Integration of Solid-State Anaerobic Digestion and Hydrothermal Carbonization

The purpose of this research was to develop an integrated biorefinery process of solid-state anaerobic digestion (SS-AD) and hydrothermal carbonization (HTC) for the co-production of methane and hydrochar using elephant dung (ED) as substrate. With a leachate recirculation rate of 4 times/day, the SS-AD presented the highest cumulative methane yield of 83.2 ± 1.7 NmL/g volatile solid (VS)_added and VS removal efficiency of 53.9 ± 0.3%. In subsequent HTC, the maximum higher heating value (HHV) of 10,078.5 ± 288.5 MJ/ton dry wt. was achieved for the digested ED without leachate recirculation under HTC temperature of 170 °C. In addition, the maximum mass and energy yields were 76.3 ± 0.8% and 84.1 ± 0.3%, respectively. The produced hydrochar had higher HHV compared to the raw digestate. Moreover, the HHV of the hydrochar was higher, and ash content was identical to conventional coal (lignite). An assessment of a full-scale elephant-sanctuary waste management scheme integrating SS-AD and HTC indicates that more than 10,078.5 ± 288.5 MJ of energy and 563.0 ± 5.2 kg dry weight of hydrochar could be recovered per ton dry weight of ED. The developed elephant dung management platform could enhance energy yield from ED while addressing environmental issues.