The Risk Assessment Of Heavy Metal Pollution In Forest Land Utilization Of Organic Fertilizer From Sewage Sludge In Beijing

The application of sewage sludge in urban drainage is an effective way for sludge reclamation,but the heavy metals contained in the sludge may have potential pollution risks to the forest environment.The paper takes sludge products produced by Beijing Gaobeidian,Gaoantun,Xiaohongmen and Huaifang sewage treatment plants as research materials.Soil heavy metal content of three main afforestation tree species in Beijing,Fraxinus chinensis,Sophora japonica and Pinus tabulaeformis,was studied.The method of combining timely monitoring with simulation control experiment is adopted.The seasonal variability of heavy metals in sludge products and the migration and accumulation of heavy metals in soil and shallow groundwater were studied.The purpose of this study is to provide a theoretical basis for the formulation of strategies for the application of drainage sludge products in forestland in Beijing.The main conclusions of the study are as follows:(1)Through the study of the seasonal variability of sludge produced by four sewage treatment plants in Gaobeidian,Gaoantun,Xiao hongmen and Huaifang in Beijing,the seasonal variation characteristics of different heavy metals were found to be different.The nemero composite pollution index showed that only the nickel content in sludge products reached the alarming value in summer,and the heavy metal content and pollution risk were higher in each treatment plant in summer,but lower in winter.It shows that in Beijing area,the heavy metal content of sludge in summer should be taken as a reference value to discuss the reasonable utilization amount of sludge in land use,so as to reduce the potential environmental pollution risk of forest application.(2)The urban sewage sludge products are applied to Fraxinus chinensis,Sophora japonica and Pinus tabulaeformis forest land according to the application rate of 0,1,2,3,4,5kg / m2.By detecting the content of eight heavy metals in the soil including copper,zinc,cadmium,chromium,nickel,lead,mercury,and arsenic,and comprehensively evaluating the environmental pollution,it was found that The content of cadmium exceeds the limit of the "Soil Environmental Quality Standards Agricultural Land Soil Pollution Risk Control Standard(Trial)"(GB / 15618-2018).Based on the comprehensive evaluation methods of pollution,it is concluded that the risk of soil pollution is the lowest under the 2kg /m2 fertilization rate.The optimal application rates of ash and pine forest land are 3kg / m2 and 4kg / m2 from the perspective of soil heavy metal environmental risk.According to the evaluation of land accumulation index,it is concluded that the three forest lands need to be protected from the pollution risks of nickel,chromium,cadmium and mercury.(3)The experimental study on simulated leaching of soil column shows that The copper content in the mixed soil leaching solution of the sludge product has a tendency to decrease with the increase in the amount of fertilization.The content of mercury and arsenic fluctuated less under different fertilizer levels,and the contents of zinc,chromium,lead,and cadmium were all below the detection limit.And the content of all tested heavy metals is far below the requirements of "Groundwater Quality Standards"(GB-14848-2017).Therefore,the application of a certain amount of sludge products will not have a significant impact on the content of heavy metals in shallow groundwater.