Long-term Production and Recovery of Medium-Chain Carboxylates from Source-Separated Organics

Abstract

Source-separated organics (SSO) are widely processed via anaerobic digestion to produce biogas, yet alternative conversion pathways could generate higher-value products. Here, we demonstrate long-term continuous production and recovery of medium-chain carboxylic acids (MCCAs) from SSO via microbial chain elongation using a bench-scale anaerobic bioreactor operated for 911 days. The bioreactor was fed with SSO samples collected from two full-scale municipal organics processing facilities in Toronto, Canada, capturing facility-specific and seasonal variability in SSO composition. MCCA production was closely associated with the availability of lactate as an electron donor, which varied with SSO preprocessing operations and outdoor collection temperatures. To mitigate product inhibition, an in-line extraction system using hollow-fiber polydimethylsiloxane (PDMS, also known as silicone polymer) membranes was integrated with the anaerobic membrane bioreactor, providing a robust and solvent-free alternative to solvent-based extraction methods. Selective extraction reduced MCCA product inhibition when lactate availability was not limiting, resulting in maximum MCCA yields and production rates of 0.31 g MCCA g-1 VSfeed and 0.84 g L-1 day-1, respectively, with notable octanoic acid production (up to 20% of total MCCA). Acidification of the alkaline extract produced a phase-separated MCCA-rich oil (0.88 g MCCA g-1 oil) without the addition of downstream separation steps. Microbial community analysis of the bioreactor revealed enrichment of putative chain-elongating bacteria, including Eubacterium and Pseudoramibacter species, while shifts in SSO feedstock microbiomes influenced substrate availability and product spectra. These results demonstrate the feasibility of sustained MCCA production from municipal organic waste streams and highlight opportunities to integrate chain elongation with existing anaerobic digestion infrastructure.