What is Bioremediation?
Why is Bioremediation Important?
Bioremediation is defined as the use of microorganisms to degrade, break down, transform, and/or essentially remove contaminants from soil and water. It is a natural process that utilizes bacteria, fungi, and plants to alter contaminants by stimulating the growth of these organisms by using the contaminants as a source of food and energy. This is possible because the metabolic processes of these organisms are able to use the contaminants as an energy source, making the pollutants harmless or less toxic to the environment.
Bioremediation is a strong cleanup strategy because it accelerates the breakdown of organic pollutants. It enhances the original biodegradation processes found in nature. Depending on the situation, it may an eco friendly and less expensive method than normal cleaning processes. BIoremediation is especially important for two main reasons. First, bioremediation uses no chemicals which means it poses no significant harm to surrounding wildlife in the area. Usually one of the main issues with traditional treatments are chemicals which can travel and damage surrounding water supplies. However, bioremediation creates no issues with that. Another importance for bioremediation is that it allows waste to be recycled. The waste is treated and the contamination becomes neutralized forming a non-toxic substance in the environment. This means bioremediation allows more waste to be recycled unlike chemicals that create waste that needs to be disposed properly.
In-Situ
Pros
Cons
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Possibility to completely transform organic contaminants into innocuous substances (carbon dioxide, water, etc)
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Accelerated ISB can provide volumetric treatment for both dissolved and sorbed contaminants
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In-situ bioremediation often costs less than other remedial options
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Areal zone of treatment using bioremediation can be larger than with other remedial technologies because the treatment moves with the plume, allowing it to reach areas that otherwise would be inaccessible
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Reduced potential for cross-media transfer of contaminants
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Reduced risk of human exposure to contaminated media
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Less intrusion because few surface structures are required with MNA
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MNA has lower overall remediation costs compared to active remediation
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When bioremediation is halted at an intermediate compound, the intermediate may be more toxic and/or mobile than the parent compound
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Some contaminants cannot be biodegraded
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Inappropriate application can cause air pipes to be clogged from increased microbial growth due to added nutrients, electron donors and/or acceptors
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May be difficult to implement completely in low-permeability or heterogeneous aquifers
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Heavy metals and toxic concentrations of organic compounds may interfere with indigenous microorganisms
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MNA may require institutional controls and a longer time frame
In-situ bioremediation (ISB) is the application of bioremediation in the affected area as opposed to moving the contaminants to an outside area for treatment. Originally, in-situ bioremediation was developed as a less costly, more effective alternative to the traditional pump-and-treat methods for treating aquifers and contaminated soils, but has expanded into the treatment of explosives, inorganics, and toxic metals. ISB can be categorized in one of two ways. The first is by the type of metabolism, either aerobic or anaerobic. The contaminants are either degraded via an aerobic (oxygen) pathway, anaerobically (without oxygen), or by either. The second is by the degree of human intervention. A high degree of human intervention is known a accelerated ISB. Accelerated ISB may require more nutrients, electron donors, or electron acceptors. Aerobic ISB may need more oxygen in the form of an electron acceptor while anaerobic ISB typically needs an additional electron donor and possibly an electron acceptor. On the other hand is monitored natural attenuation (MNA) which is the application of bioremediation with essentially no human intervention. This is a multi-step approach as the indigenous microorganisms need to be able to degrade/transform the contaminants with no human intervention and the area needs to be evaluated for long term monitoring. The evaluation is used to assess the fate and transport of the contaminants when compared to the predictions using analytical and/or numerical models.
Ex-Situ
Pros
Cons
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Suitable for a wide range of contaminants
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Relatively simple to assess from site investigation data
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Flexibility with respect to volumes
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Not applicable to heavy metal contamination or chlorinated hydrocarbons
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Non-permeable soils require additional processing
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May require large areas for treatment beds
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Contaminants must be aerobically biodegradable
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Dependent on temperature, weather, and material
Ex-situ bioremediation is the transportation of excavated soil to an outside treatment area and aerating the contaminants to enhance the degradation with the use of indigenous microbial populations. In ideal aerobic conditions, specific microorganisms can utilize the organic contaminants and some pesticides as a source of carbon and energy and ultimately degrade them into carbon dioxide and water. Normally, the nutrient requirement is tested and amends are made for the basic nutrients and organic substrate of the soil if any elements are lacking. Oxygen is an essential factor to allow microbial populations to develop the necessary cultures for sustaining degradation. Ex-situ can be performed in two ways. The first is the Slurry-Phase Bioremediation in which the contaminated soil is mixed with water and other indicators in a bioreactor to keep the microorganisms in contact with the pollutants. Then, oxygen and nutrients are added for microorganisms to be in an ideal environment for breaking down the contaminants. Afterwards, the water is separated and the soil is tested and replaced in the environment. The second method is Solid-Phase Bioremediation where the contaminated soil is treated in an aboveground treatment center. The conditions within the center are controlled for optimal treatment to take place. While this type of treatment is easy to maintain, it requires a large amount of space and the process of decontamination will take longer compared to slurry-phase bioremediation.
Importance of Electron Donors and Acceptors
The biological process of metabolism is based on the transfer of electrons from one substance to another, resulting in a net gain in usable energy for the organism. The transferring of electrons requires a “donor” material that is the "food” and an “acceptor” material. For higher organisms, the last electron donor is oxygen. In natural environments, food is limited, causing the indigenous microbial population to compete for the available food. By releasing an organic electron donor into the environment, the system becomes unbalanced as the microbial population begins to compete for any available electron acceptors. The basic objective of enhanced bioremediation is to bring balance back into the system.Most common organics like petroleum products act as electron donors and degrade quickly with enough electron acceptors; however; other organics such as chlorinated solvents are not good as electron donors, but are better under anaerobic conditions as electron acceptors instead. To create a successful project, you must first determine your goals in the project. Then, you determine whether or not the contaminants will degrade faster through aerobically or anaerobically. A good aid in deciding is the Handbook of Environmental Degradation Rates. The third step is to measure the mass of the contaminants and any other factors that can interfere such as different organics or electron acceptors.
When the contaminants can be degraded faster aerobically, enhancing bioremediation can be accomplished through the addition of electron acceptors. Many different electron acceptors can be introduced, but the most efficient is oxygen. Other electron acceptors include iron, nitrate, and sulfate. Most of the time, the form of the electron acceptor is not important as long as there is a sufficient amount to meet the needs of the project. In all aerobic cases, the ability to provide a satisfactory and evenly distributed amount of electron acceptors is important for the success of the project.
Aerobic Bioremediation
Anaerobic Bioremediation
For chlorinated solvents, an electron donor is added to begin the process. As the substance is metabolized in anaerobic conditions, an electron is released and replaces the chlorine atom on the chlorinated solvent in a process known as reductive dechlorination or halorespiration.
Importance of Enzymes
Bioremediation mainly depends on the microorganism’s ability to enzymatically attack the pollutants and convert them into harmless products. In the past, this was only effective when the environmental conditions allowed for microbial growth and activity, so bioremediation often involved a manipulation of the contaminated area. Now, the increasing implementation of enzymes allow for a green, sustainable alternative for the remediation of soil and groundwater. Enzymes are the chemicals within living organisms that assist in accelerating the breakdown of hazardous compounds. With recent advancements in biology, the isolation and purification of enzymes is possible, allowing them to be grown and harvested under ideal conditions and then introduced into the contaminated site. This means that the subsurface conditions of the environment does not need to be manipulated for the growth of the bacteria needed to remove the contaminants. Since the enzymes are selective, time and money is saved because there is no longer the need to be treating things other than the targeted pollutant.