Effects of silver nanoparticles on microbial community structure in activated sludge

Due to the antimicrobial properties and the widespread use of Ag nanoparticles (NPs) in commercial products, the prevalent Ag NPs in waste streams can decrease the effectiveness of biological wastewater treatments. To determine the effects of Ag NPs on the complex microbial communities present in activated sludge, detailed knowledge of the Ag NPs toxicity on microorganism communities is necessary. Experiments were performed to determine the effects of 1mg/L Ag NPs on microbial communities in activated sludge. Activated sludge samples with and without gravity settling were compared to evaluate the impact of activated sludge flocs structure on the response of microbial communities to Ag NPs. The effects of Ag NPs on the entire microbial community in activated sludge were analyzed using 16S rRNA gene based polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). The results suggest that certain microbial species in the intact activated sludge were highly sensitive to Ag NPs treatment, although no reduction in cell culturability was detected through heterotrophic plate counts (HPCs) during the 24h Ag NPs treatment. Conversely, one log unit reduction in the HPCs with no microbial community structure changes was observed for unsettled activated sludge flocs (intact activated sludge treated by 3h gravity-settling) after 24h Ag NPs treatment. This study strongly suggests that Ag NPs can impact the activated sludge microbial community and cell culturability depending on the physical structure of the activated sludge flocs, the spatial distribution of microorganisms in activated sludge flocs, and the community structures in the activated sludge.