Waste Water Glossary

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A stimulus severe enough to rapidly induce an effect; in aquatic toxicity tests, an effect observed in 96 hours or less is typically considered acute. When referring to aquatic toxicology or human health, an acute effect is not always measured in terms of lethality.

Advanced Primary Treatment  During wastewater treatment at the Point Loma Wastewater Treatment Plant, ferric chloride and organic polymers are added to the sedimentation tanks to help waste particles bond together in large enough masses to settle out.

Aerobic treatment unit (ATU)
A mechanical wastewater treatment unit that provides secondary wastewater treatment for a single home, a cluster of homes, or a commercial establishment by mixing air (oxygen) and aerobic and facultative microbes with the wastewater. ATUs typically use a suspended growth process (such as activated sludge-extended aeration and batch reactors), a fixed-film process (similar to a trickling filter), or a combination of the two treatment processes.

Animal Feeding Operation (AFO)
Lot or facility (other than an aquatic animal production facility) where the following conditions are met:

Animals (other than aquatic animals) have been, are, or will be stabled or confined and fed or maintained for a total of 45 days or more in any 12-month period, and Crops, vegetation, forage growth, or post-harvest residues are not sustained in the normal growing season over any portion of the lot or facility. 

Anthropogenic Relating to the science of man.

Policies which ensure protection of water quality for a particular water body where the water quality exceeds levels necessary to protect fish and wildlife propagation and recreation on and in the water. This also includes special protection of waters designated as outstanding natural resource waters. Antidegradation plans are adopted by each state to minimize adverse effects on water.

Amino acids Compounds, containing within their structure at least one amino group (NHy) and one carboxylic group (-COOH)). Sub-units of amino acids make up proteins.

Amplitude The magnitude of the maximum displacement of an oscillating sound wave.

Anaerobic digestion  Digestion of organic matter by anaerobic microbial action, resulting in the production of methane gas.

Anaerobiosis The presence of life in an anaerobic environment.

Anion A negatively charged ion.

Anion exchange capacity The ability to exchange positively charged particles of two or more compounds, measured in milliequivalents per 100 grams.

Applicant Type
Indicate whether you, the applicant, are a federal, state, tribal, private, or other public entity. This question does not refer to the where the facility is located. For example, if this is a privately owned facility that is located in Indian country, select "private" from this list.

Aquaclude Rocks and soils which transmit water with difficulty, e.g. clay, shale and unfractured granite.

Aquifer Rocks and soils which transmit water with ease through their pores and fractures, e.g. limestone, sandstone and fractured granite.

Aromatic compounds Compounds which contain a six-membered ring of carbon atoms, known as a benzene ring, e.g. vitamin B.

Atmospheric boundary layer A region near the earth in which the relative velocity increases from zero with elevation, due to the motion of the atmosphere relative to the earth’s surface.

Atmospheric inversion A term which describes the rising of warm air above cold air when two air masses of different temperatures, humidity and pressure meet.

Attached growth Fixed microbial growth on the media surface in a trickling filter.

Attentuate and disperse landfill sites The traditional type of landfill site from which the leachate produced seeps through soil assures and pores into the underlying saturated zone, where it is diluted.

Attrition A gradual abrasion.

Auto-ignition temperature The temperature to which a reactive mixture must be raised so that, under certain conditions of pressure and after a specific period of time, that mixture will spontaneously ignite.

Autotrophic A term applied to organisms which produce their own organic constituents from inorganic compounds utilizing energy from sunlight or oxidation processes.

Available water content The water available in the soil for plant use, i.e. the difference between the permanent wilting point and the field capacity.

Average Monthly Discharge Limitations
The highest allowable average of daily discharges over a calendar month, calculated as the sum of all daily discharges measured during that month divided by the number of days on which monitoring was performed (except in the case of fecal coliform).

Average Weekly Discharge Limitation
The highest allowable average of daily discharges over a calendar week, calculated as the sum of all daily discharges measured during a calendar week divided by the number of daily discharges measured during that week.

Avogadro number The number of atoms of carbon in exactly twelve grams of the carbon-12 isotope, i.e. 6.023x1023.

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Bag filter A row of fabric bags through which a gas stream is passed for the removal of particulate matter.

Baling A compaction process in which the volume of waste requiring to be stored or transported is reduced.

Bandspreading The spreading of fertilizers in thick bands, 300 mm apart.

Baseflow Water which enters streams from persistent, slowly varying sources and maintains streamflow between water-input events.

Basidia The reproduction cell of the fungal group, Basidiomycetes. which contains the mushroom puffballs and rust.

Bearing capacity A measure of the load per unit area that a material can withstand before failure.

Benthic Of the bottom.

Bentonite A natural clay whose particles form a skin of very low permeability on an excavated soil face The skin supports the soil hydrostatic pressure, thereby promoting stability.

Benzene ring The basic structure of benzene: six carbon atoms arranged in a ring, each with a hydrogen atom attached.

Best Available Technology Economically Achievable (BAT)
Technology-based standard established by the Clean Water Act (CWA) as the most appropriate means available on a national basis for controlling the direct discharge of toxic and nonconventional pollutants to navigable waters. BAT effluent limitations guidelines, in general, represent the best existing performance of treatment technologies that are economically achievable within an industrial point source category or subcategory.

Best Conventional Pollutant Control Technology (BCT)
Technology-based standard for the discharge from existing industrial point sources of conventional pollutants including BOD, TSS, fecal coliform, pH, oil and grease. The BCT is established in light of a two-part "cost reasonableness" test which compares the cost for an industry to reduce its pollutant discharge with the cost to a POTW for similar levels of reduction of a pollutant loading. The second test examines the cost-effectiveness of additional industrial treatment beyond BPT. EPA must find limits which are reasonable under both tests before establishing them as BCT.

Best Management Practices (BMPs)
Schedules of activities, prohibitions of practices, maintenance procedures, and other management practices to prevent or reduce the discharge of pollutants to waters of the United States. BMPs also include treatment requirements, operating procedures, and practice to control plant site runoff, spillage or leaks, sludge or waste disposal, or drainage from raw material storage.

Best Practicable Control Technology Currently Available (BPT)
The first level of technology-based standards established by the CWA to control pollutants discharged to waters of the U.S. BPT effluent limitations guidelines are generally based on the average of the best existing performance by plants within an industrial category or subcategory.

Best Professional Judgment (BPJ)
The method used by permit writers to develop technology-based NPDES permit conditions on a case-by-case basis using all reasonably available and relevant data.

Binary fission A form of reproduction of micro-organisms in which the cell mass is passed on as two new individuals to the succeeding generation and the biomass is retained within the population.

A test used to evaluate the relative potency of a chemical or a mixture of chemicals by comparing its effect on a living organism with the effect of a standard preparation on the same type of organism.

Biochemical oxygen demand (BOD) A measure of the amount of oxygen used by bacteria in the

Biocide A chemical toxic or lethal to living organisms.

Biodegradable Capable of decomposition by living matter.

Biodiversity The infinite range of living organisms found within an ecosystem.

Biome A major regional ecological community, characterized by distinct life forms and principal plant or animal species.

Biosolids The semi-solid end product of wastewater treatment.

Biosphere The part of the earth and the atmosphere in which life can occur.

Biotope The smallest geographical unit of the biosphere or of a habitat, characterized by its biota, that can be defined by convenient boundaries.

Bluff body A body which is of angular, rather than aerodynamic, shape.

BOD - Biological Oxygen Demand - Rate at which organisms use oxygen in water and wastewater.

Bound water A thin film of water held by adhesion to the surface of soil particles.

Buoyancy The upward force that acts on a body which is totally immersed in a fluid and is equal to the weight of the fluid displaced by the body.

Bubbling bed Expansion and fluidization of the sand of a fluid-bed incinerator caused by high rate passage of air.

Budding A type of asexual reproduction in which new cells are formed as outgrowths of a parent cell.

Buffer A solution which undergoes only a slight change in pH when H+ or H- ions are added to it.

Buffer stripping The cultivation of narrow strips of land across the slope of the land rather than parallel to it, with the aim of reducing soil erosion.

Bulking agent A low density material, usually domestic refuse, straw or woodchips which is mixed with compost to permit air circulation while the compost is digesting.

Buttress zone Zone of protection.

The intentional diversion of wastestreams from any portion of a treatment (or pretreatment) facility.

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Can velocity The velocity of the gas in the passages between the filter units in the filter house of a gas filter.

Capilliarity The rise of water in tubes of small bore due to the adhesion between the water molecules and the surface of the vessel wall.

Capilliary suction time (CST) A laboratory-determinable parameter defining the dewaterability rate of a wastewater sludge.

Capilliary water Water held in soil micropores by weak capilliary forces.

Capsid A protein coat surrounding the nucleic acid of a simple virus.

Capsule A layer of well-organized materials lying outside and adhering to the bacterial cell wall.

Carotenes See carotenoids.

Carotenoids A group of plant pigments of an orange, yellow or red color which assist in photosynthesis, absorb light in the violet – blue range but whose presence is usually masked by chlorophyll. They contain the groups carotenes and xanthophylls.

Categorical Industrial User (CIU)
An industrial user subject to national categorical pretreatment standards.

Categorical Pretreatment Standards
Limitations on pollutant discharges to publicly owned treatment works promulgated by EPA in accordance with Section 307 of the Clean Water Act that apply to specified process wastewaters of particular industrial categories.

Catalyst A substance which alters the rate of a chemical reaction but which is not used up and is unchanged chemically at the end of the reaction.

Catchment A natural drainage basin which channels rainfall into a single outflow.

Cation A positively charged ion.

Cation exchange capacity The ability to exchange negatively charged particles of two or more compounds, measured in milliequivalents per 100 grams.

Cavity zone A region within which there is little mixing of air.

Cellular storm A rainfall event consisting of a number of discreet rainfall-bearing cells (clouds).

Cell wall The outer supporting layer of a plant cell made by the protoplast and consisting largely of cellulose.

