BIOREMEDIATION OF CHLOROPHENOL-CONTAMINATED SOIL BY COMPOSTING IN FULL SCALE

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

Chlorophenols are recalcitrant compounds that have been used for decades to impregnate wood, and many residues can be found in the environment long after the use of chlorophenols have been discontinued. Chlorophenols are soluble in water and may leach from contaminated soil to groundwater. Therefore the contaminated sites must be cleaned up to prevent further spread.

A three-year full-scale bioremediation (520 m³) was performed in 1995-1997 to test the applicability of using induced straw compost as an inoculum in comparison to addition of bark chips and nutrients to chlorophenol-contaminated soil. The site was a sawmill site, where the commercial chlorophenol mixture, Ky-5, had been used as a wood preservative in a dipping basin. The soil was excavated and treated in the vicinity.

The composting of chlorophenol-contaminated soil was very efficient during the first year and resulted in 96, 92 and 90 % removal of chlorophenols in 126 days with starting chlorophenol concentrations of 960, 740 and 29 mg (kg dry weight)^-1, respectively. The compost piles froze completely during winter and during the next 5 months of activity the chlorophenol concentrations decreased with 45, 70 and 50%, respectively. According to the results of our earlier studies, the mechanism of chlorophenol removal was complete mineralization without harmful side reactions as (bio)methylation or polymerization.

The basic and substrate induced respiration rates were measured during the composting to follow the overall microbial activity. The respiration rates were the highest at the beginning of the composting. Gas measurements from the compost piles showed that oxygen was consumed and CO₂ and water released during the most active chlorophenol degradation period of composting. The total number of bacteria, as determined by microbial counts on TGY medium, remained at the same level during the composting period. The number of bacteria growing on plates with PCP as a sole carbon source decreased after 12 weeks of composting.

The use of straw compost as inoculum did not enhance the chlorophenol degradation in comparison to addition of bark chips and nutrients. Frequent mixing and control of the nutrient level enhanced the chlorophenol degradation of the indigenous microbes in the contaminated soil. The chlorophenol-degrading microorganisms originating from contaminated soil probably benefited from the enhanced general microbial activity in the piles by cometabolism or synergism.

Temperature data-loggers were used to follow the temperatures in compost piles on-line. The average temperature in the compost piles was 35 °C at its highest and it was well above the ambient day temperature. During the winter, the degradation of chlorophenols stopped since the piles were completely frozen. Moreover, the revival of chlorophenol-degrading microorganisms after winter was not very efficient. There may be many reasons for that: 1) Cold climate, 2) Changes in microbial community and/or compost environment; 3) Absence of a possible cosubstrate needed for degradation; 4) Bioavailability.

Bioremediation of chlorophenol-contaminated sawmill soil using composting without bioaugmentation is a cheap and feasible method, but outdoor composting is limited to work efficiently only during summer season when the ambient temperature exceeds 10 °C. The drawback of cold climate could be overcome by either constructing the piles indoors (e.g. airdome), or supplying extra heating (underground-wires for electric heating).

Laine, M. M. 1998. Bioremediation of chlorophenol-contaminated sawmill soil. Dept. Appl. Chemistry and Microbiol., University of Helsinki, Finland. Dissertationes Biocentri Viikki Universitatis Helsingiensis 8/1998. Academic dissertation.