Contradictory effects of silver nanoparticles on activated sludge wastewater treatment

•Ag-NPs, especially freshly prepared, can have positive effects.•Ag-NPs can help to maintain microbial community diversity in activated sludge.•Improved sludge settleability can be important in the positive effects observed.•The hormesis model may need to be considered for the toxicology of Ag-NPs.

Increased amount of nano-silver will be released into domestic and industrial waste streams due to its extensive application. However, great controversy still exists on the effects of silver nanoparticle (Ag-NP) on biological wastewater treatment processes and a toxicology model has not been built yet. Four sequencing batch reactors with activated sludge has been run for over three months with different silver species at a concentration of 1 mg Ag/L in influent. Both freshly prepared Ag-NPs and aged Ag-NPs were tested with released silver ion as control. Results in this study showed that Ag-NPs, especially freshly prepared Ag-NPs, can help to maintain or even increase the diversity of microbial community in activated sludge and the biomass concentration even under long-term treatment. It indicates that the hormesis model need to be considered for the toxicology of Ag-NPs.