Fe3O4-Enhanced Continuous Production of Medium-chain Fatty Acids from Sludge: Metagenomic Perspective on Microbial Synergy.

Fe3O4 exhibits promising potential in enhancing chain elongation (CE) for medium-chain fatty acids (MCFAs) production from sludge, yet its application in continuous flow processes remains underexplored. In this study, MCFAs were continuously produced from sludge via a Fe3O4-enhanced CE process. Optimal performance was achieved with 20 g/L Fe3O4, 5.8 g COD/L/d ethanol loading rate (ELR), and 3-day hydraulic retention time (HRT), yielding 11.4 ± 0.8 g COD/L MCFAs and a production rate of 3.7 ± 0.1 g COD/L/d. n-Caproate and n-caprylate accounted for 40.9 % and 54.4 % of total MCFAs, respectively. Fe3O4 addition halved CE start-up time and increased MCFA production fourfold. Metagenomic analysis identified Clostridium kluyveri, Petrimonas sp., Alcaligenes faecalis, Proteiniphilum acetatigenes, and Anaerosalibacter sp. as biomarkers enhancing the fatty acid biosynthesis (FAB) cycle. These taxa could increase malonyl-ACP availability, improve reductive reaction efficiency, and promote acetyl-CoA generation, facilitating a streamlined microbial community with Pseudomonas caeni. This investigation offers strategic insights and data support for the high-value utilization of excess sludge.