Co-hydrothermal carbonization of sewage sludge and banana stalk: Fuel properties of hydrochar and environmental risks of heavy metals

This study investigated co-hydrothermal carbonization (co-HTC) of sewage sludge (SS) and banana stalk (BS), mainly focusing on the variations in hydrochar fuel properties and migration behavior of heavy metals (HMs) responding to different reaction temperature and SS/BS mixing ratios. This work established reliable mathematical models to integrate discrete experimental points and further obtain continuous response surfaces. Moreover, coupling with the two-way analysis of variance, the relationship between hydrochar properties and process parameters could be intuitively described. Statistical analysis (P-value) revealed that the mixing ratio was the most dominant factor affecting various hydrochar characteristics. The synergistic effects, mainly induced by Maillard and Mannich reactions, would contribute to superior hydrochar yield, C and N contents, higher heating value, and energy yield compared with their calculated values. Thermogravimetric analysis indicated that co-HTC with BS was a viable avenue to improve hydrochar combustibility index S to an acceptable level, with figures elevating from 1.34–1.58 to 27.71 (10−7×min−2×℃−3). Additionally, co-HTC of SS and BS could generate efficient synergistic HMs immobilization effects, promoting the transformation of HMs from direct/potential eco-toxic fractions to non-toxic fraction, thereby largely reducing ecological risks of hydrochar. These findings provided referential information for harmless and resource utilization of SS and BS.