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.