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Symbols and Acronyms

facultative   Having the ability to live under different conditions; for example, with or without free oxygen.

facultative bacteria  Bacteria that can grow and metabolize in the presence, as well as in the absence, of dissolved oxygen.

facultative lagoon  A lagoon or treatment pond with an aerobic upper section and an anaerobic bottom section so that both aerobic and anaerobic biological processes occur simultaneously.

Fahrenheit  A temperature scale in which 32° marks the freezing point and 212° the boiling point of water at 760-mm Hg. To convert to centigrade (Celsius), subtract 32 and multiply by 0.5556.

false filter bottom  A type of underdrainage system consisting of a porous or perforated floor suspended above the true bottom of the filter. See also underdrain.

fats (wastes)  Triglyceride esters of fatty acids; erroneously used as a synonym for grease.

fecal coliform  Aerobic and facultative, Gram-negative, non-spore-forming, rod-shaped bacteria capable of growth at 44.5 °C (112 °F), and associated with fecal matter of warm-blooded animals.

fecal indicators  Fecal coliform, fecal Streptococci, and other bacterial groups originating in human or other warm-blooded animals, indicating contamination by fecal matter.

fecal Streptococci  The subgroup of enterococci that is of particular concern in water and wastewater. See also Enterococci.

feeder  A circuit conductor between the service equipment or switchboard and the branch circuit overcurrent device.

fermentation  Changes in organic matter or organic wastes brought about by anaerobic microorganisms and leading to the formation of carbon dioxide, organic acids, or other simple products. See also biological oxidation.

ferric chloride (FeCl3)  A soluble iron salt often used as a sludge conditioner to enhance precipitation or bind up sulfur compounds in wastewater treatment. See also coagulant.

ferric sulfate [Fe2(SO4)3]  A water-soluble iron salt formed by reaction of ferric hydroxide and sulfuric acid or by reaction of iron and hot concentrated sulfuric acid; also obtainable in solution by reaction
of chlorine and ferrous sulfate; used in conjunction with lime as a sludge conditioner to enhance ­precipitation.

ferrous chloride (FeCl2)  A soluble iron salt used as a sludge conditioner to enhance precipitation or bind up sulfur. See also coagulant.

ferrous sulfate (FeSO4·7H2O)  A water-soluble iron salt, sometimes called copperas; used in conjunction with lime as a sludge conditioner to enhance precipitation.

field groundwater velocity  The actual or field velocity of groundwater percolating through water-bearing material. It is measured by the volume of groundwater passing through a unit cross-sectional area in unit time divided by the effective porosity. Also called effective groundwater velocity, true groundwater velocity, actual groundwater velocity.

field moisture capacity  The approximate quantity of water that can be permanently retained in the soil in opposition to the downward pull of gravity. It may be expressed as a percentage of dry weight or in inches for a given depth of soil. The length of time required for a soil to reach field moisture capacity varies considerably with various soils, being approximately 24 to 48 hours for sandy soils, 5 to 10 days for silt clay soils, and longer for clays. Also called capillary capacity, field carrying capacity, maximum water-holding capacity, moisture-holding capacity, normal moisture capacity.

field permeability coefficient  The rate of flow of water, in gallons per day (gpd) or liters per second (L/s), under prevailing conditions, through each 1 ft (0.3 m) of thickness of a given aquifer in a width of 1 mile (1.6 km), for each 1 ft/mile (0.19 m/km) of hydraulic gradient. Also called hydraulic conductivity.

filamentous growth  Intertwined, thread-like biological growths characteristic of some species of bacteria, fungi, and algae. Such growths reduce sludge settleability and dewaterability.

filamentous organisms  Bacterial, fungal, and algal species that grow in thread-like colonies resulting in a biological mass that will not settle and may interfere with drainage through a filter.

filter  A device or structure for removing solid or colloidal material, usually of a type that cannot be removed by sedimentation, from water, wastewater, or other liquid. The liquid is passed through a filtering medium, usually a granular material but sometimes finely woven cloth, unglazed porcelain, or specially prepared paper. There are many types of filters used in water and wastewater treatment. See also pressure filter.

filter aid  Solid particulate media (for example, diatomaceous earth) added to a filter to improve the rate of filtration; also used colloquially to describe flocculents in water treatment; same as filtration aid. See also coagulant or flocculent aid.

filter bed  (1) A type of bank revetment consisting of layers of filtering medium of which the particles gradually increase in size from the bottom upward. Such a filter allows the groundwater to flow freely, but it prevents even the smallest soil particles from being washed out. (2) A tank for water filtration that has a false bottom covered with sand, such as a rapid sand filter. (3) A pond with sand bedding, as a sand filter or slow sand filter. (4) The media that comprise a trickling filter.

