Response surface optimization of co-hydrothermal carbonation parameters of municipal sludge and enteromorpha

The increasing amount of municipal sludge poses a serious threat to the environment. Traditional treatment methods still face various challenges in terms of harmlessness and resource utilization. Hydrothermal carbonization is considered an effective method for treating municipal sludge, but its low carbon and high ash characteristics limit the application of hydrochar as a fuel. To address this issue, the co-hydrothermal carbonization of municipal sludge and enteromorpha (a high-carbon and low-ash waste biomass) is proposed as a potential solution. In this study, co-hydrothermal carbonization experiments of municipal sludge and enteromorpha were conducted under various reaction conditions, including temperatures ranging from 160 to 280 ℃, residence times ranging from 30 to 150 minutes, and municipal sludge proportions ranging from 0 to 100%. The impacts of these process parameters on energy recovery efficiency (ERE) and electricity consumption index (ECI) were investigated. The response surface method was adopted to optimize the three main factors (temperature, time, and municipal sludge proportion) affecting ERE and ECI. The results indicated that the response surface models established by Central Composite Design exhibited a good fit (model P<0.05, corresponding lack-of-fit p>0.05), with minimal deviation between predicted values and actual values. Municipal sludge proportion had a significant impact on ERE (P<0.05), while time exerted an extremely significant influence on ECI (P<0.0001). Temperature also had a significant impact on ECI (P<0.05). Through mathematical model optimization, the optimal conditions for achieving the highest ERE (0.470) and the lowest ECI (165.61) were determined to be 216 ℃, 83 minutes, and a 64% municipal sludge proportion, respectively. Additionally, a higher ERE (0.463) and lower ECI (171.60) could be obtained simultaneously at 208 ℃, 62 minutes, and a 52% municipal sludge proportion. To validate the results, three repeated experiments were conducted, and the errors between the experimental values and predicted values were found to be less than 5%. The findings of this study provide a scientific basis for the resource utilization and high-value utilization of municipal sludge and enteromorpha. By optimizing the co-hydrothermal carbonization process, it is possible to enhance energy recovery efficiency and reduce electricity consumption, contributing to the sustainable management of municipal sludge and the preservation of the environment.