An absorption process by which one substance is taken in and retained in the interior rather than on the external surface of another, sometimes occurring by coprecipitation.

Odor Threshold

The minimum odor of a water sample that can just be detected after successive dilutions with odorless water. Also called <i>threshold odor</i>.


Air or vapor given off or expelled as a byproduct or result of an operation or treatment process.

Offset (or Droop)

The difference between the actual value and the desired value (or set point); characteristic of proportional controllers that do not incorporate reset action.

Offstream Uses

Water withdrawn from surface or groundwater sources for use at another place.


The standard unit for measuring resistance to the passage of an electrical current.     One ohm is equal to the resistance created between two points on a conductor when a potential difference of one volt creates a current of one ampere.     An ohm is also equal to the resistance offered by a column of mercury which is 1.0<sup>2</sup>mm in cross sectional area and 106.3 cm long at 0 degrees C.     Electrical resistance in a solution is often related to the electrolyte concentration in the solution.

Oil of Vitriol

An old and now obsolete term once used for sulfuric acid (H,<sub>2</sub>SO<sub>4</sub>).


Having a strong affinity (liking) for oil, and thereby exhibiting the characteristic of adsorbing oily-type substances.     Activated carbon is an oleophilic material. The opposite of oleophobic.


Having a strong aversion (dislike) for oil, and thereby exhibiting the characteristic of repelling oily-type substances.     Water is oleophobic. The opposite of oleophilic.


(H<sub>2</sub>SO<sub>4</sub> + SO<sub>3</sub>)  The Latin word for oil.     Oleum is a solution of sulfuric acid and sulfur trioxide that is a form of fuming sulfuric acid.     The term is used to describe a stage in the production of sulfuric acid.

Olfactory Fatigue

A condition in which a person's nose, after exposure to certain odors, is no longer able to detect the odor.

Oligo-dynamic Action

The bacteriostatic action exerted by very small amounts of heavy metals such as copper, silver, and zinc which deactivates bacteria and creates a hostile environment for the growth of bacterial colonies.


Reservoirs and lakes which are nutrient poor and contain little aquatic plant or animal life.


Study of cancer.

One-Hit Model

Mathematical model based on the biological theory that a single "hit" of some minimum critical amount of a carcinogen at a cellular target namely, DNA can initiate an irreversible series of events, eventually leading to a tumor.


The fertilized egg form of parasitic sporozoa protozoa such as <i>Giardia, Cryptosporidium,</i> and <i>Cyclospora</i> that is encapsulated in a tough shell.     The oocyst is an environmentally resistant transmissible form of certain protozoan parasites that is excreted in the feces of an infected host and carried viably in unfiltered water supplies.


The capacity of matter to block the passage of light or other radiant energy such as heat.     A measure of opacity might be the percentage of light transmission through a plume. Opacity is the opposite of transparency, and an object with a high degree of opacity is said to be opaque.

Operating Cycle

In filtration applications, the complete filtration process consisting of filter service, backwash and rinse, and return to service.    As relates to ion exchange, the cycle of service run, backwash and regeneration, slow rinse, fast rinse, and return to service.

Operating Pressure

The manufacturer's specified range of pressure expressed in pounds per square inch (psi) within which a water processing device or water system is designed to function.     A range of 30 to 100 psi is often indicated.     Also called <i>working pressure</i>.

Operating Temperature

The manufacturer's recommended feedwater or inlet water temperature for a water treatment system.

Operation and Maintenance Costs

The ongoing, repetitive costs of operating a water system; for example, employee wages and costs for treatment chemicals and periodic equipment repairs.

Optimal Corrosion Control Treatment

The corrosion control treatment by a public water system that minimizes the lead and copper concentrations at users' taps while ensuring that the treatment does not cause the water system to violate any national primary drinking water regulations.


Of the mouth; through or by the mouth.


1. Having the characteristics of, or being derived from, a living organism, plant, or animal.     2. Containing carbon (although a few very simple carbon compounds such as the carbon oxides, the carbides, carbon disulfide, and metallic carbonyls and carbonates are considered inorganic).     Over 6,000,000 carbon-containing organic compounds have been identified and named.

Organic Chemical

A chemical having a carbon-hydrogen structure.

Organic Iron

Iron that is bound or complexed with organic compounds, such as naturally occurring humic and fulvic acids.     In waters laden with a high level of these vegetation decay products, iron is sometimes present in an organic form. The decaying process of vegetation produces humic and fulvic acids. Iron will react with these natural complexation or chelating agents to form either an insoluble colloid with humic acid and humin, or a soluble complex chelate with fulvic acid and tannin.     Organic iron can be present in a colorless form, but most often occurs as a yellow, yellowish- brown, or pink color. The humic acid and humin colloids developing the color seem to be permanently suspended in the water due to their particle size, normally less than 0.1 micron.     Organic iron can also be referred to as <i>tannin, heme iron, complexed iron,</i> or <i>pink iron</i>.