Centralized Wastewater Treatment System
A managed system consisting of collection sewers and a single treatment plant used to collect and treat wastewater from an entire service area. Traditionally, such a system has been called a publicly owned treatment works (POTW).

Certifying Official
All applications, including NOIs, must be signed as follows:

1.     For a corporation: By a responsible corporate officer. For the purpose of this Part, a responsible corporate officer means: (i) a president, secretary, treasurer, or vice-president of the corporation in charge of a principal business function, or any other person who performs similar policy- or decision-making functions for the corporation, or (ii) the manager of one or more manufacturing, production, or operating facilities, provided, the manager is authorized to make management decisions which govern the operation of the regulated facility including having the explicit or implicit duty of making major capital investment recommendations, and initiating and directing other comprehensive measures to assure long term environmental compliance with environmental laws and regulations; the manager can ensure that the necessary systems are established or actions taken to gather complete and accurate information for permit application requirements; and where authority to sign documents has been assigned or delegated to the manager in accordance with corporate procedures.

2.     For a partnership or sole proprietorship: By a general partner or the proprietor, respectively; or

3.     For a municipality, state, federal, or other public agency: By either a principal executive officer or ranking elected official. For purposes of this Part, a principal executive officer of a federal agency includes (i) the chief executive officer of the agency, or (ii) a senior executive officer having responsibility for the overall operations of a principal geographic unit of the agency (e.g., Regional Administrator of EPA).

Chemical oxygen demand (COD) A quick chemical test to measure the oxygen equivalent of the organic matter content of wastewater that is susceptible to oxidation by a strong chemical.

Chemisorption Adsorption involving very strong bonding forces

Chemotrophic A term applied to organisms which produce their own organic constituents from inorganic compounds utilizing the energy obtained from the oxidation of hydrogen sulphide.

Chloracne A widespread acneform eruption due to exposure to compounds such as dibenzofurans, dibenzodioxins and chlorodiphenyls.

Chloramine A compound composed of chlorine and ammonia.

Chlorination A disinfection technique used in water treatment, involving the addition of Cl2 gas, chlorine dioxide, sodium hypochlorite or calcium hypochlorite.

Chlorofluorocarbons (CFCs) Compounds containing chlorine, fluorine or bromine, used as aerosol propellants, refrigerants, foaming agents and solvents and which, on decomposition by sunlight, produce oxides of chlorine responsible for the removal of ozone from the stratosphere.

Chlorophyll A photosynthetic plant pigment which absorbs red and blue light but reflects green light. The chlorophyll molecule has a square head, magnesium at the center and a long tail.

Chlorophyll a The most important of the pigments in chlorophyll, found in all photosynthetic plants except bacteria.

Chlorophyll b One of the constituent pigments of chlorophyll, found in higher plants and green algae.

Chloroplast A chlorophyll-containing, cytoplasmic body of plant cells where photosynthesis occurs.

Chromatophore A plastic containing colored pigment.

A stimulus that lingers or continues for a relatively long period of time, often one-tenth of the life span or more. Chronic should be considered a relative term depending on the life span of an organism. The measurement of a chronic effect can be reduced growth, reduced reproduction, etc., in addition to lethality.

Cilia Whiplike structures of 5 – 20 µm length which allows bacterial mobility by beating with a swimming action.

Circulating bed Recovery of solids from the gas phase of a fluidized bed combustion reactor, followed by reinjection into the sand bed.

Class V Injection Well
A shallow well used to place a variety of fluids at shallow depths below the land surface, including a domestic onsite wastewater treatment system serving more than 20 people. USEPA permits these wells to inject wastes below the ground surface provided they meet certain requirements and do not endanger underground sources of drinking water.

Closed loop recycling The remanufacture of a new product from a retired product of the same type.

Coagulation The water/wastewater treatment process of destabilizing colloidal particles to facilitate particle growth during flocculation by either double-layer compression, charge neturalization, interparticle bridging or precipitate enmeshment.

Coarse fish Fish, e.g. mullet, which are able to tolerate low oxygen levels.

Coepod species A phylum containing the Crustacea, i.e. small freshwater and marine animals, of which some plankton is composed.

Cogeneration - Co-production of electrical and thermal energy, also called combined heat and power (CHP).

Conductivity A measure of the ability of a solution to conduct an electrical current and is proportional to the concentration of ions in the solution.

Coliforms Non-pathogenic bacteria present in the intestines of warm-blooded animals, water and wastewater, whose numbers indicate contamination.

Colloids Very small particles in suspension, e.g. clays.

Combined Sewer Overflow (CSO)
A discharge of untreated wastewater from a combined sewer system at a point prior to the headworks of a publicly owned treatment works. CSOs generally occur during wet weather (rainfall or snowmelt). During periods of wet weather, these systems become overloaded, bypass treatment works, and discharge directly to receiving waters.

Combined Sewer System (CSS)
A wastewater collection system which conveys sanitary wastewaters (domestic, commercial and industrial wastewaters) and stormwater through a single pipe to a publicly owned treatment works for treatment prior to discharge to surface waters

Combustion A high temperature process involving the decomposition of organics in an excess of air.

Completely mixed reactor An aeration tank in which, on entering, the influent wastewater is dispersed immediately throughout the reactor volume.

Compliance Schedule
A schedule of remedial measures included in a permit or an enforcement order, including a sequence of interim requirements (for example, actions, operations, or milestone events) that lead to compliance with the CWA and regulations.

Composite Sample
Sample composed of two or more discrete samples. The aggregate sample will reflect the average water quality covering the compositing or sample period.

Composting The biological stabilization of wastes of biological origin under controlled conditions.

Compound A substance, the molecules of which consist of two or more different kinds of atoms.

Compression settling Particles are present in such a high concentration that they touch each other and settling can occur only by compression of the particle mass.

Concentrated Animal Feeding Operation (CAFO)
An AFO that is defined as a Large CAFO or as a Medium CAFO. . ., or that is designated as a CAFO. . . Two or more AFOs under common ownership are considered to be a single AFO for the purposes of determining the number of animals at an operation, if they adjoin each other or if they use a common area or system for the disposal of wastes.

Condensation point That level above the earth’s surface to which a parcel of unsaturated air must ascend before becoming saturated.

Constructed wetland A biological wastewater treatment system which utilizes plants for the degradation of organic waste.

Contact stabilization A wastewater treatment plant in which there are two tanks, one for the adsorption of organic matter onto the suspended solids and another for oxidation of the adsorbed materials.

Containment landfill sites The modern landfill site, in which the leachate generated is contained by bottom liners, collected and treated.

Contaminated site A landfill into which hazardous polluting waste has been dumped.

Contour ploughing Ploughing across the slope of the land rather than with it, to prevent soil erosion.

Convection Transport of heat by vertical movement of a heated body.

Convective precipitation When a parcel of air which is less dense than the air surrounding it, rises, it cools and loses moisture which falls to the earth as rain.

Conventional Septic System
A wastewater treatment system consisting of a septic tank and a typical trench or bed subsurface wastewater infiltration system.

Coriolis force A transverse force, caused by the movement of the earth about the sun, which causes a build-up in the level of water to the right of a tidal current in the northern hemisphere and to the left in the southern hemisphere.

Corona The upper portion of a body part.

The numeric values and the narrative standards that represent contaminant concentrations that are not to be exceeded in the receiving environmental media (surface water, ground water, sediment) to protect beneficial uses.

Criteria pollutant Emissions to the urban air traditionally seen as polluting, e.g. carbon monoxide (CO2), sulfur dioxide (SO4).

Cryogenic Producing very low temperatures.

Cyanide A highly poisonous salt of hydrocyanic acid, used frequently in the extraction of gold and silver.

Cyclone separator A means of purifying an air stream by using both gravitational and centrifugal forces.

Cytotoxic Damaging to cell structure and cell division

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Daily Discharge
The discharge of a pollutant measured during any 24-hour period that reasonably represents a calendar day for purposes of sampling. For pollutants with limitations expressed in units of mass, the daily discharge is calculated as the total mass of the pollutant discharged during the day. For pollutants with limitations expressed in other units of measurement (e.g., concentration) the daily discharge is calculated as the average measurement of the pollutant throughout the day.

Daily Maximum Limit
The maximum allowable discharge of pollutant during a calendar day. Where daily maximum limitations are expressed in units of mass, the daily discharge is the total mass discharged over the course of the day. Where daily maximum limitations are expressed in terms of a concentration, the daily discharge is the arithmetic average measurement of the pollutant concentration derived from all measurements taken that day.

Daphnid species The phylum containing the Branchiopodia, i.e. marine solitary, benthic animals with a shell of two valves.

Denitrification The chemical reduction of nitrate and nitrite to gaseous forms: nitric oxide, nitrous oxide and dinitrogen: NO3- NO2- NO N2 O N2

Deoxyribose nucleic acid (DNA) A large organic molecule found in the cell nucleus, containing a phosphate group, five-carbon sugars (deoxyribose) and four different nitrogenous bases in a repetitive structure.

Detritivores Organisms which feed on fragmented particulate organic matter.

Development Document
A report prepared during the development of an effluent limitation guideline by EPA that provides the data and methodology used to develop limitations guidelines and categorical pretreatment standards for an industrial category.

Dewatering of sludge A mechanical unit operation which increases the dry solids concentration of the sludge from 3.9 percent after digestion to 25 – 30 percent thereby ensuring that the sludge effectively behaves as a solid for handling purposes.

Diffusion The process by which gases and liquids spread themselves throughout any space into which they are put.

Digestion - Process by which organisms break down sludge, creating as by-products methane gas, carbon dioxide, solid organic material and water. (Aerobic digestion takes place in the presence of oxygen and anaerobic digestion takes place with the absence of oxygen.)

Dilute-phase bed The stage in fluidized bed combustion at which the bubbling of the reactor bed becomes so great that the boundary between the bed and the gas above it becomes indistinct.

Dimiclic A term to describe a lake whose thermocline is disrupted due to two periods of free circulation or overturn per year in the lake.

