Hydrothermal carbonization of sewage sludge for hydrochar production: optimization of operating conditions using Box-Behnken design coupled with response surface methodology

Hydrochar was produced from sewage sludge via the hydrothermal carbonization (HTC) process. The effects of carbonization temperature (150–300 °C), solid load (10–30%), and reaction time (30–120 min) were investigated using response surface methodology (RSM) based on the Box-Behnken factorial design. The optimum operating conditions for hydrochar production were determined by maximizing three key responses: solid yield, higher heating value (HHV), and energy yield. The results showed that the HTC process increased the carbon content from 30.02 wt.% to 37.53 wt.% while decreasing the hydrogen content from 5.26 wt.% to 3.18 wt.% and oxygen content from 25.55 wt.% to 9.48 wt.%, affecting the HHV and energy yield of the hydrochar. As for the optimization, the findings indicate the suitability of a quadratic model, revealing the most favorable conditions to be the temperature of 150 °C, the solid load of 30%, and the reaction time of 30 min, resulting in hydrochar with a solid yield of 71.24%, an HHV of 17.90 MJ/kg, and an energy yield of 94.81%. Of all the process parameters, temperature had the most significant influence on all responses. Further characterization using Fourier transform infrared spectroscopy (FTIR) demonstrated enhanced conversion of sewage sludge to hydrochar, evidenced by a notable reduction in peak intensities of solid product compared to the parent material. Scanning electron microscopy (SEM) analysis confirmed structural and porosity changes in sewage sludge post-HTC treatment. Thermogravimetric analysis (TGA) indicated remarkable improvements in the combustion performance of hydrochar, including increased ignition temperature (241.91 °C) and burnout temperature (688 °C). The results of this optimization study offer valuable insights for the scalable implementation of the HTC process in efficiently producing solid fuel from sewage sludge.