The integration of bioelectrical systems to a bioethanol production plant was investigated on small scale. A reactor was constructed with the aim to be operated as a microbial fuel cell (MFC) and a microbial electrolysis cell (MEC). The reactor was characterized in terms of power output and internal resistance with S. cerevisiae as biocatalyst. An internal resistance of 954 Ohm was measured for a maximum power output of 176 mW/m3 anode.
Electrical currents were measured with the MFC using S. cerevisiae as biocatalyst. Five glucose concentrations ranging from 20 g/l to 170 g/l were tested in presence and absence of the mediator Methylene blue. The mediator enhanced the current production but for small glucose concentrations but decreased also the ethanol production yields. In a general way higher glucose concentrations increased the number of coulombs produced, this was observed in presence and absence of the mediator.
Four set-ups were tested to produce hydrogen in MEC mode. The MEC was first operated in single chamber mode, both anode and cathode were immersed in a S. cerevisiae fermentation, hydrogen production was not detectable but a high amount of carbon dioxide. For the second set-up a proton exchange membrane was added between the half cells and a phosphate buffer pH 7.0 was used as catholyte. 2.0 V, 3.0 V and 4.0 V were applied, traces of hydrogen were detected under 3.0 V and 4.0 V. However applying such high voltages impacted negatively ethanol production. For the third set-up HCl 0.5 M and 2.0 M were used as catholyte. 0.48ml of hydrogen was produced with HCl 0.5 M and 0.67 ml with HCl 2.0M under 1.3 V. However, we observed a drastic decrease of ethanol yields and S. cerevisiae viability when using such acidic catholytes. Finally, fermentation wastes were recovered at the end of a fermentation and digested in a MEC using Shewanella oneidensis MR1 as biocatalyst.
For the last series of experiments, the valorization of fermentation wastes through methane production was tested. Wastes were recovered from ethanolic fermentations realized with S. cerevisiae. The wastes were then digested in an electromethanogenesis cell having active biofilms on the electrodes. The wastes were successfully converted into methane, highest conversion rate of the COD into methane was 91.1%.
