Pulsed feeding strategies for enhanced in situ lactate-to-caproate conversion during self-buffered chain elongation of urban organic waste

The valorization of urban biowaste is central to circular bioeconomy strategies; however, acidogenic fermentation and chain elongation (CE) of real waste streams require exogenous electron donors and chemical buffering, limiting long-term sustainability. Achieving stable production of medium-chain fatty acids (MCFAs), particularly caproic acid (C6), under chemicals-free conditions remains challenging. This study investigates long-term fermentation and CE in a single mesophilic bioreactor treating urban substrates, without external electron donors or buffering agents. A thermally pretreated mixture of food waste and waste activated sludge was processed in a 5.5 L semicontinuous reactor operated under pulsed feeding, enabling endogenous, lactate-driven CE. Two feeding frequencies were tested at a constant organic loading rate of 10 gVS L−1 per feed: twice and three times weekly. In both cases, the system exhibited stable performance, with pH self-regulation in a mildly acidic range (≈5.0–5.4) driven by coupled lactate production and consumption. Total fatty acid concentrations remained consistent (11.9–12.8 gCOD L−1), with even-chain species dominating the product spectrum (acetic ∼33%, caproic ∼30%, butyric ∼21%). Lower feeding frequency maximized caproic acid productivity (31.0 ± 1.5 mmolC L−1 d−1), whereas higher feeding frequency increased annualized caproate production by 29.5%, indicating a trade-off between per-cycle yield and cumulative throughput. Microbial analyses revealed a selective enrichment of lactate-producing Olsenella alongside CE-associated Caproiciproducens and Pseudoramibacter taxa. Biomethane potential tests of the fatty-acid-rich broth yielded 0.46–0.49 m3CH4 kg−1VS, demonstrating effective downstream anaerobic digestion. Overall, these results demonstrate a robust, additive-free CE–AD platform for integrated waste-to-chemicals and energy recovery.