The stability of ethanol production from organic waste by a mixed culture depends on inoculum transfer time

Mixed Culture Fermentation is a promising route for bioethanol production from organic wastes. Yet, achieving a stable ethanologenic activity in undefined mixed cultures remains a challenge. This work aimed to retain ethanol production from organic municipal solid waste by microbial communities enriched from sheep rumen and anaerobic sludge mixtures, under low process control (initially aerobic conditions and initial pH ≤ 5.5). To find a stable operating window, sequential inoculum transfer intervals were evaluated (14 and 3-days). Soluble fermentation product profiles and changes in the prokaryotic communities were monitored. The originally inoculated batches always produced high ethanol concentrations (60 mM; 0.070 LEtOH/Kgwaste), equivalent to 1/6 of the current corn grain-based ethanol industrial production process. Fermentative activity and community richness significantly decreased in both transfer times regimes tested. However, the 3-day transfer interval led to a stable community which consistently produced ethanol (30 mM) as its main soluble fermentation product. Originally inoculated and 3-day transferred communities consistently enriched for a solventogenic Clostridium and an acid-tolerant Pseudomonas species. Ethanologenesis, as a dominant catabolic process, is an inherent property of these mixed culture fermentations, and its maintenance across successive transfers is critically dependant on the inoculum transfer time. 1 1 Abbreviations. EtOH: bioethanol/ethanol, OMSW: Organic fraction of Municipal Solid Waste, MCF: Mixed Culture Fermentation