Removing toxins from soil has become one of the most vital and dynamic areas of modern construction. Here's a guide to the latest techniques on the market and an insight into how microbes are eliminating the poisons at a former colliery site in Yorkshire – by eating them.
Brownfield sites are in demand. The government has asked housebuilders to build 60% of the 4 million homes required in England and Wales by 2016 on sites that have previously been developed.

The problem is that the land on which many of these schemes will be constructed has been contaminated by what was there before, rendering it uninhabitable. The key to using brownfield sites is to rid them of these contaminants – a process known as remediation.

Over the past decade there has been a huge increase in the number of methods used for cleaning up soil. This boom has been driven by ever-tougher regulations: planning and building control requirements are stricter, environmental legislation is tighter and the rules on dumping contaminated soil in landfills are more restrictive. On top of that, the public is more aware of environmental issues.

Here we look at how the site of a former colliery and coking plant is being transformed into squeaky-clean land for a sports centre – and, overleaf, we give a rundown of some of the remediation techniques being used today.

From filthy dirty to squeaky clean: The bioremediation of Askern colliery
"We've built two giant compost heaps here," explains Tom Hayes, project manager of land remediation specialist Ecologia Environmental Solutions, gesturing at two piles of earth, each swathed in a bright green polyethylene blanket. This pair of giant, plastic-covered hummocks are fundamental to the process of transforming a polluted former colliery into an uncontaminated sports and recreation ground using a process known as bioremediation.

In its previous life, as well as housing a colliery, the Askern site near Doncaster was home to a coking works, where coal was burned under controlled conditions to transform it into coke for Sheffield's steel industry. The coking plant and colliery closed in 1991.

Unfortunately, the site's industrial legacy has left the soil heavily contaminated with hydrocarbon-based toxic compounds. "It is a similar residue to the toxic cocktail found in a cigarette butt," Hayes says.

In 2000, regional development agency Yorkshire Forward set out to transform the site. It commissioned consulting engineer Carl Bro to carry out a survey of the disused colliery to assess the extent of harmful chemicals in the ground. Using a combination of historic records, on-site investigations and chemical analysis of soil samples, the engineers mapped the type, location, and depth of pollution concentrations.

The engineers also discovered that the bedrock beneath the site was limestone. "Because limestone is permeable, it will allow contaminants from the soil to be washed through into the nearby river," explains Hayes. If pollution of the nearby watercourses was to be avoided, the contaminants had to be removed from the soil.

Contractor Mowlem successfully tendered for the 78-week remediation contract, and subcontracted the work to licensed bioremediation specialist Ecologia. Some of the land was so contaminated that treatment was impossible.

Instead, it was dug up and transported to special landfill sites. However, most of the site had low-to-medium levels of pollution, which meant that it could be treated in situ. And, because the contamination was hydrocarbon-based, the soil could be cleaned using bioremediation. Bioremediation is a natural process that uses the microbes that live in the soil to eat harmful contaminants, which they turn into water and carbon dioxide. The process takes place naturally in the soil, but it would have taken many years before all the contaminants were consumed – and by then the pollution could have found its way into the watercourses.

To speed up remediation, the right conditions must be created for the bacteria to grow and multiply. To do this, Ecologia needed to increase the amount of oxygen present in the soil. Two large diameter plastic pipes were laid flat on the ground, parallel to each other. These were connected at one end and a series of smaller diameter secondary pipes with perforations along their lengths connected between them like the rungs in a ladder.

The soil to be treated was tipped over the tubes to form two separate piles of earth, each served by its own system of pipes. A fan was then connected to the systems to suck air through the soil, increasing the amount of oxygen available to the microbes and encouraging them to grow. Sucking air through the soil had the additional advantage of removing contaminated moisture. Before being discharged, the air/moisture mix was passed through a collection tank where a cyclone unit removed the contaminants.

So far, the process is working. Hayes says the piles of earth would normally be at a temperature somewhere between 10°C and 12°C. However, the bacteria's frenzied activity has pushed it up to about 30°C.

Regulation of the amount of water in the soil is also important for the bacteria's health. This is achieved simply removing the green polyethylene covers to allow rain to fall on the mounds, and by covering them when the soil is sufficiently moist.

When the microbes have finished their work and eaten all the contaminants, the mounds of earth will simply be bulldozed into the landscaping scheme. The microbes have been working for 12 weeks, and Hayes reckons that in another eight they will get through all 22,000 m3 of soil.