Valorization of maize silage digestate from two-stage anaerobic digestion by hydrothermal carbonization

This study proposed a novel bioeconomy concept of combining two-stage anaerobic digestion (TSAD) and hydrothermal carbonization (HTC). The maize silage digestate from the acidification reactor (AR) of the TSAD was hydrothermally carbonized at 180, 200, and 220 degrees C for 30 min. This was followed by the biogas potential test of the process water and the co-digestion test of the hydrolysate (the liquid product from the AR) and the process water (180 degrees C). The results show that higher HTC temperatures resulted in lower yield, volatile matter content, and comprehensive combustibility index of the hydrochars. The COD, TOC, and specific methane yield of the process water decreased when increasing the HTC temperature. The modified Gompertz equation successfully described the methane formation of the aqueous samples. The process water from HTC at 180 degrees C obtained the highest specific methane yield (278 mL CH4/g COD). The highest total energy recovery from hydrochar and process water (methane production) was also achieved at 180 degrees C. The co-digestion test shows the feasibility of circularly coupling TSAD and HTC, while further optimization is still needed.