filter bottom  (1) The underdrainage system for collecting the water that has passed through a rapid sand filter and for distributing the wash water that cleans the filtering medium. (2) The underdrainage system supporting the graded gravel of a biological bed. It may consist of specially fabricated tile or concrete blocks containing waterways and slots in the top for conveying the underdrainage, or it may consist of inverted half tile.

filter cake  The solids collected on the surface of a mechanical filter. It also applies to spent cake removed from a diatomaceous earth filter.

filter clogging  The effect occurring when fine particles fill the voids of a sand filter or biological bed, or when growths form surface mats that retard the normal passage of liquid through the filter.

filter cloth  A fabric stretched around the drum of a vacuum filter.

filtered wastewater  Wastewater that has passed through a mechanical filtering process but not through a trickling filter bed.

filter efficiency  The operating results of a filter as measured by various criteria such as percentage reduction in suspended matter, total solids, BOD, bacteria, or color.

filter flooding  The filling of a trickling filter to an elevation above the top of the medium by closing all outlets in order to reduce or control filter flies.

filter gallery  A gallery provided in a treatment plant for the installation of conduits and valves and used as a passageway to provide access to them. See also pipe gallery.

filter loading  Organically, the pounds (kilograms) of BOD in the applied liquid per unit of filter bed area or volume per day. Hydraulically, the quantity of liquid applied per unit of filter bed area or volume per day.

filter media  (1) Material through which water, wastewater, or other liquid is passed for the purpose of purification, treatment, or conditioning. (2) A cloth or metal material of some appropriate design used to intercept sludge solids in sludge filtration. (3) Particulate (sand, gravel, or diatomaceous earth) or fibrous (cloth) material placed within a filter to collect suspended particles.

filter ponding  The formation of ponds on the surface of trickling filters, caused by excessive biofilm growth, media degradation, or inadequate ventilation. Sometimes called filter pooling.

filter press  A plate and frame press operated mechanically to produce a semisolid cake from a slurry. See also plate press.

filter rate  The rate of application of material to some process involving filtration, for example, application of wastewater sludge to a vacuum filter, wastewater flow to a trickling filter, or water flow to a rapid sand filter.

filter run  (1) The interval between the cleaning and washing operation of a rapid sand filter. (2) The interval between the changes of the filter medium on a sludge dewatering filter.

filter strainer  A perforated device inserted in the underdrain of a rapid sand filter through which the filtered water is collected and through which the wash water is distributed when the filter is washed. Also called a strainer head.

filter underdrain  A system of subsurface drains to collect water that passes through a sand filter or biological bed. See also filter bottom.

filter wash  The reversal of flow through a rapid sand filter to wash clogged material out of the filtering medium and relieve conditions causing loss of head. Also called backwash.

filtrate  The liquid that has passed through a filter.

filtration  The process of contacting a dilute liquid suspension with filter media for the removal of suspended or colloidal matter, or for the dewatering of concentrated sludge.

final effluent  The effluent from the final treatment unit of a wastewater treatment plant.

final sedimentation  The separation of solids from wastewater in the last settling tank of a treatment plant.

fire flow  The rate of flow, usually expressed in gallons per minute (gpm) or cubic meters per second (m3/s), that can be delivered from a water distribution system at a specified residual pressure for fire fighting. When delivery is to fire department pumpers, the specified residual pressure is generally 20 psi (138 kPa).

fire pressure  The pressure necessary in water mains when water is used for fire fighting; applied to cases in which the pressure for fire fighting is increased above that normally maintained for general use.

fire-service connection  A pipe extending from a main to supply a sprinkler, standpipe, yard main, or other fire protection system.

fire system  A separate system of water pipes or mains and their appurtenances installed solely to furnish water for extinguishing fires.

first-stage BOD  That part of oxygen demand associated with biochemical oxidation of carbonaceous material. Usually, the greater part of the carbonaceous material is oxidized before the second stage (active oxidation of the nitrogenous material) takes place.

five-day biochemical oxygen demand (BOD5)  A standard test to assess wastewater pollution due to organic substances, measuring the oxygen used under controlled conditions of temperature (20 °C) and time (5 days).

fixed distributor  A distributor consisting of perforated pipes or notched troughs, sloping boards, or sprinkler nozzles that remain stationary when the distributor is operating. See also distributor.

fixed solids  The residue remaining after ignition of suspended or dissolved matter.

flame arrester  (1) A device incorporating a fine-mesh wire screen or tube bundle inserted in a vent or pipe and designed to resist the flashback of flame. (2) Device consisting of a multiple number of corrugated stamped sheets in a gas-tight housing. As a flame passes through the sheets, it is cooled below the ignition point.

