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.