During thermal desorption, contaminated soil is heated in a rotary kiln at temperatures of up to approximately 600 °C, depending on the material and pollutant load, in the absence of oxygen. Organic pollutants are either destroyed or converted into a gaseous state and subsequently rendered harmless in a multi-stage exhaust gas treatment system. The result is a mineral material that, after testing of pollutant levels, can often be reused. The following sections answer the most important questions about the process.
Which pollutants are removed from soil by thermal desorption?
Thermal desorption removes a broad spectrum of organic and inorganic pollutants from mineral materials. These include mineral oil hydrocarbons (MKW), BTEX compounds (benzene, toluene, ethylbenzene, xylene), polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), phenols, cyanides, nitroaromatics, and per- and polyfluoroalkyl substances (PFAS/PFOS). Mercury (Hg) and dioxins or furans (PCDD/PCDF) can also be treated using this process.
The decisive advantage lies in the breadth of the pollutant spectrum. Many sites exhibit multiple contaminations — such as former gasworks, industrial brownfields, or military properties — where various pollutants are present in the soil simultaneously. Thermal treatment is one of the few processes that reliably treats such complex pollutant mixtures in a single process step.
The treatment temperature is important in this context: volatile compounds such as mineral oil hydrocarbons are driven off at lower temperatures, while semi-volatile substances such as PCDD/PCDF or PCB require higher temperatures. The plant is configured according to the pollutant characteristics of the input material.
How does the rotary kiln work in thermal soil treatment?
The rotary kiln is the heart of thermal soil treatment. After mechanical pre-treatment, the contaminated soil is fed into the rotating kiln and heated there at temperatures of up to approximately 600 °C, depending on the material and pollutant load. The continuous rotational movement of the kiln ensures that the material is mixed evenly and heated thoroughly. The process runs in the absence of oxygen, so no uncontrolled combustion takes place.
The principle of thermal desorption is based on the fact that pollutants are released from the soil matrix at sufficiently high temperatures and converted into a gaseous state. Organic compounds are either directly destroyed or driven off as pollutant gases, which are then treated in the exhaust gas cleaning system.
Before entering the rotary kiln, the material undergoes mechanical pre-treatment. Coarse components such as stones or construction rubble are screened out, and the material is brought to a suitable grain size. This pre-treatment ensures that heat input in the kiln is uniform and that the treatment is effective.
What happens to the pollutants after desorption in the exhaust gas?
The pollutants driven off in the rotary kiln enter a multi-stage exhaust gas treatment system as raw gas. This includes dust removal, thermal post-combustion, flue gas scrubbing, and an activated carbon filter. Only after this multi-stage cleaning process is the purified gas released into the atmosphere. Compliance with statutory emission limit values is continuously monitored.
Thermal post-combustion is particularly important here: in an afterburner chamber, the pollutant gases are completely oxidized at very high temperatures, destroying organic compounds. Flue gas scrubbing then removes acidic gas components and heavy metal compounds from the exhaust gas stream. The final activated carbon filter binds any remaining residual contaminants before the purified gas leaves the plant.
This multi-stage system ensures that even when treating particularly hazardous pollutants such as dioxins or PFAS, no environmentally relevant emissions are generated. The plant is approved under the Federal Immission Control Act (BImSchG) and is subject to regular regulatory inspections.
Which soils and wastes are suitable for thermal desorption?
In principle, all mineral materials contaminated with organic or certain inorganic pollutants are suitable for thermal desorption. These include contaminated soils, dredged material, construction rubble, and process residues such as slags, ashes, sludges, and dusts. The key requirement is that the material is thermally stable and that the pollutants can be mobilized by heat.
Typical sources of suitable treatment material include:
- Decommissioned industrial sites with mineral oil hydrocarbon or PAH contamination
- Former gasworks sites with tar and phenol contamination
- Military properties with explosive or fuel contamination
- Chemical plants with contamination from chlorinated hydrocarbons or PCB
- Construction sites with unexpected soil contamination
Materials with a very high organic content are not suitable, as these would combust in the kiln, nor are wastes whose pollutants cannot be mobilized by thermal action. As part of material flow management, an advance assessment is carried out to determine which treatment process is best suited for the respective material.
Is thermally treated soil reusable after desorption?
In many cases, thermally treated soil can be reused after desorption. Following treatment, the material is analytically tested to determine whether the pollutant levels fall below the applicable limit values for the intended reuse. If the results are within the permissible values, the material can be used as a recycled construction material, for example in road construction or as backfill material.
Reusability depends on several factors: the original pollutant load, the treatment temperature achieved, and the requirements at the intended site of use. In some cases, thermal treatment also alters the physical properties of the material, such as grain size or load-bearing capacity, which must be taken into account when planning the reuse.
The recycling of treated soils is an important aspect of resource efficiency. Rather than permanently disposing of pollutant-contaminated material in landfill, it can be returned to the economic cycle after successful treatment. More on how soil is treated as a resource is shown in the corresponding sustainability approach.
When is thermal desorption the right remediation method?
Thermal desorption is the right remediation method when soils are contaminated with organic pollutants that can be mobilized by heat, and when complete and verifiable pollutant elimination is required. It is particularly suitable for complex multiple contaminations where other methods such as biological or chemical treatment are insufficient or act too slowly.
The method is especially recommended in the following situations:
- High pollutant concentrations that exceed the capacity of other methods
- Time-critical remediation projects where rapid results are required
- Sites with mixed contamination from different pollutant classes
- Projects where the treated material is to be reused afterwards
- Regulatory requirements mandating complete pollutant destruction
Compared to landfill solutions, thermal desorption offers the advantage that pollutants are actually eliminated rather than merely contained. This creates legal certainty for property owners and those responsible for remediation. Compared to in-situ methods that treat soil on site, thermal desorption stands out for its high process reliability and the comprehensive documentability of treatment results.
How ARE Deutzen GmbH supports thermal soil treatment
ARE Deutzen GmbH has been operating one of the most powerful thermal soil treatment plants in Germany since 1994. With over 30 years of experience and more than 2.5 million tonnes of mineral materials treated, the company offers a reliable and technically mature solution for demanding remediation projects. Services include:
- Thermal desorption of contaminated soils, construction rubble, and mineral waste in a rotary kiln
- Treatment of a broad pollutant spectrum, including mineral oil hydrocarbons, PAH, PCB, PFAS, dioxins, and mercury
- Mechanical pre-treatment and interim storage of input material
- Complete material flow management from delivery through to recycling or disposal
- Analytical support and comprehensive documentation of treatment results
The plant is approved under the BImSchG and certified to ISO 9001, 14001, 50001, and 18001. ARE Deutzen GmbH is also a certified waste management company (EfB). If you are planning a remediation project or need to safely dispose of contaminated soil, get in touch and discuss your project directly with the specialists at ARE Deutzen GmbH.