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Process
The goal of bioremediation is to create a functional environment for a microbial community that is able to degrade specifically contaminated areas [1]. Bioremediation uses naturally selected organisms that can degrade xenobiotic chemicals. These xenobiotic chemicals include PAHs, PHCs, and others which are unnatural and difficult to degrade. These include gasoline, diesel fuel, crude oil and creosote, pesticides and their derivatives, halogenated aromatic hydrocarbons, and chlorinated solvents such as methylene chloride, trichloroethylene, and vinyl chloride [1]. The microbes must be able to withstand the contaminated environment and the contaminants themselves. The microbes that are unable to survive in these contaminated environments die leaving the ones that are able to metabolize the chemicals to repopulate. These naturally selected microbes breakdown the xenobiotic contaminants into organic matter such as CO2, water, and fatty acids. The growth of the microbial populations involved in bioremediation can be enhanced by adding nutrients and oxygen [1].

When selecting which microbial community would be most effective both the characteristics of the environment and those of the contaminants themselves must be considered. Oxygen and water content in the environment are the main characteristics of interest which dictate microbe effectiveness. Other environmental factors include pH, temperature, organic and inorganic soil content, and the presence of metals. Important characteristics of the contaminants include the structure of the contaminant, its solubility, redox potential, relative toxicity, and its interaction with soil and sediment [1].

Advantages
Bioremediation can be used on hazardous sites with less risk than other techniques. It can be done in situ which reduces risk of exposure from excavation [1]. Bioremediation can accomplish what other methods cannot while being cheaper and more effective. Other methods cost approximately two or three times more to transport and incinerate similar volumes of waste compared to bioremediation. Also, the byproducts produced from other methods require landfilling, such as ash from incineration [1].

The market from bioremediation is sizable. Currently, annual sales of remediation products are $7-$10 million. The estimated potential market value may reach $200 million annually. Bioremediation products include dried microbial inocula, liquid microbial inocula, and nutrient additives [1].

Disadvantages
Polluted sites can differ in many ways making bioremediation difficult in terms of microbiological and physiochemical parameters, which need to be considered when choosing the best microbial community [1]. Selection of the microbial community can be limited by the following factors: lack of oxygen, lack of moisture, toxic levels of waste, unfavorable pH levels, unfavorable temperatures, and lack of nutrients. These characteristics of the environment dictate which microbes and enzymes as well as how much are to be implemented. The same is true for characteristics of the polluting chemicals such as: sorption equilibrium, irreversible sorption, incorporation into humic material, the chemical structure, relative solubility, and net concentration of the chemical [1].

A disadvantage of bioremediation is the breadth of understanding required. It is a field requiring expertise in many facets. In addition to the complexity of selecting appropriate microbial communities, the soils and sediments are complex mixtures of clay, sand, and silt with many microscopic organisms. The knowledge on bioremediation technologies is hard to access because many technologies are owned as property [1]. Bioremediation is relatively young compared to conventional methods so there is also a lack of experience. Also, other methods usually require less time than bioremediation [1].

Bioremediation is not effective for removal of metals [1]. Recalcitrant species that are not degraded by bioremediation increase in proportion which can increase recalcitrance [1].

Case Studies
After 30 days 77 + 3.8% polycyclic aromatic hydrocarbons were removed at a moisture content of 40% and a C:N ratio of 60:1 [2].