Dioxin Tetrachlorodibenzoparadioxin (TCDD), a highly toxic and environmentally persistent product of the manufacture of the pesticide 2,4,5-T.

Direct contact condenser The vapor stream is in direct contact with the drying medium, hot air or gas. The drying medium (hot air or flue gas) leaves the drier with the water vapor coming from the sludge. The drying temperature is 80 – 150 ºC.

Directivity index The difference between the measured sound power level and the value based on the assumption of uniform radiation in all directions.

The Regional Administrator or State Director, as the context requires, or an authorized representative. When there is no approved state program, and there is an EPA administered program, Director means the Regional Administrator. When there is an approved state program, "Director" normally means the State Director.

Discharge Monitoring Report (DMR)
The form used (including any subsequent additions, revisions, or modifications) to report self-monitoring results by NPDES permittees. DMRs must be used by approved states as well as by EPA.

Discrete settling Particles settle as independent units, without interaction of flocs.

Disinfection The removal or inactivation of pathogenic organisms.

Dissolved oxygen A measure of the amount of oxygen dissolved in water, expressed as either:

(i) mg/1 – which is the absolute amount of oxygen dissolved in the water mass

(ii) as percentage saturation of the water with O2 (% sat)

Dissolved solids The total colloidal and suspended solids in a liquid. Any particle passing a 1.2 µm filter is defined as dissolved.

Dominant group The highest ranking group in a social order of dominance sustained by aggressive or other behavioural patterns.

Downflow column e.g. Sand filtration where water flows through the filter by gravity. Also used in anaerobic digestion, where the wastewater enters at the upper levels and flows down through a packed medium. Opposite to upflow column.

Downwash The drawdown of a plume after emission due to a low pressure area downwind of the stack.

Dry absorption A method of controlling acids in flue gas emissions, by injection of dry calciumhydroxide into the gases leaving the furnace of an incinerator.

Dry weather flow The combination of wastewater and dry weather infiltration flowing in a sanitary sewer during times of low precipitation.

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Ecology That branch of science dealing with living organisms and their surroundings.

Ecosystem A community of interdependent organisms together with the environment which they inhabit and with which they interact, e.g. a pond.

Ecotron A controlled, in-house, ecological experiment to recreate a particular ecosystem.

ECU The EU unit of monetary currency.

Effluent The outflow from a sewage treatment plant.

Effluent Limitation
Any restriction imposed by the Director on quantities, discharge rates, and concentrations of pollutants which are discharged from point sources into waters of the United States, the waters of the contiguous zone, or the ocean.

Effluent Limitations Guidelines (ELG)
A regulation published by the Administrator under Section 304(b) of CWA that establishes national technology-based effluent requirements for a specific industrial category.

Electrical double layer A name given to the combination of the Stern layer and the diffused layer of both negatively and positively charged ions which surround it.

Electron Negatively charged particle contained within an atom, the weight of which is about two thousand times less than that of the hydrogen atom.

Electrostatic precipitation A means of purifying an air stream by attraction and adhesion of ionized particles to an electrode.

Element A substance, the molecules of which have all the same atoms.

Endotoxin An environmental toxin which attacks the endocrine glands, i.e. kidney, liver, etc.

Environmental impact assessment (EIA) A review to which all commencing projects must be subjected with regard to their impact on the environment.

Enzyme A substance produced by living cells which acts like a catalyst in promoting reactions within the organism.

Epilimnion The zone in a stratified lake just below the near-surface water in which temperature decreases rapidly with depth.

Epilithic Relating to organisms growing on rocks or on other hard, inorganic substances.

Equilibrium concentration The concentration of the dissociated ions when the rates of both backward and forward reactions are equal.

Equilization basin A holding tank within which variations in sewage inflow rate and liquid nutrient concentrations are averaged.

Equivalence The number of protons donated in an acid-base reaction or the total change in valence in an oxidation-reduction reaction.

Eucaryotic cell A cell whose nucleus is enclosed by a membrane, e.g. algae, higher plants and animals.

Euphotic zone The surface zone of large lakes through which sufficient light penetrates for photo-synthesis to occur.

European Communities (EC) A precursor to the EU, created by the merger of the European Coal and Steel Community, the European Economic Community and the European Atomic Energy Community.

European Economic Community (EEC) An organization established in 1957 under the Treaty of Rome to co-ordinate the activities of its member countries in the coal and steel industry, the establishment of a common market and the pooling of atomic energy resources.

European Union (EU) A supranational organization which replaced the EEC in 1993, with the objective of peace and prosperity for its members by achieving complete economic and political union.

Eurytopic A term describing an organism which is tolerant of a wide range of habitats.

Eurotrophic A term describing freshwater bodies which are rich in plant nutrients and therefore highly productive.

Eutrophication An increase in the concentration of nutrients in an aquatic ecosystem, causing:

(i) the increased productivity of autotrophic green plants, leading to the blocking out of sunlight

(ii) elevated temperatures within the water body

(iii) depletion of the world’s oxygen resources

(iv) increased agal growth

(v) reduction in the level of and variety of fish and animal

Evaporation The changing of liquid water from rivers, lakes, bare soil and vegetative surfaces into water vapor.

Evapotranspiration A collective term for all the processes by which water in the liquid or solid phase at or near the earth’s land surfaces becomes atmospheric water vapor.

Exothermic reaction A chemical reaction during which heat is liberated.

Extended aeration Involves an aeration period of more than 24 hours and a high rate of return sludge to allow cell decay during the endogenous respiration phase of the growth curve.

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Facultative aerobes/anaerobes Having the ability to live either with or without oxygen.

Fickian diffusion Molecular diffusion, governed by Fick’s law, which says that the rate of flow of molecules across a unit area of a certain plane is directly proportional to the concentration gradient.

Field capacity The amount of water which can be held in the soil against the force of gravity, i.e. Water which will not drain freely out of the soil.

Filtration A process whereby suspended and colloidal matter is removed from water and wastewater by passage through a granular medium.

Five-day biochemical oxygen demand (BOD5) A measure of the amount of oxygen used by bacteria to degrade organic matter in a sample of wastewater over a 5 day period at 20 ºC, expressed in mg 1-1.

Fixed bed A bed of dry carbon which recovers volatile organic carbons from an air stream.

Flagellae Whiplike structures of 100 – 200 µm length which allow bacterial mobility by undulating in planar or helical waves.

Flash point The lowest temperature at which a flammable vapor/air mixture exists at the surface of a combustible liquid.

Flocculation The water treatment process in which particle collisions are induced in order to encourage the growth of larger particles.

Flotation A process by which suspended matter is lifted to the surface of a liquid to facilitate its removal. Frequently done by the bubbling of air through the liquid.

Flow duration curve A means of summarizing temporal variability by averaging precipitation over a selected time period.

Flowing well When the groundwater is flowing in a confined aquifer, it is under hydrostatic pressure. Should a standpipe be inserted into the aquifer, the water will rise in the standpipe.

Flue gases Gas by-products of the incineration process whose temperature is a measure of incinerator efficiency and whose constituents may be polluting.

Flue gas scrubber Equipment used for the removal of suspended particulates and acid gases from flue gas emissions.

Fluidized bed combustion An incineration technique in which waste is destroyed by combustion on a bubbling bed.

Fluoridation The addition of fluoride to drinking water within the limits 0.7 – 1.2 mg/1-1 to help prevent the occurrence of tooth decay.

Foaming agent Anti-foaming chemicals added to wastewater in the aeration tank to disperse the contaminating foam caused by the action of the surface aerators and the presence of detergents in the wastewater.

Food/micro-organism ratio (F/M) A measure of the organic loading rate of a wastewater treatment system, i.e. the ratio between the daily BOD load and the quantity of activated sludge in the system (microbes).

Four-Zero-One (401) (a) Certification
A requirement of Section 401(a) of the Clean Water Act that all federally issued permits be certified by the state in which the discharge occurs. The state certifies that the proposed permit will comply with state water quality standards and other state requirements.

Fugitive emissions Emissions from non-point sources, e.g. loading/unloading, transferring, trans-porting, storing and processing of materials.

Fumigating A term describing a plume from an emission stack which is trapped by a stable inversion above the stack mouth, thereby hitting the ground level very close to the stack.

Functional group A group of atoms on which the characteristic properties of a particular homologous series depends, e.g. the alkanes, alcohols and esters.

Fundamentally Different Factors (FDF)
Those components of a petitioner's facility that are determined to be so unlike those components considered by EPA during the effluent limitation guideline and pretreatment standards rulemaking that the facility is worthy of a variance from the effluent limitations guidelines or categorical pretreatment standards.

Furans Compounds causing chloracne, liver damage and liver cancer. Strictly C4H4O, but more commonly one of a range of polychlorinated dibenzofurans that are produced as contaminants from the incomplete incineration of chlorinated hydrocarbons.

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Gamete A mature cell, involved in reproduction.

Gas chromatography A process whereby compounds become separated by being physically carried by a gas over a liquid of a high molecular weight.

Gas flaring The burning of recovered landfill gas from a stack under controlled conditions to help eliminate the discharge of harmful constituents to the atmosphere.

Gasification A high temperature process involving the decomposition of organics in the absence of oxygen. Some of the energy stored as chemical energy from the organic material will be released as burnable gas.

Genotype The genes which an organism possesses or the genetic make-up of an organism.

Grab Sample
A sample which is taken from a wastestream on a one-time basis without consideration of the flow rate of the wastestream and without consideration of time.

Grit Chamber - Tank in which the flow of wastewater is slowed, allowing heavy solid materials such as pebbles, sand, coffee grounds and eggshells to sink to the bottom.

Groundborne vibrations Vibrations caused by the reaction of tyres of heavy vehicles with irregularities in the road surface.

Groundwater Water under a pressure greater than atmospheric pressure which is present in the saturated zone of the soil.

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Haematins A group of colored plant pigments, including the red pigment, haematochrome.

