Potential nanosilver impact on anaerobic digestion at moderate silver concentrations

Silver nanoparticles (AgNPs, nanosilver) entering the sewers and wastewater treatment plants (WWTPs) are mostly accumulated in the sludge. In this study, we determined the impact of AgNPs on anaerobic glucose degradation, sludge digestion and methanogenic assemblages. At ambient (22 °C) and mesophilic temperatures (37 °C), there was no significant difference in biogas and methane production between the sludge treated with AgNPs at the concentrations up to 40 mg Ag/L (13.2 g silver/Kg biomass COD) and the control. In these anaerobic digestion samples, acetate and propionic acid were the only detectable volatile fatty acids (VFAs) and they were depleted in 3 days. On the other hand, more than 90% of AgNPs was removed from the liquid phase and associated with the sludge while almost no silver ions were released from AgNPs under anaerobic conditions. Quantitative PCR results indicated that Methanosaeta and Methanomicrobiales were the dominant methanogens, and the methanogenic diversity and population remained largely unchanged after nanosilver exposure and anaerobic digestion. The results suggest that AgNPs at moderate concentrations (e.g., ≤40 mg/L) have negligible impact on anaerobic digestion and methanogenic assemblages because of little to no silver ion release.