BIOREMEDIATION BY COMPOSTING IN BIOPILES.

Kirsten S. Jørgensen, Jukka Puustinen and M. Minna Laine, Finnish Environment Institute, Research Laboratory, Hakuninmaantie 4-6, FIN-00430 Helsinki, Finland.

Contaminated soil is often poor in organic matter and has a general low microbial activity. Usually the bacterial community is adapted to the presence of the contaminant, but other environmental conditions such as nutrient availabilty and oxygen concentration may by unfavourable, and thus the microbial degradation of the contaminant is slow in situ. Composting technologies can be applied to clean up contaminated soil ex situ. By adding an organic matrix to contaminated soil the general microbial acitvity is enhanced and also the activity of specific degraders, which may be found in the contaminated soil or may be introduced along with the organic material. We have tested the application of this technology in field scale using bark chips and straw compost as the introduced organic matter for the clean up of chlorophenol contaminated soil and hydrocarbon contaminated soil. We also tested the efficacy of commercially and non-commercially available inocula. The composting of chlorophenol-contaminated soil was performed first in small field scale (4 x 13 m³ of soil) and then in large scale (520 m³). The aim was to test the applicability of using straw compost (phase I mushroom compost), which was adapted for chlorophenol degradation, as inoculum in comparison to addition of bark chips and nutrients to contaminated soil. The composting was performed in three windrows and the ratio of materials were 2 parts of contaminted soil to 1 part of bark or straw compost. The piles were turned every 3 to 4 weeks. The highest temperature was around 30° C and the temperature remained well above the ambient temperature, thus prolonging the period with high microbial activity. The starting concentrations of chlorophenols were from 800 to 900 mg (kg dry weight)^-1 or from 30 to 40 mg (kg dry weight)^-1. In all cases the chlorophenol degradation was more than 90 % during a period from 2 to 5 months. The use of straw compost as inoculum did not enhance the chlorophenol degradation in comparison to addtion of bark chips to the contaminated soil. Parallel bench scale compsts showed that around 60 % of pentachlorophenol was mineralized. There were no harmful side reactions such as biomethylation or polymerization. We observed, however, that surprisingly high amounts of polychlorinated dibenzo-p-dioxins and dibenzofurans were present in the contaminated soil as impurities in the chlorophenolic waste and these remained constant throughout the composting time. Composting of petroleum hydrocarbons were performed in field scale (5 x 40 m³ ) using only bark chips as the bulking agent and in this case we tested two commercially available inoculants and the effect of the level of added nutrients. The mineral oil degradation rate was most rapid during the first months, which correlated well with the microbial activity measured as respiration. During 5 months of composting of the mineral oil decreased in all the piles from approximately 2400 mg to 700 mg (kg dry weight)^-1, which was about 70 % of the mineral oil content. There was no major effect of the added inoculants. In conclusion, composing technology can be used to effectively remove contaminats from soil. In this type of treatment with addition of a large amount of organic matter, the general microbial activity is enhanced and no particular effect of added inocula was observed.