Enhancement of polycyclic aromatic hydrocarbons removal during anaerobic treatment of urban sludge

Anaerobically stabilized sewage sludge has potential to partially substitute synthetic fertilizers. The main risk with the recycling of urban sludge on agricultural soils is the accumulation of unwanted products, such as trace metals and organic micropollutants. In this context, the polycyclic aromatic hydrocarbons (PAHs) are particularly monitored because of their toxic properties at low concentrations and their high resistance to biological degradation. The aim of the present study was to optimize PAHs removal during anaerobic digestion of contaminated sewage sludge. Thirteen PAHs were monitored in laboratory-scale anaerobic bioreactors under mesophilic (35°C) and thermophilic (55°C) methanogenic conditions. Abiotic losses were statistically significant for the lightest PAHs, such as fluorene, phenanthrene and anthracene. It was shown that PAH removal was due to a specific biological activity. Biological PAHs removal was significantly enhanced by an increase of the temperature from 35°C to 55°C, especially for the heaviest PAHs. Bioaugmentation experiment was also performed by addition of a PAH-adapted bacterial consortium to a non-acclimated reactor. Significant enhancement of PAHs removal was observed. It was finally shown that PAH removal efficiencies and methanogenic performances were closely linked. The rate of biogas production may be used as an indicator of bacterial activity on PAH removal.