| With the processing of civilization and increase of environmental quality standard in China,the amount of municipal wastewater drastically increased which largely increased the production of municipal sewage sludge.Our country has produced more than 5.00×107 tons of municipal sewage sludge with 80%moisture content in 2018,and their producing amount will beyond 6.00×107 tons in 2019.More than 67%of sewage sludge were treated and disposed by landfill or land application,and but the high moisture content of sewage sludge severely limit their treatment and disposal.Meanwhile,abundant antibiotic resistance genes(ARGs)can be detected in sewage sludge,and its antibiotic resistance has been one of the biggest problem of public health in the world.However,the traditional treatment technologies of sewage sludge,including anaerobic digestion and aerobic composting,were ineffective in eliminating ARGs,and thus ARGs could release into soil environment during the subsequent disposal of sewage sludge,which led to potential environmental risk.Therefore,it is urgent to find the appropriate treatment technologies of sewage sludge to effectively remove moisture and ARGs from sewage sludge,which reduce the cost,difficulty and potential environmental risk during its treatment and disposal.The mechanical dewatering of sewage sludge is indispensable step of its treatment and disposal,and but sewage sludge could not be directly dewatered due to its poor dewaterability.Thus,sewage sludge must be conditioned prior to its dewatering for improving its dewaterability,including dewatering rate and dewatering extent.In recent years,many studies found that some sewage sludge conditioning treatments could resulted in the cell lysis of sludge microbial cells to improve sludge dewatering extent,and interestingly the cell lysis of sludge microbial cells also promote the elimination of ARGs in sewage sludge.In summary,the present study compared effects of different conditioning treatments on sewage sludge dewaterability and ARGs in sewage sludge,to find a appropriate conditioning treatment which simultaneously improve sewage sludge dewatering rate and dewatering extent,and effectively reduce the environmental risk of ARGs in sewage sludge.This study firstly developed a filtration-compression cell to accurately evaluate sewage sludge dewatering rate and dewatering extent;secondly compared effects of different conditioning treatments on sewage sludge dewatering rate and extent,including bioleaching,chemical acidification,ultrasonication,microwave,freezing/thawing,and chemical conditioning by cationic polyacrylamide(CPAM);finally evaluated the effectiveness of four sludge conditioning methods(namely chemical conditioning with polyacrylamide(PAM),chemical conditioning with Fe[?]/CaO,bioleaching conditioning,and chemical acidification conditioning)in eliminating 46 target ARGs and intIl in municipal sewage sludge,and detected the relative and absolute abundance of ARGs and intI1 during the composting of raw sludge and the above conditioned sludge.The related results are shown as follow:(1)A filtration-compression cell was made in this study to simulate the real dewatering process,which can comprehensively evaluate sewage sludge dewaterability at the filtration and expression stages of its compression dewatering.The investigating results indicate that the addition of Fe3+ can improve sewage sludge dewatering rate at the filtration and expression stages,and but has little influence on its dewatering extent.Oppositely,the chemical acidification conditioning can improve sewage sludge dewatering extent,and but cannot improve its dewatering rate at the filtration and expression stages.Thus,the evaluating system of sewage sludge dewaterability based on the filtration-compression cell can accurately evaluate the dewatering rate and dewatering extent of sewage sludge,which is meaningful to selecting appropriate and effective conditioning approaches for improving sewage sludge dewaterability.(2)Chemical acidification conditioning effectively improves sewage sludge dewatering extent,and but seriously reduces its dewatering rate.During the chemical acidification of sewage sludge,serious lysis of microbial cells in sewage sludge was induced to release a large amount of bound water,which improve sewage sludge dewatering extent.However,the lysis of microbial cells in sewage sludge also led to the drastic increase of outer layer FPS content and serious deflocculating of floes in sewage sludge,which severely reduce sewage sludge dewatering rate.Further studies revealed that either decreasing outer layer EPS content or modifying the deflocculated flocs in sewage sludge could improve the dewaterability of acidified sewage sludge,especially,only 5 mg/g dry solid(DS)of anionic synthetic organic polyelectrolyte polyacrylic acid(PAA)could condition acidified sewage sludge to improve its dewatering rate at both filtration and expression dewatering stages via re-flocculating the deflocculated flocs in acidified sewage sludge.