Bio-augmentation (addition of micro-organisms to contaminated materials and sites) and bio-stimulation (modification of contaminated materials and sites to enhance the growth of indigenous micro-organisms) are two approaches to treat contaminated soil. While good reasons exist for both methods having the potential to be successful bioremediation strategies, different studies showed seemingly contradicting results and there is still a need to generalise findings so that advice can be given on remediation strategies to use depending on the type of contamination and soil. As poly-aromatic hydrocarbon (PAH) contaminated sites are typically of large size, economic concerns must not be ignored when looking for remediation techniques. Composting is an easy to implement and inexpensive method that has been found to be effective in removing contaminants from soil under different conditions. Results from different studies are, however, difficult to compare due to differing conditions they were carried out under as well as due to different soil types and contamination levels. The objective of this study was to establish the suitability of bacterial and fungal additives in a composting process for bio-remediation of soil with a high clay content contaminated with mainly large and more difficult to access PAHs. During co-composting of contaminated soil microbial communities, physical and chemical conditions, and contaminant concentration and availability change constantly and influence each other.
Changes in temperature are characteristic for a composting process and Influence several other factors such as microbial community development and contaminant availability. Temperature-profiles were applied and changes in process characteristics were analysed by monitoring different physico-chemical characteristics; microbial community changes were monitored using phospholipid-fatty-acid (PLFA) analysis; GC-MS analysis was used to investigate changes in solvent-extractable PAH concentrations and PAH volatilisation. Firstly different types of manures were chosen as bacteria dominated inocula to be used as co-composting agents. Results showed differences in microbial community composition as well as some differences in physico- chemical characteristics of the treatments, but no correlation with PAH removal could be detected. While horse and cattle manure addition was slightly more successful in removing low (LMW) and medium molecular weight (MMW) PAHs, high molecular weight (HMW) PAH concentrations showed increases, but were slightly decreased with chicken manure addition.
Secondly two different white-rot fungi (Pleurotus ostreatus and Trametes versicolor), inoculated in sawdust, were added to the contaminated soil both singly and in combination. While physico-chemical characteristics were mainly not affected, P. ostreatus removed all measured PAHs by a larger extent than the other treatments. It was also noted that the highest rate of PAH removal appeared at the beginning of the experiment when fungal biomass was high. In a third experiment the hypothesis that a combination of fungi and bacteria is most suitable to remediate contaminated soil was tested. P. ostreatus inoculated sawdust was added to the contaminated soil and chicken manure was added at three different stages during the experiment. The results showed that manure addition at the beginning of the composting process delayed PAH removal, but PAH removal rates were greater in the fungus and manure amended treatments compared to the control treatment. Additionally, the effect of ageing on the removal behaviour of PAHs was investigated by adding two deuterated PAHs to the soil (the LMW PAH Acenaphthene-D10 and the MMW PAH Benz[a]anthracene-D12). Results show no advantage of fungus and manure addition. However, differences to the removal of the weathered PAHs Acenaphthene and Benz[a]anthracene show the importance of bioavailability and indicate that temperature might not play such an important role when dealing with a fresh contamination compared to one where the contaminated soil has aged. The results of this study showed that addition of P. ostreatus inoculated sawdust enhanced the removal of PAHs from a soil where simple oxygen introduction did not show further PAH reduction. However additional amendment with chicken manure as a source of bacteria did not show further reductions in PAH concentrations.