Optimization Of Energy Recovery Efficiency In Hydrothermal Carbonization Of Sewage Sludge And Pine Sawdust Coupling With Anaerobic Digestion Of The Produced Wastewater

The output of organic solid waste in China is huge,but the resource utilization rate has long been insufficient.Sewage sludge（SS）is a major by-product of the wastewater treatment process.SS contains high levels of organic matter and is considered a promising alternative energy source.Hydrothermal carbonization（HTC）can be used to convert SS into hydrochar,removing most of the N and S from SS and reducing NO_x and SO₂ emissions.However,the high ash content and low calorific value of SS make the hydrochar obtained after HTC still not up to the standard of fuel coal.Co-hydrothermal carbonization（Co-HTC）of SS with lignocellulosic biomass waste（e.g.,pine sawdust,PS）can significantly increase the calorific value of hydrochar and make it have better fuel performance.Moreover,the large amount of organic hydrothermal carbonization wastewater generated can be methanated by anaerobic digestion（AD）to improve energy recovery efficiency.In this paper,Co-HTC and medium temperature AD were carried out to investigate the effects of process parameters（raw material blending ratio,raw material-liquid-solid ratio,HTC temperature and HTC reaction time）on the Co-HTC of SS blended with PS and AD of HTC wastewater.In this study,hydrochar was obtained by Co-HTC of SS with PS,and methane gas was further obtained by AD of HTC wastewater,using single-factor experiments focusing on the energy conversion characteristics of the organic components of the feedstock under different HTC conditions（hydrothermal temperature,residence time and original feed liquid-solid ratio）to obtain the mass and energy yields of hydrochar and methane and their influencing factors.The maximum energy yield of hydrochar was91.45%under the conditions of hydrothermal temperature of 160℃,original feed liquid-solid ratio of 10:1 and residence time of 2 h.On the other hand,the wastewater obtained at a lower hydrothermal reaction temperature（160°C）showed the highest cumulative methane yield of 304.16 m LCH₄/g COD.In this study,more than 60%of the energy in SS and PS can be recovered by coupling the HTC-AD process.At mild reaction intensity（hydrothermal temperature of 160°C or residence time of 0 h）,the energy efficiency can reach more than 80%.Optimizing the energy yield benefits of the coupled HTC and AD（Co-HTC-AD）process is key to improving the operating economics of the process.On the basis of single-factor experiments and based on the principle of central composite design,the response surface methodology（RSM）was used to study the energy yield and operation parameter optimization of Co-HTC-AD process using a mixture of SS and PS as raw materials.The operating conditions include PS proportion,original feed liquid-solid ratio,HTC temperature and residence time.The PS proportion,HTC temperature and residence time had significant effects on the total energy yield（p<0.05）.Increasing the PS proportion increased the hydrochar yield（HCY）and higher heating value（HHV）of hydrochar,and the cumulative methane production from AD of HTC wastewater.Increasing the HTC temperature decreased HCY but increased HHV and decreased cumulative methane production.Extending the residence time resulted in a significant decrease in the HCY despite the increase in the degree of HTC,and the toxic substances generated in the HTC wastewater inhibited AD,resulting in a decrease in the total energy yield of hydrochar and methane.Changing the original feed liquid-solid ratio had some effect on the total energy yield,but the overall trend was not significant.The optimal conditions were determined by the RSM as follows:PS proportion of 75%,liquid-solid ratio of 7:1,HTC temperature of 190°C and residence time of 3 h.At this time,the total energy yield of SS-PS blend was the highest,which could reach 89.61%,among which the energy yields of hydrochar and methane were 86.46%and 3.15%,respectively.HTC wastewater showed good AD performance,and after 16 days after digestion,the methane yield was 255.64 m LCH₄/g COD.