Bioaugmentation refers to the addition of high-performance biodegradative microorganisms that enhance in situ degradation of contaminants. In the case of chlorinated solvents, the most accepted form of bioaugmentation involves the use of mixed anaerobic cultures that contain Dehalococcoides ethenogenes or closely related strains that can reductively dehalogenate the target contaminants, typically via the process of dehalorespiration. Successful application of the technology requires (1) proper selection of the technology as the best remedy for a site; (2) an appropriate microbial culture; (3) a mechanism to produce a sufficient quantity of the organisms, while maintaining culture integrity; (4) a means for delivering the organisms to a site and into the contaminant plume; and (5) a means for monitoring survival and performance of the added organisms. The application of bioaugmentation technology has the potential to reduce both the time and cost associated with remediating groundwater contaminated with chlorinated volatile organic compounds (CVOCs), and it has become widely used as an in situ treatment alternative.
The primary objectives of this field demonstration were to evaluate the amount of culture needed to effectively remediate a CVOC-contaminated plume, to determine the effect of inoculum dose on remedial time, to evaluate the effect of site characteristics on the effectiveness of the technology, and to evaluate the ability to increase and maintain an elevated pH for successful bioremediation.
Bioaugmentation refers to the addition of high performance biodegradative microorganisms that enhance in situ degradation of contaminants. In the case of chlorinated solvents, the most accepted form of bioaugmentation involves the use of mixed anaerobic cultures that contain Dehalococcoides ethenogenes or closely related strains that can reductively dehalogenate the target contaminants, typically via the process of dehalorespiration. Successful application of the technology requires: (1) proper selection of the technology as the best remedy for a site; (2) an appropriate microbial culture; (3) a mechanism to produce a sufficient quantity of the organisms, while maintaining culture integrity; (4) a means for delivering the organisms to a site and into the contaminant plume; and (5) a means for monitoring survival and performance of the added organisms. These factors will be evaluated during this project by performing laboratory studies and field demonstrations with the ultimate goal of producing a guidance document to aid the Department of Defense (DoD) and Department of Energy (DOE) in the selection and application of bioaugmentation for chlorinated solvent remediation.
A chlorinated ethene groundwater plume present in the MAG-1 Area at Fort Dix, New Jersey, was selected for the field demonstration component of this project. Bioaugmentation using Shaw Environmental Inc.’s (Shaw) SDC-9 Dehalococcoides (DHC)-containing culture was performed in three separate groundwater recirculation loops, with one loop bioaugmented with 1 L of culture, the second loop bioaugmented with 10 L of culture, and the third loop bioaugmented with 100 L of culture. A fourth “control” loop was not bioaugmented. Groundwater monitoring was performed to evaluate DHC growth and migration, dechlorination kinetics, and aquifer geochemistry.
Results for the loops inoculated with 1 L and 100 L of culture showed similar rates of dechlorination. Trichloroethene (TCE) concentrations in the test loop performance monitoring wells declined significantly during the demonstration, with TCE decreases in these wells ranging from 90 to 100%. cis-1,2-dichloroethene (cDCE) concentrations in test loop performance monitoring wells declined between 73 and 99% and were generally trending downward at the end of the demonstration period, while cDCE concentrations in the control loop increased during the demonstration. Transient increases (followed by decreases) in vinyl chloride (VC) were observed in five of the six test loop performance wells, with VC in two of the wells below detection at the end of the demonstration. VC was not observed in the control loop monitoring wells. Ethene data collected during the demonstration clearly indicated that complete degradation was occurring within the three test loops that were bioaugmented with SDC-9 and not within the control loop that received only electron donor, buffer, and nutrients. Final DHC concentrations in these two test loops ranged from 1.8 x 107 to 2.0 x 109 cells/L. The greatest downgradient DHC concentrations were achieved in the test loop with the greater level of CVOC contamination, rather than the loop with the greatest inoculation.
Results of this demonstration showed that many factors, including groundwater flow velocity, contaminant concentration, groundwater chemistry, and heterogeneity of the subsurface, can affect the amount of culture needed to effectively treat CVOC-contaminated aquifers. As a result, precisely determining the amount of culture needed for a given site still requires a site-by-site evaluation. The amount of culture needed cannot be reliably determined solely by estimating the volume of water to be treated, which is currently the approach commonly used by culture vendors. In this demonstration, significantly different amounts of DHC-containing culture were added to the test treatment loops, but the final treatment results were comparable. The lowest amount of culture, however, was added in a treatment loop with the greatest VOC concentration and in situ growth of the culture aided in distribution of DHC and efficient treatment of the aquifer. Conversely, the greater amount of culture was added in a treatment loop with lower CVOC concentrations, and growth of the added culture was limited by the rapid degradation of the needed electron acceptors (i.e., CVOCs); distribution of the culture was presumably dominated by transport of the added culture. Ultimately, distributed DHC concentrations in both treatment loops were similar, and in both loops, treatment was effective. The loop inoculated with 10 L of culture showed slower dechlorination kinetics and DHC migration/growth compared to the other two test loops due to persistent low pH conditions that were not adequately adjusted by adding buffer.
Because the results of this study demonstrated that many factors affect the amount of culture needed for effective treatment and that selecting the amount of culture needed cannot reliably be based solely on the amount of groundwater to be treated, a 1-D model was developed to aid practitioners in determining the amount of culture needed. Importantly, the 1-D model reasonably described the results of the demonstration. Consequently, the model appears suitable for evaluating the effect of different DHC dosages on treatment times and effectiveness, and it will be a useful design tool for planning bioaugmentation applications. To make the model more accessible to remediation practitioners, it is currently being incorporated in a widely used fate and transport model package.
This project demonstrated that CVOC-contaminated aquifers can be effectively remediated by using active groundwater recirculation, bioaugmentation with Shaw’s SDC-9 consortium, and pH adjustment. Results of the field demonstration provide a detailed evaluation of the use of a groundwater recirculation design for the distribution of groundwater amendments (including a TCE-degrading microbial culture), use of buffering agents to control in situ pH, and an application model to enable practitioners to plan bioaugmentation applications and predict their performance. As such, critical design and implementation issues regarding microbial dosage requirements, remedial time frames, and system optimization have been addressed and are being made available to environmental professionals and stakeholders.
The two major challenges encountered during the demonstration were pH adjustment of the aquifer and injection well fouling. pH adjustment, however, may not be required during most applications provided the aquifer has sufficient natural buffering capacity. Well fouling typically is of less concern during passive or semi-active application of the technology, and it may be reduced in aquifers that do not require extensive buffer addition or by using an improved injection well design.
In addition, as observed during performance of model simulations, a DHC attachment detachment factor plays a significant role in determining the relative importance of DHC dosage on bioaugmentation kinetics. Thus, the impact of DHC dosage on bioaugmentation performance likely will need to be evaluated on a site-by-site basis. However, the model developed during this project can assist in predicting the effect of different cell dosages on in situ performance of the cultures.