Hydrothermal Passivation Treatment Of Heavy Metals In Municipal Sludge

With the continuous construction of urban wastewater treatment plant,the output of sludge is increasing gradually.How to dispose the sludge properly is an important problem to be solved urgently.In the long run,the sludge land use is one of the effective ways to realize the sludge recycling.However,before the land use,it is necessary to deal with the heavy metals in the sludge and reduce the ecological risk.Hydrothermal treatment can effectively realize sludge stabilization,especially the treatment under low temperature conditions has broad prospects for development,if combined with appropriate passivation agent,can effectively control the ecological risk of heavy metals,contribute to the implementation of land use of sludge.In this research,the effects of hydrothermal treatment with low temperature(<150?)on the heavy metals in sewage sludge were studied from the following three aspects:concentration of heavy metals,speciation and ecological risk assessment.At the same time,the factors affecting the behavior of heavy metals,such as reaction temperature,reaction time,the type and amount of the passivation agent were studied.The results show that:(1)Hydrothermal treatment can reduce the moisture content of sludge and reduce the sludge.At the same time,the concentration of heavy metals in the sludge was enriched,and the enrichment rate was up to 130%under the condition of 140? and 8h,which has increased the difficulty stabilization of heavy metals in some degree.(2)After hydrothermal treatment,the proportion of heavy metals entering the liquid phase is kept below 2%.The form of heavy metals in the solid phase has undergone certain changes,especially in 140? and 8h conditions,the percentage of residue(T5)of Cu,Cd and Cr increased by 5.12%,2.64%and 3.36%respectively.The T5 states proportion of Pb and Hg decreased,mainly because of the migration and transformation from the direct risk state(T1+T2)and the non-risk state(T5)to the potential risk state(T3+T4).The index of geoaccumulation and the potential ecological risk index increased slightly with the decrease of soil water content,and the risk of Cd and Hg was the highest.(3)After adding passivation agent,the proportion of heavy metals entering the liquid phase decreased to 1.5%,and more heavy metals remained in the solid part.The effects of passivation on the form of heavy metals were large,the reductive state(T4)ratio of Cu and Zn,and the fraction of the residue of Cd,Pb,Cr and Hg(T5)increased markedly.The addition of hydroxyapatite increased the direct ecological risk of Cr,while the addition of urea reduced the direct ecological risk of all six heavy metals.For Cd,Pb,Cr and Hg,the two kinds of passivation agents can reduce the indirect ecological risk,and enhanced the stability of sludge in the environment.According to the results of ecological risk assessment,it can be seen that the passivation agent has a good stabilization effect on Cd and Hg,which can effectively reduce the risk level in the environment and make the sludge more stable.Overall,the stabilization effect of hydroxyapatite was slightly better than that of urea.In short,the hydrothermal treatment with passivation agents can significantly reduce the bioavailability and environmental mobility of heavy metals in sludge,which makes the sludge more stable.The stabilizing effect is closely related to reaction time,reaction temperature,type and dosage of passivation agent.