Glossary

Acre-Foot - A volume term referring to that amount of liquid, 1 acre in area, 1 foot deep (43,560 cubic feet).

Activated Sludge - Sludge particles produced by the growth of organisms in aeration tanks.

Aerated Pond - A wastewater treatment pond in which mechanical or diffused-air aeration is used to supplement the oxygen supply. Often called an aerated basin of aerated lagoon.

Aeration Tank Concentration (ATC) - The concentration of solids under aeration or another term for MLSS.

Aerobic - A condition characterized by the presence of free dissolved oxygen in the aquatic environment.

Aerobic Bacteria - Bacterial that live and reproduce only in an environment containing oxygen (not chemically combined H2O).

Aerobic Stabilization - The stabilization and conversion of organic matter through metabolism by bacteria in the presence of dissolved oxygen.

Algae - Primitive one- or many-celled plants, usually aquatic, that produce their food by photosynthesis.

Algicide - Any substance or chemical applied to kill or control algal growths.
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Alkalinity - Alkalinity of a water is a measure of its capacity to neutralize acids, in other words, to absorb hydrogen ions without significant pH change. Alkalinity represents the content of carbonates, bicarbonates and hydroxides in water.

Anaerobic Bacteria - Bacteria which grow in the absence of free dissolved oxygen. They obtain their oxygen by chemically breaking down organic compounds which contain combined oxygen.

Anaerobic Decomposition - The breakdown of complex organic matter by bacteria in the absence of dissolved oxygen.

Aquatic Vegetation - That vegetation which will grow in or near water.
Bacteria - A group of single-celled microscopic organisms lacking chlorophyll. They use dissolved organic nutrients (BOD) as a food source.

Benthic Layer - The accumulation of sludge, including silt, minerals, organic matter and cell mass on the bottom of a lagoon.

Biochemical Oxygen Demand - The quantity of oxygen required by bacteria to biologically oxidize organic material under aerobic conditions, usually expressed in mg/L. The organic matter serves as food for the bacteria and energy is released to the cell during its oxidation.

Blanket Thickness (BLT) - The measurement from the bottom of the clarifier to the top of the sludge blanket, usually determined with a core sampler or electronic interface detector.

Bulking - A condition in which biological solids do not concentrate and will not settle well. In activated sludge, this is characterized by high (>150) SVI. ASB’s are less prone to bulking, though highly loaded basins can experience settling problems (bulking).

CO2 - The chemical formula for carbon dioxide. This gas is produced during bacterial respiration.

Chemical Oxygen Demand (COD) - The amount of oxygen required for the chemical oxidation of organic material using chemicals as oxidants, usually expressed in mg/L.

Clarifier - Tank in which suspended solids are allowed to separate from water by gravity settling.

Clarifier Solids Detention Time (CSDT) - The time sludge remains in a primary clarifier as defined by sludge inventory divided by removal rate.


Coliforms - A group of bacteria that inhabit the intestinal tract of man and warm blooded animals and may be found in plants, soil and air and the aquatic environment. Coliform bacteria are used as an indication that disease-causing organisms may be present

Conductivity - The conductivity of a solution is a measure of its ability to conduct an electrical current. It is a property attributable to the ions in solution, and conductivity increases as ion concentration increases.

Detention Time - The time required to fill a tank at a given flow or the theoretical time required for a given flow of wastewater to pass through a tank or basin.
Denitrification - The reduction of nitrates to nitrites and nitrogen gas.

Depth of Blanket (DOB) - The distance from the clarifier water surface to the top of the sludge blanket. When subtracted from the total clarifier depth, this should equal the blanket thickness (BLT).

Dissolved Oxygen (DO) - Dissolved molecular oxygen usually expressed in mg/L, ppm, or percent saturation.

Disinfection - A process designed to kill most organisms in wastewater.

Diurnal - Having a daily cycle.

Effluent - A liquid flowing out of a chamber, treatment unit, or basin.

Facultative Bacteria - Bacteria that metabolize under aerobic or anaerobic conditions. They can utilize dissolved or combined oxygen, such as nitrate.

Field Transfer Rate (FTR) - The actual oxygen transfer rate applied in the field.
Flocculation - Gathering together of fine particles to form larger particles that are heavier and more easily settled.

Food-to-Microorganism Ratio (F/M) -A mathematical calculation of the mass of food, measured as BOD, divided by the mass of solids under aeration, or MLVSS.

Fungi - Simple or complex organisms without chlorophyll. The simpler forms are one-celled; higher forms have branched filaments and complicated life cycles. Examples are molds, yeasts, and mushrooms.

Grab Sample - A single sample not necessarily taken at a set time or flow. An instantaneous sample.

H2S - The chemical formula for hydrogen sulfide. This gas is produced during the anaerobic fermentation of effluent containing sulfur compounds, such as sulfate (SO4=) and sulfite (SO3-). It has a "rotten egg" odor.

Hydraulic Loading - The volume of flow per day per unit area.

Influent - The liquid entering a process unit or operation.

Inorganic Matter - Chemical substances of mineral origin. Generally, not containing carbon.

