Vacuum-enhanced stripping allows the extractive acidogenic fermentation at thermophilic conditions

Acetic (A), propionic (P), isobutyric (IB), butyric (B), isovaleric (IV), and valeric (V) acids, known as Volatile Fatty Acids (VFAs), can be selectively produced by microbial mixed culture (MMC) through fermentation of waste materials with methane-arrested processes. Extracting VFAs during acidogenesis can overcome product inhibition while obtaining high-purity VFAs. Being volatile and acidic, VFAs can be recovered by stripping from the fermentation broth and contacting the stripping flow with an alkaline trap (e.g. CaCO3). The stripping performance depends on concentration, pH, actual Henry solubility (Hs) of VFAs and stripping kinetics. Experimental results showed that, within actual conditions of thermophilic acetogenesis, Hs is lower than expected, namely 1047 ± 314, 529 ± 93, 179 ± 16, 268 ± 32, 127 ± 10 and 154 ± 20 (M/atm) respectively for A, P, IB, B, IV, and V. Low absolute pressure (0.15 bar) enhances the stripping rate at 50–55 °C. With 30 gVFA L−1 at pH ≈ pKa, vacuum enhanced stripping allows the extraction of 2.5 gCODVFA L−1 h−1 from a 10 g/L solution of A, P, and B, with an extraction rate for B > P > A. An extractive fermentation process was finally tested by using food waste analogue (i.e. chickpea flour and vegetable oil) and a newly developed laboratory-scale device that combines a standard anaerobic digester with a vacuum stripping device; maintaining the pH at 5.2, a 74 % yield (gCODVFA/gCOD) was achieved within 15 days, demonstrating the efficacy of the stripping method for the extraction of VFAs from fermentation systems.