Direct fermentation of sweet potato to produce maximal hydrogen and ethanol

The feasibility of biofuels (hydrogen and ethanol) production from sweet potato without any pretreatment was evaluated using response surface methodology. Batch experiments were carried out with cow dung and anaerobic sewage sludge seeds at a temperature of 35 °C, pH of 4.5–8.5 and substrate concentrations of 50–250 g/L. The maximum hydrogen production rate of 184.3 mmol/L/d, maximum specific hydrogen production rate of 37.5 mmol/gVSS/d and maximum hydrogen yield of 1.32 mmol/gCODsubstrate was obtained with cow dung seed at pH 6.5 with a sweet potato concentration of 150 g/L. The maximum ethanol concentration of 12.2 gCOD/L was obtained at sweet potato 150 g/L and an initial cultivation pH 8.5 with cow dung seed. The main soluble microbial products (SMPs) were ethanol, acetate and butyrate with small quantities of propionate at all pH values by both seeds. The maximum SMP of 34.6 gCOD/L with a total volatile fatty acids concentration of 26.5 gCOD/L was obtained at pH 7.4 and sweet potato concentration 200 g/L with cow dung seed. Cow dung was a good seed source for directly producing biohydrogen and ethanol from sweet potato under mesophilic condition.

► Sweet potato has hydrogen and ethanol production potential using dark fermentation.
► At pH 6.5 and 150 g/L sweet potato, cow dung gave peak hydrogen production rate.
► At pH 8.5 and 150 g/L sweet potato cow dung gave peak ethanol concentration.
► Main organic acids were acetate and butyrate at all pH values.