Filth, dirt and poison | Features

The dirty dozen: The latest remediation techniques
Biological treatments
Phytoremediation
Technology	The transformation or removal of contaminants by plants, which absorb harmful chemicals from the ground and store them in their roots, stems or leaves. Afterwards, the plants are harvested and destroyed.
Where it is done	In situ.
Application	Sites with low pollution levels and long-term remediation programmes.
Contaminants eliminated	Pesticides, chlorinated solvents, some hydrocarbons and metal.
Bioremediation
Technology	Microbes that are found naturally in the earth consume chemicals such as oil, converting them into water and carbon dioxide. However, the right conditions must be present for the bacteria to multiply. The soil must be at the right temperature, and there must be enough nutrients and enough oxygen. In some cases, additional oxygen and nutrients are pumped underground, in others the soil is dug up and the process takes place above ground where the right conditions are easier to achieve.
Where it is done	Both.
Application	Requires very little equipment, and exploits natural processes.
Contaminants eliminated	Petrol and oil.
Soil vapour extraction
Technology	Vapours are formed when some chemicals evaporate. If a vacuum is applied to the soil, the vapours can be pulled out, collected and treated. Sometimes vapour extraction is combined with air sparging (pumping air into the ground) to speed evaporation.
Where it is done	In situ.
Application	Remediation of petrol spills or around structures where excavation is not possible.
Contaminants eliminated	Solvents and fuels.
Thermal desorption
Technology	Thermal desorption uses heat to remove harmful chemicals from the soil by transforming them into gases for safe disposal. Soil is dug, crushed and dried before being placed in a desorber – a large oven where the process takes place.
Where it is done	Ex situ.
Application	Dry, heavily polluted soils.
Contaminants eliminated	Fuels, coal tar, solvents.
In situ thermal treatment
Technology	Heating polluted soil to destroy the chemicals, or change them into gases that move more easily through the soil. Heating is achieved using: steam, hot air or hot water injection; electrical resistance (passing an electric current through the soil to heat the ground water to steam and evaporate chemicals); radio-frequency heating using radio waves; thermal conduction using heated wells or a heated blanket on the surface.
Where it is done	In situ.
Application	One of few methods to destroy NAPLs (organic liquids with a low solubility in water). Works well with clays.
Contaminants eliminated	Fuels and chlorinated solvents, NAPLs
Electrokinetics
Technology	A low-voltage direct current is used to treat metal contaminants. Metal ions migrate toward the cathode where the fluids containing the metal are pumped to the surface and treated.
Where it is done	In situ.
Application	Where metal contaminants occur below the water table.
Contaminants eliminated	Metals.
Fracturing
Technology	Not really a treatment in itself, fracturing is used to break up dense ground as part of other clean-up methods. The cracks create paths through which the harmful chemicals can be removed and treated. Fractures can be created by forcing liquid or air into the ground, or explosives can be used.
Where it is done	In situ.
Application	Dense compact ground, or even rock if explosives are used.
Contaminants eliminated	–
Chemical treatments
Oxidation
Technology	By adding chemicals called oxidants to the soil, pesticides and fuels can be converted into water and carbon dioxide. The oxidants used are usually hydrogen peroxide or potassium permanganate. Soil is treated by drilling a series of wells and pumping the liquid oxidant into the ground. To speed up the process, oxidants are often pumped down one well, up through another, and recirculated.
Where it is done	In situ.
Application	Sites where pollution is buried too deep for other methods of remediation to be used.
Contaminants eliminated	Pesticides, solvents and fuels.
Soil flushing
Technology	By pumping chemicals or water into the ground harmful chemicals can be flushed out. The chemicals are then pumped to the surface, through wells, where they are treated. Some chemicals like solvents and heating oil do not dissolve easily in water – instead, surfactants (like washing-up liquid) and solvents (like alcohol) are mixed with the water and pumped down wells to help dissolve these chemicals.
Where it is done	In situ.
Application	Works best in permeable soils, particularly if layer beneath the polluted stratum is made of an impermeable material such as clay.
Contaminants eliminated	Solvents, heating oils.
Soil washing
Technology	Soil is first dug then sifted to remove large objects before being placed in a scrubbing unit where water and sometimes detergents are added. The mixture is passed through sieves, mixing blades and water sprays, which dissolve some pollutants and separate the silt from clay, where most pollutants are concentrated. These can then be washed again or another treatment method used to remove the pollutants.
Where it is done	Ex situ.
Application	Suited to larger grained soils like sand and gravel.
Contaminants eliminated	Fuels, metals and pesticides.
Solvent extraction
Technology	Solvent extraction uses a plant called an extractor where dug soil is mixed with a solvent. The solvent dissolves the chemicals and is then transferred to a separator, where the chemicals are removed before the solvent is reused.
Where it is done	Ex situ.
Application	For chemicals that do not dissolve in water.
Contaminants eliminated	Oil, grease and PCBs.
Stabilisation
Technology	Contaminants can be locked into the soil by mixing in fly ash, cement or lime as a re-agent. These form a stable, solid mass, resistant to weathering.
Where it is done	Both.
Application	Soils with a high clay content.
Contaminants eliminated	Heavy metals.