Bench-scale studies of reactor-based treatment of fuel-contaminated soils

Abstract

Biological treatment of hazardous wastes in groundwater and soils has recently assumed great importance. In particular, biotreatment of wastes from accidental spills or underground storage tank leaks has generated interest in bioremediation as a natural, economical mechanism for site decontamination. Because of drawbacks related to batch systems, and the successful use of continuous flow treatment of wastewater for several decades, it was felt that continuous treatment of such soils would be a feasible alternative treatment technique. To this end, bench-scale bioreactor treatability studies were conducted. This study used contaminated soil made in the laboratory using No. 2 diesel fuel and sand. Contamination levels studied were from 1335 to 6675 mg (TPH) as derived from No. 2 fuel oil per kg sand. Variation in mean cell age was obtained between reactors, with sufficient nutrients and oxygen made available to ensure the fuel oil organics were the only limit to microbial growth. A theoretical biokinetic model was formulated based on Monod's theory of limiting substrate and continuous cultures. Biokinetic constants and removal efficiencies were evaluated. The off-gases CO 2 and volatile hydrocarbons were monitored to allow mass balance analysis of the process. The solids retention times for evaluating final TPH concentration of 100 mg/kg were also calculated. The results of this investigation showed that continuous bioreactor treatment is a viable option in the treatment of diesel-contaminated sandy soils. Removal efficiencies of up to 91% were attained at a loading of 1335 mg TPH/kg wet sand, operated at a biological solid retention time (BSRT) of 60 days. Experiments also showed that TPH desorption and volatilization were not rate-limiting in the overall removal process. Sand-to-moisture ratios in excess of 3:1 were also shown to retard TPH removal rates very little. However, biokinetic constants were found to vary over a range of values. This was particularly true at varying diesel loading levels. Nevertheless, significant removal efficiency (up to 86%) was noted at the highest loading level tested, 6675 mg TPH/kg wet sand.