Energy valorization processes of glycerol for the production of biogas, biohydrogen and/or electric current through microbial fuel cell

This study focused on the valorization of glycerol which is an important by-product of the biodiesel industry corresponding to 10 % of the produced biodiesel amount. This fact contributed to the increase of the global production of biodiesel, to a point at which the industries which traditionally consumed glycerol could not absorb. This situation should be overcome through new outlets on glycerol exploitation. Usually, glycerol is treated by chemical processes in order to form new chemical compounds.On the other side, biochemical processes like anaerobic digestion and fermentation or the technology of microbial fuel cells could potentially transform glycerol into methane, hydrogen and electric current respectively. These processes, which are the subject of this Ph.D, are preferable to their chemical counterparts due to the low energy demand and reduced environmental pollution.The anaerobic digestion process was conducted in a conventional CSTR reactor and in a high rate reactor, the PABR. The experiments dealt with the effect of glycerol concentration on the methane production rate. The obtained results showed that the CSTR could not withstand organic loadings above 0.25 g COD/L/d, however PABR operated at organic loading 10 times higher than CSTR such as 3 g COD/L/d and resulted to a methane production rate of 0.982 ± 0.089 L/L/d. A model was developed for both the CSTR and the PABR digesters. Fermentative hydrogen production was conducted successfully in batch reactors. The effect of the initial glycerol concentration and initial pH on hydrogen production was studied. A maximum yield, 27.3 mL H2/ g COD glycerol, was obtained when glycerol concentration was 8.3 g COD/L and the pH 6.5. Moreover, the fermentation of glycerol took place in a CSTR in order to investigate the continuous production of hydrogen. Hydrogen production was unstable, possibly due to the washout of proper biomass from the reactor.For electricity generation from glycerol, an H-type microbial fuel cell was used in batch mode. The effect of the initial glycerol on the electric current was studied. A maximum Coulombic efficiency (CE) 34.09% was obtained at a glycerol concentration of 3.2 g COD/L. A further increase of glycerol drove to a drop of the CE. Probably, this happened since the electrochemical microorganisms were inhibited by the high glycerol concentration