Synergistic 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate and Sodium Percarbonate Pretreatment for Enhanced Medium-Chain Fatty Acid Recovery from Waste-Activated Sludge: Mechanisms and Microbial Insights

Abstract Medium-chain fatty acids (MCFAs) offer a sustainable route for valorizing waste-activated sludge (WAS) into higher-value products, yet their bioproduction is constrained by the rigid extracellular polymeric substance (EPS) barrier and inefficient chain elongation (CE). Here, we develop a synergistic pretreatment strategy employing 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][OTf], a representative ionic liquid) and sodium percarbonate (SPC) to sequentially enhance sludge solubilization and targeted MCFA production. The maximum MCFA yield reaches 5910 mg COD/L in the combined group, 1.3-, 1.1-, and 0.7-fold higher than the control (2537 mg COD/L), [Emim][OTf] alone (2888 mg COD/L), and SPC alone (3409 mg COD/L) groups, respectively. Mechanistically, SPC acted as the primary oxidant to disrupt EPS and cell membranes, while [Emim][OTf] synergistically amplified this effect, as evidenced by enhanced protein solubilization and the progressive mineralization of organic nitrogen to ammonium. The combined pretreatment enriched acidogens (e.g., Proteiniclasticum, 96.8-fold increase), CE-associated Bacillota and Petrimonas (1.1- and 24-fold increases), and MCFA release-associated thioesterase-encoding genes (e.g., fluoroacetyl-CoA thioesterase, 0.5-fold increase). These findings demonstrate that [Emim][OTf] not only activates SPC-mediated sludge disintegration but also facilitates CE, as evidenced by a 1.7-fold increase in MCFA selectivity in model substrate tests, offering a dual-functional strategy for the efficient recovery of MCFA from WAS.