Haloform A basic organic unit of the halogen group.

Halogen The reactive members of Group 7 of the Periodic Table, including chlorine, bromine, fluorine and iodine.

Hardness in water The sum of the calcium and magnesium ion concentrations. A hard water will leave a scale on the inside of kettles and will form a scum rather than a lather with soap.

Hazardous waste A substance which exhibits ignitability, reactivity, corrosivity, and/or toxicity.

Headworks - Area of treatment plant where influent begins treatment.

Heat of adsorption Adsorption is the process of retaining a gas molecule by either physical or chemical means onto an adsorbent (a solid, e.g. activated carbon). The heat change taking place during this process (loss of heat of gas, increase in temperature of adsorbent) is the heat of adsorption.

Heat of condensation The quantity of heat required to bring about a phase change from a gas to a liquid.

Heat of solution The heat change which takes place when one mole of a substance is dissolved in excess solvent.

Heavy metal Inorganic species of large atomic weight. Usually chromium (Cr3+), lead (Pb2+), mercury (Hg2+ ), zinc (Zn2+), cadmium (Cd2+) and barium (Ba2+).

Herbivores Animals which feed on plant material only, e.g. rabbits.

Heterotrophic A term applied to organisms which need ready-made food materials from which to produce their own constituents and to obtain all their energy.

High rate aeration An increased rate of aeration of MLSS in an activated sludge system requiring less activated sludge and shorter aeration periods.

Homogenous Consisting of only one phase.

Humus The vegetative upper layers of the soil.

Hydraulic conductivity See permeability.

Hydrograph A graph of stream discharge versus time.

Hydrolysis The breakdown of high molecular compounds to low molecular compounds.

Hydrophilic Displaying an affinity for water.

Hydrophobic Displaying an aversion for water.

Hydrothermal vent An opening in the earth through which heated or superheated water is ejected.

Hydraulic jump An area of turbulence and of loss of energy associated with the transmission from shooting to tranquil flow.

Hydrological cycle The endless recirculatory transport process of the earth’s water resources, linking the atmosphere, the land and the oceans.

Hyetograph A graph of water input to a catchment versus time.

Hypha A tubular filament which is the basic unit structure of most fungi and some bacteria.

Hypolimnion The lower layer of water in stratified lakes which retains the winter temperature.

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Ion Atoms or groups of atoms which have either lost or gained electrons and so have become either positively or negatively charged.

Ion exchange Ion exchange can be illustrated by the following reaction: Ca2+ + Na2 Z CaZ + 2Na+.

Incineration Chemical oxidation at high temperatures where organic material is converted into heat energy, flue gas and slag.

Inclusion body Organic or inorganic bodies containing glycogen, protein or lipids, present in the cytoplasm of a bacterium.

Indirect contact condenser A condenser in which there is no direct contact between the heating medium and the vapor stream, but a partition divides the two. The water vapor is removed separately from the heating medium. The drying temperature is 100 – 250 º C.

Indirect Discharge
The introduction of pollutants into a municipal sewage treatment system fro any nondomestic source (i.e., any industrial or commercial facility).

Industrial Sources
Non-municipal, or industrial sources, often generate wastewater that is discharged to surface waters. The types of wastewaters generated at a facility depend on the specific activities undertaken at a particular site, and may include manufacturing or process wastewaters, cooling waters, sanitary wastewater, and stormwater runoff.

Influent - Untreated wastewater when it flows into a treatment plant.

Influent Screens - First step in the wastewater treatment process. The screen consists of bars between ¾ inch and 3 inches apart. Wastewater is allowed to pass through the bars, but large solid objects such as rocks, cans, plastic bags and sticks are stopped.

Instantaneous Maximum Limit
The maximum allowable concentration of a pollutant determined from the analysis of any discrete or composite sample collected, independent of the flow rate and the duration of the sampling event.

Invertebrate Animal without cranium and spinal column.

Irrigation requirement The difference in volume between effective precipitation and evapotranspiration.

Isohyet A line on a map connecting areas of equal precipitation.

lsocyanate Derivatives of nitrogen-substituted carbamic acids, containing carbon, oxygen and an organic amine group.

Isotropic A substance whose physical properties are the same in all directions.

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Jute Fiber from the bark of some plants, used mostly for sacking.

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Karst Landforms of chemically weathered limestone, characterized by underground channels and caverns, swallow holes and open joints.

Ketone An organic compound containing three carbon atoms, one of which is double-bonded to an oxygen atom, the other two each attached to three hydrogen atoms e.g. acetone.

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Landfill A repository in the ground for unwanted waste.

Landfill gas This is produced principally from the anaerobic decomposition of biodegradable organic waste and includes ammonia, carbon dioxide, carbon monoxide, hydrogen, hydrogen sulfide, methane, nitrogen and oxygen.

Landfill liner Used to limit the movement of leachate and landfill gases from the landfill site. Can be made of natural clay material or composite geomembrane and clay materials.

Lapse rate The rate of temperature change with height for a parcel of dry air rising adiabatically.

Large Concentrated Animal Feeding Operation (Large CAFO)
An AFO is defined as a Large CAFO if it stables or confines as many or more than the numbers of animals specified in any of the following categories:

    • 700 mature dairy cows, whether milked or dry;
    • 1,000 veal calves;
    • 1,000 cattle other than mature dairy cows or veal calves. Cattle includes but is not limited to heifers, steers, bulls and cow/calf pairs;
    • 2,500 swine, each weighing 55 pounds or more;
    • 10,000 swine, each weighing less than 55 pounds;
    • 500 horses;
    • 10,000 sheep or lambs;
    • 55,000 turkeys;
    • 30,000 laying hens or broilers, if the AFO uses a liquid manure handling system;
    • 125,000 chickens (other than laying hens), if the AFO uses other than a liquid manure handling system;
    • 82,000 laying hens, if the AFO uses other than a liquid manure handling system;
    • 30,000 ducks (if the AFO uses other than a liquid manure handling system); or
    • 5,000 ducks (if the AFO uses a liquid manure handling system)

Large Construction Activity
A large construction activity includes clearing, grading, and excavating resulting in a land disturbance that will disturb equal to or more than five acres of land or will disturb less than five acres of total land area but is part of a larger common plan of development or sale that will ultimately disturb equal to or more than five acres. Large construction activity does not include routine maintenance that is performed to maintain the original line and grade, hydraulic capacity, or original purpose of the site.

Latent heat of evaporation The quantity of heat required to bring about a phase change from a liquid to a vapor.

Leachate Liquid, composed of external rainfall, groundwater, etc. which has percolated through solid waste and has extracted both biological and chemical, dissolved or suspended materials.

Legumes Legumes are specific plants, e.g. clover, soybeans and lupins, which carry nodules on their roots and, together with bacteria of the genus Rhizobium are responsible for the biological fixation of nitrogen in the soil.

Life cycle assessment (LCA) The assessment of the steps in a product life cycle, including: raw materials acquisition, bulk material processing, materials production, manufacture, assembly, use, retirement and disposal.

Ligand Molecules of a complexing agent in a complex ion, i.e. an aggregate formed when a metal ion bonds to several other ions or molecules which cluster around it. In the reaction AgC1 + 2NH3 Ag(NH3)2+ + Cl-, NH3 is the ligand.

Light compensation point The depth in a sea or lake below which, because of low light intensities, plants use up more organic matter in respiration than they make during photosynthesis.

Liquid injection incineration A method of incineration of liquid waste by high-rate injection into a combustion chamber.

Lithotrophic A term describing organisms which use inorganic compounds as electron donors in their energetic processes.

Littoral zone The shore of a lake to a depth of about 10 meters.

Local Limits
Conditional discharge limits imposed by municipalities upon industrial or commercial facilities that discharge to the municipal sewage treatment system.

Lofting A term describing a plume from an emission stack which remains aloft due to a stable inversion below the mouth of the stack.

Lower explosive limit (LEL) The concentration at which a gas forms an explosive mixture with air.

Lysis The rupture of cells.

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Magnetic separation A process which utilizes the magnetic properties of ferrous metals to extract them from the waste stream.

Major Facility
Any NPDES facility or activity classified as such by the Regional Administrator, or in the case of approved state programs, the Regional Administrator in conjunction with the State Director. Major municipal dischargers include all facilities with design flows of greater than one million gallons per day and facilities with EPA/State approved industrial pretreatment programs. Major industrial facilities are determined based on specific ratings criteria developed by EPA/State.

Masking agent A substance which will remove an offending odor from an air stream by decomposition or conversion to an organic salt.

Mass-Based Standard
A discharge limit that is measured in a mass unit such as pounds per day.

Materials recovery facility (MRF) Depots where reusable waste material is recovered.

Mean cell residence time (MCRT) The average time a single microbe will remain in an activated sludge system and is calculated by: Total mass of cells/Rate of cell wastage

Meiosis A type of cell nuclear division in which the daughter nuclei receive only half the original number of chromosomes in the parent nucleus.

Membrane process The removal of dissolved solids from water by passage through a membrane of minute pore diameter (3*10-10 m).

Mesophilic temperatures Those temperatures in the range 10 – 45 ºC.

Mesotrophic A term to describe waters having intermediate levels of the minerals required by green plants.

Methanogenesis Intermediate compounds are converted to the final products of methane and carbon dioxide.

Methanogenic bacteria Obligate anaerobes and methanobacteria (e.g. methanosarcina, methanobacilli) which produce methane gas from the decomposition of acids and alcohols:


CO2 + H2O + NH3 NH4HCO3

Method Detection Limit (MDL)
Defined as the minimum concentration of a substance that can be measured and reported with 99 percent confidence that the analyte concentration is greater than zero and is determined from analysis of a sample in a given matrix containing the analyte.

MGD - Million Gallons per Day - 694.4 gallons per minute.

Micro-organisms Neither plant nor animal, these are small, simple organisms which are either unicellular or multicellular, consisting of protozoa, algae, fungi, ricettsiae, viruses and bacteria.

