Apparent hydrogen consumption in acid reactors: observations and implications

Investigations of hydrogen production by dark fermentation have received increasing attention as a green fuel production process. Research focus is mainly on yields and rates of hydrogen production under different operation conditions. The importance of hydrogen consumption is addressed here, based on results from lab-scale reactors. Experiments were run using mixed cultures and a variety of operating conditions: HRT 6-40 hours; temperature 25-55 degrees C. Initial hydrogen yields between 0.8-1.5 mol H(2)/mol glucose and approximately 50% H(2) in headspace was observed, followed by a decrease in hydrogen production as the culture matures, resulting in hydrogen yields down to 0.02 mol H(2)/mol glucose. It is concluded that hydrogen or "hydrogen equivalents" consumption is significant, especially in reactors with high biomass concentration and/or high sludge age. Sustainable H(2) production by dark fermentation alone is therefore not likely to be developed. The results suggest that it is possible to control and avoid significant H(2) production in dark fermentation. Minimizing H(2) production can be useful in preparation of organic feed for other bio-fuel production processes, such as methanogenic processes and bio-electrochemical H(2) production.