Multivariate insights into mechanisms of volatile fatty acid accumulation affecting solid-state anaerobic digestion: Process characteristics, microbial community, and metabolic analyses

Solid-state anaerobic digestion (SSAD) is a promising technology, but vulnerable to volatile fatty acid (VFA) accumulation mainly composed of acetate and propionate. This study was performed to evaluate the syntrophic effects of VFAs and investigate the micro-factors for the decreasing methane production under adding different concentrations (2, 4, and 6 mg/g) of acetate and propionate in comparative batch SSAD of pig manure and corn straw. The results showed that mild propionate accumulation could help to mitigate acetate accumulation, while acetate accumulation could inhibit propionate degradation by affecting crucial redox reactions in propionate degradation pathway based on enzyme genes activities. As the levels of VFA accumulation increased, methane production was considerably reduced due to the inhibition of acetoclastic and hydrogenotrophic pathways and interspecies formate transfer. Furthermore, dominant Clostridium member in SSAD could tolerate acid environment, causing severe VFA accumulation by recovering hydrolysis and acidification under adding 6 mg/g acetate or propionate.