A glossary of typical terms used in biological wastewater treatment and sewage systems.

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Activated Sludge

Sludge withdrawn from the secondary clarifier in the activated sludge process, consisting of micro-organisms, non-living organic matter, and inorganic materials.

Activated Sludge Process

A common method of disposing of pollutants in biological wastewaters. In the process, large quantities of air are bubbled through wastewaters that contain dissolved organic substances in open aeration tanks. Bacteria and other types of microorganisms present in the system need oxygen to live, grown, and multiply in order to consume the dissolved organic "food" or pollutants in the waste. After several hours in a large holding tank, the water is separated from the sludge of bacteria and discharged from the system. Most of the activated sludge is returned to the treatment process, while the remainder is disposed of by one of several acceptable methods.


The process of bringing about intimate contact between air and a liquid.

Aeration Tank

A chamber for injecting air into water.

Aerobic Bacteria

Bacteria that require free (elementary) oxygen for growth.

Anaerobic Bacteria

Bacteria that grow in the absence of free oxygen and derive oxygen from breaking down complex substances.

Biological Oxidation

The process by which bacteria and other types of microorganisms consume dissolved oxygen and organic substances in biological wasterwater.  The energy released is then used to convert organic carbon into carbon dioxide and cellular material. 

Biochemical Oxygen Demand (BOD)

A quantitative measure of the oxygen needed by bacteria and microorganisms for the biological oxidation of organic wastes in a unit volume wastewater. BOD is generally measured in milligrams per liter (mg/l) of oxygen consumed over a five-day period. Although complete biological decomposition of organic waste requires about 20 days, the five-day BOD is about two-thirds of the total oxygen required and, therefore, is a practical measure of waste concentration. In waste treatment language, BOD is most frequently stated as the percentage removed during treatment, or remaining after treatment.

Chemical Oxygen Demand (COD)

A quantitative measure of the amount of oxygen required to oxidize all organic compounds in a unit volume on wastewater, non-biodegradable as well as the BOD. The COD level can be determined more readily than BOD, but this measurement does not indicate how much of the waste can be decomposed by biological oxidation.


The application of chlorine to water, sewage, or industrial wastes, generally for the purpose of disinfection, but frequently for accomplishing other chemical or biological wasterwater treatment results.


The agglomeration of colloidal or suspended matter brought about by the addition of some chemical to the liquid, by contact, or by other means.

Coliform Organisms

A group of bacteria recognized as indicators of fecal pollution.  (see also escherichia coliform).

Combined Sewer

Carries both sanitary sewage and storm water run-off.

Diffused Air

Method of aeration.


The biochemical decomposition of organic matter that results in the formation of mineral and simpler organic compounds.

Dissolved Air Flotation

Method of removing oil and suspended solids.

Dissolved Solids

Solids physically suspended in sewage that cannot be removed by proper laboratory filtering.


The liquid that comes out of a treatment plant after completion of any treatment process.

Escherichia Coliform

A species of bacteria found in large numbers in the intestinal tract of warm-blooded animals.


The agglomeration of smaller particles in a gelatinous mass that can be more easily removed from the liquid than the individual small particles.


The coming together of coalescing of minute particles in a liquid.


Heavy, inorganic matter, such as sand or pebbles.


Introduction of the presence of pathogenic organisms in potable water supply. This is determined in two ways:

  1. Bacterial Count - Number of bacteria developed under controlled conditions after 25 hours incubation period. In unpolluted waters count is frequently less than 10 per milliliter.
  2. Coliform Index - Escherichia Coli is an organism normally found in the intestinal tract of man and animals but rare elsewhere.  Indicators of this organism family most reliable as index of pollution, purification efficiency and potability of water.

Inorganic Material

Material that will not respond to biological action (sand, cinders, stone). Non-volatile fraction of solids.

Integrated Fixed Film Activated Sludge (IFAS)

A suspended growth system that provides additional biomass within a biological wastewater treatment system to meet more stringent effluent parameters or increased loadings.

Mechanical Aeration

Method of aeration.