flange  A projecting rim, edge, lip, or rib.

flap gate  A gate that opens and closes by rotation around a hinge or hinges at the top side of the gate.

flap valve  A valve that is hinged at one edge and opens and shuts by rotating about the hinges. See also check valve.

flash dryer  A device for vaporizing water from partly dewatered and finely divided sludge through contact with a current of hot gas or superheated vapor. It includes a squirrel-cage mill for separating the sludge cake into fine particles.

flash mixer  A device for uniform, quick dispersal of chemicals throughout a liquid.

flash point  The temperature at which a gas, volatile liquid, or other substance ignites.

flat-crested weir  A weir with a horizontal crest in the direction of flow and of appreciable length when compared with the depth of water passing over it.

flight  A scraper in a rectangular sedimentation tank with blades that move sludge along the bottom of the tank to a collection point. As the flights return, scum is collected on the surface of the tank and pushed to an outlet point.

float control  A float device that is triggered by changing liquid levels that activates, deactivates, or alternates process equipment operation.

float gauge  A device for measuring the elevation of the liquid, the actuating element of which is a buoyant float that rests on the surface of the liquid and rises or falls with it. The elevation of the surface is measured by a chain or tape attached to the float.

floating cover  A gas-tight metal cover floating on the sludge in a digestion tank, with guides to assist in smooth vertical travel as the sludge level changes.

float switch  An electrical switch operated by a float in a tank or reservoir and usually controlling the motor of a pump.

float valve  A valve, such as a plug or gate, that is actuated by a float to control the flow into a tank.

floc  Collections of smaller particles agglomerated into larger, more easily settleable particles through chemical, physical, or biological treatment. See also flocculation.

flocculant  Water-soluble organic polyelectrolytes that are used alone or in conjunction with inorganic coagulants, such as aluminum or iron salts, to agglomerate the solids present to form large, dense floc particles that settle rapidly.

flocculating tank  A tank used for the formation of floc by the gentle agitation of liquid suspensions, with or without the aid of chemicals.

flocculation  In water and wastewater treatment, the agglomeration of colloidal and finely divided suspended matter after coagulation by gentle stirring by either mechanical or hydraulic means. For biological wastewater treatment in which coagulation is not used, agglomeration may be accomplished biologically.

flocculation agent  A coagulating substance that, when added to water, forms a flocculent precipitate that will entrain suspended matter and expedite sedimentation; examples are alum, ferrous sulfate, and lime.

flocculator  (1) A mechanical device to enhance the formation of floc in a liquid. (2) An apparatus for the formation of floc in water and wastewater.

flood flow  The discharge of a stream during periods of flood.

flood frequency  The frequency with which the maximum flood may be expected to occur at a site in any average interval of years. Frequency analysis defines the “n-year flood” as being the flood that will, over a long period of time, be equaled or exceeded on the average once every n years. Thus, the 10-year flood would be expected to occur approximately 100 times in a period of 1 000 years, and of these, 10 would be expected to reach the 100-year magnitude. Sometimes expressed in terms of percentage of probability; for example, a probability of 1% would be 100-year flood; a probability of 10% would be a 10-year flood.

flood-protection works  Structures built to protect lands and property from damage by floods.

flotation  (1) Separation of suspended particles, or oil and grease, from solution by naturally or artificially raising them to the surface, usually with air. (2) Thickening of waste activated sludge by injecting air into it and introducing the mixture into a tank where the air buoys the sludge to the surface.

flow  (1) The movement of a stream of water or other fluid from place to place; the movement of silt, water, sand, or other material. (2) The fluid that is in motion. (3) The quantity or rate of movement of a fluid discharge; the total quantity carried by a stream. (4) To issue forth or discharge. (5) The liquid or amount of liquid per unit time passing a given point.

flow-control valve  A device that controls the rate of flow of a fluid.

flow equalization  Transient storage of wastewater for release to a sewer system or wastewater treatment plant at a controlled rate to provide a reasonably uniform flow for treatment.

flowrate  The volume or mass of a gas, liquid, or solid material that passes through a cross section of conduit in a given time; measured in such units as kilograms per hour (kg/h), cubic meters per second (m3/s), liters per day (L/d), or gallons per day (gpd).


flow recording
  Documentation of the rate of flow of a fluid past a given point. The recording is normally accomplished automatically.

flow regulator  A structure installed in a canal, conduit, or channel to control the flow of water or wastewater at the intake or to control the water level in a canal, channel, or treatment unit. See also rate-of-flow controller.

flow sheet  A diagrammatic representation of the progression of steps in a process showing their sequence and interdependence.

