Fermentation of a complex organic feedstock to medium chain carboxylates by a natural microbial consortium : investigation on the substrate influence and the process limitations

The depletion of natural non-renewable resources pushes towards better using renewable bio-resources. Medium chain carboxylic acids (MCCAs, C6 to C10) are useful molecules so far produced by petro-chemistry. An opportunity to produce MCCAs from biowastes has recently been identified: bacteria can ferment organic matter into short chain carboxylic acids (SCCAs), then elongate them into MCCAs. In previous experiments in our team, MCCAs were produced in batch by a mixed microbial culture (MMC) from brewer's spent grain (BSG), used as complex organic feedstock. With the supply of a H2+CO2 gas mixture, the MCCAs were produced in two phases, and no plateau concentration was reached. The aim of the present work was to know whether the two-step MCCA production is reproducible, when and why the MCCA production would stop, and how to avoid the end of the production. It was also to identify the influence of the substrate composition on the fermentation profile. Batch fermentations supplied with H2/CO2 77/23 v/v at 1.3 bar and with a MMC as inoculum were performed with 2 different substrates: BSG and 4-day acidogenic mixed liquor (AML), in addition to fermentations with no organic feedstock (NOF) or, i.e., H2O. These experiments allowed to reproduce the 2nd phase of MCCA production and to reach a 2nd plateau at 15.1-19.3 gCOD_MCCAs/l after about 30 days. The highest MCCA concentration was 19.3 gCOD/l. All conditions tested produced MCCAs, in descending order of production: BSG, AML, NOF. After performing the fermentations under batch mode, the switch to fed-batch and semi-continuous modes at different times of fermentation was tested. The semi-continuous fermentations (HRT of 7 days) of BSG and AML presented a continuous MCCA production, the production from AML being larger. The H2O-fed (NOF) semi-continuous fermentation stopped producing MCCAs when the SCCA concentration fell below 5 gCOD/l. The results demonstrated that the two-step MCCA production is reproducible and stops when the MCCAs reach concentrations up to 15.1-19.3 gCOD/l, which may correspond to the inhibitory concentration for microorganisms. Passing to a semi-continuous system could allow to keep the MCCAs ongoing under this concentration. Continuous MCCA productions were observed as long as SCCAs remained > 5 gCOD/l. Mean production rates of 0.93 and 0.45 kgCOD_MCCAs/(m^3*d) were obtained with AML and BSG, respectively.