Bioaugmentation Versus pH Adjustment in High-Load Food Waste Anaerobic Digestion: Divergent Microbial Responses and Methanogenesis Regulation

High organic loading is known to destabilize anaerobic digestion (AD). This study compared bioaugmentation and pH adjustment under increasing organic loading rate (OLR: 2.0, 4.0 and 6.0 gVS L−1 d−1), focusing on the responses of microbial structure, metabolic pathways, and energy metabolism. Results demonstrated that bioaugmentation maintained stable methane production of 400.54 ± 10.08 and 374.15 ± 24.32 mL·g-VS−1 at 4.0 and 6.0 gVS L−1 d−1, respectively, whereas control and pH-adjusted reactors failed at 4.0 gVS L−1 d−1. The acidified system restored methane yield from 86.30 to 382.13 mL·g-VS−1 after bioaugmentation, whereas pH adjustment and feeding cessation were ineffective, failing to produce methane within 25 days. Microbial analysis showed bioaugmentation enriched Methanosarcina, enhanced hydrogenotrophic/methylotrophic methanogenesis, and strengthened syntrophy with syntrophic propionate-oxidizing bacteria (SPOB), reducing volatile fatty acid accumulation via reinforced syntrophic propionate/butyrate oxidation. Upregulation of osmoregulatory (nha, kdp, proP) and energy metabolism genes (eha, mvh, hdr) maintained osmotic balance and energy supply under high load. In contrast, pH adjustment downregulated SPOB and propionate oxidation genes, causing persistent acid inhibition. This study elucidated the distinct regulatory effects of bioaugmentation and pH adjustment on high-load AD systems, providing actionable strategies for both maintaining operational stability in high-load reactors and recovering methanogenesis in acid-inhibited systems.