Effect of gas sparging on continuous fermentative hydrogen production

The effect of gas sparging on continuous fermentative H₂ production was investigated in completely stirred-tank reactors (CSTR) using internal biogas, N₂ and CO₂ with various flow rates (100, 200, 300 and 400 ml/min). The sparging with external gases of N₂ and CO₂ showed higher H₂ yield than the control of no sparging and internal biogas sparging. It indicated that the decrease of H₂ partial pressure by external gas sparging had a beneficial effect on H₂ fermentation. Especially, CO₂ sparging was more effective in the reactor performance than N₂ sparging, accompanied by higher production of H₂ and butyrate. The best performance was obtained by CO₂ sparging at 300 ml/min, resulting in the highest H₂ yield of 1.68 mol H₂ / molhexose_consumed and the maximum specific H₂ production rate of 6.89 L H₂ / g VSS/day. Compared to N₂ sparging, there might be another positive effect in CO₂ sparging apart from lowering H₂ partial pressure. High CO₂ partial pressure had little effect on H₂-producing bacteria but inhibitory effect on other microorganisms such as acetogens and lactic acid bacteria which were competitive with H₂-producing bacteria. Only H₂-producng bacteria, such as Clostridium tyrobutyricum, C. proteolyticum and C. acidisoli were isolated under CO₂ sparging conditions based on 16S rDNA analysis by PCR-DGGE.