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


Taking of water from river, reservoir or borehole. Abstractions of surface water and groundwater in England and Wales are subject to an Environment Agency licence. Licence holders include water undertakers, general industry (particularly electricity generating companies) and agricultural concerns.

Acid rain

Rain that has a flamboyantly low pH, due to contact with atmospheric pollutants such as sulphuric oxides.


The quantitative capacity of water to neutralize a base, expressed in ppm or mg/L calcium carbonate equivalent. The number of hydrogen atoms that are present determines this. It is usually measured by titration with a standard solution of sodium hydroxide.

Activated carbon

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

Activated coal

This is the most commonly used adsorption medium, produced by heating carbonaceous substances or cellulose bases in the absence of air. It has a very porous structure and is commonly used to remove organic matter and dissolved gases from water. Its appearance is similar to coal or peat. Available in granular, powder or block form; in powder form it has the highest adsorption capacity.

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.


Separation of liquids, gases, colloids or suspended matter from a medium by adherence to the surface or pores of a solid.

Advanced oxidation process

One of several combination oxidation processes. Advanced chemical oxidation processes use (chemical) oxidants to reduce COD/BOD levels, and to remove both organic and oxidisable inorganic components. The processes can completely oxidise organic materials to carbon dioxide and water, although it is often not necessary to operate the processes to this level of treatment.

Advanced wastewater treatment

Any treatment of sewage water that includes the removal of nutrients such as phosphorus and nitrogen and a high percentage of suspended solids.

Advanced water treatment

The level of water treatment that requires an 85-percent reduction in pollutant concentration, also known as tertiary treatment.

Aerated lagoon

A water treatment pond that speeds up biological decomposition of organic waste by stimulating the growth and activity of bacteria, which are responsible for the degradation.

Aeration tank

A tank that is used to inject air into water.


A process that takes place in the presence of oxygen, such as the digestion of organic matter by bacteria in an oxidation pond.


Very small liquid or solid particles dispersed in air.


The keenness with which an ion exchanger takes up and holds on to a counter-ion. Affinities are very much affected by the concentration of the electrolyte surrounding the ion exchanger.


A process of bringing smaller particles together to form a larger mass.

Aggresive water

Water that is soft and acidic and can corrode plumbing, pipes and appliances.


Single- or multi-celled organisms that are commonly found in surface water, such as duckweed. They produce their own food through photosynthesis. The algae population is divided up into green algae and blue algae, of which the blue algae are very damageable to human health. Excessive algae growth may cause the water to have undesirable odours or tastes. Decay of algae diminishes oxygen supplies in the water.

Algal blooms

Periods of enlarged algal growths that affect water quality. Algal blooms indicate potentially hazardous changes in the chemistry of water.


Alkalinity means the buffering capacity of water; the capacity of the water to neutralize itself. It prevents the water pH levels from becoming too basic or acid. It also adds carbon to water. Alkalinity stabilizes water at pH levels around 7. However, when the acidity is high in water the alkalinity decreases, which can cause harmful conditions for aquatic life.


Sediments deposited by erosion processes, usually by streams.


Asset Management Plan – the Water Industry 5 yearly investment cycle.


The 5th Asset Management Plan since privatisation, to run from 2010 - 2015.


The 6th Asset Management Plan since privatisation, to run from 2015 - 2020.


Automated Meter Reading; a system for automatically collecting and transferring data to a central database for analysis and billing.


A process that takes place in the absence of oxygen, such as the digestion of organic matter by bacteria in a UASB-reactor.

Anaerobic digestion (AD)

AD is a biological process that happens naturally when bacteria breaks down organic matter in environments with little or no oxygen. Almost any organic material can be processed with AD, including waste paper and cardboard (which is of too low a grade to recycle, e.g. because of food contamination), grass clippings, food, agricultural waste, industrial effluents, sewage and animal waste.


A negatively charged ion that results from the dissociation of salts, acids or alkalis in solution.