Membrane Bioreactor (MBR)

Biological wastewater treatment process where a selected membrane is integrated with a biological process to act as a suspended growth bioreactor.


Microscopic plans and animals such as bacteria, molds, protozoa, algae, and small metazoa.

Mixed Liquor

The combination of primary effluent and active biological solids (return sludge) in the activated sludge process that is fed into the aeration tank.

Mixed Media Gravity Filter

A filter using more than one filtering media (such as coal and sand).

Moving Bed Bioreactor (MBBR)

Method to biologically treat wastewater by circulating moving media in an aerobic sludge environment.


Any substance assimilated by organisms that promotes growth & replacement of cellular constituents.

Organic Matter

The waste from homes or industry of plant or animal origin. Volatile fraction of solids.

Organic Material

Material that can be broken down by bacteria (fats, meats, plant life).


The conversion of organic material to a more stable form using bacteria, chemicals, or oxygen.

Oxidation Ponds or Lagoons

Holding ponds designed to allow the decomposition of organic wastes by aerobic or anaerobic means.

pH Value

A convenient method of expressing small differences in the acidity or alkalinity of solutions. Neutrality = pH 7.1; lower values indicate increasing acidity, higher values indicate increasing alkalinity.

Potable Water

Water fit for human consumption.


Synthetic chemicals used as a coagulant aid.

Primary Waste Treatment

Mechanical separation of solids, grease, and scum from wastewater. With the aid of flocculating agents, primary treatment can eliminate 50 to 65% of the suspended solids. Solids removed by primary treatment may comprise as much as 30 to 40% of the original BOD of the water.

Receiving Waters

Rivers, lakes, or other water sources that receive treated or untreated wastewaters.

Secondary Waste Treatment

Processing by various types of systems that employ aeration and biological oxidation stages to decompose dissolved and colloidal organic contaminants (inorganic plant nutrients may also be partially removed).

Sedimentation Tanks

Provide a period of quiescence during which suspended waste material settles to the bottom of the tank and is scraped into a hopper and pumped out for disposal. During this period, floatable solids (fats, oils) rise to the surface of the tank and are skimmed off into scum pipes for disposal.


Largely the water supply of a community after it has been fouled by various uses. From the standpoint of course, it may be a combination of the liquid or water-carried wastes from residences, business buildings, and institutions, together with those from industrial establishments, and with such ground water, surface water, and storm water as may be present.


A system of pipes used for collecting domestic and industrial waste, as well as storm water run-off. Lateral sewers connect homes and industries to trunk sewers, which channel waste into interceptor sewers for delivery to sewage treatment plants. Sanitary sewers carry only domestic and industrial wastewater. Storm sewers carry only storm water run-off. Combined sewers carry both.


The accumulated suspended solids of sewage deposited in tanks or basins.

Suspended Solids

Solids physically suspended in sewage that can be removed by proper laboratory filtering.

Tertiary Waste Treatment

Following secondary treatment, the clarified effluent may require additional aeration and/or other chemical treatment to destroy bacteria remaining from the secondary treating stage, and to increase the content of dissolved oxygen needed for oxidation of the residual BOD. Tertiary treatment can also be used to remove nitrogen and phosphorous.

Total Solids

The total amount of solids in solution and suspension.

Trickling Filter

An aerobic biological wastewater treatment process used as secondary treatment of sewage. Effluent from the primary clarifier is distributed over a bed of rocks. As the liquid trickles over the rocks, a biological growth on the rocks breaks down the organic matter in the sewage. The effluent is then taken to a clarifier to remove biological matter coming from the filter.


Any finely divided, insoluble impurities that mar the clarity of the water.

Waste Activated Sludge

That portion of sludge from the secondary clarifier in the activated sludge process that is wasted to avoid a buildup of solids in the system.

Waste Treatment Plant

A series of tanks, screens, filters, and other processes by which most pollutants are removed from water.

Water Pollution

A general term signifying the introduction into water of micro-organisms, chemicals, wastes, or sewage which renders the water unfit for its intended use.