What Is Indirect Thermal Desorption (ITD)?

Discover How Indirect Desorption Works to Your Advantage

Indirect thermal desorption is the process of indirectly transferring heat to oil-contaminated soil in an oxygen-deficient atmosphere. This way, the hydrocarbons vaporize without degrading in quality. After condensing the oil and water fractions, the effluent sediment is non-toxic (PHC max 0.3%).

Process overview of the Zoom Petro indirect thermal desorption system

Indirect Desorption in a Nutshell

An indirect thermal desorption unit applies heat from the outside to a nitrogen-blanketed (oxygen-deficient) chamber containing the oil-contaminated feedstock.

The thermal energy vaporizes the water and oil phase while fully preserving the quality of the hydrocarbons.

A downstream distillation unit then condenses and separates the vapor into pure oil and water.

The remaining sediment, that is essentially free of oil residue, is cooled down before being released from the system.

This separation process provides several unique advantages over traditional oil reclamation approaches like "soil washing":

  • No limitations in feedstock: Handles all kinds of oil waste, even hard sludge.
  • Purer effluents due to the system's evaporation and distillation effect.
  • No reliance on proprietary treatment chemicals ensures a consistent high quality of the effluent oil, water, and solids.
  • Minimal water consumption, as feedstock is not mixed with water. Ideal for arid desert environments.

Now, let me show you step by step how indirect desorption turns oil-contaminated soil into clean oil, water, and sediment:


With indirect thermal desorption, the higher the solid content, the faster the separation process.

For this reason, slop oil with a high water content is first run through a pre-treatment system consisting of a rapid separator and an oil/water filter membrane module.

Normal sludge from wet oil lakes, dry oil lakes, and oil piles is fed directly into the heating chamber of the indirect desorption unit (IDU).


Before any heat is applied, nitrogen gas (N2) is inserted into the sealed chamber holding the feedstock, until an oxygen-deprived, non-explosive environment is achieved. This automated process is called nitrogen-blanketing.

Next, heat is applied from the outside, vaporizing the volatile phases water and oil.

Only so much heat is applied as is necessary to evaporate the oil.

This means:

Combined with the lack of oxygen inside the chamber, the quality of the hydrocarbons is fully conserved. No quality degradation takes place.

After applying indirect heat, the effluents oil, water, and solids are processed like this:

Oil & Water: The oil and water vapors are condensed via a distillation unit located downstream of the heating chamber. The process of distillation produces especially pure water and oil:

  • Effluent water: TPH ≤ 50 ppm
  • Effluent hydrocarbons: BS&W < 1%

The high purity of the effluents allows for safe discharge or reuse of the treated water, while the recovered hydrocarbons can be sold as valuable product.

Solids: At the end of the heating process, the solid matter in the chamber doesn't require any further separation.

Before being released from the system, the solid matter is directed through a closed-loop cooling system.

Inside the cooling system, the same water constantly circulates from the sediment chamber to the heating tower and back.

Such as closed-loop cooling design saves large amounts of water and is especially useful when operating in arid environments where water is a precious resource.

With a total hydrocarbon content (THC) of only 0.3%, the sediment can be safely disposed of, used as road base, or mixed with soil and re-used in agriculture.

Exhaust Gas Removal: Different ITDU manufacturers employ different mechanisms to deal with exhaust gases.

In case of the Zoom Petro indirect thermal desorption unit, any remaining volatile organic compounds (VOC) and other non-condensable gases (NCG) are separated and reused as a heating source, thus fully eliminating their toxic impact on the environment.

In the same way, exhaust gases from the heating process undergo a two-step process of alkaline spraying and active charcoal absorption before finally being discharged.

As a result, Zoom Petro ITDU fully adheres to environmental regulatory requirements wherever the system operates.

Where Is the Heat Coming From?

Depending on the design, electricity, oil, or gas can be used to heat the feedstock. The more heat sources the equipment accepts, the better it's suited for off-grid operation in remote areas.

For example, a Zoom Petro indirect thermal desorption unit doesn't require an external power supply and runs on either oil, gas, or both, depending on what is available on-site.

ITDU Limitations

Indirect thermal desorption is an established and reliable technology.

However, if the oil waste recovery system using this technology isn't designed properly, problems can occur.

The main issue is blocking feedstock.

On the other hand, Zoom Petro ITDU is completely non-blocking, which is the #1 requirement for operation in isolated and remote areas.

Moreover, not all indirect thermal desorption units have technology in place to treat by-product such as potentially toxic non-condensable gases.

This limitation has led to oil recovery projects being shut down due to violation of air pollution standards at the treatment site.

As described in the previous section, Zoom Petro ITDU avoids this issue by eliminating all volatile organic compounds and thoroughly cleaning exhaust gases before discharge.

The reason this process works so well is that adhering to air quality standards was an integral part of the initial design, instead of being bolted on later.

What About Other Remediation Technologies?

Zoom Petro ITDU uses an indirect thermal desorption unit at its core.

This is because indirect thermal desorption provides several distinct benefits over other oil waste remediation technologies, such as:

Soil Washing

What Is Soil Washing?

In soil washing, oil-contaminated soil is mixed with water and heated, before going through a system of 2-phase (decanter) or 3-phase (tricanter) centrifuges.

Depending on the type of oil waste, the addition of rigidly controlled chemical additives is needed to enhance the treatment effect.


  • Reliance on (proprietary) washing agents to meet the required separation parameters.
  • Reliance on an uninterrupted supply of water.
  • Variable soil conditions, e.g., silt or clay, can lead to inconsistent results.
  • Unable to recover high-viscosity hydrocarbons

Direct Thermal Desorption

What Is Direct Thermal Desorption?

Direct thermal desorption is a destructive remediation technology that uses a form of incineration to degrade contaminants like hydrocarbons via pyrolysis and oxidation.


  • No oil is recovered.
  • Combustion produces large amounts of polluting exhaust gas.


What Is Bioremediation?

Bioremediation describes a process that uses microorganisms to degrade or remove hydrocarbons (like oil) and other organic contamination from soil.


  • No oil is recovered
  • Long treatment time.
  • Often unpredictable: The treatment result heavily depends on the soil and weather conditions.
  • Not feasible in dry or cold climates.
  • High PHC levels above 5% inhibit microbial growth.