Impact Of Sludge Conditioning On The Abundances And Transfer Abilities Of Antibiotic Resistance Genes In Sewage Sludge

With the widespread use of antibiotics in livestock farming and the medical industry,public health problems caused by antibiotic resistance genes(ARGs)in the environment have drawn more and more attention.ARGs in the environment induce the emergence of antibiotics resistance bacteria(ARB),which pose a threat to human health.ARGs are difficult to be degraded naturally in the environment and can be transported in water environments,thus accumulating in urban sewage treatment plants through water cycles and eventually accumulating in sludge.During the sanitary landfill or land-application of sludge,ARGs will be released to the environment and bring certain environmental risks.Therefore,sludge treatment is an important step in controlling the spread of ARGs in the environment.At present,there are a large number of studies on the effects of conventional sludge treatment technology on ARGs,mainly focusing on anaerobic digestion technology and aerobic composting treatment.Although anaerobic digestion exhibits removal effect on ARGs,its application has greater limitations because of its longer treatment cycle and lower efficiency.The aerobic composting of sludge even increases the abundance of ARGs.The sludge conditioning dewatering is an important step in the sludge treatment and disposal.Sludge conditioning can effectively kill pathogenic microorganisms in the sludge,but the impact on ARGs is still unclear.Therefore,this study explored the impact of four sludge conditioning methods(chemical conditioning with polyacrylamide(PAM),chemical conditioning with CaO,chemical acidification conditioning and bioleaching conditioning)on ARGs in sewage sludge,and tried to explain it from three aspects including the abundance of ARGs,abundance of ARB and transfer abilities of ARGs.It is expected to find a sludge conditioning method that can effectively limit the spread of ARGs in the environment.Through a series of experiments,the results are shown as follow:(1)The chemical conditioning with PAM had no significant effect on the aminoglycoside ARB in both sludge cake and filtrate.Chemical acidification conditioning,bioleaching conditioning and chemical conditioning with CaO could remove ARB from sludge cake by destroying microbial cells.Bioleaching conditioning and chemical conditioning with CaO could also completely remove ARB from the filtrate,but chemical acidification conditioning led to the release of sludge extracellular polymeric substances(EPS)and the increase of suspended solids(SS)in the filtrate,which increased the amount of ARB in the filtrate.Although chemical acidification conditioning,bioleaching conditioning,and chemical conditioning with CaO could reduce ARB in the sludge cake,the density of aminoglycoside ARB increased substantially as the sludge cake was placed for a longer period of time.(2)The number of subclass and abundance of aminoglycosides ARGs in sludge were the highest.Chemical conditioning with CaO,chemical acidification conditioning and bioleaching conditioning significantly reduced the absolute abundances of ARGs in the sludge,and chemical conditioning with PAM had no significant effect.Chemical conditioning with PAM and chemical conditioning with CaO had little effect on the relative abundances of ARGs.Chemical acidification conditioning and bioleaching conditioning treatment reduced the relatives abundances of aminoglycoside ARGs,but increased the relative abundance of other types of ARGs.Meanwhile,chemical acidification conditioning and bioleaching conditioning significantly changed the microbial community structure in the sludge.It indicated that chemical conditioning with CaO,chemical acidification conditioning and bioleaching conditioning could effectively control the absolute abundances of ARGs in sludge,but chemical acidification conditioning and bioleaching conditioning increased the relative abundances of some ARGs due to changes in the microbial community structure in the sludge.(3)There was no conjugative transfer of ARGs in the filtrate of the conditioned sludge.In the sludge cake,conditioning with PAM could not affect the conjugative transfer of ARGs.Chemical conditioning with CaO completely inhibited the transfer of three kinds of ARGs(gentamicin,erythromycin and tetracycline).Chemical acidification conditioning and bioleaching conditioning could reduced the transfer frequency of erythromycin and gentamicin ARGs,but it increased the transfer frequency of tetracycline ARGs.Five potential host plasmids for the tetracycline ARGs were screened from the tetracycline transconjugants.(4)During sludge conditioning,the relative abundance of the genes tet(34),genes tetO-2,plasmids pLDW-9 and plasmids Pap 13 were significantly correlated with the frequency of tetracycline transconjugants.At the same time,the relative abundances of plasmids pLDW-9 and Pap 13 were significantly correlated with the relative abundances of genes tet(34)and tetO-2,which indicated that plasmids pLDW-9 and Pap13 carrying tetracycline ARGs tet(34)and tetO-2 were involved in the transfer.Therefore,chemical conditioning with CaO could greatly remove ARGs and ARB in sludge and completely inhibited the transfer abilities of ARGs.It was the most effective conditioning approach that can control the spread of ARGs in conditioned sludge.Conditioning with PAM was incapable of affecting ARGs in conditioned sludge.Although chemical acidification conditioning and bioleaching conditioning could remove ARB and most ARGs,they increased the transfer frequency of tetracycline ARGs due to their inability of reducing plasmids pLDW-9,Pap 13 and tetracycline ARGs tet(34)and tetO-2 in conditioned sludge.