Study On Upgrading Sewage Sludge And Release Characteristics Of Heavy Metals In Hydrothermal Treatment Process

Sludge is a by-product produced in the sewage sludge treatment process having high moisture,multi-pollutant(heavy metals,bacteria,etc)and low calorific values that are the main obstacles for its clean utilization.The hydrothennal treatment is a new technology to deal effectively with sludges.This paper studied physico-chemical characteristics of the sewage sludge before and after hydrothennal treatment,the migration,transformation,and stabilization of heavy metal under different temperature and residence time.Characteristics of combustion and heavy metal release of the solid residues were conducted.These results provide experimental database for its further industrial application.The physico-chemical characteristics of the solid products were analyzed by the Fourier transform infrared spectrometry(FTIR),X ray diffraction analysis(XRD),and X ray fluorescence spectrometry(XRF).The results showed that the hydrothermal treatment has an obvious effect on dehydration of sewage sludge.As the reaction temperature was 280 ℃,the mositure content was below 50%.Meantime,some compound containing sulfur,nitrogen and chlorine in sludge were removed also,thus reducing pollutant emission in the post-process for heat utilization.More than 80%of the mineral elements existed in the solid products(except Na)after the hydrothennal treatment.Some mineral elements were easily accumulated in the liquid products at higher reaction temperature.Some ones of Al,Ca,Fe,Mg was easily enriched in the solid products as the residence time increased,but Na and K was easily kept in the liquid products.By using the European Community Bureau of Reference(BCR)three State continuous sampling method and Toxicity Characteristic Leaching Procedure(TCLP)method,the behaviors of migration and transformation of heavy metals(As,Cd,Zn,Cr,Cu,Pb,Ni,Mn)in the hydrothennal treatment process were investigated,and the stabilization mechanism of heavy metals was explained.The results showed that most of heavy metals are existed in solid product,and a few in liquid product(except As).The process of hydrothennal treatment promoted heavy metal to migrate from unstable form(acid-soluble and reduction state)to relatively stable form(oxidation state)and stable form(residual state),thus realizing heavy metals stabilization.Through the hydrothermal treatment,the extraction toxicity of heavy metals(except Pb and Cd)in sewage sludge was sharply declined,especially at the reaction temperature 280℃.Ni,Cu,Zn and As reduced 86.69%,97.18%,51.78%and 75.9%,respectively.There existed three processes in the tranformation of heavy metals:dissolution process or desorption process,intracellular adsorption and chelated absorption process,and precipitation process.They were influenced by physico-chemical characteristics of the sewages sludge,hydrothermal environment and combination forms of heavy metal.The thermogravimetric analysis was used to investigate the combustion chracteristics and the heavy metal release behaviors of the solid products.The results indicated that igniting temperature(Ti)and burnout temperature(Th)were increased.and peak temperature(Tm)was declined.The activation energy of solid products was higher than that of raw sludge during volatilization combustion process,however the activation energy of solid products was lower than that of raw sludge during char combustion process.The release characteristics of heavy metals showed that the heavy metals in solid products would be more existed in the combustion residues in the relatively stable or more stable forms.