Mineralization The process by which organic N is reconverted to mineral form by a wide variety of heterotrophic organisms – bacteria, fungi and actinomycetes.

Mining (Coal)
Coal mining employs basically the same traditional mining techniques used in hard rock mining - underground and surface ("strip") mining. One of the more efficient but environmentally destructive methods for mining coal involves "strip" mining. This technique is analogous to the open pit mining techniques used in hard rock mining whereby the soil and rock above the coal seam are removed to expose the seam. The seam is then blasted and the coal is scooped up by huge front end loaders or electric shovels and transported to a coal processing plant. These coal preparation plants use a variety of physical (e.g., screening) and chemical (e.g., flotation using high gravity liquids) methods to separate the raw coal from all of the non-combustible waste rock and minerals (e.g., pyrite). The coarser waste rock is piled up adjacent to the mined out area and the finer coal tailings coming from the preparation plant are discharged as a thick slurry into a man-made impoundment. After coal mining operations have ceased, the mine is reclaimed by dumping the waste rock into the pit, regrading the area to approximate the original contours of the land and then replanting the area using native grasses and trees.

Mining (Hardrock)
Traditional hardrock mining usually involves digging tunnels and adits (horizontal entrances into hillsides) to reach lodes of mineral-rich ore. Traditional underground mining and open pit mining require that the ore-bearing rock be removed and then put through a milling and extraction plant to extract the desired minerals. After the ore body is exhausted, miners move on leaving behind huge mounds of finely-ground tailings and coarser waste rock, as well as an underground tunnel complex and/or open pit.

Mining (Mountaintop)
A relatively new variant of strip mining technology. This mining technique is common West Virginia and eastern Kentucky where there is enough topographic relief that is highly dissected such that the adjacent valleys can serve as repositories for overburden (soil and waste rock). Bulldozers remove all topsoil and vegetation from the top of the mountain. The bedrock above the coal is then blasted to break it up for removal. Huge draglines (the bucket can hold 15-20 pickup trucks) then remove the overburden from on top of the coal seam and dump the waste rock ("spoil") into adjacent valleys. Once exposed, the coal seam is then blasted and front end loaders scoop up the coal and load it into huge dump trucks (capable of carrying 100 tons) that haul the raw coal to the coal preparation plant.

Mining (Non-metals)
Non-metallic minerals include salt, gypsum, potash, phosphate, borax and other minerals used in the chemical industry. Sand and gravel extracted for glass making and construction (highways, buildings, and dams) is also regarded as a form of non-metal mining activity. These minerals are extracted primarily using open pit mining techniques, but may also employ underground mining techniques when the deposit is located hundreds or thousands of feet below the surface. Non-metal minerals and associated waste rock are generally of lower toxicity and more manageable than hardrock and coal mining wastes, but still have the potential to create environmental pollution from rainwater running off the piles

Mixing Zone
An area where an effluent discharge undergoes initial dilution and is extended to cover the secondary mixing in the ambient water body. A mixing zone is an allocated impact zone where water quality criteria can be exceeded as long as acutely toxic conditions are prevented.

Mixed liquor suspended solids (MLSS) The microbial suspension in the aeration tank containing living and dead micro-organisms and inert biodegradable matter, the operating concentration of which may vary in the range 1500 to 4000 mg/1

Modified Permit (also known as a "Waiver") - Permit granted by the U.S. Environmental Protection Agency which allows the Point Loma Wastewater Treatment Plant to operate at an Advanced Primary Level rather than a Secondary Treatment level. The Modified Permit was first granted in 1994 and renewed in 2002.

Mole A mole of any substance is that amount of it which contains the Avogadro Constant number of particles. A mole of any substance is equal to its molecular mass or atomic mass expressed in grams.

Molecular diffusion The drifting of molecules under random kinetic motion from a low concentration region to a high concentration region.

Monomielic A term to describe a lake having a single period of free circulation or overturn per year.

Morphology The study of the form of animals and plants.

Mouse system Software for the hydrodynamic and hydrochemical design of a wastewater collection system from the Danish Hydraulic Institute.

Multi-Sector General Permit (MSGP)
The Stormwater Multi-Sector General Permit (MSGP) authorizes the discharge of stormwater from industrial facilities, consistent with the terms of the permit, in areas of the United States where EPA manages the NPDES permit program.

Municipal Separate Storm Sewer System (MS4)
A conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains):

1.       Owned and operated by a state, city, town, borough, county, parish, district, association, or other public body (created by or pursuant to state law) having jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes, including special districts under state law such as a sewer district, flood control district or drainage district, or similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under section 208 of the Clean Water Act (CWA) that discharges to waters of the United States;

2.       Designed or used for collecting or conveying stormwater;

3.       Which is not a combined sewer; and

4.       Which is not part of a publicly owned treatment works (POTW).

Municipal Sources
POTWs collect domestic sewage from houses, other sanitary wastewater, and wastes from commercial and industrial facilities. POTWs discharge conventional pollutants, and are covered by secondary treatment standards and state water quality standards. POTWs also produce biosolids during the treatment process.

Mutagenic Causing alteration of the genetic material of an organism, leading to inherited differences.

Mycelium A mat of branching hyphae found particularly in actinomycetes.

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National Pollutant Discharge Elimination System (NPDES)
A national program under Section 402 of the Clean Water Act for regulation of discharges of pollutants from point sources to waters of the United States. Discharges are illegal unless authorized by an NPDES permit.

National Pretreatment Standard or Pretreatment Standard
Any regulation promulgated by the EPA in accordance with Sections 307(b) and (c) of the CWA that applies to a specific category of industrial users and provides limitations on the introduction of pollutants into publicly owned treatment works.

Negative project A commencing project for which an Environmental Impact Assessment will not be required.

Negative sorting Manual sorting of waste to remove the unwanted fractions. Is recommended only for dry waste.

Neutrality An ion or ion group which has an equal number of electrons and protons, i.e. neither a positive nor negative overall change.

New Discharger
Any building, structure, facility, or installation: a. From which there is or may be a discharge of pollutants; b. That did not commence the discharge of pollutants at that particular site prior to August 13, 1979; c. Which is not a new source; and d. Which has never received a finally effective NPDES permit for discharges at that site.

New Source
Any building, structure, facility, or installation from which there is or may be a discharge of pollutants, the construction of which commenced: a. After promulgation of standards of performance under Section 306 of the CWA which are applicable to such source; or b. After proposal of standards of performance in accordance with Section 306 of the CWA which are applicable to such source, but only if the standards are promulgated in accordance with Section 306 of the CWA within 120 days of their proposal. c. Except as otherwise provided in an applicable new source performance standard, a source is a new source if it meets the definition in and i. It is constructed at a site at which no other source is located; or ii. It totally replaces the process or production equipment that causes the discharge of pollutants at an existing source; or iii. Its processes are substantially independent of an existing source at the same site. In determining whether these processes are substantially independent, the Director shall consider such factors as the extent to which the new facility is integrated with the existing plant; and the extent to which the new facility is engaged in the same general type of activity as the existing source.

Niche The ecological role of a species in a community.

Nitrification The conversion of the ammonium ion, NH4+, into the nitrite ion, NO3+. It occurs in two steps:

(i) 2NH4+ + 3O2 = 2NO2- + 2H2O + 4H+ by the bacteria genus Nitrosomonas

(ii) 2NO2 + O2 = 2NO3- by the bacteria genus Nitrobacter.

Nonconventional Pollutants
All pollutants that are not included in the list of conventional or toxic pollutants. Includes pollutants such as chemical oxygen demand (COD), total organic carbon (TOC), nitrogen, and phosphorus.

Non-point source pollution Pollution from diffuse and not easily identifiable sources, e.g. a field.

Normality A concentration unit which is defined as: number of equivalents of solute/number of liters of solution.

Nucleic acid See Deoxyribose nucleic acid and Ribonucleic acid.

Nucleoid An irregularly shaped region in the procaryotic cell containing the genetic material.

Nucleus The well-defined region surrounded by the cell wall which contains the chromosomes, i.e. the materials of inheritance of the cell.

Nutrient removal Tertiary treatment introduced to remove some of the trace compounds and elements contained in most domestic wastewaters, e.g. inorganic ammonia, nitrates, phosphates and sulfates, which are little affected by conventional treatment processes.

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Octave band The interval between a given frequency and twice that frequency within the audible frequency range.

Odor threshold The minimum level or value of an odor necessary to elicit a public response.

Off-specification stock Stock which is produced by errors in processing, leading frequently to customer dissatisfaction.

Olfactometer An instrument used to measure relative odour levels.

Oligotrophic A term describing freshwater bodies which are poor in plant nutrients and are therefore unproductive.

Open loop recycling The manufacture of a new and simpler product from a retired, downcycled, more complex product.

For the purpose of the 2003 Construction General Permit and in the context of stormwater associated with construction activity, any party associated with a construction project that meets either of the following two criteria:

5.       The party has operational control over construction plans and specifications, including the ability to make modifications to those plans and specifications

6.       The party has day-to-day operational control of those activities at a project which are necessary to ensure compliance with a SWPPP for the site or other permit conditions (e.g., authorized to direct workers at a site to carry out activities required by the SWPPP or comply with other permit conditions).

Organochlorine A highly persistent and carcinogenic compound containing one or more chlorine atoms found in pesticides such as DDT.

Organoleptie parameters These are properties which can be detected by the human senses – eyes, nose and mouth.

Organotrophic A term describing organisms which use organic compounds as electron donors in their energy-producing processes.

Overflow rate The rate at which water is drawn off from the surface of primary and secondary clarification tanks. It is an important tank design parameter and is derived from the analysis of settling particles.

Overland flow The lateral movement of water over the ground surface due to gravitational forces.

Oxidizing waste A waste which loses electrons in an oxidation-reduction reaction, thereby becoming reduced itself.

Oxidation A process in which there is loss of electrons from an element or ion.

