Decontamination and valorization of sludges deriving from biological wastewater treatment plants through anaerobic digestion

Organic pollutants such as PAHs, PCBs and pesticides tend to accumulate in the primary and secondary sludges of biological plants treating domestic and industrial wastewaters. Thus, those sludges have to be submitted to a dedicated treatment, that generally consists in their thermal destruction. Anaerobic digestion could represent an alternative technology for the disposal of such sludges, as it can mediate both their decontamination and their valorization, through the production of a biogas with a high content of methane. In this study, two sludges coming from domestic water treatment plants were spiked with PCBs or with a mix of PAHs and chlorinated aromatic compounds and employed in the development of anaerobic microcosms. The effects of the anaerobic digestion on the sludges were evaluated in terms of methane production, biodegradation of the occurring contaminants and depletion of pathogenic microrganisms, both in mesophilic or thermophilic conditions. The contribute to the digestive process of the sole fermentative eubacteria or of the sole methanogenic and sulfate-reducing bacteria, along with the possibility of enhancing its performances by stimulating the indigenous anaerobic consortium with yeast extract, were also determined. Methane production and xenobiotic removal were more efficient under mesophilic conditions, while a complete disinfection of the sludge was achieved at 55°C. Yeast extract allowed to obtain better performances under mesophilic conditions both in terms of sludge decontamination and disinfection. All the selected microbial population seemed to be involved in the biodegradation of the contaminants.