A site in electrolysis where metal goes into solution as a cation leaving behind an equivalent of electrons to be transferred to an opposite electrode, called a cathode.


Growing in water, living in water or frequenting water.


Something made up of water.

Aqueous solubility

The maximum concentration of a chemical that dissolves in a given amount of water.


A layer in the soil that is capable of transporting a significant volume of groundwater.


A type of hydrocarbon that contains a ring structure, such as benzene and toluene. They can be found for instance in gasoline.

Asset life

The time from the date of installation (when new) of an asset (or part) until the asset (or part) has to be replaced. The remaining asset life is recorded from the present. Asset lives for the current asset base are estimated and only known exactly after the asset has been replaced.


The ability of water to purify itself of pollutants.

Assimilative Capacity

The capacity of natural water to receive wastewaters or toxic materials without negative effects and without damage to aquatic life or humans who consume the water.


The action of one particle rubbing against the other in a filter media or ion exchange bed that can in time cause breakdown of the particles.

Available chlorine

A measure of the amount of chlorine available in chlorinated lime, hypochlorite compounds, and other materials.

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Back Pressure

Pressure that can cause water to backflow into the water supply when a user's waste water system is at a higher pressure than the public system.

Back siphonage

Reverse seepage of water in a distribution system.


The flow of water in a medium in a direction opposite to normal flow. Flow is often returned into the system by backflow, if the wastewater in a purification system is severely contaminated.


Reversing the flow of water back through the filter media to remove entrapped solids.

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.

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.

Bathing waters

Areas designated under EC directive 76/160/EEC. Number of bathers is main criterion.

Bed load

Sediment particles resting on or near the channel bottom that are pushed or rolled along by the flow of water.

Benthic zone

The lower region of a body of water including the bottom.


Salts containing the anion HCO3-. When acid is added, this ion breaks into H2O and CO2, and acts as a buffer.


Chemicals that hold short fibres together in a cartridge filter.


The increase in concentration of a substance in living organisms, as they take in contaminated air, water, or food, due to slow metabolization and excretion.

Biochemical oxygen demand (BOD)

The quantity of dissolved oxygen in water (mg/l) consumed under test conditions during a given period (5 days) through the microbiological oxidation of biodegradable organic matter present in wastewaters. One of the standard tests used to characterise effluent quality.


A chemical that is toxic to microrganisms. Biocides are often used to eliminate bacteria and other single-cell organisms from water.


Ability of substance to decompose without adding chemicals.

Biodegradable pollutants

Pollutants that are capable of decomposing under natural conditions.


Population of various microrganisms, trapped in a layer of slime and excretion products, attached to a surface.

Biological contaminants

Living organisms such as viruses, bacteria, fungi and mammal and bird antigens that can cause harmful health effects to humans.

Biological filtration

A process in which settled sewage uniformly trickles downward through a bed of inert material such as slag, moulded plastics or clinker, thus permitting contact with the biological film with which the surfaces of the medium are coated so that oxidation and clarification take place.

Biological oxidation

Decomposition of complex organic materials by microrganisms through oxidation.

Biologically activated carbon

Activated carbon that supports active microbial growth, in order to aid in the degradation of organics that have been absorbed on its surface and in its pores.


The use of living organisms to test the suitability of effluents for discharge into receiving waters and to test the quality of such waters downstream from the discharge.


The biological treatment of wastewater and sludge, by inducing the breakdown of organics and hydrocarbons to carbon dioxide and water.

Biosolids (sewage sludge)

Produced from the treatment of sewage. Biosolids have to comply with EC and UK legal standards and the Safe Sludge Matrix for agricultural use.


All living organisms in a region or ecosystem.


Conversion of a substance into other compounds by organisms; including biodegradation.


Water that contains waste of humans, animals or food.

Blind spots

Any place on a filter medium where fluids cannot flow through.


