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Wastewater Treatment Terms

Wastewater Treatment Terms

Wastewater Treatment Terms

Clear Water Environmental is happy to provide the information you need regarding wastewater treatment plant design. When describing facilities and treatment processes, certain wastewater treatment terms come up. Here are some useful wastewater treatment terms you will hear in descriptions of wastewater treatment systems and wastewater treatment plant design.

Activated Sludge:

“Activated sludge” is a term used for a flocculent culture of organisms that are brown in color and are grown in aeration tanks under controlled conditions. It also refers to the sludge floc that is produced in raw or settled wastewater by the growth of bacteria and other organisms in the presence of dissolved oxygen.

Activated Sludge Process:

A common process of removing pollutants in wastewater. In the activated sludge process, air is injected into the wastewater that contain dissolved organic substances in aeration reactors. Bacteria and other micro-organisms present in the system need oxygen to live, grow, and multiply so they may consume the dissolved organic “food” or pollutants in the wastewater. After several hours in the aeration portion of the wastewater treatment system, the water is separated from the sludge and discharged to further treatment and disposal. A majority of the activated sludge is recycled to the treatment process, while a portion is removed from the treatment process and disposed of.


The process of raising or maintaining the dissolved oxygen level in wastewater. Aeration is typically accomplished using fine bubble diffusers.

Aeration Tank or Aeration Reactor:

A tank where aeration takes place and the conditions are controlled to promote the growth of aerobic bacteria.

Aerobic Bacteria:

Bacteria that requires free oxygen for growth.


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


An environment that is lacking in oxygen, especially molecular oxygen, nitrates and nitrites. The decomposition by microorganisms of waste organic matter in wastewater in the absence of dissolved oxygen is called anaerobic.

Anaerobic Bacteria:

Bacteria that grows in the anaerobic environment by deriving 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.


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 Oxidation:

The process by which micro-organisms such as bacteria, consume dissolved oxygen and organic substances in wastewater, using the energy released to convert organic carbon into carbon dioxide and cellular material.

Biochemical Oxygen Demand (BOD):

A quantitative measurement of the oxygen required 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 (BOD5). 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 pollutant concentration. BOD is typically stated as a concentration prior to and after treatment resulting in a percentage of BOD removal.

Bulking Sludge:

A situation that occurs in activated sludge plants where the sludge inhabits excessive volumes and will not concentrate readily. This condition refers to an inability of the sludge to settle and a resulting loss over the clarifier weir. Bulking sludge in activated sludge aeration tanks is primarily caused by an elevated suspended solids (SS) content. Sludge bulking in the final clarifier of an activated sludge plant may be triggered by improper balance of the BOD loading, SS concentration in the mixed liquor, or the amount of air used in the aeration reactors.

Chemical Oxygen Demand (COD):

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


The application of chlorine to water, sewage, or other wastes, generally for disinfection. Chlorination takes place in a tank called the “chlorine contact chamber” which is sized to provide a predetermined contact period to allow for a thorough disinfection process. Chlorination is typically followed by de-chlorination to neutralize the chlorine in the treated water prior to discharge.

Coliform Organisms:

A bacteria group that are indicators of fecal pollution.

Composite Sample:

Samples for laboratory tests should be representative of the wastewater. The best method of sampling is proportional composite sampling over a period during the day. Composite samples are a better representation because the flow and characteristics of the wastewater are in constant flux. A proper composite sample will yield a more accurate representation of the actual wastewater conditions.


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

Diffused Air:

The primary method of aeration. Air is passed through fine bubble diffusers which increases the surface area of water that comes into direct contact with air bubbles.

Dissolved Oxygen (DO):

The oxygen that is dissolved in water, wastewater, or other liquid. DO is measured in milligrams per liter.

Dissolved Solids:

Solids absorbed in sewage that cannot be removed by filtering or settling.


The water that exits out of a treatment plant or treatment reactor after completion of any treatment process.

Escherichia Coliform:

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

Extended Aeration:

A version of the activated sludge process which provides additional detention time in the aeration portion of the treatment process allowing for aerobic sludge digestion within the aeration system.


The collection of smaller particles in gelatinous mass that can be more readily settles from the wastewater than the individual small particles.


