Terms and technologies A-Z

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A

Absorbent
A substance, a solid or a liquid, that allows another substance, a liquid or a gas, to permeate it.

Absorptive Refrigeration
Absorption refrigeration uses a source of heat to provide the energy needed to drive the cooling process.

Activated Carbon
A solid adsorbent material that is used to remove organic pollutants from liquid or gas streams.
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Activated Sludge
(1) The flocculent mass of microorganisms, mainly bacteria, that develops when sewage or liquid effluent is aerated; (2) A continuous process in which a liquid effluent is aerated in a tank to reduce the BOD and ammoniacal nitrogen.
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Adsorbent
A substance, usually porous, that allows the molecules of a gas or liquid to adhere to its large surface area.

Aeration
The introduction of air into a liquid so that gaseous oxygen dissolves into the liquid.

Aerator
A mechanical device that transfers oxygen from air into solution.

Aerobic
(1) Microorganisms that require oxygen for their respiratory processes; (2) An environment in which oxygen is available.

Aerosol
Very small liquid or solid particles dispersed in a gas or air, for example a mist or fume; may be formed by blowing liquid through a nozzle.

Air Flotation
The separation of solids and oils from liquid effluents by their attachment to very small air bubbles which float the material to the surface.
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Air Knives
A non-thermal, mechanical de-watering technique, an air knife is a highly effective process drying application.

Ammoniacal Nitrogen
Nitrogen combined with hydrogen in the form of ammonia (NH3) or the ammonium ion (NH4+); present in sewage, toxic to fish and restricted in discharges to about 5·0-10·0 mg/l.

Anaerobic Digester
A tank or vessel which excludes oxygen and in which a sludge or liquid is modified by the action of anaerobic bacteria.
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Anaerobic
(1) Microorganisms that do not require oxygen for their respiratory processes; (2) An environment in which oxygen is not available.

Anoxic
Describes a condition in which a liquid has become devoid of oxygen and in which certain microorganisms can obtain oxygen from nitrate or nitrite ions.

Attrition Scrubbing
A process that grinds soil particles together in the presence of a liquid to remove surface contamination.

Autothermal
Used to describe the combustion or digestion of a material of sufficient calorific value or with effective heat recovery so that little or no additional fuel is needed after the initiation stage.

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B

Bag Filter
Textile or sintered polymer filters used to remove dust and fume particles from gas streams.
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Balancing Tank
Provides sufficient storage volume to permit a non-uniform flow of waste water to be collected, mixed and pumped forward to a treatment system at a uniform rate.
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Belt Press
A device in which waste sludge is compressed between two moving belts, thickening and dewatering it.
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Biochemical Oxygen Demand (BOD)
Measure of the amount of oxygen (in mg) required by aerobic bacteria to decompose the biodegradable organic material in 1 litre of an effluent. (See also COD and TOC)

Biocide
A chemical that is used to kill selected living organisms; for example pesticides, herbicides, fungicides.

Biodegradable
Describes a substance that can be decomposed by microorganisms.

Biofilm, biological film or microbial film
A layer of microorganisms that develop on surfaces and attach to it confined in a biofilm that is used in trickling filters, biotowers, membrane surfaces and contactors and oxidises organic material from effluent.

Biofilm has a reverse effect when encountered in heat exchangers and membrane filtration units where it is unwanted as it leads to a problem of biofouling. Biofouling reduces the heat flux first and the permeate flux second, leading to problem of corrosion and degradation of the material, production of bad odours and contamination problems amongst other things.

Biofilter
Removes odours from gaseous emissions by passing the gas through layers of peat, heather or similar substrate on which microorganisms grow.
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Biofuel Boilers
Biofuel boilers will burn waste wood, sawdust, cardboard and short rotation coppice, amongst other things.

Biogas
Gases produced by microorganisms. In anaerobic conditions this usually refers to the mixture of methane (CH4) and carbon dioxide (CO2), produced by the anaerobic digestion of sludges or organic material in landfill sites. This can be used to generate heat or power.

Biological Filter
A packaged treatment system that removes BOD and suspended solids from relatively small flows of sewage or industrial wastewater by passing the effluent over plastic media on which aerobic biofilm develops.
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Biomass
The total amount of organic material of biological origin in a given area or volume. This definition excludes any fossil fuel (coal, petroleum, natural gas, tars...). Usually biomass refers to plants grown for use as biofuel, but it also includes plant or animal matter used for production of fibres, chemicals or heat. This comprises biodegradable wastes from any biological sources.

