A single contaminant value is not sufficient for the suitability assessment of contaminated soils, because the suitability of a soil for thermal treatment always depends on several factors simultaneously: the type and concentration of contaminants, the contaminant mixture, the soil matrix, and the physical properties of the material. Only the interplay of these parameters allows a reliable assessment of whether and how a soil can be treated. The following sections explain which analytical values are truly decisive and what happens when the data basis remains incomplete.
What factors determine the suitability of a soil for thermal treatment?
The suitability of a soil for thermal treatment is determined by the interplay of contaminant type, contaminant concentration, soil matrix, and physical properties such as particle size distribution and moisture content. No single parameter is decisive on its own. Only a comprehensive assessment of all relevant parameters provides a reliable picture.
In practice, this means: a soil with a comparatively low PAH content can still be difficult to treat if high moisture contents or cohesive components are present at the same time, impairing heat input in the rotary kiln. Conversely, a soil with higher contaminant concentrations can be well suited if the matrix has favorable thermal properties.
For thermal soil treatment, at least the following factors must therefore always be assessed:
- Type and concentration of contaminants present
- Composition of the contaminant mixture
- Soil type and particle size distribution
- Moisture content and organic fraction
- Content of foreign materials such as construction rubble or slag
What is meant by a contaminant mixture and why is it decisive?
A contaminant mixture refers to the simultaneous presence of several different contaminants in a soil, for example mineral oil hydrocarbons (TPH) together with PAHs, BTEX, or chlorinated hydrocarbons. The mixture is decisive because different contaminants have different boiling points, reactivities, and degradation mechanisms and can influence one another.
In thermal desorption, contaminants are released from the soil matrix by heat — typically at temperatures of up to approximately 600 °C, depending on the material and contaminant load — and either destroyed or converted to a gaseous state. This process is well plannable for individual substance groups. However, if several contaminant classes are present simultaneously, the treatment temperature and residence time must be selected so that all relevant compounds are reliably captured.
The contaminant mixture is particularly relevant when volatile compounds such as BTEX occur together with semi-volatile substances such as PCBs or PCDD/PCDF. In such cases, a single analytical value for one substance group is not sufficient to correctly define the treatment conditions. The exhaust gas purification system must also be designed for the entire contaminant spectrum.
How does the soil matrix influence the result of the suitability assessment?
The soil matrix influences the suitability assessment because it determines how well heat can penetrate the material, how the contaminants are bound within it, and whether the soil can be mechanically prepared for the rotary kiln. Heavy, cohesive soils with a high clay content and high moisture content require, for example, more intensive pre-treatment and longer residence times in the kiln.
Sandy, highly permeable soils can generally be thermally treated more easily and uniformly. Soils with a high organic fraction or an elevated content of construction rubble and foreign materials, on the other hand, place special demands on mechanical pre-treatment and kiln operation.
For the suitability assessment, this means: contaminant analysis alone says nothing about whether the soil can be introduced into the rotary kiln at all or what pre-treatment steps are necessary. Mechanical-physical pre-treatment is an integral part of the overall process and must be considered in the suitability assessment.
What analytical values are required for a complete suitability assessment?
For a complete suitability assessment of contaminated soils, in addition to contaminant concentrations, physical parameters such as moisture content, particle size distribution, and organic fraction are also required. In addition, information on the origin of the material and the history of the site is necessary in order to fully assess the contaminant spectrum.
In detail, a meaningful soil analysis typically includes the following parameters:
- Contaminant levels for all relevant substance groups: TPH, BTEX, PAHs, PCBs, PFAS/PFOS, heavy metals such as mercury, cyanides, nitroaromatics, and chlorinated hydrocarbon compounds
- Moisture content and dry matter content
- Loss on ignition as a measure of the organic fraction
- Particle size distribution and soil type
- pH value and, where applicable, content of foreign materials
Incomplete analyses that capture only individual contaminant groups regularly result in treatment conditions being incorrectly assessed. This can jeopardize both treatment quality and compliance with emission limit values.
What happens if the suitability assessment is based on incomplete data?
If the suitability assessment is based on incomplete data, there is a risk that the soil will be introduced into the facility under incorrect treatment conditions. This can result in contaminants not being fully driven off, emission limit values being exceeded, or facility operations being disrupted.
In practice, this manifests, for example, when a soil is declared as TPH-contaminated but actually also contains chlorinated solvents that can release chlorine compounds during combustion. Such surprises during facility operation are not only technically problematic but can also have legal consequences under the Federal Immission Control Act (Bundes-Immissionsschutzgesetz).
Furthermore, incomplete data can result in a soil being rejected that could have been treated without issue with correct preliminary characterization. This causes unnecessary delays in the remediation project and additional costs for all parties involved. Thorough preliminary analysis is therefore not a bureaucratic obligation but an economically sound investment.
When should a suitability assessment be carried out before delivery?
A suitability assessment should always be carried out before the delivery of contaminated soils, ideally already in the planning phase of a remediation project. The earlier reliable analytical data are available, the better treatment capacities can be planned, transport routes organized, and costs calculated.
An early suitability assessment is particularly important in the following situations:
- For soils from decommissioned industrial or gasworks sites with a complex contaminant spectrum
- For materials from military or chemical facilities
- When multiple contaminations with different contaminant classes are to be expected
- For large quantities where rejection at the facility would have significant logistical consequences
- When tight project deadlines must be met
Material flow management therefore begins well before the actual delivery. Close coordination between the client, the analytical laboratory, and the treatment facility is the key to a smooth process.
How ARE Deutzen GmbH supports the suitability assessment
ARE Deutzen GmbH has been supporting civil engineering companies and industrial clients for over 30 years in the safe disposal of contaminated soils and mineral waste. For the suitability assessment, this means in concrete terms:
- Expert advice on the required analytical work already in the project planning phase
- Evaluation of analytical results based on many years of experience with complex contaminant mixtures
- Clear feedback on which additional analyses are required when the data basis is incomplete
- Coordination of delivery, interim storage, and treatment from a single source
- Legally compliant processing as a certified waste management company in accordance with § 52 KrWG
If you are planning a remediation project and would like to know whether your soils are suitable for thermal desorption, contact ARE Deutzen directly. Our contact persons will advise you quickly and straightforwardly on all questions relating to the suitability assessment and the next steps.