Successful Treatment of Low PAH-Contaminated Sewage Sludge in Aerobic Bioreactors

摘要

Background, Aims and Scope. Polycyclic Aromatic Hydrocarbons (PAHs) are known for their adverse and cumulative effects at low concentration. In particular, the PAHs accumulate in sewage sludge during wastewater treatment, and may thereafter contaminate agricultural soils by spreading sludge on land. Therefore, sludge treatment processes constitute the unique opportunity of PAH removal before their release in the environment. In this study, the ability of aerobic microorganisms to degrade light and heavy PAHs was investigated in continuous bioreactors treating trace-level PAH-contaminated sludge. Methods. Several aerobic reactors were operated under continuous and perfectly mixed conditions to simulate actual aerobic sludge digesters. Three sterile control reactors were performed at 35 deg C, 45 deg C or 55 deg C to assess PAH abiotic losses under mes-ophilic and thermophilic conditions. Three biological reactors were also operated at 35 deg C, 45 deg C or 55 deg C. Furthermore, 250 mM methanol were added in an additional mesophilic reactor (35 deg C). All reactors were fed with long-term PAH-contaminated sewage sludge, and PAH removal was assessed by inlet/outlet mass balance. In this study, PAH compounds ranged from 2 to 5-unsubstituted aromatic rings, i.e. respectively from Fluorene to Indeno(123cd)pyrene. Results and Discussion. Significant abiotic losses were observed for the lightest PAHs (fluorene, phenanthrene and anthracene), while biodegradation occurred for all PAHs. More than 80 percent of the lightest PAHs were removed. Biodegradation rates inversely correlated with the increasing molecular weight, and seemed limited by the low bioavailability of the heaviest PAHs (only 50 percent of removal). The enhancement of PAH bioavailability by increasing the process temperature or adding methanol was tested. A temperature increase from 35 deg C to 45 deg C and then to 55 deg C significantly enhanced the biodegradation of the heaviest PAHs from 50 percent to 80 percent. However, high abiotic losses were observed for all PAHs at 55 deg C, which was attributed to volatilization. Optimal conditions were found at 45 deg C considering the low abiotic losses and the high PAH biodegradation rates. Similar performances were achieved by addition of methanol in the sludge. It was concluded that increasing temperatures or addition of methanol favored PAH diffusion from solids to an aqueous compartment, and enhanced their bioavailability to PAH-degrading microorganisms. Conclusion. In this study, the use of long-term acclimated aerobic ecosystems showed the high potential of aerobic microorganisms to degrade a wide range of PAHs at trace levels. However, PAH biodegradation was likely controlled by their low bioavailability. Two aerobic processes have been finally proposed to achieve efficient decontamination of sewage sludge, at 45 deg C or in the presence of methanol. The PAH concentrations in reactor outlet were lower than the French requirements, and allow the treated sludge to be spread on agricultural land. Recommendations and Outlook. The two proposed aerobic processes used physical or chemical diffusing agents. The global ecological impact of using the latter agents for treating trace level contamination must be considered. Since methanol was completely removed during the process, no additional harm is expected after treatment. However, an increase of temperature to 45 deg C could drastically increase the energy demand in full-scale plants, and therefore the ecological impact of the process. Moreover, since bioavailability controls PAH biodegradation, efficiency of the processes could also be influenced by the hydraulic parameters, such as mixing and aeration rates. Further experimentations in a pilot scale are therefore recommended, as well as a final assessment of the global environmental benefit of using such aerobic processes in the bioremediation of trace level compounds.