Self-generated ethanol in stillage fermentation for carboxylate upgrade

Introduction
Volatile fatty acids (VFA) are intermediates of the biogas process, thus can be produced from waste streams, and have a higher economic value as they can be used as industrial precursors and bulk chemicals. Several specific organisms present in mixed cultures have the capacity to elongate short VFA to caproate or others with a higher value using ethanol as a reducing agent. In this study we investigated different operational conditions to enhance VFA production from stillage of a bio-ethanol biorefinery. The main goals were to i) increase the titers of VFA; and ii) evaluate the ethanol requirements for carboxylate upgrade.

Methods
Two continuous reactors were fed with distilled stillage and operated at pH 5.5 and 6.5. They run for 300 days at different HRT. Batch tests with inoculum from reactor 5.5 were performed with different proportions of beer and stillage.

Results
A decrease of HRT improved the production rate of VFA from 2.7 to 8.8 g COD L-1d-1, but final concentration and composition remained similar within the same reactor (Fig.1). A decrease of pH inhibited methane production by VFA present in acidic form, but also reduced VFA concentrations in the broth. pH reduction induced ethanol production and therefore, a higher presence of longer carboxylates.
Mixing beer (ethanol-rich stream) in a fermentation broth included active yeast in the system and this used VFA to generate more ethanol available for chain elongation (Fig.2). However, high concentrations of ethanol inhibited VFA production.
Conclusions
Low pH induces ethanol generation in stillage fermentation and the introduction of small portions of beer can enhance the generation of ethanol and thus elongate carboxylates.