Torrefaction as a Strategy for Solid Fuel Production from Wastewater Sludge of the Meat Industry

Waste sludge from the treatment of meat processing wastewater poses significant environmental concerns. This study examines the viability of upgrading two sludge fractions, screenings (S) and flotation sludge (FS), into solid fuel through torrefaction. The materials were thermally treated at temperatures ranging from 250 to 450 °C under an inert nitrogen atmosphere. The effects of torrefaction on physicochemical properties, thermal behaviour, and functional group composition were systematically evaluated using proximate analysis, higher heating value (HHV) determination, thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). The results indicate that increasing torrefaction temperature leads to a substantial enhancement in mass loss due to progressive devolatilization. Concurrently, there is a reduction in moisture uptake and an improvement in hydrophobicity. Proximate analysis revealed a decrease in volatile matter (35% for FS and 31% for S) and a corresponding increase in fixed carbon (from 0.46 to 1.80% for FS and from 6.43 to 20.60% for S). The S fraction demonstrated a better fuel ratio (1.27%) compared to the FS one (0.06%), indicating more favourable fuel properties. TGA results demonstrated improved thermal stability of torrefied samples. FTIR analysis confirmed the progressive removal of polar functional groups (O–H, C=O, N–O) and the formation of more carbon-rich and hydrophobic structures. Torrefaction at temperatures of at least 350 °C effectively upgrades both sludge fractions into more stable and energy-dense materials. The findings indicate that torrefaction is a promising pathway for the valorisation of meat processing wastewater sludge and its conversion into a sustainable solid fuel.