| With the development of municipal wastewater treatment plant in Shaanxi Province,the amount of excess sludge is increasing year by year,and its safe disposal and utilization has become a hot issue.In this paper,a number of municipal sewage treatment plants were selected in the south and north of Shaanxi.Sludge samples were collected from Feng-Kuping period to determine the total amount of heavy metals,such as Cu,Cr,Cd,Pb,Zn and Ni,as well as their five different forms of distribution and nutrient components such as nitrogen and phosphorus.Establish muddy database and carry out ecological security assessment.The experiment of sludge in agricultural field was started,and the concentration of heavy metals in different parts of maize plant was determined under the condition of different dosage of sludge,the enrichment effect and ecological risk of plant were studied,and the feasibility of sludge farming was discussed.The experimental results provide technical support for government decision-making and environmental protection industry investment.The main findings are as follows:The average contents of total nitrogen,total phosphorus and organic matter in sludge from southern and northern Shaanxi reach the requirement of farm manure,and have higher agricultural value.The average content of Cd,Cr,Pb,Cu,Zn in the sludge was 1.63,100.47,35.67,103.57,493.52,26.65(mg/kg).respectively,and the Zn content in 11%of sludge samples in southern Shaanxi was higher than the limit value of grade A in the sludge samples(Standard for Control of pollutants in Agricultural sludge)33%in sludge samples.In northern Shaanxi,only 20%of the sludge samples exceeded the pollutant control standard of agricultural sludge,and the sum ratio of organic bound state and residual concentration of heavy metals in sludge was more than 50%,indicating that most of the metals existed in the sludge in stable form.In the three periods,the unstable state of heavy metals in sludge was relatively high in the high water period,especially in the southern part of Shaanxi Province,where the contents of Cr,Pb,Zn and Ni were obvious,and the proportion of heavy metal instability in the high water period was 17%-47%.The contents of Zn and Ni in Cd,Cr,Pb,Zn in northern Shaanxi are obvious,and the proportion is 19%-46%.In the ecological risk assessment of the unstable heavy metals in the two regions during the periods of high water supply,the comprehensive analysis of the results of each evaluation showed that Cd in both regions showed high risk,and the south of Shaanxi was more serious.The annual evaluation values of the three indexes are higher than that of northern Shaanxi.In northern Shaanxi,there are higher ecological risks in the assessment of Nemero index and land accumulation index Zn.In the effective state assessment,the Cd of the two regions exceeded three evaluation index criteria and Zn exceeded the Nemero index and the ground accumulation index standard,among which Cd exceeded the standard more seriously in southern Shaanxi and was 1.03times higher than that in northern Shaanxi.Zn pollution in northern Shaanxi is 6.28times higher than that in southern Shaanxi.The above heavy metals should be monitored in the process of sludge farming.The content of Cd and Pb in the rhizome and leaf of maize completely meet the national standard of feed hygiene,the content of Cr in the root exceeded the standard,the content of Cr in the stem met the requirements of the country when the amount of sludge was less than 2.4kg/m~2,and the content of Cr in the leaf completely met the requirement of the country.The content of Cr in contaminated corn must be concerned when it is used as feed.The content of Cu,Zn,Cd,Cr,Ni in corn grain can meet the national food safety standard when the sludge dosage is less than 7.2kg/m~2.The content of Pb should be paid more attention to when corn is used as raw material for human food. |
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