Dark Fermentation Optimization by Anaerobic Digested Sludge Recirculation: Effects on Hydrogen Production

The addition of small amounts (5 - 10 %) of H-2 to rich CH4 biogas improves the quality of gas combustion while reducing CO2 emissions. For this reason, many researchers have recently been focused to optimize the two phase anaerobic digestion process (dark fermentation, DF and anaerobic digestion, AD) to concurrently produce H-2 and CH4 from biowaste and organic residues. In this paper the results of a two phases thermophilic AD process treating biowaste for H-2 and CH4 production are presented: in this process, nor physical neither chemical pre-treatment of inoculum, was used, but recirculation of anaerobic digested sludge to the DF reactor was exploited to control the pH in the optimal H-2 production range of 5-6. The experiment was carried out in bench scale using a stirred reactor (CSTR), applying an organic loading rate of 16 KgTVS/m(3)d, and an hydraulic retention time of 3 days in DF phase. Four different recirculation conditions were tested keeping constant the ammonia concentration (about 500 mg/L) through a separation process by evaporation. The aim was to investigate the influence of ammonia in the biological process of hydrogen production via dark fermentation and to research the recirculation condition that allow to reach the best yields and a stable process. The optimal hydrogen production (SHP of 0.03 m(3)/Kg TVS fed, 30 % H-2 in the off - gas) was found using a recirculated sludge ratio of 0.66. This allowed for a stable pH value around 5.5.