Bio-clean for contaminated land

Contaminated land has commonly been treated through simple containment, excavation and disposal. However, bioremediation is a fast developing area of technology that can be used to clean up organic chemical contamination.

Under new legislation, businesses may be required to clean up any contaminated land they own. While the more usual 'dig and dump' approach will remove the problem from site (and sight), bioremediation promises to solve the problem once and for all, often at much lower cost.

Bioremediation of soil and water uses the naturally occurring bacteria and other microbes in the soil to break down contamination. This would happen naturally in some cases, but intervening to encourage and enhance this activity speeds up the whole process. Bioremediation techniques can take slightly longer to complete than conventional treatments. However, the significantly lower costs and environmental benefits often outweigh this short delay.

BIO-WISE, the bioremediation programme sponsored by the Department of Trade and Industry, has produced a range of free publications to provide companies with information to decide whether bioremediation is the best option for their needs. BIO-WISE's latest publication, 'Contaminated land remediation: A review of biological technology', provides an easy to follow explanation of bioremediation techniques.

This publication will help companies find out if bioremediation is suitable for their land and groundwater clean-up.

The success of a remediation project depends on understanding the nature of the problem. A good site investigation is essential before any remediation decision is made. The assessment needs to take into account the level of contamination, the constraints imposed by the site itself and the potential microbial activity. If bioremediation is considered at the outset, then time to use this technique can be built in to the development framework. It cannot be considered as an afterthought.

Looking at landfarming

Landfarming is suitable for degrading polyaromatic hydrocarbons (PAHs) and involves excavating the contaminated soil. The soil is contained, usually within a bunded and impermeably lined area, and is regularly tilled and irrigated, with nutrients added to promote degradation. Covering the soil maintains the optimum temperature of 15°-30°C. Routine sampling continues until the contamination falls to the target level.

Lattice Property (formerly BG Property) decided that landfarming was the best option for the remediation of an old town-gas works in Sheffield. The site had been used to manufacture town gas until the 1960s. As there were no time restraints, the company considered both landfarming and landfilling to clean up the coal tar contamination. With the cost of the former coming in at between £30-£40/m3, compared with typically between £40-60/m3 for landfilling, the company opted for landfarming.

After 52 weeks, the PAH concentrations in the material had decreased from 4700 mg/kg to the target level of 1000 mg/kg. The contaminated material was made less hazardous and retained for use on site to backfill the excavations. Consequently, there was no need to landfill or to import clean material, so road journeys were also reduced.

Hydrocarbons can also be effectively degraded using composting (or soil banking). Long windrows are formed outdoors on an impermeable surface such as concrete. The rows are covered to maintain heat and the soil is turned periodically to aerate the contaminated soil. Composting can treat larger volumes of soil than landfarming and is more effective in colder climates. Adding organic matter will improve the process by increasing the microbial population.

RJB Mining used this method to remediate an old coke works contaminated mainly with coal tar, heavy metals, phenols and ammonia. RJB's risk-based assessment showed that only 20 per cent of the initial estimated quantity required treatment or disposal. RJB chose to bioremediate the majority (about 15 000m3) as it was mauch more expensive to dispose of the soil in landfill. Encapsulation of the soil was also not practical, as it would have resulted in long-term liabilities, and would not remove the contamination from the site.

Composting broke down the PAHs and treatment was completed in 30 weeks. Using biological treatment saved RJB Mining around £5 million when compared to the cost of landfilling the same quantity of soil, and it also reduced the environmental risks of transporting the waste.

In some cases, in-situ bioremediation will be effective, depending on the extent and depth of the contamination. As the name implies, it does not involve the excavation or removal of any soil. In-situ techniques can range from long term monitoring of how natural processes reduce contamination to active intervention via bioventing and biosparging. The latter two techniques aid remediation of soil and groundwater contaminated to a greater depth.

Bioremediation is a tried and tested method of remediating contaminated land and BIO-WISE can help and advise UK companies on the best way forward. It is vital that managers make an informed decision about whether bioremediation is the right option.

BIO-WISE has a panel of 30 Industrial Biotechnology Specialists, who can give up to four hours of free advice on specific applications or issues. It can also provides lists of biotechnology suppliers, publishes a range of literature including verified case studies and holds free events and seminars, and offers grant funding through the Demonstrator Project competition.

To order a free copy of any of the case histories, or for any other query, call the Helpline on 0800 432100, visit the web site (www.dti.gov.uk/biowise) or email (biowise.help@aeat.co.uk).