Organic Matter

Substances consisting of, or derived from, plant or animal matter as opposed to inorganic matter which is derived from rocks, ores, and minerals.     Organic matter is characterized by its carbon-hydrogen structure.


Term often used to describe any (or all) of the compounds, natural or man-made, with chemical structures based upon carbon.     Examples are hydrocarbons, wood, sugars, proteins, methane, plastics, petroleum-based compounds, solvents, pesticides, herbicides, trihalomethane (THM), and trichlorethylene (TCE).


Any form of animal or plant life.


Affecting perceptions that are stimulated by senses in the eye, ear, skin, nose, or mouth.     Used to describe subjective characteristics such as flavor, odor, color, appearance, and related factors of food and water.


1. An opening, such as a hole or vent in something    2. In water treatment, usually an opening through which water can pass or a restricted opening placed in a pipe line to provide a means of controlling or measuring flow.   For example, a flow controller.

ORP (Oxidation-Reduction Potential)

The electrical potential required to transfer electrons from one compound or element (the oxidant) to another compound or element (the reductant): used as a qualitative measure of the state of oxidation in water treatment systems.


1. A prefix used in inorganic chemistry to mean the most hydrated form of an acid or its salt, in contrast to a less hydrated form which is indicated by the <i>meta-</i> prefix. For example, H>3</sub>PO<sub>4</sub> is orthophosphoric acid; HPO<sub>3</sub> is metaphosphoric acid.    2. In organic chemistry, <i>ortho, meta,</i> and <i>para</i> are terms related to the position of atoms or radicals in the benzene ring of aromatic organic compounds.


1. A salt that contains phosphorus as PO<sub>4</sub><sup>3-</sup>.  2. A product of the hydrolysis of condensed (polymeric) phosphates, i.e., the most hydrated or reverted ("broken down") and stable form of phosphate compounds.  Orthophosphates cannot sequester metal ions and hardness, but they can form passivating films on anodic sites to suppress electrochemical corrosion reactions in metallic plumbing.  3. A nutrient required for plant and animal growth.


Orthotolidine is a colorimetric indicator of chlorine residual.     If chlorine is present, a yellow-colored compound is produced. This reagent is no longer approved for chemical analysis.


The Williams-Steiger Occupational Safety and Health Act of 1970 (OSHA) is a law designed to protect the health and safety of industrial workers and also the operators of water supply systems and treatment plants.     OSHA also refers to the federal and state agencies which administrator the OSHA regulations.


The natural tendency for water to spontaneously pass through a semipermeable membrane separating two solutions of different concentrations (strengths).     The water will naturally pass from the weaker (less concentrated) solution containing fewer particles of dissolved substance to the stronger (more concentrated) solution containing more particles of a dissolved substance. Thus, natural osmosis causes the stronger solution to become more diluted and tends to equalize the strength of the solutions on both sides of the membrane.

Osmotic Pressure

The pressure and potential energy difference which exists between two solutions on either side of a semipermeable membrane because of the tendency of water to flow in osmosis.     Every 100 ppm (mg/L) of TDS generates about one pound per square inch of osmotic pressure.     This osmotic pressure must first be overcome by water pressure for a reverse osmosis membrane to become effective.

Osmotic Stability

An expression of the ability of an ion exchange resin to resist physical degradation due to volume changes (shrinkage and swelling) imposed by repeated applications of dilute and concentrated solutions.

Overall Pump Efficiency

The combined efficiency of a pump and motor together.     Also called the <i>wire-to-water efficiency</i>.


The pumping of water from a groundwater basin or aquifer in excess of the supply flowing into the basin.     This pumping results in a depletion or "mining" of the groundwater in the basin.

Overflow Rate

One of the guidelines for the design of settling tanks and clarifiers in treatment plants.     Used by operators to determine if tanks and clarifiers are hydraulically (flow) over- or underloaded.     Also called surface loading.     Overflow Rate (GPD/sq.ft) = Flow (GPD)/Surface Area (sq.ft)


Operating a filter or ion exchange system beyond its predetermined exhaustion point.     This means the filter or system is unlikely to be as effective as it should be, and it probably needs some regeneration to restore capacity (ion exchange), or a cleansing, backwashing, or media/element replacement to reduce headloss and restore capacity (mechanical, adsorption, or neutralization filter).


