Simultaneous biomethane and hydrochar recovery from washed elephant dung: The effects of inoculum source, substrate to inoculum ratio, and hydrothermal temperature

• The biorefinery integrating anaerobic digestion and hydrothermal carbonization was set up. • The elephant dung’s solid and liquid organic substrates were separated by distilled water. • The highest energy produced from anaerobic digestion of the liquid substrate was 2,220.1 ± 53.03MJ/ton dry wt. • The hydrothermal carbonization of the solid substrate could produce 12,067.0 ± 452.1 MJ/ton dry wt. • The platform converts elephant waste to renewable energy and drives a carbon–neutral society. This research aims to develop the appropriate biorefinery process integrating anaerobic digestion (AD) and hydrothermal carbonization (HTC) to recover the highest energy from the pretreated elephant dung. Initially, the raw elephant dung was physically processed by washing with water to generate the liquid waste, i.e., washing water (WW), and solid waste, i.e., washed fiber (WF). The appropriate substrate-to-inoculum ratio (SIR) and the inoculum source of the AD of WW were determined and the HTC temperature of WF was also examined. The results indicated that the AD of WW with the SIR of 1:2 and anaerobically digested swine manure as the inoculum presented the highest methane and energy yields of 412.3 ± 9.9 N mL/g VS added and 2,220.1 ± 53.03 MJ/ton dry wt., respectively. For HTC of WF, the optimum condition was the hydrothermal temperature of 170 °C at the residence time of 60 min. The highest hydrochar and energy yields were 76.8 % ± 1.9 % dry wt. and 12,067.0 ± 452.1 MJ/ton dry wt., respectively. Thus, this biorefinery process could simultaneously treat elephant camp-derived waste and produce clean energy.