Improving the biotreatment of hydrocarbons-contaminated soils by addition of activated sludge taken from the wastewater treatment facilities of an oil refinery

Addition of activated sludge taken from the wastewater treatment facilities of an oil refinery to a soil contaminated with oily sludge stimulated hydrocarbon biodegradation in microcosms, bioreactors and biopile. Microcosms containing 50 g of soil to which 0.07% (w/w) of activated sludge was added presented a higher degradation of alkanes (80% vs 24%) and polycyclic aromatic hydrocarbons (PAHs) ( 77% vs 49%) as compared to the one receiving only water, after 30 days of incubation at room temperature. Addition of ammonium nitrate or sterile sludge filtrate instead of activated sludge resulted in a similar removal of PAHs but not of alkanes suggesting that the nitrogen contained in the activated sludge plays a major role in the degradation of PAHs while microorganisms of the sludge are active against alkanes. Addition of sludge also stimulated hydrocarbon biodegradation in 10-kg bioreactors operated during 60 days and in a 50-m(3) biopile operated during 126 days. This biopile treatment allowed the use of the soil for industrial purpose based on provincial regulation (C criteria). In contrast, the soil of the control biopile that received only water still exceeded C criteria for C-10-C-50 hydrocarbons, total PAHs, chrysene and benzo[ a] anthracene. The stimulation effect of sludge was stronger on the 4-rings than on 2-rings PAHs. The soil of the biopile that received sludge was 4 - 5 times less toxic than the control. These results suggest that this particular type of activated sludge could be used to increase the efficiency of the treatment of hydrocarbon-contaminated soils in a biopile.