Thermal soil treatment and mechanical-physical soil treatment differ fundamentally in their mode of action: thermal desorption permanently destroys or removes contaminants from the soil through high temperatures, while mechanical-physical processes separate, concentrate, or immobilize contaminants without chemically altering them. Which process is suitable depends primarily on the type and concentration of the contamination and the requirements for the treatment result. The following sections answer the most important questions about both processes.
Which contaminants can each process actually eliminate?
Thermal soil treatment is suitable for a broad spectrum of organic and inorganic contaminants, including mineral oil hydrocarbons (MKW), BTEX, phenols, PAH, PCB, PFAS/PFOS, PCDD/PCDF, cyanides, nitroaromatics, and chlorinated hydrocarbon compounds. Mercury (Hg) can also be reliably removed by thermal desorption. Mechanical-physical processes, on the other hand, are particularly suitable for soils contaminated with heavy metals and slightly contaminated materials, but reach their limits with volatile organic compounds.
A key difference lies in the depth of contaminant removal. Thermal processes destroy contaminants or convert them into the gaseous state, which is then treated in a multi-stage exhaust gas purification system. The result is a largely contaminant-free mineral. Mechanical-physical methods separate contaminated fractions from less contaminated ones or bind contaminants without eliminating them. For complex mixed contaminations involving multiple contaminant classes, thermal soil remediation is therefore generally the more effective choice.
How does thermal desorption work technically?
In thermal desorption, contaminated soil is heated in a rotary kiln at temperatures of up to approximately 600 °C under oxygen exclusion after mechanical pre-treatment. The heat releases volatile and semi-volatile contaminants from the soil material and converts them into the gaseous state or destroys them directly. The resulting contaminated gas is then treated in a multi-stage exhaust gas purification system.
The exclusion of oxygen is technically critical: it prevents uncontrolled combustion of the soil material and enables the targeted transfer of contaminants into the gas phase. The exhaust gas purification typically includes dedusting, thermal post-combustion, flue gas scrubbing, and activated carbon filters. This multi-stage system ensures that statutory emission limits are reliably met. The treated soil material leaves the kiln as a clean mineral that can be reused. The thermal soil treatment by ARE Deutzen is based on more than 30 years of operational experience with exactly this process.
How do mechanical-physical soil treatment processes work?
Mechanical-physical soil treatment processes separate, sort, or immobilize contaminants through physical processes such as screening, washing, flotation, sedimentation, or soil washing. The goal is to separate contaminated fractions from clean soil material or to reduce contaminant mobility without chemically altering the contaminants themselves.
Typical areas of application include soils contaminated with heavy metals, where contaminants are bound to fine-grain fractions and can be separated by wet classification. Slightly contaminated soils with locally limited pollution can also be treated cost-effectively through mechanical processing. However, the separated contaminant-containing fractions must subsequently be disposed of safely. Mechanical-physical processes therefore generally produce a concentrate or residue that must be further treated or landfilled. More information on how these processes are used as part of a comprehensive disposal concept is explained on the page for mechanical-physical treatment.
When is thermal treatment better than mechanical-physical processes?
Thermal soil treatment is the better choice when organic contaminants are present in high concentrations, when multiple contaminant classes occur simultaneously, or when the treatment objective requires complete contaminant elimination. Particularly at contaminated sites with complex contaminations, such as decommissioned gasworks, industrial facilities, or military properties, thermal desorption is the most reliable process.
The following criteria favor thermal soil remediation:
- High contaminant concentrations that would overwhelm mechanical processes
- Volatile organic compounds such as MKW, BTEX, or PAH that cannot be separated mechanically
- Mixed contaminations from multiple contaminant groups
- Strict requirements for the purity of the treated material
- Contaminants such as PFAS, PCB, or PCDD/PCDF that require specific treatment temperatures
Mechanical-physical processes, on the other hand, are economically viable when soils are only slightly contaminated, when contaminants are well bound to specific grain sizes, or when pre-sorting to reduce the quantity requiring thermal treatment is beneficial.
Can both processes be combined?
Yes, combining both processes is often practical and is specifically used in larger remediation projects. Mechanical-physical pre-treatment first separates slightly contaminated from heavily contaminated fractions. Only the highly contaminated portions are then thermally treated. This reduces the quantities to be treated and therefore the overall project costs.
A typical approach looks like this: incoming soil material is first screened and classified. Coarse, slightly contaminated fractions can be directly recycled or landfilled after inspection. Fine-grained, highly contaminated portions are fed into thermal desorption. This integrated material flow management allows economically and ecologically optimized treatment of even large quantities of material. Information on the holistic management of material flows can be found on the page for material flow management.
What happens to the soil after treatment?
After thermal desorption, the treated soil is present as a mineral material that is free of the organic contaminants previously present. Depending on its quality and composition, this material can be reused as a construction material, for example as fill or bulk material in civil engineering. Recycling is contingent on compliance with applicable limit values, in particular the Mantelverordnung.
Mechanically-physically treated soils are also available for recycling after processing, provided that contaminant levels fall below the permissible limit values. Separated residues such as sludges or fine-grain concentrates, however, must be disposed of separately or subjected to thermal post-treatment. In any case, the goal of both processes is to return the material to the recycling cycle and thereby conserve natural resources. Further background on the sustainable use of soils and minerals is provided on the page on the topic of soil.
How ARE Deutzen helps in choosing the right soil treatment process
ARE Deutzen GmbH specializes in the treatment of contaminated mineral waste and has been operating a thermal soil treatment plant at its Deutzen site in western Saxony since 1994. The company supports civil engineering companies, industrial customers, and remediation contractors from the initial characterization of the material through to the verified freedom from contaminants of the treated soil. Services include:
- Thermal desorption in a rotary kiln at up to approximately 600 °C, depending on the material and contaminant load, for a broad spectrum of contaminants
- Mechanical-physical pre-treatment for fractionation and quantity separation
- Integrated material flow management for complex remediation projects
- Interim storage of incoming materials
- Disposal of process residues such as slags, ashes, sludges, and dusts
- Certified waste management company under the Circular Economy Act with all required BImSchG permits
If you are planning a remediation project or need to dispose of contaminated soils safely and in compliance with the law, contact ARE Deutzen directly. The experienced team will advise you on the technically and economically optimal solution for your specific contamination situation.