skip to Main Content

Glossary of Terms Related to Water Treatment


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

Activated carbon

a solid adsorbent material that is used to remove organic pollutants from liquid or gas streams

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


a substance, usually porous, that allows the molecules of a gas or liquid to adhere to its large surface area


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


(1) microorganisms that require oxygen for their respiratory processes; (2) an environment in which oxygen is available


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

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


(1) microorganisms that do not require oxygen for their respiratory processes;

(2) an environment in which oxygen is not available


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


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

Bag filter

textile or sintered polymer filters used to remove dust and fume particles from gas streams. Used also on liquid waste to provide a final polish or remove floc particles


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

Belt press

a device in which waste sludge is compressed between two moving belts, thickening and dewatering it

Biochemical oxygen demand(BOD) MORE INFO

the amount of oxygen (in mg) required by aerobic bacteria to decompose the biodegradable organic material in 1 litre of an effluent


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


describes a substance that can be decomposed by microorganisms

Biofilm, biological film or microbial film

a thin slimy layer of microorganisms that develops on the surface of the media in trickling filters, biotowers and contactors and oxidises organic material from effluent


removes odours from gaseous emissions by passing the gas through layers of peat, heather or similar substrate on which microorganisms grow

Biogas a mixture of methane (CH4) and carbon dioxide (CO2), produced by the anaerobic digestion of sludges or organic material in landfill sites; 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


the amount of organic material of biological origin in a given area or volume


a treatment that enhances the ability of naturally occurring or cultured microorganisms to degrade organic pollutants in soil


an above ground structure containing plastic media covered in microbial film that reduces the BOD/COD/Ammonia of industrial liquid effluent flowing over it


an outer wall or tank designed to retain the contents of an inner tank in the event of leakage or spillage


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


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


a device that separates solid material, sludges or oil from liquid effluents by rapid rotation

Chemical oxygen demand (COD)   MORE INFO 

the amount of oxygen (in mg) required for the complete chemical oxidation of organic and inorganic material in 1 litre of an effluent

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) system designed to use the heat produced during the generation of electricity; can be coupled to the incineration of solid waste

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

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

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 air flotation (DAF) a method of dissolving pressurised air into a liquid to form small bubbles that will float material to the surface

Drag-out bath container of water in which metal plated objects are rinsed to remove excess plating solution

Drainfield consists of long underground perforated pipes or tiles connected to a septic tank. The network of pipes is laid in gravel-filled trenches (2-3 feet wide), or beds (over 3 feet wide) in the soil. Liquid waste or effluent flows out of the tank and is evenly distributed into the soil through the piping system. The soil below the drainfield provides the final treatment and disposal of a septic tank effluent. After the effluent has passed into the soil, most of it percolates downward and outward, eventually entering the groundwater. A small percentage is taken up by plants through their roots, or evaporates from the soil.

Dust solid particles about 1-10µm in size

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

Electrokinetic decontamination a method of removing metals and other inorganic pollutants from contaminated soils using electrodes

Electrolysis a chemical change brought about by the passage of an electric current through a liquid; used in plating of metals, oxidation of cyanide

Electronic Water Disinfection a method of sterilising water by introduction of precise amounts of copper and silver; suitable for cooling towers (Legionella control), potable water, swimming pool and effluents

Electrostatic precipitator equipment in which particles in gas streams are passed through ionisers and become charged, facilitating their subsequent removal

Eutrophication is a process taking place in natural waters. The process is characterized by development of an environment rich in nutrients and consequent proliferous plant production. Man-made eutrophication of inland water courses and shallow coastal waters is caused by excessive discharge of nutrients, especially phosphorus (P) and nitrogen (N) in the form of PO 4 and NO 3 .

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

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

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

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

FOG (Fats Oils and Grease) wastewater components from food preparation, animal or vegetable processing; largely insoluble in water, high COD and likely to float

Fume solid particles smaller than 1µm in a gaseous emission

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

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

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
Hydrocyclone a device that separates sand, silt and suspended solid particles from a liquid using centrifugal forces generated by the liquid flow

Hydrogen sulphide (H 2 S) a foul smelling gas, a product of the reduction of organic sulphur by anaerobic organisms

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

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, silver-copper, ultraviolet light

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

Methane (CH 4 ) 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

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

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

Mist very small droplets of liquid suspended in a gas

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

Nitrifying bacteria aerobic bacteria that convert ammonia (NH 3 ) into nitrate (NO 3 -)

Oil interceptor
 used to intercept rainwater or spillages contaminated with oil and to retain the liquid for subsequent separation

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

Ozone (O 3 ) 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

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

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 indicates 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

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

Reed Bed Managed wetlands used for the treatment of wastewaters; commonly planted with phragmites australis and other species to facilitate BOD removal below soil surface

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 filtration system that uses selectively permeable membranes with extremely small pores to separate ions and particles

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

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

Sand filter separates suspended solids and associated BOD/COD from liquids by passing the liquid through a bed of sand

Screens equipment that separates solid material from liquid by allowing the liquid to pass through constrictions (the screen) of chosen size

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

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

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 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

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

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

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

Submicron particles particles of less than 1µm in size, present in smoke and fumes

Sulphur dioxide (SO 2 )Sulphur dioxide (SO 2 ) toxic gas produced during the combustion of sulphur compounds in fossil fuels; component of acid rain

Surface aerator equipment in which blades or vanes rotate about a vertical or horizontal shaft and entrain air to mix and aerate wastewater

Suspended solids solids of organic and inorganic origin present in liquid effluents; concentration in (mg/l) measured by filtration followed by drying at 105 -C

Thermal oxidation high temperature incineration of pollutants such as VOCs and odours in gas streams

Trickling filter a bed of gravel or pebble media through which clarified effluent percolates and microbial activity removes BOD, suspended solids and ammoniacal nitrogen

Turbidity a measure of the cloudiness of a liquid that is caused by the presence of fine suspended solids
Ultrafiltration the separation from a liquid of particles of 0·005-0·1 µm in size by pumping the liquid through a synthetic membrane at high pressure

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

UV ozone a powerful wastewater treatment process employing ozone in solution and ultraviolet light; oxidises COD and toxic substances, removes colour and disinfects

Vapour extraction a technique that makes use of a partial vacuum to remove volatile and semi-volatile contamination, such as hydrocarbons, from soils

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

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

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

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

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

Back To Top