An anaerobic/aerobic sequential bioremediation system (Fig. 7) for removal of PCE is now introduced. It has been demonstrated that sequential anaerobic/aerobic biodegradation of PCE is feasible if the proper conditions can be established. The anaerobic process can potentially completely dechlorinate PCE. However, conversion of vinyl chloride (VC) to ethylene is the slowest step in this process. Of the chlorinated ethenes, VC is the most amenable to treatment by aerobic methanotrophic processes. Therefore, a two-step process is thought to be the most efficient. The first step is anaerobic, which rapidly dechlorinates PCE and trichloroethylene (TCE) to break down products 1, 2-dichloroethylene (DCE) and VC. Since the anaerobic dechlorination of DCE and VC to ethylene can be quite slow, a second aerobic step is
implemented that can more quickly complete the remediation process. The schematic diagram in Fig. 7 illustrates this technology.
In practical operation of an anaerobic/aerobic sequential bioremediation system, care must be taken to create and maintain the proper in situ conditions for chlorinated ethene degradation in an aquifer. Carbon and mineral nutrients should be injected and delivered into an aquifer contaminated with PCE or TCE. Groundwater chemical conditions should be monitored within and downgradient of the anaerobic treatment zone to gage the efficiency of the anaerobic process. If volatile organic compound analyses show that the resulting downgradient breakdown products include TCE, DCE, or VC, oxygen and methane will be added to the groundwater to stimulate aerobic degradation by indigenous methanotrophic bacteria. It has been demonstrated that this anaerobic/aerobic sequential bioremediation technology removes PCE, TCE, DCE, and VC from groundwater. The readers are referred to ABB Environmental Services, Inc., Wakefield, MA, United States, for the details of this commercially available process.
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