Evaporation. (1) The process by which water passes from a liquid state, at
temperatures below the boiling point, to vapor. It is the principal process by which
surface or subsurface water is converted to atmospheric vapor. (2) The quantity of
water that is evaporated; the rate is expressed in depth of water, measured as liquid
water, removed from a specified surface per unit of time--generally in inches or
centimeters per day, month, or year.
Evapotranspiration. Water withdrawn from soil by evaporation and/or plant
transpiration. Considered synonymous with consumptive use.
Facultative Bacteria. Bacteria which can adapt themselves to growth in the presence,
as well as in the absence, of oxygen.
Fahrenheit. Pertaining to the Fahrenheit thermometer scale. Water freezes at 32F
and boils at 212F.
Filter. A device or structure for removing solid or colloidal matter (which usually cannot
be removed by sedimentation) from water, or other liquids or semiliquids, by a straining
process whereby the solids are held on a medium of some kind (granular,
diatomaceous earth, woven, porous, etc.) while the liquid passes through.
Filter Bottom. The underdrainage system for collecting the water that has passed
through a filter and for distributing the wash water that cleans the filter medium.
Filter, Diatomite. A filter employing diatomaceous earth as the filtering material.
Filter, Gravity. An open filter, the operating level of which is placed near the hydraulic
grade line of the influent and through which the water flows by gravity.
Filter, Pressure. A filter of the closed type, having a vertical or horizontal cylinder of
iron, steel, wood, or other material inserted in a pressure line.
Filter, Rapid Sand. A filter used in the purification of water. Water which has been
previously treated usually by coagulation and sedimentation is passed downward
through a filtering medium, consisting of a layer of relatively coarse sand, or prepared
anthracite coal, or other suitable material; usually from 24 to 30 inches thick, resting on
a supporting bed of gravel or porous medium such as Carborundum. The filtered water
is removed by an underdrainage system which also distributes the wash water during
backwashing. Filter rates commonly range form 2 to 3 gallons per minute per square
foot of filter area. Also called mechanical filter.
Filter Rate. The rate of application of material to some process involving filtration; for
example, waterflow to a rapid sand filter.