When Is Thermal Desorption Preferable to Soil Washing?

Thermal desorption is preferable to soil washing when contaminants are organic in nature and cannot be dissolved from the soil using water or solvents. Particularly for high-molecular-weight compounds such as PAHs, PCBs, PFAS, or dioxins, thermal treatment is the more reliable and often the only approved method. The following sections examine the key criteria for selecting the appropriate process in detail.

Which contaminants can only be reliably removed by thermal desorption?

Contaminants such as PAHs, PCBs, PCDD/PCDF (dioxins and furans), PFAS/PFOS, mercury, nitroaromatics, and chlorinated hydrocarbons can be reliably removed from mineral soils by thermal desorption. These compounds are either water-insoluble, strongly bound to soil particles, or toxic enough that complete destruction is strictly required rather than mere relocation.

During thermal treatment, contaminated soils are heated in a rotary kiln to temperatures of up to approximately 600 °C, depending on the material and contaminant load. Under exclusion of oxygen, volatile contaminants evaporate or are thermally destroyed. The resulting contaminant gas is then treated in a multi-stage exhaust gas purification system that includes thermal afterburning, flue gas scrubbing, and activated carbon filters.

Soil washing, on the other hand, cannot remove these compounds, or can only do so incompletely, because they barely transition into aqueous phases. In the case of mercury contamination, for example, thermal desorption is the recognized standard method in Germany, as Hg can be reliably converted to a gaseous state and separately separated.

How does thermal desorption work compared to soil washing?

Thermal desorption uses heat to physically separate contaminants from the soil material or to chemically destroy them. Soil washing, by contrast, relies on mechanical separation and the use of water to dissolve contaminants from the soil and transfer them into an aqueous phase. Both processes aim at the remediation of contaminated soils, but differ fundamentally in their operating principle and area of application.

Thermal desorption: Heat input as the cleaning principle

In the thermal process, the soil passes through a rotary kiln after mechanical pre-treatment. The high temperatures cause organic contaminants to evaporate or decompose. The process is independent of the water solubility of the contaminants and also works with fine-grained, clayey soils that are unsuitable for soil washing.

Soil washing: Mechanical separation and solubility as prerequisites

Soil washing first separates coarse-grained material from fine-grained material and then washes out the contaminant particles using water or surfactant solutions. The process works well with sandy, highly permeable soils containing water-soluble or easily mobilizable contaminants such as heavy metal salts. However, soils with a high fine-grain content and poorly soluble organic compounds represent a fundamental limitation of this approach.

When is soil washing the better choice?

Soil washing is the better choice when the soil is sandy and easily classifiable, the contaminants are water-soluble or mobilizable by surfactants, and a full thermal treatment would be economically disproportionate. Typical areas of application include soils with heavy metal contamination such as lead or zinc, as well as readily soluble inorganic compounds.

Three factors are decisive for the choice of process: soil structure, the type of contaminants, and the required remediation targets. If the fine-grain content is below approximately 15 to 20 percent and the contaminants are readily water-soluble, soil washing may be economically and technically viable. However, as soon as organic contaminants dominate or the fine-grain content is high, soil washing reaches its limits.

In practice, both processes are also combined: mechanical-physical pre-treatment first separates the grain-size fractions. Coarse-grained material and sand are washed, while the contaminated fine-grain fraction is fed to thermal soil treatment.

Which soil types are suitable for thermal desorption?

Thermal desorption is fundamentally suitable for all mineral soil types, including sands, gravels, silts, and clays, as well as construction rubble and other mineral waste. The process is therefore considerably less restricted than soil washing and can also be used with fine-grained, cohesive soils.

What matters less is the soil type itself and more the composition: the moisture content of the material affects the energy requirements of the plant. Very wet soils require a longer residence time in the kiln or pre-drying. Organic admixtures such as roots or peat can complicate treatment but do not fundamentally exclude it.

In addition to conventional soils, dredged material, slags, ashes, sludges, and process residues from industrial plants can also be treated. This makes thermal desorption a particularly flexible process in the material flow management of complex remediation projects.

What happens to the soil after thermal treatment?

After thermal treatment, the result is a contaminant-free or contaminant-reduced mineral mixture that, depending on the remediation outcome and applicable regulations, can be reused as a recycled material or properly disposed of. The recovery of the material as a usable construction material is the target goal of soil remediation.

After desorption, the treated material is tested for compliance with the officially established limit values. If the classification values of the relevant regulations, such as the Federal Soil Protection and Contaminated Sites Ordinance (BBodSchV), are met, the material can be used as a recycled construction material, for example in road construction or earthworks.

Thermal treatment barely alters the mineral structure of the soil. Organic soil components are destroyed, which limits its suitability as agricultural land. For civil engineering purposes, however, the material generally remains well usable. Thermal desorption thus makes a concrete contribution to resource conservation, as processed material reduces the use of primary raw materials.

What regulatory requirements apply to the choice of treatment process?

In Germany, the choice of treatment process for contaminated soils is governed by the Federal Soil Protection Act (BBodSchG), the Federal Soil Protection and Contaminated Sites Ordinance (BBodSchV), and the Circular Economy Act (KrWG). The guiding principle is the best possible elimination of contaminants combined with resource conservation.

Authorities and expert assessors define the remediation targets that must be achieved as part of the remediation planning. This determines which process is technically suitable and legally permissible. Not every process is officially recognized for every contaminant. For certain types of contamination such as PFAS or dioxins, thermal desorption is in many cases the only approved treatment method.

Plant operators require a permit under the Federal Immission Control Act (BImSchG) to operate thermal desorption plants. This permit defines emission limit values and operating parameters that must be consistently complied with and documented. Certifications to ISO 9001, 14001, and 50001 support the demonstration of systematic quality management to authorities and clients.

When awarding remediation services, clients should ensure that the waste management company holds all required permits and is certified as an approved waste management company (EfB) in accordance with § 52 KrWG. This ensures legal compliance of the disposal and protects the client from liability risks.

How ARE Deutzen GmbH helps with choosing the right soil treatment process

ARE Deutzen GmbH has specialized in the thermal treatment of contaminated mineral soils and waste since 1994. With over 30 years of experience and more than 2.5 million treated tonnes, the company offers a sound technical foundation for demanding remediation projects. ARE Deutzen supports civil engineering companies, industrial clients, and project managers in process selection and project execution with the following services:

Thermal desorption in a rotary kiln at temperatures of up to approximately 600 °C, depending on the material and contaminant load, for a broad spectrum of contaminants, including PAHs, PCBs, PFAS, dioxins, mercury, and chlorinated hydrocarbons
Mechanical-physical pre-treatment for classification and preparation of the soil material
Material flow management for soils, dredged material, construction rubble, slags, ashes, and sludges
Interim storage of contaminated material directly at the Deutzen site
Permitted operation under BImSchG, certified to ISO 9001, 14001, and 50001, approved as a certified waste management company under § 52 KrWG

If you are planning a remediation project and would like to clarify the question of process selection in concrete terms, speak directly with the specialists at ARE Deutzen. On the contact page you will find the right contacts for a fast and expert initial consultation.