Activated-sludge process: Wastewater treatment process that uses activated sludge to biologically convert non-settleable (suspended, dissolved, and colloidal) organic materials to a settleable product using aerobic and facultative microorganisms; typically followed by clarification and sludge return. Active aeration: See aeration, active. Additive: Product added to a sewage treatment system marketed to improve performance.


  • active: Introduction of air via either mechanical means or diffused aeration; see also aeration, passive. Aeration, diffused: Process of introducing air bubbles under pressure into a treatment unit using a compressor or blower and a diffuser.
  • mechanical: Process of introducing air into a treatment component by physical agitation using a device such as a paddle, paddle wheel, spray nozzle or turbine.
  • passive: Process of introducing air into a treatment component without mechanical means; see also aeration, active. Aeration chamber: Chamber or tank in which wastewater is brought into contact with air to facilitate biological degradation such as in (but not limited to) the activated sludge process.

Aerobic: Having molecular oxygen (O2) as a part of the environment, or a biological process that occurs only in the presence of molecular oxygen; See also anaerobic and anoxic.

Anaerobic: Absence of molecular oxygen (O2) as a part of the environment, or a biological process that occurs in the absence of molecular oxygen; bound oxygen is present in other molecules, such as nitrate (NO3 – ) sulfate (SO4 +) and carbon dioxide CO2; See also aerobic and anoxic.

Anoxic: condition in which all constituents are in their reduced form (no oxidants present); conditions in a septic tank are generally anaerobic, but not anoxic; see also aerobic and anaerobic.

Baffle: Physical barrier placed in a component to dissipate energy, direct flow, retain solids and FOG, and/or draw water from a specific depth.

Bed: Below-grade soil treatment area consisting of an excavation greater than three feet wide containing distribution media and more than one lateral; typically installed in an excavation 18 to 36 inches below original ground elevation; utilizes pressure or gravity distribution; a final cover of suitable soil stabilizes the completed installation, supports vegetative growth, and sheds runoff; see also trench.

Biochemical oxygen demand (BOD): Amount of oxygen required by bacteria while stabilizing, digesting, or treating wastewater under aerobic conditions; an indirect measure of the amount of organic matter in wastewater; a measure of the relative strength of wastewater expressed in mg/L.

Biofilter: Media filter in which the media used is biological in origin (i.e., peat or coir).

Biosolids: Dewatered, primarily nutrient-rich organic material generated as a byproduct of biological wastewater treatment processes that can be recycled (such as for use as a soil amendment); see also residuals and septage.

Blower, air: Device that uses a fan to deliver air to a component; does not substantially compress air.

Blackwater: Portion of the wastewater stream that originates from toilet fixtures, dishwashers, and food preparation sinks; see also graywater.

Detention time: Average length of time a unit volume of wastewater or a suspended particle remains in a tank or chamber; mathematically, it is the volume of water in the tank divided by the flow rate through the tank (assuming ideal hydraulic conditions).

Disinfection: Process used to destroy or inactivate pathogenic microorganisms in wastewater to render them non-infectious.

  • chlorine: Process used to inactivate microorganisms by the addition of chlorine in the form of sodium hypochlorite; see also chlorinator and chlorine.
  • ozone: Process used to inactivate microorganisms via the application of ozone to wastewater.
  • ultraviolet (UV): Process used to inactivate microorganisms by irradiating them with ultraviolet light to disrupt their metabolic activity, thus rendering them incapable of reproduction

Dispersal: Spreading of effluent over and into the final receiving environment.

Effluent: Liquid flowing out of a component or device.

Emerging contaminants: Newly identified compounds or substances that have the potential to adversely affect public health or the environment and for which there is no currently published health standard.

FOG (fats, oils, and grease): Constituent of sewage typically originating fromfoodstuffs (animal fats or vegetable oils) or consisting of compounds of alcohol or glycerol with fatty acids (soaps and lotions), typically measured in mg/L.

Filter, activated carbon: Device filled with a porous form of carbon that is used to decolorize liquids, recover solvents, and remove toxins and odors from water and air.

Filter, media: Device that uses materials designed to treat effluent by reducing BOD and/or removing suspended solids in an unsaturated environment; biological treatment is facilitated via microbial growth on the surface of the media.

