The basic premise of bioremediation (biotreatment) is to enhance microbes capable of degrading the target compounds through physical means (mixing & aeration) and chemical means (addition of limiting mineral nutrients). The microbes to be used in the bioremediation may be indigenous (already present) or external (proven degraders of other origin). In the case of soil bioremediation, the mechanism may follow two paths. The first, ex situ (from place) involves excavation and treatment which requires placing the soil into a treatment area where mineral nutrients and external organisms (if necessary) may be added to the soil usually followed by mixing to assure distribution throughout the soil. Periodic mixing and addition of more nutrients (and/or organisms) serves to insure homogeneity while aerating 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. This mixing is often accomplished in one of two ways: by placing the soil into the treatment area to a depth of 12 - 18 inches and mixing periodically with a tiller or sim ilar machine; or placing the soil in windrows approximately 15 feet wide and 5 feet high and mixing with a windrow turner or composting machine which straddles the windrow. While mixing has been the more common method to insure aeration, another technique, referred to as a biopile, is gaining popularity. The biopile technique places the soil into treatment cells and uses a network of perforated or slotted piping to allow fresh air to be drawn through the soil using a vacuum exhaust system. This technique requires less equipment and in many cases less treatment area.
The second soil treatment method, in situ (in place), refers to treating the contaminated soil "as is", that is, without removing it (excavation). This is often the only choice when contamination has migrated under buildings or roadways. Here, mineral nutrients and often 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.
In situ bioremediation may also be used to treat contaminated groundwater, whether soil contamination is also involved or not. The groundwater treatment of very volatile compounds, e.g., gasoline VOA's, has typically been accomplished by pumping the GW up into surface air strippers where the volatiles  are physically cleaned from the water and the effluent returned either to the ground or to some other disposal outlet. To expedite the remediation process, microbes and mineral nutrients may be added to the post stripper water before it returns to the ground thereby adding microbes and minerals into the contaminated groundwater where biodegradation may take place in the ground. As air emission requirements become stiffer, the air emitted from the stripper tower and/or associated vacuum extraction system may be passed through carbon or a biofilter before being released into the atmosphere. Where the groundwater contamination is less volatile or where atmospheric emissions are less desirable, the groundwater may be treated in what would be described as a "pump and treat" in situ system. The groundwater is pumped from a recovery well and passes through a bioreactor where, over a brief period, mixing, mineral nutrient addition, aeration and perhaps microbial addition takes place before the GW is returned to the ground through infiltration galleries, trenches or other means. Microbes carried from the above ground bioreactor may continue their activities in the ground.
The end result of all of these techniques is the same. Over time, the contaminating material is converted to innocuous carbon dioxide and water, thereby returning the soil and/or groundwater to "clean" standards.
Indigenous versus inoculated
Most soils, especially shallow soils with included 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 contamination sites may show the presence of organisms capable of attacking the contaminating material if other 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 from similar origin (soil) will: 1) offset the possible  absence of indigenous degraders; 2) speed the contaminant removal; and 3) 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 such high populations. Finally, the addition of external organisms (often referred to as bioaugmentation) and essential mineral nutrients will enhance any capable indigenous organisms, thereby taking advantage of any help they might provide in the overall bioremediation process. |
