The generally limited performance of conventional pump-and-treat systems for remediation of chlorinated solvent-contaminated groundwater has led to consideration of chemically enhanced-flushing methods, such as cyclodextrin enhanced flushing (CDEF) for treatment of dense non-aqueous phase liquid (DNAPL) source zones. Cyclodextrins are non-toxic, modified sugars that form complexes with hydrophobic pollutants such as trichloroethene (TCE). To demonstrate the potential of CDEF under near full-scale operational conditions, specific objectives of this project included (1) evaluate the cost and performance of CD-enhanced removal of DNAPL from groundwater; (2) test unrefined, liquid CD as a substitute for CD powder; (3) evaluate a membrane technology for recovering and reusing CD; (4) identify the most appropriate wastewater treatment technology; and (5) conduct Partitioning Tracer Tests (PTT) pre- and post-CDEF for mass balance assessment.
CDEF typically begins with the injection of a water-based CD solution. This solution is flushed through the contaminated aquifer and then extracted. Conventional injection and extraction wells can be used to control the flow field of the flushing solution. This application scheme is similar to conventional pump-and-treat systems; however, due to the advantageous solubility-enhancing properties of the CD solution, mass removal rates are faster and consequently, remediation times are shortened.
The performance targets for this demonstration were to remove more than 90% of the DNAPL mass and to reduce the initial aqueous TCE concentration by 99%. The overall duration of the demonstration was 4 months, during which approximately 32.5 kilograms (kg) of TCE and 1,1,1-trichloroethane (TCA) plus an estimated 3 kg of 1,1-dichloroethene (DCE) and an unknown amount of other contaminants were removed. The decrease in DNAPL saturation was approximately 70 to 81%. TCE concentrations in the reference wells declined between 38.5 to 99.4% (average of 77.3%) from their pre-CDEF levels. The highest aqueous TCE concentrations measured during the CDEF demonstration were greater than 200 milligrams per liter (mg/L) or up to 9 times higher than the average pre-treatment TCE concentrations. Even higher solubility enhancements (up to 19 times) were observed for 1,1,1-TCA. These values demonstrate that CDEF significantly enhanced the contaminant removal rates. Effluent treatment by air stripping lowered the TCE concentration in the effluent below the maximum contaminant level (MCL) of 5 micrograms per liter (μg/L).
The cost of the CDEF technology was evaluated based on the following two treatment schemes: (1) injection/extraction of CD solution using several wells (I/E test) and (2) application of CDEF in multi-well push-pull mode (CPPT). Overall, the CPPT approach generated only half the costs of a comparable I/E system. The full-scale implementation of a hypothetical site (10 times larger than the demonstration) generated costs comparable to other conventional or innovative remediation technologies. The main cost savings are associated with the shortened remediation times that can be realized using CDEF.
However, there are sites where the CDEF approach may not be appropriate or must be used in combination with other technologies. For example, CDEF has been primarily used for the removal of residual NAPL. If free-moving NAPL is encountered inside a well, other technologies, such as free-product skimming, should be applied prior to CDEF. Also, CDEF should not be expected to bring contaminant concentrations to below MCL, although it may lower the contaminant concentration enough to permit the application of otherwise unfeasible remediation approaches (e.g., enhanced bioremediation). (Project Completed – 2004)