| The sewage treatment capacity of urban sewage treatment plants has increased year by year,and the amount of sludge produced has increased sharply.The sludge contains a certain amount of heavy metals.Heavy metals are difficult to degrade and easy to migrate.Once they enter the environment,they are likely to cause harm to the environment and human body.Therefore,heavy metals in sludge become one of the restrictive factor for sludge disposal.Poyang Lake Eco-economical District is a special economic zone in Jiangxi Province,with developed industry and dense population.At present,heavy metals in soil,water and other media in Poyang Lake Eco-economical District have been studied,but heavy metal pollution in sludge from urban sewage treatment plants in this region is relatively lacking.Therefore,paying attention to the characteristics of heavy metal pollution of sludge from urban sewage treatment plants in Poyang Lake Eco-economical District,discussing the possible sources of heavy metal pollution of sludge and evaluating its pollution degree can provide a reference basis for the utilization of sludge from urban sewage treatment plants in Poyang Lake Eco-economical District.In this study,11 dewatered sludge from sewage treatment plants in typical cities of Poyang Lake Eco-economical District such as Nanchang,Yingtan,Jingdezhen,Jiujiang and Gongqingcheng were selected as the research objects.The contents of Cu,Zn,Pb,Cd,Cr,Ni,Hg and As in the sludge were determined by the national standard method,and the different forms of heavy metals were extracted by BCR continuous extraction method,and the heavy metal pollution of sludge was analyzed by different source analysis method and combined with multiple pollution assessment methods.The main conclusions are as follows:(1)The average contents of heavy metals in the sludge of typical urban sewage treatment plants in Poyang Lake Eco-economical District are Zn,Cu,Pb,Cr,Ni,As,Cd and Hg,which are all higher than the soil background values of Jiangxi Province,and the average contents of Cd in the sludge are 50 times higher than the background values.The coefficient of variation of heavy metals Cu,Zn,Pb,Cd and Cr exceeds100%,belonging to strong variation.The contents of all heavy metal elements in Nanchang,Jingdezhen and Gongqingcheng are within the limits of the A-level standard sludge agricultural standard in GB 4284-2018.The elements of Cu,Zn,Pb,Cd,Cr,As in Yingtan and Cu,Pb,Cd,As in Jiujiang exceed the limits.(2)The result of source analysis shows that the heavy metals Cu,Zn,Pb,Cd,Cr and As in the sludge of the study area have strong correlation,and the pollution sources are similar.Among them,Cu,Zn,Pb,Cd and Cr may be greatly affected by the industrial distribution nearby,and the main source may be the industrial sector.The content of As in the detergent containing As is high,so As may come from domestic sewage,and Ni and Hg have little correlation with other heavy metals,Ni may come from the electronic electroplating industry.Hg is affected by the biological and pharmaceutical industry.The content of Hg in the pharmaceutical waste water is high,so Hg may come from the pharmaceutical industry.(3)The results of the comprehensive evaluation show that the average value of the Nemerow comprehensive pollution index of heavy metals in the sludge of typical urban sewage treatment plants in Poyang Lake Eco-economical District is 1.62,indicating that the heavy metals in the sludge are slightly polluted as a whole,which is based on the agricultural standards of sludge.The average geo-accumulation index of the whole study area is 1.96,reaching the moderate pollution level,and the pollution load index is 5.82,the degree of pollution is moderate pollution.The background values adopted by the two evaluation indexes were all the soil background values of Jiangxi Province,so the study area is at the moderate pollution level.The comprehensive potential ecological risk index of the whole study area is 467.43,and the pollution level is strong risks.Among the eight heavy metals,Cd and Hg are very strong risks.Although the content of Cd and Hg in the sludge are not high,their potential ecological risks are very harmful due to their high toxicity coefficient of heavy metals,and finally the comprehensive potential ecological risks of the study area are strong.(4)The results of morphological analysis and evaluation show that the heavy metals Cu in the sludge of the study area are mainly in the oxidizable fraction,Zn and Cd are mainly in the exchangeable fraction,and Pb,Cr,Ni,Hg and As are mainly in the residue fraction.The Individual Contamination Factor shows that Zn and Cu are highly polluted,Pb,As and Cd are moderately polluted,and the other heavy metals are at low pollution level.The Global Risk Index method shows that the average value of the comprehensive risk index of heavy metals in the sludge of typical urban sewage treatment plants in Poyang Lake Eco-economical District is 148.38,indicating that the heavy metals in the sludge are low risk on the whole.The Risk Assessment Code(RAC)method shows that Cd and Zn in the heavy metals in the sludge in the study area are high risks,As is medium risks,Cu,Ni and Hg are light risks,and Pb and Cr are no risks.(5)The total non-carcinogenic risk index of children in typical cities of Poyang Lake Eco-economical District is 4.02E-01,and the total non-carcinogenic risk index of adults is 1.45E-01,both of which fail to reach the risk threshold 1,indicating that heavy metals in sludge will not have very serious non-carcinogenic risk for children and adults.The total risk index of carcinogenesis in children is 1.32E-04,and the total risk index of carcinogenesis in adults is 4.68E-05,indicating that the carcinogenic risk caused by heavy metals in sludge to adults is not significant,which is acceptable,but the carcinogenic risk to children is beyond the acceptable range. |
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