Cleaning Contaminated Sites Using Heat

Cleaning contaminated sites using heat

Heating contaminated soil to around 100 degrees centigrade could prove to be the future method for removing pollutants such as petrol, oil and organic solvents from soil and groundwater. The heat changes the behaviour of the substances so dramatically that previously inaccessible contaminants can now be removed. Clayey soil is heated using electric current while sandy soil is heated by blowing steam into it. The contaminants are collected by sucking water and air out of the soil. Soil heating can be undertaken for less than DKK 200 per tonne, typically in under half a year. All indications are that the soil and groundwater can be remediated completely.

Americans in the lead with in situ remediation

Background and purpose In recent years - especially in California - some very interesting projects have been undertaken to remediate contaminated sites that we would have been powerless facing in Denmark. Very difficult contamination with large amounts of petrol, oil, wood preservative (creosote) and organic solvents have been removed without excavating the soil. The Americans heat the soil to approx. 1000C with electric current or steam.

In Denmark, we have hitherto based remediation of contaminated sites on excavation, pumping of water and air, and on biological degradation in the soil. All these methods are very slow compared with remediation based on heating, however. In addition, they fail to work on many types of contamination, especially liquid pollution that has spread to below the groundwater table.

Literature studies and visits to institutions

The study As conditions in Denmark differ from those in the USA, studies have been made of the possibilities for transferring the promising soil

1 heating techniques to Danish conditions. It is especially the Danish moraine clay and the numerous small sites contaminated by dry- cleaning plants that present a challenge. A comprehensive literature study was therefore undertaken in the American EPA over the period 1995-97 and visits were made to a number of leading institutions in the USA, among others the Lawrence Livermore National Laboratory (California), SteamTech Environmental Services (Bakersfield, California) and the US EPA National Risk Management Research Laboratory (Ada, Oklahoma).

Thermal methods are robust and rapid

Main conclusions The Americans have successfully remediated large numbers of contaminated sites, both large and small. Rinsing with steam from boreholes drilled around the contamination has proven very effective. Layers comprised of clay or silt are heated by placing vertical electrodes in the soil and switching on the current.

Soil heating has proven effective towards petrol, oil, creosote and chemical mixtures including the organic solvents TCE and PCE, which are frequently found in Denmark. The heating methods will also work well in Denmark. The process involves heating the whole contaminated site up to the boiling point of the contaminants in question. The temperature has then to be maintained at that level for several weeks while pressure and flow are adjusted. One has also to ensure the collection of all evaporated substances and finally, to avoid the dispersal of liquid contamination to colder areas. The Danish moraine clay can be heated using alternating current. More coarse layers can advantageously be heated with steam, which can rapidly flush the soil layers.

Soil heating can be undertaken at a lower cost than traditional remedial pumping, excavation and disposal. This is largely due to the short processing time since sites can be remediated in a matter of a few months rather than many years.

2 Figure 2. The water that is pumped up from the heated soil can be as hot as 1000C. It is therefore necessary to cool the water in a heat exchanger. The most common type is a pipe heat exchanger where water and coolant run in opposite directions in parallel, closely packed arrays of pipes. The air flow and water flow are typically combined before being led to the heat exchanger in order to remove as much liquid from the air as possible.

Thermodynamics gets things started - the rest is hard work

Project results The hotter it is, the more often the contaminants will be in gaseous form. Research shows that heating from 10 to 1000C will typically increase volatility 20-fold. This tremendous shift in the equilibrium towards the gaseous phase makes it possible to collect the contaminants by "sucking" them from the soil (vacuum extraction).

Liquid contaminants boil at temperatures under 1000C because the boiling point of a mixture of two substances is lower than that of the lowest boiling point of the two. This means that heating the soil to 90- 950C ensures that the soil only boils in places where liquid contaminants are still present.

3 Other thermodynamic phenomena aiding remediation are that the ability of the substances to adhere to soil particles is considerably reduced, that diffusion (i.e. the random movement of the substances from high to low concentration areas) accelerates, and that liquid contaminants more rapidly dissolve in water at high temperatures.

Once these methods are acknowledged, the rest is mainly just hard work, even though the methods have not previously been used in Denmark. American full-scale demonstration projects have shown that the key to effective remediation is to achieve uniform heating of the whole contaminated site. Efficiency is greatest when heating from the periphery towards the centre of the contamination. This prevents dispersal of the substances through condensation in colder areas.

The thermal methods are extremely robust. A typical problem is that we do not know the precise extent of the pollution. This is due to the fact that liquid pollutants are very difficult to measure with existing methods. Experience shows, though, that boiling (and hence remediation) is more effective in those parts of the soil where the liquid contaminants are located.

Once the liquid contaminants have been boiled off leaving behind the dissolved and adsorbed contaminants, the water can be heated further so as to boil the water between the soil particles. This can lead to very rapid remediation.

The heating process requires greater control in the field than traditional methods. It is important to be able to follow developments in the ground at the same time as they take place. Steam injection needs particularly careful monitoring because the steam can spread very rapidly if it finds an easy route.

For monitoring purposes, two methods are particularly well suited: Detailed measurement of temperature and pressure, and measurement of changes in soil conductivity caused by the steam and the increasing temperatures (the so-called ERT method (electrical resistance tomography)).

4 There are a number of things one has to take into consideration. Thus it is important to keep an eye on whether fractures appear in the soil where the air can be pressed out. This can be beneficial for remediation, but can damage the soil's carrying capacity. As soil typically contains microorganisms (bacteria, yeast, fungi), their survival has to be investigated. It appears that some of the organisms can survive even protracted boiling and thereafter continue to degrade the contaminants. Survival probably takes place in special resting stages. Finally, the heating process necessitates extra safety precautions since working with 1000C hot mixtures of air, water and pollutants can be dangerous.

An in-depth analysis shows that soil heating is not particularly expensive compared with traditional methods, primarily because soil can be heated for under DKK 200 per tonne and remediation can be undertaken in the course of a few weeks or months. This saves money on long-term pumping and treatment of the waste products. In addition to being the only known method for removing a number of very difficult contaminants, soil heating is not prohibitively expensive.

It is recommended that key heating technologies be transferred to Danish conditions. This particularly applies to electrical heating, steam injection and combinations where one concomitantly heats clay and sand at stratified sites. The transfer of methods to monitor the processes is also recommended, including both ERT to follow steam fronts, water saturation and warm water, and rapid, cheap methods to log the temperature. Soil heating appears to be extremely promising for remediating Danish sites contaminated with liquid contaminants both above and below the groundwater table.

Project title: Udredningsprojekt om mulighederne for at bruge opvarmnings-teknikker (joule heating) til oprensning (Study of the possibilities for using joule heating for cleaning up

5 contaminated soil

Performing organizations: Institute for Environmental Technology, Technical University of Denmark, Nellemann, Nielsen & Rauschenberger A/S, and Danish Land Development Service

Printed publication: Termisk assisterede oprensninger (Thermally assisted remediation). Environmental Project 409, Danish EPA 1998. ISBN 87-7909-031-1. Price: DKK 90

Electronic publication: http://www.mst.dk/udgiv/Publikationer/1998/87-7909-134- 2/html/default.htm

Financing: Danish EPA, Programme for Development of Technology, Soil and Groundwater Contamination

Further information: Soil Contamination Division, Danish EPA. Phone: +45 3266 0100