ENERGY VALORIZATION OF RESIDUES OF DARK ANAEROBIC PRODUCTION OF hYDROGEN

Hydrogen production, via Dark Anaerobic Fermentation, leads to a negative net energy balance because of the difference between the energy produced as hydrogen and the direct ones (heat and electricity) consumed to produce it. The residual metabolites (Volatile Fatty Acids and alcohols) at the end of hydrogen production are oxidized compounds with a high-energy value. This paper deals with experimental tests that were conducted to recovery the energy embedded in these metabolites. Two proofs of the tests have been realized following two strategies: i) production of biogas containing methane by methanogen microorganisms from liquid metabolites; ii) production of additional H2 from acetate as main constituted of Volatile Fatty acids (VFAs) by Microbial Electrolysis Cell (MEC) with a cation exchange membrane (CEM). Both technologies have shown to be feasible and offer a double positive effect: the total produced energy increased and a beneficial effect, due to the achievement of a greater abatement of the organic wastewater load in the effluent, was obtained. A detailed energy analysis has showed that the second methanogenic stage increases the overall energy efficiency of the two-stage process to a great extent compared to the H2 stage on its own. The introduction of a second step leads to a positive net energy production for the methanogenic step, while additional research is necessary for the case of hydrogen production by MEC, as this technology is still at the infancy stage.