Integration of Solid-state Anaerobic Digestion and Hydrothermal Carbonization of Elephant Dung for Elephant Sanctuary Waste Management

Abstract

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 VSadded and VS removal efficiency of 53.9 ± 0.3%. In subsequent HTC, the maximum HHV of 10078.5 ± 288.5 MJ/ton dry wt. was achieved for the digested ED without leachate recirculation under HTC temperature of 170 ºC. In addition, the optimum 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 than and ash content was identical to a conventional coal (lignite). An assessment of a full-scale elephant-sanctuary waste management scheme integrating SS-AD and HTC indicates that more than 10078.5 ± 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 proposed waste management platform could lower the energy cost for elephant sanctuary by the produced hydrochar while addressing its environmental issues.