Contaminated site assessors can evaluate the thermal treatability of a soil at an early stage by systematically assessing the contaminant spectrum, soil matrix, and moisture content, and by initiating targeted preliminary investigations. The foundation is a structured analysis of the relevant contamination parameters during the site investigation phase. The following sections answer the most important questions that arise in practice.
Which contaminant parameters determine thermal treatability?
The thermal treatability of a contaminated soil depends primarily on the boiling point, volatility, and thermal stability of the contaminants present. Organic compounds such as TPH, BTEX, PAHs, phenols, and chlorinated hydrocarbons can be effectively desorbed or decomposed at temperatures up to approximately 600 °C, depending on the material and contaminant load. Heavy metal compounds such as mercury are also thermally desorbable, but require a specific process management approach.
Assessors should distinguish between volatile, semi-volatile, and low-volatility contaminants when conducting their evaluation. Particularly relevant are:
- Highly volatile compounds (BTEX, light TPH): desorbable at low temperatures
- Semi-volatile compounds (PAHs, phenols, PCBs): require higher process temperatures
- Low-volatility or thermally stable compounds (PCDD/PCDF, PFAS/PFOS): require specific temperature ranges and high-performance exhaust gas treatment
- Cyanides and nitroaromatics: thermally decomposable, but with increased demands on process monitoring
Where mixed contaminations are present, treatability is generally achievable provided all relevant contaminants react within the permissible temperature range. Assessors should capture the full contaminant spectrum and not rely solely on indicator parameters, as unknown co-contaminants can influence process management.
How do soil matrix and moisture content affect suitability?
Alongside the contaminant spectrum, soil matrix and moisture content are the most important physical factors influencing thermal treatability. Soils with a high clay content tend to clump in rotary kilns, while sandy substrates are heated more evenly. A moisture content above 25 to 30 percent significantly increases energy demand and can reduce throughput.
Assessors should systematically record the following matrix properties:
- Particle size distribution: Coarse-grained materials (gravels, sands) are generally well suited; fine fractions below 0.063 mm can cause problems during dust separation
- Clay mineral content: High clay content requires mechanical pre-treatment (e.g., crushing, screening)
- Moisture content: Soils with high water content must be dewatered or dried prior to thermal treatment
- Organic fraction: High levels of natural organic matter can affect combustion in the kiln and complicate process management
- Interfering materials: Metal parts, construction debris, or wood remnants must be removed before feeding into the rotary kiln
Mechanical-physical pre-treatment can significantly improve the suitability of many soils. Assessors should therefore evaluate not only the raw contamination, but also the processability of the material.
What preliminary investigations are necessary for a suitability assessment?
A reliable suitability assessment requires not only chemical contaminant analysis, but also soil-physical and mineralogical investigations. The minimum requirements include a complete contaminant characterization, a particle size analysis, and the determination of water content and loss on ignition.
In practice, the following investigation sequence is recommended:
- Contaminant analysis: Comprehensive screening for all relevant organic and inorganic parameters, not just suspected indicator contaminants
- Particle size analysis: Determination of the fine fraction content and grading curve to assess processability
- Water content and dry mass: Basis for the energy balance and logistics planning
- Loss on ignition: Indicator of the organic fraction and the thermal reactivity of the material
- Eluate analysis: Assessment of leachability for classifying the treated material after desorption
The earlier this data is available, the more reliable the suitability assessment. Assessors who collect soil-physical parameters during the detailed investigation phase avoid surprises when the disposal services are later put out to tender.
When should an assessor prefer thermal desorption over other methods?
Thermal desorption is the method of choice when organic contaminants are present at high concentrations, in-situ remediation is technically not feasible, or the site requires rapid and complete contaminant removal. It is particularly suitable for multiple contaminations and for contaminants that are not amenable to biological or chemical-physical treatment methods.
Specific situations in which thermal desorption is clearly preferable:
- Heavily contaminated soils with PAH, PCB, or TPH levels that preclude biological treatment
- Sites under time pressure, as thermal processes are completed significantly faster than biological methods
- Complex contaminant mixtures where no single alternative method addresses all parameters
- PFAS or mercury contaminations for which few alternative treatment methods are approved
- Remediation of decommissioned industrial or gasworks sites with typical multiple contaminations
Thermal desorption is less suitable for soils with a very high heavy metal content (except Hg), as these metals are not removed by desorption. In such cases, a combination of thermal treatment and subsequent stabilization or landfilling should be considered. An overview of the possibilities of thermal soil treatment provides further detail on the process and its limitations.
What sampling errors distort the suitability assessment?
Sampling errors are one of the most common causes of an incorrect assessment of thermal treatability. Non-representative samples lead to an under- or overestimation of contaminant concentrations, and consequently to incorrect conclusions about the appropriate treatment method.
The most common sources of error in practice:
- Insufficient number of samples: Heterogeneous contaminations are not fully captured; hotspots remain undetected
- Incorrect sampling depth: Contaminants that have accumulated in deeper layers are not captured by near-surface sampling
- Loss of volatile compounds: Samples for BTEX or light TPH must be sealed airtight and transported under refrigeration; any delay distorts the measured values
- Composite sample formation without a concept: Uncontrolled mixing of samples from different areas obscures the spatial distribution of contaminants
- Missing documentation of the sample matrix: Without information on moisture, color, odor, and consistency, the analytical results are difficult to interpret
A structured sampling plan based on the contamination history of the site is the most important prerequisite for reliable analytical results. Assessors should not delegate sampling without providing clear specifications for sample containers, the cold chain, and transport times.
How can laboratory tests validate the on-site assessment?
Laboratory tests for thermal treatability make it possible to validate the assessment on the basis of representative samples before large quantities of material are transported to the facility. Soil samples are thermally treated at laboratory scale and subsequently analyzed for residual contaminant levels.
Useful laboratory investigations to support the suitability assessment include:
- Thermogravimetric analysis (TGA): Provides information on mass changes at increasing temperatures and helps determine the optimal process temperature range
- Laboratory desorption tests: Small quantities of soil are treated at defined temperatures; residual levels indicate whether the target values are achievable
- Eluate analysis of the treated material: Determines leachability after desorption and forms the basis for the recovery or disposal decision for the output material
- Gas phase analysis: For unknown contaminant mixtures, the composition of the desorption gases can provide indications of reaction products that could place a burden on the exhaust gas treatment system
Laboratory tests are particularly recommended when the contaminant spectrum is unusual, very high contaminant concentrations are present, or the material is highly heterogeneous. They reduce the risk of surprises during plant operation and provide a solid basis for tendering the disposal services. A well-prepared material flow management approach begins precisely here: with reliable data from the investigation phase.
How ARE Deutzen supports assessors in evaluating thermal treatability
ARE Deutzen GmbH has specialized in the thermal treatment of contaminated mineral waste since 1994 and has more than 30 years of practical experience with complex contaminant spectra. Contaminated site assessors who make contact at an early stage benefit from this experience when evaluating the treatability of their specific material.
ARE Deutzen offers concrete support:
- Expert initial consultation on treatability based on the available analytical data
- Assessment of the contaminant spectrum, soil matrix, and moisture content with regard to plant suitability
- Coordination on sampling and the necessary analytical parameters for a reliable suitability assessment
- Support in planning the material flow from the site to the thermal treatment facility in Deutzen
- Assistance with documentation and regulatory communication as a certified waste management company
The earlier an assessor clarifies treatability, the smoother the subsequent tendering and project execution will be. Get in touch early with the contacts at ARE Deutzen and jointly determine whether your material is suitable for thermal desorption.