Properties of Ethanol-Driven Chain Elongation for Caproic Acid Production Under Different pH Conditions: Effect of Inoculum Sources

Caproic acid (CA) production through ethanol-driven chain elongation (CE) is a promising pathway to valorize organic wastes. However, the effect of pH and inoculum source on substrate conversion properties and microbial communities was not fully explored. In this study, performance of caproic acid production with anaerobic methanogenic sludge (AMS), aerobic sludge (AS) and chain elongation sludge (CES) at different pH conditions (uncontrolled (UN), 5, 6, and 7) were investigated. It was found that microorganisms in all inocula could degrade ethanol, but the consumption rate was different. The AS mainly used substrate for biogas production, without CA accumulation, while AMS and CES could synthesize butyrate and caproate with ethanol and acetate as substrates. At pH UN and 5, excessive ethanol oxidation (EEO) was activated and transformed ethanol into acetate resulting in low CA yield. Increasing pH to 7, the AMS produced more caproate and achieved a higher CA yield (0.36 g-COD/g-COD) than that of CES (0.33 g-COD/g-COD). Microbial communities in raw inocula were different, which led to distinct substrate conversion pathways. After fermentation, Anaerolineaceae was the dominate family in AMS, while Corynebacteriaceae and Dysgonomonadaceae dominated in the reactor with CES, explaining the distinct caproate yield in both reactors. The results of this study provided useful information for constructing ethanol-driven CE processes from organic wastes.