Effect Of Heavy Metals On The Performance Of SBR Reactor And Microbial Flora

In this study,three types of heavy metals including Cu2+,Hg2+ and Ag+ were studied by simulating typical wastewater treatment systems in the laboratory.Under short-term impact,long-term impact and recovery conditions,they affect the treatment performance of conventional pollutants in the system,key microbial enzymes activities,molecular weight and composition of extracellular polymers substances,and microbial diversity and community structure.It is intended to clarify the close relationship between reactor processing efficiency and microbial driven feedback mechanisms from multiple levels and from multiple perspectives.The main conclusions of the study are as follows:(1)The short-term shock test results show that the heavy metals Cu2+,Hg2+ and Ag+ have no significant effect on the treatment efficiency of the SBR system at low concentrations(<0.08 mM).However,as the shock concentration increases,the removal rates of DOC,NH4+-N,and PO43--P gradually decrease,and Hgng has a more pronounced effect on system performance.The specific oxygen uptake rate of activated sludge and microbial enzymes analysis data showed that the changes of microbial soil dehydrogenase(sDHA)activity and adenosine triphosphate(ATP)content were correlated with DOC removal rate.The reasons of heavy metal’s effect on system denitrification ability are related to the inhibition of some biological denitrification key enzymes.With the increase of the impact concentration of heavy metals,it was shown that higher concentrations of heavy metals(>0.11 mM)would cause oxidative damage to cells,but the integrity of the microbial cell surface was not significantly damaged.Based on the short-term shock data,the long-term shock concentrations of heavy metals Cu2+,Hg2+ and Ag+ were 0.05mM,0.0mM and 0.02mM,respectively.(2)The experimental results of the effects of long-term impact on the treatment performance of conventional contaminants in the reactor show that the removal rate of DOC,NH4+-N,and PO43--P gradually reduce along with the shock time from the third day after shock.During the typical cycle of after 15 days and 30 days of shock,it is found that Cu2+ has the most serious effect on the removal rate of DOC.Cu2+ and Hg2+ obviously inhibited the oxidation of ammonia.While Ag+ mainly inhibited the oxidation of nitrite.Cu2+ had an inhibitory effect on anaerobic phosphorus release and aerobic phosphorus uptake,relatively.The effects of Hg2+ and Ag+ were not obvious.The data of the impact recovery period showed that after the 60 days of heavy metal was stopped,the system completely recovered the DOC and PO43-P removal performance.The NH4+-N treatment performance of the reactor impacted by Hg2+ basically recovered,but the reactors denitrification ability impacted by the Cu2+ and Ag+ gradually recovered and still not fully recovered to the pre-impact level.(3)The experimental results of the effect of long-term shock on extracellular polymer substances of activated sludge show that the organic carbon content of the extracellular polymer substances is increased after the shock of three heavy metals such as Cu2+,Hg2+ and Ag+.In the change of molecular weight distribution,the content of biopolymers in SMP increased,and the humic substances and building blocks of EPS increased.Different heavy metals have different effects on components in SMP and EPS.The results of three-dimensional fluorescence spectroscopy showed that Cu2+ had the most severe effects on SMP,and the effect of Hg2+ on EPS is most obvious.In the recovery period,the extracellular polymer substances components and levels of the activated sludge system gradually recovered,and they basically recovered to similar levels with the control group at 60 days after recovery.(4)The experimental results of the effect of long-term shock on microbial activity and cytotoxicity-related enzyme activity showed that the long-term impact of Cu2+,Hg2+,Ag+ caused the inhibition of microbial denitrification-related enzyme activities to varying degrees.The effect of Cu2+ was the most significant.The results of enzyme activity characterizing cytotoxicity showed that with the accumulation of heavy metal ions under continuous impact conditions,the ROS level of cells continued to increase,especially in the Cu2+ and Ag+ shock groups.At the same time,the activity of anti-oxidation kinase should increase with the ROS level.After being impacted by Cu2+ for 30 days,the release of LDH from cells increased significantly,that is the cell surface integrity.The SEM scanning electron microscopy analysis of activated sludge morphology showed that the surface of the sludge was intact and compact before the impact,and most of the Cocci.After being impacted by heavy metal ions,the cell surface becomes rough and the bacterial mass becomes loose,with the majority of bacilli and filamentous bacteria.With the accumulation of heavy metals in the system,when the impact of Cu2+ and Hg2+ was restored for 15 days,the surface of the bacteria could still be observed uneven and some cells were significantly broken.(5)The experimental results about the effects of long-term impact on microbial diversity and community structure showed that the number of OTUs in the sample was significantly reduced in the Cu2+ impact group at 15 days and 30 days.Alpha diversity analysis showed that the Chao1 index,ACE index and Shannon index were significantly reduced,indicating that Cu2+ significantly reduced the microbial community richness and diversity.During the recovery period,the diversity gradually recovered;the Beta diversity analysis showed that the species composition of samples after 15 days and 30 days of Cu2+ shock and 60 days after Ag+ recovery was different from other groups.Principal component analysis based on the abundance of OTU also found that the Cu2+impacted samples had the largest changes in community composition before and after impact,and the changes were most significant at 15 days and 30 days after impact.At the same time,the microbial community structure has undergone significant changes after long-term heavy metal impact.An the level of phylum,at 60 days after shock recovery,the relative content of dominant bacteria in the impact group was significantly reduced,while the Actinobacteria bacteria content increased.At the level of genus,the dominant bacterium Zoogloea in the heavy metal impact group was significantly reduced at the 15th day of shock,At 15 days and 30 days after shock,the contents of the genus Candidatus_Xiphinematobacter and Dechloromonas increased in the Cu2+shock group.