The design of thermal soil treatment depends on several key project parameters: type of contaminant, contaminant concentration, soil material properties, total quantity, quality requirements for the treatment outcome, and project schedule and logistics. Each of these parameters influences which process temperatures, throughput rates, and post-treatment steps are required. The following sections explain the role each factor plays in planning a thermal desorption project.
Which types of contaminants determine the treatment process?
The type of contaminants present in the soil is the most fundamental parameter in designing thermal soil treatment. Different contaminants have different boiling points, thermal stabilities, and decomposition behaviors, which directly affect the required treatment temperature and exhaust gas purification technology.
Volatile compounds such as mineral oil hydrocarbons (TPH), BTEX, or lighter PAHs can be driven out of the soil material at relatively low temperatures. Semi-volatile or thermally stable compounds such as PCBs, PCDD/PCDF, PFAS, or mercury (Hg), on the other hand, require higher process temperatures and particularly high-performance exhaust gas treatment. Cyanides and nitroaromatics can form specific intermediate products during thermal treatment that must be safely captured in the exhaust gas stream.
Where mixed contamination is present — i.e., soils containing multiple contaminant groups simultaneously — process design is always governed by the most demanding contaminant. Complete contaminant analysis is therefore essential before quantities, temperatures, or throughput rates are determined.
How does contaminant concentration influence plant design?
The concentration of contaminants in the soil material largely determines how intensive and time-consuming the treatment process must be. Heavily contaminated soils require longer residence times in the rotary kiln, lower throughput rates, and a more highly dimensioned exhaust gas treatment system to reliably comply with statutory emission limit values.
Particularly in the case of organic contaminants at high concentrations, heat release in the kiln increases, making process control more complex. At the same time, the quantity of contaminants in the exhaust gas stream increases, placing higher demands on the afterburner, flue gas scrubber, and activated carbon filter. Conversely, low-concentration soils allow higher throughput rates and shorter treatment times, which has a positive effect on project costs.
For a realistic plant design, it is therefore important to have not only individual measured values but ranges of contaminant concentrations from representative soil samples, as concentration variations within a project can be considerable.
Which material properties of the soil are relevant for treatment?
In addition to the contaminants themselves, the physical and chemical properties of the soil material determine how efficiently thermal desorption proceeds. The most important parameters include particle size distribution, moisture content, organic content, and mineralogical composition.
Particle size distribution and moisture content
Fine-grained soils such as clays and silts have a higher internal surface area and retain contaminants more strongly than coarse-grained sands or gravels. They require longer residence times in the rotary kiln to ensure complete contaminant desorption. A high moisture content significantly increases energy requirements, as the water must be evaporated before actual desorption can take place. Highly water-saturated materials can also place a strain on conveying equipment and mechanical pre-treatment systems.
Organic content and mineralogical composition
A high proportion of organic matter in the soil affects the heat balance in the kiln and can lead to unwanted combustion reactions during thermal treatment if the process is not precisely controlled. Mineralogical composition is relevant because certain minerals can react at high temperatures or encapsulate contaminants within their matrix, reducing desorption efficiency.
Why is the total quantity of material to be treated decisive?
The total quantity of contaminated material is a central economic and logistical planning parameter. It determines the project duration, resource deployment, and the overall cost-effectiveness of thermal treatment.
Small quantities of a few hundred tonnes require a different project organization than large-scale projects involving several tens of thousands of tonnes of material. With large quantities, continuous delivery schedules can be established that optimize plant operation and reduce costs. For smaller projects, careful coordination of delivery quantities is important to minimize storage and waiting times.
Furthermore, the total quantity influences capacity planning within material flow management: interim storage, mechanical pre-treatment, and thermal treatment must be coordinated with one another to prevent bottlenecks and ensure the project is completed on schedule.
Which requirements for the treatment outcome influence the process?
The target treatment objective defines which residual contaminant levels are acceptable in the treated material, and thereby directly determines process intensity. The stricter the limit values for the treatment outcome, the higher the demands on temperature, residence time, and exhaust gas treatment.
In Germany, quality targets for treated soil are generally governed by the relevant precautionary and trigger values of the Federal Soil Protection and Contaminated Sites Ordinance (BBodSchV), as well as project-specific regulatory requirements. If the treated material is to be recycled after soil remediation, stricter requirements often apply than for landfill disposal.
The specifications for the treatment outcome must be definitively clarified before the project begins, as they directly affect the process parameters of the rotary kiln. A subsequent tightening of quality requirements can have significant implications for costs and schedule. Quality management plays a central role here in systematically monitoring and documenting compliance with the agreed target values.
How do project schedule and logistics factor into the design?
Project schedule and logistics are not secondary planning considerations — they directly influence how the thermal treatment plant must be operated. A tight schedule requires higher throughput rates, which is only achievable given suitable material characteristics and sufficient plant capacity.
The distance between the remediation site and the treatment plant determines transport costs and delivery frequency. Short delivery intervals enable more consistent plant operation, while irregular deliveries can lead to idle periods or overload situations. Close coordination between the client, transport companies, and plant operator is therefore important as early as the project planning phase.
Seasonal factors such as frost periods or heavy rainfall events can affect the trafficability of remediation sites and the condition of the material, and should be taken into account in the schedule. Regulatory approval lead times for the transport of hazardous waste are also a logistical parameter that influences project planning.
How ARE Deutzen GmbH helps with the design of your thermal treatment
ARE Deutzen GmbH brings over 30 years of experience in the assessment and treatment of contaminated mineral waste, and supports clients from the early project phase in clarifying all design-relevant parameters. Services specifically include:
- Assessment of contaminant analysis and material characterization as the basis for process design
- Capacity planning and coordination of delivery quantities and schedules
- Thermal desorption in the rotary kiln at temperatures up to approximately 600 °C, depending on material properties and contaminant load, with multi-stage exhaust gas treatment
- Coordination of interim storage, mechanical pre-treatment, and thermal treatment from a single source
- Documentation of treatment results in accordance with regulatory and project-specific requirements
Whether brownfield remediation, revitalization of an industrial site, or disposal of mineral process residues: ARE Deutzen GmbH works with you to develop a technically sound and timely solution. Contact us now and describe your project so that we can develop the right approach for your specific contamination situation.