Bioremediation
A treatment that enhances the ability of naturally occurring or cultured microorganisms to degrade organic pollutants in soil, water and air.
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Biotower
An above ground structure containing plastic media covered in microbial film that reduces the BOD of industrial liquid effluent flowing over it.
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Bund
An outer wall or tank designed to retain the contents of an inner tank in the event of leakage or spillage.

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C

Cadmium
A toxic heavy metal to which the most stringent discharge standards apply; a prescribed red list compound with an environmental quality standard (EQS) of 5 -g/l.

Catalyst
A substance which increases the rate of a chemical reaction without being consumed in the reaction.

Catalytic Incinerator
A device in which the use of catalysts allows the combustion of solvent vapours and other VOCs to be carried out at relatively low temperatures.
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Centrifuge
A device that separates solid material, sludges or oil from liquid effluents by rapid rotation.
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Chemical Oxygen Demand (COD)
The amount of oxygen (in mg) required for the complete chemical oxidation of organic and inorganic material in 1 litre of an effluent. (See also BOD and TOC)

Chlorinated Hydrocarbons
Organic compounds containing the elements carbon, hydrogen and chlorine, implicated in low level ozone production and high level ozone destruction.

Chromium
A toxic heavy metal that may contaminate groundwater; stringent environmental discharge limits apply.

Coagulation
A process that alters the surface charge on dispersed colloidal particles in a liquid so that they are able to agglomerate; the first stage in floc formation.

Combined Heat and Power (CHP)
A device that allows the cogeneration of heat and electricity and can be coupled to the incineration of solid waste.

Composting
The aerobic decomposition of biodegradable organic matter under controlled conditions into a soil-like substance called compost.
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Condenser
Removes high concentrations of solvent vapours from gas streams by cooling, allowing both the liquid solvent fraction and the gas to be recycled or otherwise treated.
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Contact Stabilisation
An activated sludge process that produces less surplus sludge; separated sludge is aerated vigorously before it contacts and stabilises raw sewage.

Cryptosporidium
A protozoan parasite causing diarrhoea in humans and other animals; the pathogen is spread in water as spherical oocysts 5 µm diameter, resistant to chlorine treatment.

Cyanides
Toxic inorganic chemical compounds containing cyanide (-CN) groups and classed as special waste.

Cyclone
A device in which a gas stream is spun and coarse grit and dust particles are thrown outwards to the cyclone wall where they are guided into a hopper.
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D

Denitrification
Biological process that reduces nitrates ultimately into N2 nitrogen gas.

Desorption
The removal of adsorbed gas or liquid molecules from the surface of a solid.

Desulphurisation
The removal of sulphur compounds from flue gas in the form of solid sulphur or as calcium sulphate (gypsum).

Digestion
The enzymatic breakdown of large insoluble organic molecules into small soluble organic molecules which can be absorbed and used by either aerobic and anaerobic microorganisms.

Dioxins
A group of toxic organic substances, containing carbon, hydrogen, oxygen and chlorine, resistant to biodegradation, produced when organic material is incinerated at low temperatures.

Dissolved Solids (DS)
See Total Dissolved Solids (TDS)

Drag-out Bath
Container of water in which metal plated objects are rinsed to remove excess plating solution.

Dry Solids (DS)
Dry solids are the solid recovered buy drying the sample at 105°C.

Dust
Solid particles about 1-10µm in size.

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E

Effluent
(1) A flow containing polluting material; (2) Liquid waste from sewage treatment, industry, agriculture.

Electrochemical Cell
A device that removes metals from aqueous solutions by plating them as solid metal on the cathode or oxidises organic pollutants at the anode.
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Electrokinetic Decontamination
A method of removing metals and other inorganic pollutants from contaminated soils using electrodes.
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Electrolysis
A chemical change brought about by the passage of an electric current through a liquid; used in plating of metals, oxidation of cyanide.

Electrostatic Precipitator
Equipment in which particles in gas streams are passed through ionisers and become charged, facilitating their subsequent removal.
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F

Ferric Chloride or Sulphate
Iron-containing substances used to coagulate and flocculate in effluent treatment processes.

