| With the increase of urban domestic sewage production and treatment rate,the treatment and disposal of sewage sludge has become a new environmental problem.Sewage sludge is rich in nutrients and contains a large amount of toxic and harmful substances.If improperly disposed,it will inevitably lead to serious"secondary pollution"problems.In this study,through sampling and analysis of the sludge generated by 32 sewage treatment plants in Shaanxi Province,the contents of nutrients and heavy metals in sewage sludge of different water periods and the potential pollution risk status of sewage sludge for agricultural use were obtained.Furthermore,combined with field experiments and indoor soil column leaching tests,the effects of sludge application on the contents of nutrient and heavy metal in farmland tillage soil,the enrichment characteristics of heavy metals in various parts of wheat plants after sludge application and the effects of precipitation and irrigation leaching on the migration and accumulation of heavy metals in soil during long-term repeated application of sludge were determined.The following conclusions are drawn:（1）The contents of N,P nutrients and OM in urban sludge in Shaanxi Province were abundant,but the TK content was not high.The distribution characteristics of the total nutrient content of sludge generally showed low water period>flat water period>abundant water period,in which the TN and OM contents were very significantly affected by the water period(PJ-T<0.001),while the TP and TK contents were not significantly affected by the water period(PJ-T>0.05).（2）The distribution of heavy metal contents in urban sludge of Shaanxi Province showed log-normal.The contents of 6 kinds of heavy metals from large to small were:Zn>Cu>Cr>Pb>Ni>As.There was no significant difference in the heavy metal contents of sludge in different water periods(PANOVA>0.05),indicating that the heavy metal contents of sludge were less affected by urban non-point source pollution.（3）The results of potential ecological risk index method showed that there were medium potential ecological risks of Cu,Zn and As,and Pb,Ni and Cr had low potential ecological risks.The comprehensive potential ecological risk index of the six heavy metals was 228.04,which was at the second level of the four-level evaluation,indicating a medium ecological risk.Overall,the urban sludge in Shaanxi Province was suitable for application to forest land,pasture grassland and cultivated land without growing edible crops,and land improvement for medium alkaline soil.Meanwhile,special attention should be paid to the fact that the As content must be within the standard limit value.（4）The application of sludge can significantly increase the content of available nutrients in the soil of farmland soil,especially in the surface layer of 010cm.The contents of available nitrogen and available phosphorus increased greatly,while the contents of available potassium and organic matter increased little.In addition,the application of sludge from 10t/hm2 to 60t/hm2 significantly increased the contents of Cu,Zn,Pb and Cd in the 10 cm thick soil of the farmland,and the contents of Cr and Ni also increased,but the increase was relatively small.The increase of the content of six heavy metals in the soil layer below 10 cm was small,indicating that the higher or lower amount of sludge application did not cause excessive accumulation of heavy metals in the lower soil of the farmland.（5）According to the enrichment of heavy metals in various parts of wheat under different sludge application rates,the contents of six heavy metals in various parts of wheat were basically positively correlated with the amount of sludge applied.From the enrichment of single heavy metal elements in different parts of wheat plants,Zn had the highest enrichment concentration in wheat bran and the lowest enrichment concentration in wheat stalks;Cu,Pb,Cr,Ni and Cd had the highest enrichment concentration in wheat roots,Cu had the lowest enrichment concentration in stems,and Pb,Cr,Ni and Cd had the lowest enrichment concentration in wheat flour.（6）Soil column leaching test results show that organic matter,nitrogen and phosphorus nutrients would be lost with the leaching of water,and the leaching amount increased with the increase of leaching water.The available potassium in the soil would gradually migrate to the lower end of the soil column with the leaching of water,and accumulate in the soil below the soil column.In addition,the leaching of rainfall and irrigation water has little effect on the migration performance of heavy metals,and the continuous surface addition of sludge didn’t cause the accumulation of heavy metals in deep soil,but only enriched in the soil of 8 cm in the surface layer. |
