Silver nanoparticles (AgNPs) are largely discharged into sewers and mostly accumulated in the sediments and sludge. The toxicity of AgNPs to environmental microorganisms has attracted great attention. However, the effect of AgNPs on anaerobic ammonium-oxidizing (anammox) granules remains unknown. Here we present the underlying promotion mechanism of AgNPs on anammox granules from a morphological and molecular biology perspective. Our results demonstrate a positive effect of AgNPs on the proliferation of anammox bacteria. AgNPs resulted in a change in the three-dimensional structure of anammox granules and led to larger pore size and higher porosity. In addition, the diffusion capacity of the substrate and metal ions was enhanced. Furthermore, the expression of anammox-related enzymes, such as nitrite oxidoreductase (NirS), hydrazine dehydrogenase (Hdh), and hydrazine synthase (HZS), was upregulated. Therefore, the growth rate and the nitrogen removal performance of the anammox granules were improved. Our findings clarify the underlying mechanism of AgNPs on anammox granules and provide a promising method for the treatment of AgNPs-rich wastewater.
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