In line with its traditional role as a soil fertilizer, compost has been used as an additive in soil remediation. Composting mostly takes place in fixed-bed reactors (Fig. 11.3a). Basically, the compost is added to stimulate microbial breakdown. In experiments, soil contaminated with hydrocarbons has been mixed with compost in various ratios (soil-compost ratios of 2:1, 3:1, and 4:1). In 3-L test batch reactors, the hydrocarbon degradation was >90% after a period of 44 d. Compared with the results in the absence of added compost, soil-compost systems had a much faster degradation rate and a lower end concentration (Lotter et al., 1990). In addition to composting, experiments focusing on the use of white-rot fungi have also been carried out (Schaeffer et al., 1995).
Another example of composting contaminated soil (in a column) was presented by Gorostiza et al. (1998). In a soil column the degradation of pentachlorophenol with and without added compost was tested; compost addition significantly enhanced the degradation rate (residence time about 60 d).
On a larger scale (in Finland), composting systems have been implemented by mixing contaminated soil with spruce bark chips as a bulking agent, lime, and nutrients beforehand. The bed is constructed as a biopile and can be turned by a tractor-drawn screw mixer. Degradation temperature rose during composting to as high as 44 °C. During five months the mineral oil concentration dropped from 2400 to 700 mg kg-1 (Puustinen et al., 1995).
In another experiment, four biopiles of 10 m3 each were created to treat chloro-phenol-contaminated soil. Chalk, commercial fertilizer (NPK), and bark chips (as a bulk aeration agent) were added. After two months 80% of the contaminant was removed (Laine and Jorgensen, 1995).
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