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In-situ Anaerobic Bioremediation

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  GROUNDBREAKING TECHNOLOGY: IN-SITU ANAEROBIC BIOREMEDIATION FOR TREATMENT OF CONTAMINATED SOIL AND GROUNDWATER Kelly A. Fernandes, Undergraduate Student, Northeastern University, Boston, MA, 02115, US A   Abstract Anaerobic in-situ bioremediation is a technique often used to cleanse contaminated soil and groundwater. Anaerobic in-situ bioremediation is a phrase with distinct terms all having relevance in the application of this technique. Anaerobic mplies the absence of dissolved oxygen, while in-situ simply means that the environmental cleansing occurs with out removing, displacing, or significantly disturbing the specimen or surrounding area. Bioremediation is a term used to describe the biological use of microbes or plants to detoxify the environment. In order to properly implement this complex process, one must have an understanding of microbiology, biochemistry, genetics, metabolic processes, and structure and function of natural microbial communities. Keywords:  bioremediation, groundwater treatment, in-situ BACKGROUND d anaerobic in-situ bioremediation has been an accepted practice in groundwater clean up only since the 1980's, but has been proven to be very effective for treating soil and groundwater contamination all over the world. Anaerobic in-situ bioremediation makes use of organisms to remove pollutants from contaminated sites. The application of biological treatment will lead to clean up of the hazardous chemicals, which are usually organics. Typically indigenous microorganisms consume and transform the compound until the pollutant is completely degraded, and the wastes are harmless. Bacterial or fungal strains can also be implemented rather than indigenous microorganisms. In conclusion, in-situ bioremediation is responsible for satisfying the following objective: ã Transforming toxic materials into non-toxic chemicals and compounds without  producing additional toxins in the process. Bioremediation treats several classes of chemicals: Fuel Hyrdrocarbons, PAH's (polychlorinated aromatic hyrdrocarbons), crude oil compounds, coal compounds, PCB’s (polychlorinated biphenyls), chlorinated solvents (i.e. TCE), pesticides, and many more. Anaerobic bioremediation is particularly useful in the biodegradation of persistent wastes and chemical compounds, like TCE, PCB's, and Fuel Hydrocarbons. There are several reasons why bioremediation is the preferred method for soil and groundwater contamination treatment. First, bioremediation is usually a more cost- effective solution compared to other alternatives. Bioremediation costs about ½ that of incineration, and saves approximately 60% to 90% of landfill costs. In addition to these cost benefits, contaminated soil and groundwater can be treated simultaneously, further reducing possible operating costs. Employing an in-situ approach to the bioremediation is also an advantage in itself. In-situ treatment reduces contaminant exposure to personnel. Also, contaminant transportation and disposal accidents are completely eliminated when implementing in-situ  CSCE/EWRI of ASCE Environmental Engineering Conf. Niagara 2002 2 treatment. Over all, in-situ bioremediation has few environmental impacts and worries, since complete degradation of contaminants occurs in the subsurface. Therefore, making in-situ  bioremediation an ideal method of contamination clean up for leaking underground storage tanks, where fuels and oils are the contaminants, like many environmental issues that we are encountering in the twenty-first century. Techniques There are two techniques being currently used around the world for the implementation of in-situ bioremediation. These two different techniques are dependent on several factors. One of  paramount importance are microorganisms. Existing anaerobes in the subsurface, versus the lack there of, can determine the technique used for this process. Biostimulation is the process where existing anaerobes are stimulated with enzymes and catalysts to speed up metabolic  processes. When useful anaerobes are not present in the subsurface, they must be added. This is called bioaugmentation. Both of these techniques are successful and efficient. The two techniques must only be differentiated between because of existing conditions in the contaminated zone. This type of data would be collected in the initial stage of bioremediation: Site Investigation and Feasibility Studies (see below). THEORY Most bioremediation projects rely on the introduction of an electron acceptor into the contaminated zone to stimulate the growth of microorganisms which will digest contaminants and render non-toxic wastes. There are several chemicals and compounds that can serve as electron acceptors (energy sources) and allow the organisms to metabolically degrade the contaminants. Oxidation and reduction (redox) processes