Bioremediation of diesel-contaminated soil by heated and humidified biopile system in cold climates

Abstract

Abstract Bioremediation is a proven and widely accepted technology for the remediation of soil contaminated with various mixtures of petroleum hydrocarbons (PHC) in temperate climates but its application in cold climates has received far less attention. Low ambient temperature is one of the main factors limiting microbial degradation of the organic contaminants in such locations. Heating the soil should therefore enhance bioremediation as laboratory studies have shown increased biodegradation rates in Arctic soils with increasing soil temperature. A biopile is one of the many bioremediation techniques to treat hydrocarbon-contaminated soil where the soil is piled over an air distribution system and aerated. The air distribution system can also be used to provide heat to the soil in order to optimize soil temperature when conditions are limiting. However, heating the soil by forced air may cause excessive drying of the soil which may inhibit microbial activity and promote volatilization of the contaminants rather than their biodegradation. During treatment of hydrocarbon contaminated soils in a biopile system, biodegradation is preferred over volatilization and should be optimized. Few field-scale studies have tested the use of heated biopile systems in cold climates. This paper examines the effect of humidifying the air for the treatment of PHC-contaminated soil by an aerated/heated biopile system. Three biopiles were constructed with soil freshly contaminated with diesel fuel (initial total petroleum hydrocarbon concentration, [TPH], ~ 11,000 mg/kg dry weight) and operated for 10 months in Kingston, ON, Canada. One biopile was heated with an aerating/heating system previously tested in the Arctic. A second biopile was also aerated and heated but received water by humidifying the air prior to entering the soil pile. A third biopile was passively aerated by pipes protruding from the soil pile. TPH, available nutrients content, and pH were monitored by periodic collection and analysis of soil samples. Volatilization of hydrocarbon compounds was measured by trapping them on activated charcoal. Temperature and moisture were monitored continuously with a datalogger. Hydrocarbon concentrations in soil and charcoal samples were measured by solvent extraction followed by gas chromatography flame ionization detection (GC/FID) analysis. Significant TPH reduction was observed in all systems. TPH reduction followed first-order kinetics for the first two-third of the treatment. The humidified system maintained optimal soil moisture content and produced significantly lower final TPH than the other two treatments (~ 300 mg/kg dry weight). Findings suggest that humidifying the air enhanced biodegradation and minimized volatilization. The removal of hydrocarbons of different carbon chain lengths was investigated by GC/FID analysis. Three hydrocarbon fractions were defined ( nC15) based on equivalent straight-chain alkane ranges. Results from this analysis showed that all fractions were removed during treatment, but the overall data suggest that biodegradation was dominant for the highest molecular weight fraction.