The soil matrix is the decisive factor when choosing between thermal desorption and soil washing. Coarse-grained, sandy soils are well suited for soil washing, while fine-grained, clayey, or silty soils as well as organically contaminated substrates generally require thermal treatment. The type of contaminants present plays an equally important role as the physical composition of the material. The following sections systematically examine the most important influencing factors.
How does the soil matrix affect contaminant binding?
The soil matrix determines how firmly contaminants are bound to the soil material and how readily they can be released from it. Fine-grained soil components such as clay and silt have a very large surface area and bind contaminants significantly more strongly than coarse-grained sands or gravels. The stronger the binding, the more difficult it is to mobilize contaminants through aqueous processes.
Organic soil components considerably amplify this effect. Humus, peat, or other organic substances form particularly strong interactions with hydrophobic contaminants such as PAHs, TPHs, or PCBs. In such soils, contaminants are often deeply embedded in the soil structure and can barely be fully mobilized by physicochemical methods such as soil washing. Thermal treatment, on the other hand, overcomes these binding forces through heat input regardless of the type of binding.
For which soil types is thermal desorption suitable?
The thermal desorption process is particularly suitable for fine-grained, clayey, and silty soils as well as for substrates with a high organic content. Mixed soil types with varying particle size distributions or highly heterogeneous materials can also be reliably treated thermally. The process is largely independent of soil texture, as heat acts uniformly throughout.
In addition, thermal treatment is suitable for materials that contain not only soil but also construction rubble, slags, ashes, or other mineral waste. The ARE Deutzen facility processes such materials after mechanical pre-treatment in a rotary kiln at temperatures up to approximately 600 °C under oxygen exclusion, depending on the material and contaminant load. Contaminants are either destroyed or converted to a gaseous state and subsequently rendered harmless in a multi-stage exhaust gas treatment system.
Even at very high contaminant concentrations or in soils with multiple contaminant groups present simultaneously, thermal desorption remains effective where other processes reach their limits.
When is soil washing the better choice?
Soil washing is advantageous when the contaminated soil consists predominantly of sand or gravel, the fine particle content is low, and the contaminants are readily water-soluble or easily mobilizable. In these cases, contaminants can be efficiently separated from soil particles by washing, and the cleaned coarse-grained material can subsequently be reused.
A further advantage of soil washing lies in material recovery: with a suitable soil matrix, a large proportion of the treated material can be returned as a clean recycled product. This makes the process economically attractive when the contaminant load is concentrated in the fine fraction and this fraction can be separated out.
However, soil washing quickly reaches its limits once the fine particle content exceeds approximately 30 percent, as the fine fraction strongly binds contaminants and is difficult to wash. In such cases, a significant proportion of contaminated material remains and must be disposed of separately.
Which contaminants can be removed by desorption, and which by washing?
Thermal desorption is suitable for volatile and semi-volatile organic contaminants as well as for certain inorganic compounds. These include TPHs, BTEX, PAHs, phenols, PCBs, PFAS, PCDD/PCDF, cyanides, nitroaromatics, chlorinated hydrocarbons, and mercury. These contaminants vaporize or decompose at the temperatures applied and are subsequently rendered harmless in the exhaust gas treatment system.
Soil washing, on the other hand, is primarily suitable for water-soluble or easily mobilizable contaminants such as certain heavy metals, volatile chlorinated hydrocarbons (VCHs) in sandy soils, or mineral oil compounds with sufficient water solubility. Heavy metals such as lead, zinc, or cadmium can be transferred into the aqueous phase by soil washing and then removed from the wash water.
Important: Contaminants such as PAHs or PCBs, which are hydrophobic and barely soluble in water, generally cannot be removed sufficiently by soil washing. Here, thermal treatment is the more reliable process.
What happens when the soil matrix is mixed?
With mixed soil matrices — that is, materials containing both coarse- and fine-grained fractions with different contaminant profiles — careful preliminary investigation and often a combination of processes is necessary. In practice, the material is first mechanically processed to separate particle size fractions and to target treatment specifically at each fraction.
Coarse fractions with low contaminant loads may, where appropriate, be treated by soil washing or directly recycled. The fine fraction with higher contaminant concentrations, however, is frequently directed to thermal desorption. This approach maximizes material recovery and minimizes the volume of material requiring thermal treatment.
For efficient material flow management, it is essential to analytically characterize the soil before selecting a process. Particle size distribution, organic carbon content, moisture content, and contaminant spectrum are the key input parameters.
What other parameters influence the choice of process beyond the matrix?
In addition to the soil matrix, other factors play an important role in the decision for or against a treatment process. The moisture content of the material affects both the energy demand of thermal treatment and the efficiency of soil washing. Very moist soils often need to be pre-dried before thermal treatment.
Other relevant parameters include:
- Contaminant concentration: Very high concentrations can render individual processes uneconomical or technically unsuitable.
- Contaminant spectrum: The simultaneous presence of multiple contaminant classes favors thermal desorption, as it covers a broad spectrum.
- Material volume: Large quantities require high-capacity facilities with the appropriate permits and capacities.
- Recovery objective: If the treated material is to be reused, the cleaning performance and output quality must meet the respective requirements.
- Legal framework: Approved processes, limit values, and waste disposal documentation requirements vary depending on the contaminant and disposal route.
Logistical aspects such as transport routes, interim storage, and the availability of suitable treatment facilities in the region also influence the decision. A well-founded analytical preliminary investigation combined with the experience of a specialized waste management company forms the basis for a technically and economically sound process selection.
How ARE Deutzen helps in choosing the right soil treatment process
ARE Deutzen GmbH has more than 30 years of experience in treating contaminated mineral waste and supports civil engineering companies as well as industrial clients in identifying the appropriate process for their specific situation. The company offers concrete services along the entire process chain:
- Analytical preliminary investigation and assessment of the soil matrix and contaminant profile
- Thermal treatment of contaminated soils, construction rubble, and mineral waste in a rotary kiln at temperatures up to approximately 600 °C under oxygen exclusion, depending on the material and contaminant load
- Mechanical-physical pre-treatment for fractionation and preparation of the material
- Material flow management for mixed materials with varying treatment requirements
- Safe interim storage and waste disposal documentation as a certified waste management company
- Treatment of a broad contaminant spectrum, including TPHs, PAHs, PCBs, PFAS, mercury, and chlorinated hydrocarbons
If you are facing a remediation project and are unsure which process is suitable for your soil matrix and contaminant profile, speak directly with the experts at ARE Deutzen. Get in touch now and receive an expert assessment for your project.