Staged modulation using synergistic alkaline biochar-ferrate enhances medium-chain fatty acid production from waste activated sludge

Medium-chain fatty acids (MCFAs) are carbon-neutral alternative to petroleum-derived chemicals, offering sustainable valorization for waste activated sludge. Current bioproduction systems, however, face a critical dual bottleneck stemming from complex sludge matrices and inefficient carbon flux regulation. Here, we develop a stage-optimized modulation strategy employing alkaline biochar (AlkBC)-ferrate to sequentially enhance sludge solubilization and targeted MCFA-producing fermentation. Maximum MCFA production reaches 10495.0 mg chemical oxygen demand L^-1, 20.6-, 15.2- and 2.3-fold higher than the control, AlkBC-alone and ferrate-alone groups, respectively. Mechanistically, AlkBC initiates ferrate activation to produce metastable Fe(IV)/Fe(V) intermediates via physical adsorption, electron donation, and oxygen-functionalized coordination. High-valent Fe species oxidation coupled with AlkBC-elevated alkalinity efficiently disrupts sludge polymerized structure and drives bioconversion of released organics. Simultaneously, AlkBC is structurally reconfigured due to ferrate oxidation with surface-loaded Fe₂O₃ as active component, substantially enhancing chain elongation. Furthermore, AlkBC-ferrate enriches key functional bacteria while suppressing methanogens, steering carbon flux towards MCFA production.