A build-up of particles in a filter medium, that prevents fluids from flowing through.

Blue flag beaches

Beaches which meet a number of detailed criteria for cleanliness and facilities including compliance with the EC directive's guideline bathing water standards.


Biodegradable Municipal Waste.


Biological Oxygen Demand.


The amount of dissolved oxygen consumed in five days by bacteria that perform biological degradation of organic matter.


Best Practicable Environmental Option.

Brackish water

Water that is neither falls in the category of salt water, nor in the category of fresh water. It holds the middle between either one of the categories.

Breakpoint chlorination

Addition of chlorine to water until there is enough chlorine present for disinfection of water.


Crack or break in a filter bed that allows the passage of floc or particulate matter through a filter.


Highly salty and heavily mineralised water, containing heavy metal and organic contaminants.


A substance that reacts with hydrogen or hydroxyl ions in a solution, in order to prevent a change in pH.


Bathing Water Directive.


Bathing Water Regulations.

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Solid dewatered residue on a filter media after filtration.

Calcium hypo chlorite

A chemical that is widely used for water disinfection, for instance in swimming pools or water purification plants. It is especially useful because it is a stable dry powder and can be made into tablets.

Candle filter

A relatively coarse aperture filter, designed to retain a coat of filter medium on an extended surface.

Carbon accounting

This is the general term used to describe the accounting undertaken to measure the amount of greenhouse gas emissions (measured in carbon dioxide equivalent units) emitted or avoided being released into the atmosphere by a company’s activities.

Carbon Reduction Commitment (CRC)

The Carbon Reduction Commitment (CRC) is a government scheme applying mandatory (October 2009) emissions trading to cut carbon emissions from large commercial and public sector organisations, including supermarkets, hotel chains, government departments, large local authority buildings and water companies.

Carbonate hardness

Hardness of water caused by carbonate and bicarbonate by-products of calcium and magnesium.


Area drained by a river or river system. Also area drained by a sewerage system.


The Competition Commission considers merger references. It is also the body to which companies can appeal if they disagree with Ofwat decisions on price limits, licence amendments or accounting guidelines.


A separation process, which uses the action of centrifugal force to promote accelerated settling of particles in a solid-liquid mixture.

Cess Pit

A holding tank for wastewater storage which is emptied on a regular basis. No treatment of the wastewater occurs in the cess pit. Used by outlying properties remote from the mains sewerage systems.

Check valve

A valve that allows water to stream in one direction and will then close to prevent development of a back-flow.

Chemical Oxygen Demand (COD)

The quantity of oxygen equivalent to the amount of oxidising agent consumed in oxidising the majority of organic matter present in wastewaters. Does not distinguish between the organic matter liable to be degraded readily by biological means and the more intractable forms. Can include the oxygen demand of some inorganic substances such as sulphides. Measured by testing a sample of the wastewater with potassium dichromate and sulphuric acid.


A chemical complex that consists of chlorine and ammonia. It serves as a water disinfectant in public water supplies in place of chlorine because chlorine can combine with organics to form dangerous reaction products. In which forms chloramines exist depends on the physical/ chemical properties of the water source.


A water purification process in which chlorine is added to water for disinfection, for the control of present microrganisms. It is also used in the oxidation of compound impurities in water.


Combined heat and power. A technology used to generate both heat and power in a single process e.g. burning of biogas produced during sludge treatment.


Destabilisation of colloid particles by addition of a reactive chemical, called a coagulant. This happens through neutralization of the charges.


Liquid particles in suspension that unite to create particles of a greater volume.

Coarse screen

A screen used for removing gross solids from domestic or industrial wastewater, with spaces between the bars at least 50mm wide.

Coastal waters (United Kingdom)

Defined in the Water Act 1989 as any waters which are within the area which extends landwards from baselines from which the breadth of the territorial sea is measured as far as the limit of the highest tide and in the case of any rivers, as far as the tidal limit of the river.

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