Dark Fermentation of Arundo donax Hydrolysate for Hydrogen Production

The exploitation of mixed cultures for the dark fermentation of hydrolysates of lignocellulosic biomass is hindered by low hydrogen yields due to the presence of inhibitors of bacterial growth (phenols, furans) and to proliferation of bacteria not involved in the production of hydrogen (e.g. lactic acid bacteria) or consuming hydrogen (methanogens, acetogens). Initially, a non-methanogenic anaerobic consortium has been tested for the the dark fermentation of a synthetic medium with glucose as sole carbon source (yields average at 0.66 mol H2/mol glucose). The effect of single inhibitors (p-coumaric acid, furfural) added to the synthetic medium has been evaluated in terms of hydrogen yields and kinetics. 0.2 g/L of p-coumaric acid limited both kinetics and yield of H2 production (dropping to 0.16 mol H2/mol glucose), whereas 2.0 g/L furfural prolonged the lag phase of about one day without significantly affecting the overall yield. The control of medium pH has then been exploited for the enhancement of hydrogen yields. Keeping pH at 6.0-6.5 has shown to be an efficient way to prevent the outgrowth of lactic acid bacteria during dark fermentation of glucose, improving H2 yields (1.33 mol H2/mol glucose). Finally, comparable hydrogen yields (1.40 mol H2/mol glucose equivalents) have been observed during the dark fermentation of Arundo donax hydrolysate with pH control, notwithstanding the presence of inhibitor compounds in the medium.