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Acid Test

A testing laboratory is playing an important role in a contaminated
land clean-up contract in Derbyshire. Adrian Greeman reports.

Fast turnaround and tight logistics for soil sampling and analysis have been crucial to a huge soil remediation project at the Avenue Coking Works in Derbyshire. A £172.3M clean-up project for the 80ha of poisoned land began in 2010 and work is about to move into a new phase.

This involves implementing a sophisticated soil testing regime to help contractors sort the worst-contaminated soils, containing tars and heavy metals, from those which can be cleaned by washing or bioremediation.
Work has begun in earnest on the main phase of the contaminated land clean-up at the site. This involves burning off the worst of the tar and heavy metal pollutants in a huge soil heating thermal desorption plant.

Desorption is the chosen method for dealing with the most contaminated material at the site, including silt from two vast dumping lagoons of waste from the coke and chemical production works which operated here for decades until its closure 17 years ago.

Around 200 technicians work at the laboratory and a total of 3,000 tests are possible

The process will also deal with 250,000m3 of further chemical waste dumped on top of them.
The giant furnaces, powered by gas from a dedicated and specially laid mains pipe, will heat soils to 600oC, driving off phenols, benzenes, and numerous open-chain hydrocarbons as well as heavy metals, arsenic and cyanides. The gaseous output is variously filtered, precipitated and absorbed.

“Tuning” the machine for its two-year task, during which time it will handle more than 25t an hour of material on a continuous 24-hour basis, has been a key focus on site during commissioning work. Assembly of the unit was completed in October, says Marcus Foweather, project director for the VSD Avenue consortium which carried out the project for client the East Midlands Development Agency.

“It is critical to adjust the machine for the particular soils; you must avoid having too much calorific value in the throughput or it will overheat, but equally there must not be too much moisture cooling it, and not too much contamination,” he explains. Mercury levels are particularly significant, he says.

Mix to Feed

Sludges from the site and especially the lagoons, are mixed with a granular material to produce a homogenous mix to feed into the plant and adjusting the mix is important. Sampling the input constantly and, equally, verifying the output has been critical during testing to make continual adjustments and for this the company has been using a long-term analysis programme set up with Alcontrol, a major testing laboratory based in Flintshire, Wales.Around 200 analysts and 100 administrative staff work at the laboratory, which carries out a wide range of tests from simple quantitative analysis to the more detailed work. A total of 3,000 tests are possible, though for the Avenue project a subset of tests for the main contaminants is used.

High-pressure liquid chromatography

“Methods include gas chromatography, measuring total petroleum hydrocarbons, high-pressure liquid chromatography for phenols, acid extraction and inductively coupled plasma emission for heavy metals,” says marketing manager Hazel Davidson. Most of her analysts are degree level chemists with a number of more specialist staff for the complex tests.

Testing is not only important during commissioning, but will be necessary throughout the operation of the plant.

Sampling is also important for the other work at the site - only the most contaminated material will go through the expensive and energy intensive desorption process, and where possible bioremediation or soil washing is being used. As a result the operator needs to assess all the different materials on a more or less daily basis to determine processing methods and to ensure heavy earth moving plant is kept continuously busy, with results required in less than a week from collection.

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