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Karolina clean-up

REMEDIATION : Thermal desorbtion is being used on one of Europe's biggest remediation projects, in the Czech Republic.

The Czech city of Ostrava lies at the centre of the republic's former industrial heartland, a history that has left it with severe contamination.

The 70,000m 2Karolina site, only metres from one of the main squares, adjoined a huge coal mining area and for 150 years up to the 1980s, was home to heavy industry. This included coking plants and smelting and metal processing units which have left tar and hydrocarbon contamination as well as very high levels of heavy metals including mercury.

Now undergoing a mixed use redevelopment, the site required massive remediation. This CK1. 6bn (£29M) clean-up project is funded by the Czech Ministry of the Environment. OKD Rekultivace (OKD Reclamation), the Czech Republic's largest mining company, is the main contractor.

Before soil treatment began the heavily contaminated water table was isolated by installing a 12m deep sheet pile wall around the perimeter of the 250m by 350m site. A high capacity dewatering system was installed and water pumped from the ground to a series of active carbon filters. From the filters the water was checked and then discharged into local watercourses.

With this complete, surveying and analysis of the site started in late 1997. Excavation began in early 1999, with soil treatment getting under way in May that year.

Site geology comprises demolition fill over a 2m layer of coal and coke dust. This is underlain by a 1-1. 5m layer of clay and then a 2-4m layer of gravel. Beneath this is another layer of clay. Over the years the gravel has acted as a collector for the contaminants.

Excavation is being carried out in phases using hydraulic excavators and a fleet of up to six Volvo A25 articulated haulers. Working on a 10m by 10m grid in 2m thick layers, the material is analysed before it is excavated. Depending on the result of the analysis, the material is stockpiled, processed by biodegradation or processed by a thermal desorbtion plant.

Treatment depends primarily on the concentration of poly aromatic hydrocarbons (PAH). If contamination levels are less than 20mg/kg the material is considered safe and stockpiled for later reinstatement. If the PAH level is 20100mg/kg the material is processed by biodegradation or taken to landfill.

Material with concentrations of more than 100mg/kg can be processed by biodegradation, but as levels increase the process becomes ineffective. Soil analysis at Karolina revealed contamination as high as 40,000mg/kg. So most of the material is transported 3km to the thermal desorbtion plant designed and operated by European Remediation Services (ERS). Here the material is blended to ensure soil particles less than 50mm diameter are passed into the thermal desorbtion process. The soil is then fed into the process feed hopper, which processes up to 40t an hour.

Contaminated soil enters a rotary desorber which heats the soil in two phases. The first unit is a cylindrical processor with a gas burner that heats the soil to about 200degreesC. From here the preheated material is passed into the second unit where three gas burners heat the material to 500-550degreesC. At these temperatures the soil gives up (desorbs) its contaminants and releases up to 50,000m 3of gas an hour.

Cleaned soil is cooled, analysed and stockpiled ready for reuse.

The contaminant-rich gas is cleaned before being released back into the atmosphere. First it is passed through two cyclone separators to extract dust particles which are fed back to the rotary desorber for retreatment.

The dust-free gas is then piped to a ceramic filter containing 2600 filter elements. From here the remaining gas passes to an oxidiser where it is heated to 900degreesC. After filtering and oxidation the main harmful gas that remains is sulphur dioxide.

As the gas leaves the oxidiser it is cooled from 500degreesC to 80-90degreesC. It is then fed through a plastic grid system where it is mixed with sodium hydroxide to produce sodium sulphates and water. The water is evaporated into the atmosphere and the sodium sulphates are processed off-site.

By June this year, 250,000t of material had been processed with a 'significant' proportion of it reused on site. By 2005, when the project ends, more than 500,000t of contaminated material will have been recycled and over 720,000m 2of soil processed.

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