Bioremediation refers to the use of entire organisms (mostly soil microorganisms) or selected constituents of microbial cells (mostly enzymes) for chemical transformations. Bioremediation transforms a toxic substance into a harmless or less toxic substance. Ideally, the toxic substance is transformed into carbon dioxide and water. If the toxic substance contains a metal or a halogen, such as chlorine or fluorine, there will be additional side-products (perhaps the free metal atom or its ion or a halide ion). Mineralization is the term used to describe the complete degradation of a chemical substance to water and carbon dioxide. Bioaugmentation, another frequently used term, involves the deliberate addition of microorganisms that have been cultured, adapted, and enhanced for specific contaminants and conditions at the site.
Microorganisms used in bioremediation include aerobic (which use free oxygen) and anaerobic (which live only in the absence of free oxygen). Aerobic microbes have been the organisms of choice for degrading hazardous wastes.
Bioremediation is practiced in two modes - in situ and ex situ. In situ bioremediation involves the use of microorganisms to degrade wastes at the site (both on and below the surface) and avoid excavation of contaminated soil and transfer to different locations. Surface remediation is used to treat the top parts of the soil through aeration by the addition of microorganisms, nutrients, and water. Subsurface bioremediation uses microorganisms already in the soil and groundwater and adds oxygen and nutrients. Ex situ treatment involves the excavation of contaminated soil and its transfer to appropriate treatment sites, i.e., bioreactors. The contaminated soil is aerated and treated with nutrients to provide an active environment for the microorganisms of choice. Treatment continues until the soil is sufficiently clean and can be returned to the site. Ex situ techniques are varied but can involve slurry-phase treatments that combine contaminated soil or sludge in bioreactors or solid-phase treatments that involve placing contaminated soils in lined treatment beds. Bioremediation of water or leachate includes treatment with special bioreactors or filters that contain an active film of microorganisms. The choice of method involves many factors, including the contaminant, the site, and the costs that can be borne. Ex situ treatment is usually very expensive. Most often, the microorganisms are expected to reproduce in situ.