Oxidation ditch A ring-shaped channel, 1 – 1.5 m deep, around which wastewater circulates at 0.3-0.6 m/s-1, is aerated by mechanical rotors and undergoes biological treatment by the resident microbes.

Oxidation number The charge which an atom of an element has, or appears to have, in a compound, e.g. chlorine has an oxidation number of – 1.

Oxygen sag curve The longitudinal profile of oxygen concentration in a river.

Ozone A triatomic gas, particularly prevalent in the stratosphere, formed by the reaction:

O + O2 + energy O3 + energy.

Ozone-depleting gases Gases (e.g. oxides of nitrates, chlorine nitrate, halocarbons and water vapor) which cause destruction of the ozone layer, thereby allowing increased amounts of sunlight to reach the earth.

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PAH Polycyclic aromatic hydrocarbons.

PAN Peroxyacetyl-nitrate.

Parasite An organism which lives on or in another living organism of a different species (the host), from which it obtains food and protection, e.g. tapeworms, greenflies.

Parr Fry, i.e. young fish.

Partial pressure The pressure a single gas within a contained mixture of gases would exert if it were the only gas in the container.

Peat bed filter Odorous compounds are removed from an air stream by passage through a bed of uncompacted peat from the upper layer of a bog.

Percentage exceedance The inverse of the return period, i.e. a frequency (e.g. 90 percent) of occurrence of a flood event or rainfall whose associated depth is exceeded that percent (90 percent) of the time.

Periplasmic space A space between the plasma membrane and the outer membrane of a biological cell, sometimes filled with a loose network of peptidoglycan.

Permanent wilting point The water content of the soil beyond which plants cannot exert sufficient suction to extract moisture.

Permeability The rate at which a fluid flows through a porous medium under the hydraulic head operating within the medium. Usually, the greater the porosity, the greater the permeability.

Permitting Authority
The United States Environmental Protection Agency (EPA), a Regional Administrator of the Environmental Protection Agency, or an authorized representative. Under the Clean Water Act, most states are authorized to implement the NPDES permit program.

Pesticide A material used for the mitigation, control or elimination of plants or animals detrimental to human health or economy.

pH A measure of the acidity or basicity of a solution i.e. the negative of the logarithm of the hydrogen ion concentration.

Phagocytose Ingestion of solid particulate matter by a cell.

Phenol An organic compound with a hydroxyl (OH) group bonded directly to a benzene ring.

Phenotic compound Compounds containing a phenol group, i.e. those containing hydrogen, six carbon atoms joined by alternating single and double bonds, and a hydroxyl group attached to the first carbon atom.

Photo-oxidation Oxidation initiated by sunlight.

Photolytic process A process in which radiant energy causes chemical decomposition.

Phototrophic An orientation response to light.

Physico-chemical parameters Instrumental methods of analysis such as turbidimetry, colorimetry, polarography, adsorption spectrometry, spectroscopy and nuclear radiation.

Physiology The science of functioning of living organisms.

Phytoplankton Plankton consisting of photosynthesising plants, such as algae.

Plasma arc destruction A method of incineration in which very hot plasma, heated by the conversion of electrical to thermal energy, pyrolyses and atomizes waste.

Plasma membrane A membrane of 5 – 10 nm containing proteins and lipids, surrounding the cytoplasm of all cells.

Plastid A membrane-bounded body found in the cytoplasm of most plant cells. See chloroplast.

Point-source pollution Pollution from sources which are easily identified, e.g. slurry tank.

Pollination The transfer of pollen, usually by insects or wind, from the anther of a stamen (male part of the flower) to the stigma of a carpel (female part of the flower).

Pollutant, Conservative
Pollutants that do not readily degrade in the environment, and which are mitigated primarily by natural stream dilution after entering receiving bodies of waters. Included are pollutants such as metals.

Pollutant, Non-Conservative
Pollutants that are mitigated by natural biodegradation or other environmental decay or removal processes in the receiving stream after in-stream mixing and dilution have occurred.

Polyampholite A type of polymer.

Polyaromatic compounds (PAHs) Long chain compounds, very persistent in nature, containing the hydroxyl group in a cyclic structure.

Polychlorinated biphenyls (PCBs) A generic term covering a family of chlorinated isomers of biphenyl found in sewage outfalls and industrial and municipal solid wastes.

Polyelectrolytes Long-chain molecules used in the conditioning of sludge which, by neutralizing surface charges, cause bridging across fine particles or floes to form larger particles.

Polymer Giant molecules built up from thousands of smaller molecules, combined together to form a repetitive structure.

Porosity The proportion of void spaces in the soil. The porosity of fine soils, e.g. clay, is low, whereas that of coarser gravelly soils is higher.

Positive project A commencing project for which an Environmental Impact Assessment is considered essential.

Practical Quantification Limit (PQL)
The lowest level that can be reliably achieved within specified limits of precision and accuracy during routine laboratory operating conditions.

Precipitation The depth of rainfall plus the water equivalent of snow, sleet and hail falling during a given measurement period.

Precipitation reaction A physical or chemical reaction which results in the precipitation of one of the products formed.

Predator An organism which lives by killing and consuming other living things, e.g. sparrows, rabbits.

The reduction of the amount of pollutants, the elimination of pollutants, or the alteration of the nature of pollutant properties in wastewater prior to or in lieu of discharging or otherwise introducing such pollutants into a publicly owned treatment works

Primary consumer Organisms which feed directly on the primary producers. These include herbivores, detritus feeders, scavengers and decomposers (animals which feed on dead plant remains).

Primary pollutant Air pollutants which are emitted from an identifiable source, e.g. carbon monoxide from the car engine.

Primary producer Organisms which are capable of using solar energy to make food by the process of photosynthesis, e.g. plants.

Primary succession An ecological succession commencing in a habitat or on a substrate that has never previously been inhabited.

Primary Treatment
The practice of removing some portion of the suspended solids and organic matter in a wastewater through sedimentation. Common usage of this term also includes preliminary treatment to remove wastewater constituents that may cause maintenance or operational problems in the system (i.e., grit removal, screening for rags and debris, oil and grease removal, etc.).

Procaryotic cell A cell which lacks a distinct nucleus, e.g. bacteria.

Process Wastewater
Any water which, during manufacturing or processing, comes into direct contact with, or results from the production or use of any raw material, intermediate product, finished product, byproduct, or waste product.

Production-Based Standard
A discharge standard expressed in terms of pollutant mass allowed in a discharge per unit of product manufactured.

Profundal zone The zone of a lake lying below that depth at which the light compensation point occurs

Project thresholds Pre-determined levels relating to project size, production or site required which, jf exceeded by a commencing project, define the necessity for an Environmental Impact Assessment.

Proposed Permit
A state NPDES permit prepared after the close of the public comment period (and when applicable, any public hearing and administrative appeals) which is sent to EPA for review before final issuance by the state.

Protista The microbial kingdom to which unicellular or cell groups of eucaryotic organisms which lack true tissues, e.g. protozoa, belong.

Protozoa Aquatic, free-living and parasitic organisms, these are the most basic of all animals, with only one single cell and measuring no more than 5 – 1000 µm in size.

Proteins Substances containing the elements carbon, hydrogen, oxygen, nitrogen and occasionally sulfur, whose main function is cell growth and repair.

Proton Positively charged particle contained within the nucleus of an atom.

Protoplasm The living matter of a cell, comprising both the nucleus and the cytoplasm.

Pseudopodium A temporary protrusion of cytoplasm from the surface of a cell which serves for both cell motion and ingestion.

Psychrophilic temperatures Those in the range 0 – 10 ºC.

Publicly Owned Treatment Works (POTW)
A treatment works, as defined by Section 212 of the CWA, that is owned by the state or municipality. This definition includes any devices and systems used in the storage, treatment, recycling, and reclamation of municipal sewage or industrial wastes of a liquid nature. It also includes sewers, pipes, and other conveyances only if they convey wastewater to a POTW treatment plant . Privately-owned treatment works, Federally-owned treatment works, and other treatment plants not owned by municipalities are not considered POTWs.

Pyrolysis A high temperature process involving the decomposition of organics without oxygen, air at steam. Burnable gas is released as a by-product.

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Quench tank A tank containing water to cool ashes and unburned materials which fall from the grates into a residue hopper during combustion.

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Radiation The emission of rays and particles characteristic of radioactive substances.

Radical An element or atom, or group of these, normally forming part of a compound and remaining unaltered during that compound’s ordinary chemical changes.

Radioactive cloud An artificially generated atmospheric cloud containing radioactive compounds.

Rapid gravity filter A filter used in water treatment which removes suspended solids from water by passing it through a sand bed, where the solids collect as a surface mat and in the sand interstices. The water should previously have been treated by coagulation, flocculation and sedimentation.

Raw sewage - Untreated wastewater.

Recharge The process of renewing underground water by infiltration.

Reclaimed water - Product produced by tertiary treatment of wastewater.

Redox potential The oxidizing or reducing power of a reactant.

Reduced groundwater Groundwater which contains no oxygen.

Reduction A process in which an atom or ion gains electrons.

Regeneration rate The rate of reproduction of bacteria, the method of which is usually by binary fission.

Regolith All loose earth material above the underlying soil rock.

Retention time The length of time a wastewater remains in a clarification tank, an important design parameter in the optimization of settling of suspended solids.

Return activated sludge Settled activated sludge from the clarifier which is returned to the aeration tank to ensure an active population of microbes will be mixed with the incoming wastewater.

Return period The long-term average of the intervals between successive exceedances of a flood magnitude.

Reverse osmosis A membrane process in which solutions of two different concentrations are separated by a semi-permeable membrane. An applied pressure gradient greater than the osmotic pressure ensures flow from the more concentrated to the less concentrated solution.

Reynold’s number The ratio of inertial to viscous forces in a fluid, the value of which will determine whether the Quid flow is turbulent or viscous.

Ribonucleic acid (RNA) A large organic molecule found in the cell cytoplasm containing a phosphate, five-carbon sugars (ribose) and four nitrogenous bases in a repetitive structure.

