| AgNPs are widely used in nanomaterials because of their broad-spectrum antibacterial and antiviral properties,but they also increase the risk of releasing AgNPs into the environment.AgNPs are directly discharged into environmental water bodies during production,use,and waste disposal,and eventually flow into sewer systems and wastewater treatment plants,affecting the normal life activities of activated sludge,degrading the effluent quality of wastewater treatment plants,and further polluting natural water bodies and posing risks to ecosystems.The highest detected concentration of AgNPs in wastewater treatment plants worldwide is as high as 20.2 mg/L.In this paper,the SBR reactor was set up with different concentrations of AgNPs,and the effects of different concentrations of AgNPs on the inhibition of the biological nitrification process and biological mechanism were investigated by short-term exposure,long-term exposure,and high-throughput sequencing techniques.The concentration of Ag~+released from the highest concentration of AgNPs was also used as a control group to compare and analyze the source of toxicity of AgNPs inhibiting the biological nitrification process.The main results obtained in this paper are as follows:(1)AgNPs are effective inhibitors of the biological nitrification process in SBR systems.Short-term experimental results showed that low concentrations of AgNPs(<0.5mg/L)and 0.3 mg/L Ag~+had almost no effect on the nitrogen removal efficiency of the SBR system during the aerobic nitrification stage.However,with the gradual increase of AgNPs concentration,the nitrogen removal efficiency of the SBR system was gradually suppressed,and when the AgNPs concentration was 3.0 mg/L,the nitrogen removal efficiency in the system was almost zero.In the anaerobic denitrification stage,the effluent COD concentration increased with the increase of AgNPs concentration.(2)AgNPs affect the activity of nitrifying bacteria,sludge performance,EPS,and activated sludge morphology in the SBR system.The results of long-term experiments showed that AgNPs at 0.5 mg/L had no significant effect on the activity of nitrifying bacteria in the SBR system,but the nitrogen removal efficiency of the SBR system gradually decreased with increasing AgNPs concentration(>0.5 mg/L).In addition,Ag~+at 0.2 mg/L had almost no effect on the SBR system.The relative biomass changes in the AgNPs and Ag~+control groups at 0.5 mg/L were not significantly different from the blank control group,but the inhibition of microbial activity and growth and reproduction gradually increased with increasing AgNPs concentration(>0.5 mg/L).AgNPs affect the composition of EPS within the SBR system,with an overall decreasing trend under long-term exposure,and the most obvious decreasing trend for AgNPs at 2.0 mg/L,with a significant decrease in PN and PS content.SEM analysis showed that AgNPs and Ag~+at0.5 mg/L had little effect on the flocculent structure,and as the concentration of AgNPs(>0.5 mg/L)increased,the cells began to collapse and crack to varying degrees.(3)Both AgNPs and Ag~+altered microbial diversity,species composition,and community structure.Low concentrations of AgNPs(0.5 mg/L)and Ag~+at 0.2 mg/L stimulated microbial diversity to increase against external shocks,but high concentrations of AgNPs(>0.5 mg/L)decreased microbial diversity.At the phylum and genus level,AgNPs at 0.5 mg/L promoted the growth of nitrifying bacteria,and AgNPs at 2.0 mg/L showed inhibitory effects on nitrifying bacteria.Lef Se analysis showed that both AgNPs and Ag~+changed the dominant species in the system.In addition,correlation network analysis showed that microorganisms in the SBR system mostly showed synergistic interactions to achieve organic matter removal together.Meta Cyc network database analysis showed that there were mainly 7 metabolic pathway functions in the primary metabolic function pathway,and biosynthesis showed significant superiority;there were57 metabolic pathway functions in the secondary metabolic function pathway,and the biosynthesis of cofactors,cofactors,electron carriers,and vitamins dominated the secondary metabolic pathway. |
