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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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