(3)The bioleaching conditioning simultaneously improve sewage sludge dewatering rate and dewatering extent,which was not achieved by other physical/chemical conditioning approaches in this study,including ultrasonication,microwave,freezing/thawing,or by adding the chemical conditioner cationic polyacrylamide(CPAM).During the bioleaching of sewage sludge,the decrease of sludge pH severely damaged sludge microbial cell to release lots of water,which enhanced the dewatering extent of sewage sludge,and the bio-oxidized Fe3+ effectively flocculated the damaged sludge flocs and absorbed outer layer EPS in sewage sludge to improve the sludge dewatering rate.Therefore,the bioleaching conditioning improve sewage sludge dewatering extent due to the decrease of sewage sludge pH,and improve sewage sludge dewatering rate due to the appearance of Fe3+.(4)The chemical conditioning with PAM and aerobic incubation treatment did not remove ARGs and intI1 from the sewage sludge.The chemical acidification reduced the absolute abundances of most ARGs and intI1,but increased their relative abundances.However,the chemical conditioning with Fe[?]/CaO and bioleaching conditioning reduced both the absolute and relative abundances of most ARGs and removed a majority of extracellular ARGs in the sewage sludge.Therefore,sludge conditioning can be an important sludge treatment process in attenuating antibiotic resistance in sewage sludge,and bioleaching and chemical conditioning with Fe[?]/CaO can be employed as effective conditioning ways to eliminate ARGs in sewage sludge which potentially limiting their release to the environment.(5)The chemical acidification,chemical conditioning with Fe[?]/CaO and bioleaching conditioning severely damaged microbial cell to reduce total bacterial biomass in sewage sludge,and accordingly the bacterial hosts carrying ARGs and intI were effectively damaged to attenuate most ARGs and intI1 in sludge.Meanwhile,the bacteria belonging to the genus of Clostridium,Microbacterium,and Prevotella,as the main potential hosts of floR,vanC-03,tet(34)and tetM-01,had stronger surviving ability in sewage sludge conditioned the chemical acidification,chemical conditioning with Fe[?]/CaO and bioleaching conditioning,which led to the drastic increase of their relative abundance after the above conditioning treatments.Therefore,sludge conditioning treatments could reduce the absolute abundance of ARGs via killing their bacterial hosts,and the sludge bacterial community in conditioned sludge determined the relative abundances of residual ARGs.(6)Raw sludge,bioleached sludge and PAM-conditioned sludge could be effectively composted and their compost products were stable,mature and phytotoxin-free,and but the composting effectiveness of Fe[?]/CaO-conditioned sludge was poor possibly due to the addition of lots of inorganic matters in to sewage sludge.In addition,the relative and absolute abundances of all ARG subtypes in the compost product of bioleached sludge were greatly lower than that in the compost product of raw sludge,which was not obtained in the compost product of other conditioned sludge.During the composting of raw sludge,PAM-conditioned sludge and Fe[?]/CaO-conditioned sludge,the horizontal gene transfer of ARGs led to the increase of ARGs abundance in their composting product.However,the horizontal gene transfer of ARGs was inhibited during the composting of bioleached sludge,which kept low abundance of ARGs in its composting product.Thus,the bioleaching conditioning of sewage sludge can effectively eliminate ARGs in its compost product.Therefore,sewage sludge conditioning can be an important treatment process in improving sewage sludge dewaterability and attenuating antibiotic resistance in sewage sludge.Especially,the bioleaching conditioning not only improve sewage sludge dewatering rate and dewatering extent,but also effectively eliminate most of ARGs and intI1 in sewage sludge and the composting product of bioleached sludge.The results of this study have an important guiding significance for the treatment and disposal of sewage sludge. |
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