Wastewater Terms and Glossary Wastewater Treatment Glossary Definitions

Ecologix is a rich resource of wastewater treatment knowledge. Contact us and we'd be glad to discuss any of the terms provided below.

Activated Sludge:

The term "activated sludge" refers to a brownish flocculent culture of organisms developed in aeration tanks under controlled conditions. It is also Sludge floc produced in raw or settled waste water by the growth of zoological bacteria and other organisms in the presence of dissolved oxygen. Activated sludge is normally brown in color.

Activated Sludge Process:

A common method of disposing of pollution in 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 micro-organisms present in the system need oxygen to live, grow, 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 or method of bringing about intimate contact between air and a liquid.

Aeration Tank:

A chamber for injecting air into water.

Aerobic Bacteria:

Bacteria that requires free (elementary) oxygen for growth.


The capacity of water to neutralize acids, a property imparted by the water's content of carbonates, bicarbonates, hydroxides, and occasionally borates, silicates, and phosphates. Alkaline fluids have a pH value over 7.


A biological environment that is deficient in all forms of oxygen, especially molecular oxygen, nitrates and nitrites. The decomposition by microorganisms of waste organic matter in wastewater in the absence of dissolved oxygen is classed as anaerobic.

Anaerobic Bacteria:

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


A biological environment that is deficient in molecular oxygen, but may contain chemically bound oxygen, such as nitrates and nitrites.


Bacteria are microscopic living organisms They are a group of universally distributed, rigid, essentially unicellular, microscopic organisms lacking chlorophyll. They are characterized as spheroids, rod-like, or curved entities, but occasionally appearing as sheets, chains, or branched filaments.

Biological Oxidaiton:

The process by which bacteria and other types of micro-organisms consume dissolved oxygen and organic substances in waster water, using the energy released to convert organic carbon into carbon dioxide and cellular material.

Biochemical Oxygen Demand (BOD):

A quantitative measure of the oxygen needed by bacteria and micro-organisms for the biological oxidation of organic wastes in a unit volume of waste water. 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.

Bulking Sludge:

A phenomenon that occurs in activated sludge plants whereby the sludge occupies excessive volumes and will not concentrate readily. This condition refers to a decrease in the ability of the sludge to settle and consequent loss over the settling tank weir. Bulking in activated sludge aeration tanks is caused mainly by excess suspended solids (SS) content. Sludge bulking in the final settling tank of an activated sludge plant may be caused by improper balance of the BOD load, SS concentration in the mixed liquor, or the amount of air used in aeration.

Chemical Oxygen Demand (COD):

A quantitative measure of the amount of oxygen required to oxidize all organic compounds in a unit volume on waste water - 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 biological or chemical 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.

Composite Sample:

To have significant meaning, samples for laboratory tests on wastewater should be representative of the wastewater. The best method of sampling is proportional composite sampling over several hours during the day. Composite samples are collected because the flow and characteristics of the wastewater are continually changing. A composite sample will give a representative analysis of the wastewater conditions.


A biological process by which nitrate is converted to nitrogen gas.

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 Oxygen (DO):

The oxygen dissolved in water, wastewater, or other liquid. DO is measured in milligrams per liter. If the DO of a sample of water is 2 mg/L, it means that there are 2lbs of oxygen in 1 mil lb of water.

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.

Extended Aeration:

A modification of the activated sludge process which provides for aerobic sludge digestion within the aeration system.


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


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


In wastewater, a group of substances, including fats, waxes, free fatty acids, calcium and magnesium soaps, mineral oils, and certain other non-fatty materials.


Heavy, inorganic matter, such as sand or pebbles.

Inorganic Material:

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


Introduction of presence of pathogenic organisms in potable water supply.


Microscopic plants 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 Liquor Suspended Solids (MLSS):

The milligrams of suspended solids per liter of mixed liquor that are combustible at 550 degrees Centigrade. An estimate of the quantity of MLSS to be wasted from the aeration tank of an extended aeration plant may be determined by the rate of settling and centrifuge tests on the sludge solids.

Mixed Media Gravity Filter:

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


The conversion of nitrogen matter into nitrates by bacteria.


Nitrogen is present in wastewater in many forms: total Kjeldahl nitrogen, ammonia nitrogen, organic nitrogen.

Nitrogen Cycle:

The cycle of life, death, and decay involving organic nitrogenous matter is known as the nitrogen cycle. In the nitrogen cycle ammonia is produced from proteins.


Any substance assimilated by organisms that promotes growth and 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).


A simple compound of phosphorous and oxygen that is soluble in water.


A biological environment which is aerobic.


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.


A large compound formed of several orthophosphate molecules connected by phosphate-storing microorganisms.

Primary Waste Treatment:

Mechanical separation of solids, grease, and scum from waste-water. With the aid of flocculating agents, primary treatment can eliminate 50% t 65% othe suspended solids. Solids removed by the primary treatment may comprise as much as 30% t40% othe original BOD of the water.

Raw Wastewater:

Wastewater before it receives any treatment.


A tank where a wastewater stream is mixed with bacterial sludge and biochemical reactions occur.

Receiving Waters:

Rivers, lakes, or other water sources that recieve treated or untreated waters.

Return Sludge:

Settled activated sludge returned to mix with incoming raw or primary settled wastewater. When the return sludge rate in the activated sludge process is too low, there will be insufficient organisms to meet the waste load entering the aerator.

Return Activated Sludge:

Activated return sludge is normally returned continuously to the aeration tank. Recycling of activated sludge back to the aeration tank provides bacteria for incoming wastewater. Its should be brown in color with no obnoxious odor and is often also returned in small portions to the primary settling tanks to aid sedimentation. Settled activated sludge is generally thinner than raw sludge. Some activated sludge will be wasted to prevent excessive solids build up.

Secondary Waste Treatment:

Secondary treatment is part of the primary treatment in that the wastewater continues from the equalization tank and sludge holding zone where it loses the most solids. From the primary stages it passes to the aeration zone where it continues to be broken down and separated from any solids. After aeration the wastewater will pass to the clarifier and disinfection zones. Some plants will include a tertiary treatment that typically involves clorination or UV treatment.

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 te 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 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.

Sludge Age:

In the activated sludge process, a measure of the length of time a particle of suspended solids has been undergoing aeration, expressed in day. It is usually computed by dividing the weight of the suspended solids in the aeration tank by the weight of excess activated sludge discharged from the system per day.

Sludge Digestion:

The purpose of sludge digestion is to separate the liquid from the solids to facilitate drying. The proper pH range for digested sludge is 6.8 - 7.2.

Sludge Index:

Properly called sludge volume index (SVI). It is the volume in millimeters occupied by 1 g of activated sludge after settling of the aerated liquid for 30 minutes.

Sludge Reaeration:

The continuous aeration of sludge after initial aeration for the purpose of improving or maintaining its condition.

Splitter Box:

A division box that splits the incoming flow into two or more streams. A device for splitting and directing discharge from the head box to two separate points of application.

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. This is done with clorination and often times UV treatment.

Total Solids:

The total amount of solids in solution and suspension.

Trickling Filter:

An aerobic biological 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 mark 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 Sludge:

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


Domestic wastewater is 99.9% water and 0.1% solids. Fresh wastewater is usually slightly alkaline. If the pH of the raw wastewater is 8.0, it indicates that the sample is alkaline. If wastewater has a pH value of 6.5, it means that it is acid. Wastewater is said to be septic when it is undergoing decomposition.

Water Pollution:

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