Chlorinated solvents such as tetrachloroethene (PCE) and trichloroethene (TCE) are present in groundwater as dense non-aqueous phase liquids (DNAPLs) at many U.S. Department of Defense (DoD), Department of Energy (DOE), and related contractor facilities. DNAPLs have low aqueous solubilities, but even these values may exceed regulatory criteria by as much as five orders of magnitude. As a result, these compounds only slowly dissolve in groundwater and act as long-term sources of groundwater contamination.
Laboratory experimentation and field applications have demonstrated that in situ chemical oxidation (ISCO) with permanganate (MnO4-) is an effective technique for degrading chlorinated solvents. The principal benefit of ISCO using permanganate is that it aggressively enhances dissolution and destruction of the target contaminants within a relatively short period of time, but the economic benefits of this technology diminish as the mass of target chemicals decreases. To reduce overall remediation costs, it was suspected that ISCO potentially could be coupled with a less costly mass removal technology such as in situ bioremediation (ISB).
The main objectives of this project were to assess the technical feasibility of sequential application of ISCO and ISB, to evaluate the effects of this combined treatment on overall cost and performance, and to identify the optimal timing of the transition from ISCO to ISB. Unfortunately, during the course of the demonstration conducted at Launch Complex 34, an unused launch facility at the Kennedy Space Center in Florida, technical problems limited the ability to meet these objectives fully. Biofouling caused significant downtime during operations, and processes were severely impacted by a series of hurricanes. As a result, the demonstration was terminated earlier than planned, though significant results were still obtained.
ISCO typically involves injection and/or recirculation of a concentrated oxidant solution to promote rapid oxidation of the target chemicals. Permanganate attacks the carbon-carbon double bonds in chlorinated ethenes (e.g., TCE), mineralizing the target compound to inorganic products such as carbon dioxide, water, and chloride. ISB involves increasing microbial activity through biostimulation or bioaugmentation to enhance degradation of chlorinated solvents. Both technologies can be used to treat chlorinated solvents, so it was expected that the combined treatment approach would reduce the duration and cost of remediation at chlorinated solvent sites relative to application of either technology alone or in conjunction with other technologies.
The demonstration was designed to be completed in three operational phases: (1) baseline with groundwater circulation alone, (2) biostimulation with the addition of electron donor, and (3) bioaugmentation with the addition of electron donor and the bacterial culture KB-1™. During the demonstration, groundwater was recirculated through the pilot test area (PTA) at a constant groundwater velocity. Each phase was operated for sufficient duration to establish a near “steady-state” rate of TCE removal under each of the different operating conditions.
Organisms present during the baseline phase did not dechlorinate TCE despite the presence of Dehalococcoides in the PTA, and there was an apparent inhibition of dechlorination in the presence of manganese dioxide during the biostimulation and the bioaugmentation phases. In microcosms using materials from the PTA during the biostimulation and bioaugmentation phases, dechlorination through to ethane was observed concurrent with methanogenesis, which was consistent with dechlorination activity observed in the PTA.
The extent of dehalogenation increased over the course of the pilot study, with cis-dichloroethene (cDCE) and vinyl chloride (VC) concentrations increasing in the biostimulation phase and further increasing in the bioaugmentation phase. Ethene concentrations were highest in the bioaugmentation phase and in post-treatment sample events. Correspondingly, the amount of dechlorination increased from the baseline phase through biostimulation and bioaugmentation (where cDCE accumulation was observed) and was highest post-demonstration, with VC accumulation observed. Based on the accumulation of cDCE, VC, and ethene during the biostimulation and bioaugmentation phases, it was apparent that degradation rates of TCE increased during the demonstration, although the chloroethene mass flux and discharge values did not increase significantly over the demonstration. As a result, a substantial reduction in the remedial duration would not be anticipated.
The equipment used to execute the demonstration required only minimal maintenance, with the exception of biofouling control measures. Biofouling of the injection wells impacted the implementation of the demonstration and resulted in downtime during operations; however, the biofouling control knowledge gained during this demonstration could be applied to other ISB recirculation projects. Specifically, the use of regular biofouling control agents would reduce in-line and in-well biofouling issues and the elimination of storage tank(s) from the process equipment would help to limit opportunities for undesired biogrowth. The demonstration downtime was also negatively impacted by hurricanes and ultimately resulted in the early termination of the demonstration.