Microwave pretreatment and fermentation conditions affect acidogenic fermentation of wheat straw and horse manure

This study aimed to investigate the influence of microwave pretreatment and fermentation parameters on acidogenic fermentation of wheat straw and horse manure. Four different inoculation strategies were tested on wheat straw fermentation, including horse dung, cow dung, aerobic sludge, and wheat straw indigenous inoculum at two different pH (6 and 10). The best yield was achieved with horse dung, reaching 0.172 ± 0.005 gCOD/gVS after seven days at pH 6. However, for horse manure fermentation, lignocellulosic substrate suitable for fermentation without external inoculation, a pretreatment was needed to deactivate methanogenic archaea and solubilize sugars. Microwave parameters such as power (200 or 1000 W), liquid/solid (L/S) ratio (6 or 17) and energy (7 Wh/gVS or 12 Wh/gVS) were assessed on substrate solubilization and metabolic yield. The best pretreatment condition, at 200 W for 15 min with a L/S ratio of 17, led to a metabolic yield of 0.134 ± 0.029 g COD/gVS after seven days. This condition suppressed methanogenic activity while increasing soluble sugars and Chemical Oxygen Demand (COD) by 44 % and 58 %, respectively. Finally, increasing volatile solid (VS) concentration during batch fermentation from 14 to 28 gVS/L of horse manure led to a significant metabolic profile change. Butyrate production was quadrupled for the 200 W pretreatment while total metabolite yield was unchanged. Overall, this study demonstrates that microwave pretreatment is an effective strategy to inhibit methanogenesis and enhance substrate solubilization, without significantly promoting volatile fatty acid production. In addition, modifying VS concentration can induce a notable metabolic shift.