What Role Does Treatment Duration Play in Comparing Thermal and Biological Processes?

Thermal soil treatment is generally significantly faster than biological methods: while a rotary kiln treats contaminated soil within minutes to a few hours, biological methods such as windrow composting or in-situ bioremediation often require months to several years. The key difference lies in the mechanism of action: thermal desorption destroys contaminants or transfers them directly into the gas phase through high temperatures, whereas biological methods rely on natural degradation by microorganisms. The following sections examine which factors specifically influence treatment duration and what this means for planning, costs, and permitting.

Why does thermal desorption take significantly less time than biological methods?

Thermal desorption is faster because it physically and chemically destroys contaminants through direct heat input, rather than waiting for biological degradation processes. In a rotary kiln, contaminated soil is treated at temperatures of up to approximately 600 °C, depending on the material and contaminant load. The residence time of the material in the kiln is typically only a few minutes. Biological methods, on the other hand, are tied to the metabolic activity of microorganisms, which depends on temperature, moisture, nutrient availability, and contaminant concentration.

With biological methods such as windrow treatment or land farming, the soil must be regularly turned, irrigated, and aerated over weeks and months. In-situ methods, in which the soil remains on site and is treated by introduced microorganisms or nutrients, can even take several years. The treatment duration is difficult to predict, as it depends heavily on the specific site conditions.

Thermal methods, by contrast, offer a plannable, reproducible treatment time. This makes them particularly suitable when a site is under time pressure — for example, because a subsequent use is planned or excavated soil from ongoing construction work must be disposed of promptly.

Which contaminants determine which method reaches the goal faster?

The type of contamination is the most important factor when choosing the faster method. Thermal desorption has a clear advantage with volatile and semi-volatile organic contaminants such as TPH, BTEX, PAHs, PCBs, PFAS/PFOS, phenols, cyanides, nitroaromatics, and chlorinated hydrocarbons. These substances can be efficiently removed from the soil by heat and subsequently rendered harmless in the exhaust gas treatment system. Mercury (Hg) and dioxins (PCDD/PCDF) can also be safely treated thermally.

Biological methods achieve good results with easily degradable organic compounds of low toxicity — for example, light mineral oil contamination in sandy, well-aerated soils. In such cases, bioremediation can be economically attractive, even if it requires more time.

With complex contaminant mixtures, high initial concentrations, or difficult-to-degrade substances such as PAHs or PCBs, however, biological methods reach their limits. Treatment not only takes longer, but often fails to achieve the required target values. Thermal methods reliably deliver demonstrable results in these cases, regardless of the initial concentration.

How does treatment duration affect the overall costs of a remediation project?

A shorter treatment duration significantly reduces the overall costs of a remediation project, because it lowers storage, personnel, and logistics costs. Anyone who must stockpile soil for months or regularly turn it ties up land, machinery, and personnel. Added to this are costs for monitoring, sampling, and reporting, which accrue multiple times over long treatment periods.

With thermal soil treatment, the per-tonne treatment costs are higher than with simple biological methods. However, this difference is put into perspective when overall project costs are considered: shorter standstill times, reduced planning effort, and reliable scheduling for subsequent use can offset or even exceed the additional cost.

Particularly for projects with fixed deadlines — such as infrastructure measures or property sales — the rapid availability of treated material is an economic advantage. Delays caused by unmet target values or unforeseen extensions of the biological treatment period, on the other hand, can cause significant additional costs. Material flow management plays an important role here: coordinated planning of all material flows from delivery to disposal helps avoid unnecessary waiting times and additional costs.

When is a longer treatment duration acceptable for biological methods?

A longer treatment duration for biological methods is acceptable when the contamination is easily degradable, the target values are achievable, and there is no time pressure from subsequent use or statutory deadlines. Typical scenarios are large-scale, lightly contaminated sites with no immediate need for action, where cost-effective, area-intensive treatment over several growing seasons is economically sensible.

Ecological considerations can also speak in favor of biological methods: with suitable soils and contaminants, the soil structure is preserved, and microorganisms convert contaminants into natural compounds without the transport effort or emissions associated with thermal treatment.

However, a realistic assessment of achievable remediation targets is crucial. If biological methods fail to meet the required limit values after long treatment periods, significant additional effort arises. In such cases, early thermal treatment would have been the more economical and safer choice. A thorough initial characterization of the soil and a clear definition of objectives are therefore prerequisites for any method decision.

What role does treatment duration play in permitting and project planning?

Treatment duration is a central element in the permitting and project planning of soil remediation, because authorities require concrete schedules and milestone documentation. Remediation plans must contain realistic deadlines, and delays must be justified and re-approved. Short, plannable treatment periods significantly facilitate coordination with authorities.

Thermal methods offer a structural advantage here: treatment time is largely calculable based on the technical parameters of the kiln, throughput capacity, and residence time. This facilitates the preparation of remediation plans and gives clients, authorities, and subsequent users planning certainty.

Biological methods, on the other hand, often require a more flexible permitting framework. Authorities typically require regular progress checks, and if target values are not achieved within the planned timeframe, supplementary approvals or changes to the method are necessary. This not only extends the project duration but also increases the administrative burden on both sides.

For projects with statutory deadlines — such as authority-ordered remediation or in the context of property transactions — the reliability of treatment duration is therefore often a decisive criterion when selecting a method. Early coordination with the responsible authorities and careful documentation of the chosen method are advisable in every case. Information on quality management in the treatment of contaminated soils helps in understanding the requirements for documentation and proof of performance.

How ARE Deutzen GmbH supports the selection of the right method

ARE Deutzen GmbH has specialized in the thermal treatment of contaminated mineral waste since 1994 and has more than 30 years of experience with complex remediation projects. The company offers concrete support with method selection and project planning:

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 TPH, BTEX, PAHs, PCBs, PFAS/PFOS, mercury, dioxins, and chlorinated hydrocarbons
Clear, plannable treatment times for reliable project planning and coordination with authorities
Holistic material flow management from delivery through mechanical-physical pre-treatment to disposal or recovery
Certified waste management company with permits under the Federal Immission Control Act and certifications to ISO 9001, 14001, and 50001
Over 2.5 million tonnes of treated mineral materials as evidence of technical competence and plant reliability

If you are planning a remediation project and would like to know which method is suitable for your specific contamination and schedule, contact ARE Deutzen GmbH directly. Further information on thermal soil treatment as well as contact options can be found on the website. The team will advise you objectively and solution-orientedly so that your project can be implemented on time and in compliance with regulations.