Bioremediation is a process by which microorganisms, fungi, and plants metabolize pollutant chemicals.52,53 It has been used to treat oxygenates in soil and groundwater both in situ and ex situ. Generally, an engineered bioremediation system stimulates the biodegradation of contaminants through the introduction of electron acceptors (typically oxygen), electron donors (substrates or food sources), nutrients, or microbes54 that are acclimated to the contaminated soil or groundwater. These amendments are either introduced to the subsurface in situ or are added to extracted groundwater or excavated soil. A description of the various types of amendments is provided below.55
1. Electron acceptors—Oxygen is the most common electron acceptor used to promote biodegradation and is added in different ways including in sparged air, through injection of a solid or liquid that generates oxygen or through in situ electrochemical generation. Other electron acceptors, including nitrate, sulfate, and iron (III) compounds, may be added to support anaerobic biodegradation.
2. Electron donors—In direct biodegradation pathways, the contaminant acts as the electron donor or substrate. However, during cometabolic degradation, a different electron donor is metabolized, resulting in the consequential oxidation of the contaminant. In some contaminated plumes, other electron donors, such as other constituents of gasoline, may also be present. In cases where they are not, and cometabolic degradation pathways are desired, electron donors may be added.
3. Nutrients—Nutrients, such as nitrogen and phosphorus and other trace elements, are necessary for cell growth because they are key biological building blocks.52,53 Addition of nutrients as a supplement helps ensure that concentrations of nutrients do not become a limiting factor for bioremediation.
4. Bioaugmentation—Bioaugmentation involves the addition of supplemental microbes to the subsurface where organisms able to degrade specific contaminants are deficient. Microbes may be grown from populations already present at a site or "seeded" from cultures grown in aboveground reactors or available commercially as cultivated strains of bacteria known to degrade specific contaminants. The application of bioaugmentation technology is highly site-specific and dependent on the microbial ecology and physiology of the subsurface.56
This section focuses on engineered in situ remediation technologies that use microorganisms to biodegrade pollutant chemicals. In situ bioremediation technologies are configured to either directly inject supplements into the contaminated media; to place the supplements in the pathway of ground-water flow; or to extract contaminated groundwater, amend it with supplements, and recirculate the amended groundwater through the contaminated zone.
The design and configuration of in situ bioremediation systems vary widely based on site-specific conditions. A treatability study or pilot-scale testing is often performed to determine the type and amount of amendments required to create and maintain the conditions optimal for biodegradation as well as to select the type of engineered system that is most suitable to introduce the amendments to the subsurface.57 Some of the key considerations for various types of engineered in situ remediation systems are summarized below.56 A discussion of ex situ applications of bioremediation, which focuses on biological treatment of extracted groundwater, is included in the section about treatment of extracted groundwater.
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