| 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. | |
| Finished Water | Water that has been treated and is ready to be delivered to customers. | |
| Fixed Solids | The suspended or dissolved solids remaining after ignition, usually at 600oCl; 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. | |
| 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. | |
| 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. |