Different Electron Donors Drive the Variation in the Performance of Medium-Chain Fatty Acid Production from Waste-Activated Sludge

Producing high-value medium-chain fatty acids (MCFAs) via anaerobic fermentation is an emerging green biotechnology to recover bioenergy from waste-activated sludge (WAS). Electron donor (ED) is a key driver affecting the MCFA production profiles. This study investigated and compared the potential of using ethanol, lactate, and a combination of both as ED supplies (Co-EDs) for enhancing MCFA production during anaerobic WAS fermentation. Adopting ethanol as the sole ED was the optimal strategy to attain the highest MCFA production (3352.7 +/- 564.7 mg of COD/L) and selectivity (20.7%) from WAS. Although lactate dosage promoted WAS degradation, more organics were converted to short-chain fatty acids (SCFAs) rather than MCFAs, resulting in the lowest MCFA production (1034.8 +/- 303.6 mg of COD/L) afterward. Lactate- and Co-EDs-added systems enriched hydrolytic bacteria and SCFA producers to a higher level than the reactor with an ethanol supplement. Nevertheless, the richest abundance of Clostridium_kluyveri, a key MCFA producer, was attained in the ethanol-added system. Further metatranscriptomic analysis mapped all expressed metabolic pathways involved in MCFA production and revealed that no synergistic effect was observed in the Co-ED system. The insufficient electron acceptor (EA) synthesis at the initial stage induced the inefficiency in ED usage for CE, rendering a huge waste of ED (i.e., lactate) via the competitive acrylate pathway. In comparison, the gene abundance related to the CE cycle was the highest in the ethanol system. This study provided useful information and insights for enhancing the MCFA yield by optimizing ED composition from various substrates.