Initiating the Caproate Production from Waste Activated Sludge without External Electron Donors by the Oriented Microbial Hydrolysis

Medium-chain fatty acids (MCFA) are scarcely produced when waste activated sludge (WAS) is used as the sole substrate, limiting their practical development. Production typically requires external electron donors following chemical or physical pretreatments. This study demonstrates that an enriched MCFA-producing consortium (MPC) can initiate MCFA production from WAS without any external electron donors. In the WAS group, the COD conversion ratios from WAS to SCFA (short-chain fatty acids) and MCFA were 6.0% and 0%, respectively. After dosing MPC in WAS, these ratios increased to 20.7% and 4.6%, respectively. These results were attributed to the higher hydrolytic activity in MPC compared to WAS; for instance, the relative activity of alginate lyase in WAS was only 2.8% of that in MPC. <i>Bacteroides</i> (33.1%) was identified as the key bacterium in MPC due to its role in secreting multihydrolases and intracellular enzymes to degrade uronic acids. Conversion of the hydrolysates to MCFA occurred predominantly via the fatty acid biosynthesis (FAB) pathway, as evidenced by the comprehensive identification of FAB-related genes and active enzymes via multiomics analysis. The initiating mechanism is thus attributed to the high activity of both multihydrolases and the FAB pathway without external electron donors. Overall, this study demonstrates the oriented microbial method, coupling the hydrolytic efficiency of FAB-related bacteria and the favorable thermodynamics of the reverse β-oxidation (RBO) pathway in the future, to support the recovery of MCFA from WAS.