Bioremediation in general refers to the use of microorganisms to remove undesirable compounds from soil, sludge, ground water or service water so that these sources will be returned to their clean and natural state.
The basic premise of bioremediation is to enhance microbes capable of degrading the target compounds through physical means (mixing and aeration) and chemical means (addition of mineral nutrients). The microbes to be used in bioremediation process may be indigenous (already present) or external (proven degraders of other origin). In the case of soil biotreatment, the mechanism may follow two paths. First, the excavation and treatment requires replacing the soil into the treatment area where mineral nutrients and external organisms may be added to the soil followed by missing to assure distribution throughout the soil. Periodic mixing and addition of more nutrients and/or organisms serve to achieve homogeneity and the aeration of the soil. This scenario is often referred to as land farming, where microbes are in a sense being farmed, or tilled in to facilitate the degradation of the contaminating material. Second, the soil treatment method, in situ, refers to the treating of the contaminated soil without removing it. Here mineral nutrients and organisms are added into the ground through wells, infiltration galleries or other means to allow the organisms to degrade the contaminating material where it is found.
Indigenous versus Inoculated
Most soils, especially shallow soils which include vegetation, will have an indigenous microbial population. The presence among that indigenous population of organisms with sophisticated degradation capabilities is highly variable. Long-term, chronic contaminated sites may show the presence of organisms capable of attacking the contaminated material if conditions of aeration and adequate mineral nutrition are present. Short term or emergency (spill) contamination sites are less likely to contain significant degradation populations. The addition of external microbes of known degradation capabilities will:
- Offset the possible absence of indigenous degraders
- Speed the contaminate removal
- Allow degradation to reach lower levels (greater removal) since artificially high numbers of organisms will be maintained in the soil where low nutrient levels would otherwise be unable to maintain high populations
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