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Dirty business

A mammoth piece of equipment is being used to treat the legacy of contaminated land at an old manufacturing facility in south east England. Alexandra Wynne reports.

The village of Yalding, nestled in the Kent countryside, has been home to a 40ha agro-chemical manufacturing plant for over 100 years. Today, the now-decommissioned facility is being overhauled to make way for a multi-use development, but not without some serious remediation work first.

A number of different companies used the site to produce pesticides and, because of previously accepted (but possibly inadequate) standards and leaks in drainage systems, over time large areas became contaminated.

EDSR – a firm that deals with contaminated land remediation and sustainable regeneration of industrial sites – was awarded the £20M contract to come up with a battle plan for client Syngenta (owner of the manufacturing facility).

Its solution is a thermal desorption unit (TDU), which is doing the bulk of the work to remediate these areas. The machine heats the soil, vaporising the chemicals into gas. Since July 2006, excavators have been removing material to a maximum depth of 6m and feeding it into the unit for treatment. The TDU is working 24 hours a day, working through an average of 32t of soil/h.

The material comes from strata typically comprising up to 3m of made ground on about 4m of gravels overlying weald clay. Material is dug out and samples are then tested for moisture and mercury content. This TDU is unable to cope with heavy metals so if the mercury exceeds 15mg/kg, such material has to be sent for landfill.

This is less than ideal on a project that aims to reduce the environmental impact of the remediation work, although EDSR says the TDU method has saved over 36,000 lorry movements that would have been needed to remove contaminated soil. But it is necessary – EDSR site agent Saul Disbury says it would have been difficult to obtain thermal desorption equipment that could cope with heavy metals and most likely would have meant doubling the £3M spent on the TDU.

Any retrieved soil that has a moisture content over 17% is dosed with lime before going into the facility, which helps break the material down into smaller pieces. Optimal treatment is obtained when particles are less than 50mm, so the unit has a hopper with slots that help to further fragment any larger lumps before it feeds soil on to a shaker belt that draws it into the primary treatment unit of the TDU.

This part is a 3m diameter cylindrical drum with a flame inside that burns the contaminant out of the soil and into a gas. This gas stream is then drawn into a secondary treatment unit and hit with an even hotter flame – up to 1400˚C – which destroys the contaminants.

The gas is then cooled down ready to be released into the environment as steam. This happens in the next section of the TDU – an evaporative cooling tower, which Disbury says is similar to drawing air through a vacuum cleaner bag and reduces the gas temperature down to 200˚C. The emissions are now harmless to human health – limits are controlled by Environment Agency conditions.

The newly sterilised soil is held in 750t capacity bays, re-hydrated and tested over a five-day period before given the all-clear to be reused as fill across the site.

In addition to the impact on soils from materials kept at the facility over the years, engineers have had to cope with the potential of flooding on site, inflaming concerns that in such an event, contamination could spread.

Two main watercourses border the area, both of which are used for leisure activities and have caused flooding in the past. These watercourses ultimately flow into the River Medway.

To ensure against any leaching outside the site perimeter, workers installed a permeable reactive barrier along much of the site as a catch-all for any groundwater that might otherwise run off and contaminate the Medway. This water is treated to dilute the contaminants and stored and reused where needed on the project.

EDSR managing director Tom Brankley says one of the biggest challenges on the job was getting to grips with operating the TDU: "This equipment is far, far more sophisticated than I originally imagined. I thought it would be 'press a button and you're away'." He says that during the early stages it took a team of up to five people to operate the TDU's software alone.

To help smooth out a potentially steep learning curve, the firm brought in a team from the US. "We're not re-inventing the wheel here and the TDU is only going to suit large scale projects," Brankley says. "But we involved a team from the US with a proven track record and it's turning out to be a Rolls-Royce of a job that's working in a sustainable manner too." The decision to run the TDU on re-claimed fuel, of which it gets through 55,000l/d, is helping reduce the job's environmental impact.

EDSR is due to finish on site in October, and hopes by then its success will encourage future uptake of the thermal desorption remediation method in the UK in line with its popularity abroad.

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