In-situ thermal treatments of soils are usually applied in conjunction with other technologies. The process involves heating contaminated soils, rock and groundwater to exploit resulting physical and chemical effects on the contaminants present.
To understand the benefits of most in-situ thermal treatments, it is necessary to understand a little about the ‘phases’ that organic compounds exist in.
Consider a common contaminant existing in a porous medium, like sand. The contaminant is not alone in the pores and will share space with air (usually termed soil vapour to distinguish it), and water. Water saturation within pores will vary of course, but for this example we will look at a single pore space in the unsaturated (vadose) zone wherein there is also some soil vapour present.
The organic contaminant present in the soil pore exists in a number of different ‘phases’ simultaneously. Some will be in the ‘liquid phase’ as a globule in contact with the sand grains, some in the ‘vapour phase’ as vapour in the air within the pore space, some in the ‘dissolved phase’ within the water present in the pore space and some will be in the ‘solid phase’ where molecules are actually attached to the surface of the sand grains. All of these phases are in contact with each other within the soil pore.
At steady state, where the conditions in the soil pore have been constant for a long period, all phases will be in equilibrium with each other. Molecules may freely move between phases but the overall mass balance between phases will remain constant until a change is imposed from outside.
Cleansing every pore
Physical remediation techniques work by removing contaminant mass from each pore space. For example, soil vapour extraction works on the basis of removing the soil vapour from the pore, taking with it the contaminant present in the vapour phase. Extracted soil vapour is replaced with air drawn into the pore space that has less or no organic contaminant within it. This creates an imbalance between the phases in the pore space that cannot last long.
The organic contaminant molecules will pass from the other phases to the vapour phase to bring that phase back into equilibrium. If this process is continually repeated, the organic contaminant mass is gradually removed as its molecules keep moving into the vapour phase to try and maintain equilibrium.
If we understand this, the effect of heating the soil pore space can be appreciated. Raising the contaminant temperature, through heating the soil, makes it easier for the organic contaminant to enter the vapour phase because the vapour pressures of organic compounds generally increase as temperature increases. Vapour pressure is a measure of how readily the compound will enter the vapour phase from the liquid phase.
Henry’s law says that at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial of that gas in equilibrium with that liquid. This law increases with temperature and expresses the same concept between vapour phase and dissolved phase. Thus applying heat speeds up the process of organic contaminants changing phases.
This means physical soil treatment processes such as soil vapour extraction can be significantly speeded up and lower remediation targets can be achieved.
This example is just one effect harnessed to assist remediation. Other effects are also used, notably reducing viscosity by increasing petroleum hydrocarbon temperature in the liquid phase. Total fluids pumping combined with ground heating can recover tars and heavy fuel oils present in the subsurface. Without heat there is often no significant recovery for such contaminants and the only alternative is ‘dig and dump’.
Thermal treatments can advantageously shorten in-situ remediation programmes and enable the use of otherwise ineffectual in-situ approaches.
There is a compelling commercial argument too. For example, a soil vapour extraction plant with a power consumption rate of 20kW/h would consume 116800kW in eight months. At £0.09/kWh, this would cost £10,512. With the application of heat we estimate that the operation period would typically be reduced by 70%. With an appropriately sized steam flooding system, the fuel cost would be around £10,000, the total energy cost by adding heating in this instance would be around 25% greater than SVE alone.
However, this can be quickly recouped through savings related to reduced contract preliminaries, and plant and equipment hire. Then there’s the fact that the site is released for development far sooner.
A recent project of ours illustrates the energy costs of thermal treatment. A former Post Office depot bounded by an SSSI estuary and a school had creosote contamination in the subsurface from activities related to the preservation of telegraph poles. Steam flooding of the saturated and vadose zone, combined with dual phase extraction, provided a rapid remediation option that could meet low site-specific remedial target values. Remediation and regulatory sign off was achieved within the tight six week deadline.
An estimated 1,200m3 of contaminated soils were treated in-situ. Fuel cost for both a steam boiler and the generator supplying electricity to the dual phase plant was £8,196, or about £4 per tonne of soil treated.
There is scope to improve the carbon footprint of techniques such as steam flooding, and we are currently looking into the possibility that bio-diesel could be used to power the boilers. Renewable energy suppliers could also provide the electricity supply needed to run the remediation plant.
In the right circumstances then, thermal remediation can be a very attractive option.