Thermal Hydrolysis-Driven Lactate Enrichment and Caproate Specificity in Self-Buffered Mixed Waste Fermentation: Mechanistic Insights into a Long-Term Integrated Chain Elongation and Anaerobic Digestion Process

Abstract This study demonstrates a sustainable continuous production of medium-chain fatty acids (MCFAs) from real waste streams, achieving stable caproate synthesis without external buffering or electron donor addition. This new process couples microbial chain elongation of mixed food waste (FW) and waste activated sludge (WAS) with downstream anaerobic digestion (AD), using thermal hydrolysis (TH) of WAS as a key enabling step. Both acidogenic reactors─one fed with FW + WASraw and one with FW + WASTH─exhibited high and stable performance, with acidification degrees of 79–84% and total fatty acid (C2–C8) concentrations of 12 ± 1 gCOD L–1. Notably, the TH-assisted system achieved a consistent and selective caproate (C6) production of 4.7 ± 0.5 gCOD L–1 with 24% higher productivity compared to the untreated condition. Process robustness was ensured under unbuffered conditions, with tight pH control (5.1–5.3) mitigating undissociated C6 toxicity and supporting long-term stability. High-frequency monitoring revealed a rapid substrate turnover with a dynamic lactate-driven metabolism, where endogenous lactate sustained efficient transition to elongation, enabling continuous carbon flux toward MCFAs. Coupling the system with AD ensured complete valorization of fermentation residues, achieving methane yields of 0.43 ± 0.02 Nm3 kg–1 VSbroth and stable operation despite the short HRT of 15 days. Metals tracking showed that heavy metals remained largely confined to the solid phase, while Mn, Na, K, Ca, Mg, and P, mobilized during fermentation, re-precipitated during AD, resulting in a nutrient-rich, agriculturally safe digestate. Overall, this work demonstrates a robust, self-sustaining, and scalable platform for urban biowaste valorization through integrated MCFAs production and biomethane recovery.