Ribosomes Tiny bodies containing RNA responsible for protein synthesis and found in the bacterial cell cytoplasm.

Riffle area An area of the river bed covered by grains of too large a size and weight to be carried by the water and so were dispersed.

Root zone The soil layer from which plant roots can extract water during transpiration.

Rotary screen See trommel.

Rotary kiln incineration A process by which waste enters an inclined, rotating kiln, is mixed with air and combusted.

Rotating biological contactor A form of biological treatment in which fixed media is grown on circular discs mounted on a horizontal axle. These discs are partially submerged in wastewater while the axle rotates, allowing bio-oxidation of the wastewater, using oxygen from the air.

Roughing filter A high-rate trickling filter of depth 1 – 2m, hydraulic loading 10-40m3/m2/d and organic loading 0.32 – 1.0 kgBOD/m3/d through which wastewater may be passed prior to an activated sludge treatment.

Run off - Rainfall or other water that is not absorbed into the soil and drains off the street or land. This runoff moves through the Storm Drain system, not the sewerage system.

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Salmonoids Belonging to the salmon family, Salmonidae.

Sanitary Sewer
A pipe or conduit (sewer) intended to carry wastewater or water-borne wastes from homes, businesses, and industries to the POTW.

Sanitary Sewer Overflows (SSO)
Untreated or partially treated sewage overflows from a sanitary sewer collection system.

Saprophytic An organism which obtains food by absorbing dissolved organic materials resulting from organic breakdown and decay.

Schmutzdeck The surface mat of suspended particles which forms on the surface of a slow sand filter.

Scoping The second stage of an Environmental Impact Assessment which decides the key issues for review within the EIA.

Screening (i) The final sorting stage necessary for high-quality compost, during which uncomposted particles such as wood, glass or plastic are removed by passing through a fine mesh. (ii) The first stage of an Environmental Impact Assessment (EIA) in which the projects to be subjected to an EIA are chosen.

Scrubbing A process by which suspended particles and acid gases are removed from a flue gas stream, the former by absorption onto liquid droplets and the latter by diffusion into the liquid phase.

Scum - Floatable material in wastewater made up of mainly fats, cooking oil and grease which are skimmed off during the treatment process.

Scum well A box used to store the scum which forms on the surface of a wastewater in a clarification tank. Scum is usually drawn off by a horizontal, slotted pipe that can be rotated by a lever or a screw.

Secondary consumers Organisms which feed on the herbivores or other primary consumers, e.g. foxes, lions.

Secondary Industry Category
Any industry category which is not a primary industry category.

Sampling and analyses performed by a facility to determine compliance with a permit or other regulatory requirements.

Secondary pollutant Air pollutants which are formed in the atmosphere by chemical reactions, e.g. ozone.

Secondary production The assimilation of organic matter by a primary consumer.

Secondary succession An ecological succession that takes place in an area where a natural community existed and was removed.

Secondary Treatment - Second stage of wastewater treatment that uses biological process in which bacteria consume organic matter, then settle out as sludge.

Sedimentation - Process by which solid material settles out of a liquid. Sludge settles out of wastewater in sedimentation tanks.

Selective catalytic reduction (SCR) A pre-combustion method of decomposition of NO, in an ajar stream to nitrogen and water by injection of ammonia into the catalytic bed of a combustion chamber.

Selective non-catalytic reduction (NSCR) A post-combustion method of decomposition of NOx in an air stream to nitrogen and water by injection of ammonia downstream of a combustion chamber.

Semi-dry absorption process A method of controlling acids in flue gas emissions, by injection of a calcium hydroxide and water solution into the gases leaving the furnace of an incinerator.

Sensible heat That portion of the heat radiated by the sun which is required to heat the earth.

Sensitive area A water body which may intermittently suffer eutrophication.

Sequencing batch reactor A time-stepped batch process for the biological treatment of liquid hazardous waste.

Sere A term used to describe a succession of communities, each following one after the other and finally reaching a stable state.

Settling chamber The purification of an air stream by reducing the velocity of the gas so that the particles drop out by gravity.

Settling tank A rectangular or circular tank in which particle velocities within the liquid are sufficiently reduced to allow the suspended material to be removed from the liquid by gravity settling.

Settling velocity This is the velocity at which a particle will fall to the bottom of a settling tank and is equal to the surface overflow rate for a rectangular tank.

Severance The physical or psychological division of an existing community or property due to traffic development.

Sewage Wastewater and other refuse such as faeces, carried away in sewers.

Sewerage System of pipes and treatment plants which collect and dispose of sewage in a town.

Sheath A hollow, tubelike structure found in most bacteria surrounding a chain of cells.

Significant Industrial User (SIU)
An indirect discharger that is the focus of control efforts under the national pretreatment program; includes all indirect dischargers subject to national categorical pretreatment standards, and all other indirect dischargers that contribute 25,000 gpd or more of process wastewater, or which make up five percent or more of the hydraulic or organic loading to the municipal treatment plant.

Sisal Strong, durable white fiber of agave used in the making of ropes.

Slag The fused bottom ash produced by the incineration process containing incombustibles, the ash fraction of combustibles and any undestroyed pollutants.

Sloughing A term which describes the falling off of the slime layer of micro-organisms on the media of a trickling filter due to the development of anaerobic conditions and lack of food caused by an increase in slime thickness.

Slow sand filter A filter which removes suspended solids from raw water by passing it through a sand bed, where the solids collect as a surface mat and in the sand interstices. Filtration rates are in the order of 2 – 51/m2/min.

Sludge The accumulation of solids resulting from chemical coagulation, flocculation and sedimentation after water or wastewater treatment.

Sludge bulking A phenomenon caused when a large number of filamentous micro-organisms present in the mixed liquor interferes with the compaction of the floe and produces a sludge with a poor settling rate.

Sludge conditioning Addition of chemicals, polyelectrolytes or heat treatment to improve the rate of dewatering.

Sludge dewatering The mechanical unit operation used to reduce the moisture content of sludge to 70 – 75 percent and thus ensure that the remaining sludge residue effectively behaves as a solid for handling purposes.

Sludge stabilization The process of destroying or inactivating pathogens.

Sludge volume index (SVI) A measure of the ability of sludge to settle, coalesce and compact on settlement.

Small Concentrated Animal Feeding Operation (Small CAFO)
An AFO that is designated as a CAFO and is not a Medium CAFO.

Small Construction Activity
Clearing, grading, and excavating resulting in a land disturbance that will disturb equal to or more than one acre and fewer than five acres of total land area but is part of a larger common plan of development or sale that will ultimately disturb equal to or fewer than five acres. Small construction activity does not include routine maintenance that is performed to maintain the original line and grade, hydraulic capacity, or original purpose of the site.

Smog Dense, smoky fog, the formation of which is promoted by reactions between unsaturated hydrocarbons and oxides of nitrogen in the presence of sunlight and under stable meteorological conditions.

Soilbed A large tract of land, the microbes within which remove odorous compounds from an air stream

Soil horizons The soil layers seen in a vertical soil profile, characteristic of soil-forming processes over time.

Soil profile A vertical cut through the soil revealing a sequence of horizons.

Soil suction Water pressure within a soil which is less than atmospheric pressure.

Solid waste All the wastes arising from human and animal activities which are normally solid and are discarded as useless or unwanted.

Solubility product The equilibrium constant for a reaction involving a precipitate and its constituent ions, e.g. for magnesium sulfate MgSO4 = Mg+2 + SO4-2, the solubility product = [Mg+2][SO4-2]

Solute A substance dissolved in a fluid.

Solution The conversion of a solid or gas into liquid form by mixing with a solvent.

Solvent A liquid capable of or used for dissolving something.

Sound exposure level (SEL) Used to express the energy of isolated noise events, the SEL is that constant level in decibels lasting for one second which has the same amount of acoustic energy as a transient noise.

Sound intensity The average sound power per unit area normal to the direction of propagation of a sound wave.

Sound power The rate, measured in watts, at which energy is transmitted by oscillating sound waves.

Spates A river in flood.

Specific flux A measure of rate of flow per unit area.

Specific resistance to filtration (SRF) A laboratory-determinable wastewater sludge parameter.

Spectrophotometry An instrumental method of measuring the intensity of light in various parts of the spectrum.

Spill Prevention Control and Countermeasure Plan (SPCC)
A plan prepared by a facility to minimize the likelihood of a spill and to expedite control and cleanup activities should a spill occur.

Spore A unicellular or multicellular microscopic body involved in plant, bacteria and protozoan reproduction.

Stable inversion On moving downwards through the atmosphere, a cool parcel of air becomes heated and less dense than the surrounding air, thereby being pushed back up. It finds itself in a stable position – wanting neither to move up nor down.

Stabilization pond A quiescent, diked pond in which wastewater undergoes biological treatment under microbial action.

Standard Industrial Classification (SIC) Code
A code number system used to identify various types of industries. The code numbers are published by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. A particular industry may have more than one SIC code if it conducts several types of commercial or manufacturing activities onsite.

Statement of Basis
A document prepared for every draft NPDES permit for which a fact sheet is not required. A statement of basis briefly describes how permit conditions were derived and the reasons the conditions are necessary for the permit.

Stenotopic A term describing an organism which is tolerant of a narrow range of habit.

Step feed aeration An aeration system in which a portion of the sewage load is added at each of several inlets, thus spreading out the oxygen demand over the length of the tank so that oxygen utilization is more efficient.

Stern layer The innermost ion layer tightly attached to the surface of a colloidal particle.

Stratosphere The temperature-constant region of the atmosphere above the troposphere which contains oxygen and ozone.

EPA's computerized STOrage and RETrieval water quality data base that includes physical, chemical, and biological data measured in waterbodies throughout the United States.

Storm Drain - A pipeline or channel system that carries surface water and/or runoff to public waters, but does not feed into sewer system.

Stormwater runoff, snow melt runoff, and surface runoff and drainage.

