Soil contamination is a matter of concern in many locations.

Soil may include natural formation material such as dirt, sand, and rock, as well as fill material. Soil may be contaminated with chemical, biological, and/or radioactive compounds.
Contamination of soil may occur in a variety of ways, such as material spillage, leakage from storage vessels, and landfill seepage. Public health concerns may arise if contaminants migrate into aquifers or into air.
Soil contaminants may also migrate into the food supply through bioaccumulation in various species in a food chain.
 There are many ways to remediate contaminated soil. “Remediating soil” means treating the soil to reduce contaminant levels within the soil or to remove contaminants from the soil.

Generic Thermal Desorption Process
IN SITU PROCESS
An ex-situ method of remediating contaminated soil is to excavate the soil and then process the soil in a separate treatment facility to reduce contaminant levels within the soil or to remove contaminants from the soil. Alternatively, contaminated soil may be remediated in situ.
Thermal desorption is a physical separation process and is not designed to destroy organics. Wastes are heated to volatilise water and organic contaminants. A carrier gas or vacuum system transports volatilised water and organics to the gas treatment system. The bed temperatures and residence times designed into these systems will volatilise selected contaminants but will typically not oxidise them.
Thermal desorption is a term applied to many different types of soil remediation technologies. All of these technologies consist fundamentally of a two-step process, as illustrated in the figure above.
In Step 1, heat is applied to a contaminated material, such as soil, sediment, sludge, or filter cake, to vapourise the contaminants into a gas stream that, in Step 2, is treated to meet regulatory requirements prior to discharge. A variety of gas treatment technologies are used to collect, condense, or destroy these volatised gases. Thermal desorption is fundamentally a thermally induced physical separation process. Contaminants are vapourised from a solid matrix and are transferred into a gas stream where they can be more easily managed in Step 2.
Options used to manage or treat the contaminant-laden gas stream may consist of condensation, collection, or combustion. For the first of these two options, the condensed or collected contaminants usually are treated off site at some time subsequent to Step 1.
For the third option, combustion, treatment occurs on site, immediately after the gases exit Step 1 of the process.
Two common thermal desorption designs are the rotary dryer and thermal screw. Rotary dryers are horizontal cylinders that can be indirect- or direct-fired. The dryer is normally inclined and rotated. For the thermal screw units, screw conveyors or hollow augers are used to transport the medium through an enclosed trough. Hot oil or steam circulates through the auger to indirectly heat the medium.
All thermal desorption systems require treatment of the off-gas to remove particulates and contaminants.
Particulates are removed by conventional particulate removal equipment, such as wet scrubbers or fabric filters. Contaminants are removed through condensation followed by carbon adsorption, or they are destroyed in a secondary combustion chamber or a catalytic oxidiser. Most of these units are transportable.
Three types of thermal desorption are available and briefly described as follows:
• Direct fired: Fire is applied directly upon the surface of contaminated media. The main purpose of the fire is to desorb contaminants from the soil though some contaminants may be thermally oxidised.
• Indirect fired: A direct-fired rotary dryer heats an air stream which, by direct contact, desorbs water and organic contaminants from the soil.
• Indirect heated: An externally fired rotary dryer volatilises the water and organics from the contaminated media into an inert carrier gas stream. The carrier gas is later treated to remove or recover the contaminants.

Contaminant
HTTD
High temperature thermal desorption (HTTD) is a full-scale technology in which wastes are heated to 320 to 800  deg C (600 to 1,472 deg F). The technology has proven it can produce a final contaminant concentration level below 5 mg/kg for the target contaminants identified. HTTD can be utilised practically to all soil types according to our experience and to all kind of organic contaminants and volatile metals

LTTD
In low temperature thermal desorption (LTTD), wastes are heated to between  90 and 320 deg C (200 to 600 deg F). LTTD is a full-scale technology that has been proven successful for remediating petroleum hydrocarbon contamination in all types of soil. Contaminant destruction efficiencies in the afterburners of these units are greater than 95 per cent. The same equipment could probably meet stricter requirements with minor modifications, if necessary. Decontaminated soil retains its physical properties.
The applicability of HTTD and LTTD for various environmental contaminants are as shown in the table above.
The remediation process starts by screening contaminated soil, where stones over 100 mm are removed. Screening is done over a hopper, so extra transportation and loading is unnecessary.
Soil is fed from hopper in batches of 2 to 6 tonnes to desorber drum. Soil is treated in desorber drum by combined effect of mechanical mixing and hot vapour for three to 20 minutes.
The treatment capacity of the unit is between 40-80 tonnes per hour, depending of the soil type and contamination level.
The treated soil is stored intermediately during the time of sampling and analysing the results of its quality.
Remediated soil can be utilised according to conditions stated in the permit, even into construction material. Typical, thermally treated soil can be used for landscaping and for other purposes.  The gas flow generated by the remediation process (ca 45.000 cu m/h) flows through an oxidiser.
Thermal desorption works well at sites with dry soils and certain types of pollution, such as fuel oil, coal tar, chemicals and preserve wood, and solvent. Sometimes thermal desorption works where some other clean-up methods cannot – such as at sites that have a lot of pollution in the soil. 
Thermal desorption can be a faster clean-up method than most. This is important if a polluted site needs to be cleaned up quickly so it can be used for other purposes. The equipment used for thermal desorption often costs less to build and operate than equipment for other clean-up methods using heat.
Al Osais Industrial & Structural Supply is contemplating business co-operation with a leading European environmental company for similar technology to be used in the Middle East in general and Saudi Arabia in particular in order to provide state-of-the-art solutions to the environmental problems faced today by oil and petrochemical chemicals companies in the region.