Simultaneous hydrogen and ethanol production from sweet potato via dark fermentation

The indigenous bacteria, cow dung and sewage sludge were used as seed microflora sources to ferment sweet potato into hydrogen and ethanol under batch cultivation at a temperature of 37 °C. Hydrogen and ethanol production was evaluated based on their yields (HY and EY) and production rate (HPR and EPR) at various sweet potato concentrations (30–240 g/L), sweet potato particle diameters (<1.19 mm to >6.38 mm) and initial cultivation pH conditions (4.0–9.0). The experimental results indicate an interesting finding that sweet potato fermentation without adding external seed (i.e., the fermenter contained only the indigenous bacteria which were identified to have seven species) could produce hydrogen and ethanol (EtOH). The indigenous bacterial community analysis showed that Klebsiella oxytoca, Clostridium fimetarium, Grimontella senegalensis, and Enterobacter asburiae (or Escherichia coli) were present at peak ethanol production and two more species Ruminococcus schinkii and Lactovum miscens were present at peak hydrogen production. ternally seeding strategy could enhance the hydrogen production from sweet potato with peak cumulative H2 production potential (HP) of 120 mmol H2/L-reactor using sewage sludge. However, the cow dung seed showed more ethanol production than hydrogen with peak concentration of 12,146 mg COD/L. Increasing the sweet potato concentration could enhance the HPR and HP values. Maximum HY of 1.24 mol H2/mol hexose was obtained at a substrate concentration of 150 g/L with an initial cultivation pH of 6.7 and particle diameter <1.19 mm using sewage sludge. The HPRmax, HY and HP values were affected slightly by the variations in sweet potato particle size. The peak total energy production yield of 1625 J/g sweet potato was obtained with sweet potato concentration of 150 g/L at pH 8.5 using sewage sludge.