Technical Resources

This Week in Washington


Search All WEF Access Water Knowledge Centers!

You can narrow your search by selecting one or more of the options below:


To find WEFTEC materials please visit WEFTEC.ORG!


A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

Symbols and Acronyms

absorption   (1) Taking up of matter in bulk by other matter, as in dissolving of a gas by a liquid. (2) Penetration of substances into the bulk of the solid or liquid. See also adsorption.

absorption capacity  A measure of the quantity of a soluble substance that can be absorbed by a given quantity of a solid substance.

acclimation  The dynamic response of a system to the addition or deletion of a substance until equilibrium is reached; adjustment to a change in the environment.

accuracy  The absolute nearness to the truth. In physical measurements, it is the degree of agreement between the quantity measured and the actual quantity. It should not be confused with “precision,” which denotes the reproducibility of the measurement.

acid  (1) A substance that tends to lose a proton. (2) A substance that dissolves in water with the formation of hydrogen ions. (3) A substance containing hydrogen which may be replaced by metals to form salts.

acid-forming bacteria  Microorganisms that can metabolize complex organic compounds under anaerobic conditions. This metabolic activity is the first step in the two-step anaerobic fermentation process leading to the production of methane.

acidity  The quantitative capacity of aqueous solutions to neutralize a base; measured by titration with a standard solution of a base to a specified end point; usually expressed as milligrams of equivalent calcium carbonate per liter (mg/L CaCO3); not to be confused with pH. Water does not have to have a low pH to have high acidity.

acre-foot (ac-ft)  A volume of water 1-ft deep and 1 ac in area, or 43 560 cu ft (1 233.5 m3).
activated carbon
  Adsorptive particles or granules usually obtained by heating carbonaceous material in the absence of air or in steam and possessing a high capacity to selectively remove trace and soluble components from solution.

activated carbon adsorption  Removal of soluble components from aqueous solution by contact with highly adsorptive granular or powdered carbon.

activated carbon treatment  Treatment process in which water is brought into contact with highly adsorptive granular or powdered carbon to remove soluble components; process may be applied to raw water, primary effluent, or chemically clarified wastewater for nonspecific removal of organics, or to secondary effluent as a polishing process to remove specific organics.

activated sludge  Sludge particles produced by the growth of organisms in the aeration tank in the presence of dissolved oxygen.

activated-sludge loading  The pounds (kilograms) of BOD in the applied liquid per unit volume of aeration capacity or per pound (kilogram) of activated sludge per day.

activated-sludge process  A biological wastewater treatment process that converts nonsettleable (suspended, dissolved, and colloidal solids) organic materials to a settleable product using aerobic and facultative microorganisms.

adsorption  The adherence of a gas, liquid, or dissolved material to the surface of a solid or liquid. It should not be confused with absorption.

adsorption water  Water held on the surface of solid particles by molecular forces with the emission of heat (heat of wetting).

advanced waste treatment  Any physical, chemical, or biological treatment process used to accomplish a degree of treatment greater than that achieved by secondary treatment.

aerated contact bed  A biological treatment unit consisting of stone, cement-asbestos, or other surfaces supported in an aeration tank, in which air is diffused up and around the surfaces and settled wastewater flows through the tank; also called a contact aerator.

aerated pond  A natural or artificial wastewater treatment pond in which mechanical or diffused air aeration is used to supplement the oxygen supply.

aeration  (1) The bringing about of intimate contact between air and a liquid by one or more of the following methods: (a) spraying the liquid in the air; (b) bubbling air through the liquid; and (c) agitating the liquid to promote surface absorption of air. (2) The supplying of air to confined spaces under nappes, downstream from gates in conduits, and so on, to relieve low pressures and to replenish air entrained and removed from such confined spaces by flowing water. (3) Relief of the effects of cavitation by admitting air to the affected section.

aeration period  (1) The theoretical time, usually expressed in hours, during which mixed liquor is subjected to aeration in an aeration tank while undergoing activated-sludge treatment. It is equal to the volume of the tank divided by the volumetric rate of flow of the wastewater and return sludge. (2) The theoretical time during which water is subjected to aeration.

