Summary and Conclusions, Miljøstyrelsen

Rensning af jord med blandingsforureninger ved hjælp af termiske jordbehandlingsanlæg

Summary and Conclusions

A facility for off-site treatment of contaminated soil by Low Temperature Thermal Desorption (LTTD) has been tested in the period January-March 2001. The test has been performed by the Danish EPA and the Environmental Protection Department of the City of Copenhagen in cooperation with the company RGS90 Jordrens.

The scope of the test is three-fold:

To investigate the effect of low temperature thermal desorption on heavy oil and tar components in different types of soils

To highlight changes in the soil that might affect the possibilities for reuse of the treated soil with special regard to geotechnical characteristics and leaching of heavy metals.

To perform an economic and environmental evaluation of low temperature thermal desorption

The test has been performed on a semi-mobile treatment facility with a nominal capacity of 15 tons pr. hour. In the process, contaminated soil is heated in a fuel-oil heated rotary kiln to a temperature of approx. 550-600 °C. Residence time in the kiln is approx. 15 minutes. Flue gas is cleaned in a bag filter, evaporated tar and oil components are destructed in an afterburner and the gas is cleaned in a two-step gas scrubber. Filter dust and effluent water from the scrubbers are recycled to the treated soil.

The tests have basically been performed as 24 hrs continuous runs, where different types of soils have been treated. Prior to the test runs, the soil has been sieved through a 40 mm rotary screen in order to remove rocks and to create a homogenous batch of soil for the test.

Two soil types have been tested. Batch no. 1 is a clayish fill soil with a pronounced content of organic matter, whereas batch no. 2 is a clayish soil containing less organic matter. Batch no. 1 is regarded to be a typical representative of surplus soil from rural construction works, and is contaminated with heavy metals as well as tar and oil components. Batch no. 2 is solely contaminated with tar and oil components.

Before and after treatment of the two batches, a large number of soil samples have been taken. Soil characteristics have been measured through sieve tests and compacting testing. The analytical programme comprises general chemical soil parameters such as TOC, carbonate content, pH and CEC, as well as environmental screening for heavy metals, tar components and mineral oils. Changes in the leaching of heavy metals from the soils have been studied through batch-tests and column-test.

Economic and environmental performance of the facility has been investigated during the test runs by monitoring the consumption of oil, electricity and water, and through sampling and analysis of flue-gas and discharge water.

The findings of the tests can be summarized as follows:

Efficiency with regard to removal of organic contaminants is very high. A 96-99% reduction of contaminant levels is observed on both tar and oil components.

After treatment, the soil particles have been completely covered with soot. GC-MS analysis of the soil reveals that the formed soot is of pyrogenic origin, but it cannot be conclusively established whether soot-formation is a result of inadequate combustion of fuel oil in the kiln or a result of pyrolysis of natural organic components in the soil itself.

Although the general effect on the physical characteristics of the treated soil appears to be pronounced, only minor changes in the geotecnical properties of the soil can be detected.

Leaching of heavy metals is generally reduced as a result of the treatment. Leaching of lead, copper, zink and cadmium is initially reduced by 37 - 97%. In opposition, leaching of barium and arsenic is increased as a result of the treatment. Column tests show that these effects are lasting over a very long period of time. Changes in leaching pattern is believed to be caused partly by increased pH values in the soil and partly by the physical changes, where soot-covering and aggregation of soil particles reduce the dissolution rates along the particle surfaces.

From an environmental perspective, low temperature thermal desorption is a very resource-consuming process, where approx. 45 kg of fuel oil is spent on each ton of contaminated soil. The environmental benefit, on the other side, is a product with low levels of organic contaminants and a general reduced level of heavy metal leaching. It is therefore a suitable treatment method for non-volatile organic persistent contaminants, given possibilities for a controlled reuse is present.

The cost of low temperature thermal desorption treatment has in the test runs been in the magnitude of 100 EUR pr. ton, of which fuel oil consumption has been approx. 35%. It should be noted that the oil consumption during the test has varied with up to 50%, depending on moisture and organic contaminant contents in the treated soil.