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| Resource Library > Technology Transfer > Programs and Initiatives > Enhanced In Situ Anaerobic Bioremediation > Overview |
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Chlorinated solvents in groundwater, primarily trichloroethene (TCE) and perchloroethene (PCE) and their anaerobic degradation daughter products, continue to pose a significant technical and costly challenge to the Department of Defense (DoD). Since 1993, AFCEE Technology Transfer has been teaming with industry, academia, and the US EPA to address this formidable problem. AFCEE Technology Transfer has funded extensive research and demonstration work across the nation to better understand and document the mechanisms responsible for the natural breakdown of these contaminates in the environment. This leading-edge work was documented in the EPA's Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. This work, together with that of numerous institutions and research organizations, has resulted in an understanding of the mechanisms and processes responsible for the breakdown of these compounds that was not at all understood only a decade ago.
Although much has been learned there is still much to understand regarding the incomplete mineralization of these relatively recalcitrant compounds. We now understand that the primary mechanism for the breakdown of PCE and TCE is anaerobic reductive dechlorination or halorespiration. This mechanism is simply the replacement of chlorine ions with hydrogen ions that are produced from the fermentation of a natural or manmade organic substrate. This reductive dechlorination process is biologically mediated and fully dependent on favorable environmental conditions and the presence of organisms capable of completing the reductive dechlorination process.
The nation-wide natural attenuation initiative work sponsored by AFCEE has highlighted the various types of plume behavior where some demonstrate complete dechlorination and others show little or no dechlorination. Because of the understanding regarding the importance of adequate organic carbon for fermentation and also the correct microbial communities, several vendors have started offering various organic amendments and/or microbial inoculants to stimulate the complete dechlorination at sites that are stalled at some daughter product such as cis-1,2 dichloroethene (DCE) or vinyl chloride (VC).
Unfortunately, the research has not advanced to the point that one can easily determine the following:
- which organic substrate to add or if it makes a difference;
- if you add a best substrate, will the site eventually completely dechlorinate or will it stop or stall at a daughter product;
- if there is adequate substrate present will the native microbial community be adequate to facilitate complete dechlorination or will a microbial injection be necessary to augment the site; and
- does a particular site favor a particular substrate and/or microorganism and how do you tell?
With vendors offering different substrates or inoculants, it is very difficult for the Air Force project manager to know what to do. To this end, AFCEE Technology Transfer has embarked on another leading-edge program to attempt to answer these difficult but very important questions, the Enhanced In Situ Bioremediation Initiative.
AFCEE Technology Transfer Enhanced In Situ Anaerobic Bioremediation Initiative
Enhanced in situ bioremediation can cost-effectively remediate contaminated sites if (1) the site can be engineered to provide appropriate growth conditions favorable to existing microorganisms, and (2) contaminants are susceptible to enhanced bioremediation (e.g., chlorinated solvents, perchlorate). However, although enhanced in situ bioremediation has been applied at over 600 sites to date, it has yet to gain widespread regulatory acceptance as a proven technology. This is primarily due to lack of consistency in achieving remedial objectives. To further prove the technology, AFCEE Technology Transfer, through the Enhanced In Situ Anaerobic Bioremediation Initiative, has generated several successful forms of the technology, including edible oil injection, molasses injection, hydrogen injection, passive in situ bioreactors, biogeochemical investigations, and biobarriers AFCEE has successfully installed and achieved extremely promising results from several fielded systems at numerous Air Force sites. To date, AFCEE has field-tested 24 enhanced in situ anaerobic bioremediation systems at 18 bases (Figure 1).
| Figure 1. AFCEE Technology Transfer Enhanced in Situ Anaerobic Bioremediation Field Sites |
The objective of the Enhanced In Situ Anaerobic Bioremediation Initiative is to develop and demonstrate processes for distributing organic amendments into a contaminated aquifer for the purpose of enhancing in situ reductive dehalogenation of chlorinated solvents in groundwater. Additional future investigations include investigating abiotic contributions, electron shuttles, and application for emerging contaminants such as perchlorate. In addition, AFCEE has developed and made available principles and practice documents, guidance documents, protocols, work plans, final reports, cost and performance reports, presentations, and other helpful information about the technology.
The most recent and most exciting development is the release of the AFCEE Technology Transfer Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents document. The objectives of the document are to describe the scientific basis of enhanced anaerobic bioremediation and to summarize relevant site selection, design, and performance criteria for various engineered approaches to stimulate and enhance in situ biodegradation of chlorinated solvents. The document is not intended as a protocol to implement enhanced bioremediation. Rather, it provides the information necessary to make informed decisions as to when enhanced anaerobic bioremediation is appropriate, and what specific enhanced bioremediation approaches may be suitable for achieving remedial goals. The document provides the RPM with the tools required to assess the application of enhanced anaerobic bioremediation at their sites and to identify optimum approaches, particularly when soliciting and reviewing enhanced bioremediation services.
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