Effect of biostimulation on community level physiological profiles of microorganisms in field-scale biopiles composed of aged oil sludge
Highlights
► Field-scale biopiles were constructed for the bioremediation of aged oil sludge. ► Bulking agent, cotton stalk, raised microbial metabolic activity and diversity. ► Nutrients revealed suppressing effect on microbes for its large amount of application. ► Homogeneity among layers was shown in the biopiles with bulking agent. ► The highest TPHs removal was achieved by the middle layer of bulking agent pile.
Introduction
Oil sludge, which is a mixture of petroleum product or waste, solid particles and water, is mainly generated by sedimentation and accumulation in crude oil storage tanks and transportation pipelines of oilfields. It was classified as a kind of hazardous waste by the Ministry of Environmental Protection of China. The amount of oil sludge discharged by the petrochemical industry in China was 3 × 106 tons per year (Jin et al., 2011). During one year, more than 105 tons of oil sludge were discharged by Shengli oilfield, which was the site of this field experiment (Li et al., 2003). Oil sludge was stacked in the open air with no other means of disposal before its negative effects on the environment were realized. The volatile components and moisture content of the oil sludge decreased during its exposure outdoors, and a large quantity of aged oil sludge was generated. Many components of oil sludge are generally considered toxic, mutagenic and carcinogenic (Liu et al., 2009). The release of oil sludge may cause extensive environmental disturbances, such as soil pollution and surface water/ground water contamination. In view of the above-mentioned facts, treatment of oil sludge has been incorporated on the agenda of environmental science.
The disposal of oil sludge is currently a prevalent environmental issue. In 1992, the United States Environmental Protection Agency (US EPA) published a final rule (57 FR 37194, 37252), which established treatment standards under the land disposal restrictions program for various hazardous wastes, including hydrocarbons (Lin et al., 1996). Physical, chemical and biological technologies are applied to the treatment of oil sludge. As the field of bioremediation develops, more and more limitations to the use of physical and chemical methods, such as those employing solvent extraction/washing, ultrasonic devices, thermochemistry and supercritical water, are exposed. In comparison, biological treatments are environmentally friendly and cost-effective. More than 50% of Superfund sites chose biological technology as the preferred in situ remediation method. The Fourth International In Situ and On-Site Bioremediation Symposium in 1997 introduced the biopile to the public as an important technique of bioremediation. A biopile refers to the piling up of materials to be treated by bioaugmentation and biostimulation (Jørgensen et al., 2000). Treatment can be improved with bulking agent, nutrients, moisture adjustment and air blowing to enhance the activities of indigenous or extraneous microorganisms. The construction and operation of a biopile were the focus of research at the early stage (Fahnestock et al., 1998). Further exploration revealed the characteristics of microorganisms in biopiles. Furthermore, the use of biopiles under extreme climatic conditions, such as in subAntarctica area, was studied (Delille et al., 2007).
After persistent research in this field, the biopile is now a fully developed technical system. However, oil sludge, especially aged oil sludge, is seldom treated using a biopile system. There is little data showing the variance among the layers within field-scale biopiles. Further studies are still necessary to expand the choices available for bioremediation.
Shengli oilfield, the second biggest oilfield in China, was chosen as the site for this study. Four biopiles of aged oil sludge were constructed in the oilfield, and different biostimulation strategies were applied to each. Microbial community level physiological profiles (CLPPs) in the contaminated environment and the efficiency of bioremediation of the aged oil sludge were the focus of this study. Microbial metabolic activity and diversity, enumeration of culturable bacteria and total petroleum hydrocarbons (TPHs) were chosen as parameters to describe the biological and physicochemical properties of the oil sludge. According to these parameters, the responses of CLPPs to biostimulation and the removal rate of TPHs were estimated.
Section snippets
Site and experimental design
An outdoor oil sludge storage site in Shengli oilfield, Shandong Province, China, was chosen for this field trial. The mean annual temperature is 12.8 °C, and temperatures range from −3.0 °C in January to 26.6 °C in July. Quantities of oil sludge produced as a result of different processes of oil exploitation had been transported in last 20 years to the site and mixed together in a pool without further treatment.
Aged oil sludge was dug out of the pools and mixed thoroughly using an excavator.
Analysis of microbial CLPPs
The initial samples and samples taken on day 220 were used to analyze microbial CLPPs. The AWCD, Shannon index and principal component analysis (PCA) reflected the characteristics of microbial metabolism.
The AWCD values revealed the microbial metabolic activities in the aged oil sludge (Rodriguez and Toranzos, 2003). The effects of the four treatments (three different biostimulation strategies and the control) on microbial activity within each layer are shown in Fig. 1a–c. During the first 100 h
Conclusions
After 220 days of bioremediation, the microbial metabolic activity and diversity, as well as the numbers of two bacteria, were significantly enhanced by the addition of bulking agent. The addition of a large amount of urea led to a suppressing effect on both metabolic activity and diversity. Furthermore, homogeneity among the layers was seen in the biopiles with bulking agent. The maximum removal of TPHs (49.62%) occurred in the middle layer of pile B. However, the quantity of TPHs in the middle
Acknowledgements
This research was supported by the Sino-Russia International Cooperation Program (Grant No. 2008DFR90550), Young Scientists Fund of NSFC (Grant No. 40901249) and Special Environmental Research Funds of MEP for Public Service (Grant No. 201009015). Appreciation for site management and cooperation is gratefully expressed to Shengli oilfield.
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