Filter Press
A device that dewaters sludge by compressing it between membranes or cloth lined plates to produce a cake.
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Filtration
Mechanical process of separating solids from a fluid (air, water, oil).
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Fine Bubble Diffuser
An aeration device in which air is blown through a porous ceramic or synthetic membrane to create small bubbles in a liquid effluent.
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Flocculation
The agglomeration of coagulated particles to form a floc which can settle or float; may be assisted by biological, chemical or mechanical means.

Fluidised Bed
A body of solid particles through which gas or liquid is blown or pumped upwards causing separation and movement; a means of increasing reaction rates.
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Fluidised Bed Scrubber
A device that removes pollutant particles or trace gases from a gas stream, in which plastic spheres are wetted by a scrubbing liquor and fluidised by the contaminated gas stream.

Fume
Solid particles smaller than 1µm in a gaseous emission.

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G

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H

Halogenated Hydrocarbons
Organic compounds containing carbon, hydrogen and halogen atoms (fluorine, chlorine, bromine and iodine); implicated in stratospheric ozone loss.

Hazardous Wwaste
Waste that originates from a specified waste stream or has hazardous properties or contains hazardous substances as defined in the Hazardous Waste Directive.

Heat Exchanger
Device used for efficient heat transfer from one medium to another without mixing both media that are always kept separated. A radiator is an example of a heat exchanger. They are classified according to the flow arrangement, co-current, counter current or cross flow.

Heavy Metals
Certain metals, used industrially and harmful to living organisms, for which discharge and emission standards are set; including cadmium, chromium, copper, lead, mercury, nickel, zinc.

High Performance Filter
A device that separates particles 2-500 µm in size from relatively clean liquid streams.
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Hydrocyclone
A device that separates sand, silt and suspended solid particles from a liquid using centrifugal forces generated by the liquid flow.
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Hydrogen Sulphide (H2S)
A foul smelling gas, a product of the reduction of organic sulphur by anaerobic organisms.

Hydrolysis

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I

Incineration
The controlled burning of municipal, clinical and chemical waste material and sewage sludge, commonly in two stages, to produce minimum amounts of gas and ash.

Inorganic
Substances such as sand, clay and metals whose molecular structures do not contain carbon-hydrogen (C-H) bonds except as carbonates and similar.

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J

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K

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L

Leachate
Liquid that has percolated through a solid mass; soil leachate may be high in nitrate; landfill leachate may be high in BOD, ammonia, salinity and toxic substances.

Legionella Pneumophila
A water-borne pathogenic bacterium which causes Legionnaires' Disease; may be present in cooling systems and spread by droplets; controlled by chlorination, ozone, ultraviolet light.

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M

Media
Stones, pebbles or plastic that provides a substrate for the growth of a film of microorganisms in wastewater treatment; plastic, ceramic or metal shapes or surfaces that improve gas-liquid contact in gas scrubbing.

Membrane Filtration
A separation technology suitable for a range of industrial processes in water and waste water treatment processes.
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Methane (CH4)
Highly calorific gas; 60 % of biogas; produced during anaerobic biological processes; may contribute to global warming.

Microfiltration
The use of microporous filters operating under pressure to remove particles or microorganisms of 0·1-5·0 µm size from process and effluent liquids.
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Microorganisms
Microscopic living creatures; bacteria, protozoa, fungi and algae.

Microwave Disinfection Unit
Equipment that shreds clinical waste and disinfects it with steam and microwave radiation.
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Mist Eliminator
Unit that separates small droplets of liquid (mist) from gas streams by impingement on a mesh or plate surface; part of a wet scrubber.
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Mist
Very small droplets of liquid suspended in a gas.

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N

Nanofiltration
A separation technology suitable for a range of industrial processes in water and waste water treatment processes.
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Nitrate Sensitive Area
A defined area, designated by statutory order, within which measures are taken to minimise the leaching of nitrates from agriculture into groundwater.

Nitrate (NO3-)
The most oxidised form of nitrogen; formed from the oxidation of ammonia by aerobic bacteria; present in fertiliser; the preferred form of nitrogen in discharges to rivers.

Nitrification-dentrification
Biological process aimed at removing nitrate and ammonia from a water stream by first oxidising ammonia into nitrate and by reducing nitrates produced by dentification to produce nitrogen gas N2.

Nitrifying Bacteria
Aerobic bacteria that convert ammonia (NH3) into nitrate (NO3-).

