Optiminization Research On The Evalution Model Of The Pollution And Evironmetal Risk Of Heavy Metals In Municipal Sewage Sludge

The centralized treatment of municipal wastewater is a common trend in the development of domestic and foreign,and sludge is the main by-product of centralized sewage treatment.Sewage sludge contains a certain amount of heavy metals,which has become one of the important factors restricting the comprehensive utilization of sewage sludge due to its bioaccumulation and,low biodegradability.It is clear that the pollution characteristics of heavy metals in sewage sludge and environmental risk can provide important theoretical basis and data support for the advanced treatment and comprehensive utilization of sewage sludge.At present,the assessment methods of the pollution degree and ecological risk of heavy metals in sewage sludge have different classification,which can lead to large differences in evaluation results.Furthermore,the evalution methods of pollution degree and ecological risk of heavy metals in sewage sludge based on their chemical speciation are not perfect.Study on the health risk assessment of heavy metals in the sewage sludge is not deeply immplemented.Aiming at the above problems,this paper,takeing four major sewage treatment plant(HongGuTan,QingShanHu,XiangHu and ChaoYang)sludge in Nanchang city as the research object,carried out the following studies:Firstly,the occurrence characteristics of six kinds of heavy metals in sewage sludge were analyzed,including Zn,Cu,Cd,Cr,Ni and Pb.The results showed that the total content of heavy metals in the sludge from QingShanHu sewage treatment plant was higher,and the total content of heavy metal Cd and Ni in four sewage sludges exceeded the standard limit of acid soil.The heavy metals of Zn,Cd and Ni are mainly in exchangeable/carbonate fraction(F1)and reducible state(F2).Heavy metals Cu and Cr are mainly in oxidizable metals(F3).Pb mainly existed in residual fraction(F4).The leaching amounts of heavy metals Zn,Pb and Cd in the sewage sludge exceeded the corresponding standard value.In addition,except HongGuTan sewage treatment plant,the leaching content Ni in other sewage sludge were higher than the standard value.The leaching rates of heavy metals Zn,Ni and Cd were relatively higher.Secondly,on the basis of assessment methods applied to the pollution degree and ecological risk of heavy metal in sewage sludge(Igeo,ER,RI and RAC),extra assessment methods were introduced,including CF,ICF,DC,GCF and GRI.Meanwhile,a unified classification was established.The pollution degree and ecological risk of heavy metals in four sewage sludges in Nanchang city were assessed.The results showed that the pollution level and ecological risk assessment level of Cd belong to II-IV level(medium to high pollution level and ecological risk of heavy metals).The comprehensive pollution level and ecological risk of heavy metals based on DC and RI belong to IV grade,indicating high pollution and high ecological risk.The correlation analysis of single heavy metal pollution assessment methods(Igeo,CF and ICF)showed that the results of Cu,Zn,Ni and Cr were positively correlated,and the results of Pb and Cd were negatively correlated.The correlation analysis of single heavy metal ecological risk assessment methods(ER and RAC)showed that the evaluation results of Zn and Ni were positively correlated,and the results of Cu,Pb and Cd were negatively correlated.There was a positive correlation between the degree of comprehensive pollution(DC and GCF)and ecological risk(RI and GRI).Third,combined with the health risk assessment model of heavy metals in soil,the health risk assessment model of heavy metals in sewage sludge was optimized and then applied to four sewage sludges in Nanchang city.The results showed that the adult's and children's daily total exposure(CDIrisk)of heavy metal in sewage sludge from QingShanHu is the highest,followed by XiangHu,Chao Yang,HongGu Tan.The children's hand-mouth exposure dose(CDIinh)was significantly higher than that in adults,but the touched skin exposure dose(CDIdermal)was lower than that in adults.The health risk of five non carcinogenic heavy metals(Cu,Zn,Pb,Ni and Cr)in adults and children was lower than that of non carcinogenic risk threshold,but the carcinogenic risk of children with carcinogenic heavy metals Cd was slightly higher than the threshold of carcinogenic risk.