Stormwater Discharge-Related Activities
Activities that cause, contribute to, or result in stormwater point source pollutant discharges, including excavation, site development, grading, and other surface disturbance activities; and measures to control stormwater, including the siting, construction, and operation of BMPs to control, reduce, or prevent stormwater pollution.

Supernatant The partially purified water, high in suspended solids and ammoniacal nitrogen, which is released during the digestion process and whose quality and amount is dependent on the type and settling quality of the waste and on the digester system efficiency.

Surface tension The minimization of the surface of a free body of liquid due to the unbalanced attractions exerted by the liquid and the air on the liquid surface molecules.

Surge channel A channel or basin designed to take excess flow.

Suspended growth The free-moving, aerobic, microbial culture used in the biological treatment of wastewater by the activated sludge process.

Suspended solids Solids in suspension in a water or wastewater which can be removed by filtration.

Suspension A substance consisting of particles suspended in a medium.

Sustainable development Projects undertaken with care to preserve and manage resources, use genetic engineering with responsibility, search for technical alternatives to existing energy sources and control land, water and air pollution.

Synoptic storm A storm covering several hundred miles, associated with frontal activity and/or intense low pressure centers.

Synthetic organics Man-made, organic compounds, some of which are carcinogenic, including surfactants pesticides, cleaning solvents and trihalomethanes.

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Tapered aeration An aeration system which equalizes the quantity of air supplied to the demand for air exerted by the micro-organisms as the liquor flows through the aeration tank.

Technology-Based Effluent Limit
A permit limit for a pollutant that is based on the capability of a treatment method to reduce the pollutant to a certain concentration.

Temperate climate A climate not exhibiting extremes of either heat or cold, e.g. the Irish climate.

Tempering A process which brings metals to the proper hardness and elasticity by heating after quenching.

Teratogenic Causing developmental malformations.

Tertiary consumer Organisms which feed on secondary consumers, e.g. man.

Tertiary Treatment - The use of filtration to remove microscopic particles from wastewater that has already been treated to a Secondary Level. Anthracite coal is the filter medium used by the MWWD.

Thermal drying An operation which involves reducing the water content of sludge by vaporization of water to air, resulting in a granular dried product of 92 – 95 per cent dry solids concentration.

Thermal plume Heated effluent from an outfall, usually less dense than the receiving water, causing increased growth rates and species changes due to local warming.

Thermocline A horizontal temperature discontinuity layer in a lake in which the temperature falls by at least 1 ºC per meter depth.

Thermophilic temperatures Those in the range 45 – 75 ºC.

Thickening of sludge A process which facilitates disposal of sludge by increasing the solids content to approximately 4 per cent.

Thiocyanates Pseudohalide ions, formed from the oxidation of a CN- group, containing an SCN- group.

Threshold of hearing 10 –12 W/m –2 , i.e. the lowest sound intensity to which the human ear can respond.

Tiered Permit Limits
Permit limits that only apply to the discharge when a certain threshold (e.g., production level), specific circumstance (e.g., batch discharge), or timeframe (e.g., after 6 months) triggers their use.

Tiered Testing
Any of a series of tests that are conducted as a result of a previous test's findings.

Total Maximum Daily Load (TMDL)
The sum of the individual wasteload allocations (WLAs) for point sources and load allocations (LAs) for nonpoint sources and natural background.

Total Organic Carbon (TOC)
Measures the amount of organic carbon in water.

Total Suspended Solids (TSS)
A measure of the filterable solids present in a sample.

Toxic Pollutant
Pollutants or combinations of pollutants, including disease-causing agents, which after discharge and upon exposure, ingestion, inhalation or assimilation into any organism, either directly from the environment or indirectly by ingestion through food chains, will, on the basis of information available to the Administrator of EPA, cause death, disease, behavioral abnormalities, cancer, genetic mutations, physiological malfunctions, (including malfunctions in reproduction) or physical deformations, in such organisms or their offspring. Toxic pollutants also include those pollutants listed by the Administrator under CWA Section 307(a)(1) or any pollutant listed under Section 405(d) which relates to sludge management.

Toxicity Reduction Evaluation (TRE)
A site-specific study conducted in a stepwise process designed to identify the causative agent(s) of effluent toxicity, isolate the sources of toxicity, evaluate the effectiveness of toxicity control options, and then confirm the reduction in effluent toxicity.

Toxicity Test
A procedure to determine the toxicity of a chemical or an effluent using living organisms. A toxicity test measures the degree of effect on exposed test organisms of a specific chemical or effluent.

Toxin A specific poison of biological organic origin.

Transfer station A location to accomplish transfer of solid wastes from collection and other small vehicles to larger transport equipment, with the aim of economizing on waste transportation.

Transmissivity A measure of the rate of flow of water through a water-bearing rock.

Trapping A term describing a plume from an emission stack which is trapped by a stable inversion above the stack mouth, but due to mixing below the mouth level, hits the ground downwind rather than beside the stack.

Trickling flter A biological reactor in which micro-organisms, growing as a slime on the surface of fixed media, oxidize the colloidal and dissolved organic matter in wastewater using atmospheric oxygen which diffuses into the thin film of liquid as the wastewater is trickled over the slimed surfaces at regular intervals.

Transpiration The loss of water vapor from the surface of the plant due to evaporation.

Trommel A rotary screen used to separate out the various size fractions of municipal solid waste.

Trophic levels One of the hierarchical strata of the food web characterized by organisms which are the same number of steps removed from the primary producers.

Tropopause The interface between the troposphere and stratosphere.

Troposphere The layer of atmosphere extending from the earth’s surface to the stratosphere.

TSS - Total Suspended Solids - Solids in water that can be trapped by a filter. TSS can include a wide variety of material, such as silt, decaying plant and animal matter, industrial wastes, and sewage.

Tubificid worm (Potamothrix hummonensis) A benthic worm, tolerant of low oxygen conditions, belonging to the genus Tubifex.

Tundra A vast, level, treeless region with an arctic climate and vegetation.

Turbidity The clarity of water, i.e. a measure of the accumulation of collidal particles, determined by light transmission through the water.

Turbulent mixing When a flow of liquid or air becomes large, the streamlines become irregular and parcels of the flowing substance begin to move in a highly irregular path while maintaining a net downstream velocity.

Abbreviation for Treatment Works Treating Domestic Sewage. Includes all POTWs and other facilities that treat domestic wastewater, and facilities that do not treat domestic wastewater, but that treat or dispose of sewage sludge.

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Ultrafiltration Filtration technique used in water treatment to separate out bacteria larger than 1 - 100 µm.

Upflow column Where the water/wastewater flows upward under pressure through a column or tank, instead of downward by gravity. When used in water filtration, it is akin to the backwashing process in rapid gravity filters. Also used in aerobic digestion of industrial wastewaters where the column is packed with aggregate or synthetic material.

An exceptional incident in which there is unintentional and temporary noncompliance with the permit limit because of factors beyond the reasonable control of the permittee. An upset does not include noncompliance to the extent caused by operational error, improperly designed treatment facilities, inadequate treatment facilities, lack of preventive maintenance, or careless or improper operation.

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Vadose zone The entire zone of negative water pressures above the water table, the lowest portion of which is permanently saturated by capilliary rise.

Valency The number of electrons which an atom of an element must either lose or gain to achieve a noble gas structure.

Van der Waals forces The forces which exist between the molecules in a crystal.

Vibrating screen Used to remove undersized components of municipal solid waste.

Virion A mature virus.

Volatile acid A fatty acid with, at most, six carbon atoms which are water soluble.

Volatile solid Solids, frequently organic, which volatilize at a temperature of 550 ºC.

Vortex shedding Turbulent eddies which are shed from the downstream corners of buildings in a wind/water environment.

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Waiver - See Modified Permit

Wasteload Allocation (WLA)
The proportion of a receiving water's total maximum daily load that is allocated to one of its existing or future point sources of pollution.

Waste minimization The general trend in developed countries to reduce the quantities of waste material produced.

Water Quality Criteria
Comprised of numeric and narrative criteria. Numeric criteria are scientifically derived ambient concentrations developed by EPA or states for various pollutants of concern to protect human health and aquatic life. Narrative criteria are statements that describe the desired water quality goal.

Water Quality Standard (WQS)
A law or regulation that consists of the beneficial use or uses of a waterbody, the numeric and narrative water quality criteria that are necessary to protect the use or uses of that particular waterbody, and an antidegradation statement.

Water Quality-Based Effluent Limit (WQBEL)
A value determined by selecting the most stringent of the effluent limits calculated using all applicable water quality criteria (e.g., aquatic life, human health, and wildlife) for a specific point source to a specific receiving water for a given pollutant.

Waters of the United States
All waters that are currently used, were used in the past, or may be susceptible to use in interstate or foreign commerce, including all waters subject to the ebb and flow of the tide. Waters of the United States include all interstate waters and intrastate lakes, rivers, streams (including intermittent streams), mudflats, sand flats, wetlands, sloughs, prairie potholes, wet meadows, playa lakes, or natural ponds.

Watershed Line between the headstreams of river systems, dividing one catchment from another.

Water table The level of water within the soil at which the pore water pressure is equal to the atmospheric pressure.

Wavelength The horizontal distance between two successive wave crests or between two Wave troughs or between any two corresponding points on the wave surface.

Waveperiod The time taken for two successive wave crests or two wave troughs or any two corresponding points on successive waves to pass a fixed point in space.

Areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas.

Wet oxidation A method whereby waste, either dissolved in water or emulsified, is oxidized at very high temperatures and pressures.

Whole Effluent Toxicity (WET)
The total toxic effect of an effluent measured directly with a toxicity test.

Windrow A form of composting in which pretreated refuse is laid out in heaps with a triangular cross section of 2 – 3 m width at the base and a height of 2 m and turned at regular intervals.

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Zeta potential A measure of the charge on a colloidal particle.

Zone settling Particles are so close together that interparticle forces hinder the settling of neighbouring particles, causing all the particles to remain in a fixed position relative to each other and to settle at a constant velocity.