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O

Oil interceptor and Separator
Used to intercept rainwater or spillages contaminated with oil and to retain the liquid for subsequent separation.
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Oocysts
Persistent reproductive spores of Cryptosporidium; excreted by infected animals and spread in unfiltered water; highly resistant to chlorine but destroyed by heat or ozone.

Organic
Substances such as proteins, sugars, wood and plastics with molecular structures containing carbon-hydrogen (C-H) bonds.

Organochlorines
Organic compounds containing chlorine, including PCBs, DDT and lindane; accumulate in fatty tissue and may be toxic.

Oxidation
The chemical or biochemical change that occurs when a substance combines with oxygen, for example during combustion and respiration; the release of carbon dioxide and energy from organic compounds.

Oxygen System
Injects oxygen into liquid effluent; increases aerobic microbial activity and the rate of removal of BOD, COD and ammoniacal nitrogen.
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Ozone (O3)
A naturally occurring unstable form of oxygen; attenuates harmful ultraviolet light in the stratosphere; a pollutant in the atmosphere at ground level; a powerful oxidising agent and disinfectant.

Ozoniser
Equipment that generates ozone from oxygen or air by electrical discharge; used in the treatment and disinfection of water and wastewater.
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P

Packed Tower Scrubber
Cleans gases with a scrubbing liquor as they pass through inert media; removes soluble and noxious gases and mist droplets larger than 5 µm.
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PCBs
Polychlorinated biphenyls; very persistent, toxic substances, stored in fatty tissue; previously widely used as fire-resistant dielectric fluids in transformers and capacitors; now prohibited.

Pentachlorophenol (PCP)
A powerful biocide, used to preserve cotton and wood; toxic and persistent; release to the environment now heavily restricted.

pH
A logarithmic scale (0-14) that reflects acidity or alkalinity; pH 7 indicate.s neutrality; acidic solutions have lower pH values; alkaline solutions have higher values

Plate Scrubbers
Remove dust, noxious gases and odours from gaseous emissions by. passing them through perforated plates flooded with scrubbing liquor
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Pollutant
A chemical or substance that causes harm in the environment.

Polychlorinated Biphenyls
See PCBs.

Polyelectrolytes
Natural or synthetic water-soluble macromolecules that assist in the flocculation of dispersed solids.

Potable Water
Water of a quality suitable for drinking.

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Q

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R

Reed Beds
A very efficient filter that incorporates aerobic, anoxic and anaerobic zones where large populations of microorganisms break down contaminants.
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Red List
A list used in the UK that identifies the most persistent and toxic substances so that their discharge to water and accumulation in living organisms may be strictly controlled.

Reverse Osmosis
A high pressure unit that uses membrane filtration to desalinate water to provide drinking water on ships, oil rigs and arid areas.
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Rotary Scrubber
A device that separates particulate contamination from a gas stream by impaction onto a wet rotor or into a fine spray of scrubbing liquor.
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Rotating Biological Contactor
An aerobic treatment system that rotates plastic media with fixed microbial film alternately through liquid effluent and air to reduce the BOD value.
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S

Sand Filter
Separates suspended solids and associated BOD from liquids by passing the liquid through a bed of sand.
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Screens(Parabolic)
This is a type of course filtration that separates solid material from liquid by allowing the liquid to pass through constrictions (the screen) of chosen size.
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Septic Tank
Container that receives untreated sewage, holds back floating scum and retains and digests heavy solids but allows clarified liquor to proceed forward for further treatment or discharge.

Settleable Solids
The settleable solids test is the measurement of the volume of solids in one litre of sample that will settle to the bottom of an Imhoff cone during a specific time period. The test indicates the volume of solids removed by settling in sedimentation tanks, clarifiers or ponds. The settleable solids test indicates whether the primary and secondary processes are functioning properly. Testing for settealabe solids is important to assess the settling tank and clarifier pound.

Settlement Tank
Container that allows sufficient retention time for the separation of organic and inorganic solids from liquid and produces sludge and clarified liquor both of which may receive further treatment.
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Sewage
Liquid effluent from domestic and industrial activity.

Sludge
The wet solids that can be settled from an untreated liquid effluent (primary sludge); or from aerobically treated effluent (secondary sludge).

