The duration of thermal soil treatment depends on several factors and cannot be stated as a blanket figure. The decisive factors are primarily the type and concentration of contaminants, the physical properties of the material, and the overall scope of the remediation project. The following sections examine the most important influencing variables in detail.
What affects treatment time in the rotary kiln?
Treatment time in the rotary kiln is determined primarily by the residence time of the material at the required treatment temperature. This residence time depends on the contaminant limit values to be demonstrated, the thermal stability of the contaminants, and the material throughput rate of the plant. All three variables are in direct interaction with one another.
In the rotary kiln, contaminated material is treated at temperatures of up to approximately 600 °C under oxygen exclusion, depending on the material and contaminant load. The rotation of the kiln ensures uniform heat transfer and controlled material transport. Higher contaminant concentrations or thermally more resistant compounds require longer residence times or higher temperatures, which reduces overall throughput and extends the treatment duration for a defined quantity of material.
Plant-specific parameters such as kiln rotation speed, inclination angle, and heating output also influence how quickly material passes through the kiln. These parameters are set on a project-specific basis to ensure both treatment quality and efficiency.
How does the type of contaminant affect treatment duration?
The type of contaminant is one of the most important factors determining treatment duration in thermal desorption. Volatile compounds such as mineral oil hydrocarbons (TPH) or BTEX can be driven out of the soil at lower temperatures and shorter residence times. Semi-volatile or chemically stable compounds such as PCDD/PCDF, PCB, or certain PFAS compounds require higher temperatures and longer treatment times.
Volatile contaminants
Compounds such as BTEX, TPH, or phenols have comparatively low boiling points. They transition to a gaseous state at moderate temperatures and are subsequently destroyed in the downstream thermal afterburner. Treatment time for such materials is generally shorter, enabling higher material throughput.
Semi-volatile and persistent organic contaminants
Compounds such as PAH, PCB, PCDD/PCDF, or PFAS/PFOS are thermally more stable. They require higher treatment temperatures and a longer dwell time in the kiln so that the contaminants are completely driven out or destroyed. Mercury (Hg) also represents a special case: it must be selectively driven out at lower temperatures and then safely separated from the exhaust gas stream, which imposes particular control requirements.
It is also crucial whether mixed contamination is present. Soils with multiple contaminant classes simultaneously must be treated according to the most thermally demanding contaminant, which tightens the treatment parameters overall.
What role does the nature of the input material play?
The physical properties of the contaminated material significantly influence treatment duration. Moisture content, particle size, and homogeneity of the input material determine how efficiently heat can penetrate the material and how uniformly contaminants are driven out.
Materials with high water content must first be dried before actual contaminant desorption begins. This consumes thermal energy and extends the effective residence time. Clay-rich or silty soils tend to bind contaminants more strongly and release them more slowly than sandy substrates. Coarse-grained materials with large particles require longer heat exposure to sufficiently heat even the core of the particles.
For this reason, mechanical-physical pre-treatment typically includes crushing and classification of the material. By reducing particle size, the surface area is increased, heat transfer is improved, and treatment time is reduced. Removing interfering materials such as reinforcing steel or plastics is also part of the preparation, as these can impair kiln operation.
How long does a typical remediation project take in total?
The total duration of a remediation project involving thermal soil treatment does not depend solely on the kiln treatment. It comprises several phases: analysis and permitting planning, excavation and transport, mechanical pre-treatment, thermal treatment, and final analysis and documentation. Depending on the project scope and contamination situation, a project can take anywhere from a few weeks to several years.
For the thermal treatment itself: plant throughput determines how much material can be treated per day. For large volumes of contamination amounting to several tens of thousands of tonnes, the treatment phase extends over months accordingly. Smaller projects involving a few thousand tonnes, on the other hand, can be completed within a few weeks.
Logistics are also important: continuous material deliveries must be coordinated with plant operations. Delays in excavation, changed analytical results, or permitting conditions can affect the schedule. Structured material flow management helps keep material flows predictable and avoid idle times.
When does treatment duration unexpectedly extend?
Unexpected extensions of treatment duration frequently occur when the actual contamination situation deviates from the original planning. Typical causes include higher contaminant concentrations than forecast, the appearance of previously unknown contaminants, or an inhomogeneous distribution of contaminants in the excavated material.
Additional factors that can lead to delays:
- Changed material characteristics: If material with significantly higher moisture content or different particle composition appears during excavation, treatment parameters must be adjusted.
- Mixed contamination: Unexpected combinations of inorganic and organic contaminants require a reassessment of the treatment strategy.
- Analytical verification: If clearance measurements show that treated material has not yet reached the required limit values, it must be retreated or directed to an alternative disposal route.
- Plant engineering adjustments: Changes in the contaminant spectrum may necessitate adjustments to the exhaust gas cleaning system or kiln control.
Thorough preliminary investigation of the contaminated site significantly reduces the risk of such surprises. Nevertheless, a degree of flexibility in project planning remains essential, as brownfield sites often have a heterogeneous history.
How ARE Deutzen GmbH supports thermal soil treatment
ARE Deutzen GmbH has specialized in the treatment of complexly contaminated mineral waste since 1994 and has more than 30 years of experience with demanding remediation projects. The thermal soil treatment plant at the Deutzen site enables the reliable treatment of a wide range of contaminants, including TPH, BTEX, PAH, PCB, PFAS/PFOS, PCDD/PCDF, and mercury.
For civil engineering companies and industrial customers, ARE Deutzen GmbH specifically offers:
- Thermal desorption in the rotary kiln at temperatures of up to approximately 600 °C under oxygen exclusion, depending on the material and contaminant load
- Mechanical-physical pre-treatment to optimize treatment efficiency
- Interim storage and material flow management for predictable material flows
- Multi-stage exhaust gas cleaning to reliably comply with all emission limit values
- Project support from analysis through to disposal documentation
If you are planning a remediation project or have questions about treatment duration and associated costs, speak directly with the specialists at ARE Deutzen GmbH. Via the contact page, you can reach the team for an individual project assessment.