Milli - An expression used to indicate 1/1000 of a standard unit of weight, length or capacity (metric system).
milliliter (mL) = 1/1000 liter (L)
milligram (mg) = 1/1000 gram (g)
millimeter (mm) = 1/1000 meter (m)

Milligrams per Liter (mg/L) - A unit of concentration on weight/volume basis, milligrams per liter. Equivalent to ppm when speaking of water or wastewater.

Mixed Liquor Suspended Solids (MLSS) - The mass of solids contained in the mixture in an aeration tank, determined by laboratory analysis in which solids are separated by filtration and expressed in mg/L.

Mixed Liquor Volatile Suspended Solids (MLVSS) - The volatile or organic portion of the mixed liquor suspended solids. Determined by laboratory analysis in which mixed liquor suspended solids are ignited at 600°C. The portion that burns away is "volatile."

Nitrification - The biochemical conversion of ammonia to nitrate.

Nutrients - Substances which are required to support living plants and organisms. Generally refers to nitrogen and phosphorus in the context of wastewater treatment. Nitrogen and phosphorus are commonly fed to mill wastewater to enhance its treatability. A theoretical nutrient balance of 100:5:1 (C:N:P) is required for efficient biological oxidation of BOD.

Organic - Substances of plant or animal origin. All organic compounds contain carbon. In relation to wastewater treatment, the term "organic" usually refers to biodegradable compounds, as measured by the BOD test.

Organic Loading - The number of pounds of BOD added to treatment unit per day.

Oxygen Available - That part of the oxygen available for aerobic stabilization of organic matter. Includes dissolved oxygen and that available in nitrites or nitrates, peroxides, ozone, and certain other forms of oxygen.

Oxygen Depletion - The loss of oxygen from water or wastewater due to biological, chemical, or physical action.

Pathogenic Organisms - Disease causing organisms (typhoid, cholera, dysentery).

pH - Expresses the intensity of acidity or alkalinity of a liquid. The pH scale is logarithmic and ranges from 0-14. Neutral pH is 7, acid pH is below 7, alkaline pH is above 7. Technically, it is the logarithm of the reciprocal of the hydrogen ion concentration.

Photosynthesis - A process in which chlorophyll-containing plants produce complex organic (living) materials from carbon dioxide, water, and inorganic salts, with sunlight as the source of energy. Oxygen is produced in this process as a waste product.

Population Equivalent – For municipal wastewater treatment facilities, the calculated population, which would normally contribute the same amount of biochemical oxygen demand as the wastewater. A common base is 0.2 lb. (0.09 kg) of 5-day BOD per capita per day.

Primary Treatment - A process in wastewater treatment which allows those substances that readily settle or float to be separated.

Respiration - The biological activity of utilizing oxygen and releasing carbon dioxide.

Settlometer - A wide-mouthed graduated cylinder used to evaluate settling properties of activated sludge.

Settled Sludge Concentration (SSC) - The concentration of settled sludge based on the volume occupied in the settlometer.

Settled Sludge Volume (SSV) - The volume occupied by activated sludge in a settlometer.

Sludge Concentration Curves - Plots of the SSC data calculated from the settlometer test data.

Secondary Treatment - Process that biologically Both activated sludge systems and aerated stabilization basins (ASBs) are considered secondary treatment systems. Technically, a treatment process that removes > 85% of the BOD5.

Settleable Solids - Those solids which will settle out when a sample of wastewater is allowed to stand quietly for a one-hour period. Measured in an Imhoff cone, expressed in mL/L.

Short-Circuiting - The hydraulic conditions in a tank, chamber, or basin where time of passage is less than that of the theoretical detention time (based on volume vs. flow.)

Sludge Banks - The accumulation of solids including silt, mineral, organic, and cell mass material that is produced in an aquatic system. Also called the benthic layer.

Sludge Settling Curves - Plots of the SSV data generated from the settlometer test
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Sludge Residence Time (SRT) - Another term for MCRT, expressing the length of time bacteria spend in the secondary system.

Sludge Volume Index (SVI) - The volume in milliliters occupied by one gram of activated sludge after 30 minutes of settling. This is used as a measurement of sludge quality.

Stabilization - The process of reducing a material using a biological and chemical means to a form that does not readily decompose.

Standard Methods - Methods of analysis prescribed by joint action of the American Public Health Association (APHA), American Water Works Association (AWWA), and Water Environment Federation (WEF).

Supersaturation - The situation in which water holds more oxygen at a specified temperature than normally required for saturation.

Suspended Solids - The concentration of insoluble materials suspended or dispersed in water or wastewater. Generally expressed in mg/L on a dry weight basis and determined by filtration methods. Also called total suspended solids (TSS).

Temperature - The degree of hotness or coldness as measured by a thermometer. Temperature affects the growth rate of microorganisms in the treatment system.

Total Solids - The solids in water that included suspended solids (filterable) and dissolved solids (non-filterable). Determined on weighing after drying at 103ø Celsius.

Toxicity LC50 - The concentration lethal to 50% of the test organisms during a specified time. This is usually expressed as a dilution percentage of the whole effluent. An LC50 of 100% means that an undiluted (100%) effluent does not kill more than 50% of the test species.

Ultimate Compaction - Expression used to describe when no further settling occurs and the sludge concentration is at its maximum.

Volatile Solids - The quantity of solids in water that represent a loss in weight upon ignition at 550ø Celsius.