Filter, sand: Media filter which uses sand of particular specifications as the media.

Filter, trickling: Type of media filter which uses a variety of media such as rigid plastics of varying shapes, stone, or tire chips; includes a clarifier in its configuration and may include a recirculation mode.

Filtration: Removal of suspended materials using processes such as sieving, stagnation, adsorption, absorption, and possibly biochemical degradation. Final cover: Soil with characteristics suitable for stabilizing the surface of system components, supporting vegetative growth and (in some cases) facilitating gas exchange.

Final treatment and dispersal: Last treatment component (or combination of components) through which effluent is returned to the hydrologic cycle via a soil treatment area or a discharging outfall.

Gravel: Rounded or subrounded rock fragment that is between 0.1 inch (2 millimeters) and 3 inches (76 millimeters) in diameter.

Greywater: Water captured from non-food preparation sinks, showers, baths, spa baths, clothes washing machines, and laundry tubs; see also blackwater.

Infiltration: 1. Entry of water or effluent into the soil; 2. Undesirable inflow or seepage of water into a system component; for example, infiltration of surface water into a tank through a leaking pipe, pipe penetration, or through an access riser/tank seam that is not water-tight.

Lagoon: Constructed basin lined with either soils with very low permeability or a synthetic material, surrounded with berms and which contains at least three feet of wastewater which utilizes sunlight, wind or mechanical aeration, and natural bacteria to break down waste via physical, chemical, and biological processes. Lagoon, evaporation: Lagoon where wastewater is stored and the water is allowed to evaporate over time. Lagoon, storage: Lagoon where some form of wastewater is stored before it is either conveyed to another component for further processing or is reused. Land application: Process in which biosolids or liquid waste treatment residuals are spread over, sprayed onto, or injected into the soil.

Lifecycle cost: Total cost of a system over its design period including capital costs and ongoing operation and maintenance costs; expressed as a total present value or a monthly value over the expected life; costs in future years are discounted to the present.

Mound: Above-grade soil treatment area designed and installed with at least 12 inches of clean sand (ASTM C-33) between the bottom of the infiltrative surface and the original ground elevation; utilizes pressure distribution; a final cover of suitable soil material stabilizes the surface and supports vegetative growth.

Nitrification: Biological oxidation of ammonium (NH4 +) to nitrite (NO2 – ) and nitrate (NO3 – ), or a biologically induced increase in the oxidation state of nitrogen.

Non-potable: Water that is not known to be safe to drink because it may either contain pollutants, contaminants, minerals, or infectious agents or may contain harmful constituents due to it not being a “permitted” source of drinking water.

Performance standard, operation-based: Specific, measurable, and enforceable standard that establishes minimum frequency of and requirements for operation and maintenance activities and reporting the operational status of a system.

Pharmaceutical and personal care products (PPCP): Chemical substances such as prescription or over-the-counter therapeutic drugs, fragrances, cosmetic, sunscreen agents, diagnostic agents, among others.

Porosity: 1. Open space or interstices in rock, other earth materials or synthetic media; 2. Ratio of the open space to the total volume often described as a percentage.

Potable water: Water that is safe for human consumption; presumed to meet safe drinking water standards.

Pretreatment: Any component or combination of components that provides treatment of wastewater prior to conveyance to a final treatment and dispersal component or reuse; often, this treatment is designed to meet primary, secondary, tertiary, and/or disinfection treatment standards.

Processing Tank: Term applied to a septic tank when it is configured to receive a combination of raw sewage and recirculated effluent in order to enhance nitrogen removal.

Reactor: Container or tank in which controlled chemical and biological reactions used for the treatment of wastewater are carried out.

Recirculating tank: Dosing tank that mixes effluent from two or more components within the treatment train and allows a portion of partially treated effluent to pass through one or more treatment components again.

Septic tank: Water-tight, covered receptacle for treatment of sewage; receives the discharge of sewage from a building, separates settleable and floating solids from the liquid, digests organic matter by anaerobic bacterial action, stores digested solids through a period of detention, allows clarified liquids to discharge for additional treatment and final dispersal, and attenuates flows.

Settling: Process of subsidence and deposition of suspended matter carried by a liquid; typically accomplished by reducing the velocity of the liquid below the point at which it can transport the suspended material.

