Bioremediation is applied to MTBE and other oxygenates in systems that range in complexity from not being engineered at all (natural biodegradation) to systems that are completely engineered, including the addition of conditioned microorganisms (bioaugmentation) and of nutrients as well as cosubstrates and electron acceptors (biostimulation). Further, these systems can be based on aerobic or anaerobic pathways, or a sequential combination of these pathways.
The rate at which natural biodegradation of MTBE and other oxygenates will occur at a site is affected by a number of site conditions, including groundwater chemistry, presence of other contaminants, and number of native microbes capable of degrading MTBE or other oxygenates. Whether the contaminated zone is aerobic or anaerobic (nitrate reducing, iron reducing, sulfate reducing, or methanogenic), and other chemical parameters (e.g., pH, alkalinity, and inorganic content) will determine what types of microbes may be able to grow and what type of biodegradation pathway may be followed. Figure 24.6 depicts the oxidative zones that may be present in a plume at a petroleum-contaminated site and illustrates how each of the anaerobic and aerobic pathways listed in Table 24.12 may be part of the natural biodegradation process.
Multiple microbes that are capable of biodegrading oxygenates have been identified at sites contaminated with MTBE and other oxygenates. Whether such microbes are present at a specific site will affect the viability of natural biodegradation without the need for bioaugmentation. Where these microbes are present, natural biodegradation or limited biostimulation, such as air sparging to increase oxygen levels, may be effective in reducing the concentrations of MTBE and other oxygenates. However, other conditions must be conducive to support significant natural biodegradation.
Typically, only those sites that have aerobic conditions in the contaminated zone because of shallow water tables and high rates of groundwater recharge have achieved significant natural biodegradation of MTBE and other oxygenates.65
In some cases, the presence of other contaminants, such as benzene, has been shown to facilitate the natural biodegradation of MTBE and other oxygenates through cometabolism. However, contaminants such as BTEX may also inhibit the biodegradation of oxygenates through the depletion of electron acceptors or nutrients, or may be preferentially used because of the relatively slow growth of oxygenate-degrading microorganisms.2466 In addition, sites contaminated with alcohols such as ethanol may also inhibit the biodegradation of ether-based oxygenates such as MTBE through the depletion of electron acceptors or nutrients.67
Fully engineered systems for the bioremediation of oxygenates typically incorporate both biostimulation and bioaugmentation to accelerate the biodegradation process. Most commonly, these systems are based on the aerobic pathway so that the biostimulation component includes the addition of oxygen, through air/oxygen sparging or addition of oxygen-releasing chemicals, as well as the addition of nutrients. The addition of oxygen through one of these means can be used to make the entire contaminant zone aerobic and thereby provide more uniform conditions for accelerated biodegradation. Maintaining high oxygen levels is especially important to effective aerobic biodegradation in that oxygenate-degrading organisms have been shown in research studies to require a higher concentration of oxygen. The bioaugmentation component is achieved by adding microbial cultures that are conditioned to degrade oxygenates either by being grown on these contaminants or by culturing isolated species that have the required enzymes. Bioaugmentation is often critical to the success of an engineered bioremediation system in that microorganisms capable of degrading oxygenates may not be present natively and are slow growers.5568
The use of anaerobic pathways may have engineering advantages under certain conditions, such as in treating oxygenate contamination in deep aquifers or source zones. Recent research and field studies have focused on the various anaerobic pathways for the biodegradation of MTBE and other fuel oxygenates.60 However, the results of these studies in terms of oxygenate degradation efficiency have been variable and no single anaerobic pathway has demonstrated consistent success for degrading oxygenates to end products, even in the laboratory environment.69 Therefore, site-specific treat-ability studies and pilot testing are generally performed if bioremediation using anaerobic pathways is to be considered at a site.
Engineered systems may also incorporate the addition of a cosubstrate to help establish an active microbial community and thereby accelerate the biodegradation of oxygenates. Hydrocarbon gases, such as propane and butane, are one type of cosubstrate that has been used in field applications due to the simplicity of injecting and diffusing a hydrocarbon gas.62 Some proprietary technologies are based on the use of specific cosubstrates and strains of microbes.
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