fluidized bed reactor  A pressure vessel or tank that is designed for liquid–solid or gas–solid reaction. The liquid or gas moves upward through the solids particles at a velocity sufficient to suspend the individual particles in the fluid. Applications include ion exchange, granular activated carbon adsorbers, and some types of furnaces, kilns, and biological contactors.

flushing  The flow of water under pressure in a conduit or well to remove clogged material.

foam  (1) A collection of minute bubbles formed on the surface of a liquid by agitation, fermentation, and so on. (2) The frothy substance composed of an aggregation of bubbles on the surface of liquids and created by violent agitation or by the admission of air bubbles to liquid containing surface-active materials, solid particles, or both. Also called froth.

food-to-microorganism (F:M) ratio  In the activated-sludge process, the loading rate expressed as pounds of BOD5 per pound of mixed liquor or mixed liquor volatile suspended solids per day (lb BOD5/d/lb MLSS or MLVSS).

foot valve  (1) A valve placed at the bottom of the suction pipe of a pump that opens to allow water to enter the suction pipe, but closes to prevent water from passing out of it at the bottom end. (2) A valve with the reverse action attached to the drainage pipe of a vacuum chamber. It allows water to drain out, but closes to hold the vacuum.

forced aeration  The bringing about of intimate contact between air and liquid where the air, under pressure, is applied below the surface of the liquid through diffusers or other devices that promote the formation of small bubbles.

force main  A pressure pipe joining the pump discharge at a water or wastewater pumping station with a point of gravity flow.

formazine turbidity unit (FTU)  A standard unit of turbidity based on a known chemical reaction that produces insoluble particulates of uniform size. The FTU has largely replaced the JTU. Also known as nephelometric turbidity unit.

fouling  A gelatinous, slimy accumulation resulting from the activity of organisms in the water. Fouling may be found on concrete, masonry, or metal surfaces, but tuberculation is found only on metal surfaces.

Francis turbine  A reaction turbine of the radial inward-flow type.

free available chlorine  The amount of chlorine available as dissolved gas, hypochlorous acid, or hypochlorite ion that is not combined with an amine or other organic compound.

free available residual chlorine  That portion of the total residual chlorine remaining in water or wastewater at the end of a specified contact period that will react chemically and biologically as hypochlorous acid or hypochlorite ion.

freeboard  The vertical distance between the normal maximum level of the surface of the liquid in a conduit, reservoir, tank, or canal and the top of the sides of an open conduit or the top of a dam or levee, which is provided so that waves and other movements of the liquid will not overflow the confining structure.

free flow  A condition of flow through or over a structure where such flow is not affected by submergence or the existence of tailwater.

free oxygen  Elemental oxygen (O2).

free-swimming ciliate  Mobile, one-celled organisms using cilia (hair-like projections) for movement.

free water  Suspended water constituting films covering the surface of solid particles or the walls of fractures, but in excess of pellicular water; mobile water is free to move in any direction under the pull of the force of gravity and unbalanced film pressure.

frequency  (1) The time rate of vibration or the number of complex cycles per unit time. (2) The number of occurrences of a certain phenomenon in a given time. (3) The number of occasions on which the same numerical measure of a particular quantity has occurred between definite limits. (4) The number of cycles through which an alternating current passes per second. Frequency has been generally standardized in the electrical utility industry in the United States at 60 cycles per second (60 Hz).

fresh-air inlet  A specially constructed opening usually provided with a perforated cover to facilitate ventilation of a wastewater line.

fresh sludge  Sludge in which decomposition is little advanced.

fresh wastewater  Wastewater of recent origin containing dissolved oxygen.

friction factor  A measure of the resistance to flow of fluid in a conduit as influenced by wall roughness.

friction head  The head loss resulting from water flowing in a stream or conduit as the result of the disturbances set up by the contact between the moving water and its containing conduit and by intermolecular friction. In laminar flow, the head loss is approximately proportional to the first power of the velocity; in turbulent flow to a higher power, approximately the square of the velocity. While, strictly speaking, head losses such as those caused by bends, expansions, obstructions, and impact are not included in this term, the usual practice is to include all such head losses under this term.

friction loss  The head loss resulting from water flowing in a stream or conduit as the result of the disturbances set up by the contact between the moving water and its containing conduit and by intermolecular friction. See also friction head.

fungi  Small, nonchlorophyll-bearing plants that lack roots, stems, or leaves; occur (among other places) in water, wastewater, or wastewater effluents; and grow best in the absence of light. Their decomposition may cause disagreeable tastes and odors in water; in some wastewater treatment processes they are helpful and in others they are detrimental.
fuse  A protective device that carries the full current of a circuit. If the current is higher than the fuse rating, it contains a substance that will melt and break the current. Fuses cannot be reset but must be replaced.