aeration tank  A tank in which wastewater or other liquid is aerated.

aerator  A device that brings air and a liquid into intimate contact. See diffuser.

aerobic  Requiring, or not destroyed by, the presence of free or dissolved oxygen in an aqueous environment.

aerobic bacteria  Bacteria that require free elemental oxygen to sustain life.

aerobic digestion  The breakdown of suspended and dissolved organic matter in the presence of dissolved oxygen. An extension of the activated-sludge process, waste sludge is stored in an aerated tank where aerobic microorganisms break down the material.

aerobic lagoon  An oxygen-containing lagoon, often equipped with mechanical aerators, in which wastewater is partially stabilized by the metabolic activities of bacteria and algae. Small lagoons (less than 0.5 ac [0.2 ha] and less than 3-ft [0.9-m] deep) may remain aerobic without mechanical aeration. See also anaerobic lagoon.

aerosol  Colloidal particles dispersed in a gas, smoke, or fog.

agglomeration  Coalescence of dispersed suspended matter into larger flocs or particles.

agitator  Mechanical apparatus for mixing or aerating. A device for creating turbulence.

air-bound  Obstructed, as to the free flow of water, because of air entrapped in a high point; used to describe a pipeline or pump in such condition.

air chamber  A closed pipe chamber installed on the discharge line of a reciprocating pump to take up irregularities in hydraulic conditions, induce a uniform flow in suction and discharge lines, and relieve the pump of shocks caused by pulsating flow.

air-chamber pump  A displacement pump equipped with an air chamber in which the air is alternately compressed and expanded by the water displaced by the pump, resulting in the water being discharged at a more even rate.

air diffuser  Devices of varied design that transfer oxygen from air into a liquid.

air diffusion  The transfer of air into a liquid through an oxygen-transfer device. See diffusion.

air gap  The unobstructed vertical distance through the free atmosphere between the lowest opening from any pipe or outlet supplying water to a tank, plumbing fixture, or other device, and the flood-level rim of the receptacle.

air lift  A device for raising liquid by injecting air in and near the bottom of a riser pipe submerged in the liquid to be raised.

air-lift pump  A pump, used largely for lifting water from wells, from which fine pressured air bubbles are discharged into the water at the bottom of the well. The bubbles reduce the density of the water at the bottom, allowing the denser surrounding water to push it up in the discharge pipe to the outlet. Also called an air lift.

air stripping  A technique for removal of volatile substances from a solution; employs the principles of Henry’s Law to transfer volatile pollutants from a solution of high concentration into an air stream of lower concentration. The process ordinarily is designed so that the solution containing the volatile pollutant contacts large volumes of air. The method is used to remove ammonia in advanced waste treatment.

algae  Photosynthetic microscopic plants that contain chlorophyll that float or are suspended in water. They may also be attached to structures, rocks, etc. In high concentrations, algae may deplete dissolved oxygen in receiving waters.

algal assay  An analytical procedure that uses specified nutrients and algal inoculums to identify the limiting algal nutrient in water bodies.

algal bloom  Large masses of microscopic and macroscopic plant life, such as green algae, occurring in bodies of water.

alkali  Generally, any substance that has highly basic properties; used particularly with reference to the soluble salts of sodium, potassium, calcium, and magnesium.

alkaline  The condition of water, wastewater, or soil that contains a sufficient amount of alkali substances to raise the pH above 7.0.

alkalinity  The capacity of water to neutralize acids; a property imparted by carbonates, bicarbonates, hydroxides, and occasionally borates, silicates, and phosphates. It is expressed in milligrams of equivalent calcium carbonate per liter (mg/L CaCO3).

alkyl benzene sulfonate (ABS)  A type of surfactant, or surface active agent, present in synthetic detergents in the United States prior to 1965. ABS was troublesome because of its foam-producing characteristics and resistance to breakdown by biological action. ABS has been replaced in detergents by linear alkyl sulfonate, which is biodegradable.

