Thermal desorption and soil washing are two fundamentally different methods for treating contaminated soils. The key difference lies in the operating principle: thermal desorption destroys or removes contaminants through heat, while soil washing dissolves contaminants from the soil using water and chemical agents. The choice of the appropriate method depends primarily on the type of contaminants, the soil structure, and the remediation objective. The following sections explain both methods in detail and show when each approach is appropriate.
How does thermal desorption work on contaminated soils?
In thermal desorption, contaminated soil is heated to high temperatures in the absence of oxygen, causing contaminants to vaporize or be destroyed. The method is particularly suitable for organic contaminants such as mineral oil hydrocarbons, PAHs, or BTEX and achieves very high cleaning performance even with complex contaminations.
The soil is first mechanically pre-treated — for example, screened and crushed — before entering a rotary kiln. There it is heated to temperatures of up to approximately 600 °C, depending on the material and contaminant load. Since no oxygen is supplied, the material does not combust; instead, the contaminants transition to a gaseous state or are thermally destroyed. The resulting exhaust gas is then cleaned in a multi-stage system comprising dust removal, thermal post-combustion, flue gas scrubbing, and activated carbon filters. The treated soil can often be reused as a construction material after cleaning. More information on this method is available on the page about thermal soil treatment.
How does soil washing work and for which contaminants is it suitable?
Soil washing is a wet-chemical process in which contaminants are dissolved from the soil using water, surfactants, or other chemical agents. The method works well for soils contaminated with heavy metals and for contaminations that concentrate on fine soil particles.
In the process, the soil is first separated by grain size. Coarse fractions such as gravel and sand can generally be washed effectively and reused after treatment. Fine-grained components such as silt and clay bind contaminants more strongly and are more difficult to clean. The wash water must be treated after use, as it contains the dissolved contaminants. Soil washing is particularly effective for:
- Heavy metals such as lead, zinc, or cadmium
- Volatile chlorinated hydrocarbons in sandy soils
- Mineral oil contaminations in coarse-grained soils
- Cyanides in certain soil matrices
In highly cohesive or humus-rich soils, soil washing reaches its limits, as the contaminants are too firmly bound to the soil matrix.
What are the most important differences between the two methods?
The most important difference between thermal desorption and soil washing lies in the operating principle: thermal desorption uses heat to remove or destroy contaminants, while soil washing is based on physico-chemical extraction. This results in significant differences in applicability, energy requirements, and treatment outcomes.
- Contaminant spectrum: Thermal desorption covers a broader range of organic contaminants, including persistent compounds such as PCDD/PCDF, PCB, or PFAS. Soil washing is superior for heavy metals.
- Soil type: Thermal desorption works on nearly all mineral soils regardless of fine particle content. Soil washing loses efficiency significantly with high silt and clay content.
- Energy requirements: Thermal desorption is more energy-intensive, as high temperatures must be generated. Soil washing requires less energy but larger volumes of water.
- Waste streams: Thermal desorption produces cleaned mineral materials and exhaust gases that are treated. Soil washing generates contaminated wash water and a contaminant-rich fine fraction that requires further treatment.
- Treatment depth: Thermal desorption generally achieves more complete contaminant destruction, especially for persistent organic compounds.
When should thermal desorption be preferred over soil washing?
Thermal desorption should be preferred over soil washing when the soil is contaminated with persistent organic pollutants, has a high fine particle content, or when complete contaminant destruction is required. Thermal treatment is also often the safer choice for mixed contaminations involving multiple contaminant classes.
Specific situations in which thermal desorption has a clear advantage:
- Contaminations with PAHs, PCBs, PCDD/PCDF, PFAS, or mercury
- Soils with high clay or silt content where soil washing is not efficient
- Remediation projects at former gasworks, coking plants, or chemical facilities with complex contaminant mixtures
- Projects where the treated soil is to be reused as recycled material
- Cases where statutory limit values require complete contaminant destruction
In cases of exclusively heavy metal contamination in sandy, coarse-grained soils, soil washing may be more cost-effective. Both methods should therefore always be selected on the basis of a thorough contaminant analysis and soil characterization.
Can thermal desorption and soil washing be combined?
Yes, thermal desorption and soil washing can be meaningfully combined in certain projects. A typical combination involves the soil first being mechanically and physically treated and separated by grain size. Coarse fractions are washed, while fine-grained or organically contaminated portions are thermally treated.
This combined approach is particularly useful for soils with mixed contamination — that is, when both heavy metals and organic contaminants are present. Pre-sorting can reduce the proportion of material that must be thermally treated, which can lower overall costs. Thoughtful material flow management is crucial for efficiently directing the various fractions and assigning the optimal treatment method to each. Mechanical and physical pre-treatment can also serve as a first step to optimize the material throughput of both primary methods.
What permits and certifications are required for these methods?
Facilities for thermal soil treatment and soil washing in Germany are subject to the Federal Immission Control Act (BImSchG) and require the corresponding immission control permit. Operators must also be recognized as a certified waste management company (EfB) pursuant to Section 52 of the Circular Economy Act.
The following requirements are relevant in practice for safe and legally compliant operation:
- BImSchG permit: Mandatory for all facilities falling under the 4th BImSchV, which applies to thermal treatment facilities and larger soil washing plants.
- Certification as a waste management company (EfB): Proof of professional competence, reliability, and technical equipment.
- Quality management systems: Standards such as ISO 9001 for quality management, ISO 14001 for environmental management, and ISO 50001 for energy management are industry-standard benchmarks that provide additional assurance to clients and authorities.
- Waste law documentation: Disposal certificates, consignment notes, and register-keeping in accordance with the Waste Tracking Ordinance are mandatory for the acceptance and treatment of hazardous waste.
Compliance with these requirements is not a formality but the foundation for the safe, proper, and harmless treatment of contaminated soils. A consistent quality management system ensures that all statutory requirements are permanently met.
How ARE Deutzen GmbH supports soil remediation
ARE Deutzen GmbH has specialized in the thermal treatment of contaminated mineral soils and waste since 1994. As a certified waste management company with an immission control permit and over 30 years of experience, the company offers a reliable solution for demanding remediation projects. Services include specifically:
- Thermal desorption in a rotary kiln at temperatures of up to approximately 600 °C, depending on the material and contaminant load, for a broad range of contaminants
- Treatment of soils containing Hg, MKW, BTEX, PAHs, PCBs, PFAS/PFOS, PCDD/PCDF, and other compounds
- Mechanical and physical pre-treatment and grain size separation
- Material flow management for soil, dredged material, construction rubble, and process residues
- Interim storage and coordination with partner facilities for waste that cannot be thermally treated
- Certified operation in accordance with ISO 9001, 14001, 50001, and 18001
If you are planning a remediation project or need to properly dispose of contaminated soil, contact ARE Deutzen directly. The contact persons will provide expert advice on method selection, delivery quantities, and permit requirements.