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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.

Coastal zone

Lands and waters near the coast, whose uses and ecology are affected by the sea.

Coliform bacteria

A group of bacteria found in the intestine and faeces of most animals. Coliforms can sometimes be found in untreated water. The treatment process removes them and disinfection prevents their reappearance in the distribution system. In water receiving discharges, faecal coliform bacteria are used to indicate the presence of sewage.

Coliform index

A rating of the purity of water based on a count of coliform bacteria.


Matter of very small particle size, in the range of 10-5 to 10-7 in diameter.

Combined sewer

A sewer system that carries both sewage and rain water runoff.

Commercial water use

Water used for hotels, restaurants, office buildings and other commercial facilities.

Communication pipe

That part of the pipe which conveys water from the main to the customers house and which is in the road or footpath.

Composite sample

A series of water samples taken over a given period of time and weighted by flow rate.

Consumer Council for Water (CCWater)

CCWater represents the interests of water and sewerage consumers in England and Wales. It replaced WaterVoice on 1 October 2005. CCWater is funded directly by the Department for Environment, Food and Rural Affairs, which recovers the costs from the companies.


Consumers refers to individuals or households that purchase and use goods and services generated within the economy. In this case we are referring to those who use water and sewerage services.

Contact time

The length of time a substance is in contact with a liquid, before it is removed by filtration or the occurrence of a chemical change.


Any foreign component in a substance, for example in water.

Controlled waters

Inland, coastal, territorial and groundwater to which British pollution control legislation applies.


The Carbon Reduction Commitment is a new scheme, announced in the Energy White Paper 2007, which will apply mandatory 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) by 1.1 Mega Tonnes Carbon per year by 2020. The UK-wide emissions trading scheme is due to start in 2010.

Cross flow filtration

A process that uses opposite flows across a membrane surface to minimize particle build-up.


A microorganism in water that causes gastrointestinal illness in humans. It is commonly found in untreated surface water and can be removed by filtration. It is resistant to disinfectants such as chlorine.


Coastal Water Outfall

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