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Assessment and Remediation of Contaminated Sediments (ARCS) Program

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Pilot-Scale Demonstration of Thermal Desorption for the Treatment of Ashtabula River Sediments

Final Report

Prepared by
U.S. Army Engineer District, Buffalo

For the
Assessment and Remediation of Contaminated Sediments (ARCS) Program
U.S. Environmental Protection Agency
Great Lakes National Program Office
Chicago, Illinois



This report discusses the results of a pilot-scale project which demonstrated the effectiveness of using low temperature thermal desorption to remediate contaminated Ashtabula River sediments. The Ashtabula River sediments are contaminated with polychlorinated biphenyls (PCBs), hexachlorobenzene, hexachlorobutadiene and other chlorinated compounds, as well as heavy metals. Approximately 10 to 12 cubic yards of sediment were dredged from the river and treated on-site using a mobile thermal desorption unit during September 1992.

Sediments with varying contamination levels were processed at different solids content, thermal desorption unit residence times, and temperatures to determine the effect of these parameters on removal efficiencies. Additionally, each process stream was individually weighed and extensive sampling was conducted so that contaminant partitioning in the waste streams could be determined, along with a comprehensive mass balance. A particular focus of the project was to determine the extent of mercury, dioxins, and furans that were vented from the processor through the stack.

The sediment was processed under three different nominal thermal unit residence times: 60, 90 and 120 minutes. This resulted in a minimum average sediment temperature of 179deg.C during the 60 minute residence time run and a maximum temperature of 332deg.C for the 120 minute residence time run.

Removal of chlorinated volatile compounds exceeded 92% under all conditions and, in general, these compounds were captured by the carbon adsorber. On average, 86 percent of total PCBs (as Aroclor 1248) were removed from the sediment and subsequently captured primarily in the condensate produced by the process. Heavy metals, with the exception of mercury, tended to pass through the system with the sediment without being volatilized. Thermal desorption removed 73.3 to 96.8 percent of the mercury in the sediment, with this element being captured primarily by the carbon adsorber.

Trace amounts of some of the constituents of interest were detected in the process off-gas. This includes mercury at an average of 0.014 nanograms per dry standard cubic meter; total dioxins ranging from 0.0071 to 0.0825 nanograms per dry standard cubic meter; and total furans ranging from 0.0029 to 0.2669 nanograms per dry standard cubic meter. These levels are below generally accepted standards for air emission from thermal treatment units.

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