fecal - Matter containing or derived from animal or human wastes or feces.

filter- A device or system for the removal of solid particles (suspended solids).

filter area - The effective area through which water passes through filter media, often expressed in square feet. (See cross sectional area.)

filter medium - (See medium.)

filtrate - The effluent liquid from a filter.

fines - Extremely small particles of filter media or ion exchange material, often the result of breakage or chemical or physical deterioration.

fixed solids - The suspended or dissolved solids remaining after ignition, usually at 600^oCl; usually due to inorganic matter which is not volatilized at the ignition temperature.

fixture - In plumbing, a permanently installed device in which water is used, such as a faucet or toilet.

fixture unit - An arbitrary unit assigned to different types of plumbing fixtures, and used to estimate flow rate requirements and drain capacity requirements.

floc - An agglomeration of finely divided suspended particles in a larger, usually gelatinous particle the result of physical attraction or adhesion to a coagulant compound.

flocculation - The process of causing a "floc" to form after treatment with a coagulant by gentle stirring or mixing. (See coagulation.)

flow control- A device designed to limit or restrict the flow of water or regenerant; may include a throttling valve, an orifice of fixed diameter, or a pressure compensating orifice.

flow rate- The quantity of water or regenerant which passes a given point in a specified unit of time, often expressed in gallons per minute.

fluoridation- The addition of a fluoride compound to a water supply for the reduction in incidence of dental caries.

flush tank- A tank or chamber in which water is stored for rapid release.

flush valve- A self-closing valve designed to release a large volume of water when tripped.

FMA - Abbreviation for free mineral acidity (see mineral acidity).

fouling - The process in which undesirable foreign matter accumulates in a bed of water conditioning media, clogging pores and coating surfaces and thus inhibiting or retarding the proper operation of the bed.

free available chlorine - The concentration of residual chlorine present as dissolved gas, hypochlorous acid or hypochlorite, not combined with ammonia or in other less readily available form.

freeboard- The vertical distance between a bed of filter media or ion exchange material and the overflow or collector for backwash water; the height above the bed of granular media available for bed expansion during backwashing; may be expressed either as a linear distance or a percentage of bed depth.

free carbon dioxide - Carbon dioxide present in water as the gas, or as carbonic acid, but not that combined in carbonates or bicarbonates.

free chlorine- See free available chlorine.

free mineral acidity - See mineral acidity.