Wet torrefaction of sewage sludge: parametric evaluation on solid fuel production and assessment of liquid by-product potential

In this study, solid fuel was produced from sewage sludge through wet torrefaction at different solid loadings (5, 10, 15, and 20%), reaction times (30, 60, 90, and 120 min), and temperatures (150, 200, 250, and 300°C). The physicochemical characteristics of the hydrochar were evaluated based on proximate and ultimate analyses, alongside the higher heating value (HHV). The liquid by-products were examined for pH, nitrogen (N), phosphorus (P), potassium (K), and total organic carbon (TOC). The process yielded a maximum solid output of 81.00% and HHV of 16.94 MJ/kg 20% solid loading, 150°C and 30 min. Increasing the reaction temperature from 150 to 300°C reduced the TOC and P content from 19.80 to 11.30 g/L and from 2060 to 1949mg/L, respectively. The liquid by-products showed increased N and K concentrations, with N rising from 1150 to 2508 mg/L and K from 269 to 276 mg/L, indicating the potential application of the liquid phase as a liquid fertiliser. The findings suggest that wet torrefaction at 20% solid loading, 30 min, and 150°C enhances hydrochar fuel characteristics, yielding an HHV of 16.94 MJ/kg and carbon content of 36.36 wt.%, making it a viable renewable energy source.