Effects Of Physicochemical Pretreatment On The Formation Of Hydrothermal Carbonization Products And Heavy Metal Pollution Risk Of Municipal Sewage Sludg

In recent years,the annual output of sewage sludge in China has been increasing due to the continuous improvement of sewage wastewater collection rate and treatment rate.Sewage sludge has the characteristics of high moisture content and high pollution risk,making conventional disposal methods inadequate.Conventional disposal methods are difficult to meet the needs of sludge disposal.Hydrothermal carbonization(HTC),a technology suitable for the treatment and disposal of wet organic waste,can utilize the large amount of water contained in sewage sludge as a reaction medium to decompose organic pollutants in sludge during HTC and convert them into hydrochar.However,the sludge directly undergoes HTC reaction,and the prepared hydrochar has the problems of low yield and poor performance.In this study,a novel process combining physicochemical pretreatment(ultrasonic/Fenton pretreatment)and HTC was proposed to improve the yield and performance of hydrochar by changing the surface structure and organic composition of sludge through physicochemical pretreatment.Firstly,the effects of physicochemical pretreatment on the yield and performance of hydrochar were investigated.The results showed that the yield of hydrochar after ultrasonic and Fenton pretreatment increased by 11.25%and 12.35%,respectively,compared with direct HTC.Physicochemical pretreatment had an effect on the calorific value,energy density,energy yield,specific surface area and morphology of hydrochar.Compared with direct HTC,the energy yield of hydrochar after ultrasonic and Fenton pretreatment increased by 39.46%and 53.79%,respectively.Ultrasonic and Fenton pretreatment increased the specific surface area of hydrochar from 15.78 m~2/g to 19.47 m~2/g and 21.10 m~2/g,respectively.In addition,Fenton pretreatment made the surface pore structure of hydrochar more abundant,with more porosity and fragmentation.Secondly,the mechanism of physicochemical pretreatment on the subsequent HTC of sludge was explored,the results show that its influence mechanism is related to many factors:(1)Physicochemical pretreatment enhanced the mass transfer effect of HTC process and enhanced the reaction activity of sludge in HTC by improving the apparent morphology and structure of sludge.(2)Physicochemical pretreatment promotes the rapid decomposition of macromolecular organic components in sludge,while small molecular organic matter can be hydrolyzed more quickly and easily to form intermediate products,and further form hydrochar.(3)Ultrasonic pretreatment also reduced the sludge particle size,and the smaller material particle size made the HTC reaction more sufficient,thereby increasing the yield of hydrochar.Finally,the effects of physicochemical pretreatment on the dissolution of heavy metals and environmental risks of hydrochar were investigated.The results showed that under the optimal physicochemical pretreatment parameters,the contents of heavy metals Zn,Pb,Cr and Cu in hydrochar decreased.The contents of Zn,Pb,Cr and Cu in CHC decreased by 89.52%,31.12%,23.79%and 68.96%,respectively,and the contents of Zn,Pb,Cr and Cu in FHC decreased by70.07%,21.21%,8.51%and 15.5%,respectively.Physicochemical pretreatment did not increase the leaching toxicity of heavy metals Zn,Pb,Cr and Cu in hydrochar under different leaching time and p H value of leaching solution.After physicochemical pretreatment,the pollution degree of heavy metals Zn and Cu in hydrochar decreased.