Effect Of Zero-valent Iron On Tetracycline Resistance Gene In Aerobic Biological Treatment Of Sewage And Hydrothermal Treatment Of Sludge

The accumulation of antibiotics in the environment will promote the enrichment of antibiotic resistance genes?ARGs?in environment and threaten human health.Antibiotics discharged during the daily use flow into the sewage treatment plant through the drainage pipe network,making the sewage biological treatment system become an important place for ARGs enrichment and dissemination.Excess sludge is the main reservoir of ARGs,and discharging it directly would pose a great threat to the environment.Zero-valent iron is widely used in sewage treatment,and the released iron ions are mainly enriched in the activated sludge.This paper investigates the effect of zero-valent iron on the enrichment of ARGs in sewage aerobic biological treatment.The influencing mechanism was explored from sludge morphology changes,extra-cellular polymeric substance?EPS?and microbial community structure.At the same time,the effect of iron on the reduction of ARGs during the hydrothermal treatment of sludge is discussed,and the mechanism is explored from the aspect of the content of organic matter in hydrothermal supernatant,the protection of iron and organic matter on DNA during hydrothermal reaction.Since hydrothermal supernatant has ARGs which released from sludge phase,this paper further conducts experiment on the reduction of ARGs in hydrothermal supernatant via advanced oxidation processes,and explores the effect of hydrothermal supernatant on ARGs abundance when it returns to sewage biological treatment system.Concluded as follow:In aerobic wastewater treatment systems,TCs can significantly increase the absolute abundance and relative abundance of ARGs in sewage and sludge.Zero-valent iron can promote the enrichment of ARGs in the first 40 days.After 40 days,the promotion effect of zero-valent iron is not obvious,and the abundance of some ARGs even declined.Mechanism analysis shows that TCs and zero-valent iron would change the sludge surface structure and the EPS content.TCs can destroy the integrity of the sludge surface structure,which would reduce the EPS content,and let the cells to become dispersed.The spheroid bacteria will appear on the surface of the zero-valent iron group sludge.The content change rule of sludge EPS was:control group>Fe group>Fe+TCs group>TCs group.The EPS of TCs group was significantly reduced,which could promote the horizontal transferring of ARGs between microorganisms,thus promoting the enrichment of ARGs in sewage and sludge.At the initial stage?20d?,the introduction of zero-valent iron made the biodiversity of sludge significantly improve.However,the sludge microbial diversity decreased probably due to the accumulation of iron in sludge at 90 days.The introduction of TCs further reduces the diversity.At 20 days,the relative abundances of Bacteroidetes and Planctomycetes in TC-ARGs in the zero-valent iron group were significantly increased,meaning that zero-valent iron may promote the enrichment of ARGs.At 90 days,the relative abundance of Comamonas in all reactor increased,which was showed:TCs+Fe>Fe>TCs>blank,which was consistent with the relative abundance of tetX in each reactor.After hydrothermal reaction of sludge,the abundance of ARGs in sludge phase decreased significantly,while the ARGs in the liquid phase increased remarkably.The abundance of ARGs in solid and liquid phase in the Sludge-Fe group was both lower than that in the Sludge-C group.When adding Fe²⁺,Fe³⁺and reduced iron powder directly into the control sludge,they also promoted the reduction of some ARGs?tetA,tetC,tetM?during hydrothermal treatment of sludge.And the promoting effects of Fe²⁺and Fe³⁺were more obvious.Based on the sum of ARGs in sludge phase and liquid phase,the total amount of ARGs in the Sludge-Fe group was reduced more,and the proportion of ARGs in the hydrothermal supernatant was higher.The organic matter in the hydrothermal supernatant could protect the DNA.The iron can reduce the content of organic matter in supernatant during hydrothermal treatment,thus reducing the protection effect of organics on DNA and improving the ARGs reducing efficiency.During the treatment of hydrothermal supernatant,three advanced oxidation processes?Fenton,UV/H₂O₂ and O₃/H₂O₂?showed the reduction effect on ARGs in hydrothermal supernatant.Among them,Fenton had the best reduction effect.After hydrothermal reaction and advanced oxidation processes,ARGs of Sludge-Fe group was obviously lower than that of Sludge-C group,indicating that the introduction of zero-valent iron into the sewage biological treatment system benefited the reduction of ARGs in sludge.Hydrothermal supernatant returning to the sewage biological treatment system would slightly reduce the degradation rate of COD,and the ARGs abundance would increase to some extent.The hydrothermal supernatant of Sludge-C group increases the abundance of ARGs of sludge in sewage biological treatment system by0.39-0.82 logs,while only 0.17-0.6 logs increased for hydrothermal supernatant of Sludge-Fe group.