Formatted Title
How to Manage the Thermal Remediation of a Chemical Waste Landfill? Chemistry is Key!
Background/Objectives
Treatability by thermal desorption is well established for many contaminants and any gaps can be filled by conducting lab-scale treatability studies. But how do you design a thermal remediation for a complex mixture found in high concentrations in a former chemical waste landfill?
A 1960’s and 70’s chemical waste landfill holding over 100 tons of contaminants (mercury, highly toxic pesticides as well as intermediates, impurities, elemental sulphur, and solvents) is up for remediation. During thermal remediation toxic and flammable chemicals are evaporated (some are formed during heating). The big question is how to approach this remediation?
Approach/Activities
Lab-scale treatability testing has been performed at 300 and 350 °C.
Based on the lab-scale results a pilot test was conducted treating 40 tons of soil in a sealed steel box. The operational strategy was based on thermal and chemical reaction simulation and was adapted to measurements and findings as the test progressed.
Based on the testing, simulations have been made for the full-scale project to answer the all important question: Should the soil be excavated and thermally treated in a pile or is the project suited for in situ thermal desorption?.
Results/Lessons Learned
The post-treatment soil analysis from lab and pilot testing has shown that the organic contaminants are remediated to below detection limits at 200 °C, whereas the mercury (mainly sulfide) requires 350-500 °C to reach the 1-1.5 mg/kg total mercury range. The heat treatment process seems to influence the temperature required to reach the mercury target.
Pilot testing has shown that a thermal desorption remediation can be operated safely in spite of exothermic reactions, evolution of flammable gasses and toxic pesticides. Regarding the soil volume as a chemical reactor and carefully controlling the heating rate according to prior simulations allowed safe operation without excessive LEL levels. A further advantage of the chemical reactor approach was that 99.99% degradation of pesticides in the soil was accomplished minimizing the amount extracted and greatly improving the chemical safety during operation.
The pilot test revealed some challenges not apparent from lab-scale testing: The sulfur content made operation difficult and impaired extraction of mercury ultimately increasing the target temperature.
High temperature thermal conductive heating remediation has proven to be an efficient and safe approach to the complex mixed contamination for at the Danish Groyne 42 site containing highly toxic pesticides mixed with mercury bound as mercury sulfide and a number of other contaminants. The pilot-scale test results have formed the basis for an in situ full-scale remediation design.