DDT
The first chlorinated hydrocarbon insecticide (chemical name: dichloro-di-phenyl-trichloroethane). It has a half-life of 15 years and can collect in fatty tissues of certain animals. It was used in massive quantities after World War II. Although its toxicity is generally low, its persistence and accumulation in food chains led to a ban on its use in many countries.

Decomposition
The breakdown of matter by bacteria and fungi, changing the chemical makeup and physical appearance of materials.

Decontamination
Removal of harmful substances such as noxious chemicals, harmful bacteria or other organisms, or radioactive material from exposed individuals, rooms and furnishings in buildings, or the exterior environment.

DEFRA
Department for Environment, Food & Rural Affairs.

Density
The mass of unit volume of a substance, expressed in units such as kg. m-3.

Desulphurization
Removal of sulphur from fossil fuels to reduce pollution.

Dibenzofurans
A group of highly toxic organic compounds, often associated with dibenzo-p-dioxins.

Dioxin
Any of a family of compounds known chemically as dibenzo-p-dioxins.

Dissolved Oxygen (DO)
The oxygen freely available in water, vital to fish and other aquatic life and for the prevention of odours. DO levels are considered a most important indicator of a water body's ability to support desirable aquatic life. Secondary and advanced waste treatment is generally designed to ensure adequate DO in waste-receiving waters.

Dissolved Solids
Disintegrated organic and inorganic material in water. Excessive amounts make water unfit to drink or use in industrial processes.

Distillation
The act of purifying liquids through boiling, so that the steam condenses to a pure liquid and the pollutants remain in a concentrated residue.

Duct
An enclosed structure through which gases travel