Clean-up of extremely toxic pollutants at the Spolana chemical works in the Czech Republic will see the site completely enshrouded and the buildings maintained at negative pressure to prevent release of contaminants.
Manufacturing at Spolana, which sits on the bank of the River Elbe north of Prague, dates back a hundred years.
Like many chemical facilities in the region, production centred on chlorine chemistry and the electrolysis of brine. PVC was one of the main products but insecticides, herbicides and fungicides were also made.
The production processes have left extremely high levels of contamination - dioxin and associated materials - in some of the buildings, the ground and groundwater.
These buildings, complete with plant, drums of chemicals, waste and other materials, were abandoned more or less overnight more than 30 years ago when serious health problems among workers were identified.
Evidence of such problems, related to dioxin contamination, was being found at other sites around the world. Dioxin is a carcinogen and thought to affect immune and reproductive systems.
'Apart from the human exposure risks, one of the key drivers for clean-up was the risk of pollution to the river, ' says Chris Wynne, director of Thermal & Chemical Soil Remediation (TCSR).
The UK remediation firm is part of the joint venture team that will remediate the site, BCD CZ.
It includes SITA Bohemia of the Czech Republic and the Suez Group of France. A go-ahead for the project is expected in May.
Contamination at Spolana is clearly a serious risk to the environment. Dioxin concentrations in soils of more than 1M. ng/kg have been identified, with over 20M. ng/kg found in dust residues in buildings.
These concentrations vastly exceed those identified in other high-profi remediation schemes around the world, including the notorious Times Beach project in Missouri, near St Louis, US.
The small river town was closed in 1982 by the US Environmental Protection Agency when waste oil sprayed on roads to suppress dust was found to contain high levels of dioxin. The town was demolished and it took 17 years to incinerate 265,000t of dioxin-contaminated soil from the area.
At Spolana, one of the largest on-going remediation projects in Europe, the clean-up will involve a combination of thermal and chemical treatments. A number of risk analyses and feasibility studies undertaken over the last few years will provide the basis for a robust remediation design, Wynne says.
Concentrations of such high toxicity pollutants have seldom been seen at any previous site remediation, he adds. This severity will dictate the operating and health and safety procedures for site workers.
The most serious potential hazard to the local population is from dust emissions, so before any remediation starts, the contaminated buildings and surrounding area will be sealed off inside a steel framed enclosure.
Buildings will also be maintained at a negative pressure relative to that outside the enclosure, to prevent any contaminants being released.
In the fi rst phase, covering building contents, the plaster and dust will be removed using industrial vacuum cleaners and hand and hydraulic tools. Once these have been removed demolition will start.
The rate of demolition and excavation will be determined by the feeding rates of contaminated material to the treatment plants.
The site sits on the River Elbe floodplain, where ground conditions are typically construction fill over alluvial sandy silt. Groundwater is shallow, less than 3m down, and is directly linked to river levels.
'Soil was found to be grossly contaminated but only to relatively shallow depths, so only the top 1m to 1.5m is being removed and treated, ' Wynne says. 'Nonetheless the area affected is large and the total treatment volume is about 12,000m 3 (23,000t).' TCSR technical director John Fairweather adds: 'We are only excavating down to groundwater level at most. Groundwater contamination is complex, also resulting from other contaminated areas within the site. It does not make sense to treat only the groundwater in this area.' It will be treated in the future as a separate and 'complete' solution, he explains.
Cleanup criteria have been set at 0.2parts per billion (ppb) for soil and 5ppb for building rubble. In practice, materials will be treated simultaneously so that 0.2ppb will apply throughout, Wynne says.
'Within the buildings and main enclosed areas all materials will be excavated and removed for treatment down to agreed levels, ' Fairweather says. 'These levels differ underneath more or less intact concrete slabs or under storage tanks which have clearly leaked.
'The excavated areas will be tested and further excavation made if required. Outside the core area, where additional site investigation has exposed further contamination, excavation will proceed where dioxins are found to be over 5ng/g and/ or persistent organic pollutants are greater than 30mg/kg.' Most of the contaminated materials - about 30,000t of soil, demolition rubble, glass and fabric - will be thermally treated in an indirectly heated, gas-fi red, rotary kiln at temperatures up to 650¦C.
Wynne says the quantity of materials may at fi rst seem too small to justify the expense of the technology adopted - 'however the concentration levels of dioxin are extreme'.
Desorbed chemicals from the thermal treatment process are concentrated in the dust and condensate removed as part of a fully contained off-gas control system. The concentrated contaminants are then treated in a second stage process.
As incineration at the site is politically unacceptable, the concentrates, and pure waste chemicals still stored around the buildings, are being treated using the base catalysed decomposition (BCD) process.
The process was originally developed by the Environmental Protection Agency in the US. In it, chlorinated organic chemicals, including dioxins and other persistent organic pollutants, are destroyed to levels below analytical detection limits.
Wynne says the BCD process is simple and maintains good quality control. Since its first use in 1991, more than 100,000t of contaminated materials have been treated. Pure chemicals and residues can be fed directly to the BCD reactor.
'All soils are being treated using the BCD process, involving thermal desorption as a first stage and then BCD treatment of the residual condensate, ' Wynne says.
Fairweather adds: 'Treated soil will be stored on site and then used as inert backfill to landscape the site and infill holes at the end of the job.' Metal parts, ranging from drums and tools to full-scale manufacturing plant items, will be cut into manageable pieces on site, but cannot be handled in the thermal desorber, and so will be treated in a stationary high temperature furnace.
This plant is being used elsewhere to decontaminate metal parts such as artillery shells, which had been filled with chemical warfare agents.
Any water from treatment, mechanical work or decontamination of protective clothing will be treated in a batch waste water treatment plant, using flocculation, oxidation with hydrogen peroxide, followed by carbon filtration.
Before remediation permits could be applied for, an extensive technical scale pilot plant trial was carried out in late 2002 on a plot close to the contaminated site, using both a desorber plant and a BCD reactor.
The area was sealed off using double skinned tents, plastic sheets and steel plates laid on a concrete slab.
'More than a tonne of the different contaminated materials and chemicals was treated, ' says Fairweather. Samples of all the materials and chemicals expected to be treated at the site were tested and showed removal and destruction levels below the limits of detection, Wynne adds.
Finalising environmental impact and permit requirements for the full-size plant with the public and regulatory authorities is now being carried out. Full approval of the construction and operation of the plant is expected to be granted in May.