Sludge,as a by-product of urban sewage treatment plants,seriously affects people’s lives and the ecological environment.Using sludge as a raw material,the use of hydrothermal carbonization technology to prepare carbon materials rich in oxygen-containing functional groups has become a research hotspot in recent years.In this paper,based on the original functional groups of hydrochar,it is grafted and modified to prepare a coagulant to treat actual domestic sewage.First,by controlling the hydrothermal conditions,hydrochar with different temperatures（160°C～250°C）were prepared,and the reaction time was fixed at 4h,named HTC160-4,HTC170-4,HTC180-4,HTC190-4,HTC200-4,HTC210-4,HTC220-4,HTC230-4,HTC240-4,HTC250-4.The functional group content of HTC170-4,HTC190-4,HTC210-4,and HTC230-4 were semi-quantitatively characterized by FTIR and XPS,and it was found that the C-OR functional group content of hydrochar at 210℃was the most,with the functional group accounting for29%and the strongest peak intensity.The content of functional groups on the surface of hydrochar was calculated by Boehm titration experiment,and it was found that at 170℃,4h of hydrochar had the most phenolic hydroxyl functional groups,with a content of0.463mmol/g.As the Boehm titration is more accurate,the hydrochar at 170°C for 4h was selected for modification.The reactions were carried out by using polyethyleneimine grafting hydrochar monomer-initiated graft copolymerization hydrochar,and cationic modified hydrochar.The three prepared materials were put into the sewage to explore the flocculation effect,and it was found that the cationic modification best improved the hydrophobicity of the hydrochar,and the COD removal effect of the sewage was the best,reaching 67.2%.SEM,BET,FTIR,and Zeta potential were used to characterize the performance difference between the original hydrochar and the modified hydrochar.It was found that the pore structure of the hydrochar was reduced after the polymer was loaded,and the BET decreased from 14m~2/g to 8m~2/g.The hydroxyl content is reduced,and the modified hydrochar has a positive charge on the surface of p H 2～10,and has good acid and alkali resistance.The cationic modified material absorbs the organic pollutants in the sewage through the functions of charge neutralization,compression of the electric double layer,and adsorption bridging.Then put the modified material Y-HTC into the sewage to conduct a coagulation experiment to explore the best coagulation conditions,that is,the best dosage,p H,and settling time.The results show that the optimal dosage is 3g/L,and it is more economical when it is neutral and the sedimentation time is 40min.The COD removal rate of domestic sewage is 76.2%,and the turbidity removal rate is 97.8%.After determining the cationic modification method,the operating conditions of the modification process need to be optimized,and the influence of reaction temperature,reaction time,and ratio of reaction should be explored.According to the removal effect of turbidity,COD,UV254,it is found that when the temperature is 70℃,the reaction time is1h,and the ratio of hydrothermal carbon to modifier is 4:1,the sewage treatment effect is better.The removal rate is 98.4%,and the removal rate of COD is 77.8%.As time goes by,the polyquaternary ammonium salt reacts more fully with the hydrochar,and the positive charge becomes stronger.As the amount of modifier increases,the degree of substitution of the hydrochar active group hydroxyl groups by the polyquaternary ammonium salt increases,thereby increasing the cationicity.The changes of hydrochar functional groups,surface morphology and surface charge were characterized by FTIR,SEM,and Zeta potential.It was found that the optimized Y-HTC particles became smaller,the pore structure increased,the pores were deeper,and the surface charge increased.Through recycling experiments,it is found that the modified hydrochar still has a certain flocculation effect after re-hydrothermal carbonization.Hydrothermal carbonization realizes sludge reduction,and the preparation of materials used in organic matter enrichment can realize sludge resource utilization,and can produce certain economic benefits.