Recycling cinder in efficient methane production from wheat straw via solid-state anaerobic digestion (SS-AD)

Solid-state anaerobic digestion (SS-AD) has received widespread attention due to its high methane productivity and organic loading, as well as its almost nonexistent wastewater emission. However, local acidification of the reaction system resulted in low material transfer efficiency due to low free water content leading to low energy conversion efficiency. Cinder is a byproduct of the coal chemical industry that has the potential to enhance the SS-AD performance due to its porous structure, electrocatalytic activity, and high trace elements concentration. The effect of cinder addition on SS-AD of wheat straw at 35 degrees C was studied herein. The cumulative methane of 2.5%, 5.0%, and 10.0% cinder groups were 41.25, 33.34, 28.94, and 26.68 mL/g VS, respectively, which increased 54.61%, 24.96%, and 8.47% compared to control (CK), with 2.5% cinder obtained the highest methane production. Cinder considerably enhanced volatile fatty acids conversion, particularly the propionic acid. The modified Gompertz model was used to predict the methane production potential and found to be able to accurately fit the characteristics of gas production. The maximum methane production of three test groups (2.5%, 5.0%, and 10.0%) was predicted to increase by 53.28%, 26.60%, and 11.56%, respectively, compared to CK. Cinder greatly enhanced the abundance of Methanosarcina, Fibrobacteres, and Spirochaetoma, which contributed to the switch from hydrogenophilic to acetylated methanogenesis. The prediction of the abundance of enzymes associated with microbial metabolism verified that cinder contributed to the greatest abundance of enzymes working in the hydrolysis phase, including peptidases, pyruvate metabolism, and genes encoding methane metabolism. Based on the concept of circular economy, this study presents a significant reference for synergistic and efficient disposal of industrial and agricultural wastes with high-value utilization.