Hydro thermal carbonization (HTC) of sewage sludge: process optimization through RSM and assessing its energy potential

Sewage sludge is a by-product of municipal wastewater treatment, and its production is increasing with population growth and urbanization. Hydrothermal carbonization (HTC) is a promising method for managing sewage sludge due to its applicability and cost-effectiveness. The HTC process produces hydrochar, a stabilized carbon material derived from sewage sludge. In this study, sewage sludge was subjected to HTC at optimized conditions of 200 degrees C and 6 h using the response surface methodology (RSM) statistical tool. The structural morphology of the hydrochar was analyzed using Scanning Electron Microscopy and Transmission Electron Microscopy, which revealed a fluffy and coarse surface with deeper fragments, indicating the presence of carbon microspheres. Fourier Transform Infrared (FTIR) spectra showed that the hydrochar retained similar functional groups as the original sewage sludge, such as hydroxyl, aliphatic methyl, amino, ketonic, amide, and ether groups, with minimal shifts and reductions in vibrational peaks.The higher heating value (HHV) of the sewage sludge increased from 16.24 to 19.41 MJ kg(-1) after HTC, indicating energy densification with a factor of 1.07. However, when hydrochar was blended with coal, the HHV gradually decreased. Thermo-gravimetric analysis was performed to assess the co-combustion effectiveness, and it showed that a blend of 50% coal and 50% sewage sludge-derived hydrochar exhibited extended heat generation with minimal reduction in HHV compared to other blends, reaching a value of 23.16 MJ kg(-1). These findings suggest that hydrochar produced from sewage sludge through HTC could serve as an alternative energy source in industries, reducing the reliance on coal. Furthermore, the combustion behavior of hydrochar was significantly altered after the HTC process, highlighting its potential for energy generation. By utilizing hydrochar, the industry can contribute to the sustainable management of sewage sludge while reducing greenhouse gas (GHG) emissions associated with coal combustion.