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

Food to Microorganism Ratio (F:M):

The ratio of microorganisms to pollutants in the activated sludge process. By maintaining this ratio at the appropriate level, the biomass will consume high percentages of the food. This minimizes the loss of residual food in the treated effluent. In simple terms, the more the biomass consumes, the lower the biochemical oxygen demand (BOD) will be in the discharge.


In wastewater, a group of substances, including fats, waxes, free fatty acids, calcium and magnesium soaps, mineral oils, and certain other non-fatty materials. Grease is typically an issue with wastewater treatment plants that treat water from restaurants or food preparation facilities.


Heavy, inorganic solids, such as sand or pebbles. Grit is typically removed during the first or primary stages of the wastewater treatment process.


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

Mixed Liquor (MLR):

The mixture of effluent and active biological solids (return sludge) in the activated sludge process that is recycled into the aeration tank(s).

Mixed Liquor Suspended Solids (MLSS):

The concentration of suspended solids in the aeration reactors expressed in mg/L. MLSS is an important part of the activated sludge process to ensure that there is a sufficient quantity of active biomass available to consume the applied quantity of organic pollutant at any time.


The conversion of nitrogen matter into nitrates by bacteria. Typically takes place in an anoxic reactor.


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


Any material consumed by organisms that promotes growth.

Organic Matter:

The waste from homes or other activity originated from plants or animals. Organic matter is the volatile fraction of solids.

Organic Material:

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


A biological environment which is aerobic.


Using bacteria, chemicals, or oxygen for the conversion of organic material to a more stable form.

Oxidation Ponds or Lagoons:

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

pH Value:

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

Potable Water:

Water suitable for drinking by humans.

Primary Wastewater Treatment:

The mechanical separation of trash, solids, grease, grit and scum from wastewater. Primary treatment is accomplished by grit chambers and screens. Proper primary treatment helps to protect the downstream equipment from objects that can foul or damage them.

Raw Wastewater:

Wastewater prior to any treatment.


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

Receiving Waters:

Rivers, lakes, or other water sources where effluent is discharged to.

Return Activated Sludge (RAS):

Return activated sludge or “RAS” is returned continuously to the aeration tanks. Recycling of activated sludge back to the aeration tanks provide bacteria for incoming wastewater. Activated sludge should be brown in color with no obnoxious odor. Settled activated sludge is generally thinner than raw sludge. The treatment plant operator will waste a portion of the plant’s activated sludge to prevent excessive solids build up in the treatment process.

Secondary Wastewater Treatment:

Secondary wastewater treatment follows the primary treatment portion of the facility. The secondary portion of the treatment process involves the aeration reactors where the wastewater continues to be separated from solids. After the aeration reactors, the wastewater will pass to the clarifier for final solids separation. Many plants will include a tertiary treatment process after secondary that involves chlorination or UV treatment.


Clarifiers provide a period of quiescence during which suspended waste material settles to the bottom of the tank and is removed for recycling to the aeration reactors or wasted to the sludge holding tank. During this period, floatable solids (fats, oils) also rise to the surface of the tank and are skimmed off for disposal.


The water supply of a community after it has been contaminated by various uses. 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 possibly ground water, surface water, and storm water. Also see “wastewater”.


The settled and collected suspended solids of sewage. Sludge is deposited in tanks or basins prior to disposal.

Sludge Age:

In the activated sludge process, sludge age refers to the length of time suspended solids undergo aeration. Sludge age is expressed in days. It is computed by dividing the weight of the suspended solids in the aeration tanks by the weight of sludge wasted from the system per day.

Sludge Index (SVI):

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. SVI describes the settling and compacting characteristics of sludge in the treatment process.

Suspended Solids:

Solids that are physically suspended in sewage that can be removed by settling or filtering.

Tertiary Waste Treatment:

After secondary treatment, the clarified effluent may undergo additional aeration and/or other chemical treatment to accomplish the complete destruction of the bacteria remaining following the secondary treatment. This is done with chlorination and often UV treatment.

Total Solids:

The total quantity of solids in solution and suspension.


A term used to describe “murkiness” of water. High turbidity levels will result in cloudiness of the water and potentially inhibit UV treatment.

Waste Activated Sludge (WAS):

The portion of the sludge from the secondary clarifier that is wasted to avoid excessive buildup of solids in the wastewater treatment system.


The water supply of a community after it has been contaminated by various uses. 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 possibly ground water, surface water, and storm water. Wastewater is said to be septic when it is undergoing decomposition. Also see “Sewage”.

Water Pollution:

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