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Occurrence Characteristics Of Heavy Metals In Municipal Sewage Sludge In China And Establishment And Application Of Vulnerability Index Ecological Risk Assessment System

Posted on:2023-10-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y M GengFull Text:PDF
GTID:1521307088974469Subject:Geological Resources and Geological Engineering
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Ecological risk assessment of heavy metals(HMs)in municipal sewage sludge(MSS)has always been the focus of attention.In this paper,the Ei-AHP model and RAC-AHP model were respectively constructed based on the principle of analytic hierarchy process and the combination of potential ecological risk index and ecological risk coding.The content of heavy metals in sludge,toxicity and biological validity and soil background values were normalized to build an urban vulnerability index model of heavy metal sludge,and coupled by Arc GIS,a comprehensive ecological risk assessment system based on the vulnerability index method was established.In the presented study,de-watered MSS samples were collected from 40 MSTPs located in 40 cities of China in the 2016 sampling campaign.Based on the analysis of HMs contents in the MSS samples,spatial distribution and temporal variance of HMs in MSS in China were discussed.The BCR protocol was used to investigate the occurrence forms of HMs in MSS,followed by the estimation of transport potentials.By applying the newly established vulnerability index method,the ecological risk of HMs in MSS and the ecological risk caused by heavy metals contained in sludge for agricultural use were evaluated.The main results are presented as follows.(1)Combined with Ei-AHP model and the RAC-AHP model,the contents of HMs in MSS,biotoxicity,bioavailability and soil background values were considered.Coupled by Arc GIS,a comprehensive evaluation system was established on the basis of the vulnerability index method,which could give a more accurate evaluation of the level of ecological risk of HMs in MSS.(2)The median values of HMs in MSS decreased in the order Zn>Cu>Cr>Pb>Ni>As>Hg>Cd.As and Pb in all the SS samples met the requirement of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant(GB18918-2002),and the attainment rates for the other 5 elements were also satisfying(>92.5%).In general,the contents of HMs in MSS followed a decreasing trend from south to north,and from east to west of China.The average value of HM content in MSS of different scale cities is in the following order: type Ⅰ big cities > type Ⅱ big cities > megalopolis > super-large cities > medium and small cities.A comprehensive analysis of historical data since 1992 revealed an obvious decreasing trend in HM contents of MSS along with time.(3)Obvious differences in the occurrence forms were observed among HMs in MSS.As was evenly distributed among all the four fractions.Cd and Zn mainly occurred as reducible forms.Cr occurred primarily in residual fraction,followed by oxidizable and reducible fractions.Cu mainly occurred as oxidizable forms.Ni in MSS occurred mainly in exchangeable fraction.Pb was mainly distributed in the reducible and residual fractions,with the exchangeable fraction accounting for the lowest portion.The migration potentials of the 7 heavy metals in MSS followed an order of Cd > Zn >Ni > Pb > As > Cu > Cr.(4)With the vulnerability index method,the ecological risk of heavy metals in sewage sludge from 40 cities in China was comprehensively assessed.The risk assessment results showed that among these cities,Ningbo is "risk-free",Liaoning,Hohhot and other five cities belong to the "low risk",Suzhou,Taiyuan and other 20 cities belong to "medium risk",11 cities such as Zhengzhou,Guiyang belong to "high risk",Xinyu and Dalian belong to "extremely high risk";areas with higher pollution levels are concentrated in the central and southeastern regions and several cities along the coast.The evaluation result was lower than that of ER method.The vulnerability index method takes into account both heavy metal content and bioavailability of sludge,and considers Ei and RAC results in the selection of weight,so the evaluation results are more scientific.(5)The risk of soil pollution is generally at a low level when agricultural use is carried out in accordance with the Standards for Pollutants Control in Agricultural Sludge(GB4284-2018).After 10 and 20 years of sludge agricultural use,the potential ecological risk increased,but all the other elements except Cd belonged to "low risk".According to the evaluation results,most urban sludge in China can be used in agriculture,but some cities still have certain ecological risks,mainly due to the high content of Cd in sludge.Therefore,corresponding attention should be paid to agricultural use to ensure agricultural safety.39 figures,31 tables,184 references.
Keywords/Search Tags:occurrence characteristic, analytic hierarchy process, vulnerable index method, ecological risk assessment, municipal sewage sludge
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