Exploitation of sweet sorghum biomass for biofuel production using mixed acidogenic and methanogenic cultures and pure cultures of ruminococcus albus

Full text: The present study focuses on the exploitation of sweet sorghum biomass for gas biofuel production in continuous and batch systems. Sweet sorghum is an annual C4 plant of tropical origin, well-adapted to sub-tropical and temperate regions and highly productive in biomass. It is rich in readily fermentable sugars and thus it can be considered as an excellent raw material for biohydrogen production from many different fermentative microorganisms. Extraction of free sugars from the sorghum stalks was achieved using water at 30 degrees centigrade. After the extraction process a liquid fraction (sorghum extract), rich in sucrose, and a solid fraction (sorghum cellulosic-hemicellulosic residues or sorghum bagasse), containing the cellulose and hemicelluloses, were obtained. A two-step continuous process was developed for the biological hydrogen production and the subsequent production of biogas from sweet sorghum extract. In the first reactor sugars were fermented to hydrogen, volatile fatty acids and alcohols b mixed acidogenic culture derived from the indigenous microfauna of sweet sorghum. The hydrogen producing reactor was operated at five different hydraulic retention times (HRT), i.e 24h, 12h, 8h, 6h and 4h. The HRT of 12h proved to be the most effective leading to the production 10.4 L H2/kg sweet sorghum biomass. Subsequently, the effluent was fed to the methanogenic reactor, where all the residual organic compounds were digested by an acclimated methanogenic culture derived from activated sludge. The operation of the methanogenic reactor was studied at three different HRTs, i.e 20d, 15d and 10d with the latter being the most prosing leading to the production 35.2 L CH4/kg sweet sorghum biomass. Both continuous and batch cultures were used for the investigation of hydrogen production from sweet sorghum biomass using Ruminococcus albus. R. albus is an important, fibrolytic bacterium of the rumen that can hydrolyse both cellulose and hemicellulose and ferment disaccharides, hexoses and pentoses. A continuous stirred tank reactor operated at 42h HRT was used for hydrogen production from sweet sorghum extract, whereas the exploitation of the whole sorghum biomass, in one step and two step processes was investigated using batch reactors. The hydrogen yield obtained from sorghum extract treated with R. albus was as high as 2.1-2.6 mol hydrogen per mol of glucose consumed. Hydrogen yield of sorghum bagasse fermented with R. albus reached 2.6 mol hydrogen per mol of glucose equivalent consumed. In total, the productivity of sorghum biomass (that of sorghum extract plus that of sorghum bagasse) using R. albus reached 60 L hydrogen per kg of sorghum biomass and it was the same in both cases of one and two step processes. (authors)