Enhancing Thermophilic High-Solid Anaerobic Digestion of Swine Manure Using Ammonia-Stripped Biogas Slurry Reflux Amended with Waste Iron Powder and Biochar

Ammonia stripping has been widely used to recover ammonia nitrogen from biogas slurry; however, the inhibitory effects on biogas production cannot be fully eliminated if ammonia-stripped biogas slurry (ASBS) is recycled back to the high-solid anaerobic digestion (AD) of animal wastes. This study investigated the performance of swine manure AD with recycling of ASBS and confirmed that there was no positive effect on increasing biogas production for ASBS recycling in the swine manure AD system under high solids (15%). The lowest accumulated methane yield was 133.9 mL/g-VS when swine waste was diluted with only raw biogas slurry (RBS), which was 9.2% lower than that of the water group (C0). Notably, the performance of AD was enhanced by adding rice husk biochar (RHB), waste iron powder (WIP), or their combination with ammonia-stripped biogas slurry (ASBS) reflux in the swine manure AD system. By adding 9.0 g/L of RHB, the biogas yield increased by 21.1%, and the total ammonia concentration (TAN) reduced by 15.1% compared to ASBS reflux alone (C1). The methane content reached a maximum of 75.2%, which was 12.8% higher than C1, while the methane yield was 1.5-times higher with the addition of 9.0 g/L of WIP. Correspondingly, the TAN was reduced, while the degradation of volatile fatty acids (VFAs) and total chemical oxygen demand (TCOD) increased. Both WIP and RHB can provide great potential to reuse biogas slurry in AD with a higher biogas yield and organic degradation rate. This approach facilitates source reduction in biogas slurry and nutrient recovery, while providing insights for reducing water consumption in manure treatment processes and enhancing biogas production efficiency.