alum, aluminum sulfate  [Al2(SO4)3 3 18H2O] Used as a coagulant in filtration. Dissolved in water, it hydrolyzes into Al(OH)2 and sulfuric acid (H2SO4). To precipitate the hydroxide, as needed for coagulation, the water must be alkaline.

ambient  Generally refers to the prevailing dynamic environmental conditions in a given area.

ammonia, ammonium  (NH3, NH41) Urea and proteins are degraded into dissolved ammonia and ammonium in raw wastewaters. Typically, raw wastewater contains 30 to 50 mg/L of NH3. Reactions between chlorine and ammonia are important in disinfection.

ammonia nitrogen  The quantity of elemental nitrogen present in the form of ammonia (NH3).

ammoniator  Apparatus used for applying ammonia or ammonium compounds to water.

ammonification  Bacterial decomposition of organic nitrogen to ammonia.

amoeba  A group of simple protozoans, some of which produce diseases such as dysentery in humans.

ampere  The unit of measurement of electrical current. It is proportional to the quantity of electrons flowing through a conductor past a given point in one second and is analogous to cubic feet of water flowing per second. It is the current produced in a circuit by one volt acting through a resistance of one ohm.

amperometric  Pertaining to measurement of electric current flowing or generated, rather than by voltage.

anaerobic  (1) A condition in which free and dissolved oxygen are unavailable. (2) Requiring or not destroyed by the absence of air or free oxygen.

anaerobic bacteria  Bacteria that grow only in the absence of free and dissolved oxygen.

anaerobic digestion  The degradation of concentrated wastewater solids, during which anaerobic bacteria break down the organic material into inert solids, water, carbon dioxide, and methane.

anaerobic lagoon  A wastewater or sludge treatment process that involves retention under anaerobic ­conditions.

anion  A negatively charged ion attracted to the anode under the influence of electrical potential.

anionic flocculant  A polyelectrolyte with a net negative electrical charge.

anoxic  Condition in which oxygen is available in the combined form only; there is no free oxygen. Anoxic sections in an activated-sludge plant may be used for denitrification.

antagonism  Detrimental interaction between two entities. See also synergism.

antichlors  Reagents, such as sulfur dioxide, sodium bisulfite, and sodium thiosulfate, that can be used to remove excess chlorine residuals from water or watery wastes by conversion to an inert salt.

anticorrosion treatment  Treatment to reduce or eliminate corrosion-producing qualities of a water.

appurtenances  Machinery, appliances, or auxiliary structures attached to a main structure enabling it to function, but not considered an integral part of it.

aqueous vapor  The gaseous form of water. See water vapor.

area drain  A drain installed to collect surface or stormwater from an open area of a building.

automatic recording gauge  An automatic instrument for measuring and recording graphically and continuously. Also called register.

automatic sampling  Collecting of samples of prescribed volume over a defined time period by an apparatus designed to operate remotely without direct manual control. See also composite sample.

autothermal thermophilic aerobic digestion  As part of the aerobic digestion process, heat is evolved. In a contained vessel, the sufficient heat is generated to maintain temperatures in the thermophilic range. At higher temperatures, detention time requirements are reduced for a given solids reduction resulting in an end product that is relatively pathogen free.

autotrophic organisms  Organisms including nitrifying bacteria and algae that use carbon dioxide as a source of carbon for cell synthesis. They can consume dissolved nitrates and ammonium salts.

available chlorine  A measure of the total oxidizing power of chlorinated lime, hypochlorites, and other materials used as a source of chlorine as compared with that of elemental chlorine.

average  An arithmetic mean obtained by adding quantities and dividing the sum by the number of ­quantities.

average daily flow  (1) The total quantity of liquid tributary to a point divided by the number of days of flow measurement. (2) In water and wastewater applications, the total flow past a point over a period of time divided by the number of days in that period.

average flow  Arithmetic average of flows measured at a given point.

average velocity  The average velocity of a stream flowing in a channel or conduit at a given cross section or in a given reach. It is equal to the discharge divided by the cross-sectional area of the section or the average cross-sectional area of the reach. Also called mean velocity.

axis  A line about which a figure or a body is symmetrically arranged, or about which such a figure or body rotates.