Different roles of syngas and ethanol in promoting caprylate production from short-range sludge anaerobic fermentation broth in microbial electrosynthesis: performance and mechanism
Carbon chain extension (CE) in microbial electrosynthesis (MES) is considered a promising resource recovery technology to convert short-chain fatty acids (SCFAs) in waste activated sludge into high-value medium-chain fatty acids (MCFAs) efficiently. However, a significant challenge lies in the insufficient availability of electron donors for this biotechnological application. This study selected ethanol and syngas as external electron donors and utilized various types of sludge fermentation broths as substrates to examine the effects of single and multiple electron donors on the production, selectivity, and electrochemical performance of medium-chain fatty acids in the MES systems, alongside studying the microbial response. The results indicated that ethanol had a more favorable effect on CE efficiency compared to syngas. Furthermore, the combined use of both electron donors demonstrated the most optimal enhancement of MCFAs yield and electron transfer efficiency. At the genus level, Acetobacterium, a member of Clostridia class, predominated in all experimental groups, especially in those supplemented with syngas. Simultaneous supplementation of syngas and ethanol led to a high resource recovery rate through synergistic interactions among various bacterial groups, such as homoacetogenic bacteria, EAB, and CEB, proving the potential to achieve the highest economic profitability.