Biological remediation - no longer a black art
The first ex-situ bioremediation project of the 21st Century under the CL:AIRE initiative has been successfully completed at Askern Colliery, near Doncaster, for the regional development agency, Yorkshire Forward.
Specialist environmental contractor/consultant, Ecologia Environmental Solutions of Sittingbourne Research Centre, Kent, holds the Mobile Plant Licence which was employed, and also designed and fabricated, in house, the plant and equipment. Construction commenced in March 2002 under sub-contract to Mowlem plc, and the satisfactory remediation of 22,000 cubic metres of soil was completed in October. The site was the former colliery and coking plant, where coal had been burnt for decades to power the Sheffield steel industry furnaces. As they fell silent in the 70s, so did the collieries in turn, and Askern finally closed at the start of the nineties. A legacy of industrial dereliction hung over the area for that decade, but now the regional development agency, in conjunction with English Partnerships' coalfield regeneration programme, is seeking to revitalise the commercial and environmental heartbeat. Biological remediation
Ecologia's role on this £3.5m regeneration project was to deal with the 'medium' contaminated hydrocarbon and phenolic soils, with only the worst fraction having to go off-site to landfill. Because of the organic nature of the pollutants and the granular consistency of the colliery spoil upon which the wastes had been tipped, the option for biological remediation seemed to be a cost-effective (and environmental) choice to Yorkshire Forward and their consultant, CarlBro of Leeds. But the nature of an underlying limestone geology meant that significant concern was felt about certain PAH and phenolic contaminants affecting groundwater, so stringent remedial targets were set for these.
Site-specific targets for certain compounds meant that the ex-situ bioremediation method chosen would need careful evaluation and monitoring, and some 'smart' thinking. Based on experience from previous similar projects in the southeast, and pilot studies in Ecologia's own laboratories, a scheme was developed that used static biopiles with forced aeration to stimulate bio-activity. This would use air extraction, not injection, to enhance microbial activity in the soils. Adjusting techniques already employed in Ecologia's parent base in Italy, the UK scientific staff designed and fabricated their system from widely-sourced components, and received mobile plant licence approval in good time from their 'home' office in Kent. They then set off to build it…
However, South Yorkshire is a different location to the North Downs, and several natural inundations and local informal lending systems led to slow early progress. But by May, the climate and the neighbours had been tamed, the pipework laid, the extraction equipment, separators and controls tested, and Mowlem had delivered their required soil for remediation, and quite a bit more besides… Two large green-sheeted piles of soil were ready for the switch-on, once extensive baseline characterisation had taken place. Only once the process had started, did these baseline laboratory tests reveal that the starting point for the process had got a bit more heavily contaminated!Natural processes
Bioremediation is a natural process that uses the microbes that live in the soil to convert certain organic contaminants into less harmful compounds, excreting water and carbon dioxide as they run through their life cycles. It has long been recognised, in scientific fields, that these natural processes can take place more rapidly when there are optimal conditions for replication and growth. In the case of contaminated soil, the amount of oxygen present is a limiting factor as is sufficient nutrient. To enhance these factors, Ecologia established an air flow through their buried pipework system in the order of 7000 cubic metres per hour, and provided a nutrient mix into the spoil when deposited. This mixture incorporated both specific nutrients and composted sewage sludge to add organic matter. Proprietary bacterial products were not added as the bacteria population present at the site was sufficient.
For the first two months it appears that there was very little volatilisation from the static piles, but that also the microbial activity was consistently at a lower level than expected. As there was sampling in 22 different sub-zones of the main piles, then the specific degradation in each 'packet' could be systematically monitored. Changes could then be made to airflow rates or nutrient addition in each sub-system. The more heavily contaminated spoil in some sections could also be determined to be remediating at too slow a rate, by this means.
In early August, the pipeline configuration was adjusted and the air flow ratcheted up to more than double previous flow rates. This point can be seen at week 13, which is one week after the change. The Oxygen level has significantly changed, and it was immediately noted that the biopile internal temperatures rose from 20 to 30 degrees C. To confirm that this activity was biological in nature, CL:AIRE agreed that respiration testing within certain locations would be a useful scientific tool. The results of this analysis will be presented in the full CL:AIRE report.Target concentrations
After 20 weeks, all but two of the sub-piles had successfully achieved the target peak concentrations. Average concentrations within the sub-piles had also reached Ecologia's working targets of 1000mg/kg TPH and 1mg/kg Phenol. Whilst TPH is often the main contaminant of concern, and activity, for bioremediation, a substantial reduction in the Phenol concentrations is a point of significant note on the Askern project. Phenolic & PAH contaminants from coke plants and gasworks have long been considered to be more resistant to 'swift' bioremediation, and of more risk to the environment, however bacterial degradation pathways for these molecules are known and can be achieved with the correct conditions. Ecologia have demonstrated, under the scientific rigour of a CL:AIRE project, that this problem may be mitigated by use of the appropriate technology and technique management.