Study On The Migration Regulation Mechanism Of Carbon Nitrogen And Sulfur Elements In The Hydrothermal Treatment Of Sewage Sludge

Hydrothermal carbonization（HTC）of municipal sewage sludge（SS）is a new sludge treatment technology,which is considered as a very promising thermochemical technology for the disposal of organic waste with high water content,and it can achieve the reduction,harmlessness and resource utilization of SS.HTC received more attention recently due to its advantages in dewatering and reclamation of SS.However,SS treatment by HTC was often affected by reaction temperature,residence time,dewatering conditioners and external additives.In this study,SS was treated by HTC at different temperature and time firstly to obtain the operation condition.Then three typical flocculants including polyacrylamide（PAM）,polyaluminum chloride（PAC）and FeCl₃ were used flocculate activated sludge and the derived sludge was submitted to HTC treatment to study their influence on HTC process.Finally,fly ash and H₂O₂assisted HTC was performed to produce high-grade biochar fuel with low N and S contents.The main conclusions obtained in this study were as follows:The effects of temperature and time on SS treatment by HTC were studied at 120℃to 240℃for 0 min to 120 min.The products yields,physicochemical properties and functional groups of hydrochar,as well as the water quality of the derived liquid were analyzed.The results show that the reaction temperature influence the hydrothermal conversion of SS more obviously than residence time.With the increase of temperature,the coalification degree of hydrochar was improved with decreased product yields.While the N and S removal rate and ash content in hydrochar increased significantly with the increase of temperature.The N and S removal rate increased from23.74%and 31.02%to 74.34%and 56.6%,respectively,when the temperature increased from 120℃to 240℃.However,the N and S removal rate increased marginally when the temperature was higher than 210℃.Therefore,the HTC treatment condition in this study was set at 210℃for 60 min.The effects of flocculants（including PAM,PAC and FeCl₃）on physicochemical properties of hydrochar and derived liquid were investigated.The results suggested that the flocculants including PAM,PAC and FeCl₃ can be transferred from the liquid phase to the hydrochar during the HTC process.The monomers generated from PAM thermal degradation can be absorbed on hydrochar,and the Al³⁺and Fe³⁺derived from the hydrolysis of PAC and FeCl₃ were in the form of oxides,hydroxides or salts in hydrochar.Moreover,flocculants can affect the conversion of N,S and P in SS,and the thermochemical properties of the derived hydrochar show obvious differences.The influence mechanism of flocculants on element conversion can be attributed to the precipitation of phosphate by metal ions,p H change caused by fluccoluants hydrolysis and the catalysis effect caused by the ions.Fly ash（FA）and hydrogen peroxide（HP）assisted HTC were conducted to improve the removal efficiency of N and S from SS for high grade fuel hydrochar production.The removal rate and distribution of N and S in hydrochar were evaluated,and the characteristics of the wastewater were measured to illustrate the conversion mechanism of N and S.The results indicated that a positive synergistic effect of FA and HP on N and S removal was observed during HTC process.With the assistance of FA（10%of the raw sludge）and HP（48 g/L）,the N and S removal rates increased from 69.53%and 49.92%in the control group to 81.71%and62.83%,respectively.Mechanism analysis suggested that the·OH radicals can destroy the biopolymers such polysaccharides and proteins in SS,which facilitated the N and S removal from SS.In addition,FA used as catalysts will reduce the energy requirement for N and S removal.Therefore,FA and HP assisted HTC improved the removal efficiency of N and S from SS,and the derived hydrochar with low N and S contents was more suitable for use as solid biofuels.