Influence Of Nanoscale Zero Valent Iron And Magnetite Nanoparticles On Sludge Anaerobic Digestion Performance And Resistance Genes Reduction

With the growth of urban population and the acceleration of urbanization,the output of municipal sewage is larger than before,which is accompanied by a large amount of waste activated sludge（WAS）.Moreover,WAS contains lots of emerging pollutants such as antibiotics,which is conducive to the enrichment and proliferation of antibiotic resistance genes（ARGs）.If sludge is discharged without disposal,it will have a great impact on the environmental microbial community.As an important way of sludge harmless and reduction,anaerobic digestion（AD）is used to utilize and convert the organic matter in sludge into clean energy methane through the decomposition of microbe.However,the traditional AD process still has some problems,such as the difficulty of sludge hydrolysis and low efficiency of organic matter degradation.In addition,the ARGs could not be effectively removed after the traditional anaerobic digestion.In this study,the enhancement effect of nanoscale zero valent iron（NZVI）and magnetite nanoparticles（Fe₃O₄NPs）was investigated at three levels,including biogas production,microbial community evolution and resistance gene reduction.The main conclusions of this study are as follows:1.The hydrolysis performance and gas production efficiency in different anaerobic digester with different types and concentrations of iron nanoparticles were investigated in this study.The optimal dosage of Fe₃O₄NPs and NZVI was 0.5 g/L and 1.0 g/L,whose cumulative biogas production was increased by 24.44%and 21.66%,respectively.The p H value of all the digesters remained at a relatively stable level,fluctuating between 7.37 and 7.81 during 100-day AD process,and no obvious rancidity was observed in any digesters.Moreover,both the addition of Fe₃O₄NPs and NZVI promoted the hydrolysis process of sludge,and the removal rate of VS and s COD was also improved compared with the control group.The addition of two kinds of iron nanoparticles enhanced the performance of anaerobic digestion.2.The evolutions of microbial community in different digesters were investigated based on the 16S rRNA amplicon sequencing technology.The correlation and response mode between major environmental factors and microbial genus were also explored during AD process.It was found that the addition of two kinds of iron nanoparticles affected the structure of microbial community,especially the archaea community.The addition of Fe₃O₄NPs greatly increased the abundance of Methanosaeta,which is an obligate acetoclastic methanogens.And the abundance of Methanobacterium,a hydrogenotrophic methanogens,had increased with the addition of both Fe₃O₄NPs and NZVI.The redundancy analysis（RDA）of microbial community and environmental factors revealed that dominan genera,such as Gelria,Lutispora and Proteiniclasticum didn’t have a good correlation with biogas production.While the less abundant vadin BC27 and UCG-013 are closely related to biogas production,suggesting that these may be the key microbial community driving biogas production in anaerobic digesters.3.The abundance of five representative ARGs（ermF,ermA,ermT,aac（6’）-IB,blaOXA-1）and class I integron（intI1）in sludge was measured through real-time quantitative PCR（q PCR）.It was found that the average reduction rates of aac（6’）-IB and blaOXA-1 reached95.69%and 55.18%,respectively,but the abundance of three macrolide resistance genes（ermF,ermT,ermA）was enriched after 100-day anaerobic digestion.Different kinds of resistance genes showed distinct evolution trends during AD process.In addition,most of the ARGs had lower abundance in the NZVI group than in the control group.This may attribute to the cytotoxicity of NZVI.The vertical gene transfer was the main mechanism of ARGs proliferation in this study.NZVI can interact with key components of the cell membrane,such as functional proteins,causing a damage of cell membrane and destroying the cell integrity of potential hosts.Therefore,the vertical gene transfer of ARGs completed by the reproduction of hosts was weakened,which was conducive to the reduction of ARGs in the sludge.