Slurry

  1. Can be a very thick suspension of solid in a liquid (like plaster or cement before setting), paper pulp
  2. Mixture of urine, faeces and wash water from cattle or pig rearing.

Soil Washing
Water-based volume reduction process in which contaminated soil or dredgings are separated into a small contaminated fraction, for further treatment or disposal, and a larger cleaned fraction.
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Special Waste
Waste that contains substances considered to be dangerous to life as listed in the Control of Pollution (Special Waste) Regulations 1980; applies within the UK.

Static Tubular Aerator
A system in which air is blown into vertical tubes submerged in a liquid, promoting mixing and oxygen transfer.
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Submerged Aerated Filter
A device in which highly aerated effluent flows through inert media that is completely submerged; fixed microbial film reduces the BOD and ammoniacal content of the effluent.
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Submerged Biological Contactor
an aerobic treatment system in which plastic media that is largely submerged in effluent is rotated by rising air, the fixed microbial film on the media reducing the BOD value.
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Submicron Particles
Particles of less than 1millionth of a metre in size, present in smoke and fumes.

Sulphur Dioxide (SO2)
Sulphur dioxide produces acid rain by tranforming into sulphuric acid, especially in the presence of nitrogen oxide.

Surface Aerator
Equipment in which blades or vanes rotate about a vertical or horizontal shaft and entrain air to mix and aerate wastewater.
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Suspended Solids
Solids of organic and inorganic origin present in liquid effluents; concentration in (mg/l) measured by filtration followed by drying at 105 oC.

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T

Thermal Oxidation
High temperature incineration of pollutants such as VOCs and odours in gas streams.
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Total Dissolved Solids (TDS)
This is an expression for the combined content of all inorganic and organic substances contained in a liquid which are present in a molecular, ionized or micro-granular suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two micrometers.

Total Organic Carbon (TOC)

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Total Solids (TS)
TSS+TDS

Total Suspended Solids (TSS)
is the faction of very small solids particle colloids that remain in suspension this is usually measured by by filtering 1 litre of water on a pre weight 2µm filter and measuring the weight og the residue retain and expressed in (mg/l).

Trickling Filter
A bed of gravel or pebble media through which clarified effluent percolates and microbial activity removes BOD, suspended solids and ammoniacal nitrogen.
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Turbidity
A measure of the cloudiness of a liquid that is caused by the presence of fine suspended solids.

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U

Ultrafiltration Units
A technology using membrane filtration to disinfect liquid through the direct removal of bacteria, viruses and pathogens.
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Ultrapure Water
Water from which most other materials have been removed, producing a resistivity of at least 18 M-cm, and making it sufficiently pure for the most exacting microelectronic, pharmaceutical and medical uses.

Ultraviolet Light System
A disinfection system in which light of 254 nm, produced by mercury lamps, damages microorganisms by disrupting their genetic material. This can be used in water, air or surface treatment.
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UV Ozone
A powerful wastewater treatment process employing ozone in solution and ultraviolet light; oxidises COD and toxic substances, removes colour and disinfects.
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V

Vapour Extraction
A technique that makes use of a partial vacuum to remove volatile and semi-volatile contamination, such as hydrocarbons, from soils.
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Venturi Aerator
A submerged jet aeration device in which the liquid to be aerated is pumped through a nozzle within a larger diameter tube, causing air entrainment and vigorous liquid mixing.
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Venturi Scrubber
A device that removes submicron particles and reactive trace gases from gas streams by passing the gas at high velocity through the scrubbing liquor ensuring vigorous mixing.
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Volatile Organic Compounds (VOCs)
Substances such as solvents that are liquid at room temperature, but vaporise significantly and take part in the generation of ozone pollution in the lower atmosphere.

Volatile Solids (VS)
In water treatment they are different from VOC . They are those solids in water (or other liquid) that are lost on ignition of the dry solids at 550°C.

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W

Waste Minimisation
The analysis and revision of the use of materials, processes, equipment and procedures in order to reduce the unnecessary use of energy and resources, to reduce the generation of effluents and wastes and to recycle where practical.

Waste Incineration
Incineration of waste at high temperatures in two stages to promote efficient combustion to an ash and carbon dioxide.
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Wet Air Oxidation
A process that treats waste water containing high strength or hazardous organic chemicals by a combination of high temperature and pressure that creates highly reactive hydroxyl radicals.
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X

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Y

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Z