The almost spontaneous mixing of all layers of water in a reservoir or lake when the water temperature becomes similar from top to bottom.     This may occur in the fall/winter when the surface waters cools to the same temperature as the bottom waters and also in the spring when the surface waters warm after the ice melts.     Also called <i>turnover</i>.


The process in which a molecule, atom, or ion loses electrons to an oxidant. The oxidized substance (which lost the electrons) increases in positive valence.   Oxidation <u>never</u> occurs alone, but always occurs as part of the oxidation-reduction (redox) reaction. The reduced substance gains electrons and thereby decreases in positive valence.

Oxidation (electrodialysis)

A chemical reaction which occurs at the anode and results in the loss of electrons from the anodic material.

Oxidation (ion exchange)

Specific attack on the cross linking of the co-polymer of ion exchange resins by an oxidant (chlorine, peroxide, ozone, or others) leading to degradation (loss of structure of resin beads) and shortening of the resin life.

Oxidation Pond (waste water treatment)

In wastewater treatment, a man-made lake or body of water within which wastes are consumed by bacteria, and where this biological oxidation process is enhanced by the transfer of oxygen to the water from the air.     Oxidation ponds are most frequently used together with other waste treatment processes.

Oxidation-Reduction (Redox) Reaction

The combination of the processes involved in the flow of electrons from a reducing agent (reducer) to an oxidizing agent (oxidant).   The total number of electrons lost by one substance is the same as the total number of electrons gained by another substance. Oxidation and reduction always occur together simultaneously and are really opposite sides of the same reaction, often called the <i>redox reaction</i>.     In earlier years, oxidation referred to the combining of a substance with, or addition of, oxygen, and reduction meant the loss or reduction of oxygen. However, as chemistry became more advanced, it was seen that the real key to what was happening was the loss or gain of electrons.     Now, the definitions accepted are as follows:     Oxidation is the <u>loss</u> of electrons from the reducing agent (which is said to have "been oxidized" in the process). Since electrons carry negative charges, oxidation results in an increase of positive valence.   Reduction is the acquiring of electrons (the ones lost in the oxidation process) by the oxidizing agent (which is said to have "been reduced" in the process). Because electrons (carrying negative charges) have been acquired, reduction results in a lowering (a reduction) of positive valence.   It may be helpful to remember that the word "agent" refers to an active substance that produces or brings about some effect. Therefore, the oxidizing agent is the substance that brings about oxidation; the reducing agent is the substance that brings about reduction.

Oxidation-Reduction Potential (ORP)

The electrical potential required to transfer electrons from one compound or element (the oxidant) to another compound or element (the reductant); used as a qualitative measure of the state of oxidation in water treatment systems.


To increase a molecule or ion in positive valence; to lose electrons to an oxidizing agent.

Oxidizing Agent

A chemical substance that gains electrons (i.e., is <i>reduced</i>) and brings about the oxidation of other substances in chemical oxidation and reduction (redox) reactions.     Examples of oxidizing agents include oxygen, ozone, chlorine, peroxide.

Oxidizing Filter

A type of filter used to change the valence state of dissolved molecules, making them insoluble and, therefore, filterable.     For example, a filter that oxidizes ferrous iron, manganous manganese, and/or anionic sulfur by use of catalytic media such as manganic oxides and then filters the oxidized precipitates out of the water.


To impregnate or combine with oxygen such as the forced draft step in aeration.     Maximum oxygen exposure.


The process of feeding ozone into a water supply for the purpose of decolorization, deodorization, disinfection, or oxidation.

Ozone Destruction

The step by which a component unit of an ozonation system destroys all or some of the ozone present in the off-gas being vented.

Ozone Enrichment

A step in the ozonation process in which more ozone is added to a gas which previously contained ozone.

Ozone Half-Life

The period of time required for 50 percent of a given quantity of ozone to decompose at a specific temperature and pressure.


A very strong oxidizing agent which is unstable and must be generated on site.     Ozone is a highly reactive form of oxygen and can be produced by sending a high voltage electrical discharge through air or oxygen (such as occurs in a lightning storm). Some degree of ozone can also be produced by certain types of ultraviolet lamps.     Ozone is an excellent oxidizing agent and bactericide.


A compound which occurs as a byproduct of ozonation.


1. The oxidation of an organic material by ozone.    2. The use of ozone as a tool in analytical chemistry to locate double bonds in organic compounds.


A region in the upper atmosphere containing a relatively high concentration of ozone which absorbs certain wavelengths of solar ultraviolet radiation that are not screened out by other substances in the atmosphere.

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Updated on Thu, 02 Jul 2020 by Jonathan

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