Short-chain fatty acids production from maize silage: impact of stepwise hydraulic retention time reduction on microbial adaptation and process output

The conversion of maize silage, byproduct of bioethanol production, into short-chain fatty acids (SCFAs) through anaerobic fermentation (AF) is a promising approach. Due to the diverse microbial metabolisms involved, precise control of operational parameters is crucial to maintain the stability of the AF and achieve maximum process outputs. This study investigates the impact of the stepwise hydraulic retention time (HRT) reduction on the stability of microbial communities and SCFAs production. To address this, a 40-L semi-pilot continuous stirred tank reactor was operated under different HRT conditions: 15, 12.5, 10, and 7.5 days. Results showed that the maximum bioconversion of 28% g CODSCFAs/g CODin (expressed as chemical oxygen demand [COD]) was reached at HRTs higher than 7.5 days, while the highest total SCFAs concentration of 14.3 g/L was observed at HRT of 15 days. SCFAs derived from maize silage mainly consisted of acetic, propionic, butyric, and valeric acids, maintaining a consistent profile across all evaluated HRTs. Canonical correspondence analysis results indicated synergistic and cross-feeding interactions among key microbial bacteria involved in AF such as unclassified Clostridiales, Megasphaera, Lachnospiraceae, Ruminococcaceae, and Prevotella for all tested HRTs. This finding indicated that these microorganisms were capable of maintaining their function and stability across different HRT conditions.