Filter Rate Controller. An automatic device inserted in the effluent pipe of a filter to
maintain the rate of flow constant throughout the filter run.
Filter, Slow Sand. A filter used in the purification of water where water without previous
treatment is passed downward through a filtering medium consisting of a layer of sand
or other suitable material; usually finer media than for a rapid sand filter, and from 24 to
40 inches thick. The filtered water is removed by an underdrainage system and the
filter is cleaned by scraping off and replacing the clogged layer. It is characterized by a
low rate of filtration, usually from 3 to 6 million gallons per day per acre of filter area.
Fines. The finer grained particles of a mass of soil, sand, or gravel.
Finished Water. Treated water.
Fire Demand. The quantity of water and rate of flow required for firefighting purposes;
based on the types and sizes of structures to be protected and the duration of the fire.
Fixed Installation. An installation which, through extended use, has gained those
structures and facilities not initially found or intended for use at a "temporary" standard
Flashboard. A temporary barrier, of relatively low height and usually constructed of
wood, placed along the crest of the spillway of a dam to allow the water surface in the
reservoir to be raised above the spillway level to increase storage capacity.
Construction is such that it can be readily removed or lowered, or carried away by high
flow or floods.
Floc. Small gelatinous masses which are accumulations of microparticles, bacteria,
and other organisms; formed in a liquid by the addition of chemical coagulants, or by
the gathering together of particles by mixing.
Flocculation. The formation of flocs subsequent to the process of coagulation.
Flume. An open conduit, constructed of wood, masonry, metal, and constructed on a
grade; sometimes elevated.
Flume, Control. A flume arranged for measuring the flow of water or other liquids;
generally including a constricted section wherein a minimum energy head exists at all
Flume, Parshall. A device for measuring the flow of liquid in an open conduit.
Fluoridation. The addition of a chemical to increase the concentration of fluoride ions
to a predetermined limit causing a reduction in the incidence of dental cavities.