Sewage collection system: System of piping, lift stations, and other appurtenances that receives and conveys wastewater either by gravity or pressure.

Trace organic contaminants (TOrCs): Organic compounds originating from residential and non-residential sources, such as ingredients in drugs, pesticides, consumer products, and industrial process agents (usually present in concentrations much lower than one mg/L) which may have adverse ecological and/or human health effects.

Treatment: Method, technique, or process designed to remove solids and/or pollutants from wastewater.

  • Treatment, aerobic: Digestion of organic matter in an environment containing molecular (or dissolved) oxygen (O2). Treatment, advanced secondary: Level of treatment that achieves 95% reduction in BOD and TSS, generally to levels below 10 mg/L.
  • Treatment, anaerobic: Digestion of organic matter in an environment without molecular (or dissolved) oxygen (O2).
  • Treatment, biological: Process involving the metabolic activities of bacteria and other microorganisms in the breakdown of complex organic materials into simpler, more stable substances.
  • Treatment, chemical: Process involving the addition of chemicals to obtain a desired result, such as precipitation, coagulation, flocculation, pH adjustment, disinfection, or sludge conditioning.
  • Treatment, physical: Treatment which involves only physical means of solid-liquid separation, such as filtration, flotation, and sedimentation; chemical and biological reactions do not play an important role in physical treatment.
  • Treatment, primary: Physical treatment processes involving removal of particles, typically by settling and flotation with or without the use of coagulants; (e.g. a grease interceptor or a septic tank provides primary treatment); see also treatment, physical.
  • Treatment, secondary: Biological and chemical treatment processes designed to remove organic matter; a typical standard for secondary effluent is BOD and TSS less than or equal to 20 mg/L each on a 30-day average basis. Treatment, tertiary: Advanced treatment of wastewater for enhanced organic matter removal, pathogen reduction, and nutrient removal; typical standards for tertiary effluent vary according to regulatory requirements. Treatment train: Site-specific combination of components that make up a wastewater treatment system; a simple example of a treatment train is a septic tank and a soil treatment area.

Urine-separating device: Toilet fixture designed to separate urine from other waste materials.

Wastewater, commercial: Non-toxic, non-hazardous wastewater from commercial establishments, including but not limited to commercial food preparation operations, that is similar in composition to domestic wastewater, but which may have one or more of its constituents exceed typical domestic ranges.

  • Wastewater, domestic: Water or liquid-carried waste from plumbing fixtures, appliances and devices such as toilets, bath, laundry, and dishwashers; see also, wastewater, residential-strength.
  • Wastewater, high-strength: 1. Influent having BOD5 greater than 300 mg/L; and/or TSS greater than 200 mg/L; and/or fats, oils, and grease greater than 50 mg/L entering a pretreatment component (as defined by NSF Standard 40 testing protocol); 2. Effluent from a septic tank or other pretreatment component that has BOD5 greater than 170 mg/L; and/or TSS greater than 60 mg/L; and/or fats, oils, and grease greater than 25 mg/L and is applied to an infiltrative surface.
  • Wastewater, industrial: Water or liquid-carried waste from an industrial process resulting from industry, manufacture, trade, automotive repair, vehicle wash, business or medical, activity; this wastewater may contain toxic or hazardous constituents.

Wastewater recycling: Reclamation process of collection and treatment of wastewater on-site for return and use back into the same site; for example, collection and reclamation of graywater from an establishment for subsequent toilet flushing in that same establishment; see also wastewater reuse.

Wastewater reuse: Reclamation process of collection and treatment of wastewater for the deliberate application of that treated wastewater for a beneficial purpose such as turf irrigation; see also wastewater recycling.

Wastewater treatment system: Assembly of components for collection, treatment and dispersal of sewage or effluent.

Wastewater treatment system, cluster: Wastewater treatment systems designed to serve two or more sewage-generating dwellings or facilities with multiple owners; typically includes a comprehensive, sequential land-use planning component and private ownership.

Wastewater treatment system, decentralized: wastewater treatment system for collection, treatment, and dispersal/reuse of wastewater from individual homes, clusters of homes, isolated communities, industries, or institutional facilities, at or near the point of waste generation.

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