Storm water, surface water, and groundwater infiltration also may be included in the wastewater that enters a wastewater treatment plant. The term "sewage" usually refers to household wastes, but this word is being replaced by the term "wastewater".
Wastewater Treatment Plant : A facility containing a series of tanks, screens, filters, and other processes by which pollutants are removed from water. Most treatments include chlorination to attain safe drinking water standards.
Watercourse : A channel in which a flow of water occurs, either continuously or intermittently.
Water Meter : A water volume measuring and recording device, furnished and / or installed by a user and approved by the city.
Water Quality Standard (WQS) : The combination of a designated use and the maximum concentration of a pollutant which will protect that use for any given body of water. For example, in a trout stream, the concentration of iron should not exceed 1 mg / L.
Water Table : The boundary between the saturated and unsaturated zones. Generally, the level to which water will rise in a well (except artesian wells).
Weak Acid : An acid that does not ionize completely under the conditions of interest. Examples include acetic acid, carbonic acid, and hypochlorous acid.
Weir : A wall or plate placed in an open channel and used to measure the flow of water.
Wellhead Protection Area : A protected surface and subsurface zone surrounding a well or well field that supplies a public water system and through which contaminants could likely reach well water.
Wetlands : Areas that are soaked or flooded by surface or ground water frequently enough or for sufficient duration to support plants, birds, animals, and aquatic life. Wetlands generally include swamps, marshes, bogs, estuaries, and other inland and coastal areas, and are federally protected. Wetlands frequently serve as recharge / discharge areas and are known as "nature's kidneys" since they help purify water. Wetlands also have been referred to as natural sponges that absorb flood waters, functioning like natural tubs to collect overflow. Wetlands are important wildlife habitats, breeding grounds, and nurseries because of their biodiversity. Many endangered species as well as countless estuarine and marine fish and shellfish, mammals, waterfowl, and other migratory birds use wetland habitat for growth, reproduction, food, and shelter. Wetlands are among the most fertile, natural ecosystems in the world since they produce great volumes of food (plant material).
Wildlife Refuge : An area designated for the protection of wild animals, within which hunting and fishing are either prohibited or strictly controlled.
Wood Treatment Facility : An industrial facility which treats lumber and other wood products for outdoor use. The process involves use of chromated copper arsenate and other toxic chemicals which are regulated as hazardous materials.

X...

Xenobiotic : A term for non-natural or man-made substances found in the environment (i.e., synthetics, plastics).

Y...

Yield Coefficient : Defined as the ratio of the mass of cells formed to the mass of substrate consumed, measured during any finite period of logarithmic growth.

Z...

Z-list : OSHA's Toxic and Hazardous Substances Tables (Z-1, Z-2, and Z-3) of air contaminants; any material found on these tables is considered hazardous.
Zone of Saturation : The layer beneath the surface of the land in which all openings are filled with water.
Zone Settling : See hindered settling.
Zoogleal Film : A complex population of organisms that form a "slime growth" on a trickling-filter media and break down the organic matter in wastewater.
Zoogleal Mass : Jelly-like masses of bacteria found in both the trickling filter and activated sludge processes.