occur in the subsurface and consume organic matter. Often times in anaerobic conditions, denitrification is the most efficient  bioremediation process used. See Table 1 below. G is a measure of the energy yield for these processes. It is clear that aerobic oxidation has a high-energy yield, but very low solubility in water, resulting in a slower reaction. To remediate a large volume would require limitless amounts of atmospheric oxygen to get acceptable results, which would be very expensive. Denitrification has a high-energy yield as well, and NO -3  solubility in water is very high, requiring much less of a nitrate solution to receive acceptable results. In addition, aerobic remediation isn't always a possibility depending on the contaminant. Anaerobic treatment is much more effective when degrading persistent wastes. In most situations, the preferable method for anaerobic bioremediation is by denitrification. The following equation is more appropriate for modeling the denitrification  process (the above equations depict ideal laboratory conditions and distilled water): C 6 H 12 O 6  + 4NO 3   -     6CO 2  + 6H 2 O + 2N 2  Denitrification is the bacterial mediated reduction of nitrate and nitrite to nitrogen gas. Most denitrifying bacteria are anaerobes and will only proceed in the absence of dissolved oxygen (this condition is typical in the subsurface).  Fernandes, 2002 - Student Technical Paper Session 3 Table 1:  Redox reactions with accompanying data relative to bioremediation Source: Proceedings of the 1998 Petroleum Hydrocarbons and Organic Chemicals in Groundwater: Prevention, Detection, and Remediation, 1998. Redox Reaction Simplified Equation Electron Acceptor G (kJ) Solubility (mg/L)   Aerobic Oxidation CH 2 O+O 2 =CO 2 + H 2 O O 2 502 11 Denitrification CH 2 O+NO 3- +H + =N 2 +HCO 3 + H + + H 2 0  N0 3- 477 930000 Manganese Reduction CH 2 O+2MnO 2 + 3H + = 2Mn 2 +HCO 3 + 2H 2 O MnO 2 340 Insoluble Iron Reduction CH 2 O+ 7H + +   4Fe(OH) 3 =4Fe 2+  +HCO 3 + 10H 2 O Fe(OH) 3 116 Insoluble Sulfate Reduction   CH 2 O+SO 2-4 = HS -  + HC0 3  + H + S0 4- 105 2000 Methane Fermentation CH 2 O + CO 2  = CH 4  + CO 2 CO 2 93 Miscible DESIGN PROCESS Prior to design and implementation of a remediation project there are two very important stages: Compliance Analysis, and Site Investigation and Feasibility Studies. Both of these steps of the general design procedure also outline the relevant information and data needed for design. Compliance Analysis involves examining the regulations governing environmental quality, clean up techniques, and remediation goals or MCLs for the region. Site Investigation and Feasibility Studies both involve the gathering of initial data. Site investigations tend to be more general and less scientific. Feasibility Studies use site-specific data to determine whether this particular type of treatment is even a viable option, while Site Investigation just involves gathering the initial findings about the site: Where? What? How? The following is typical data acquired both in Site Investigation and Feasibility Studies:  CSCE/EWRI of ASCE Environmental Engineering Conf. Niagara 2002 4 ã Type and volume of material to be treated ã Transport and fate characteristics of waste ã Source of pollution ã Concentrations and distribution of contaminant ã Microbiology background: ã  Naturally occurring bacteria in subsurface ã If not, how to select anaerobes ã What type of catalyst for microorganisms ã Biodegradability of contaminants ã Soil type and properties ã Environment suitable for growth of bacteria ã Chemical reactivity of contaminants with respect to: ã Electron acceptor ã Enzymes ã Site topography ã Groundwater flow ã Geologic properties Role of Bacteria It is only appropriate to now discuss the importance and characteristics of bacteria. How is it that a simple, single-celled organism can eliminate such hazardous wastes? First, only special types of bacteria that can carry on bacterial digestion in some way can be considered for this type of treatment. Some of these bacteria are found only in a specific environment and require specialized types of food. In order to properly carry out waste digestion, bacteria must: ã Consume organic wastes. ã Digest the waste completely and quickly with out producing odors and toxic gas. ã  Not be pathogenic. ã Grow and reproduce in the given environmental conditions of the contaminant. Under ideal conditions, bacteria can produce new generations every 20-30 minutes, reproducing logarithmically. Eventually, these special bacteria species completely deplete their food source (the contaminant), sometimes with the added help of enzymes as catalysts. For these reasons, it is necessary to have a strong background in microbiology. Identifying the appropriate bacteria, either to add to the subsurface, or naturally occurring, is an important step in the remediation process.
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