Thermal and biological soil treatment differ fundamentally in their mode of action: while thermal desorption destroys contaminants through high temperatures or drives them out of the soil, biological soil treatment uses microorganisms to break down organic contaminants. Which method is appropriate depends primarily on the type and concentration of the contamination. The following sections explain both methods and show when each soil remediation method is most effectively applied.
How does thermal desorption work in soil remediation?
In thermal desorption, contaminated soil is heated in a rotary kiln to temperatures of up to approximately 600 °C, depending on the material and contaminant load. Under the exclusion of oxygen, volatile and semi-volatile contaminants are either completely destroyed or converted into a gaseous state and then safely treated in a multi-stage exhaust gas purification system. The process is suitable for a broad spectrum of inorganic and organic contaminations.
The process runs in several stages: first, the delivered material is mechanically pre-treated to separate oversized material and ensure uniform feeding of the kiln. Inside the rotary kiln, the contaminants evaporate at the treatment temperature. The resulting exhaust gas then passes through thermal post-combustion, a flue gas scrubber, and an activated carbon filter before being released into the atmosphere. The treated soil leaves the facility as clean mineral material, which can often be reused as a construction material.
A key advantage of this process is its high process reliability: treatment conditions can be precisely controlled and monitored. More on thermal soil treatment and the approved waste types can be found on the relevant process page.
How does biological soil treatment work?
Biological soil treatment uses microorganisms to break down organic contaminants in the soil. Bacteria and fungi convert hydrocarbon compounds under aerobic or anaerobic conditions into less harmful or harmless substances. The process requires that the contaminants present are biodegradable and that the living conditions of the microorganisms are actively managed.
In practice, contaminated soil is piled into windrows or placed in bioreactors. Optimal conditions for microbial degradation are created through aeration, irrigation, and, if necessary, the addition of nutrients. Typical treatment durations range from several weeks to several months depending on contaminant concentration and soil type. The process is comparatively cost-effective, but requires sufficient space and time.
Which contaminants can each process treat?
Thermal soil treatment covers a significantly broader range of contaminants than biological methods. It is suitable for hard-to-degrade and highly concentrated contaminations that microorganisms cannot or can barely break down. Biological methods, on the other hand, are limited to biodegradable organic compounds and quickly reach their limits with complex contaminant mixtures.
Contaminants for thermal desorption
Thermal desorption reliably treats a wide range of contaminations, including:
- Mineral oil hydrocarbons (TPH) and BTEX compounds
- Polycyclic aromatic hydrocarbons (PAH)
- Polychlorinated biphenyls (PCB) and chlorinated hydrocarbons
- PFAS and PFOS
- Dioxins and furans (PCDD/PCDF)
- Mercury (Hg) and cyanides
- Phenols and nitroaromatics
Contaminants for biological methods
Biological soil treatment is effective for:
- Light volatile hydrocarbons (e.g., gasoline, heating oil) at low to moderate concentrations
- Certain low-molecular-weight PAH compounds
- Biodegradable organic compounds without toxic effects on microorganisms
Contaminants such as PCB, PFAS, heavy metals, or dioxins are not or barely biodegradable. For these contaminations, thermal soil treatment is the only reliable method.
When should thermal treatment be preferred over biological treatment?
Thermal treatment should always be preferred over biological soil treatment when the contaminants are not biodegradable, concentrations are very high, or a rapid remediation result is required. Thermal desorption is also the more reliable choice when dealing with contaminant mixtures from multiple substance classes or when a construction project has tight time constraints.
Specific situations in which thermal treatment is clearly superior:
- Contaminated sites at decommissioned gasworks, coking plants, or chemical sites with PAH and PCB contamination
- Mercury contaminations that cannot be treated biologically
- PFAS-contaminated soils where biological methods do not achieve sufficient effectiveness
- Projects with short remediation deadlines where months-long bioremediation is not practicable
- Highly contaminated soils where contaminant concentrations would be toxic to microorganisms
Biological methods, on the other hand, can be economically viable when dealing with large volumes of lightly to moderately contaminated soils with readily degradable organic contaminants and when sufficient time is available for treatment.
Which method is more environmentally compatible?
Both methods are fundamentally environmentally compatible when operated properly. Biological soil treatment is considered less energy-intensive, as it does not require high temperatures. Thermal desorption requires more energy input, but ensures through multi-stage exhaust gas purification that no contaminants are released into the environment in an uncontrolled manner. For hard-to-degrade contaminants, it is the more environmentally relevant option, because biological methods cannot fully eliminate these substances.
An important aspect of environmental compatibility is the outcome of the treatment: thermally treated soil is demonstrably freed of contaminants after the process and can often be reused as a recycled material. This conserves natural resources and reduces the demand for primary raw materials. Biologically treated soil can, if degradation is successful, also be reused, although this depends heavily on the residual contaminant load and the verification requirements.
Ultimately, the decisive factor for the environmental balance is whether the chosen method actually eliminates the contaminant completely. A biological method that does not fully break down PFAS or PCB is not a sustainable solution despite lower energy consumption. Further background on the topic of soil protection and sustainability shows what standards should be applied when making this assessment.
Which method is right for a remediation project?
The choice of the right soil remediation method depends on four key factors: the type of contaminants, their concentration, the available time frame, and the requirements for the remediation objective. There is no universally superior method, but for complex and hard-to-degrade contaminations, thermal desorption is generally the safer and more reliable choice.
The following questions help in making the decision:
- What contaminants are present? Non-biodegradable compounds such as PCB, PFAS, or heavy metal compounds require thermal treatment.
- How high is the concentration? At very high contaminant levels, biological treatment is often not effective or too slow.
- How much time is available? Short project timelines favor thermal desorption.
- What remediation objective must be achieved? Strict limit values for reuse as a construction material are more easily demonstrable with thermal treatment.
- What volume needs to be treated? For very large quantities of lightly contaminated soils with suitable contaminants, bioremediation can be economically viable.
For projects with mixed waste streams, a combined approach is also worth considering: mechanical-physical pre-treatment for classification, followed by targeted thermal or biological treatment depending on the fraction. Material flow management plays an important role in efficiently controlling material streams and optimizing treatment costs.
How ARE Deutzen GmbH helps with thermal soil treatment
ARE Deutzen GmbH specializes in the thermal treatment of contaminated mineral waste and has operated one of the most powerful thermal desorption facilities in Germany since 1994. The company offers civil engineering firms, industrial clients, and public sector clients a reliable solution for demanding remediation projects where biological methods reach their limits.
Services specifically include:
- Thermal desorption in a rotary kiln at temperatures of up to approximately 600 °C, depending on the material and contaminant load, under the exclusion of oxygen
- Treatment of a broad range of contaminants, including TPH, PAH, PCB, PFAS, dioxins, mercury, and cyanides
- Multi-stage exhaust gas purification to ensure safe compliance with statutory emission limit values
- Mechanical-physical pre-treatment for optimal preparation of input material
- Material flow management for soils, dredged material, construction rubble, and process residues
- Over 30 years of experience and more than 2.5 million tonnes of treated mineral materials
If you are planning a remediation project and would like to know which method is suitable for your specific contamination, contact ARE Deutzen directly. Get in touch now and develop the right solution for your project together.