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

odor control   Prevention or reduction of objectionable odors by chlorination, aeration, or other processes, or by masking with chemical aerosols.

odor threshold  The point at which, after successive dilutions with odorless water or air, the odor of a sample can barely be detected. The threshold odor is expressed quantitatively by the number of times the sample is diluted with odorless water or air.

off-peak power  That part of the available load or energy that can be produced at off-peak hours outside the load curve when the combined primary and secondary load has fallen below plant capacity.

ohm  The unit of measurement of electrical resistance. It is that resistance through which an electromotive force of one volt will produce a current of one ampere.

oil separation  (1) Removal of insoluble oils and floating grease from municipal wastewater. (2) Removal of soluble or emulsified oils from industrial wastewater.

open centrifugal pump  A centrifugal pump in which the impeller is built with a set of independent vanes.

open channel  Any natural or artificial water conduit in which water flows with a free surface.

open-channel flow  Flow of a fluid with its surface exposed to the atmosphere. The conduit may be an open channel or a closed conduit flowing partly full.

open impeller  An impeller without attached side walls.

operators  (1) Persons employed to operate a treatment facility. (2) Mechanism used to manipulate valve positions.

organic  Refers to volatile, combustible, and sometimes biodegradable chemical compounds containing carbon atoms (carbonaceous) bonded together with other elements. The principal groups of organic substances found in wastewater are proteins, carbohydrates, and fats and oils. See also inorganic.

organic loading  The amount of organic material, usually measured as BOD5, applied to a given treatment process; expressed as weight per unit time per unit surface area or per unit weight.

organic nitrogen  Nitrogen chemically bound in organic molecules such as proteins, amines, and amino acids.

orifice  (1) An opening with a closed perimeter, usually of regular form, in a plate, wall, or partition through which water may flow; generally used for the purpose of measurement or control of such water. The edge may be sharp or of another configuration. (2) The end of a small tube such as a pitot tube or piezometer.

orifice plate  A plate containing an orifice. In pipes, the plate is usually inserted between a pair of flanges and the orifice is smaller in area than the cross section of the pipe.

orthophosphate  (1) A salt that contains phosphorus as (PO4)23. (2) A product of hydrolysis of condensed (polymeric) phosphates. (3) A nutrient required for plant and animal growth. See also nutrient, phosphorus removal.

osmosis  The process of diffusion of a solvent through a semipermeable membrane from a solution of lower concentration to one of higher concentration.

outfall  (1) The point, location, or structure where wastewater or drainage discharges from a sewer, drain, or other conduit. (2) The conduit leading to the ultimate disposal area.

outlet  A point on the wiring system at which the current is taken to supply utilization equipment.

overflow rate  One of the criteria in the design of settling tanks for treatment plants; expressed as the settling velocity of particles that are removed in an ideal basin if they enter at the surface. It is expressed as a volume of flow per unit water surface area.

overflow weir  Any device or structure over which any excess water or wastewater beyond the capacity of the conduit or container is allowed to flow or waste.

overland flow  (1) The flow of water over the ground before it enters some defined channel. (2) A type of wastewater irrigation.

overturn  The phenomenon of vertical circulation that occurs in large bodies of water because of the increase in density of water above and below 39.2 °F (4 °C). In the spring, as the surface of the water warms above the freezing point, the water increases in density and tends to sink, producing vertical currents; in the fall, as the surface water becomes colder, it also tends to sink. Wind may also create such vertical currents.

oxidant  A chemical substance capable of promoting oxidation, for example, O2, O3, and Cl2. See also oxidation, reduction.

oxidation  (1) A chemical reaction in which the oxidation number (valence) of an element increases because of the loss of one or more electrons by that element. Oxidation of an element is accompanied by simultaneous reduction of the other reactant. See also reduction. (2) The conversion or organic materials to simpler, more stable forms with the release of energy. This may be accomplished by chemical or biological means. (3) The addition of oxygen to a compound.

oxidation ditch  A secondary wastewater treatment facility that uses an oval channel with a rotor placed across it to provide aeration and circulation. The screened wastewater in the ditch is aerated by the rotor and circulated at approximately 1 to 2 ft/sec (0.3 m/s). See also secondary treatment.

oxidation pond  A relatively shallow body of wastewater contained in an earthen basin of controlled shape in which biological oxidation of organic matter is effected by natural or artificially accelerated transfer of oxygen.

oxidation process  Any method of wastewater treatment for the oxidation of the putrescible organic matter.

oxidation–reduction potential (ORP)  The potential required to transfer electrons from the oxidant to the reductant and used as a qualitative measure of the state of oxidation in wastewater treatment systems.

oxidized sludge  Sludge in which the organic matter has been stabilized by chemical or biological oxidation.

oxidized wastewater  Wastewater in which the organic matter has been stabilized.

oxygen (O)  A necessary chemical element. Typically found as O2 and used in biological oxidation. It constitutes approximately 20% of the atmosphere.

oxygenation capacity  In treatment processes, a measure of the ability of an aerator to supply oxygen to a liquid.

oxygen consumed  A measure of the oxygen-consuming capability of inorganic and organic matter present in water or wastewater. See also chemical oxygen demand.

oxygen deficiency  (1) The additional quantity of oxygen required to satisfy the oxygen requirement in a given liquid; usually expressed in milligrams per liter (mg/L). (2) Lack of oxygen.

oxygen transfer  (1) Exchange of oxygen between a gaseous and a liquid phase. (2) The amount of oxygen absorbed by a liquid compared to the amount fed into the liquid through an aeration or oxygenation device; usually expressed as percent.

oxygen uptake rate  The oxygen used during biochemical oxidation, typically ex­pressed as mg O2/L/h in the activated sludge process.

oxygen utilization  (1) The portion of oxygen effectively used to support aerobic treatment processes. (2) The oxygen used to support combustion in the degradation of sludge by incineration or wet-air oxidation.

ozonation  The process of contacting water, wastewater, or air with ozone for purposes of disinfection, oxidation, or odor control.
ozone (O3)  Oxygen in a molecular form with three atoms of oxygen forming each molecule.