Effects of initial volatile fatty acid concentrations on process characteristics, microbial communities, and metabolic pathways on solid-state anaerobic digestion

Solid-state anaerobic digestion (SSAD) is vulnerable to excess volatile fatty acids (VFA), mainly acetate and propionate. The co-effects of VFAs and microbial dynamics under VFA accumulation were investigated in SSAD of pig manure and corn straw. Adding 2 and 4 mg/g acetate or propionate caused initial increases in total VFAs, followed by decreases after day 6, resulting in ‘mild’ VFA accumulation, while adding 6 mg/g caused similarly increased VFAs, but with no subsequent decrease, causing ‘severe’ VFA accumulation and poor methanation performance. Mild propionate accumulation promoted acetate consumption, whereas acetate accumulation inhibited propionate degradation by affecting crucial redox reactions. Under severe VFA accumulation, hydrolysis and acidification mainly conducted by acid-tolerant Clostridium sp. exacerbated VFA inhibition, causing a competition between Methanosarcina and Methanosaeta, and impairments of acetoclastic and hydrogenotrophic methanogenesis and interspecies formate transfer. This study provides new insights into mechanisms of VFA accumulation in SSAD, and its effects on methanogenesis.